2015000001143.pdf
Anuncio
UNIVERSIDAD DE CORDOBA DEPARTAMENTO DE QUÍMICA INORGÁNICA E INGENIERÍA QUÍMICA APLICACIÓN DE HIDROTALCITAS Y SÓLIDOS MESOPOROSOS ORDENADOS COMO ADSORBENTES DE HERBICIDAS Memoria presentada por Dña. Rocío Otero Izquierdo para aspirar al Grado de Doctor con mención internacional Universidad de Córdoba Fdo. Rocío Otero Izquierdo Córdoba 2015 por la TITULO: Aplicación de hidrotalcitas y sólidos mesoporosos ordenados como adsorbentes de herbicidas AUTOR: Rocío Otero Izquierdo © Edita: Servicio de Publicaciones de la Universidad de Córdoba. 2015 Campus de Rabanales Ctra. Nacional IV, Km. 396 A 14071 Córdoba www.uco.es/publicaciones [email protected] TÍTULO DE LA TESIS: APLICACIÓN DE HIDROTALCITAS Y SÓLIDOS MESOPOROSOS ORDENADOS COMO ADSORBENTES DE HERBICIDAS DOCTORANDO/A: ROCÍO OTERO IZQUIERDO INFORME RAZONADO DEL DIRECTOR Y DE LA CODIRECTORA DE LA TESIS La Lcda. Rocío Otero Izquierdo ha llevado a cabo con éxito el desarrollo de la Tesis Doctoral “Aplicación de hidrotalcitas y sólidos mesoporosos ordenados como adsorbentes de herbicidas.”, bajo la dirección de los doctores abajo firmantes en los laboratorios del Departamento de Química Inorgánica e Ingeniería Química. La doctoranda ha experimentado una formación continua tanto en la práctica experimental como en la discusión de los resultados obtenidos. Los objetivos marcados en el Proyecto inicial de la Tesis Doctoral se han completado de manera sobresaliente, obteniendo resultados relevantes que han dado lugar a la publicación de cinco artículos científicos en revistas internacionales de prestigio (Q1) y diez comunicaciones en congresos científicos. Por todo ello, se autoriza la presentación de la tesis doctoral. Córdoba,14 de Abril 2015 Firma del director Fdo.: José María Fernández Rodríguez Firma de la codirectora Fdo : Mª Ángeles Ulibarri Cormenzana D. José María Fernández Rodríguez, Catedrático de Escuelas Universitarias de Química Inorgánica del Departamento de Química Inorgánica e Ingeniería Química de la Universidad de Córdoba y Dña. Mª Ángeles Ulibarri Cormenzana, Catedrática de Universidad de Química Inorgánica. CERTIFICAN Que el presente Trabajo de Investigación, titulado Aplicación de hidrotalcitas y sólidos mesoporosos ordenados como adsorbentes de herbicidas y que constituye la Memoria que presenta la Lda. Dña. Rocío Otero Izquierdo para aspirar al Grado de Doctor por la Universidad de Córdoba en Ciencias Químicas, ha sido realizado en los laboratorios del Departamento de Química Inorgánica e Ingeniería Química, bajo nuestra dirección, y reúne las condiciones exigidas para ser presentado como Tesis Doctoral. Y para que conste, expedimos y firmamos el presente certificado en Córdoba, a 14 de Abril de 2015. Fdo.: José María Fernández Rodríguez Fdo.: Mª Ángeles Ulibarri Cormenzana Mediante la defensa de esta Memoria se pretende optar a la mención de Doctor Europeo, habida cuenta de que la doctoranda reúne los requisitos exigidos por tal mención, a saber: 1- Informes de dos doctores pertenecientes a Instituciones de Enseñanza Superiores de otros países: - Silvia Real. INIFTA, Universidad de la Plata, Argentina. - Jeriffa De Clercq, Universidad de Gante, COMOC, Bélgica. 2- Un miembro del tribunal que ha de evaluar la Tesis pertenece a un centro de Enseñanza Superior de otro país. -Dr. Els de Canck, researcher of COMOC (Center for Ordered Materials, Organometallics and Catalysis). 3- La defensa de parte de esta Memoria se realizará en la lengua oficial de otro país europeo: inglés. 4- Estancia de cuatro meses en un centro de investigación de otro país europeo: -Departamento de Química Inorgánica y Química Física de la Universidad de Gante, Bélgica. A Mi Familia. AGRADECIMIENTOS Poniendo punto y seguido a esta etapa de mi carrera y haciendo una introspectiva tanto a mi trabajo como a mi vida, llego a la conclusión de que sin el apoyo necesario cualquier meta que te propongas por pequeña que ésta sea, puede ser una auténtica odisea. Por eso y porque es de bien nacido ser agradecidos, estas palabras son para todas las personas que me han apoyado de una manera u otra. Quiero expresar mi más sincero agradecimiento a mis directores de esta Tesis Doctoral, D. José María Fernández Rodríguez y Dª Mª Ángeles Ulibarri Cormenzana por todo su tiempo dedicado, ayuda y conocimientos aportados para la realización de este trabajo los cuales han sido fundamentales para mi formación como investigadora. Me gustaría agradecer a todos los miembros pertenecientes al grupo FQM 214. A las Doctoras Cristi e Ivana por todos los consejos y enseñanzas recibidas durante estos años. A mis compañeras de laboratorio por todos los buenos ratos que hemos pasado y en especial a Dikra que me inició en el campo de la investigación y con la que pasé las mejores tardes aprendiendo síntesis mientras intercambiábamos nuestras diferentes opiniones culturales. Agradecer también a todos los Doctores, licenciados o colaboradores del Departamento de Química Inorgánica por todo el compañerismo y apoyo recibido. A todos mis compañeros becarios y contratados que han pasado o están pasando por esta etapa y que me han ayudado siempre a resolver todas mis dudas y problemas. También a todos los que me brindan su amistad día a día y compartimos ratos de charla en los pasillos y laboratorios. Al profesor Francisco Romero Salguero del Departamento de Química Orgánica que sin tener ninguna obligación me ayudó como a cualquiera de sus doctorandos. Al Servicio Central de Apoyo a la Investigación (SCAI) de la Universidad de Córdoba por toda su ayuda prestada para la realización de diversas técnicas necesarias para el desarrollo de esta Tesis Doctoral. Aprovecho la ocasión para hacer una especial mención a todas las personas que hicieron que me sintiera como en casa durante mi estancia en Bélgica. Al Profesor Pascal Van der Voort del Departamento de Química Inorgánica (COMOC) de la Universidad de Gante por sus conocimientos, paciencia, motivación y sobretodo enseñarme una manera de trabajar la cual hizo cambiar mi concepto de investigación. A Dolores por ayudarme en todo desde ese primer día que pensé que en lugar de aterrizar en Bélgica había llegado al Polo Norte. Hiciste mi estancia más cómoda dentro y fuera del laboratorio. El día que me fui, sé que me deje una amiga. A todos los componentes del Grupo COMOC con los cuales aprendí mucho y pasé buenos momentos. A los proyectos AGL2008-0403-C02-02/AGR y CTM2011-25325 del Ministerio de Ciencia y Tecnología, mediante el grupo de investigación FQM-214, así como una beca de Formación de Profesorado Univesritario (FPU) finaciada por el Ministerio de Ciencia e Innovación. A mis amigas de dentro y fuera de la Facultad por estar ahí cuando más os he necesitado y que a su modo han sabido siempre ayudarme. Todo esto nunca hubiera sido posible sin mi familia, fuente de apoyo constante e incondicional en toda mi vida. A mi madre por darme siempre su mano cuando más lo he necesitado, por estar siempre a mi lado, por ser mi confidente y mi mejor amiga. Gracias por ese amor incondicional, que como ya sé, solo una madre puede dar. A mi padre por ser mi ejemplo, por sus consejos y sobretodo por mostrarme el mejor camino a seguir. Gracias por todo lo que me has dado e inculcado en la vida. A mi hermana Marí por creer siempre en mí y darme dos de las mejores alegrías de mi vida, Aitana y Paula, que siempre me han sacado una sonrisa hasta en los malos momentos. Finalmente a las dos personas más importantes, los pilares de mi vida, Juan y Rocío. A Juan por su amor, cariño y comprensión a lo largo de estos años que a veces han sido duros pero que hemos sabido superar sin ningún problema. Gracias por todo el apoyo incondicional, por ser mi amigo leal y principalmente por lo mejor que me ha pasado en la vida, Rocío, quien ha sido mi motivación para culminar este Doctorado. Para el mejor resultado obtenido en esta Tesis Doctoral, Rocío Índice CAPÍTULO I. HIPOTESIS Y OBJETIVOS ............................................................................... 1 CAPÍTULO II. INTRODUCCIÓN .......................................................................................... 9 1. GENERALIDADES.................................................................................................... 11 2. DINÁMICA DE LOS PLAGUICIDAS EN EL SUELO ..................................................... 13 2.1. Procesos de transferencia.............................................................................. 15 2.1.1. Adsorción-Desorción .............................................................................. 15 2.1.2. Otros procesos de transferencia ............................................................ 16 2.2 Procesos de transformación ........................................................................... 17 3. CONTAMINACIÓN DEL AGUA POR PLAGUICIDAS.................................................. 19 4. MEDIDAS DE ADSORCIÓN. ISOTERMAS ................................................................ 20 5. ADSORBENTES EMPLEADOS .................................................................................. 24 5.1 Hidrotalcitas .................................................................................................... 24 5.1.1 Métodos de síntesis................................................................................. 28 5.1.2. Incorporación de aniones en la estructura............................................. 31 5.1.3. Aplicaciones ............................................................................................ 32 5.2 Materiales porosos ......................................................................................... 35 5.2.1 Materiales híbridos.................................................................................. 42 5.2.1.1 Síntesis de materiales híbridos de sílice........................................... 43 5.2.2 PMOs ....................................................................................................... 44 5.2.2.1 Tipos de PMOs y sus síntesis ............................................................ 48 5.2.2.2 Aplicaciones de PMOs ...................................................................... 50 5.2.3 Mesoporosos ordenados de carbón ........................................................ 53 5.2.3.1 Tipos de síntesis ............................................................................... 54 5.2.3.2 Aplicaciones de las resinas fenólicas y carbones mesoporosos ordenados .................................................................................................... 59 BIBLIOGRAFÍA ............................................................................................................ 62 CAPÍTULO III. MATERIALES Y MÉTODOS ........................................................................ 79 1. MATERIALES ..................................................................................................... 81 1. 1 Síntesis de adsorbentes ................................................................................. 82 1.1.1 Hidrotalcita con carbonato en la interlámina.......................................... 82 1.1.2. Hidrotalcita con anión orgánico en la interlámina ................................. 82 1.1.3 Etileno y fenileno PMOs .......................................................................... 83 1.1.4 Resina fenólica y carbon mesoporosos ordenados ................................. 84 1.2 Pesticidas ........................................................................................................ 85 1.2.1 S-Metolacloro .......................................................................................... 85 1.2.2 Mecoprop-P ............................................................................................. 87 1.2.3 Nicosulfuron ............................................................................................ 88 1.2.4 Bentazona ................................................................................................ 88 2.MÉTODOS ............................................................................................................... 91 2.1 Espectroscopía de Absorción Atómica............................................................ 91 2.2 Difracción de Rayos X...................................................................................... 92 2.3 Microscopía Electrónica .................................................................................. 93 2.3.1 Microscopía Electrónica de Transmisión ................................................. 93 2.3.2 Microscopía Electrónica de Barrido......................................................... 93 2.4. Espectroscopía Fotoelectrónica de Rayos X (Xps) ......................................... 94 2.5 Espectroscopía Infrarroja con Transformada de Fourier ................................ 95 2.6 Espectroscopía Resonancia Magnética Nuclear ............................................. 95 2.7 Análisis Termogravimétrico (TGA) .................................................................. 96 2.8 Isotermas de Adsorción-Desorción de Nitrógeno........................................... 97 2.9 Potencial Zeta ............................................................................................... 100 2.10 Espectroscopía Ultravioleta Visible............................................................. 100 2.11 Tamaño de partícula ................................................................................... 101 CAPÍTULO IV. ADSORPTION OF NON-IONIC PESTICIDE S-METOLACHLOR ON LAYERED DOUBLE HYDROXIDES INTERCALATED WITH DODECYLSULFATE AND TETRADECANEDIOATE ANIONS ................................................................................... 103 CAPÍTULO V. PESTICIDES ADSORPTION-DESORPTION ON Mg-Al MIXED OXIDES. KINETIC MODELING, COMPETING FACTORS AND RECYCLABILITY. ........................................... 127 CAPÍTULO VI. ADSORPTION OF THE HERBICIDE S-METOLACHLOR ON PERIODIC MESOPOROUS ORGANOSILICAS .................................................................................. 153 CAPÍTULO VII. MESOPOROUS PHENOLIC RESIN AND MESOPOROUS CARBON FOR THE REMOVAL OF S-METOLACHLOR AND BENTAZON HERBICIDES ................................... 177 CAPÍTULO VIII. CONCLUSIONES GENERALES GENERAL CONCLUSIONS ....................... 205 CAPÍTULO IX. SUMMARY ............................................................................................. 219 CAPÍTULO X. ABREVIATURAS ....................................................................................... 229 CAPÍTULO XI. PUBLICACIONES E INDICIOS DE CALIDAD .............................................. 233 CAPÍTULO I. HIPOTESIS Y OBJETIVOS Hipótesis y objetivos La presente memoria de Tesis Doctoral se ha encuadrado en las líneas de investigación que desarrolla actualmente el grupo FQM-214 de la Universidad de Córdoba. Durante el desarrollo de esta memoria se han realizado colaboraciones con el grupo FQM-346 del Departamento de Química Orgánica de la Universidad de Córdoba, con el grupo AGR 268 del Instituto de Recursos Naturales y Agrobiología de Sevilla, IRNASE-CSIC, así como una estancia de cuatro meses en el Departamento de Química Inorgánica (COMOC) de la Universidad de Gante. La contaminación del agua se define como la introducción por el hombre en el ambiente acuático de elementos abióticos o bióticos que causen efectos dañinos o tóxicos, perjudiquen los recursos vivos, constituyan un peligro para la salud humana, obstaculicen las actividades marítimas, disminuyan la calidad del agua o los valores estéticos y de recreación [FAO, 1992]. Uno de los contaminantes que afectan principalmente a la calidad del agua son los pesticidas químicos, los cuales son usados por el hombre para eliminación de plagas, enfermedades y malezas que afectan a los cultivos agrícolas. El uso excesivo de estos compuestos químicos ha provocado un gran problema en el medioambiente. Esta Tesis Doctoral trata de aportar conocimientos sobre la utilización de materiales para la eliminación de algunos pesticidas presentes en el agua. Así, el objetivo principal del trabajo presentado en esta Memoria ha sido la preparación de diversos materiales tales como hidrotalcitas, bien calcinadas o bien modificadas con compuestos orgánicos de cadena larga, y sólidos mesoporosos ordenados para su posterior utilización como adsorbentes de pesticidas. La elección de los materiales se realizó en función del carácter iónico del pesticida así como del pH del medio en el cual se encuentran los mismos. Por ello, a pHs bajos se trabajará con compuestos mesoporosos mientras a pHs altos con hidrotalcitas. En la bibliografía existen numerosos estudios de adsorción de pesticidas usando hidrotalcitas con aniones inorgánicos en la interlámina. Sin embargo, pocos estudios abordan el uso de organohidrotalcitas para la eliminación de contaminantes orgánicos. En la bibliografía, el estudio de la relación entre la longitud de la cadena del anión orgánico en la interlámina de la organohidrotalcita y la adsorción de pesticidas es muy 3 Capítulo I limitado. Por ello, el primer objetivo ha sido la búsqueda de aniones orgánicos de cadena larga que permitan introducir en la interlámina de la hidrotalcita distintas especies químicas contaminates. En el estudio de la adsorción de pesticidas, el procedimiento de adsorcióndesorción es de importancia capital. Es habitual encontrar un procedimiento que tal vez sea muy oportuno para el estudio de liberación lenta de pesticidas en cultivos, ya que intenta reproducir las condiciones de campo, de manera que modeliza bien la desorción del pesticida en los suelos, pero no es útil para modelizar la interpretación de la reversibilidad o no del proceso de desorción, en definitiva no permite conocer claramente la naturaleza del proceso de adsorción-desorción. El procedimiento suele consistir en la eliminación de una cierta cantidad del sobrenadante por la misma cantidad de agua destilada, dejar en contacto 24 h y repitir varias veces la misma experiencia. El aspecto más negativo es no poder interpretar adecuadamente los casos en los que la isoterma de desorción se encuentra por debajo de la isoterma de adsorción, a lo cual ciertos autores denominan “artefacto experimental”. Una explicación podría ser la pérdida de adsorbente durante su manipulación. En esta memoria uno de los objetivos más importantes ha sido la búsqueda de un nuevo procedimiento de adsorción-desorción, en concreto para la rama de desorción, que permita conocer mejor la naturaleza del proceso de adsorción–desorción y que sea comparable con el procedimiento empleado en la adsorción, siendo útil para modelizar la interpretación de la reversibilidad o no del proceso de desorción. Un parámetro de gran transcendencia es la temperatura a la que se realiza el proceso de adsorción, ya que de ella va a depender la cantidad adsorbida y estará en consonancia con la naturaleza físico-química del proceso. Por esta razón, se realizarán isotermas de adsorción a varias temperaturas en una cámara de temperatura constante. Otro aspecto a tener en cuenta es el estudio de la regeneración de los adsorbentes y sus implicaciones medioambientales. La regeneración de la hidrotalcita calcinada ha sido muy estudiada en los últimos años mediante calcinación a 500°C. Sin embargo, no se encuentran demasiadas referencias del reciclado de órgano4 Hipótesis y objetivos hidrotalcitas con disolventes orgánicos. En la presente Tesis Doctoral se propondrá la utilización de disolventes orgánicos como método de regeneración, entre otros. En relación a los objetivos concretos, se investigará la adsorción de un pesticida no iónico, S-Metolacloro, en dos organohidrotalcitas diferentes (OHTs), intercaladas con aniones dodecilsulfato (HT-DDS) una y con tetradecanodioato (HT-TDD) la otra preparados usando el método de coprecipitación. Uno de los objetivo será estudiar los cambios estructurales como consecuencia de la introducción de aniones voluminosos tales como DDS y TDD, así como la variación del espaciado interlaminar por efecto de la adsorción del pesticida SMetolacloro. Se estudiará la cinética de adsorción e isotermas de adsorción a varias temperaturas, así como, la regeneración del adsorbente con disolventes orgánicos con el objetivo de analizar su reciclabilidad. La descomposición de la hidrotalcita cuando es calcinada en torno a 500°C da lugar a una mezcla de óxidos metálicos, que se caracteriza por elevadas superficies específicas y una dispersión homgenea de los mismos. La mezcla de óxidos metálicos puede rehidratarse y combinarse con aniones de la solución acuosa recuperando la estructura original. Se abordará la adsorción de un pesticida ionizable, Nicosulfuron, y un pesticida iónico, Mecoprop-P, en una hidrotalcita de Mg-Al calcinada a 500⁰C (HT-500). Se realizarán experimentos de adsorción-desorción en los que se contemple la influencia de la presencia simultánea de aniones tales como carbonato, nitrato, sulfato, cloruro o fosfato en la adsorción de los pesticidas, así como la extracción de éstos del complejo soporte-pesticida mediante los aniones citados para comprobar su eficacia como desorbentes. El estudio se completará con el análisis del proceso de regeneración de la hidrotalcita calcinada ya que diversos estudios ponen de manifiesto que se produce una disminución de la capacidad de adsorción después de cada ciclo de adsorción5 Capítulo I calcinación. Esta disminución podría estar justificada por los cambios en la morfología de las partículas después de diversos ciclos, para cuya comprobación se utilizará microscopía electrónica de trasmisión (TEM). Materiales tales como las organosilices mesoporosas periódicas (PMOs) tienen propiedades únicas para ser utilizadas como adsorbentes de moléculas orgánicas en virtud de su gran área superficial y de una estrecha distribución de mesoporos, de manera similar a sílices mesoporosas (PMS), así como de la naturaleza orgánica de sus puentes que pueden interactuar con ellos. Por esta razón se sintetizarán dos organosilices (etano-PMO y benceno-PMO) y una sílice (PMS), siguiendo un procedimiento similar. Tanto en el caso de las hidrotalcitas como de estos nuevos adsorbentes, se procederá a la modelización de las cinéticas e isotermas de adsorción mediante diversas ecuaciones matemáticas. Las ecuaciones de pseudo-primer y pseudo-segundo orden y la ecuación de Morris-Wever para el ajuste de las cinéticas, y las ecuaciones de Freundlich, Langmuir y Dubinin-Radushkevich para las isotermas de adsorción. Otro aspecto a considerar es la determinación de la capacidad máxima de adsorción de ambos soportes, procediéndose si fuera necesario a realizar adsorciones sucesivas. Finalmente, el carbón activo es el adsorbente universal empleado en la eliminación de una amplia variedad de contaminantes. Tiene una gran ventaja como es su bajo coste pero también tiene inconvenientes ya que al ser un material microporoso no permite la adsorción de moléculas grandes y además el carbón en polvo es más difícil de manipular. Los carbones mesoporosos ordenados tienen buenas propiedades de adsorción por lo que podrían ser utilizados en sustitución de los carbones activos comerciales. Se examinará la adsorción de S-Metolacloro, en función del pH, en carbón mesoporoso ordenado (MC) y en su material de partida, una resina mesoporosa fenólica ordenada (MPR), que se sintetizarán siguiendo un método de “soft template”, así como en un carbón comercial (CC) para poder dilucidar las ventajas e 6 Hipótesis y objetivos inconvenientes de éste frente a los dos nuevos materiales, procediéndose a la modelización de las cinéticas e isotermas de adsorción mediante diversas ecuaciones matemáticas. Previamente se caracterizaran estos materiales mediante el análisis de su área superficial, la distribución de tamaño de poro, la distribución de tamaño de párticula y microscopía electrónica, entre otros. En estos materiales se evaluará la regeneración mediante calcinación bajo atmósfera de nitrógeno contemplando aspectos como eficiencia o pérdida de material en cada ciclo de adsorción-desorción. Otro pesticida objeto de estudio en estos materiales será el plaguicida de postemergencia denominado Bentazona. 7 CAPÍTULO II. INTRODUCCIÓN Introducción 1. GENERALIDADES Los plaguicidas según el Real Decreto núm. 3349/83 de 30 de noviembre de 1983 sobre “Reglamento técnico-sanitario para la fabricación, comercialización y utilización de plaguicidas”, establece una serie de definiciones entre las que cabe destacar: - Combatir los agentes nocivos para los vegetales y productos vegetales o prevenir su acción. - Favorecer o regular la producción vegetal, con excepción de los nutrientes y los destinados a la enmienda de suelos. - Conservar los productos vegetales, incluida la protección de las maderas. - Destruir los vegetales indeseables. - Destruir parte de los vegetales o prevenir un crecimiento indeseable de los mismos. - Hacer inofensivos, destruir o prevenir la acción de otros organismos nocivos o indeseables distintos de los que atacan a los vegetales. Según su naturaleza química, pueden clasificarse en inorgánicos y orgánicos. Los primeros, no plantean un problema importante desde el punto de vista de su toxicidad y evolución en el suelo. Sin embargo, en lo que se refiere a los orgánicos, se ha ido desarrollando una amplia gama de productos que plantean problemas de evolución en el complejo sistema del suelo. La toxicidad de estos pesticidas esta relacionada con la manipulación de los compuestos, la toxicidad residual en alimentos y su evolución en el suelo. La evaluación de la toxicidad se lleva a cabo mediante ensayos en el laboratorio en animales de experimentación y su expresión cuantitativa se expresa mediante la dosis letal media DL50 (cantidad de pesticida necesario para causar la muerte al 50% de los individuos que componen el lote de ensayo). La toxicidad residual se refiere a los residuos en los alimentos y a la contaminación del medio biológico. Para evitar intoxicaciones, los Organismos Internacionales, como la FAO (Organización para la Agricultura y la Alimentación de las Naciones Unidas) y la OMS (Organización Mundial 11 Capítulo II de la Salud) establecen unos niveles máximos admisibles respecto a la ingestión de plaguicidas normalmente utilizados en distintos países. Existen diversos factores que influyen en la persistencia y evolución de los pesticidas en el suelo, los cuales se esquematizan en la Figura 1. Estos factores además pueden verse afectados por otras variables como la humedad, la temperatura, el pH, la materia orgánica, etc. Figure 1. Pesticide evolution in the soil. Debido al uso excesivo de los plaguicidas en la agricultura, en 2009 se adoptó un paquete de medidas sobre estos compuestos por la Directiva 2009/128/CE sobre el uso sostenible de los plaguicidas, orientada a reducir los riesgos ambientales y sanitarios y a mantener la productividad de los cultivos mejorando el control del uso y de distribución de estos. Los gobiernos aprobarán fitosanitarios a nivel nacional o mediante “reconocimiento mutuo”, obligatorio dentro de la misma zona, ya que, 12 Introducción según el nuevo Reglamento, la UE estará dividida en tres zonas (norte, centro y sur) según las condiciones agrícolas, climatológicas y ecológicas de los países. La Organización de las Naciones Unidas para la Alimentación y la Agricultura (FAO) señala que la invasión de malezas, las enfermedades de las plantas y los insectos, provocan la pérdida de entre 30% y 35% de las cosechas. Sin el uso de plaguicidas las pérdidas serían mayores. Sin embargo, debido al uso de agroquímicos todos los años resultan intoxicados alrededor de 25 millones de trabajadores agrícolas, de los cuáles mueren unos 20.000. Ésto, sin considerar los errores de diagnóstico, especialmente cuando los casos de envenenamiento, no se comunican a las autoridades o no se registran. Es por ello, que muchos investigadores estudian la eliminación de estos plaguicidas mediante técnicas como fotocatálisis [1, 2], electrolisis [3, 4] y adsorción [5-7], entre otras. Dentro de estas técnicas la que ofrece mayores ventajas es la de adsorción ya que es mucho más económica, tiene una alta eficacia en la eliminación de los plaguicidas y el manejo es mucho más sencillo. 2. DINÁMICA DE LOS PLAGUICIDAS EN EL SUELO La presencia de los plaguicidas en los suelos agrícolas se produce por diversas vías. Unas veces se debe a los tratamientos que se efectúan sobre las partes aéreas de los cultivos para combatir sus plagas, depositándose aproximadamente un 50% del producto en el suelo. Este es el caso de los insecticidas, fungicidas y algunos herbicidas. Otras veces, caso por ejemplo de los neumaticidas, desinfectantes y la mayoría de los herbicidas, el tratamiento se efectúa directamente al suelo, apareciendo el producto en cantidades mayores. Los plaguicidas una vez en el suelo, entran en un ecosistema dinámico y empiezan a moverse en el mismo, a degradarse “in situ”, a desplazarse del sistema inicial a otros sistemas o a mantenerse en él con su estructura original o más o menos degradada durante un período de tiempo variable. 13 Capítulo II La desaparición de los plaguicidas en el suelo transcurre en tres etapas. Una fase de corta duración en la que el plaguicida mantiene una determinada concentración llamada de latencia. Una segunda etapa, la cual es relativamente rápida en lo que respecta a su desaparición del suelo, denominada de disipación. Finalmente, la última etapa, conocida como persistencia de plaguicida, es más lenta y puede durar horas, días, semanas e incluso años. Una vez el plaguicida en el suelo, puede verse afectado por diferentes procesos (de tipo físico, químico o biológico) que determinan su dinámica y destino final en el suelo. Estos procesos, como se ilustra en la Figura 2, se dividen en procesos de transferencia o transporte y en procesos de degradación o transformación. Transferencia o transporte Transformación o degradación •Adsorción-desorción •Volatilización •Lixiviación •Escorrentia •Absorción •Difusión •Degradación química •Biodegradación •Fotodegradación Figure 2. Dinamics of pesticides in soils. Los procesos de transferencia o transporte son aquellos en los que el compuesto se desplaza de un medio a otro o dentro de un mismo medio sin experimentar cambio en su estructura. Dentro de estos procesos se destaca la adsorción-desorción, volatilización, lixiviación, escorrentía y difusión (Figura 3). Sin embargo, los procesos de transformación o degradación, implican la modificación del plaguicida en un nuevo compuesto que puede ser de igual, menor o mayor toxicidad. Estos procesos se subdividen en degradación fotodegradación. 14 química, biodegradación y Introducción De todos los mecanismos implicados en la evolución de los plaguicidas en el suelo, la adsorción-desorción es el proceso más importante por influir directa o indirectamente en la magnitud y efecto de los otros. Figure 3. Principal factors affecting pesticides 2.1. Procesos de transferencia 2.1.1. Adsorción-Desorción La adsorción del plaguicida en el suelo consiste en la acumulación del mismo en la interfase sólido-agua o sólido-aire, siendo la desorción el proceso inverso [8]. El adsorbato es el compuesto que se une a la fase sólida, mientras que el adsorbente es la fase sólida en sí. El proceso de adsorción es ampliamente usado ya que es muy fácil de diseñar y de operar así como relativamente simple a la hora de regenerar el adsorbente [9]. Se pueden encontrar varios tipos de interacciones en la adsorción de pesticidas. Pueden darse interacciones de Van der Waals (proceso mediante el cual los iones de una sustancia se concentran en una superficie como resultado de la atracción electrostática en los lugares cargados en la superficie) como es el caso del pesticida Picloram en la montmorillonita [10]. Cuando un pesticida tiene comportamiento 15 Capítulo II catiónico puede intercambiarse con los cationes que saturan la materia orgánica y de esta manera quedan retenidos por fuerzas electrostáticas. Este tipo de interacción es de cambio iónico y se puede observar en la adsorción del pesticida Paraquat en suelos orgánicos [11]. Existen también las interacciones por puentes de hidrógeno que se producen por la atracción electrostática entre un núcleo de H electropositivo y pares de electrones sin compartir de átomos electronegativos. Este tipo de interacción se produce en la adsorción del pesticida Thiazafluron en suelos [12]. Por otra parte las interacciones hidrofóbicas son responsables de la adsorción en el caso de pesticidas neutros tales como Metolachlor [13] o Triadimefon [14]. Existen algunos pesticidas que pueden establecer un equilibrio rápido y reversible entre el compuesto en disolución y el compuesto adsorbido sobre la superficie del suelo. Esto ocurre, por ejemplo, con el pesticida Fenantreno al reaccionar con la materia orgánica del suelo e incorporarse a la misma por reacciones de oxidación [15]. Este tipo de interacción se denomina covalente. También se pueden observar enlaces de cambio de ligando en plaguicidas aniónicos y complejos de transferencia de carga como en la Atrazina [16]. 2.1.2. Otros procesos de transferencia La volatilización (Figuras. 2 y 3) consiste en la pérdida de un compuesto en forma de vapor. Una vez en fase gas, el compuesto puede desplazarse lejos desde el sitio inicial de aplicación. Numerosos estudios realizados con modelos explican que el movimiento de los compuestos volátiles por el suelo se ve influenciado, en mayor medida por los procesos de difusión [17, 18] los cuales dependen de las características físicas y químicas del compuesto y del suelo [19]. La difusión (Figura 2) se define como el movimiento de moléculas a causa de un gradiente de concentración. Este movimiento al azar provoca el flujo de materiales desde las zonas de mayor concentración hacia las de menor concentración. Existe una gran cantidad de pesticidas solubles en agua que se desplazan fácilmente a través del suelo. Este fenómeno es conocido como lixiviación (Figuras. 2 y 16 Introducción 3) y puede provocar la contaminación de las aguas superficiales y subterráneas, además de reducir la productividad agrícola. La lixiviación depende de las propiedades de los pesticidas, como por ejemplo la tendencia a ser degradados o a volatilizarse desde el suelo, del lugar de aplicación, incluyendo la estructura, textura y contenido de materia orgánica del suelo, así como los parámetros climáticos y finalmente también depende de la prácticas agrícolas que determinan el tipo de pesticida aplicado, el momento de aplicación y de los cultivos tratados. De todos los factores de los que depende la lixiviación, los que más contribuyen a la variabilidad de ésta son las propiedades del pesticida, seguido por las características del lugar de aplicación, el clima, el tipo de cultivo y la corriente de flujo [20]. La escorrentía (Figuras. 2 y 3) tiene lugar cuando la precipitación o el riego superan la tasa de infiltración de agua en el suelo. En este proceso de transporte, influyen diversos factores que determinan en gran medida la cuantía y características de la concentración del soluto en el agua. Las mayores concentraciones de pesticidas en el agua de escorrentía ocurren en la primera lluvia significativa después del momento de aplicación. Sin embargo, lluvias muy intensas provocan escorrentías más tempranas, con mayor velocidad de flujo y por tanto, con más energía disponible para la extracción y transporte de plaguicidas desde la superficie del suelo. Otro factor importante a considerar son las características del suelo ya que la escorrentía de las sustancias puede verse afectada por la textura y contenido de materia orgánica. La naturaleza y las características físico-químicas de los compuestos, así como la zona de aplicación, son también otros elementos que hay que tener en cuenta en el transporte producido en la escorrentía. 2.2 Procesos de transformación Como se ha comentado anteriormente, la diferencia de los procesos de transferencia con respecto a los procesos de transformación es que los primeros implican el movimiento del plaguicida de una fase a otra mientras que en los segundos 17 Capítulo II hay un cambio de estructura en el plaguicida. Dentro de los procesos de transformación se encuentran la degradación química, la biodegradación y la fotodegradación. La degradación química depende de entre otros factores, del “compartimento” medioambiental, es decir aire, tierra o agua, en el cual los compuestos son dispersados y transportados. Ésta, tiene lugar hasta la mineralización + - 2- completa del pesticida dando como productos finales CO2, H2O, NH4 , NO3 , SO4 , etc., dependiendo de cada pesticida en particular aunque en muchas ocasiones, la transformación no es total y el plaguicida se degrada en otros productos cuya toxicidad, movilidad y efectos sobre el medio son prácticamente desconocidos [21]. Si los productos obtenidos tras la degradación son menos tóxicos que la sustancia original se trata de una inactivación o destoxificación, si por el contrario, el producto de degradación resulta con mayor toxicidad que el original, se trata de una activación. La biodegradación es el resultado de los procesos de digestión, asimilación y metabolización de un compuesto orgánico llevado a cabo por protozoos, hongos y otros organismos. En la bibliografía existen numerosos estudios de biodegradación de plaguicidas entre los que se encuentran: biodegración de Endosulfan por el hongo “Aspergillus niger”[22], la biodegración de Methyl Parathion por hongos de origen marino [23] o la biodegradación de Chlorpyrifos por Pseudomonas [24] entre otros. La fotodegradación consiste en la transformación del plaguicida por la luz solar, que pueden tener lugar mediante reacciones de oxidación, reducción, hidrólisis, sustitución e isomerización. El proceso de fotodegradación depende de la intensidad y tiempo de exposición del plaguicida a la radiación solar, la presencia de catalizadores fotoquímicos que pueden favorecer la descomposición, el pH del suelo y el tipo, estado y grado de adsorción del plaguicida. Existen dos efectos diferentes en la fotodegradación: la fotólisis directa y la fotólisis indirecta. La fotólisis directa comprende la absorción directa de la luz por el pesticida seguido de la reacción química correspondiente, sin la participación de otras sustancias químicas. En la fotólisis indirecta, la energía de la luz es absorbida por otros constituyentes del agua y es o bien transmitida a un plaguicida cercano o bien permite 18 Introducción la formación de especies reactivas como radicales hidroxilo, que transforman al plaguicida. Existen especies orgánicas e inorgánicas que pueden acelerar la fotodegradación como los ácidos húmicos y fúlvicos o nitratos y nitritos [25]. 3. CONTAMINACIÓN DEL AGUA POR PLAGUICIDAS A lo largo de la historia, la calidad del agua potable ha sido un factor determinante del bienestar humano. Aunque actualmente su contaminación ha disminuido, el agua insalubre contaminada por fuentes naturales o humanas sigue causando grandes problemas a las personas que se ven obligadas a usarla, tanto para beber como para la irrigación de hortalizas y otras plantas comestibles. Hoy en día, han disminuido las epidemias ocasionadas por bacterias o virus que son causadas por agentes infecciosos transportados en el agua potable, como el cólera, la poliomielitis y otras. Las enfermedades propagadas por ella están, en general, bien controladas y el agua potable en los países tecnológicamente desarrollados está ahora notablemente libre de los agentes causantes de enfermedades que eran contaminantes muy comunes del agua hace sólo unas décadas. La principal preocupación sobre la seguridad del agua es la presencia de contaminantes químicos como productos químicos orgánicos e inorgánicos y metales pesados, procedentes de fuentes industriales, agrícolas y de la escorrentía urbana. Dentro de los contaminantes orgánicos se encuentran los herbicidas (objeto de estudio en esta Tesis Doctoral). El primer plaguicida conocido como DDT (diclorodifeniltricloroetano) se introdujo durante la Segunda Guerra Mundial y marcó el principio de un período de crecimiento muy rápido en el uso de pesticidas. La contaminación del agua por plaguicidas se produce al ser arrastrados por el agua de los campos de cultivo hasta los ríos y mares donde se introducen en las cadenas alimenticias provocando la muerte de varias formas de vida necesarias en el balance de algunos ecosistemas. 19 Capítulo II Los herbicidas se aplican en millones de hectáreas de tierra de labranza en todo el mundo. Éstos se encuentran en aguas superficiales y subterráneas y son particularmente problemáticos para las fuentes de agua potable. Sus niveles varían con la estación y con las veces que son aplicados para controlar malezas. Así, los más solubles son los que tienen mayor probabilidad de entrar en las fuentes de agua potable. El tratamiento con carbón activado o activo es el mejor medio para eliminar herbicidas y sus metabolitos de las fuentes de agua potable. Sin embargo, un problema con el carbón activo es el de la precarga, en que la materia orgánica natural en el agua satura el carbón e impide la incorporación de compuestos orgánicos contaminantes como los herbicidas. Es por ello, que el principal objetivo de esta Tesis Doctoral sea la búsqueda de nuevos materiales adsorbentes. 4. MEDIDAS DE ADSORCIÓN. ISOTERMAS En la adsorción física las moléculas están adsorbidas por fuerzas de Van der Waals y no están fijas en un lugar específico de la superficie del sólido. Esta adsorción, en general, predomina a temperaturas bajas. Sin embargo, cuando el adsorbato sufre una interacción química con el adsorbente, la adsorción se denomina química o quimisorción. Las energías de adsorción son elevadas debido a que el adsorbato forma unos enlaces fuertes localizados en los centros activos del adsorbente. Este tipo de adsorción, esta favorecida a alta temperatura. La mayor parte de los fenómenos de adsorción son combinaciones de las diferentes formas de adsorción física y químicas. La desorción es el paso del adsorbato del estado adsorbido a la fase líquida o gaseosa. La desorción puede ser reversible cuando es total o bien parcial cuando una parte del adsorbato permanece unido al adsorbente de manera irreversible. En la literatura existen varios ejemplos entre los que cabe citar la adsorción de DNP y DNOC en la hidrotalcita con cloruro en la interlámina y su producto calcinado cuyo proceso de adsorción-desorción es irreversible [26] como también ocurre con el pesticida Carbaryl en suelos [27]. Un procedimiento utilizado para caracterizar los procesos de adsorción es la realización de isotermas de adsorción de un sustrato en un soporte. El procedimiento seguido para la realización de éstas es de extrema importancia para que la información 20 Introducción que nos aporte sea la apropiada para explicar los procesos de adsorción. En ellas se hace interaccionar a una temperatura determinada cantidades conocidas de adsorbente con disoluciones con diferente concentración de pesticida. Las isotermas de adsorción corresponden al estado de equilibrio entre la concentración de pesticida en fase líquida (Ce) y la concentración en fase sólida (Cs), de un soluto en particular, después de un tiempo en contacto. La cantidad de pesticida adsorbido se puede obtener por un método directo o indirecto. Con el primer método se mide extrayendo y determinando la cantidad de pesticida presente en el adsorbente mientras que por el segundo método se obtiene por diferencia entre la cantidad de pesticida inicial y la concentración que hay en disolución. Según Giles [28] las isotermas de adsorción se pueden clasificar dependiendo de su forma y del tramo inicial en cuatro tipos (Figura 4). Mediante esta clasificación se puede obtener información del mecanismo de adsorción, la naturaleza del adsorbato y la superficie del adsorbente. Figure 4. Types of adsorption isotherms and representation of different subgroups according to the classification of Giles et al.1960. Isoterma de tipo S: Este tipo de isoterma presenta una primera zona donde las moléculas de soluto compiten con las moléculas de disolvente y por tanto 21 Capítulo II la intensidad de adsorción es baja y una segunda zona, con una mayor pendiente, indicando la atracción que ejercen las moléculas adsorbidas por el resto de moléculas disueltas y que favorece su adsorción. Isoterma de tipo L: Existe una gran afinidad entre el soluto y el adsorbente a bajas concentraciones y va disminuyendo a medida que aumenta la concentración. Estas isotermas se caracterizan por una disminución de la pendiente a medida que se incrementa la concentración del soluto en la disolución, debido a una disminución de los sitios de adsorción. Finalmente, termina con un “plateau” cuando el adsorbente es cubierto completamente por el soluto. Isoterma de tipo H: se caracterizan porque hay una alta afinidad entre el soluto y el adsorbente. La parte inicial de la curva es completamente vertical dado que el soluto es totalmente adsorbido. Es un caso particular de la isoterma de tipo L y suele darse en adsorbatos de alto peso molecular como micelas iónicas o especies poliméricas. Isoterma de tipo C: son isotermas lineales donde se mantiene el equilibrio entre la cantidad de soluto adsorbido y la concentración del mismo en la disolución de equilibrio. Este tipo de isotermas se obtiene con la mayoría de compuestos químicos en un rango de concentraciones bajas. Existen varias ecuaciones matemáticas o modelos de adsorción a las que se puede ajustar las isotermas de adsorción de pesticidas en suelos, entre las que destacaremos las ecuaciones de Freundlich, Langmuir, Dubinin-Radushkevich y Temkin (Tabla 1). La ecuación de Freundlich [29] es una ecuación empírica que se utiliza principalmente para modelizar la adsorción en superficies heterogéneas y asume que los sitios de enlace más intensos se ocupan primero y que la fuerza de enlace disminuye con el aumento del grado de ocupación de los sitios de adsorción. 22 Introducción La ecuación de Langmuir [30] supone que todos los sitios de adsorción son idénticos, por lo que la adsorción se produce en una superficie homogénea del adsorbente y que todos los sitios de adsorción son estérica y energéticamente independientes de la cantidad adsorbida. La ecuación de Temkin [31] se basa en que el calor de adsorción de todas las moléculas en la lámina podría disminuir linealmente debido a las interacciones entre adsorbente/adsorbato. Table 1 Fittings for adsorption isotherm. MODELO Freundlich FÓRMULA MATEMÁTICA log 𝐶𝑠 = log 𝐾𝑓 + 𝑛𝑓 log 𝐶𝑒 Kf y nf dan información de la capacidad e intensidad de adsorción respectivamente 1 1 1 = + 𝐶𝑠 𝐶𝑚 𝐶𝑚 · 𝐿 · 𝐶𝑒 Langmuir Cm es la capacidad máxima de adsorción L es la constante de adsorción de Langmuir Temkin 𝐶𝑠 = 𝐵𝑇 𝑙𝑛𝐾𝑇 + 𝐵𝑇 𝑙𝑛𝐶𝑒 KT y BT son constantes relativas a la capacidad e intensidad de adsorción respectivamente. Dubinin-Raduskevich 𝑙𝑛𝐶𝑠 = 𝑙𝑛𝑞𝑚 − 𝐾𝜀 2 qm es la capacidad de adsorción teórica K es la constante relativa a la energía de adsorción. 𝜀=RTln(1+1/Ce) es el potencial Polanyi Todos estos modelos, no permiten dilucidar el mecanismo de adsorción. Sin embargo, mediante el uso del parámetro (K) de la ecuación de Dubinin-Radushkevich [32], podemos obtener información sobre si la adsorción es de naturaleza química o física aplicando la ecuación: -0.5 E=(2K) 23 Capítulo II -1 Cuando los valores de E están entre 0 y 8 kJ·mol , la adsorción es física, si -1 están entre 8 y 16 kJ·mol se considera debida a intercambio de iones y cuando está -1 entre 20 y 40 kJ·mol se asocia con la presencia de quimisorción. 5. ADSORBENTES EMPLEADOS En el presente apartado se detallan algunos aspectos de los adsorbentes empleados en esta Tesis Doctoral, es decir, las hidrotalcitas y compuestos mesoporosos ordenados (benceno-PMO, etano-PMO, resina fenólica y carbón mesoporoso), cuya comprensión es importante para la discusión de los resultados planteados en este trabajo. 5.1 Hidrotalcitas El mineral hidrotalcita, perteneciente al grupo de las arcillas aniónicas, se descubrió en Suecia en torno a 1842 y es un hidroxicarbonato de magnesio y aluminio. A partir de ahí, diversos minerales isoestructurales con la hidrotalcita, pero con otros elementos en su composición, fueron agrupados con el nombre general de compuestos tipo hidrotalcita. La estructura de la hidrotalcita deriva de la brucita (Figura 5) por sustitución isomórfica de parte de los cationes divalentes por cationes de mayor carga pero radio 3+ 3+ 3+ similar como Al , Fe , Co , etc. Esta sustitución parcial implica que las láminas quedan cargadas positivamente. Para mantener la electroneutralidad del compuesto, se introducen aniones en la interlámina que comparten este espacio con moléculas de agua (Figura 6). Las láminas se pueden apilar dando un ordenamiento hexagonal, es decir hay dos láminas de hidróxido por celda (2H), donde cada catión metálico se encuentra sobre el de la capa inferior, o bien en un ordenamiento romboédrico con 3 láminas de hidróxido por celda unidad (3R) donde cada catión metálico superior se encuentra desplazado 2/3 de a y 1/3 b respecto al catión metálico de la lámina inferior [33]. 24 Introducción Figure 5. Structure of brucite. Las hidrotalcitas o arcillas aniónicas, también denominados hidróxidos dobles laminares, son sólidos básicos bidimensionales de composición [M x/n·mH2O II 2+ 2+ 2+ III 3+ 3+ 3+ n- II III n1-xM x(OH)2]A 2- 2- - donde M =Mg , Zn , Ni , etc., M =Al , Fe , Cr …, A =CO3 , SO4 , Cl … y x varía entre 0.2 y 0.33 [34]. En la naturaleza, las arcillas aniónicas no son tan frecuentes como las catiónicas pero sí son baratas y fáciles de sintetizar. En las hidrotalcitas sus propiedades estructurales vienen determinadas por la II III naturaleza de los cationes metálicos, por la relación M /M , por el anión interlaminar y por el contenido de agua. Naturaleza de los cationes metálicos Existen un gran número de HDLs naturales y sintéticos conteniendo varios cationes metálicos. Algunos de los cationes divalentes son: Mg, Mn, Ni, Cu, Zn y Ca y trivalentes: Al, Cr, Mn, Fe, Co. Ni, La y Ga, entre otros. La incorporación de nuevos cationes en la lámina sigue siendo uno de los retos de los procesos de síntesis en hidrotalcitas, habiéndose descrito en los últimos años la preparación de nuevos materiales. Además, es posible preparar hidróxidos dobles laminares con más de dos cationes diferentes. Zhang y col. [35] sintetizaron una hidrotalcita de ZnFeAl-Cl mediante el método de coprecipitación usando flujo de nitrógeno para poder eliminar 25 Capítulo II bromato de disoluciones acuosas. Guo y col. [36] prepararon una hidrotacita con los 2+ 2+ metáles Cu , Mg 3+ y Fe para poder reducir el arseniato en agua. En la bibliografía existen numerosas publicaciones ya que de esta manera hay casi una infinidad de combinaciones posibles. Figure 6. Structure of hidrotalcite. II Relación M /M III II III En muchos casos, la relación M /M varía según las condiciones de síntresis y la concentración de las sales [37]. Según la fórmula general el valor de x da información del grado de sustitución del catión divalente por el trivalente y por ello el exceso de carga positiva en la lámina. Por lo general, x varía entre 0.1≤x≤0.33. Al alejarse de este intervalo favoreceremos la nucleación y segregación de una fase del catión que se encuentre en exceso. Composición del anión interlaminar En principio, no hay limitaciones en el tipo de anión que puede ocupar el espacio interlaminar de un HDL. No obstante, la preparación de materiales con aniones distintos del carbonato, en diversas ocasiones presenta ciertas dificultades debido a la gran afinidad que muestran estos materiales al contaminarse por dicho anión, así como la posible inestabilidad de los aniones en el intervales de pH en el que el HDL es estable. Originalmente se sintetizaron sólidos que contenían aniones sencillos pero a medida que avanzaron los estudios la familia de HDLs fue ampliamente extendida. En 26 Introducción la tabla 2 se presentan algunos de los aniones que se introdcen en la interlámina de la hidrotalcita. Tabla 2. Some anions in the interlayer of the hydrotalcite. ÁNIÓN INTERLAMINAR Aniones inorgánicos - NO3 [29] 2CO3 [38-40] - [41, 42] Cl - F [42] - Br OH Aniones orgánicos REFERENCIA [42] - [42] 2SO4 [43] Ácido p-aminobenzoico [44] Ácido tánico [45] Ácido Dodecilsulfúrico [46, 47] Ácido sebácico [46] Ácido sulfámico [48] Ácido dodecilbencenosulfato [47] Ácido ferúlico [49] Además de aniones inorgánicos u orgánicos, pueden intercalarse en el espaciado interlaminar una gran variedad de especies, por ejemplo polímeros [50, 51], ácido húmicos [52, 53], polioxometalatos [54] o antibióticos [55], entre otros. Contenido de agua Las moléculas de agua en la hidrotalcita pueden encontrarse en la interlámina o en la superficie de las partículas. Es posible calcular el contenido de moléculas de H2O en la interlámina basándose en el número de sitios presentes en la misma, asumiendo una configuración de átomos empaquetada y restando los sitios ocupados por el anión. Por ello, un aumento en la relación de catión trivalente y divalente provoca un aumento de la concentración del anión interlaminar y, por consiguiente, un descenso en el contenido de agua interlaminar. 27 Capítulo II La hidrotalcita cuando es tratada térmicamente a temperaturas inferiores a 150ᵒC expulsa las moléculas de agua interlaminar sin alterar la estructura laminar, entre 300-500ᵒC tiene lugar la deshidroxilación de las láminas y descomposición del carbonato, entre 500 y 600ᵒC se forma una disolución sólida de Mg(Al)O y a temperaturas superiores a 600ᵒC se forma MgO y la espinela MgAl2O4. Los productos de calcinación de la hidrotalcita tienen la capacidad de reconstruir la estructura inicial al ponerse en contacto con disoluciones que contengan al anión inicial, u otro diferente. A temperaturas inferiores a 600ᵒC el proceso es reversible en presencia de agua y de aniones, lo que permite usar el material como secuestrante de aniones. Algunas de las propiedades más interesantes que caracterizan a los productos calcinados [34, 56, 57] de las hidrotalcitas son: -Entre 300 y 600ᵒC los productos de calcinación tienen la capacidad de reconstruir la estructura laminar al ponerse en contacto con disoluciones acuosas que contienen aniones. A esta propiedad se le conoce como efecto memoria. 2 -Área superficial específica elevada (entre 100-300m /g) -Distribución homogénea de los iones metálicos en los productos de calcinación, térmicamente estables, con formación mediante reducción de metales cristalinos muy pequeños y estables. -Alta capacidad de intercambio aniónico. - Propiedades ácido-base ajustables en función de la presencia de diferentes metales en la lámina. 5.1.1 Métodos de síntesis Los compuestos tipo hidrotalcitas son poco comunes en la naturaleza, mientras que a nivel de laboratorio e industrial su síntesis es relativamente sencilla y de bajo costo [37]. 28 Introducción Alguno de los métodos empleados para la síntesis de los hidróxidos dobles laminares son coprecipitación [58, 59], método de la urea [60, 61], intercambio iónico [62, 63], reconstrucción [64, 65], hidrólisis inducida [37], técnica sol-gel [66, 67], electrosíntesis [68] y métodos asistidos con tratamiento hidrotermal [69-71], ultrasonido [72, 73] y microondas [74, 75]. La coprecipitación es el método de síntesis más común en las arcillas aniónicas. Consiste en la adicción paulatina y por goteo de una disolución que contiene 2+ 3+ la mezcla de dos sales metálicas (M y M ) con anión común, en proporción adecuada a una disolución que contiene un medio acuoso. Se añade disolución alcalina simultáneamente con el fin de fijar el pH a un valor seleccionado para producir la coprecipitación de las dos sales metálicas. Los aniones interlaminares se pueden adicionar de los contraiones de los cationes constituyentes de las láminas metálicas, de los aniones hidroxilo presentes en el medio en el caso de síntesis a pHs muy alcalinos, o bien de alguna sal presente en el medio acuoso. Los parámetros que influyen en las propiedades del sólido final son la temperatura, el pH del medio, la concentración de sales metálicas en la disolución, la velocidad de adicción de los reactivos, el lavado, el secado y el envejecimiento de precipitado. Crepaldi y col. [76], demostraron que los materiales preparados por este método muestran un alto grado de cristalinidad, bajo tamaño de partícula, área superficial específica elevada y un promedio elevado de diámetro de poro. El pH durante la coprecipitación tiene un efecto crucial en la estructura, la química y las propiedades texturales de la fase obtenida. En 1998 se desarrolló un nuevo procedimiento, por Constantino y col. [77], denominado método de la urea. Este nuevo método, a diferencia de la síntesis por coprecipitación, parte de disoluciones homogéneas donde los iones OH- se producen “in situ” mediante la descomposición de la urea la cual libera amoniaco (Figura 7). De 29 Capítulo II esta manera se forma una hidrotalcita con mayor cristalinidad, grandes cristales (rango de µm) y una distribución de tamaño de partícula muy homogénea. Figure 7. Decomposition of urea Cuando un óxido tal como ZnO, NiO y CuO se pone en contacto bajo agitación con una disolución ácida de una sal trivalente como AlCl3 o CrCl3, el óxido se disuelve y se forma el HDL siempre que el pH sea tamponado por el óxido o hidróxido en suspensión. Esta síntesis se conoce como hidrólisis inducida. López y col. [78] fueron los primeros en preparar hidrotalcitas mediante el método de sol-gel. En este tipo de síntesis, el HDL tiene una gran estabilidad hasta 550ᵒC pero tienen el inconveniente de que la cristalinidad es mucho menor que la que se alcanza para las muestras sintetizadas por el método de coprecipitación. La síntesis de ciertas hidrotalcitas también ha sido llevada a cabo por métodos electroquímicos, por ejemplo, a través de la reducción de óxidos de NiCo con peróxido de hidrógeno. Aunque los resultados obtenidos en este tipo de síntesis no son muy buenos desde el punto de vista de cristalinidad del HDL, este método puede ser muy interesante en la preparación de electrodos modificados con HDLs. Además, se pueden realizar tratamientos “in situ” o después de la síntesis para controlar las propiedades texturales y estructurales. Cortas irradiaciones de microondas dan lugar a materiales con mejor cristalinidad lo mismo que ocurre por ultrasonidos o mediante el tratamiento hidrotermal. El método de síntesis en microondas se usa con objeto de evitar las altas temperaturas y los largos procesos de cristalización del método hidrotermal. En general, se obtienen compuestos de tamaño de partícula muy pequeño. 30 Introducción 5.1.2. Incorporación de aniones en la estructura Un aspecto de gran importancia en la síntesis de los HDLs es la naturaleza del anión interlaminar. Basicamente, existen tres métodos para la preparación de hidrotalcitas diferentes de carbonato y que son: coprecipitación o síntesis directa, intercambio iónico y reconstrucción. Mediante la síntesis directa se coprecipitan los cationes sobre una disolución que contiene el anión que se vaya a incorporar en la estructra de la hidrotalcita eligiendo adecuadamente el pH de trabajo. Si el pH es demasiado alto hay que evitar 2- de forma muy cuidadosa la presencia de CO3 - o incluso OH en el espaciado interlaminar y la realización de la síntesis a pH muy ácido inhibe la precipitación de los cationes metálicos. Otro método muy utilizado es el de intercambio iónico que consiste en partir - - de una hidrotalcita previamente intercalada con un anión, como Cl o NO3 , y ponerla en contacto, en forma de suspensión con una disolución que contenga el anión que se desea intercalar. Este método presenta la ventaja de dar hidrotalcitas más cristalinas que las obtenidas por los demás y además, evita en parte la disolución de los cationes laminares cuando se trabaja a pH bajo. Con este tipo de síntesis se han intercalado una gran variedad de aniones para tales como: ácido indol-3-butírico [79], el anión adenosintrifosfato [63], el anión sulfato [62], medicamentos [80, 81], polioxometalatos [54], entre otros. En 1986, Reichle propuso un método de síntesis alternativo basado en la descomposición reversible que experimentan los HDLs cuando se calcinan a temperaturas del orden de 400-600°C, denominado reconstrucción. La estructura de hidrotalcitas es estable hasta 400° y se descarbonata, formando Mg6Al2O8(OH)2 tras calcinación a aproximadamente 500-600°C, siendo este proceso reversible en presencia de agua y aniones, particularidad que puede aprovecharse para incorporar diferentes aniones en la interlámina. 31 Capítulo II El método de reconstrucción es muy utizado cuando se introducen aniones que sólo existan a pH relativamente bajo (4.5-6) ya que a pH alto producirían materiales contaminados por la presencia del anión carbonato. 5.1.3. Aplicaciones Como se ha comentado en el apartado anterior, una de las propiedades más importantes de las arcillas aniónicas es su capacidad para intercambiar aniones, lo cual permite utilizarlas para secuestrar o liberar especies en medio acuoso. Además, los óxidos obtenidos tras calcinar estos materiales presentan excelentes propiedades tales como, alta superficie específica, dispersión homogénea y termodinámicamente estable de los cationes metálicos y propiedades básicas y reductoras importantes. Es por ello, que el número de aplicaciones es extenso y esta bien establecido, lo que ha dado lugar a patentes que inlucran a los HDLs. A continuación, se presentan algunas de las aplicaciones más relevantes de estos compuestos. Los hidróxidos dobles laminares pueden ser utilizados como catalizadores en diversas reacciones químicas, soportes catalíticos o precursores de óxidos catalíticos. La selección de metales involucrados en la composición de un HDL, determinará el tipo de reacción que el compuesto laminar pueda catalizar. Así, una hidrotalcita de Mg/Al con nitrato o dodecilsulfato en la interlámina es capaz de epoxidar el limoneno en presencia de nitrilos con conversiones del 63 al 97%. Existen también reacciones en que el catalizador se activa con luz cuando contiene metales como el Cerio, que al estar disperso en las láminas, es capaz de catalizar la reacción de degradación de compuestos orgánicos. Hay una gran diversidad de composiciones y efectos catalíticos que pueden ser consultados en la bibliografía en relación a la catálisis como la oxidación de alcoholes a aldehídos [82], oxidación de estireno [83], isomerización de glucosa en fructosa [84], eterificación de glicerol a poligliceroles [85], hidrogenación de fructosa [86], etc. El campo de la medicina ha sido y sigue siendo objeto de investigación y aplicación de los HDLs. En un principio, se descubrió que los HDLs son muy útiles como antiácidos para tratamientos gástricos o úlceras duodenales. Su acción en el 32 Introducción tratamiento de las úlceras gástricas se basa en que inhiben la acción del ácido clorhídrico y la pepsina en el jugo gástrico. Años más tarde, al modificar la estructura y propiedades se consiguieron aplicaciones más sofisticadas, utilizándose mayormente como matrices huésped de moléculas con actividad biológica. Dentro del área farmacéutica, las hidrotalcitas mejoran los procesos de disolución de fármacos hidrofóbicos, participan en procesos de liberación controlada e incrementan su estabilidad térmica y mecánica. 2+ 2+ Los cationes como Ca , Mg o Al 3+ que forman las láminas, permiten convertir a las hidrotalcitas en biocompatibles, poco tóxicas y con gran capacidad de protección para el fármaco. Existen muchas investigaciones en relación al intercalado de genes en los HDLs, a la toxicidad celular y a pruebas de transferencia génica in vitro en diferentes líneas celulares. También han sido utilizadas como transportadores de biomacromoléculas. En uno de los trabajos de Choy y col. [87], usaron HDLs para encapsular moléculas de ADN mediante intercambio iónico en el agua. El compuesto resultante, se denominó bioHDL y es empleado para la liberación de las moléculas de ADN en el interior de la célula. Una de las aplicaciones más ingeniosas y quizá la de mayor repercusión industrial es su uso como componente en los materiales de PVC. El principal problema de este material es su degradación frente a la luz directa del sol, por la formación de HCl y radicales que provocan que el PVC se vuelva amarillo. La introducción de pequeñas cantidades de hidrotalcita en el material, captura el HCl mediante intercambio aniónico y por tanto mantiene la fuerza y la blancura del PVC durante más tiempo [88, 89]. Otra aplicación muy interesante es su uso como retardante de la combustión y aislante de cables con cierta resistencia al calentamiento, fruto de sus propiedades térmicas. En efecto, al calentar la hidrotalcita ésta se descompone liberando agua y CO2 lo que le proporciona cierta capacidad extintora de incendios. 33 Capítulo II La hidrotalcita debido a su estructura molecular y su capacidad para intercambiar aniones tienen propiedades para ser usadas como componente activo de cementos y morteros [90]. Además, en el campo de la química analítica los HDLs resultan muy interesantes como modificadores de la superficie de los electrodos con el fin de crear nuevos sensores amperométricos [91-93]. Desde el punto de vista medioambiental, los HDLs actúan como intercambiadores y adsorbentes. Las hidrotalcitas debido a que tiene una alta capacidad de adsorción de especies aniónicas posee una alta capacidad de intercambio aniónico y un espacio interlaminar que permite adsorber moléculas polares y una amplia gama de especies aniónicas. Sin embargo, la principal dificultad radica en evitar la presencia del anión carbonato de mayor afinidad que la mayoría de las especies - contaminantes por lo que a veces se emplean materiales intercalados con Cl en lugar 2- de CO3 . Otro aspecto a considerar es el pH, ya que a veces el intervalo de éste de la especie contaminante en disolución es incompatible con el de la hdrotalcita. Existen un gran número de publicaciones sobre la eliminación de contaminantes inorgánicos entre las que destacan: la adsorción de perclorato [94], 2+ 2+ 2+ nitrato [95], cationes metálicos [96], de Pb , Cu y Cd [53, 97], adsorción de nitrato, VI sulfato y fosfato [98] o Cr [99] entre otros. También se pueden encontrar en la bibliografía trabajos de adsorción de gases como CO2 [100] o SO2 [101]. Del mismo modo, han sido estudiados como adsorbentes de contaminantes orgánicos entre los que se pueden citar la adsorción de los pesticidas 2,4 D, Clopyralid y Picloram usando la hidrotalcita calcinada [102], adsorción del Linuron, 2,4 DB y Metamitron en la hidrotalcita con caprilato en la interlámina [103], fenoles [104] glifosato en hidrotalcitas con diferentes aniones interlaminares [105], 2,4,5 Triclorofenol en hidrotalcita modificada con dodecilbencenosulfato de sodio [106] y de tintes sintéticos [107, 108] para poder ser eliminados de las aguas naturales. Las propiedades físico-químicas de los HDLs los hacen idóneos para la adsorción de plaguicidas de tipo aniónico. Éstos tienen una movilidad en el suelo muy alta debido a la baja capacidad de los componentes del suelo a retener moléculas aniónicas [109]. Este problema no es tan importante para los plaguicidas catiónicos o 34 Introducción de base débil protonable ya que interaccionan con los componentes activos del suelo, concretamente con los minerales de la arcilla. La posibilidad de modificar la interlámina de los HDLs permite que puedan interaccionar con los cationes orgánicos. El efecto memoria de los HDLs los hace muy interesantes en lo que respecta a su reciclabilidad. Así, la hidrotalcita intercalada con la especie contaminante se calcina a 500-600°C para liberarlo. Posteriormente, el tratamiento del óxido formado con una suspensión de agua permite recuperar la estructura laminar de la hidrotalcita manteniéndose su capacidad de adsorción [110]. La adsorción de plaguicidas tiene un carácter reversible que depende de las condiciones experimentales y de la naturaleza de los aniones presentes en la disolución [111]. Existen también plaguicidas neutros que aunque son más fácilmente degradables en el ambiente siguen presentando riesgos de contaminación. Las hidrotalcitas tienen que ser modificadas con aniones orgánicos (anión sebacato, dodecilsulfato, dodecilbencenosulfato…) para poder adsorber contaminantes neutros o de baja polaridad. Existen diversos estudios de adsorción de plaguicidas en arcillas [112-114] pero son escasos en organohidrotalcitas [13, 103, 115]. 5.2 Materiales porosos Los materiales porosos merecen una singular atención debido a sus potenciales aplicaciones como adsorbentes, sistemas de intercambio iónico, membranas, soportes o catalizadores. Aunque los sólidos porosos poseen una variada composición, tienen en común el espacio accesible en el interior de su estructura. De acuerdo con la definición de la IUPAC, los materiales porosos se clasifican en función del tamaño de poro en: microporosos (<2nm), mesoporosos (2-50nm) y macroporosos (>50nm). Aunque estos materiales han sido ampliamente estudiados a lo largo del siglo XVIII, no es hasta el siglo XIX cuando se comienza a estudiar su capacidad adsorbente. El objetivo principal de los investigadores en relación a los materiales porosos es controlar el tamaño, la forma, uniformidad y periodicidad de los espacios porosos, así como los átomos o moléculas que los constituyen. El control y el ajuste preciso de 35 Capítulo II estas propiedades permiten conseguir diferentes materiales para el desempeño de una función deseada en una aplicación en particular. La sílica gel [116, 117] y el carbón activado [118, 119] han sido ampliamente utilizados como adsorbentes. El principal problema de estos materiales reside en que al ser microporosos no permiten la adsorción de moléculas grandes. Sin embargo, los compuestos mesoporosos ordenados que contienen sílica y carbón en su estructura permiten la adsorción de moléculas de gran tamaño. Estos materiales poseen alta área superficial con superficies de poro activa para su modificación o funcionalización y grandes volúmenes de poro distribuidos de manera periódica, Kato y col [120] sintetizaron en 1992 el primer material mesoporoso ordenado y en este mismo año se produjo un gran avance en la preparación de estos materiales por la Mobil Research and Development Corporation. Ellos describieron la síntesis de los primeros materiales porosos, M41S [121], los cuales eran obtenidos a partir de un precursor inorgánico en presencia de surfactantes catiónicos. Inmediatamente, las posibilidades en el uso de otros moldes porógenos se amplió a surfactantes aniónicos [122] y no iónicos [123]. Durante la primera década de los 90, estos materiales despertaron un gran interés científico y la síntesis de nuevas estructuras mesoporosas ordenadas ha sido objeto de numerosas citas bibliográficas. Los poros de estos materiales se encuentran en el intervalo de 1.5 a 10nm. Al tener poros grandes ofrecen grandes ventajas ya que estos no se bloquean tan fácilmente y pueden ser utilizados para reacciones en fase líquida a diferencia de las zeolitas comunes que por lo general se utilizan para gases y vapores. Para lograr una estructura mesoporosa ordenada, se necesita partir de una solución homogénea de un surfactante disuelto en el medio. Es muy importante conocer el comportamiento del surfactante en disolución acuosa para entender la relación entre el surfactante y la formación de la mesoestructura. Los surfactantes se caracterizan por ser moléculas orgánicas y de carácter anfifílico, formados por una cabeza polar y una cadena hidrocarbonada apolar. Cuando el surfactante se pone en contacto con agua, las cadenas hidrofóbicas se reúnen en el interior formando micelas y los grupos polares se disponen en la superficie en contacto con el medio acuoso. 36 Introducción Figure 8. Ethane and methane molecules in the hexagonal pores of MCM-41. El miembro más interesante y estudiado de esta familia es el denominado 2 MCM-41 (Figura 8). Este sólido posee altas superficies específicas (~1000 m /g), alta estabilidad térmica, tamaños de poros regulables en el rango 1-10 nm y una estrecha distribución de tamaño de los mismos. Para obtención de estos materiales se pueden seguir el proceso sol-gel o el método cristal líquido. Proceso sol-gel El proceso sol-gel es una ruta química que permite fabricar materiales amorfos y policristalinos de forma relativamente sencilla. En el estado sol hay una suspensión estable de partículas coloidales en un líquido y en el gel se forma una red porosa sólida tridimensional continua, que se expande a lo largo de un medio líquido. Mediante esta síntesis se obtienen mesoestructuras inorgánicas ordenadas basándose en la hidrólisis y policondensación de precursores de tipo alcóxido metálico M(OR)n donde R es un grupo alquilo (Figura 9). Para la síntesis de MCM-41 los precursores más utilizados son Si(OCH3)4 o TMOS y Si(OCH2CH3)4 o TEOS. 37 Capítulo II Figure 9. Diagram of sol-gel process. En esta síntesis el pH del medio juega un papel muy importante. A pHs ácidos la condensación es la etapa determinante de la velocidad ya que el número de enlaces siloxanos alrededor del átomo central de silicio aumenta. A estos pHs se producen estructuras poliméricas largas débilmente ramificadas. Sin embargo, a pHs básicos la hidrólisis es la etapa determinante y dan lugar a estructuras ramificadas. En este caso, la velocidad de la reacción aumenta con el incremento de enlaces siloxano (Figura 10). Método cristal líquido Beck y col [121] propusieron el modelo de cristal líquido (Liquid cristal templating mechanism, LCT) para la síntesis de mesoporosos. La estructura viene definida por la organización de moléculas de surfactante en fases de tipo cristal líquido mesostructuradas, bajo unas condiciones de temperatura y concentración determinadas. En estas moléculas se deposita la fase inorgánica. El ion amonio cuaternario actúa como agente director de la estructura formando las micelas que se agregan en el cristal líquido. En la Figura 11 se representan las diferentes estructuras que se pueden obtener en función de las condiciones de síntesis como la temperatura, la naturaleza de surfactante y la concentración. Así, se puede obtener una estructura hexagonal como la MCM-41, o cúbica como la MCM-48 o laminar como la MCM-50. La concentración más baja a la cual se observa la formación de micelas esféricas se 38 Introducción denomina concentración micelar crítica (cmc1). La segunda concentración micelar crítica se define como la concentración a la cual las micelas esféricas comienzan a transformarse en micelas cilíndricas (cmc2). Figure 10. Influence of pH in the synthesis. Se pueden establecer dos tipos de mecanismos según las condiciones de síntesis. Un primer mecanismo denominado “True Liquid-Crystal Templating” en el cual las moléculas se organizan en superestructuras, antes de la formación del sólido. Éste crece en torno a la estructura ordenada previa, es decir, la fase cristal líquido se forma sin requerir la presencia del precursor inorgánico. En el segundo mecanismo llamado “Cooperative Self Assembly” hay una interacción entre las moléculas orgánicas y el precursor inorgánico induciendo así a la ordenación del sistema (Figura 12). Años más tarde, varios investigadores tras el estudio de diversos métodos y técnicas se posicionaron a favor del método liquid cristal templating [124]. De acuerdo con las interacciones entre las moléculas de surfactante, la especie inorgánica y los iones presentes en el medio, se pueden distinguir diferentes mecanismos de formación de estructuras mesoporosas [122]. El pH juega un papel importante en la interacción entre estas moléculas. El punto isoeléctrico de la sílice, es decir, el pH en el que su carga es cero, es de 2. Por ello, cuando el pH es mayor de 2, 39 Capítulo II las especies de sílice tienen carga negativa. Si se emplean surfactantes catiónicos, + - como por ejemplo sales de amonio cuaternarias, el mecanismo se denomina S I donde S es el surfactante e I la especie inorgánica. Si por el contrario, se utilizan surfactantes + + aniónicos como un fosfato de cadena larga, se necesita un ión mediador como Na o K para asegurar la interacción entre ambas especies cargadas negativamente, siendo - + - S M I el mecanismo. Figure 11. Mesoporous structure depending on the temperature and concentration of surfactant Por otro lado, si las condiciones de síntesis son ácidas, pH<2, la sílice estará presente como catión. En este caso, cuando se empleen surfactantes catiónicos será - necesario añadir un ion molecular X que generalmente es un haluro. Este mecanismo + - + - + es expresado como S X I . Si se utiliza un surfactante aniónico, el mecanismo es S I . 40 Introducción Figure 12. Types of mechanisms (a) True Liquid-Crystal Templating and (b) Cooperative Self Assembly. Las interacciones comentadas anteriormente son de naturaleza electrostática pero se pueden producir enlaces de hidrógeno o de naturaleza dipolar. En esta ruta se 0 utilizan surfactantes no iónicos como Pluronic o neutros (N polióxido de etileno, S 0 0 0 0 0 aminas de cadena larga) con especies de sílice no cargadas, S I o N I o pares iónicos 0 0 S (XI) (Figura 13). La última etapa en cualquier ruta sintética utilizada es la eliminación de surfactante orgánico. Ésta se puede llevar a cabo mediante calcinación o métodos de extracción química. En el tratamiento térmico se suelen emplear rampas de calentamiento lentas hasta alcanzar los 550ᵒC. El problema de este tratamiento radica en la aparición de grietas y distorsiones por contracción de la estructura. Por ello, se suele emplear más la extracción mediante lavado con disolventes o fluidos supercríticos [125-127], la digestión por microondas [128] o el tratamiento oxidativo con ozono [129]. Sin embargo, estas alternativas suelen venir acompañadas de un proceso de calcinación ya que no se consigue la eliminación completa del surfactante. La extracción minimiza la pérdida de OH en la superficie, lo cual es de gran interés para la funcionalización de las matrices con agentes sililantes. 41 Capítulo II Figure 13. Possible mechanisms of interaction between the surfactant and the inorganic species. 5.2.1 Materiales híbridos Uno de los materiales más versátiles y estables fue presentado en 1998 por Zhao y col. [130] con la síntesis de estructuras de sílices mesoporosas bien ordenadas, denominadas SBA-15, con tamaño uniforme de poro de hasta 30nm. Para la síntesis de estos materiales se empleó un surfactante de tres bloques no iónico (EOnPOmEOn) en un medio fuertemente ácido. La calcinación a 500ᵒC produce una estructura porosa con grandes distancias interplanares “d” de 7.5 a 32 nm entre los planos (100). En comparación con MCM-41 proporciona mayor estabilidad térmica e hidrotérmica al tener un mayor espesor de pared. Esto le permite tener buenas aplicaciones en los campos de catálisis y de adsorción. Un material híbrido incluye dos tipos de funcionalidades que comúnmente suelen ser orgánica e inorgánica. Estos materiales generan un gran interés debido a la posibilidad de combinar la enorme variabilidad de funciones de los compuestos 42 Introducción orgánicos con la robustez y estabilidad térmica de los compuestos inorgánicos [131]. La integración de grupos funcionales permite ampliar su utilidad ya que han sido utilizados como catalizadores [132-134] para una amplia variedad de transformaciones químicas, como adsorbentes. [135, 136], soportes cromatográficos [137], agentes de liberación de fármacos [138-140] y sensores químicos [141, 142]. 5.2.1.1 Síntesis de materiales híbridos de sílice Generalmente existen dos vías de funcionalización superficial de los materiales de sílice: el método de post funcionalización o “grafting” y el método de co-condensación (Figura 13). El primer método, es un proceso post-síntesis que implica la condensación de organosilanos del grupo (R´O)3SiR siendo R el grupo orgánico, con los grupos silanoles libres en la superficie de los poros. Por otro lado, el método de co-condensación es un proceso de síntesis directo en el que dos precursores, uno tetraalcoxisilano [(R’O)4Si(TEOS o TMOs)] y un precursor híbrido trialcoxiorganosilano del tipo R-Si(OR´)3, reaccionan por cocondensación en presencia de un agente director de la estructura, generando una sílice mesoestructurada modificada con una gran variedad de funcionalidades (de tipo amino, tiol, alquilo, aromático o vinilo). Este proceso de funcionalización puede ocurrir en condiciones ácidas, básicas o neutras. El método de co-condensación tiene una serie de desventajas. El grado de ordenamiento mesoscópico de los productos disminuye con el incremento de la concentración (R´O)3SiR en la mezcla de reacción conduciendo a productos totalmente desordenados [143]. El contenido de funciones orgánicas en la sílica modificada normalmente no excede del 40% molar. Además, la proporción de grupos orgánicos terminales que son incorporados en las paredes de los poros es generalmente más baja que la que correspondería según la concentración de partida. Existe también una tendencia a dar reacciones de homocondensación, debido a las diferentes velocidades de hidrólisis y condensación de precursores estructuralmente diferentes. Puede darse además una reducción de diámetro de poro, volumen de poro y área superficial por el incremento en la cantidad de grupos incorporados. Por último, debe prestarse especial cuidado para no destruir la funcionalidad orgánica durante la extracción del 43 Capítulo II surfactante. Por ello, se suelen realizar métodos de extracción ya que la calcinación en la mayoría de los casos no es adecuada. Figure 14. Synthetic routes for mesoporous hybrid materials. Debido a los problemas presentados en las rutas de “grafting” y cocondensación se ha desarrollado una tercera vía de síntesis denominada PMOs (Periodic Mesoporous Organosílicas). Este método consiste en la hidrólisiscondensación de precursores organosilícicos puentes del tipo (R´O)3Si-R-Si(OR´)3, en presencia de un agente director de la estructura. Estos materiales se diferencian de los anteriores por tener incorporadas las unidades orgánicas en la estructura tridimensional de la matriz de sílice mediante enlaces covalentes quedando éstas homogéneamente distribuidas en las paredes de los poros. En esta Tesis Doctoral se han sintetizado PMOs por lo que se procederá a una descripción más detallada. 5.2.2 PMOs En 1999, tres grupos de investigación [144-146] sintetizaron los primeros materiales periódicos mesoporosos organosilícicos, PMOs. La siguiente figura muestra una visión simplificada de estos materiales. 44 Introducción Figure 15. PMOs materials El grupo de Inagaki [144] sintetizó materiales con estructura hexagonal bidimensional o tridimensional dependiendo de la temperatura y de la longitud de la cadena alquílica, con diámetro de poro de 31 a 27Å y alta área superficial. La síntesis se llevó a cabo bajo condiciones ácidas y como precursor se empleó 1,2 bis(trimetoxisilil)etano y octadeciltrimetil amonio (OCTA) como agente director de estructura. Stein y col. [145] sintetizaron una organosilica mesoporosa con alta área superficial y puentes etilenos usando los precursores 1,2 bis (trietoxisilil)etano o bis(trietoxisilil)etileno y hexadeciltrimetilamonio como surfactante. Finalmente, el grupo de Ozin [146] preparó PMOs a partir del precursor 1,2 bis (trietoxisilil)eteno y tetraortosilicato (TEOS) empleando medio básico y bromuro de hexadecilamonio como surfactante. Actualmente existe una gran variedad de PMOs sintetizados a partir de diferentes precursores organosilícicos puentes (Figura 16). Hay un número limitado de estos precursores para la síntesis de estos materiales ya que la estructura del precursor orgánico tiene que ser rígida, tal como una cadena corta o un sistema conjugado. Además se necesita una fuerte interacción entre el “template” o agente director de la estructura y el precursor. Para ello hay que ajustar una serie de parámetros como la naturaleza de “template”, el pH, las concentraciones, la temperatura, las condiciones de envejecimiento y la presencia de ciertos aditivos. Generalmente, el uso de surfactantes neutros da lugar a materiales de poro de hasta 10nm. La tabla 3 presenta los surfactantes más utilizados. Por otro lado, la adición de sales mejora el grado de orden ya que fomenta la interacción entre el 45 Capítulo II surfactante y la sílice y además permite preparar PMOs con poros grandes y altamente ordenados en un amplio rango de concentraciones ácidas. F i g u r e 1 5 Figure 16. Precursors used for the synthesis of PMOs. 46 Introducción Table 3 Some surfactants used for the synthesis of PMOs. CTAC/CTAB Cetyltrimethylammonium chloride/bromide OTAC Octadecyltrimethylammonium chloride CnTMACl/Br Alkyltrimethylammonium chloride/bromide CPCl Cetylpyridinium chloride FC4 Fluorocarbon surfactant Brij-30 Polyoxyethylene (4) lauryl ether Brij-56 Polyoxyethylene (10) cetyl ether Brij-76 Polyoxyethylene (10) stearyl ether Triton-X100 Polyoxyethylene (10) octylphenyl ether P123 Pluronic P123 poly(ethylene glycol)– poly(propylene glycol)–poly(ethylene glycol) F127 Pluronic F127 poly(ethylene glycol)– poly(propylene glycol)–poly(ethylene glycol) B50-6600 Poly(ethylene oxide)–poly(butylene oxide)– poly(ethylene oxide) PEO–PLGA– Poly(ethylene PEO glycolic acid)– oxide)–poly(lactic poly(ethylene oxide) Cn-s-m Divalent and gemini surfactants 47 acid-co- Cl-/Br- Capítulo II Un parámetro importante en la síntesis de PMOs es la acidez o basicidad en el medio de reacción porque determinan la interacción específica entre el surfactante y el precursor organosilícico y afecta a la regularidad mesostructural de los PMOs [148]. Existen otros parámetros a tener en cuenta durante la síntesis de materiales mesoporosos ordenados organosilícicos como la temperatura, el tiempo de agitación [149] y la naturaleza del precursor. 5.2.2.1 Tipos de PMOs y sus síntesis Como se ha podido comprobar en la Figura 16 existe una amplia variedad de materiales periódicos mesoporosos organosilícicos (PMOs). En este apartado, sin embargo solo se van a desarrollar los dos materiales empleados para la adsorción de SMetolacloro, es decir PMO con puentes etilénicos (denominado etano-PMO) y PMO con puentes fenileno (denominado benceno-PMO). Dentro de los materiales PMOs con puentes alquilo, el material con puentes etileno ha sido uno de los más estudiados. Como precursores se emplean 1,2bis(trimetoxisilil)etano (BTME) o 1,2-bis(trietoxisilil)etano (BTEE) [150]. La estructura de ambos se representa en la Tabla 4. Table 4. Precursors used for synthesis of ethane-PMO. BTME BTEE Inagaki y col. [144] sintetizaron este tipo de materiales PMOs usando un surfactante catiónico de naturaleza alquiltrimetilamonio bajo. Con esta síntesis se consiguió obtener eteno-PMO con ordenamiento mesoporoso hexagonal 2D (P6mm) o 3D (P63/mm), dependiendo de la composición molar de partida. Generalmente, el cloruro de alquiltrimetilamonio proporciona materiales con simetría 2D. La síntesis cuyos productos de partida son BTME, alquiltrimetilamonio, NaOH y agua permite formar materiales de alto orden con morfologías bien definidas 48 Introducción como barras hexagonales (2D-hexagonal), partículas esféricas (3D-hexagonal) y decaedros (Pm3n cúbica). Kao y col. [151] sintetizaron materiales con simetría cúbica por cocondensación de TEOS (Tetraetil ortosilicato) y BTEE y bromuro de cetiltrietilamonio como surfactante bajo condiciones ácidas. Empleando surfactantes iónicos se había conseguido hasta el momento un diámetro de poro menor a 4nm. Por ello, con el fin de poder aumentar el tamaño de poro, diversos investigadores comenzaron a emplear copolímeros de tres bloques no iónicos para la síntesis de PMOs. Como surfactantes se han utilizado una amplia variedad de óxido de fenil(polietileno) como Triton-X100, Brij56 y Brij-76. Este último proporciona una simetría hexagonal 2-D con mejores cualidades, produciendo metano, eteno y benceno-PMO multifuncionales [152]. Este surfactante puede ser usado bajo condiciones neutras en presencia de fluoruro como catalizador ya que hidroliza el precursor y promueve la formación de oligómeros de organosílica cargados negativamente. Dependiendo del precursor utilizado y la composición del gel, se pueden obtener materiales con simetría hexagonal 2D, más o menos ordenados usando como template P123. El grado de ordenamiento puede aumentarse mediante la adicción de sales inorgánicas. Como se ha comentado anteriormente, el empleo de aditivos incrementa la interacción entre la cabeza de surfactante y las especies organosilícicas. Así Zhao y col. [153] sintetizaron etano-PMO bajo condiciones ácidas y usando como surfactante F127. El empleo de KCl fue necesario para la síntesis de estos PMOs con estructura porosa de tipo caja y simetría Fm3m. En diversas ocasiones, algunas aplicaciones como la adsorción de biomoléculas o la encapsulación de nanopartículas de metal requieren de materiales con morfologías y estructuras especiales. Lu y col. [154] sintetizaron etano-PMO como partículas huecas con diámetros entre 300 y 500nm. Sus materiales tenían cavidades de aproximadamente 100-150nm y paredes mesoestructuradas de 100nm de espesor con simetría hexagonal 2D. Existen además, otras morfologías interesantes que han sido estudiadas en los últimos años [155-157]. 49 Capítulo II Para la síntesis de PMOs con puentes fenilo y derivados, al igual que para etano-PMO, se han empleado diferentes agentes directores de estructura, es decir templates catiónicos, copolímeros en bloque, etc. El grupo de investigación de Ozin [158] fue el primero que descubrió estos materiales ordenados con puentes fenilo y para ello utilizaron como precursores 1,4bis(trietoxisilil)benceno (BTEB) y 2,5-bis(trietoxisilil)tiofeno. En su síntesis emplearon cloruro de cetilpiridino (CPCL) bajo condiciones ácidas suaves. Más tarde modificaron el surfactante empleando otros como CTAB o C16TEABr. Además se mencionó por primera vez el ordenamiento de los grupos aromáticos dentro de las paredes, debido probablemente al apilamiento π-π de los mismos. Bajo condiciones ácidas y utilizando P123 se obtiene benceno-PMO con grandes poros, múltiples canales con ordenamiento hexagonal 2D mientras que usando F127 se obtienen PMOs 3D en forma de caja [159] empleando en ambos casos 1,4bis(trietoxisilil)benceno como precursor. Un importante desarrollo dentro de los PMOs fue realizado por el grupo de Inagaki quienes sintetizaron PMOs con puentes fenilo con un ordenamiento hexagonal 2D y organización (crystal-like) de sus puentes orgánicos fenilo dentro de las paredes de los poros. Este material alterna capas hidrófilas e hidrófobas compuestas por sílica y benceno [160]. Los PMOs preparados bajo condiciones ácidas generalmente, carecen de periodicidad en las paredes de los poros. Sin embargo, Sayari [161] obtuvo bencenoPMOs con un determinado orden, usando surfactantes no iónicos. La disposición 2Dhexagonal permaneció inalterada usando NaOH. Además de precursores disilánicos, se han obtenido PMOs a partir de precursores multisilánicos. Ozin y col. [162] utilizaron como precursor anillos de benceno simétricamente integrados mediante su unión a tres átomos de silicio en la estructura mesoporosa. 5.2.2.2 Aplicaciones de PMOs Los PMOs al poseer alta área superficial tienen numerosas aplicaciones ya que pequeñas cantidades de material producen efectos significativos. Una de las principales aplicaciones de estos materiales radica en su uso como catalizadores. 50 Introducción Existen diferentes estructuras de los PMOs sulfonados y su acidez depende del método de obtención, el cuál puede ser por sulfonación directa, co-condensación y postmodificación o mediante el método grafting. Hay diversos factores que afectan a la actividad catalítica como la estructura, la hidrofobicidad o la fuerza y la estabilidad de los sitios ácidos. Sin embargo, no se puede establecer una relación clara entre la actividad catalítica y la cantidad de sitios ácidos [163]. Los PMOs como catalizadores se han utilizado en la reacción de acetilación de heptanal a 1-butanol [164], dimerización de 2-fenilpropeno [165], hidroxialquilación [166] o hidrólisis de azúcar [163] entre otros. Por otro lado, una cantidad de centros metálicos como Ti, Sn, V o Nb han sido incorporados en la estructura de los PMOs para que también actúen como catalizadores. Además, la introducción de compuestos organometálicos ofrece una serie de ventajas a la “catálisis verde”, particularmente, la recuperación del catalizador, que normalmente es tóxico y caro, se evita la contaminación del producto y permite la posibilidad de operar a flujo continuo. Con este objetivo, varios complejos metálicos se han incorporado a la estructura de los PMOs. La obtención de partículas esféricas de los PMOs para ser usadas como fases estacionarias en cromatografía, ha sido objeto de numerosas investigaciones Para HPLC (High-performance liquid chromatography) es muy importante el tamaño de estas esferas (entre 3 y 10 µm) [167, 168] y el tratamiento de funcionalización postsíntesis del material silícico para producir una fase estacionaria de naturaleza apolar [169]. Recientemente se ha preparado un nuevo material híbrido para ser usado como fase estacionaria en HPLC, para ello los PMOs con puentes etileno y sílica pura fueron funcionalizados con eritromicina y vancomicina[137]. Se pueden encontrar aplicaciones en microelectrónica para estos materiales. Para ello, la permitividad relativa o constante dieléctrica k debe tener un valor bajo para prevenir la transferencia de carga y pérdidas de señal que puedan surgir por acumulación de carga indeseable entre los elementos interruptores y los circuitos de corriente. Las propiedades más importantes requeridas para un material dieléctrico 51 Capítulo II ideal, es decir, bajo k, son: elevada porosidad, estabilidad mecánica y térmica y resistencia a la humedad. Los PMOs debido a su volumen de poro grande, su elevado contenido orgánico y el fácil control de su hidrofobia, son en principio muy adecuados como dieléctricos de baja permitividad. Los PMOs con grupos funcionales orgánicos poseen elevadas áreas superficiales y tamaño y distribución de poro más homogéneos lo que los hace idóneos para su utilización como indicadores ópticos. Varios investigadores prepararon un sensor para la detección de compuestos orgánicos volátiles. Este material fue sintetizado a partir de una partícula delgada de material mesoporoso híbrido de sílice y benceno y se insertó en la estructura de un sensor de superficie fotovoltaica [170]. Adicionalmente, si los PMOs contienen grupos cromóforos orgánicos pueden ser adecuados como elementos fotoactivos. En este sentido, se han desarrollado PMOs con cromóforos orgánicos tales como viológenos, azobencenos, trifenilpirilio, diacetileno, etc. [171-173]. Las organosilicas mesoporosas ordenadas ofrecen buenas propiedades ópticas, particularmente cuando se encuentran en forma de película delgada. Inagaki y col. [174] sintetizaron un nuevo PMO usando como precursor 2,6- bis(trietoxisilil)antraceno y Rodio como catalizador. En este grupo de investigación además de su síntesis, compararon sus propiedades ópticas con el ya preparado años atrás, el 9,10 antraceno-PMO. Por otro lado, los PMOs son buenos soportes para la inmovilización de biomoléculas, tales como proteínas y péptidos. Las enzimas libres debido a su inestabilidad, su alto coste y su dificultad para ser separadas de las mezclas de reacción para su reutilización, no son siempre catalizadores ideales. Este problema se puede solucionar mediante su inmovilización sobre algún sólido como polímeros orgánicos, zeolitas, materiales mesoporosos, etc. Blanco y col. [175] compararon el uso de diferentes sílicas mesoporosas (sílica amorfa, SBA-15 y etano-PMO) como soportes de lipasa. Ellos concluyeron, tras una serie de investigaciones, que los materiales silícicos ordenados son excelentes 52 Introducción soportes para la inmovilización de esta enzima cuando la superficie es bastante hidrofóbica. Además, los PMOS tienen excelentes propiedades como biocatalizadores nanoestructurados con una mínima pérdida de actividad. Yang y col. [176] sintetizaron organosílices mesoporosas con puentes 1,4 dietilbenceno por co-condensación directa usando P123 como agente director de la estructura bajo condiciones ácidas. Se obtuvieron buenos resultados para la inmovilización de lisozima en este material. Otra de las aplicaciones principales y la que se desarrollará en esta Tesis Doctoral es su uso como adsorbente en este caso de pesticidas, aunque también se ha estudiado la adsorción de metales pesados, otros contaminantes orgánicos e incluso gases. García y col. [177] sintetizaron una organosílica mesoporosa conteniendo urea 3+ mediante el método de co-condensación y la emplearon para la adsorción de Fe . También se han sintetizado materiales mesoporosos ordenados funcionalizados con 2+ tiol para la adsorción de Hg [178]. La adsorción de compuestos orgánicos volátiles fue estudiada en 2006 en benceno-PMO con ordenamiento hexagonal 2D y cúbico 3D [179]. Años más tarde, Melde y col. llevaron a cabo la adsorción de otros gases como dióxido de silicio, cloruro de cianógeno, amoníaco y octano en etano-PMO [180]. La adsorción del gas hidrógeno fue comparada con etileno, etenileno, fenileno y bifenilo-PMO. Los PMOs que contienen electrones π tienen mayor capacidad de adsorción que etano-PMO [181]. El estudio de contaminantes que tengan anillos aromáticos en su estructura es de especial interés por las interacciones π-π entre el adsorbente y adsorbato. La tabla 5 resume los compuestos orgánicos adsorbidos en PMOs. 5.2.3 Mesoporosos ordenados de carbón Los materiales de carbono mesoporoso están considerados por muchos investigadores como “la siguiente generación de materiales mesoporosos” [182]. Estos compuestos tienen un área superficial elevada, volumen de poro grande, químicamente inertes y conducen la electricidad. Es por ello, que se han empleado en un gran número de aplicaciones como dispositivos electrónicos, separación de membranas, adsorbentes, catalizadores, componentes ópticos y sistemas de conversión de energía. 53 Capítulo II Table 5 Precursors used for the adsorption of organic compounds. PRECURSOR COMPUESTO ORGÁNICO REFERENCIA 1,4 bis (trietoxisilil)benceno Mesosulfuron [183] 1,4 bis (trietoxisilil)benceno Benceno, tolueno, orto y [184] para-xileno 1,4 bis (trietoxisilil)benceno Hidrocarburos policíclicos [185] aromáticos 1,4 bis (trietoxisilil)benceno 1,4 bis (trietoxisilil)bifenilo Nicotina [186] 4-clorofenol [187] 1,4 bis(trimetoxisililetil)benceno 4-nitrofenol, 4-clorofenol, 4- [188] 1,2-bis(trietoxisilil)etano metilfenol bis(trimetoxisililetil)benceno Trinitrotolueno bis[3(trimetoxisilil)propil]amina 1,4 bis (trietoxisilil)benceno 1,2-Bis(trietoxisilil)etano [189] 1,4 bis (trietoxisilil)benceno 5.2.3.1 Tipos de síntesis En 1999 se sintetizó por primera vez un carbón mesoporoso usando silica mesoporosa como “template”. Utilizando un proceso de carbonización y como catalizador ácido sulfúrico, se convirtió la sacarosa en carbón. La estructura del nuevo material no fue una réplica exacta del template pero permitió la formación de una estructura ordenada [190]. Este método se conoce como “hard template” Sin embargo, este tipo de síntesis consume mucho tiempo y además no es posible su producción a gran escala. Estos obstáculos hicieron a muchos investigadores buscar un nuevo método denominado “soft template” similar a la síntesis de sílicas mesoporosas [191-193]. Aunque se han usado principalmente resinas fenólicas como fuente de carbón, algunos investigadores han preferido compuestos más pequeños como resorcinol [194, 195] o phloroglucinol [196]. En el caso del método soft template la síntesis no es tan flexible como en el caso de las sílices. Lo mismo que ocurre con las sílices mesoporosas, el éxito de esta técnica se debe al autoensamblaje cooperativo de las moléculas de surfactante y de 54 Introducción precursor y la polimerización del precursor alrededor del cristal líquido. El problema radica en que no se establecen las mismas interacciones entre los precursores de carbono con las micelas debido a la baja densidad de los grupos funcionales. La primera síntesis de estos compuestos usando fenol-formaldehído como precursor fue realizada por Dai y col. [196, 197]. En estos trabajos se obtuvieron materiales mesoporosos altamente ordenados como se muestra en la Figura 17. A partir de estas publicaciones se han realizado numerosos estudios utilizando el mismo método y empleando resinas fenólicas como precursor. Las síntesis de resinas fenólicas y carbones mesoporosos ordenados se pueden dividir en cuatro etapas: formación del prepolímero mediante la adicción de formaldehido al compuesto fenólico, formación de la mesofase, eliminación del template y carbonización (Figura 18). (A) (B) Figure 17 . Electron microscopy images of the carbon film. (A) Highresolution SEM image of the surface of the carbon film with uniform hexagonal-pore array, (B) SEM image of the film cross section. OH R OH/H+ O H -Polimerización -Formación mesofase -Eliminación template -Carbonización H Figure 18. Schematic representation of ordered mesoporous phenolic resina and carbon using the soft template method. L 55 Capítulo II a formación del prepolímero de resina fenólica es llevada a cabo por un catalizador ácido o básico y polimerizando el formaldehido con el compuesto de carbono. Como se ha expuesto anteriormente se usan, normalmente, tres compuestos fenólicos que difieren en el número de grupos hidroxilo y anillos aromáticos: fenol, resorcinol y phloroglucinol (Figura 19). El fenol es uno de los más utilizados ya que es un compuesto de bajo coste y su velocidad de polimerización es fácil de controlar. El phloroglucinol tiene mayor velocidad de polimerización ya que interacciona mejor con el agente director de la estructura debido al mayor número de grupos hidroxilo. El pH y la temperatura afectan directamente a la velocidad de reacción y al mecanismo de reacción entre los dos monómeros. OH HO OH HO OH OH Phenol Resorcinol Phloroglucinol Figure 19 Materials used as carbon sources in the synthesis of phenolic resins and ordered mesoporous carbons. En medio ácido se produce una sustitución electrofílica aromática entre los monómeros mientras que a pHs altos el compuesto fenólico se desprotona formando ión fenóxido y ocurre una sustitución en el formaldehido. Así, si se trabaja en medio ácido se forma el prepolímero Novalac y en medio básico Resol los cuales se representan en la Figura 20. Ambos compuestos son fácilmente transformados en polímeros orto y para sustituidos. Una vez formado el prepolímero, se forma la red tridimensional por policondensación. En presencia de un agente director de la estructura, se produce un co-ensamblado del material. 56 Introducción OH OH CH2OH HOH2C Novalac OH OH OH H2OCH2 C HOH2C CH2OH CH2OCH2 HO OH CH2OH CH2OH Resol Figure 20. Structures Novalac and Resol prepolymers. Si se incrementa la concentración de surfactante y/o la temperatura, se forman mesofases laminares, hexagonales o cúbicas periódicas. Este proceso ocurre por la organización de moléculas asimétricas en agregados supramoleculares bien definidos mediante interacciones no covalentes siempre por encima de la concentración y temperatura micelar crítica. Para la síntesis de estos compuestos se emplean copolímeros en bloque ya que no son tóxicos, disponibles comercialmente y económicos. Los más usados son P123 (EO20PO70EO20), F127 (EO106PO70EO106) y F108 (EO132PO50EO132) donde EO son óxidos de etileno y PO óxidos de propileno. Para la síntesis de estos materiales mesoporosos se han desarrollado varias rutas. Zhang y col. [198] sintetizaron polímeros y carbones mesoporosos altamente ordenados con estructura cúbica bicontinua. Para ello, emplearon como fuentes de carbono, fenol y formaldehido; como agente director de la estructura P123 y medio básico. Cuando el material se sometió a 350ᵒC en atmósfera de nitrógeno se formó el 2 3 polímero cuya área superficial fue de 550m /g y volumen de poro 0.4 cm /g. Sin 2 embargo, a 700ᵒC el área superficial aumento a 1150m /g y el volumen de poro a 3 0.57cm /g obteniéndose el carbón mesoporoso. Un año más tarde, sintetizaron más materiales usando F127 o P123 como agentes directores de la estructura y diferentes fuentes de carbono como fenol, formaldehido o moléculas de hidrocarburos 57 Capítulo II (hexadecano o decano) en medio básico [199]. Con este tipo de síntesis, se pueden obtener materiales con diferente morfología y estructura, haciéndolos muy prometedores en un gran número de aplicaciones. Este mismo grupo de investigación, sintetizó polímeros y materiales con el método EISA (ensamblaje por evaporación inducida) utilizando etanol como disolvente [200]. El área superficial y el volumen de poro de los carbones mesoporosos 2 3 ordenados formados a 900ᵒC fueron de 968 m /g y 0.56 cm /g, respectivamente. La síntesis de estos materiales se esquematiza en la Figura 21. Estos nuevos compuestos exhiben un volumen de poro y un área superficial mayor que los propuestos por el método anterior. Figure 21. Schematic representation of the procedure used to prepare polymers and ordered mesoporous carbons [200]. Gao y col. [201] realizaron la síntesis de estos materiales con alguna modificación. Concretamente, el disolvente que se utilizó fue una mezcla de etanol agua y se llevó a cabo en medio ácido. La principal diferencia con respecto a las síntesis anteriores, radica en que una vez que se añadieron todos los reactivos, la mezcla se 58 Introducción dejó reposar hasta que se formaron dos fases las cuales se dejaron envejecer 96h. Pasado ese tiempo, la capa superior fue desechada y la fase rica en polímero se dejó agitar hasta que se formó el monolito. Finalmente fue calcinada en atmosfera de nitrógeno como en las demás síntesis. Los compuestos obtenidos después de la calcinación tienen mayor área superficial y volumen de poro que los expuestos por los anteriores autores. A diferencia de los materiales mesoporosos inorgánicos, en las resinas fenólicas y en los carbones mesoporosos se elimina el template bajo atmósfera de nitrógeno [202]. El agente director de la estructura es completamente eliminado a 13 350ᵒC en atmósfera de nitrógeno como muestran los análisis de TGA, C NMR y FTIR. No obstante, una pequeña cantidad de oxígeno durante el proceso de calcinación facilita la degradación del template y los segmentos orgánicos, dando lugar a la formación de materiales con tamaño de poro grandes, así como áreas superficiales más elevadas [203]. Por otro lado, se debe controlar la velocidad de flujo de los gases ya que una gran velocidad podría ocasionar una deformación en la estructura porosa del material. Por ello, se establece una velocidad de calentamiento entre 1 y 5ᵒC/min a 350-400ᵒC durante 4-6h. El proceso de carbonización comienza a 400ᵒC contrayéndose la estructura ligeramente. Desde 600ᵒC no se observa ninguna contracción por lo que se puede aumentar la velocidad de calentamiento. Los poros se vuelven asimétricos. Entre 600 y 800ᵒC se forman microporos y simultáneamente incrementa el área superficial [204]. Por encima de 800ᵒC la estructura comienza a distorsionarse un poco y los poros se vuelven aún más asimétricos [205]. 5.2.3.2 Aplicaciones de las resinas fenólicas y carbones mesoporosos ordenados Los carbones mesoporosos ordenados al tener alta área superficial y poros grandes y ajustables pueden ser muy prometedores para ser usados como electrodos para baterías, pilas de combustible y supercondensadores, como adsorbentes para procesos de separación y almacenamiento de gas y como soportes para una amplia variedad de procesos catalíticos. Estos materiales pueden, además, ser modificados con grupos funcionales o añadiendo metales a su estructura y pueden ser obtenidos 59 Capítulo II directamente añadiendo el metal u otro precursor a la síntesis o empleando un tratamiento post-síntesis como impregnación, adsorción, grafting, o intercambio iónico. Dai y col. [206] fabricaron películas de carbón mesoporoso conteniendo V y Co. La conductividad de estos materiales fue superior a 20 S/cm y la capacitancia específica para Co y V fueron de 113 F/g y 159 F/g respectivamente. En 2013, Guan y col. sintetizaron carbón mesoporoso con poros ordenados y desordenados mediante el método EISA bajo condiciones básicas. Estos fueron empleados como electrodos usando H2SO4 como electrolito y empleando un simple tratamiento ácido para poder ser usados [207]. Un año más tarde, Guo y col. [208] fabricaron un electrodo de carbón mesoporoso dopado con nitrógeno obteniendo altas capacitancias específicas 225 F/g mientras que el compuesto no dopado ofrecía una capacitancia de 168 F/g. Este nuevo compuesto exhibía buena estabilidad electroquímica después de 1000 ciclos a 5 A/g. El campo de adsorción comprende un gran número de compuestos orgánicos, inorgánicos y gaseosos. En 2009, Deng y col. [209] prepararon varios carbones mesoporosos ordenados por el método de soft-template dopados con los metales de transición Pd, Pt, Ni y Ru. Demostraron que la adsorción de hidrógeno aumentaba en todos los compuestos mesoporosos dopados exceptuando el carbón mesoporoso dopado con Niquel que mostró una tendencia inversa y además, el metal no afectó a la cinética de adsorción de hidrógeno. Zhang y col. [210] utilizaron el carbón mesoporoso dopado con nitrógeno para adsorber los gases CO2 y SO2. Este compuesto exhibió -1 -1 buena capacidad de adsorción, 2.43 mmolCO2·g y 119.1 mgSO2·g . Sin embargo, cuando el mismo material se activó con KOH el poder de adsorción aumentó -1 -1 significativamente a 4.3 mmolCO2·g y a 213.1 mgSO2·g . Estos materiales también han sido empleados para eliminar metales pesados. Así, Jean-Ping y col. [211] prepararon un carbón mesoporoso que fue obtenido después de eliminar la sílica con HF. Cu(II) y Cr(VI) fueron los iones empleados para realizar las pruebas de adsorción. Los resultados mostraron baja capacidad de adsorción para el Cr(VI) pero sin embargo, fue muy selectivo para la adsorción de Cu(II). Ese mismo año se sintetizó un material híbrido funcionalizado con tiol 60 Introducción empleando un copolímero en bloque. Éste mostró muy buenas propiedades para ser -1 -1 utilizado como adsorbente de Ag(I) (2.11 mmol·g ) y Pb(II) (1.6 mmol·g ) [212]. Además del carbón mesoporoso, también la resina fenólica se ha utilizado como adsorbente para eliminar compuestos orgánicos. Zhao y col. [213] prepararon una resina fenólica altamente ordenada con resol como precursor y varias sílicas mesoporosas como hard template. Los materiales obtenidos tenían áreas superficiales 2 -1 3 -1 altas (1600m ·g ) y grandes volúmenes de poro (1.12 cm ·g ). Estos materiales -1 exhibieron alta capacidad de adsorción para anilina (134.2 mg·g ) y fucsina (317.5 -1 mg·g ). Existen también estudios de adsorción de varios alcaloides (clorhidrato de berberina, colchicina y matrina) en carbones mesoporosos ordenados. La capacidad de -1 -1 adsorción para una concentración de 0.1 mg·L y 298 K fue de 450, 600 y 480 mg·g , respectivamente [214]. También existen investigaciones para eliminar tintes como el verde de metileno con estos compuestos híbridos conteniendo cobalto. Los resultados demostraron que la capacidad de adsorción dependía de la concentración de Co -1 aumentando de 2 a 90 mg·g [215]. Entre las aplicaciones a destacar de estos materiales, se encuentra su capacidad para ser usados como catalizadores. En 2014, Liu y col. [216] prepararon carbones mesoporosos obtenidos por los métodos hard template, soft template e hidrotermal. Las muestras fueron además soportadas con Pd mediante el método de impregnación. El compuesto obtenido por hard template presentaba una mejor selectividad 82.2% y una conversión de 99.4% para pasar de la oxima ciclohexanona a nitrociclohexano. Ese mismo año Gao y col. [217] sintetizaron carbón mesoporoso conteniendo Nb para ser utilizado en la deshidratación de la fructosa a 5hidroximetilfurfural. Un 100% de conversión y un 76.5% de selectividad se consiguió con el material que contenía 76.5% de Nb2O5. Finalmente, otra aplicación importante es su uso como catalizadores en la síntesis de biodiesel. Dong y col. [218] demostraron que los carbones mesoporosos + -1 ordenados sulfonados tenían una alta actividad catalítica (2.21 mmol H ·g ) a 400ᵒC 61 Capítulo II manteniendo su estructura. Para evaluar su actividad se estudió la reacción de esterificación de ácido oleico con metanol exhibiendo una conversión del 95%. BIBLIOGRAFÍA [1] S. Rafqah, P. Wong-Wah-Chung, A. Aamili, M. Sarakha, Degradation of metsulfuron methyl by heterogeneous photocatalysis on TiO2 in aqueous suspensions: Kinetic and analytical studies, Journal of Molecular Catalysis A: Chemical, 237 (2005) 50-59. [2] X. Zhu, C. Yuan, Y. Bao, J. Yang, Y. Wu, Photocatalytic degradation of pesticide pyridaben on TiO2 particles, Journal of Molecular Catalysis A: Chemical, 229 (2005) 95105. [3] Q. Lu, J. Yu, J. Gao, Degradation of 2,4-dichlorophenol by using glow discharge electrolysis, J Hazard Mater, 136 (2006) 526-531. [4] P. Jiang, J. Zhou, A. Zhang, Y. Zhong, Electrochemical degradation of p-nitrophenol with different processes, Journal of Environmental Sciences, 22 (2010) 500-506. [5] S. Aslam, P. Garnier, C. Rumpel, S.E. Parent, P. Benoit, Adsorption and desorption behavior of selected pesticides as influenced by decomposition of maize mulch, Chemosphere, 91 (2013) 1447-1455. [6] R. Otero, J.M. Fernández, M.A. González, I. Pavlovic, M.A. Ulibarri, Pesticides adsorption–desorption on Mg–Al mixed oxides. Kinetic modeling, competing factors and recyclability, Chem Eng J, 221 (2013) 214-221. [7] H. El Harmoudi, L. El Gaini, E. Daoudi, M. Rhazi, Y. Boughaleb, M.A. El Mhammedi, A. Migalska-Zalas, M. Bakasse, Removal of 2,4-D from aqueous solutions by adsorption processes using two biopolymers: chitin and chitosan and their optical properties, Optical Materials, 36 (2014) 1471-1477. [8] J. Cornejo, P. Jamet, Pesticide/soil interactions: Some current research methods. , Paris 2000. [9] K. Li, Z. Zheng, J. Feng, J. Zhang, X. Luo, G. Zhao, X. Huang, Adsorption of pnitroaniline from aqueous solutions onto activated carbon fiber prepared from cotton stalk, J Hazard Mater, 166 (2009) 1180-1185. [10] J.L. Marco-Brown, M.M. Areco, R.M. Torres Sánchez, M. dos Santos Afonso, Adsorption of picloram herbicide on montmorillonite: Kinetic and equilibrium studies, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 449 (2014) 121128. [11] D. Gondar, R. López, J. Antelo, S. Fiol, F. Arce, Adsorption of paraquat on soil organic matter: Effect of exchangeable cations and dissolved organic carbon, J Hazard Mater, 235–236 (2012) 218-223. [12] L. Cox, M.C. Hermosin, J. Cornejo, Adsorption mechanisms of thiazafluron in mineral soil clay components, European Journal of Soil Science, 46 (1995) 431-438. 62 Introducción [13] D. Chaara, F. Bruna, M.A. Ulibarri, K. Draoui, C. Barriga, I. Pavlovic, Organo/layered double hydroxide nanohybrids used to remove non ionic pesticides, J Hazard Mater, 196 (2011) 350-359. [14] R. Celis, W.C. Koskinen, M.C. Hermosin, J. Cornejo, Sorption and Desorption of Triadimefon by Soils and Model Soil Colloids, J. Agric. Food Chem., 47 (1999) 776-781. [15] W.J. Weber, Q. Huang, Inclusion of Persistent Organic Pollutants in Humification Processes: Direct Chemical Incorporation of Phenanthrene via Oxidative Coupling, Environmental Science & Technology, 37 (2003) 4221-4227. [16] N. Senesi, Binding mechanisms of pesticides to soil humic substances, Science of The Total Environment, 123–124 (1992) 63-76. [17] J.-W. Choi, F.D. Tillman, J.A. Smith, Relative Importance of Gas-Phase Diffusive and Advective Trichloroethene (TCE) Fluxes in the Unsaturated Zone under Natural Conditions, Environmental Science & Technology, 36 (2002) 3157-3164. [18] S. Jellali, P. Muntzer, O. Razakarisoa, G. Schäfer, Large Scale Experiment on Transport of Trichloroethylene in a Controlled Aquifer, Transport in Porous Media, 44 (2001) 145-163. [19] P.H. Stauffer, K.H. Birdsell, M.S. Witkowski, J.K. Hopkins, Vadose Zone Transport of 1,1,1-Trichloroethane, Vadose Zone J., 4 (2005) 760-773. [20] G. Geisler, S. Hellweg, S. Liechti, K. Hungerbühler, Variability Assessment of Groundwater Exposure to Pesticides and Its Consideration in Life-Cycle Assessment, Environmental Science & Technology, 38 (2004) 4457-4464. [21] J. Hapeman-Somich Cathleen, Chemical Degradation of Pesticide Wastes, in: Pesticide Waste Management, American Chemical Society, 1992, pp. 157-165. [22] T.S. Bhalerao, P.R. Puranik, Biodegradation of organochlorine pesticide, endosulfan, by a fungal soil isolate, Aspergillus niger, International Biodeterioration & Biodegradation, 59 (2007) 315-321. [23] N. Alvarenga, W.G. Birolli, M.H.R. Seleghim, A.L.M. Porto, Biodegradation of methyl parathion by whole cells of marine-derived fungi Aspergillus sydowii and Penicillium decaturense, Chemosphere, 117 (2014) 47-52. [24] M. Yadav, N. Srivastva, R.S. Singh, S.N. Upadhyay, S.K. Dubey, Biodegradation of chlorpyrifos by Pseudomonas sp. in a continuous packed bed bioreactor, Bioresource Technology, 165 (2014) 265-269. [25] J.R. Garbin, D.M.B.P. Milori, M.L. Simões, W.T.L. da Silva, L.M. Neto, Influence of humic substances on the photolysis of aqueous pesticide residues, Chemosphere, 66 (2007) 1692-1698. [26] D. Chaara, I. Pavlovic, F. Bruna, M.A. Ulibarri, K. Draoui, C. Barriga, Removal of nitrophenol pesticides from aqueous solutions by layered double hydroxides and their calcined products, Appl Clay Sci, 50 (2010) 292-298. [27] Y. Qiu, M. Wu, J. Jiang, L. Li, G. Daniel Sheng, Enhanced irreversible sorption of carbaryl to soils amended with crop-residue-derived biochar, Chemosphere, 93 (2013) 69-74. 63 Capítulo II [28] C.H. Giles, T.H. Macewan, S.N. Nakhwa, D. Smith, Studies in Adsorption .11. A System of Classification of Solution Adsorption Isotherms, and Its Use in Diagnosis of Adsorption Mechanisms and in Measurement of Specific Surface Areas of Solids, Journal of the Chemical Society, (1960) 3973-3993. [29] H. Freundlich, Uber die adsorption in laosungen, Z. Phys. Chem., 57 (1906) 385470. [30] I. Langmuir, THE ADSORPTION OF GASES ON PLANE SURFACES OF GLASS, MICA AND PLATINUM, J Am Chem Soc, 40 (1918) 1361-1403. [31] C. Aharoni, M. Ungarish, Kinetics of activated chemisorption. Part 2.-Theoretical models, Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 73 (1977) 456-464. [32] M.M. Dubinin, L.V. Radushkevich, Equation of the characteristic curve of activated charcoal. , Proceedings of the Academy of Sciences, Physical Chemistry Section, U.S.S.R, 55 (1947) 331-333. [33] V. Rives, M.a. Angeles Ulibarri, Layered double hydroxides (LDH) intercalated with metal coordination compounds and oxometalates, Coordination Chemistry Reviews, 181 (1999) 61-120. [34] F. Cavani, F. Trifiro, A. Vaccari, Hydrotalcite-Type Anionic Clays: Preparation, Properties and Applications, Catalysis Today, 11 (1991) 173-301. [35] Y. Zhang, S. Jing, H. Liu, Reactivity and mechanism of bromate reduction from aqueous solution using Zn–Fe(II)–Al layered double hydroxides, Chem Eng J, 266 (2015) 21-27. [36] Y. Guo, Z. Zhu, Y. Qiu, J. Zhao, Synthesis of mesoporous Cu/Mg/Fe layered double hydroxide and its adsorption performance for arsenate in aqueous solutions, Journal of Environmental Sciences, 25 (2013) 944-953. [37] C. Forano, T. Hibino, F. Leroux, C. Taviot-Guého, Chapter 13.1 Layered Double Hydroxides, in: B.K.G.T. Faïza Bergaya, L. Gerhard (Eds.) Developments in Clay Science, Elsevier, 2006, pp. 1021-1095. [38] Q. Wang, H.H. Tay, Z. Guo, L. Chen, Y. Liu, J. Chang, Z. Zhong, J. Luo, A. Borgna, Morphology and composition controllable synthesis of Mg–Al–CO3 hydrotalcites by tuning the synthesis pH and the CO2 capture capacity, Appl Clay Sci, 55 (2012) 18-26. [39] Z.-Q. Zhang, M.-C. Liao, H.-Y. Zeng, S. Xu, L.-H. Xu, X.-J. Liu, J.-Z. Du, Mg–Al hydrotalcites as solid base catalysts for alcoholysis of propylene oxide, Fuel Processing Technology, 128 (2014) 519-524. [40] N. Benselka-Hadj Abdelkader, A. Bentouami, Z. Derriche, N. Bettahar, L.C. de Ménorval, Synthesis and characterization of Mg–Fe layer double hydroxides and its application on adsorption of Orange G from aqueous solution, Chem Eng J, 169 (2011) 231-238. [41] M.A. Ulibarri, I. Pavlovic, C. Barriga, M.C. Hermosı ́n, J. Cornejo, Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity, Appl Clay Sci, 18 (2001) 17-27. 64 Introducción [42] D.G. Costa, A.B. Rocha, W.F. Souza, S.S.X. Chiaro, A.A. Leitão, Comparative Structural, thermodynamic and electronic analyses of ZnAlAn− hydrotalcite-like compounds (An&#xa0;−Cl−, F−, Br−, OH−, CO32&#xa0;− or NO3−): An ab initio study, Appl Clay Sci, 56 (2012) 16-22. [43] E. Álvarez-Ayuso, H.W. Nugteren, Emission reduction of aluminium anodising industry by production of -hydrotalcite-type compound, Chemosphere, 62 (2006) 155162. [44] Z. Yang, H. Fischer, R. Polder, Synthesis and characterization of modified hydrotalcites and their ion exchange characteristics in chloride-rich simulated concrete pore solution, Cement and Concrete Composites, 47 (2014) 87-93. [45] T.S. Anirudhan, P.S. Suchithra, Synthesis and characterization of tanninimmobilized hydrotalcite as a potential adsorbent of heavy metal ions in effluent treatments, Appl Clay Sci, 42 (2008) 214-223. [46] D. Chaara, F. Bruna, K. Draoui, M.A. Ulibarri, C. Barriga, I. Pavlovic, Study of key parameters affecting adsorption of the herbicide Linuron on organohydrotalcites, Appl Clay Sci, 58 (2012) 34-38. [47] L.D.L. Miranda, C.R. Bellato, M.P.F. Fontes, M.F. de Almeida, J.L. Milagres, L.A. Minim, Preparation and evaluation of hydrotalcite-iron oxide magnetic organocomposite intercalated with surfactants for cationic methylene blue dye removal, Chem Eng J, 254 (2014) 88-97. [48] Y. Jiang, X. Gu, S. Zhang, W. Tang, J. Zhao, The preparation and characterization of sulfamic acid-intercalated layered double hydroxide, Materials Letters, 150 (2015) 3134. [49] C. Rossi, A. Schoubben, M. Ricci, L. Perioli, V. Ambrogi, L. Latterini, G.G. Aloisi, A. Rossi, Intercalation of the radical scavenger ferulic acid in hydrotalcite-like anionic clays, International Journal of Pharmaceutics, 295 (2005) 47-55. [50] K.-B. Yoon, Y.-Y. Hwang, S.K. Noh, D.-H. Lee, Polymer Nanocomposite of Mg-Al Hydrotalcite-Type Anionic Clay Modified with Organosulfate, Polym. J, 40 (2007) 50-55. [51] M. Du, W. Ye, W. Lv, H. Fu, Q. Zheng, Fabrication of high-performance poly(vinyl alcohol)/MgAl-layered double hydroxide nanocomposites, European Polymer Journal, 61 (2014) 300-308. [52] M.A. González, I. Pavlovic, C. Barriga, Cu(II), Pb(II) and Cd(II) sorption on different layered double hydroxides. A kinetic and thermodynamic study and competing factors, Chem Eng J, 269 (2015) 221-228. [53] M.A. González, I. Pavlovic, R. Rojas-Delgado, C. Barriga, Removal of Cu2+, Pb2+ and Cd2+ by layered double hydroxide–humate hybrid. Sorbate and sorbent comparative studies, Chem Eng J, 254 (2014) 605-611. [54] Y. Guo, D. Li, C. Hu, Y. Wang, E. Wang, Layered double hydroxides pillared by tungsten polyoxometalates: Synthesis and photocatalytic activity, International Journal of Inorganic Materials, 3 (2001) 347-355. 65 Capítulo II [55] G. Carja, Y. Kameshima, G. Ciobanu, H. Chiriac, K. Okada, New hybrid nanostructures based on oxacillin–hydrotalcite-like anionic clays and their textural properties, Micron, 40 (2009) 147-150. [56] J. Rocha, M. del Arco, V. Rives, M. A. Ulibarri, Reconstruction of layered double hydroxides from calcined precursors: a powder XRD and 27Al MAS NMR study, Journal of Materials Chemistry, 9 (1999) 2499-2503. [57] C. Del Hoyo, Layered double hydroxides and human health: An overview, Applied Clay Science, 36 (2007) 103-121. [58] E. Narita, T. Yamagishi, K. Aizu, K.N. Han, The formation of layered hydrotalcitelike compounds by coprecipitation of Co(II) and Ni(II) with Al(III) in aqueous ammoniacal solutions, International Journal of Mineral Processing, 29 (1990) 267-278. [59] N. Gerds, V. Katiyar, C.B. Koch, J. Risbo, D. Plackett, H.C.B. Hansen, Synthesis and characterization of laurate-intercalated Mg–Al layered double hydroxide prepared by coprecipitation, Applied Clay Science, 65–66 (2012) 143-151. [60] A. Inayat, M. Klumpp, W. Schwieger, The urea method for the direct synthesis of ZnAl layered double hydroxides with nitrate as the interlayer anion, Applied Clay Science, 51 (2011) 452-459. [61] H.-Y. Zeng, X. Deng, Y.-J. Wang, K.-B. Liao, Preparation of Mg-Al hydrotalcite by urea method and its catalytic activity for transesterification, AIChE Journal, 55 (2009) 1229-1235. [62] A. Tsujimura, M. Uchida, A. Okuwaki, Synthesis and sulfate ion-exchange properties of a hydrotalcite-like compound intercalated by chloride ions, J Hazard Mater, 143 (2007) 582-586. [63] H. Tamura, J. Chiba, M. Ito, T. Takeda, S. Kikkawa, Y. Mawatari, M. Tabata, Formation of hydrotalcite in aqueous solutions and intercalation of ATP by anion exchange, Journal of Colloid and Interface Science, 300 (2006) 648-654. [64] S.K. Sharma, P.A. Parikh, R.V. Jasra, Reconstructed Mg/Al hydrotalcite as a solid base catalyst for synthesis of jasminaldehyde, Applied Catalysis A: General, 386 (2010) 34-42. [65] M. Muñoz, S. Moreno, R. Molina, Synthesis of Ce and Pr-promoted Ni and Co catalysts from hydrotalcite type precursors by reconstruction method, International Journal of Hydrogen Energy, 37 (2012) 18827-18842. [66] E. Ramos-Ramírez, N.L.G. Ortega, C.A.C. Soto, M.T.O. Gutiérrez, Adsorption isotherm studies of chromium (VI) from aqueous solutions using sol–gel hydrotalcitelike compounds, J Hazard Mater, 172 (2009) 1527-1531. [67] F. Prinetto, G. Ghiotti, P. Graffin, D. Tichit, Synthesis and characterization of sol– gel Mg/Al and Ni/Al layered double hydroxides and comparison with co-precipitated samples, Micropor Mesopor Mat, 39 (2000) 229-247. [68] L. Indira, M. Dixit, P.V. Kamath, Electrosynthesis of layered double hydroxides of nickel with trivalent cations, Journal of Power Sources, 52 (1994) 93-97. 66 Introducción [69] W.N. Budhysutanto, H.J.M. Kramer, D. van Agterveld, A.G. Talma, P.J. Jansens, Pre-treatment of raw materials for the hydrothermal synthesis of hydrotalcite-like compounds, Chemical Engineering Research and Design, 88 (2010) 1445-1449. [70] A. Venugopal, J. Palgunadi, J.-K. Deog, O.-S. Joo, C.-H. Shin, Hydrotalcite derived Cu-Zn-Cr catalysts admixed with γ-Al2O3 for single step dimethyl ether synthesis from syngas: Influence of hydrothermal treatment, Catalysis Today, 147 (2009) 94-99. [71] L. Wang, K. Zhang, W. Sun, T. Wu, H. He, G. Liu, Hydrothermal synthesis of corrosion resistant hydrotalcite conversion coating on AZ91D alloy, Materials Letters, 106 (2013) 111-114. [72] Y. Li, J. Wang, Z. Li, Q. Liu, J. Liu, L. Liu, X. Zhang, J. Yu, Ultrasound assisted synthesis of Ca–Al hydrotalcite for U (VI) and Cr (VI) adsorption, Chem Eng J, 218 (2013) 295-302. [73] X. Xie, X. Ren, J. Li, X. Hu, Z. Wang, Preparation of Small Particle Sized ZnAlHydrotalcite-Like Compounds by Ultrasonic Crystallization, Journal of Natural Gas Chemistry, 15 (2006) 100-104. [74] L. Wang, B. Li, C. Chen, L. Jia, Structural characterization and related properties of the stearate anions intercalated Ni–Al hydrotalcite-like compound prepared by the microwave crystallization, Journal of Alloys and Compounds, 508 (2010) 426-432. [75] J.A. Rivera, G. Fetter, P. Bosch, Microwave power effect on hydrotalcite synthesis, Micropor Mesopor Mat, 89 (2006) 306-314. [76] E.L. Crepaldi, P.C. Pavan, J.B. Valim, Comparative study of the coprecipitation methods for the preparation of Layered Double Hydroxides, Journal of the Brazilian Chemical Society, 11 (2000) 64-70. [77] U. Costantino, F. Marmottini, M. Nocchetti, R. Vivani, New Synthetic Routes to Hydrotalcite-Like Compounds − Characterisation and Properties of the Obtained Materials, European Journal of Inorganic Chemistry, 1998 (1998) 1439-1446. [78] T. Lopez, P. Bosch, E. Ramos, R. Gomez, O. Novaro, D. Acosta, F. Figueras, Synthesis and Characterization of Sol−Gel Hydrotalcites. Structure and Texture†, Langmuir, 12 (1996) 189-192. [79] D.-p. Qiu, W.-g. Hou, Synthesis and characterization of indole-3-butyric acid/hydrotalcite-like compound nanohybrids, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 336 (2009) 12-17. [80] M. del Arco, A. Fernández, C. Martín, V. Rives, Intercalation of mefenamic and meclofenamic acid anions in hydrotalcite-like matrixes, Appl Clay Sci, 36 (2007) 133140. [81] V. Ambrogi, G. Fardella, G. Grandolini, L. Perioli, Intercalation compounds of hydrotalcite-like anionic clays with antiinflammatory agents — I. Intercalation and in vitro release of ibuprofen, International Journal of Pharmaceutics, 220 (2001) 23-32. [82] L. Yumin, L. Shetian, Z. Kaizheng, Y. Xingkai, W. Yue, Catalysis of hydrotalcite-like compounds in liquid phase oxidation:: (II) Oxidation of p-cresol to phydroxybenzaldehyde, Applied Catalysis A: General, 169 (1998) 127-135. 67 Capítulo II [83] N.T. Thao, H.H. Trung, Selective oxidation of styrene over Mg–Co–Al hydrotalcite like-catalysts using air as oxidant, Catalysis Communications, 45 (2014) 153-157. [84] S. Yu, E. Kim, S. Park, I.K. Song, J.C. Jung, Isomerization of glucose into fructose over Mg–Al hydrotalcite catalysts, Catalysis Communications, 29 (2012) 63-67. [85] M.R. Anuar, A.Z. Abdullah, M.R. Othman, Etherification of glycerol to polyglycerols over hydrotalcite catalyst prepared using a combustion method, Catalysis Communications, 32 (2013) 67-70. [86] J. Zhang, S. Xu, S. Wu, Y. Liu, Hydrogenation of fructose over magnetic catalyst derived from hydrotalcite precursor, Chemical Engineering Science, 99 (2013) 171-176. [87] J.-H. Choy, S.-J. Choi, J.-M. Oh, T. Park, Clay minerals and layered double hydroxides for novel biological applications, Applied Clay Science, 36 (2007) 122-132. [88] L. van der Ven, M.L.M. van Gemert, L.F. Batenburg, J.J. Keern, L.H. Gielgens, T.P.M. Koster, H.R. Fischer, On the action of hydrotalcite-like clay materials as stabilizers in polyvinylchloride, Appl Clay Sci, 17 (2000) 25-34. [89] R. Wen, Z. Yang, H. Chen, Y. Hu, J.a. Duan, Zn-Al-La hydrotalcite-like compounds as heating stabilizer in PVC resin, Journal of Rare Earths, 30 (2012) 895-902. [90] Z. Yang, H. Fischer, R. Polder, Laboratory investigation of the influence of two types of modified hydrotalcites on chloride ingress into cement mortar, Cement and Concrete Composites, 58 (2015) 105-113. [91] E. Scavetta, B. Ballarin, M. Berrettoni, I. Carpani, M. Giorgetti, D. Tonelli, Electrochemical sensors based on electrodes modified with synthetic hydrotalcites, Electrochimica Acta, 51 (2006) 2129-2134. [92] J. Qiu, G. Villemure, Anionic clay modified electrodes: Electron transfer mediated by electroactive nickel, cobalt or manganese sites in layered double hydroxide films, Journal of Electroanalytical Chemistry, 428 (1997) 165-172. [93] E. Scavetta, D. Tonelli, M. Giorgetti, F. Nobili, R. Marassi, M. Berrettoni, AC impedance study of a synthetic hydrotalcite-like compound modified electrode in aqueous solution, Electrochimica Acta, 48 (2003) 1347-1355. [94] Y. Yang, N. Gao, W. Chu, Y. Zhang, Y. Ma, Adsorption of perchlorate from aqueous solution by the calcination product of Mg/(Al–Fe) hydrotalcite-like compounds, J Hazard Mater, 209–210 (2012) 318-325. [95] M. Islam, R. Patel, Nitrate sorption by thermally activated Mg/Al chloride hydrotalcite-like compound, J Hazard Mater, 169 (2009) 524-531. [96] X. Liang, Y. Zang, Y. Xu, X. Tan, W. Hou, L. Wang, Y. Sun, Sorption of metal cations on layered double hydroxides, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 433 (2013) 122-131. [97] D. Zhao, G. Sheng, J. Hu, C. Chen, X. Wang, The adsorption of Pb(II) on Mg2Al layered double hydroxide, Chem Eng J, 171 (2011) 167-174. [98] A. Halajnia, S. Oustan, N. Najafi, A.R. Khataee, A. Lakzian, Adsorption–desorption characteristics of nitrate, phosphate and sulfate on Mg–Al layered double hydroxide, Applied Clay Science, 80–81 (2013) 305-312. 68 Introducción [99] W. Wang, J. Zhou, G. Achari, J. Yu, W. Cai, Cr(VI) removal from aqueous solutions by hydrothermal synthetic layered double hydroxides: Adsorption performance, coexisting anions and regeneration studies, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 457 (2014) 33-40. [100] O. Aschenbrenner, P. McGuire, S. Alsamaq, J. Wang, S. Supasitmongkol, B. AlDuri, P. Styring, J. Wood, Adsorption of carbon dioxide on hydrotalcite-like compounds of different compositions, Chemical Engineering Research and Design, 89 (2011) 17111721. [101] L. Zhao, X. Li, C. Hao, C.L. Raston, SO2 adsorption and transformation on calcined NiAl hydrotalcite-like compounds surfaces: An in situ FTIR and DFT study, Applied Catalysis B: Environmental, 117–118 (2012) 339-345. [102] I. Pavlovic, C. Barriga, M.C. Hermosín, J. Cornejo, M.A. Ulibarri, Adsorption of acidic pesticides 2,4-D, Clopyralid and Picloram on calcined hydrotalcite, Applied Clay Science, 30 (2005) 125-133. [103] I. Pavlovic, M.A. González, F. Rodríguez-Rivas, M.A. Ulibarri, C. Barriga, Caprylate intercalated layered double hydroxide as adsorbent of the linuron, 2,4-DB and metamitron pesticides from aqueous solution, Applied Clay Science, 80–81 (2013) 7684. [104] A. De Martino, M. Iorio, P.D. Prenzler, D. Ryan, H.K. Obied, M. Arienzo, Adsorption of phenols from olive oil waste waters on layered double hydroxide, hydroxyaluminium–iron-co-precipitate and hydroxyaluminium–iron–montmorillonite complex, Applied Clay Science, 80–81 (2013) 154-161. [105] F. Li, Y. Wang, Q. Yang, D.G. Evans, C. Forano, X. Duan, Study on adsorption of glyphosate (N-phosphonomethyl glycine) pesticide on MgAl-layered double hydroxides in aqueous solution, J Hazard Mater, 125 (2005) 89-95. [106] H. Zaghouane-Boudiaf, M. Boutahala, C. Tiar, L. Arab, F. Garin, Treatment of 2,4,5-trichlorophenol by MgAl–SDBS organo-layered double hydroxides: Kinetic and equilibrium studies, Chem Eng J, 173 (2011) 36-41. [107] G. Bascialla, A.E. Regazzoni, Immobilization of anionic dyes by intercalation into hydrotalcite, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 328 (2008) 34-39. [108] M. Bouraada, F. Belhalfaoui, M.S. Ouali, L.-C. de Ménorval, Sorption study of an acid dye from an aqueous solution on modified Mg–Al layered double hydroxides, J Hazard Mater, 163 (2009) 463-467. [109] I.G. Dubus, E. Barriuso, R. Calvet, Sorption of weak organic acids in soils: clofencet, 2,4-D and salicylic acid, Chemosphere, 45 (2001) 767-774. [110] M.A. Ulibarri, I. Pavlovic, M.C. Hermosín, J. Cornejo, Hydrotalcite-like compounds as potential sorbents of phenols from water, Appl Clay Sci, 10 (1995) 131-145. [111] L. Patricio Cardoso, J.B. Valim, Competition between three organic anions during regeneration process of calcined LDH, Journal of Physics and Chemistry of Solids, 65 (2004) 481-485. 69 Capítulo II [112] S. Nir, Y. El-Nahhal, T. Undabeytia, G. Rytwo, T. Polubesova, Y. Mishael, O. Rabinovitz, B. Rubin, Chapter 5.2 - Clays, Clay Minerals, and Pesticides, in: B. Faïza, L. Gerhard (Eds.) Developments in Clay Science, Elsevier, 2013, pp. 645-662. [113] G. Lagaly, Pesticide–clay interactions and formulations, Applied Clay Science, 18 (2001) 205-209. [114] N. Sánchez-Jiménez, M.T. Sevilla, J. Cuevas, M. Rodríguez, J.R. Procopio, Interaction of organic contaminants with natural clay type geosorbents: Potential use as geologic barrier in urban landfill, Journal of Environmental Management, 95, Supplement (2012) S182-S187. [115] P. Wu, T. Wu, W. He, L. Sun, Y. Li, D. Sun, Adsorption properties of dodecylsulfate-intercalated layered double hydroxide for various dyes in water, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 436 (2013) 726731. [116] S. Koner, A. Pal, A. Adak, Utilization of silica gel waste for adsorption of cationic surfactant and adsolubilization of organics from textile wastewater: A case study, Desalination, 276 (2011) 142-147. [117] J. Pan, H. Yao, W. Guan, H. Ou, P. Huo, X. Wang, X. Zou, C. Li, Selective adsorption of 2,6-dichlorophenol by surface imprinted polymers using polyaniline/silica gel composites as functional support: Equilibrium, kinetics, thermodynamics modeling, Chem Eng J, 172 (2011) 847-855. [118] A.R. Khan, T.A. Al-Bahri, A. Al-Haddad, Adsorption of phenol based organic pollutants on activated carbon from multi-component dilute aqueous solutions, Water Res., 31 (1997) 2102-2112. [119] E. Gonzalez-Serrano, T. Cordero, J. Rodriguez-Mirasol, L. Cotoruelo, J.J. Rodriguez, Removal of water pollutants with activated carbons prepared from H3PO4 activation of lignin from kraft black liquors, Water Res., 38 (2004) 3043-3050. [120] T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, The preparation of alkyltrimethylammonium-kanemite complexes and their conversion to microporous materials, B Chem Soc Jpn, 63 (1992) 988-992. [121] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, A new family of mesoporous molecular sieves prepared with liquid crystal templates, J Am Chem Soc, 114 (1992) 10834-10843. [122] Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schuth, G.D. Stucky, Generalized synthesis of periodic surfactant/inorganic composite materials, Nature, 368 (1994) 317-321. [123] P.T. Tanev, T.J. Pinnavaia, A Neutral Templating Route to Mesoporous Molecular Sieves, Science, 267 (1995) 865-867. [124] O. Regev, Nucleation Events during the Synthesis of Mesoporous Materials Using Liquid Crystalline Templating, Langmuir, 12 (1996) 4940-4944. 70 Introducción [125] D. Margolese, J.A. Melero, S.C. Christiansen, B.F. Chmelka, G.D. Stucky, Direct Syntheses of Ordered SBA-15 Mesoporous Silica Containing Sulfonic Acid Groups, Chemistry of Materials, 12 (2000) 2448-2459. [126] C.-Y. Chen, H.-X. Li, M.E. Davis, Studies on mesoporous materials: I. Synthesis and characterization of MCM-41, Microporous Materials, 2 (1993) 17-26. [127] S. Kawi, Supercritical fluid extraction of surfactant template from MCM-41, Chem. Commun., (1998) 1407-1408. [128] B. Tian, X. Liu, C. Yu, F. Gao, Q. Luo, S. Xie, B. Tu, D. Zhao, Microwave assisted template removal of siliceous porous materials, Chem. Commun., (2002) 1186-1187. [129] M. T. J. Keene, R. Denoyel, P. L. Llewellyn, Ozone treatment for the removal of surfactant to form MCM-41 type materials, Chem. Commun., (1998) 2203-2204. [130] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Triblock copolymer synthesis of mesoporous silica with periodic 50 to 300 angstrom pores, Science, 279 (1998) 548-552. [131] P. Van Der Voort, D. Esquivel, E. De Canck, F. Goethals, I. Van Driessche, F.J. Romero-Salguero, Periodic mesoporous organosilicas: from simple to complex bridges; a comprehensive overview of functions, morphologies and applications Chem Soc Rev, (2013 DOI:10.1039/c2cs35222b). [132] B.W. Glasspoole, J.D. Webb, C.M. Crudden, Catalysis with chemically modified mesoporous silicas: Stability of the mesostructure under Suzuki–Miyaura reaction conditions, Journal of Catalysis, 265 (2009) 148-154. [133] D.J. Macquarrie, D.B. Jackson, S. Tailland, K. Wilson, J.H. Clark, New organically modified hexagonal mesoporous silicas: Preparation and applications in catalysis, in: S. Abdelhamid, J. Mietek (Eds.) Studies in Surface Science and Catalysis, Elsevier, 2000, pp. 275-282. [134] S. Angloher, J. Kecht, T. Bein, Mesoporous ordered silica structures modified by metal organic reagents and their application in catalytic Michael additions, Micropor Mesopor Mat, 115 (2008) 629-633. [135] D.D. Asouhidou, K.S. Triantafyllidis, N.K. Lazaridis, K.A. Matis, Adsorption of Remazol Red 3BS from aqueous solutions using APTES- and cyclodextrin-modified HMS-type mesoporous silicas, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 346 (2009) 83-90. [136] H. Zhao, W. Yan, Z. Bian, J. Hu, H. Liu, Investigation of Mg modified mesoporous silicas and their CO2 adsorption capacities, Solid State Sciences, 14 (2012) 250-257. [137] D. Pérez-Quintanilla, S. Morante-Zarcero, I. Sierra, Preparation and characterization of mesoporous silicas modified with chiral selectors as stationary phase for high-performance liquid chromatography, Journal of Colloid and Interface Science, 414 (2014) 14-23. [138] M.D. Popova, Á. Szegedi, I.N. Kolev, J. Mihály, B.S. Tzankov, G.T. Momekov, N.G. Lambov, K.P. Yoncheva, Carboxylic modified spherical mesoporous silicas аs drug delivery carriers, International Journal of Pharmaceutics, 436 (2012) 778-785. 71 Capítulo II [139] Q. Tang, Y. Xu, D. Wu, Y. Sun, A study of carboxylic-modified mesoporous silica in controlled delivery for drug famotidine, Journal of Solid State Chemistry, 179 (2006) 1513-1520. [140] Y.-f. Zhu, J.-l. Shi, Y.-s. Li, H.-r. Chen, W.-h. Shen, X.-p. Dong, Storage and release of ibuprofen drug molecules in hollow mesoporous silica spheres with modified pore surface, Micropor Mesopor Mat, 85 (2005) 75-81. [141] J. Li, D.-f. Lu, Z. Zhang, Q. Liu, Z.-m. Qi, Hierarchical mesoporous silica film modified near infrared SPR sensor with high sensitivities to small and large molecules, Sensors and Actuators B: Chemical, 203 (2014) 690-696. [142] M.P. dos Santos, A. Rahim, N. Fattori, L.T. Kubota, Y. Gushikem, Novel amperometric sensor based on mesoporous silica chemically modified with ensal copper complexes for selective and sensitive dopamine determination, Sensors and Actuators B: Chemical, 171–172 (2012) 712-718. [143] A. Stein, B.J. Melde, R.C. Schroden, Hybrid Inorganic–Organic Mesoporous Silicates—Nanoscopic Reactors Coming of Age, Adv Mater, 12 (2000) 1403-1419. [144] S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, Novel Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic Oxide in Their Frameworks, J Am Chem Soc, 121 (1999) 9611-9614. [145] B.J. Melde, B.T. Holland, C.F. Blanford, A. Stein, Mesoporous Sieves with Unified Hybrid Inorganic/Organic Frameworks, Chem Mater, 11 (1999) 3302-3308. [146] T. Asefa, M.J. MacLachlan, N. Coombs, G.A. Ozin, Periodic mesoporous organosilicas with organic groups inside the channel walls, Nature, 402 (1999) 867871. [147] A. Sayari, S. Hamoudi, Periodic Mesoporous Silica-Based Organic−Inorganic Nanocomposite Materials, Chemistry of Materials, 13 (2001) 3151-3168. [148] X.Y. Bao, X.S. Zhao, X. Li, P.A. Chia, J. Li, A Novel Route toward the Synthesis of High-Quality Large-Pore Periodic Mesoporous Organosilicas, The Journal of Physical Chemistry B, 108 (2004) 4684-4689. [149] C.H. Lee, S. Soo Park, S. Joon Choe, D.H. Park, Synthesis of periodic mesoporous organosilica with remarkable morphologies, Micropor Mesopor Mat, 46 (2001) 257264. [150] P. Van Der Voort, D. Esquivel, E. De Canck, F. Goethals, I. Van Driessche, F.J. Romero-Salguero, Periodic Mesoporous Organosilicas: from simple to complex bridges; a comprehensive overview of functions, morphologies and applications, Chemical Society Reviews, 42 (2013) 3913-3955. [151] Y.-C. Pan, H.-Y. Wu, G.-L. Jheng, H.-H.G. Tsai, H.-M. Kao, Ordered and Hydrothermally Stable Cubic Periodic Mesoporous Organosilicas with SBA-1 Mesostructures: Synthesis, Characterization, Solid-State NMR Spectroscopy, and DFT Calculations, The Journal of Physical Chemistry C, 113 (2009) 2690-2698. 72 Introducción [152] A. Sayari, Y. Yang, Nonionic oligomeric polymer directed synthesis of highly ordered large pore periodic mesoporous organosilica, Chem. Commun., (2002) 25822583. [153] L. Zhao, G. Zhu, D. Zhang, Y. Di, Y. Chen, O. Terasaki, S. Qiu, Synthesis and Structural Identification of a Highly Ordered Mesoporous Organosilica with Large Cagelike Pores, The Journal of Physical Chemistry B, 109 (2004) 764-768. [154] H. Djojoputro, X.F. Zhou, S.Z. Qiao, L.Z. Wang, C.Z. Yu, G.Q. Lu, Periodic Mesoporous Organosilica Hollow Spheres with Tunable Wall Thickness, J Am Chem Soc, 128 (2006) 6320-6321. [155] X. Liu, X. Li, Z. Guan, J. Liu, J. Zhao, Y. Yang, Q. Yang, Organosilica nanotubes: large-scale synthesis and encapsulation of metal nanoparticles, Chem. Commun., 47 (2011) 8073-8075. [156] M. Mandal, M. Kruk, Surfactant-Templated Synthesis of Ordered Silicas with Closed Cylindrical Mesopores, Chemistry of Materials, 24 (2011) 149-154. [157] Y. Zhang, M. Yu, L. Zhou, X. Zhou, Q. Zhao, H. Li, C. Yu, Organosilica Multilamellar Vesicles with Tunable Number of Layers and Sponge-Like Walls via One Surfactant Templating, Chemistry of Materials, 20 (2008) 6238-6243. [158] C. Yoshina-Ishii, T. Asefa, N. Coombs, M.J. MacLachlan, G.A. Ozin, Periodic mesoporous organosilicas, PMOs: fusion of organic and inorganic chemistry 'inside' the channel walls of hexagonal mesoporous silica, Chem Commun, (1999) 2539-2540. [159] E.-B. Cho, D. Kim, J. Gorka, M. Jaroniec, Three-dimensional cubic (Im3m) periodic mesoporous organosilicas with benzene- and thiophene-bridging groups, Journal of Materials Chemistry, 19 (2009) 2076-2081. [160] S. Inagaki, S. Guan, T. Ohsuna, O. Terasaki, An ordered mesoporous organosilica hybrid material with a crystal-like wall structure, Nature, 416 (2002) 304-307. [161] W. Wang, W. Zhou, A. Sayari, Synthesis of Periodic Mesoporous Phenylenesilica under Acidic Conditions with Novel Molecular Order in the Pore Walls, Chemistry of Materials, 15 (2003) 4886-4889. [162] M. Kuroki, T. Asefa, W. Whitnal, M. Kruk, C. Yoshina-Ishii, M. Jaroniec, G.A. Ozin, Synthesis and Properties of 1,3,5-Benzene Periodic Mesoporous Organosilica (PMO): Novel Aromatic PMO with Three Point Attachments and Unique Thermal Transformations, J Am Chem Soc, 124 (2002) 13886-13895. [163] P. Dhepe, M. Ohashi, S. Inagaki, M. Ichikawa, A. Fukuoka, Hydrolysis of sugars catalyzed by water-tolerant sulfonated mesoporous silicas, Catal Lett, 102 (2005) 163169. [164] D. Dubé, M. Rat, W. Shen, F. Béland, S. Kaliaguine, Perfluoroalkylsulfonic acidfunctionalized periodic mesostructured organosilica: a strongly acidic heterogeneous catalyst, J Mater Sci, 44 (2009) 6683-6692. [165] B. Rác, P. Hegyes, P. Forgo, Á. Molnár, Sulfonic acid-functionalized phenylenebridged periodic mesoporous organosilicas as catalyst materials, Applied Catalysis A: General, 299 (2006) 193-201. 73 Capítulo II [166] A. Karam, J.C. Alonso, T.I. Gerganova, P. Ferreira, N. Bion, J. Barrault, F. Jerome, Sulfonic acid functionalized crystal-like mesoporous benzene-silica as a remarkable water-tolerant catalyst, Chem. Commun., (2009) 7000-7002. [167] K.W. Gallis, J.T. Araujo, K.J. Duff, J.G. Moore, C.C. Landry, The Use of Mesoporous Silica in Liquid Chromatography, Adv Mater, 11 (1999) 1452-1455. [168] T. Salesch, S. Bachmann, S. Brugger, R. Rabelo-Schaefer, K. Albert, S. Steinbrecher, E. Plies, A. Mehdi, C. Reyé, R.J.P. Corriu, E. Lindner, New Inorganic– Organic Hybrid Materials for HPLC Separation Obtained by Direct Synthesis in the Presence of a Surfactant, Advanced Functional Materials, 12 (2002) 134-142. [169] Y. Ma, L. Qi, J. Ma, Y. Wu, O. Liu, H. Cheng, Large-pore mesoporous silica spheres: synthesis and application in HPLC, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 229 (2003) 1-8. [170] B. Yuliarto, Y. Kumai, S. Inagaki, H. Zhou, Enhanced benzene selectivity of mesoporous silica SPV sensors by incorporating phenylene groups in the silica framework, Sensors and Actuators B: Chemical, 138 (2009) 417-421. [171] H. Peng, J. Tang, L. Yang, J. Pang, H.S. Ashbaugh, C.J. Brinker, Z. Yang, Y. Lu, Responsive Periodic Mesoporous Polydiacetylene/Silica Nanocomposites, J Am Chem Soc, 128 (2006) 5304-5305. [172] X. Guo, X. Wang, H. Zhang, L. Fu, H. Guo, J. Yu, L.D. Carlos, K. Yang, Preparation and luminescence properties of covalent linking of luminescent ternary europium complexes on periodic mesoporous organosilica, Micropor Mesopor Mat, 116 (2008) 28-35. [173] Y. Li, B. Yan, Y.-J. Li, Sulfide functionalized lanthanide (Eu/Tb) periodic mesoporous organosilicas (PMOs) hybrids with covalent bond: Physical characterization and photoluminescence, Micropor Mesopor Mat, 132 (2010) 87-93. [174] Y. Goto, K. Nakajima, N. Mizoshita, M. Suda, N. Tanaka, T. Hasegawa, T. Shimada, T. Tani, S. Inagaki, Synthesis and optical properties of 2,6-anthracene-bridged periodic mesostructured organosilicas, Micropor Mesopor Mat, 117 (2009) 535-540. [175] E. Serra, E. Díez, I. Díaz, R.M. Blanco, A comparative study of periodic mesoporous organosilica and different hydrophobic mesoporous silicas for lipase immobilization, Micropor Mesopor Mat, 132 (2010) 487-493. [176] C. Li, J. Liu, X. Shi, J. Yang, Q. Yang, Periodic Mesoporous Organosilicas with 1,4Diethylenebenzene in the Mesoporous Wall: Synthesis, Characterization, and Bioadsorption Properties, The Journal of Physical Chemistry C, 111 (2007) 1094810954. [177] M. Benitez, D. Das, R. Ferreira, U. Pischel, H. García, Urea-Containing Mesoporous Silica for the Adsorption of Fe(III) Cations, Chemistry of Materials, 18 (2006) 5597-5603. [178] Q. Yang, J. Liu, J. Yang, L. Zhang, Z. Feng, J. Zhang, C. Li, Acid catalyzed synthesis of ordered bifunctionalized mesoporous organosilicas with large pore, Micropor Mesopor Mat, 77 (2005) 257-264. 74 Introducción [179] M.P. Kapoor, M. Yanagi, Y. Kasama, T. Yokoyama, S. Inagaki, T. Shimada, H. Nanbu, L.R. Juneja, Self-assembly of cubic phenylene bridged mesoporous hybrids from allylorganosilane precursors, Journal of Materials Chemistry, 16 (2006) 33053311. [180] B.J. Johnson, B.J. Melde, G.W. Peterson, B.J. Schindler, P. Jones, Functionalized organosilicate materials for irritant gas removal, Chemical Engineering Science, 68 (2012) 376-382. [181] M. Kubo, K. Ishiyama, A. Shimojima, T. Okubo, Effect of organic groups on hydrogen adsorption properties of periodic mesoporous organosilicas, Micropor Mesopor Mat, 147 (2012) 194-199. [182] A. Vinu, M. Miyahara, K. Ariga, Biomaterial Immobilization in Nanoporous Carbon Molecular Sieves: Influence of Solution pH, Pore Volume, and Pore Diameter, The Journal of Physical Chemistry B, 109 (2005) 6436-6441. [183] S.O. Ganiyu, C. Bispo, N. Bion, P. Ferreira, I. Batonneau-Gener, Periodic Mesoporous Organosilicas as adsorbents for the organic pollutants removal in aqueous phase, Micropor Mesopor Mat, 200 (2014) 117-123. [184] C.P. Moura, C.B. Vidal, A.L. Barros, L.S. Costa, L.C.G. Vasconcellos, F.S. Dias, R.F. Nascimento, Adsorption of BTX (benzene, toluene, o-xylene, and p-xylene) from aqueous solutions by modified periodic mesoporous organosilica, Journal of Colloid and Interface Science, 363 (2011) 626-634. [185] C.B. Vidal, A.L. Barros, C.P. Moura, A.C.A. de Lima, F.S. Dias, L.C.G. Vasconcellos, P.B.A. Fechine, R.F. Nascimento, Adsorption of polycyclic aromatic hydrocarbons from aqueous solutions by modified periodic mesoporous organosilica, Journal of Colloid and Interface Science, 357 (2011) 466-473. [186] J.H. Shin, S.S. Park, C.-S. Ha, Adsorption behavior of nicotine on periodic mesoporous organosilicas, Colloids and Surfaces B: Biointerfaces, 84 (2011) 579-584. [187] M.C. Burleigh, S. Jayasundera, M.S. Spector, C.W. Thomas, M.A. Markowitz, B.P. Gaber, A New Family of Copolymers: Multifunctional Periodic Mesoporous Organosilicas, Chemistry of Materials, 16 (2003) 3-5. [188] M.C. Burleigh, M.A. Markowitz, M.S. Spector, B.P. Gaber, Porous Polysilsesquioxanes for the Adsorption of Phenols, Environmental Science & Technology, 36 (2002) 2515-2518. [189] S.A. Trammell, M. Zeinali, B.J. Melde, P.T. Charles, F.L. Velez, M.A. Dinderman, A. Kusterbeck, M.A. Markowitz, Nanoporous Organosilicas as Preconcentration Materials for the Electrochemical Detection of Trinitrotoluene, Analytical Chemistry, 80 (2008) 4627-4633. [190] R. Ryoo, S.H. Joo, S. Jun, Synthesis of Highly Ordered Carbon Molecular Sieves via Template-Mediated Structural Transformation, The Journal of Physical Chemistry B, 103 (1999) 7743-7746. [191] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, 75 Capítulo II Hydrothermally Stable, Mesoporous Silica Structures, J Am Chem Soc, 120 (1998) 6024-6036. [192] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism, Nature, 359 (1992) 710-712. [193] Y. Wan, Y. Shi, D. Zhao, Supramolecular Aggregates as Templates: Ordered Mesoporous Polymers and Carbons†, Chemistry of Materials, 20 (2007) 932-945. [194] X. Wang, C. Liang, S. Dai, Facile Synthesis of Ordered Mesoporous Carbons with High Thermal Stability by Self-Assembly of Resorcinol−Formaldehyde and Block Copolymers under Highly Acidic Conditions, Langmuir, 24 (2008) 7500-7505. [195] S. Tanaka, N. Nishiyama, Y. Egashira, K. Ueyama, Synthesis of ordered mesoporous carbons with channel structure from an organic-organic nanocomposite, Chem. Commun., (2005) 2125-2127. [196] C. Liang, S. Dai, Synthesis of Mesoporous Carbon Materials via Enhanced Hydrogen-Bonding Interaction, J Am Chem Soc, 128 (2006) 5316-5317. [197] C. Liang, K. Hong, G.A. Guiochon, J.W. Mays, S. Dai, Synthesis of a Large-Scale Highly Ordered Porous Carbon Film by Self-Assembly of Block Copolymers, Angewandte Chemie International Edition, 43 (2004) 5785-5789. [198] F. Zhang, Y. Meng, D. Gu, Yan, C. Yu, B. Tu, D. Zhao, A Facile Aqueous Route to Synthesize Highly Ordered Mesoporous Polymers and Carbon Frameworks with Ia3̄d Bicontinuous Cubic Structure, J Am Chem Soc, 127 (2005) 13508-13509. [199] F. Zhang, Y. Meng, D. Gu, Yan, Z. Chen, B. Tu, D. Zhao, An Aqueous Cooperative Assembly Route To Synthesize Ordered Mesoporous Carbons with Controlled Structures and Morphology, Chemistry of Materials, 18 (2006) 5279-5288. [200] Y. Meng, D. Gu, F. Zhang, Y. Shi, H. Yang, Z. Li, C. Yu, B. Tu, D. Zhao, Ordered Mesoporous Polymers and Homologous Carbon Frameworks: Amphiphilic Surfactant Templating and Direct Transformation, Angewandte Chemie, 117 (2005) 7215-7221. [201] P. Gao, A. Wang, X. Wang, T. Zhang, Synthesis and Catalytic Performance of Highly Ordered Ru-Containing Mesoporous Carbons for Hydrogenation of Cinnamaldehyde, Catal Lett, 125 (2008) 289-295. [202] F. Bérubé, S. Kaliaguine, Calcination and thermal degradation mechanisms of triblock copolymer template in SBA-15 materials, Micropor Mesopor Mat, 115 (2008) 469-479. [203] Y. Meng, D. Gu, F. Zhang, Y. Shi, L. Cheng, D. Feng, Z. Wu, Z. Chen, Y. Wan, A. Stein, D. Zhao, A Family of Highly Ordered Mesoporous Polymer Resin and Carbon Structures from Organic−Organic Self-Assembly, Chemistry of Materials, 18 (2006) 4447-4464. [204] I. Muylaert, A. Verberckmoes, J. De Decker, P. Van Der Voort, Ordered mesoporous phenolic resins: Highly versatile and ultra stable support materials, Adv. Colloid Interface Sci., 175 (2012) 39-51. 76 Introducción [205] X. Li, A.B. Larson, L. Jiang, L. Song, T. Prichard, N. Chawla, B.D. Vogt, Evolution of mechanical, optical and electrical properties of self-assembled mesostructured phenolic resins during carbonization, Micropor Mesopor Mat, 138 (2011) 86-93. [206] M. Dai, L. Song, J.T. LaBelle, B.D. Vogt, Ordered Mesoporous Carbon Composite Films Containing Cobalt Oxide and Vanadia for Electrochemical Applications, Chemistry of Materials, 23 (2011) 2869-2878. [207] T. Cai, M. Zhou, G. Han, S. Guan, Phenol–formaldehyde carbon with ordered/disordered bimodal mesoporous structure as high-performance electrode materials for supercapacitors, Journal of Power Sources, 241 (2013) 6-11. [208] M. Xie, Y. Xia, J. Liang, L. Chen, X. Guo, Ordered nitrogen doped mesoporous carbon assembled under aqueous acidic conditions and its electrochemical capacitive properties, Micropor Mesopor Mat, 197 (2014) 237-243. [209] D. Saha, S. Deng, Hydrogen Adsorption on Ordered Mesoporous Carbons Doped with Pd, Pt, Ni, and Ru, Langmuir, 25 (2009) 12550-12560. [210] A. Chen, Y. Yu, Y. Zhang, W. Zang, Y. Yu, Y. Zhang, S. Shen, J. Zhang, Aqueousphase synthesis of nitrogen-doped ordered mesoporous carbon nanospheres as an efficient adsorbent for acidic gases, Carbon, 80 (2014) 19-27. [211] C. Tian, Wan Tao, Wang Dao-Jun, Zhao Jian-Qing, Ding Xiao-Chun, Wu Shi-Chao, Xue Hai-Rong, H. Jian-Ping, Selective Adsorption Behavior of Cu(II) and Cr(VI) Heavy Metal Ions by Functionalized Ordered Mesoporous Carbon, Acta Phys-Chim Sin, 20 (2010) 3249-3256. [212] X. Zhuang, Q. Zhao, Y. Wan, Multi-constituent co-assembling ordered mesoporous thiol-functionalized hybrid materials: synthesis and adsorption properties, Journal of Materials Chemistry, 20 (2010) 4715-4724. [213] Z. Wu, Y. Meng, D. Zhao, Nanocasting fabrication of ordered mesoporous phenol–formaldehyde resins with various structures and their adsorption performances for basic organic compounds, Micropor Mesopor Mat, 128 (2010) 165179. [214] Y. Li, B. Yuan, J. Fu, S. Deng, X. Lu, Adsorption of alkaloids on ordered mesoporous carbon, Journal of Colloid and Interface Science, 408 (2013) 181-190. [215] M. Dai, B.D. Vogt, High capacity magnetic mesoporous carbon–cobalt composite adsorbents for removal of methylene green from aqueous solutions, Journal of Colloid and Interface Science, 387 (2012) 127-134. [216] Y. Yan, S. Liu, F. Hao, P. Liu, H.a. Luo, Nitrocyclohexane hydrogenation to cyclohexanone oxime over mesoporous carbon supported Pd catalyst, Catalysis Communications, 50 (2014) 9-12. [217] J.-L. Gao, S. Gao, C.-L. Liu, Z.-T. Liu, W.-S. Dong, Synthesis, characterization, and catalytic application of ordered mesoporous carbon–niobium oxide composites, Materials Research Bulletin, 59 (2014) 131-136. 77 Capítulo II [218] B. Chang, J. Fu, Y. Tian, X. Dong, Soft-template synthesis of sulfonated mesoporous carbon with high catalytic activity for biodiesel production, RSC Advances, 3 (2013) 1987-1994. 78 CAPÍTULO III. MATERIALES Y MÉTODOS Materiales y métodos 1. MATERIALES Para la síntesis de los diferentes materiales adsorbentes empleados en esta Tesis Doctoral se han utilizado los siguientes reactivos: Líquidos -Etanol; C2H6O -Ácido clorhídrico; HCl -1,2-bis(trietoxisilil)etano; (CH3CH2O)3SC2H4Si(CH3CH2O)3 -1,2bis(trietoxisilil)benceno; (CH3CH2O)3SiC6H4Si(CH3CH2O)3 Sólidos -Dodecilsulfato de sodio; (C12H25NaO4S) -Ácido tetradecanodioico; (C14H26O4) -Nitrato de magnesio;Mg(NO3)2·6H2O -Nitrato de aluminio; Al(NO3)3·9H2O -Hidróxido sódico; NaOH -Carbonato de sodio; Na2CO3 -Polioxietileno (10)-estearil éter; (Brij 76); C18H37(CH2CH2O)n n=10 aprox. -Pluronic F127; EO106PO70EO106 -Resorcinol; C6H6O2 -Formaldehído; 36% en agua Gases -Nitrógeno 5.0. Envasado en botellas de acero a presión de 200 bares y con pureza de 99.99% 81 Capítulo III 1. 1 Síntesis de adsorbentes 1.1.1 Hidrotalcita con carbonato en la interlámina La síntesis de la hidrotalcita con el anión inorgánico en la interlámina fue preparada según el procedimiento descrito por Reichle [1]. Se seleccionó el anión 2- 2+ 3+ inorgánico CO3 y como metal divalente y trivalente Mg y Al respectivamente. La 2+ 3+ relación M /M fue igual a tres. Esta hidrotalcita fue calcinada a 500ᵒC dando lugar al óxido mixto de Mg-Al, el cuál fue usado como adsorbente para los pesticidas Mecoprop-P y Nicosulfuron. 1.1.2. Hidrotalcita con anión orgánico en la interlámina Se escogieron dodecilsulfato y tetradecanedioato como aniones interláminares para la síntesis de hidrotalcitas modificadas orgánicamente con el fin de poder comparar el efecto del tamaño de la cadena en la adsorción de compuestos. Para su síntesis también se utilizó el método de coprecipitación o síntesis directa y 2+ 3+ Mg y Al como metales. El dodecilsulfato de sodio C12H25NaO4S también conocido como laurilsulfato -1 de sodio, tiene un peso molecular de 288.38 g·mol y es muy soluble en agua. La molécula posee propiedades anfifílicas ya que tiene un grupo sulfato adosado a una cadena de 12 átomos de carbono. La solubilidad en agua de este compuesto es -1 independiente del pH siendo de 43 g·L . Su punto de fusión varía entre 204 y 207ᵒC. El ácido tetradecanodioico es un sólido blanco con un peso molecular de -1 258.35 g·mol . Es una molécula con 14 átomos de carbono y dos grupos ácido en sus extremos. Su pKa es aproximadamente de 4.48 y su punto de fusión 127ᵒC. La solubilidad de este compuesto depende del pH del medio. Así, a pH<5 es poco soluble -1 -1 (0.18 g·L ), a pH 6 el compuesto tiene una solubilidad de 57g·L y a pH>6 es muy -1 soluble (1000g·L ). En la Figura 22 se esquematiza el procedimiento de síntesis empleado para la obtención de las hidrotalcitas orgánicas. 82 Materiales y métodos En la síntesis de estos materiales es muy importante establecer una atmósfera 2- inerte durante el proceso debido a la alta afinidad del anión CO3 por la interlámina. Este anión se forma en medio básico cuando se pone en contacto el CO2 con el H2O. Figure 22. Schematic representation of hydrotalcite synthesis También es muy importante obtener una fase ordenada y pura. Para ello se añade la disolución de los metales muy lentamente sobre la disolución del anión interlaminar. En ciertas ocasiones es necesario el control de pH. La fórmula teórica de las hidrotalcitas orgánicas son [Mg3Al(OH)8]CH3CH2SO4·nH2O para la hidrotalcita con dodecilsulfato en la interlámina, HTDDS y [Mg3Al(OH)8]2(CO2(CH2)12CO2 para la hidrotalcita con tetradecanodioato en la interlámina, HTTDD. 1.1.3 Etileno y fenileno PMOs Para la síntesis de estos dos materiales se ha seguido el procedimiento de Burleigh y col. [2] con alguna modificación en las proporciones de ácido clohídrico y etanol en el proceso de extracción. Los precursores fueron bis(trietoxisilil)etileno y 1,4bis(trietoxisilil)benceno (Figura 23) y como surfactante Brij 76. Bis(trietoxisilil)etileno 1,4-bis(trietoxisilil)benceno Si(OEt)3 Si(OEt)3 Si(OEt)3 Figure 23. Precursors for the synthesis of PMOs. 83 Si(OEt)3 Capítulo III En la Figura 24 se esquematiza la síntesis y la extracción del surfactante mediante la adicción de una mezcla de etanol y ácido clorhídrico. Brij-76 B i Solution H2O:HCl Room temperature 24 h Organosilane 50ᵒC, 24h 90ᵒC, 24h Filtrated and dry Extraction PMO material Solution EtOH:HCl Filtrated and dry Surfactant extraction Figure 24. Experimental procedure for the preparation of periodic mesoporous organosilicas using Brij 76 as “template”. 1.1.4 Resina fenólica y carbon mesoporosos ordenados Para la síntesis de estos materiales se utilizó resorcinol y F127 como precursor. El resorcinol o benceno 1-3-diol es un sólido soluble en agua con reacción -1 ligeramente ácida cuyo pKa es 9.45 y densidad 1.27 g·mol . Su peso molecular es 110 -1 g·mol y su punto de fusión 110ᵒC. El Pluronic F127 es un copolímero en bloque cuya estructura es EO106PO70EO106. En la Figura 25 se esquematiza el proceso de síntesis llevado a cabo para estos dos materiales. Cuando se calcinó en atmósfera de nitrógeno a 380ᵒC, el material obtenido fue la resina fenólica ordenada (MPR). Sin embargo, cuando la temperatura se aumentó a 800ᵒC, el compuesto sintetizado fue el carbón mesoporoso ordenado. 84 Materiales y métodos HCl:ETOH + F127 + ≈15min Evaporation ≈20min MPR Calcination ,380ᵒC MC Calcination ,800ᵒC Aging 60ᵒC, 24h. Figure 25. Representation of synthesis of ordered phenolic resin and mesoporous carbon. 1.2 Pesticidas Los pesticidas empleados fueron S-Metolacloro, Mecoprop-P, Nicosulfuron y Bentazona cuyas estructuras se recogen en la Figura 26. N O O Cl O O N N N O O S N H O N H N O Nicosulfuron S-Metolacloro O H N O S O O OH N Cl O Mecoprop-P Bentazon Figure 26. Pesticides used in this phD. 1.2.1 S-Metolacloro El S-Metolacloro es el nombre comercial del 2 cloro-N-(2-etil-6-metilfenil)-N[(2S)-1-metoxi-2-propanil]acetamida (número 85 del CAS 87392-12-9) que fue Capítulo III suministrado por Sigma Aldrich con una pureza de aproximadamente el 100%. Es un -1 líquido amarillento no ionizable con un peso molecular 283.20 g·mol . La solubilidad -1 en agua es de 530mg·L y su punto fusión es aproximadamente -62.1ᵒC. Es una cloroacetanilida con actividad herbicida de preemergencia, selectiva, que actúa inhibiendo la división celular con lo que impide la germinación de las semillas y dificulta el crecimiento, en especial, de las raíces. Dentro del grupo de las clorocetanilinas, S-Metolacloro es uno de los pesticidas más usados en el mundo, aproximadamente representa un 4.2% del pesticida global usado. En general, la dosis empleada depende del formulado del pesticida y del tipo de preparado de este. Puede estar como suspensión concentrada (SC), dispersión oleosa (DO), granulado dispersable en agua (WG), concentrado emulsionable (EC), suspo-emulsión (SE) o concentrado soluble (SL) como queda reflejado en la Tabla 6. En Francia el isómero rac-Metolacloro fue prohibido el 30 de Diciembre de 2003. En su lugar, se comenzó a utilizar S-Metolacloro (Anexo 1 Directiva 91/414/CEE). El paso de una sustancia a otra fue motivado por la alta eficiencia de eliminación de las malezas lo que condujo a una reducción de las dosis aplicadas por los agricultores -1 -1 (2.1kg·ha en 1998 a 0.5kg·ha en 2012). Debido a su baja capacidad de adsorción y alta solubilidad en agua se detecta fácilmente en el suelo y en la superficie del agua. Por ello, es una fuente potencial de contaminación. Table 6 Formulation of S-Metholachlor according to Ministry of Agriculture, Food and Environment FORMULADO NOMBRE TIPO DE DOSIS FABRICANTE COMERCIAL PREPARADO EMPLEADA Mesotriona 4% + S- Camix SE 3-3.75 L·ha-1 SYNGENTA ESPAÑA S.A. Metolacloro 40% (P/V) S-Metolacloro 31.25% Primexta Liquid SC 3-4 L·ha-1 SYNGENTA ESPAÑA S.A. terbutilazina 18.75% gold (P/V) Tyllanex magnum S-Metolacloro 96% (P/V) Dual gold EC 0.5-1.6 L·ha-1 SYNGENTA ESPAÑA S.A. Dual gold 96 EC 86 Materiales y métodos 1.2.2 Mecoprop-P El Mecoprop es un herbicida cuya sustancia activa es el ácido fenoxipropiónico clorado que se registró en 1960. Este compuesto consiste en la mezcla de dos isómeros en igual proporción pero solo uno de ellos es activo. Por ello, en 1980, tras el descubrimiento de nuevas tecnologías se consiguió separar estos isoméros y obtener así un pesticida con el isómero activo con una pureza de aproximadamente el 95%. Este compuesto se denominó Mecoprop-P. Table 7 Formulation of Mecoprop-P according to Ministry of Agriculture, Food and Environment. FORMULADO 2,4-D a 7% (sal dimetilamina) + dicamba 2% (sal dimetilamina) + mcpa 7% (sal dimetilamina) + mecoprop-p 4,2% (sal dimetilamina) P/V Bromoxinil 12% (ester octanóico) + mecoprop-p 18% (2-etil-hexil ester) P/V Bromoxinil 12% (octanoato) + ioxinil 12% (octanoato) + mecoprop-P 36% (ester butoxietílico) P/V Bromoxinil 7,5% (ester octanóico) + ioxinil 7,5% (ester octanóico) + mecoprop-P 18,75% (ester 2-etilhexil) P/V Carfentrazona-etil 1,5% + mecoprop-p 60% (sal de magnesio) P/P Ioxinil 18% (ester octanóico) + mecoprop-p 29% (ester butoxietil) P/V MCPA 16% (sal amina) + mecoprop-P 13% (sal amina) + diclorprop-P 31% (sal amina) P/V Mecoprop-P 28% (sal potásica) + piraflufen-etil 0,45% P/P Mecoprop-P 28,75% (sal dimetilamina) P/V Mecoprop-P 60% (sal amina (esp.)) P/V Mecoprop-P 73,4% (sal de magnesio y de dimetilamina) + tribenuron-metil 1% P/P NOMBRE COMERCIAL Dicotex TIPO DE PREPARADO SL DOSIS EMPLEADA 5 L·ha-1 o 1000L·ha-1 FABRICANTE AGRIPHAR, S.A. (consultar www.magrama.es ) Driwer EC 2L·ha-1 EXCLUSIVAS SARABIA, S.A. Image I-B-M Valles EC 1-1.75 L·ha-1 NUFARM S.A. Brioxil super P EC 1.5-2 L·ha-1 ARAGONESAS AGRO, S.A Platform S WG 1Kg·ha-1 FMC CHEMICAL SPRL Certrol H Mexiron EC 1.3L·ha-1 NUFARM S.A. Duplosan super Optica Trio SL 2.5 L·ha-1 NUFARM ESPAÑA, S.A. NUFARM UK LIMITED Caravan WG 1.5-2Kg·ha-1 Herbimur Forte Merekal SL 2-4 L·ha-1 SL 1.33L·ha-1 Aralis SG 1.09 Kg·ha-1 CHEMINOVA AGRO, S.A. EXCLUSIVAS SARABIA, S.A. NUFARM ESPAÑA, S.A. DU PONT IBERICA, S.L. ESPAÑA, ESPAÑA, Mecoprop-P es el nombre comercial de ácido (R)-2-(4-cloro-o-toliloxi) propiónico con el número CAS 16484-77-8. Es un pesticida de post-emergencia con un 87 Capítulo III -1 -1 peso molecular de 214.65 g·mol . Tiene una solubilidad en agua a 20ᵒC de 860 mg·L y un pKa de 3.86. En condiciones aerobias tiene una vida media de 21 días en el campo. La dosis empleada se muestra en la tabla 7. 1.2.3 Nicosulfuron El Nicosulfuron es el nombre comercial de 2-[(4,6-Dimetoxipirimidin-2ilcarbamoil) sulfamoil]-N,Ndimetilnicotinamida (número del CAS 111991-09-4) que fue suministrado también por Sigma-Aldrich. Es un pesticida de post-emergencia del grupo -1 de las sulfonilureas con un peso molecular de 410.41 g·mol y un pKa de 4.78. Su -1 solubilidad en agua a 20ᵒC es de 7500 mg·L y su punto de fusión 145ᵒC. Este pesticida es muy usado en España para la eliminación de las malas hierbas en el cultivo del maíz. En el suelo tiene una vida media de 26 días bajo condiciones aerobias y 63 días en ausencia de oxígeno. Este pesticida muestra una elevada movilidad en suelos de textura franco arenosa y franco limosa. Sin embargo, algunos de sus metabolitos son más móviles que el propio Nicosulfuron siendo sus principales productos de degradación la piridina sulfonamida y la pirimidina amina. Además, este plaguicida es susceptible a una fotólisis lenta en la superficie de los suelos y en agua que depende del pH, siendo mayor la descomposición en condiciones ácidas. La dosis empleada de este plaguicida depende de la composición del mismo como se muestra en la tabla 8. 1.2.4 Bentazona Bentazona es el nombre comercial de 3-isopropil-1H-2,1,3-benzotiadiazin 4(3H) ona, 2,2-dióxido. Es un pesticida de post-emergencia cuyo número CAS es 25057-89-0 y un pKa de 3.28. Tiene un peso molecular de 240.3 g·mol -1 y una -1 solubilidad en agua a 20ᵒC de 570 mg·L . Su punto de fusión es de 140ᵒC. y es estable en soluciones ácidas o básicas. La Bentazona sódica es considerablemente más soluble en el agua que el compuesto original con una solubilidad de 2300 g·L -1 Este pesticida es estable a la hidrólisis pero se fotodegrada en el agua en menos de 24h. También se fotodegrada en el suelo bajo condiciones aerobias, con una vida media que fluctúa entre menos de 7 dias y 14 semanas, dependiendo de los tipos 88 Materiales y métodos de suelo y de las condiciones. Además este pesticida es muy móvil en el suelo, y por consiguiente tiene el potencial de contaminar las aguas superficiales. Las dosis empleadas se especifican en la tabla 9. Table 8. Formulation of Nicosulfuron according to Ministry of Agriculture, Food and Environment. FORMULADO Nicosulfuron 24% (P/V) Nicosulfuron 3% + mesotriona 7.5% (P/V) Nicosulfuron 4% P/V NOMBRE TIPO DE DOSIS COMERCIAL PREPARADO EMPLEADA Chaman Forte Milagro 240 SC Elumis SC 0.25 L·ha-1 DO 1-2 L·ha-1 SYNGENTA ESPAÑA S.A. Nicosulfuron 4% OD Elite M Samson DO 1.5 L·ha-1 SHARDA EUROPE B.V.B.A Nico Simun Bandera CHEMINOVA AGRO, S.A. ISK BIOSCIENCES EUROPE S.A. Nic-Sar Nicosulfuron 4% FABRICANTE EXCLUSIVAS SARABIA, S.A. 1-1.5L·ha-1 SC Nic-4 BELCHIM CROP PROTECTION ESPAÑA, S.A. ROTAM AGROCHEMICAL EUROPE LIMITED SHARDA EUROPE B.V.B.A Sajon SAPEC AGRO S.A.U Nicozea Nicosulfuron 42.9% +rimsulfuron 10.7% (P/P) Nicosulfuron 6% P/V Nicosulfuron 75% P/P TRADE CORPORATION INTERNATIONAL, S.A. CHEMINOVA AGRO, S.A. ARAGONESAS AGRO, S.A. Chaman Tomcat-maiz SC Tomcat-maiz Nicovita Nicogan Kelvin Principal WG 90g·ha-1 Elite Plus 6 OD DO Accent 75 WG WG 0.5-0.75 L·ha-1 53-83g·ha-1 DU PONT IBERICA, S.L. 89 -DU PONT IBERICA, S.L. -ISK BIOSCIENCES EUROPE S.A. - DU PONT IBERICA, S.L. Capítulo III Table 9. Formulation of Bentazon according to Ministry of Agriculture, Food and Environment. FORMULADO Bentazona 48% (sal sódica) Bentazona 48% (sal sódica) (ESP) P/V Bentazona 48% + Imazamox 2.24% P/V Bentazona 87% P/P NOMBRE COMERCIAL Basagran L Troy 480 Kaos-B TIPO DE PREPARADO SL DOSIS EMPLEADA 1.5-2 L·ha-1 SL 2 L·ha-1 Corum SL Basagran SG SG 1.12-1.88 L·ha-1 1-1.15 Kg·ha-1 90 FABRICANTE BASF ESPAÑOLA, S.L. AGRIGHEM B.V. SAPEC AGRO S.A. BASF ESPAÑOLA S.L. BASF ESPAÑOLA S.L. Materiales y métodos 2.MÉTODOS En esta sección se describen brevemente todas las técnicas llevadas a cabo para la caracterización de los materiales utilizados como adsorbentes para poder conocer la composición y sus características estructurales. Así mismo, se han empleado técnicas para poder determinar la concentración de pesticida adsorbido o bien observar las diferencias tras la adsorción. 2.1 Espectroscopía de Absorción Atómica Esta técnica data del siglo XIX, aunque no fue hasta la década de los 50 cuando fue desarrollada en profundidad por un equipo de químicos de Australia, dirigidos por Alan Walsh. Esta técnica ha permitido junto con el análisis termogravimétrico conocer la composición de las hidrotalcitas. El análisis químico de los metales se ha llevado a cabo mediante espectroscopía de absorción atómica previa disolución de las muestras con HCl concentrado (50%). En esta técnica los átomos, en el atomizador, pasan a estado gaseoso fundamental y sus electrones son promovidos a orbitales más altos por un instante mediante la absorción de una cierta cantidad de energía. Esta cantidad de energía se corresponde a una transición electrónica típica de cada elemento. Se puede calcular a partir de la ley de Beer-Lambert cuántas de estas transiciones tienen lugar y así obtener una señal que es proporcional a la concentración del elemento, ya que la cantidad de energía absorbida en la llama puede medirse con un detector. El análisis elemental de Mg y Al se realizó en un espectrofotómetro de Perkin Elmer 3100 después de disolver las muestras en HCl. Para determinar Mg se emplea un atomizador de llama con una cabeza de quemador de 10 cm, que utiliza flujo de acetileno 2.5L·min-1 como combustible y de aire 4L·min-1 como oxidante. Sin embargo, para Al se emplea un atomizador de llama con una cabeza de quemador de 5 cm, con -1 un flujo de protóxido de nitrógeno de 3.5L·min como combustible y de aire a 4 L·min 91 -1 Capítulo III como oxidante. El análisis de S, C y N fue llevado a cabo en un analizador Eurovector 3A 2010. 2.2 Difracción de Rayos X La difracción de rayos X es la técnica básica para el análisis cualitativo y cuantitativo de fases cristalinas de cualquier tipo de material, tanto natural como sintético. La difracción de rayos X se basa en la dispersión coherente de un haz monocromático de rayos X por parte de la materia y en la interferencia constructiva de las ondas que están en fase y que se dispersan en determinadas direcciones del espacio. Este fenómeno se puede explicar mediante la ley de Bragg que predice la dirección en la que se da la interferencia constructiva de las ondas que están en fase y que se dispersan en determinadas direcciones del espacio: 𝑛𝜆 = 2𝑑𝑠𝑒𝑛𝜃 Esta ecuación nos indica la relación que existe entre el espaciado entre dos planos de átomos (d), la longitud de onda de la radiación X utilizada (𝜆) y el ángulo de incidencia de los rayos X (𝜃), siendo n un número entero. Los estudios de difracción de rayos X fueron realizados en un difractómetro Siemens modelo D5000, controlado por el sistema informático DiffracPlus BASIC 4.0, provisto de un monocromador de grafito para el haz difractado y detector de tipo proporcional. La radiación utilizada fue Cu Kα1,2 (λ1 = 1.54056 Å y λ2 = 1.54439 Å) y las condiciones de registro fueron 40 kV y 30 mA. Para poder obtener difractogramas a bajo ángulo se empleó el difractómetro ARL X´TRA Thermo Scientific. En este caso la radiación fue también Cu Kα a 45kV y 44mA. 92 Materiales y métodos 2.3 Microscopía Electrónica 2.3.1 Microscopía Electrónica de Transmisión El microscopio electrónico de transmisión (TEM) es un instrumento que aprovecha los fenómenos físico-atómicos que se producen cuando un haz de electrones suficientemente acelerado colisiona con una muestra delgada convenientemente preparada. A partir del modelo construido por Knoll y Ruska en 1931, varias casas comerciales han fabricado diversos modelos de microscopios electrónicos modernos, pero todos ellos basados en el modelo original. La obtención de las imágenes se llevo a cabo con dos microscopios electrónicos pertenecientes a los Servicios Centrales de Apoyo a la Investigación (SCAI) de la Universidad de Córdoba: el JEOL JEM 1400 con resolución de 0.38nm entre puntos con voltaje de aceleración 80, 100 y 120 kV y el microscopio JEOL JEM 2010 de alta resolución (resolución de 0.194 nm entre puntos) y un voltaje de aceleración de 80, 100, 120, 160 y 200Kv. Previo a su análisis, las muestras se dispersaron en etanol, colocando la mezcla en un baño de ultrasonidos y, transcurridos 15 minutos, una microgota de suspensión se depositó sobre una rejilla de cobre recubierta de carbón suministrado por la empresa ANAME. 2.3.2 Microscopía Electrónica de Barrido El microscopio electrónico de barrido (SEM) fue inventado en 1937 por Manfred von Ardenne. Este aparato utiliza un haz de electrones para generar una imagen. Esta técnica nos permite observar y estudiar superficies de un área muy reducida de cualquier tipo de material, bien sea orgánico o inorgánico, sin necesidad de pasar por procesos de preparación muy complejos, ya que los requerimientos más importantes son que la muestra esté deshidratada y sea eléctricamente conductora. El equipamiento actual consta de un detector de electrones secundarios (SE) que proporciona imágenes topográficas de la superficie de la muestra, un detector de electrones retrodispersados (BSE) que proporciona imágenes en escala de grises 93 Capítulo III relativas a los elementos constituyentes de la muestra y un detector de energías dispersivas de rayos X (EDX) que permite el análisis cualitativo y semicuantitativo de los elementos constituyentes de la muestra. El microscopio utilizado, un Jeol modelo JMS 6400 del SCAI de la Universidad de Córdoba, emplea un voltaje de aceleración del haz de electrones de 20 kV. Las muestras colocadas sobre cinta adhesiva de cobre se unen a un portamuestras también de cobre de 15 mm de diámetro. Después se recubren, solo los sistemas no conductores, con una capa fina de oro. 2.4. Espectroscopía Fotoelectrónica de Rayos X (Xps) La espectroscopía de fotoelectrones de rayos X fue desarrollada por Kai Siegbahn a partir de 1957 y se utiliza para estudiar los niveles de energía de los electrones atómicos básicos, principalmente en sólidos. Es una espectroscopía semicuantitativa y de baja resolución espacial que se emplea para determinar la estequiometría, estado químico y estructura electrónica de los elementos que existen en la superficie de un material. Es una técnica superficial debido al corto rango de los fotoelectrones que son excitados en el sólido, alcanzando profundidades de 10 a 100Å. La energía de los fotoelectrones que abandonan la muestra es determinada mediante un analizador CHA (Concentric Hemispherical Analyser) que proporciona un espectro con una serie de picos de fotoelectrones. Así, cada elemento tiene una energía de unión de los picos (Binding Energy) y el área de éstos puede ser utilizada para determinar la composición de la superficie del material. La espectroscopia XPS trabaja a ultravacío para prever la contaminación de la muestra. Además, la muestra puede ser bombardeada con iones Argon para eliminar la posible contaminación superficial. Los espectros fueron registrados en un espectrofotómetro PHI-5700 fabricado por Physical Electronic, utilizando como fuente de excitación Mg Kα (1253.6 eV). Para la adquisición de datos el equipo cuenta con un software PHI ACCESS-ESCA V0.6F. El espectrofotómetro se calibró corrigiendo la señal con la energía de enlace de los electrones C 1s a 248.8 eV, respecto del nivel de Fermi. 94 Materiales y métodos 2.5 Espectroscopía Infrarroja con Transformada de Fourier Es una técnica que se emplea para identificar moléculas a través del análisis de sus enlaces químicos. Cada enlace químico en una molécula vibra a una frecuencia característica y, generalmente, esta frecuencia se encuentra dentro del intervalo de la radiación infrarroja. Cuando una molécula absorbe un fotón, salta de un estado fundamental a un estado excitado, y da lugar a una vibración. La espectroscopía infrarroja ha sido de gran utilidad para la caracterización de los adsorbentes así como confirmar la presencia de los plaguicidas en los productos de adsorción. Los espectros de absorción de infrarrojo de las distintas muestras se registraron con un espectrofotómetro de infrarrojo con transformada de Fourier -1 Perkin-Elmer Spectrum One 2000 en un rango de número de onda entre 400 cm y -1 -1 4000cm y con una resolución de 4cm . Los espectros obtenidos son el promedio de 64 registros mejorando así la relación señal/ruido. Las muestras, antes de realizar el espectro, se dejan en estufa a 60⁰C durante 24h. Se utiliza como blanco una pastilla de KBr. 2.6 Espectroscopía Resonancia Magnética Nuclear La espectroscopía de resonancia magnética nuclear (RMN) fue desarrollada a finales de los años cuarenta para estudiar los núcleos atómicos. En 1951, se descubrió que esta técnica podía ser utilizada para determinar las estructuras de los compuestos orgánicos. Puede utilizarse sólo para estudiar núcleos atómicos con un número impar de protones o neutrones (o de ambos). Cuando una muestra se coloca en un campo magnético, los núcleos con espín positivo se orientan en la misma dirección del campo, en un estado de mínima energía denominado estado de espín α, mientras que los núcleos con espín negativo se orientan en dirección opuesta a la del campo magnético, en un estado de mayor energía denominado estado de espín β. Existen más núcleos en el estado de espín α que en el β pero aunque la diferencia de población no es enorme sí que es suficiente para establecer las bases de 95 Capítulo III la espectroscopia de RMN. La diferencia de energía entre los dos estados de espín α y β, depende de la fuerza del campo magnético aplicado H0. Cuanto mayor sea el campo magnético, mayor diferencia energética habrá entre los dos estados de espín. Cuando una muestra que contiene un compuesto orgánico es irradiada brevemente por un pulso intenso de radiación, los núcleos en el estado de espín α son promovidos al estado de espín β. Esta radiación se encuentra en la región de las radiofrecuencias del espectro electromagnético. Cuando los núcleos vuelven a su estado inicial emiten señales cuya frecuencia depende de la diferencia de energía (∆E) entre los estados de espín α y β. El espectrómetro de RMN detecta estas señales y las registra como una gráfica de frecuencias frente a intensidad, que es el llamado espectro de RMN. El término resonancia magnética nuclear procede del hecho de que los núcleos están en resonancia con la radiofrecuencia de la radiación. Es decir, los núcleos pasan de un estado de espín a otro como respuesta a la radiación a la que son sometidos. El análisis de 29 Si MAS RMN ha sido empleado para estudiar la sílice y organosilices mesoporosas periódicas ordenadas proporcionando información acerca 29 del grado de condensación del material. Los espectros de RMN Si se han obtenido a temperatura ambiente en un espectrómetro Bruker ACP-400 a una velocidad de giro de 8 o 12 KHz y frecuencias de resonancias de 79.49 MHz, respectivamente. El pulso de excitación y tiempo de retardo para silicio fue 6 μs y 60 s. Los espectros de 29Si MAS RMN han sido deconvolucionados utilizando el programa Dmfit2006. 2.7 Análisis Termogravimétrico (TGA) Es una técnica que mide la variación de masa de un compuesto en función de la temperatura. Las variaciones de temperatura no siempre implican un cambio en la masa de la muestra; existen sin embargo, cambios térmicos que sí se acompañan de un cambio de masa, como la descomposición, la sublimación, la reducción, la desorción, la absorción y la vaporización. Estos cambios pueden ser medidos con un analizador termogravimétrico. 96 Materiales y métodos El análisis de las muestras se realizó mediante un equipo SETARAM Setsys -1 Evolution 16/18 en un rango de temperatura entre 30-1000⁰C a 5⁰C·min . 2.8 Isotermas de Adsorción-Desorción de Nitrógeno La adsorción física o fisisorción de gases es una técnica de análisis de propiedades texturales (superficie específica, volumen y tamaño de poros) basada en la interacción que tiene lugar entre un gas (adsorbato) y el sólido que se quiere caracterizar (adsorbente). La interpretación de estas isotermas mediante diferentes modelos matemáticos permite obtener valores para las propiedades texturales. Las propiedades que caracterizan texturalmente a un sólido son varias, aunque las más empleadas suelen ser: 2 -1 -Superficie específica: suele expresarse en m ·g . Para la medida de esta propiedad se emplean modelos matemáticos como Langmuir o B.E.T (Brunauer-Emmett-Teller), siendo este último el más empleado. 3 -1 -Volumen total de poros: se suele expresar en cm ·g . Hace referencia al volumen ocupado por el adsorbato, dentro del adsorbente a una presión determinada, normalmente P/P0=1. En este caso, se trata de una medida directa y expresa el volumen que ocupan los poros en una unidad másica de sólido. -Distribución de tamaño de poro: consiste en expresar el volumen de poro frente al tamaño de poro al que se adscribe. No es una medida directa, sino la consecuencia de aplicar modelos matemáticos más o menos complejos. Los modelos más utilizados para el rango de los microporos son Horvath-Kawazoe (HK) y Dubinin mientras que para el rango de los mesoporos el más empleado es Barret-Joyner-Halenda (BJH). La isoterma de adsorción es el resultado de la representación gráfica de la cantidad adsorbida, a temperatura constante, en función de la presión (o concentración) del adsorbato. La IUPAC clasifica las isotermas en seis tipos (Figura 27.): 97 Capítulo III Figure 27. Clasification of adsorption isotherm of gases. -Tipo I: la isoterma es cóncava respecto al eje de la presión relativa ( P/P0 ), -3 aumenta rápidamente a baja presión ( P/P0<1x10 ) y posteriormente alcanza un plateau de saturación horizontal. Este tipo de isotermas la muestran los sólidos microporosos. -Tipo II: esta clase de isoterma es característica de sólidos no-porosos o de adsorbentes macroporosos. La total reversibilidad de la isoterma de adsorcióndesorción, es decir, la ausencia de histéresis, es una condición que se cumple en este tipo de sistemas. -Tipo III: es característico de procesos de adsorción en sólidos no porosos en los que la interacción adsorbente-adsorbato es débil. En la práctica no es común este tipo de isotermas. -Tipo IV: a bajas presiones se comporta como la isoterma de Tipo II. Es característica de los sólidos mesoporosos. Ésta presenta un incremento de la cantidad adsorbida importante a presiones relativas intermedias y ocurre mediante un mecanismo de llenado en multicapas. -Tipo V: del mismo modo que las de Tipo III, esta clase de isotermas se obtiene cuando las interacciones entre el adsorbato y el adsorbente son débiles. La presencia de histéresis está asociada con el mecanismo de llenado y vaciado de los poros. En la práctica es poco usual encontrase con este tipo de isotermas. 98 Materiales y métodos -Tipo VI: Es muy poco frecuente. Es característico de la adsorción en multicapa de gases nobles sobre superficies altamente uniformes. Este tipo de adsorción en escalones ocurre sólo para sólidos con una superficie no porosa muy uniforme. La histéresis que aparece en el rango de multicapa de las isotermas de fisisorción se asocia normalmente con la condensación capilar en la estructura de mesoporos. Los diferentes tipos de histéresis se muestran en la Figura 28. La histéresis H1 se encuentra en materiales formados por aglomerados, agregados de partículas esféricas ordenadas de una manera uniforme así como materiales con poros cilíndricos y con un elevado grado de uniformidad del tamaño de poro. En este tipo de histéresis las ramas de adsorción-desorción son paralelas y casi verticales. Figure 28. Types of hysteresis loops En la histéresis tipo H2 la rama de desorción es completamente vertical y se observa en materiales con poros dispuestos como canales. Sin embargo, la histéresis H3 se asocia a materiales formando agregados de partículas planas, con sus poros en 99 Capítulo III forma de platos. Además esta histéresis no se estabiliza a presiones relativas a la presión de saturación. Finalmente, la histéresis H4 es característica de poros estrechos en forma de rendija, con presencia de microporosidad. Los análisis se realizaron en un analizador Belsorp-mini II. Antes de realizar el estudio, las muestras fueron desgasificadas a 120⁰C durante 24h para eliminar el agua adsorbida. 2.9 Potencial Zeta El potencial zeta es un importante indicador de la carga superficial y da información para el entendimiento y control de los fenómenos relacionados con dicha carga. Las medidas del potencial zeta pueden aplicarse para: -Cálculo del punto isoeléctrico. -Caracterización de la estructura química superficial de los sólidos. -Determinación de la energía libre de adsorción de tensioactivos sobre los sólidos. -Cálculo de la densidad de carga superficial. -Cálculo del espesor de la capa polimérica adsorbida. En nuestro caso esta técnica fue empleada para la determinación del punto isoeléctrico de los adsorbentes. Las medidas se realizaron en un Zetasiser Nano Zs de Malvern y un autotitulador MPT-2 perteneciente al SCAI de la Universidad de Málaga. Previamente, las muestras se suspendieron en una disolución de KCl 0.01M y HCl 0.1M. 2.10 Espectroscopía Ultravioleta Visible En la espectroscopía UV-Vis se irradia con luz de una longitud de onda de energía suficiente como para provocar transiciones electrónicas, es decir, promover un electrón desde un orbital de baja energía a uno vacante de alta energía. El rango de longiudes de onda de un espectro UV-Vis está aproximadamente entre 200 y 800 nm. La luz visible o UV es absorbida por los electrones de valencia, éstos son promovidos a estados excitados (de energía mayor). Al absorber radiación 100 Materiales y métodos electromagnética de una frecuencia adecuada, ocurre una transición desde uno de estos orbitales a un orbital vacío. Las diferencias de energía varían entre los diversos orbitales. Algunos enlaces, como los dobles, provocan coloración en las moléculas ya que absorben energía tanto en el visible como en el UV, como es el caso del βcaroteno. Mediante la ley de Lambert Beer (A=ε·l·C) se relaciona la absorbancia medida con la concentración de la disolución. Las medidas se llevaron a cabo en un espectrofotómetro Perkin-Elmer UV/VIS Lambda 11. 2.11 Tamaño de partícula La distribución de tamaño de partícula se ha determinado utilizando la técnica de difracción de láser. El procedimiento consiste en medir la intensidad de la luz dispersada cuando un haz láser pasa a través de una dispersión de partículas. Una serie de detectores miden con presición la intensidad de la luz dispersada por las partículas en un amplio rango de ángulos. Mediante un software adecuado se analizan los datos dispersión que permiten el cálculo de la distribución del tamaño de partícula. El equipo empleado fue un Mastersizer S (Malvern Instrument) usando etanol como dispersante. Previo a su análisis, las muestras fueron tratadas en ultrasonidos durante al menos 10min. BIBLIOGRAFÍA [1] W. T. Reichle, Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite), Solid State Ionics 22 (1986) 135–141. [2]M. C. Burleigh, S. Jayasundera, C. W. Thomas, M. S. Spector, M. A. Markowitz y B. P. Gaber, A versatile synthetic approach to periodic mesoporous organosilicas”, Colloid Polym. Sci., 282 (2004) 728-33 101 CAPÍTULO IV. ADSORPTION OF NON-IONIC PESTICIDE S-METOLACHLOR ON LAYERED DOUBLE HYDROXIDES INTERCALATED WITH DODECYLSULFATE TETRADECANEDIOATE ANIONS AND Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions Abstract The objective of the present research was to investigate the adsorption of non-ionic pesticide S-Metolachlor on two different organohydrotalcites (OHTs): one intercalated with dodecylsulfate (HT-DDS) and the other one with tetradecanedioate (HT-TDD) anions, both of them obtained by the coprecipitation method. The adsorbents and the adsorption products were characterized by X-ray diffraction and FT-IR spectroscopy. The results indicated that the adsorption of S-Metolachlor on the organohydrotalcite HT-TDD was higher than that on HT-DDS, but desorption was lower. The amount of SMetolachlor adsorbed increased with temperature, especially for HT-TDD. SMetolachlor desorption from HT-DDS and HT-TDD was characterized using two experimental procedures, which indicated the higher reversibility of S-Metolachlor adsorption from HT-DDS compared to HT-TDD. The results suggest the possibility to use both organohydrotalcites uptaking S-Metolachlor from contaminated water. Keywords: S-Metolachlor, organohydrotalcite, dodecylsufate anion, tetradecanedioate anion, adsorption, desorption, water treatment. 1. Introduction The widespread use of pesticides in modern agriculture has given rise to an increase about the environmental concern. The contamination of soil and ground water by pesticides applied to soil and swept by transport processes, such as leaching and run-off, is becoming an important environmental problem (Modabber et al., 2007). Pesticides are very hazardous pollutants, toxic to many forms of wildlife, especially aquatic organisms, insects and mammals and can persist in the aquatic environment for many years after their application (Shukla et al., 2006). A strategy to reduce the pesticide transport losses after soil application consists of the use of controlled release formulations, in which the pesticide is gradually released over time. This limits the amount of pesticide immediately available for transport processes. Beneficial effects related to the use of controlled release formulations include reduction in rapid losses of pesticide by volatilization, runoff and 105 Capítulo IV leaching. The savings in manpower and energy, by reducing the number of applications required in comparison to conventional formulations, increase safety for the pesticide applicator and lead to a general decrease in non-target effects (Gerstl et al., 1998; Celis et al., 2002). Previous research has indicated that layered double hydroxides (LDHs), also known as anionic clays or hydrotalcite-like compounds (HTs), could be suitable to smooth and even prevent the environmental impact caused by pesticides, i.e. they can be used either as filters of pesticide-contaminated water, or as supports in pesticide controlled release formulations. LDHs or HTs are brucite-like layered materials with a II III x+ n− II III general formula [M1−x Mx (OH)2] Xx/n ·mH2O, where M and M are divalent and trivalent cations respectively, and X n− is the interlayer anion, which balances the III positive charge originated by the presence of M in the layers. The layer charge is III III II determined by the molar ratio x= M / (M +M ) and can vary between 0.2 and 0.4 (Cavani et al., 1991; Costantino et al., 2009). The interest of these materials is related to their structure, high anion exchange capacity, and simplicity of their synthesis (Rives, 2001; Reichle, 1986). Similarly to clay minerals, the inorganic anions of HTs can be replaced with organic anions (Meyn et al., 1990; Clearfield et al., 1991; Zhao and Vance, 1997; Costantino et al., 2009). This simple modification changes the nature of the HT surface from hydrophilic to hydrophobic, increasing its affinity for non-ionic organic compounds, including many non-ionic pesticides (Pavlovic et al., 1996; Villa et al., 1999; Celis et al., 2000; Wang et al., 2005; Bruna et al., 2006; Costa et al., 2008). These organoclays and organo/LDHs have applications in a wide range of organic pollution control fields (Costa et al., 2008; Celis et al., 2000; Frost et al., 2008; Zhou et al., 2008; Laha et al., 2009; Bruna et al., 2009). In the present paper, we report on the adsorption of a non-ionic pesticide, SMetolachlor, on two different organohydrotalcites (OHTs): one intercalated with dodecylsulfate (HT-DDS) and the other with tetradecanedioate (HT-TDD) anions in the interlayer space, both surfactants with the same carbon chain (twelve -CH2 units) but differing in the nature of the chemical groups bonded to the HT layers. The aim of this paper was to analyze the influence of the surfactant as well as the effect of temperature on the adsorption process of S-Metolachlor to complete the study about 106 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions the behavior of this pesticide carried out previously (Chaara et al., 2011). A detailed study of the reversibility of the adsorption-desorption process was also conducted using two different experimental approaches. 2. Materials and methods 2.1. Organic anions and pesticide Dodecylsulfate (DDS) was supplied as sodium salt by Fluka, and tetradecanedioic acid (TDD) and analytical standard S-Metolachlor (2-Chloro-N-(2-ethyl-6-methylphenyl)-N[(1S)-2-methoxy-1-methylethyl]acetamide) were provided by Sigma-Aldrich. The structure of these materials is included in the following scheme: H3C O CH3 CH3 N O Cl H3C S-Metolachlor - O O S H3C O O Dodecylsulfate anion O O- O Tetradecanedioate anion O S-Metolachlor, a chloroacetamide type compound with a water solubility of 480 mg/L (25 °C), is a selective herbicide applied as pre- and post-emergence to control grasses and some broad-leaved weeds in a wide range of crops. 2.2. Synthesis of organohydrotalcites Two organohydrotalcites (OHTs), [Mg3Al(OH)8][(C12H25SO4)·4H2O] and [Mg3Al(OH)8][(C14H24O4)·4H2O], named HT-DDS and HT-TDD, respectively, were 107 Capítulo IV obtained by the coprecipitation method (Reichle, 1986). To prepare hydrotalcite with DDS in the interlayer, a solution containing 0.06 mol Mg(NO3)2·6H2O and 0.02 mol Al(NO3)3·9H2O in 100 mL of distilled water was dropped to an alkaline solution (0.16 mol of NaOH in 500mL of water) containing 0.05 mol of dodecylsulfate (DDS). The synthesis was carried out without control of pH and at room temperature. To prepare hydrotalcite with TDD in the interlayer, a solution containing 0.06 mol Mg(NO3)2·6H2O and 0.02 mol Al(NO3)3·9H2O in 100 mL of distilled water was dropped to an alkaline solution of NaOH at pH=8, containing 0.01 mol of tetradecanedioic acid (TDD); during the addition the pH was constant and the temperature 75°C. In both cases, the syntheses were carried out in inert atmosphere (N2 bubbling), to avoid CO2 dissolution and subsequent intercalation of carbonate into the LDH interlayer. The resultant suspensions were hydrothermally treated at 80°C for 24 h, and the precipitate washed with CO2-free distilled water and dried at 60°C. 2.3. Characterization of organohydrotalcites Elemental analysis of Mg and Al was performed with an AAS Perkin Elmer 3100 spectrometer after dissolving the samples in concentrate HCl, whereas S, C and N contents were determined with an elemental analyzer Eurovector 3A 2010. The water content was calculated by using a thermogravimetric analyzer SETARAM Setsys Evolution 16/18. For this purpose, 20 mg of sample was submitted to heating with a constant temperature gradient of 5°C/min in the range of 30 to 1000°C. Powder X-ray diffraction patterns (PXRD) were obtained on a Siemens D-5000 instrument with CuKα radiation and FT-IR spectra were recorded by using the KBr disk method on a Perkin Elmer Spectrum One spectrophotometer. Specific surface areas were measured from nitrogen adsorption/desorption isotherms on a Micromeritics ASAP 2020 instrument using N2 gas as adsorbate. The surface morphology of both samples was obtained using a scanning electron microscope JEOL JSM 6300. 2.4. Adsorption and desorption experiments S-Metolachlor adsorption on OHTs was studied at different initial pH values, contact times (kinetics), and initial pesticide concentration (isotherms), by suspending 108 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions duplicate samples of 20 mg of each adsorbent in 30 mL of aqueous pesticide solutions. Samples of sorbent and pesticide in water were shaken in a turn-over shaker at 52 rpm and the supernatants were centrifuged and separated to determine the concentration of S-Metolachlor by UV spectroscopy (Perkin Elmer UV-visible spectrophotometer, Model Lambda 11) at 265 nm. The amounts of pesticide adsorbed were determined from the initial and final solution concentrations. To study the adsorption at different initial pHs (5.0, 6.6 and 9.0) and contact times (1, 2, 5, 24 h), an initial pesticide concentration of 0.35 mM was used. Adsorption isotherms were obtained by the batch equilibration technique as described elsewhere (Hermosín et al., 1993; Pavlovic et al., 1997), using initial pesticide concentrations between 0.05 mM and 0.35 mM. Desorption isotherms were obtained using two different procedures: In the first one, described by Bowman (1979) and Bowman and Sans (1985), desorption was carried out immediately after adsorption from the highest equilibrium point of the adsorption isotherm. The procedure consisted of removing 15 mL of the supernatant liquid and replacing it with the same amount of distilled water. After shaking at room temperature for 24 h, the suspensions were centrifuged, and the pesticide concentration in the supernatant was determined by UV-Vis spectroscopy. This process was repeated four times (Bruna et al., 2008). In the second procedure, which was a modification of that described by Yang (1974), one unique desorption point was measured from each equilibrium point of the adsorption curve by replacing a small portion of supernatant (15 mL) with the same amount of distilled water. After shaking for 24 h, the amount of pesticide desorbed was calculated as describe above. This procedure avoids the difficulties mentioned by Bowman (1979) about the procedure used by Yang (1974). Adsorption and desorption parameters were calculated using the Freundlich equation (Eq. 1) 1/𝑛𝑓 (1) 𝐶𝑠 = 𝐾𝑓 𝐶𝑒 where Cs(µmol/g) is the amount of S-Metolachlor adsorbed at the equilibrium concentration Ce (µmol/L), and Kf and nf are the empirical Freundlich constants 109 Capítulo IV (Bowman and Sans, 1985). Hysteresis coefficients, H, were calculated according to H=nf-des/nf-ads, where nf-ads and nf-des are the Freundlich slopes of the adsorption and desorption isotherms, respectively (Barriuso et al., 1994). For the adsorbent regeneration and recyclability study, a preliminary experiment was conducted to obtain the kinetics of S-Metolachlor desorption from HT-DDS and HT-TDD using water, methanol, and ethanol as extracting solutions. Desorption kinetic curves were obtained from the highest equilibrium point of the adsorption kinetic curve. The extracting volume was 30 mL and the contact times 1, 2, 5, 24, and 30 h. After shaking, the suspensions were centrifuged, the supernatant liquid was separated and evaporated, and the residue dissolved in distilled water to determine the pesticide concentration by UV-Vis spectroscopy. Successive S-Metolachlor/methanol adsorption-desorption cycles for HT-DDS were conducted to assess the potential recyclability of this material as an adsorbent of S-Metolachlor. 3. Results and Discussion 3.1 Characterization of the organohydrotalcite sorbents 3.1.1 Elemental analysis According to the values of Mg/Al molar ratio (Table 1), a partial dissolution of Al during the synthesis of HT-DDS and Mg 2+ 3+ in HT-TDD seemed to occur. This could be attributed to the differences on the synthesis conditions (temperature, pH…), as well as, to the different acid/base properties of the organic anions (pKa = 4.46 for TDD and -3.29 for DDS). The molar ratio S/Al in HT-DDS (Table 1), which corresponds to the fraction of the positive charge of HT compensated by DDS anions, indicated that most of the layer charge (91%) was compensated by DDS anions and about 9% by other anions, probably 2- CO3 (as it will be seen below). The elemental analysis of C in HT-TDD sample (Table 1) suggests that, if all carbon atoms were as TDD anions, only 45% of the layer charge of HT will be compensated. Therefore, a part of the charge must be balanced by other 2- anions, probably CO3 , and also a small amount by nitrate anions (from the metal salts) as suggested by the N content of the sample (Table 1). From the elemental analysis and the TG data to obtain the water content (not included), the formulae for both organohydrotalcites were obtained and included in Table 1. 110 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions Fig. 1. XRD patterns of the organohydrotalcites and their adsorption products. (*diffraction peaks due to the Al sample holder). Table 1 Elemental analysis of the adsorbents and proposed formula Adsorbent HT-DDS wt. % Atomic ratios Mg Al C S N Mg/Al S/Al 15.72 4.67 29.38 4.97 0 3.74 0.91 Proposed formula [Mg0.79Al0.21(OH)2](C12H25SO4)0.20(CO3)0.00 5·0.56H2O HT-TDD 31.39 14.67 20.22 0 0.37 2.38 0 [Mg0.71Al0.29(OH)2](NO3)0.01Xn0.28/n·0.75H2 O 3.1.2 X-ray diffraction study Powder XRD patterns of the synthesized organohydrotalcites were recorded (Fig. 1). In both cases the diagram corresponds to a hydrotalcite-like structure with an important expansion of the interlayer space with respect to the inorganic hydrotalcite with carbonate anion (d003=7.8 Å) (Cavani et al., 1991). X-ray diffraction pattern of HTDDS sample (Fig. 1A) showed reflections (00l) at 26.4 Å and 13.1 Å, suggesting a vertical monolayer arrangement of dodecylsulfate anions (Clearfield et al., 1991; Bruna et al., 2006). X-ray diffraction pattern of HT-TDD (Fig 1B) showed (001) reflections at 17.5 Å and 8.5 Å suggesting a tilt position of the surfactant in the interlayer, since, according to CS Chem 3D 5.0 program, TDD height is 18.3 Å, that added to the 111 Capítulo IV hydrotalcite-layer thickness (4.8 Å) would give a value of 23.1 Å for a perpendicular arrangement of TDD anions. 3.1.3 Textural properties The scanning electron micrographs (Fig. 2) showed the surface morphology of HT-DDS and HT-TDD consisting of an agglomerate of particles, although slightly more porosity and less homogeneity in the HT-DDS particle size were observed. These results are in accordance with the low crystallinity observed in the XRD for both samples (Fig. 1) and with the low specific surface area values obtained by nitrogen 2 2 adsorption/desorption isotherms (not shown), 5.4 m /g for HT-DDS and < 1 m /g for HT-TDD. Fig. 2 SEM micrographs of samples (A) HT-DDS and (B) HT-TDD 3.2. S-Metolachlor adsorption-desorption experiments The adsorption of S-Metolachlor was not affected by the initial pH of the solution (not shown). This was attributed to the fact that the final pH values for the 112 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions equilibrated suspensions were all between 9 and 10, due to the buffer effect of hydrotalcite-like compounds (Cavani et al., 1991). For that reason, the selected initial pH for all subsequent adsorption experiments was 6.6, i.e. the original pH of the water-dissolved pesticide. The kinetic study of the S-Metolachlor adsorption on HT-DDS (Fig. 3) indicated an almost instantaneous adsorption, reaching the equilibrium in 2 h, while the pesticide adsorption on HT-TDD was slower and the plateau was not achieved. Nevertheless, taking into account that in the last case the amount of pesticide adsorbed at 24 h was only 5% greater than that adsorbed at 6 h, 24 h was assumed to be sufficient to reach an apparent adsorption equilibrium. This different behavior in the adsorption kinetics could be attributed to little changes in the morphology and specific surface area of the sorbents. Fig. 3.Time evolution of S-Metolachlor adsorption on HT-DDS and HT-TDD (Cinitial=0.35mM) The adsorption isotherms were obtained by plotting the amount of SMetolachlor adsorbed (Cs) versus the solute concentration (Ce) in the equilibrium solution and were of L-2 subtype, according to the classification of Giles et al. (1960), on both organohydrotalcites (Fig 4). The amount of S-Metolachlor adsorbed on HTDDS and HT-TDD was very similar (e.g., between 43 and 45% at an initial herbicide concentration, Ci= 0.35mM), suggesting a direct relation between the amount of 113 Capítulo IV pesticide adsorbed and the length of the surfactant alkyl chain (twelve –CH2 groups in both cases), and agree with the results of a previous work (Chaara et al., 2011) where adsorption of Metolachlor on HT-DDS was higher than that on an organohydrotalcite modified with sebacate anion (HT-SEB), with only eight -CH2 groups. The isotherms shape suggested that the Freundlich equation would be a good model to fitting, and the choice is justified by the high regression coefficients (Table 2). Table 2. Freundlich parameters for S-Metolachlor sorption (s) and desorption (d) isotherms on organohydrotalcites and hysteresis coefficient H. Kf nf R 2 H HT-DDS (s) 29.12-16.32 0.48±0.06 0.995 HT-DDS (d*) 0.07-0.06 1.94±0.27 0.973 4.04 HT-DDS (d**) 9.46-5.81 0.61±0.05 0.990 1.27 HT-TDD (s) 40.65-27.38 0.39±0.02 0.998 2.39 ±0.36 0.967 6.13 0.90±0.19 0.914 2.31 HT-TDD (d*) 0.017-9.23·10 HT-TDD (d**) 7.55-1.55 *first desorption way -4 **second desorption way The Freundlich constants (Kf and nf) together with the regression coefficients 2 (R ) were very similar for both sorbents (Table 2). Kf parameters were in the range 40.6-16.3, which indicated strong sorption between the sorbate and sorbent (Liu et al., 2010). The values of nf were less than 1, indicating a non-linear sorption of SMetolachlor on both samples. This behavior is similar to that observed by Bruna et al. (2008) for the adsorption of terbuthylazine on organohydrotalcites and Celis et al. (1999) for adsorption of imazamox on organoclays and organohydrotalcites. The temperature effect on the adsorption of S-Metolachlor by HT-DDS (Fig. 5A) and HT-TDD (Fig. 5B) was also investigated. When the temperature was increased from 10 to 30 °C, the amount of pesticide adsorbed at the initial concentration 0.35mM increased from 45.9 to 50.3% in HT-DDS and from 49.9 to 56.8% in HT-TDD. In the case of HT-DDS, the increase in pesticide adsorption is only observed clearly at high initial concentrations, i.e. at Ce values above 150 µmol/L. These results indicate 114 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions the endothermic nature of the adsorption process, as confirmed by the calculation of the thermodynamic parameters. Fig. 4 Adsorption and desorption isotherms of S-Metolachlor on HT-DDS (A, C) and HTTDD (B, D). Desorption was performed after adsorption from the highest equilibrium point of the adsorption isotherm (A, B). Desorption was performed after each point of the adsorption isotherm (C, D). o o o The change in standard free energy (ΔG ), enthalpy (ΔH ), and entropy (ΔS ) of adsorption was calculated using the following equations (Eqs. 2-3) (Özcan et al., 2005; Islam and Patel, 2007; Li et al., 2009; Islam et al., 2011): o o o ΔG =ΔH - TΔS o (2) o lnK = ΔS /R - ΔH /RT (3) where K is the equilibrium constant, R the molar gas constant and T the absolute o o temperature. Values of ΔH and ΔS were calculated from the slope and intercept of 115 Capítulo IV Van’t Hoff plots (Ln KC versus 1/T) and are summarized in Table 3. The values of K have been obtained from value of Kf from the Freundlich equation because our isotherms have been fitted to this equation. K has been recalculated to become dimensionless by o multiplying it by 1000 (Milonjic, 2007). ΔG values were calculated from Eq(2). Fig. 5 Adsorption isotherms of S-Metolachlor at different temperaturas: (A) on HT-DDS and (B) on HT-TDD. o ΔH values were positive in both cases, confirming the endothermic character o of the S-Metolachlor adsorption process, but ΔH for HT-TDD was an order of magnitude higher than that for HT-DDS (Table 3). Changes in ΔS were favorable for o both sorbents but the ΔS value was again higher for HT-TDD compared to HT-DDS. The larger contribution of the entropy than the enthalpy was responsible for the o negative value of ΔG in all cases, thus indicating that the adsorption process was o entropically controlled. The most negative ΔG value was observed for HT-TDD at 30°C coincident with the greater adsorption constant. With respect to the S-Metolachlor desorption study, the desorption isotherms obtained according to the above-described first procedure (the more widely used method (Hermosin et al., 1993; Liu et al., 2010; Pavlovic et al., 2005)), for both HT-DDS and HT-TDD showed high H values, H>>1 (Table 2), indicating a high reversibility of the pesticide adsorption process (Cruz-Guzmán et al., 2004) (Fig 4A, B). The shape of the curves, with the desorption branch falling below the adsorption isotherm branch, is probably due to the experimental procedure rather than to the adsorption-desorption 116 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions processes themselves (Koskinen et al., 1979) because that procedure, implying successive treatments with ethanol, probably could modify the solid features. Table 3 Thermodynamic parameters for S-Metolachlor sorption on organohydrotalcites HT-DDS and HT-TDD determined at different temperatures Sample T (ºC) K HT-DDS 10 20 30 11000 11020 12050 HT-TDD 10 20 30 14420 20320 38470 ΔS (KJ·mol-1·K-1) 0.090 ΔH (KJ·mol-1) 3.53 ΔG (KJ·mol-1) -21.89 -22.67 -23.67 0.204 35.32 -22.53 -24.16 -26.60 To overcome some of the limitations inherent to the successive desorption procedure, we used a second procedure to study desorption, i.e. a modification of procedure proposed by Yang (1974), where one desorption step was performed from every equilibrium point of the adsorption isotherm. Desorption curves thus obtained were very different from those obtained using the conventional successive desorption procedure (Fig. 4C, D). This procedure tries to reproduce the method carried out when N2 adsorption on any material is performed, where the hysteresis parameter close to 1 indicates high reversibility of the process and H values higher than 1 suggest irreversibility. The H value obtained for S-Metolachlor adsorbed on HT-DDS was very close to 1 (Fig. 4C), according with a high reversibility, indicating that desorption is favored. However, the hysteresis coefficient (H) for S-Metolachlor on HT-TDD is considerably greater than 1 (2.31) (Fig 4D), that indicates a certain no reversibility of the adsorption process. Desorption curve above the adsorption evidences the special difficulty to remove S-Metolachlor from the substrate HT-TDD. This new experimental procedure permits to distinguish between the behavior of S-Metolachlor in both organohydrotalcites, so that we could conclude that the reversibility of S-Metolachlor adsorption is higher for HT-DDS (H=1.27) than for HTTDD (2.31). However, the first procedure is still useful to simulate the conditions occurring in soil when, after adsorption, a pesticide is sequentially desorbed by rainfall or irrigation water. 117 Capítulo IV 3.3 Sorbent regeneration and recyclability experiments These experiments consisted of successive adsorption-desorption stages of SMetolachlor on HT-DDS and HT-TDD with the final purpose to analyze the possibility of sorbent regeneration. As a preliminary step, desorption experiments were performed with methanol, ethanol and water from the highest equilibrium point of the adsorption kinetic curves. The process was quite fast in all the cases (Fig. 6), desorption reaching the plateau in few hours. S-Metolachlor adsorbed on HT-DDS (Fig 6A) was totally removed (100%) with ethanol and almost the same happened with methanol (92.8%) while only 25.4% was desorbed using water. However, for hydrotalcite HT-TDD (Fig 6B) desorption was not completed with ethanol (62.7%) or methanol (58.0%) and was only 20.7% with water. As expected, this pesticide showed more affinity for hydrophobic solvents due to its non-ionic character. Fig. 6. Desorption kinetics of S-Metolachlor with different solvents (A) HT-DDS and (B) HT-TDD According to the above results, the study of recyclability was only performed on hydrotalcite HT-DDS using ethanol as solvent, the unique case where desorption was completed. HT-DDS adsorbent allowed only a stage of adsorption-desorption with ethanol removing completely S-Metolachlor. Although the second stage was only slightly less effective in the adsorption (95%) than the first one, only 40% of the pesticide was desorbed after the second adsorption step. These results indicate that 118 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions the possibility of regeneration of the sorbent with ethanol is limited, and new experiments should be necessary to found a more effective regeneration method. 3.4 Characterization of solids after adsorption In order to characterize the adsorption products in the “plateau” of the isotherms, powder XRD patterns and FT-IR spectra were registered to confirm the adsorption of S-Metolachlor in the hydrotalcite interlayers. After adsorption of SMetolachlor, a change in the d00l-value was observed for both organohydrotalcites (Fig 1C, D). The adsorption product of S-Metolachlor on HT-DDS showed a small decrease of the d003-value close to 1 Å (from 26.4 to 25.6 Å), (Fig. 1C). In contrast, S-Metolachlor adsorption on HT-TDD (Fig 1D) led to a very important increase in the d003-value compared with the pristine adsorbent (from 17.5 to 23.7 Å). This suggests a change in the orientation of the tetradecanedioate anion from tilt position in HT-TDD to perpendicular position after the adsorption of the pesticide, thus providing strong evidence for sorbent deformation upon adsorption of S-Metolachlor. The d003-value of the HT-TDD-S-Metolachlor adsorption product was only slightly higher than the sum of the size of TDD and the thickness of the brucite layer (23.12 Å). Taking into account that TDD anions have two negative charges versus only one for DDS, half of TDD anions will be necessary to compensate the layer charge producing a lower “congestion” of the interlayer space in the last case. The higher free space in the interlayer for HT-TDD would permit S-Metolachlor to be introduced between the organic chains causing a change in the orientation of the organic molecules, reflected in the increase in d003 space showed in the Fig. 1C. These results confirm those found by Chaara et al., (2011) for Alachlor adsorption on other organohydrotalcites. Fig. 7 shows FT-IR spectra of the adsorbents, S-Metolachlor, and adsorption products. Characteristic features of the hydrotalcite structure are the broad bands -1 - around 3460 cm , corresponding to the νO-H of hydration water molecules and free OH -1 groups of the brucite layers, and a weak band at 1650 cm corresponding to δO-H of -1 water molecules (Fig. 7A, B). Strong peaks between 2920 cm -1 and 2850 cm correspond to C-H stretching modes (Bruna et al., 2008)]. The most characteristic −1 bands of HT–DDS (Fig. 7A), are those due to the antisymmetric (1211cm ) and 119 Capítulo IV −1 symmetric (1071 cm ) stretching S=O vibration of the sulfate group (Clearfield et al., 1991; Bellamy, 1975). FT-IR of HT-TDD (Fig. 7B) shows characteristics bands at 1561 -1 -1 cm and 1421 cm corresponding to vibrations of carboxylate groups (antisymmetrical and symmetrical, respectively). Fig. 7. FT-IR spectra of (A) HT-DDS, (B) HT-TDD, (C) SMetolachlor, (D) adsorption product HT-DDS-S-Metolachlor and (E) adsorption product HT-TDD-S-Metolachlor. -1 The presence of the pesticide bands (1760 cm attributed to νC=O of the carbonyl -1 group, and 1113 cm corresponding to νC-O of the ether group and/or δC-H associated to the benzene ring) in the FT-IR spectra of the adsorption products in the “plateau” of the isotherms (Figs. 7D, E) revealed the adsorption of S-Metolachlor in both organohydrotalcites. These results are similar to those observed by Bruna et al., (2006) for the adsorption of Metamitron on an organohydrotalcite with dodecylsulfate in the interlayer. 120 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions 4. Conclusions Adsorption-desorption of S-Metolachlor pesticide by two organohydrotalcites (HT-DDS and HT-TDD) have been characterized. The organohydrotalcites presented an expansion of the interlayer space compared to the inorganic hydrotalcite with carbonate anion. X-ray diffraction patterns suggested a vertical monolayer arrangement of dodecylsulfate anions and a tilt position of the tetradecanedioate anions in the interlayer space. Adsorption of S-Metolachlor resulted in changes in the d00l-value for both organohydrotalcites. In particular, the adsorption of S-Metolachlor on HT-TDD led to a very important increase in the basal spacing, suggesting changes in the orientation of the tetradecanedioate anion from tilt to perpendicular position. The amount of adsorbed pesticide increased with the rise of the temperature from 10 to 30 °C, especially for HT-TDD. A new experimental procedure has been used to distinguish that the reversibility of S-Metolachlor adsorption is higher for HT-DDS than for HT-TDD. Only S-Metolachlor adsorbed on HT-DDS was possible to be removed with ethanol. Therefore, the study of recyclability was only performed on hydrotalcite HT-DDS using ethanol as solvent. The results indicated that the possibility of regeneration of the sorbent with ethanol was limited to one regeneration step, so that further experiments would be needed to fìnd a more effective way for effective regeneration of the sorbent. Acknowledgement This work has been partially supported by grants AGL2008-04031, AGL201123779 and CTM2011-25325 from MICINN, cofinanced with FEDER funds, and Research Groups FQM-214 and AGR-264 of Junta de Andalucía. R. Otero also acknowledges financial support from MEC-ERDF. We appreciate the technical assistance received in the SCAI (Universidad de Córdoba) from Electron Microscopy and Elemental Analysis Units. 121 Capítulo IV References Barriuso, E., Laird, D.A., Koskinen, W.C., Dowdy, R.H., 1994. Atrazine desorption from smectites. Soil Sci. Soc. Am. J. 58,1632-1638. Bellamy, J.L., 1975. The Infrared Spectra of Complex Molecules, third ed. Chapman and Hall, New York. Bowman, B.T., 1979. Method of repeated additions for generating pesticide adsorption-desorption isotherm data. Can. J. Soil Sci. 59, 435-437. Bowman, B.T., and Sans, W.W., 1985. Effect of temperature on the water solubility of insecticides. J. Environ. Sci. Health. B20, 625-631. Bruna, F., Pavlovic, I., Barriga, C., Cornejo, J., Ulibarri, M.A., 2006. Adsorption of pesticides Carbetamide and Metamitron on organohydrotalcite. Appl. Clay Sci. 33, 116–124. Bruna, F., Pavlovic, I., Celis, R., Barriga, C., Cornejo, J., Ulibarri, M.A., 2008. Organohydrotalcites as novel supports for the slow release of the pesticide terbuthylazine. Appl. Clay Sci. 42, 194-200. Bruna, F., Celis, R., Pavlovic, I., Barriga, C., Cornejo, J., Ulibarri, M.A., 2009. Layered double hydroxides as adsorbents and carriers of the herbicide (4-chloro-2methylphenoxy)acetic acid (MCPA): systems Mg–Al, Mg–Fe and Mg–Al–Fe, J. Hazard. Mater.168, 1476-1481. Cavani, F., Trifiro, F., Vaccari, A., 1991. Hydrotalcite-type anionic clays: preparation, properties and applications. Catal. Today 11, 173–30. Celis, R., Koskinen, W.C., Cecchi, A.M., Bresnahan, G.A., Carrisoza, M.J., Ulibarri, M.A., Pavlovic I., and Hermosín M.C., 1999. Sorption of the ionizable pesticide imazamox by organo-clays and organohydrotalcites. J. Environ. Sci. Health. B34, 929-941. Celis, R., Koskinen, W.C., Hermosín, M.C., Ulibarri, M.A., Cornejo, J., 2000. Tridimefon intercalations with organoclays and organohydrotalcites. Soil Sci. Soc. Am. J. 64, 36–43. Celis, R., Hermosín, M.C., Cornejo, J., Carrizosa, M.J., 2002. Clay-pesticide complexes to retard picloram leaching in soil. Int. J. Environ. Anal. Chem. 82, 503-517. 122 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions Chaara, D., Bruna, F., Ulibarri, M.A., Draoui, K., Barriga, C., Pavlovic, I., 2011. Organo/layered double hydroxide nanohybrids used to remove non ionic pesticides. J. Hazard. Mater. 196, 350– 359 Clearfield, A., M. Kieke, J. Kwan, Colon, J.L., Wang R.C., 1991. Intercalation of dodecylsulfate into layered double hydroxides. J. Incl. Phenom. Mol. Recognit. Chem. 11, 361-378. Costa, F.R., Leuteritz, A., Wagenknecht, U., Jehnichen, D., Haeussler, L., Heinrich, G., 2008. Intercalation of Mg–Al layered double hydroxide by anionic surfactants: Preparation and characterization, Appl. Clay Sci. 38, 153–164. Costantino, U., Nocchetti M., Sisani M., Vivani R., 2009, Recent progress in the synthesis and application of organically modified hydrotalcites, Z. Kristallogr. 224, 273 - 281 Cruz-Guzmán, M., Celis, R., Hermosín, M.C., Cornejo, J., 2004. Adsorption of the pesticide simazine by montmorillonite modified with natural organic cations. Environ. Sci. Technol. 38, 180-186. Frost, R., Zhou, Q., He, H., Xi, Y., 2008. An infrared study of adsorption of paranitrophenol on mono-, di- and tri-alkyl surfactant intercalated organoclays, Spectrochim. Acta Part A 69, 239–344. Gerstl, Z., Nasser, A., Mingelgrin, U., 1998. Controlled release of pesticides into soils from clay-polimer formulations. J. Agr. Food. Chem. 46 [9], 3797-3802. Giles, C.H., MacEwans, T.H., Nakhwa, N. and Smith.Studies, D., 1960. Part XI.* A System of Classification of Solution Adsorption Isotherms, and its Use in Diagnosis of Adsorption Mechanisms and in Measurement of Specific Surface Areas of Solids. J. Chem. Soc. 3973-3993. Hermosín, M.C., Pavlovic, I., Ulibarri, M.A., Cornejo, J., 1993. Trichlorophenol adsorption on layered double hydroxide: a potential sorbent. J. Environ. Sci. Health A28 (9), 1875–1888. Islam, M., Patel, R.K., 2007. Evaluation of removal efficiency of fluoride from aqueous solution using quick lime, J. Hazard. Mater.143, 303–310. Islam, M., Patel, R.,2011. Physicochemical characterization and adsorption behavior of Ca/Al chloride hydrotalcite-like compound towards removal of nitrate. J. Hazard. Mater. 190, 659-668 123 Capítulo IV Koskinen, W.C., O’Connor G.A., and Cheng, H.H., 1979. Characterization of hysteresis in the desorption of 2,4,5-T from soils. Soil Sci. Soc. Am. J., 43, 871-874. Laha, S., Tansel, B., Ussawarujikulchai, A., 2009. Surfactant-soil interactions during surfactant-amended remediation of contaminated soils by hydrophobic organic compounds: a review. J. Environ. Manage. 90, 95–100. Li, Y., Gaoa, B., Wub, T., Wanga, B., Li, X., 2009. Adsorption properties of aluminum magnesium mixed hydroxide for the model anionic dye Reactive Brilliant Red K2BP. J. Hazard. Mater. 164, 1098–1104. Liu, Y., Xu, Z., Wu, X., Gui, W., Zhu, Z., 2010. Adsorption and desorption behavior of pesticide diuron on various Chinese cultivated soils. J. Hazard. Mater. 178, 462-468. Meyn, M., Beneke, K., Lagaly, G., 1990. Anion exchange reaction of layered double hydroxides. Inorg.Chem. 29, 5201–5207. Milonjic, S.K., 2007.A consideration of the correct calculation of thermodynamic parameters of adsorption, J. Serb. Chem. Soc., 72, 136-137. Modabber Ahmed, K., Choong-Lyeal, C., Dong-Hoon, L., Man, P., L. Bu-Kug, L. JongYoon, Jyung–Choi, 2007. Synthesis and properties of mecoprop-intercalated layered double hydroxide. J. Phys. Chem. Solids. 68, 1591-1597. Özcan, A., Safa Özcan. A., 2005. Adsorption of Acid Red 57 from aqueous solutions onto surfactant-modified sepiolite. J. Hazard. Mater. B125, 252–259. Pavlovic, I., Ulibarri,M.A., Hermosín,M.C., Cornejo, J., 1996. Pesticide adsorption on organo-hydrotalcites. In: Ortega-Huertas, M., López-Galindo, A., Palomo-Delgado, I. (Eds.), Advances in Clay Minerals. Universidad de Granada, Granada, pp. 42–44. Pavlovic, I., Ulibarri, M.C., Hermosín, M.C., Cornejo, J., 1997. Sorption of an anionic surfactant from water by a calcined hydrotalcite-like sorbent. Fresenius Environ. Bull. 6, 266-271. Pavlovic, I., Barriga, C., Hermosín, M.C., Cornejo, J., Ulibarri. M.A., 2005. Adsorption of acidic pesticides 2,4-D, Clopyralid and Picloram on calcined hydrotalcite. Appl. Clay. Sci. 30, 125-133. Reichle, W.T., 1986. Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite) Solid State Ionics, 22, 135-141. Rives, V., 2001. Layered Double Hydroxides: Present and Future. Nova Science Publishers, Inc., New York. 124 Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions Shukla, G., Kumar, A., Bhanti, M., Joseph, P.E., Taneja, A., 2006. Organochlorine pesticide contamination of ground water in the city of Hyderabad. Environ. Int. 32, 244-247. Villa, M.V., Sánchez-Martín, M.J., Sánchez-Camazano, M., 1999. Hydrotalcites and organohydrotalcites as sorbents for removing pesticides from water. J. Environ. Sci. Health, Part B, 34, 509–525. Wang, B., Zhang, H., Evans, D., Duan, X., 2005. Surface modification of layered double hydroxides and incorporation of hydrophobic organic compounds. Mater. Chem. Phys. 92, 190–196. Yang, M., 1974. Processes of adsorption, desorption, degradation, volatilization and movement of O,O-diethyl O-p-nitrophenol phosphorothioate (parathion) in soils. Ph.D. Thesis. Univ. of California, Davis Zhao, H.T., Vance, G.F., 1997. Intercalation of carboxymethyl–cyclodextrin into magnesium aluminium layered double hydroxides. J. Chem. Soc. Dalton Trans. 11, 1961–1965. Zhou Q. Xi, Y., He, H., Frost, R.L., 2008. Application of near infrared spectroscopy for the determination of adsorbed p-nitrophenol on HDTMA organoclay-implications for the removal of organic pollutants from water. Spectrochim. Acta Part A 69, 835–841. 125 CAPÍTULO V. PESTICIDES ADSORPTION-DESORPTION ON Mg-Al MIXED OXIDES. KINETIC MODELING, COMPETING FACTORS AND RECYCLABILITY. Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability GRAPHICAL ABSTRACT Abstract The adsorption of the ionizable pesticide Nicosulfuron and the ionic pesticide Mecoprop-P on a calcined Mg–Al hydrotalcite (HT-500) in the presence and absence of various anions was studied with a view to assessing the potential of regenerating the adsorbent. Kinetic tests showed the adsorption of both pesticides to involve intraparticle diffusion, and the rate constant of intraparticle diffusion for Nicosulfuron to increase with increasing concentration of the compound by effect of its increased molecular size relative to Mecoprop-P. Application of the Langmuir and Dubini– Radushkevich equations to the adsorption isotherms for the two pesticides revealed easy adsorption of both via an ion-exchange process. Adsorption and desorption of the two pesticides were both affected by the presence of other anions. The adsorbents and their adsorption products were characterized by X-ray diffraction and FT-IR spectroscopies. Based on the results of regeneration tests, calcined hydrotalcite is an effective adsorbent for the removal of Nicosulfuron and Mecoprop-P from contaminated water. Keywords: Nicosulfuron, Mecoprop-P, adsorption, hydrotalcite, water treatment. Introduction 129 Capítulo V Pesticides are very hazardous pollutants that can persist in the aquatic environment for many years [1]. Contamination of soil and ground water by pesticides applied to soil and swept by transport processes such as leaching or runoff is a posing an increasingly serious environmental problem. Some researchers have suggested that the use of layered double hydroxides (LDHs), also known as “anionic clays” or “hydrotalcite-like compounds” (HTs), as filters for pesticide-contaminated water or additives in controlled-release pesticide formulations might be effective toward partially or completely avoiding the environmental impact of pesticides [2-4]. LDHs are brucite-like layered materials of II n− x+ III II III general formula [M1−x Mx (OH)2] Xx/n ·mH2O, where M and M are a divalent and a n− trivalent cation, respectively, and X is an interlayer anion countering the positive III charge arising from the presence of M in the layers. Layer charge in an LDH is dictated III III II by the mole ratio M /(M + M ), which typically ranges from 0.2 to 0.4 [5,6]. The main interest of these materials lies in their structure, high anion-exchange capacity and straightforward synthesis [3,7-9]. The adsorption efficiency of LDHs is strongly affected by the properties of their interlayer anions [10]. Thus, LDHs usually have a greater affinity for anions with a high charge density and hence tend to adsorb multivalent anions easily in relation to 2– monovalent ions [11,12]. In addition, LDHs exhibit preferential affinity for CO3 , which 2– usually hinders further ion-exchange [13]. However, interlayer CO3 ions can be removed by heating LDH at about 500 ºC to obtained calcined LDH (HT500), which can regain the original LDH structure in the presence of dissolved anions [14–16]: n– n- Mg3AlO4(OH) + 4 H2O + (1/n) X → [Mg3Al(OH)8]X1/n + OH – n- where X is the anionic species present in solution. The anion-exchange capacity (AEC) of HT500 was much higher than that of –1 the original LDH (5.8 vs 3.3 mmol·g ) [17]; this allows the adsorption efficiency of an LDH to be significantly increased by calcination [16,18]. The influence of various common inorganic anions on pesticide adsorption is one other major factor to be considered since some such anions are often encountered in pesticide-contaminated waters [10]. 130 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability Several author have studied different properties of Mecoprop and Nicosulfuron pesticides such as their degradation or its effect in the grown of different vegetables. It had been studied adsorption and desorption of Nicosulfuron in soils [1920] and clay minerals [21]. Furthermore, it had been studied the adsorption of Mecoprop on calcareous and organic matter amended soils [22], in clays [23] or onto Iron Oxides [24]. Also, Khan et al [25] have characterized the Mecoprop-LDH hybrid synthetized by the cooprecipitation method. In this work, we examined the adsorption of two widely used, ionizable pesticides (Nicosulfuron and Mecoprop-P) on a calcined hydrotalcite (HT500) with a view to elucidating their adsorption–desorption behavior and the way it is influenced by the presence of some anions. In addition, we subjected the hydrotalcite to repeated adsorption–calcination cycles in order to assess its reusability in the removal of Nicosulfuron and Mecoprop-P from contaminated water. 2. Materials and Methods 2.1 Pesticides Nicosulfuron (2-[(4,6-dimethoxypyrimidin-2-ylcarbamoyl)sulfamoyl]-N,N- dimethylnicotinamide) and Mecoprop-P [(R)-2-(4-chloro-o-tolyloxy)propionic acid] were both supplied as high-purity products by Sigma–Aldrich. These pesticides are typically used to control weeds in post-emergence treatments. Nicosulfuron is a sulfonylurea of molecular weight 410.41 g·mol–1, a high solubility in water (7500 mg·L–1 at 20 °C) and pKa = 4.78. Mecoprop-P is an aryloxyalkanoic acid pesticide of molecular –1 weight 214.65 g·mol , water solubility 860 mg·L formula of these pesticides is as follows: 131 –1 and pKa = 3.86. The structural Capítulo V NICOSULFURON H3C O N H3C O O O O S O N N H CH3 N N H CH3 N MECOPROP-P CH3 Cl O CO2H CH3 2.2. Synthesis of the hydrotalcite, and characterization of the adsorbent and adsorption products A hydrotalcite intercalated with carbonate anions and designated MgAl-LDH was prepared according to Reichle [7]. To this end, a solution containing 0.75 mol of Mg(NO3)2·6H2O and 0.25 mol of Al(NO3)3·9H2O in 250 mL of distilled water was dropped over 500 mL of another containing 1.7 mol of NaOH and 0.5 mol of Na2CO3 under vigorous stirring for 2 h. The resulting suspension was hydrothermally treated at 80 °C for 24 h, and the precipitate thus formed washed with distilled water and dried at 60 °C to obtain the solid MgAl-LDH, calcination of which at 500 °C for 3 h gave an Mg–Al mixed oxide that was named HT500. The adsorbent (HT500) and adsorption products were characterized from their XRD patterns as obtained on a Siemens D-5000 diffractometer using CuKα radiation (λ = 1.54050 Å) and their FT-IR spectra as recorded by using the KBr disc technique on a Perkin Elmer spectrophotometer. The materials were also characterized microstructurally with a JEOL 200CX TEM instrument. Elemental analyses (Mg and Al) were performed by atomic absorption spectrometry on a Perkin Elmer AA-3100 instrument, and C and N contents determined with a Eurovector 3A 2010 elemental analyzer. 2.3. Adsorption–desorption tests 132 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability Kinetic adsorption and adsorption–desorption isotherms for Nicosulfuron and Mecoprop-P were obtained by using the batch equilibration method. For kinetic adsorption, 30 mg of adsorbent for Nicosulfuron and 20 mg for Mecoprop-P were equilibrated in duplicate by shaking with 30 mL of pesticide solutions containing 0.25– 1.0 mM Nicosulfuron and 1.0–3.0 mM Mecoprop-P at room temperature in a turnover shaker operating at 52 rpm for variable lengths of time. For adsorptiondesorption isotherms, duplicate samples containing 30 mg of adsorbent for Nicosulfuron and 20 mg for Mecoprop-P were equilibrated by shaking with 30 mL of 0.5–1.5 mM Nicosulfuron and 0.5–3.0 mM Mecoprop-P at room temperature for 24 h [26]. The resulting supernatant was centrifuged and separated to determine the concentrations of Nicosulfuron and Mecoprop-P by UV–Vis spectroscopy at 239 and 285 nm, respectively, on a Perkin Elmer Lambda 11 UV–Vis spectrophotometer. Desorption was measured at each equilibrium point in the adsorption curve by replacing a small portion of supernatant (15 mL for Nicosulfuron and 7.5 mL for Mecoprop-P) with an identical volume of distilled water. After shaking for 24 h, the amount of pesticide desorbed was calculated as described above. This procedure is described in detail elsewhere [27]. Nicosulfuron and Mecoprop-P adsorption– desorption isotherms were fitted to the Langmuir [28], Freundlich [29] and Dubinin– Radushkevich (D–R) [30] equations: 𝐶𝑒 𝐶𝑠 = 1 𝐶𝑚 ·𝐿 𝐶𝑒 (1) 1/𝑛𝑓 (2) + 𝐶𝑚 𝐶𝑠 = 𝐾𝑓 · 𝐶𝑒 –1 𝑙𝑛𝐶𝑠 = 𝑙𝑛𝑞𝑚 − 𝐾𝜀 2 (3) where Cs (mmol·g ) is the amount of pesticide adsorbed per unit mass of adsorbent at –1 the equilibrium concentration Ce (mmol·L ); Cm is the adsorbent monolayer capacity -1 –1 (mmol·g ); L is the Langmuir adsorption constant (L·mmol ), which is related to the adsorption energy; Kf (mmol 1–nf nf –1 L g ) and nf are the Freundlich parameters incorporating all factors affecting adsorption; qm is the theoretical adsorption capacity 133 Capítulo V –1 2 –2 of the adsorbent (mmol·g ); K (mmol ·kJ ) is a constant related to the adsorption energy; and ε, which is equal to RT·ln [1 + (1/Ce)], is the Polanyi adsorption potential, R being the universal gas constant and T (K) the temperature. 2.4. Effects of competing anions – – 2– 2– The effects of competing anions (NO3 , Cl , SO4 , CO3 2– and HPO4 ) on the adsorption behavior of Nicosulfuron or Mecoprop-P were examined by mixing variable volumes of 0.5 M solutions of the sodium salts of the anions with 50 mL of a 1 mM pesticide solution in a total volume of 100 mL. The anion/pesticide mole ratio ranged from 0 to 100. Duplicate volumes of 30 mL of these solutions were added to 30 mg of adsorbent. For desorption tests, duplicate volumes of 30 mL of the anion solutions were added to 30 mg of adsorbent containing Nicosulfuron or Mecoprop-P in proportions such that the resulting anion/adsorbed pesticide mole ratio ranged from 0 to 100. The solutions were shaken at 52 rpm for 24 h, and the pesticide concentrations in the supernatant then determined by UV–Vis spectroscopy. 2.5. Adsorbent regeneration and recyclability Duplicate amounts of 30 mg of adsorbent for Nicosulfuron and 20 mg for Mecoprop-P were equilibrated by shaking in 30 mL of solutions containing an initial pesticide concentration of 0.5 mM at room temperature for 24 h. Then, the supernatant was centrifuged and separated to determine the concentration of Nicosulfuron and Mecoprop-P by UV–Vis spectroscopy at 239 and 285 nm, respectively, the LDH–pesticide complex being calcined at 500°C for 2 h to recover the adsorbent: HT500. The adsorbent was subjected to a total of four successive adsorption–calcination cycles. 3. Results and discussion 3.1. Adsorption–desorption tests 3.1.1. Effect of pH and adsorption kinetics The influence of pH on Nicosulfuron and Mecoprop-P adsorption on HT500 was studied at an initial value of 5, 7 or 9. Based on the results, the adsorption of 134 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability Nicosulfuron was not influenced by pH and that of Mecoprop-P only slightly. This led us to adopt pH 5 (i.e. that of the pesticide solution) for subsequent adsorption tests on Nicosulfuron and pH 7 (i.e. that of maximum adsorption) for those on Mecoprop-P. Table 1. Rate constants (Kad) obtained from adsorption curve for HT500 with different initial concentrations of pesticide Pesticide Initial concentration -1 2 Slope Intercept Kad (h ) R 0.25 -0.139 2.396 0.320 0.982 0.50 -0.130 2.384 0.300 0.972 1.00 -0.137 2.722 0.317 0.992 1.00 -0.125 3.015 0.288 0.990 2.00 -0.121 2.930 0.278 0.998 3.00 -0.124 2.968 0.286 0.995 (mmol/L) Nicosulfuron Mecoprop-P Fig. 1. Adsorption kinetics of 0.5mM Nicosulfuron (A) and 1mM Mecoprop-P (B) on HT500 135 Capítulo V Adsorption of Nicosulfuron and Mecoprop-P was fast at the start and slowed down as equilibrium was approached. The kinetics of adsorption of the pesticides on HT500 was examined via the kinetic order and an intraparticle diffusion kinetic model based on the following equations was used to fit the experimental results. The adsorption kinetics of the pesticides was elucidated by substituting the initial concentrations of Fig.1 into the pseudo first-order rate equation of Largergren [31,32]: log (Cse − Cs) = log Cse − K ad t (4) 2.303 where Cse is the maximum amount of pesticide adsorbed and Cs that adsorbed at time –1 t, both in mmol·g . Plots of log (Cse – Cs) versus t spanning different time intervals were all almost linear —the fit being much better for Mecoprop-P, however—, which testifies to the applicability of the Largergren first-order rate equation here. Table 1 shows the adsorption rate constant, Kad, for the two pesticides as calculated from the slope of each plot. Adsorption measurements of the pesticides at different initial concentrations made after a variable contact time were used to calculate their adsorption rate constant, Kad, and kinetic order. In all cases, more than 50% of the adsorbed pesticide amount occurs within 5 h., and the equilibrium was reached within 24 h in both pesticides (see Fig. 1). Both exhibited increasing adsorption with increase in their initial concentration. The mechanism behind the adsorption of the two pesticides was elucidated by analyzing the kinetic results in the light of the Morris–Weber equation for intraparticle diffusion [32-34]: 𝐶𝑠 = 𝐾𝑝 𝑡 1/2 + 𝐶 ( 5) –1 where Cs is the amount of pesticide adsorbed at time t (mmol·g ), Kp the intraparticle –1 diffusion rate constant (mmol·g ·h –1/2 ), t the agitation time (h) and C the intercept. 136 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability Fig. 2. Adsorption and desorption isotherms for Nicosulfuron (A) and Mecoprop-P (B) on calcined hydrotalcite. At least two different types of adsorption mechanisms were to be expected from the structural characteristics of the adsorbent and the use of stirring. One was transfer of the pesticides from the bulk solution into HT500 pores and the other adsorption at the outer surface of the hydrotalcite. The adsorption rate-determining step could therefore be either adsorption or intraparticle diffusion [35]. Based on the applied model, a plot of the amount of pesticide adsorbed, Cs, against the square root 1/2 of time, t , should be linear if intraparticle diffusion was involved in the process; also, if the lines passed through the origin, then intraparticle diffusion would be the rate2 determining step [36,37]. The correlation coefficients R of Table 2, obtained at Cs values spanning up to 16 h, suggest that the adsorption of Nicosulfuron and 137 Capítulo V Mecoprop-P on calcined hydrotalcite (HT500) may involve intraparticle diffusion; however, none of the curves passed through the origin, so intraparticle diffusion cannot have been the rate-determining step and other factors may have operated simultaneously to govern the adsorption process. Increasing the contact time to 24 h led to poor correlation coefficients for the intraparticle diffusion model. Therefore, adsorption of Nicosulfuron and Mecoprop-P on calcined hydrotalcite (HT500) may be followed by intraparticle diffusion for up to 16 h. Fig. 3.D-R adsorption isotherms for Nicosulfuron and Mecoprop-P on HT-500 Table 3. Freundlich and Langmuir model parameters for Nicosulfuron and Mecoprop-P adsorption onto HT500 138 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability Kp was calculated from the slope of a plot of Cs versus t 1/2 for each pesticide (see Table 2). Whereas the values for Mecoprop-P were all very similar, those for Nicosulfuron increased with increasing pesticide concentration. Removing nitrate with a Ca/Al chloride hydrotalcite [34] was previously found to result in a similar increase in Kp with increasing concentration. The dissimilar behavior of Nicosulfuron and Mecoprop-P in this respect can be ascribed to the greater steric hindrance in the former —a result of its being a bulkier molecule. 3.1.2. Adsorption–desorption isotherms Fig. 2 shows the adsorption–desorption isotherms for Nicosulfuron and Mecoprop-P adsorbed on HT500. Adsorption isotherms are useful to determine the adsorption capabilities of adsorbents, thermodynamic parameters such as the energy of adsorption and the nature of host–guest interactions. Both isotherms in Fig. 2 are of the L-type in the classification of Giles [38], therefore HT500 has a high affinity for the two pesticides. An L-shaped isotherm suggests that, as substrate sites are filled, it becomes increasingly difficult for a solute molecule to find a vacant site. Based on the adsorption data, the uptake of pesticide per gram of adsorbent, Cs, was much greater –1 –1 for Mecoprop-P (1.21 mmol·g ) than it was for Nicosulfuron (0.64 mmol·g ), probably as a result of structural, size and ionic differences between the two pesticides. Fig. 4.Adsorption of 0.5mM Nicosulfuron (A) and Mecoprop-P (B) by HT500 as a function of the anion/pesticide mole ratio 139 Capítulo V Fig. 5. Release of Nicosulfuron (A) or Mecoprop-P (B) adsorbed on HT500 by various anions at variable anion/adsorbed pesticide ratios (0.36 and 0.47 mmol g-1 Nicosulfuron or Mecoprop-P adsorbed per gram of HT500). The Freundlich and Langmuir equations were used to model the adsorption isotherms in order to identify the specific factors governing adsorption of the two pesticides. Based on the R2 values of Table 3, the Langmuir equation fitted the experimental data much better than the Freundlich equation (see inset of Fig.2). For this reason, the adsorption isotherms are discussed in terms of the Langmuir equation [Eq. 1] here. The adsorption efficiency was assessed via a dimensionless separation factor R [39,40] that was calculated from: 𝑅= 1 1+𝐿 𝐶𝑖 (6) where Ci is the initial pesticide concentration. R values from 0 to 1 denote favorable adsorption. The R values obtained at Nicosulfuron initial concentrations of 0.25, 0.5, 0.75, 1 and 1.5 mmol·L –1 –2 fell in the range (31.0–7.02)·10 ; likewise, those for 140 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability –1 Mecoprop-P at an initial concentration of 0.5, 1, 1.5 2 or 3 mmol·L spanned the range –2 (20.0–7.69)·10 . These values are suggestive of favorable adsorption, especially at low adsorbate concentrations. Fig. 6. Recyclability of HT500 containing adsorbed Nicosulfuron (A) and Mecoprop-P (B) We used the Dubinin–Radushkevich (D–R) equation [Eq. 3] to discriminate between physical and chemical adsorption. Parameters K and qm were calculated from 2 the slope and intercept, respectively, of a plot of ln Cs against ε (Fig. 3). The qm values thus obtained (Table 3) were consistent with the increased adsorption of Mecoprop-P relative Nicosulfuron noted earlier (Fig. 2). Constant K provides a measure of the mean free energy of adsorption, E, which can be calculated from [39,41] –1/2 E = (2K) (7) The mean free energy provides information about adsorption mechanisms. Thus, adsorption is physical when 0 < E < 8 kJ/mol, due to ion-exchange if 8 < E < 16 –1 –1 kJ·mol and chemical if 20 < E < 40 kJ/mol. Our values (11.72 kJ·mol for Nicosulfuron and 11.08 kJ·mol –1 for Mecoprop-P) suggest that, as confirmed by the IR spectra for the adsorption products, the two pesticides were adsorbed by ion-exchange. 141 Capítulo V Fig. 7. TEM images of HT500 before cycling (a) and after the 3rd cycle (b) Finally, desorption curves closely matched the adsorption curves for both pesticides (Fig. 2), thus testifying to the high reversibility of the adsorption–desorption process. 3.1.3. Effects of competing anions on Nicosulfuron and Mecoprop-P adsorption The effect of competing anions on the adsorption of Nicosulfuron and Mecoprop-P on HT500 is illustrated in Fig. 4. With an initial concentration of 0.5 mM of both pesticides, HT500 adsorbed 0.36 mmol·g–1 Nicosulfuron and 0.47 mmol·g–1 Mecoprop-P in the absence of competing anions, but smaller amounts in their presence. The effect was more marked on Mecoprop-P than on Nicosulfuron. The 2– 2– adverse effect on Nicosulfuron adsorption decreased in the sequence HPO4 > CO3 ≈ – – 2– 2– 2– – NO3 ≈ Cl > SO4 and that on Mecoprop-P in the sequence HPO4 > CO3 ≈ NO3 ≈ Cl 142 – Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability 2– 2– ≈ SO4 both with anion/pesticide ratios higher than 20. Therefore, HPO4 influenced 2– the adsorption of both pesticides more markedly than did CO3 2– and SO4 . This is 2– 2– consistent with the sequences proposed by Forano et al. [6] (HPO4 > CrO4 > SO4 2– 2– – – – – – 2– – and CO3 > SO4 > OH > F > Cl > Br > NO3 > I ) from previous results of Miyata [11] 2– and Yamaoka et al. [42]. However, You et al. [10] found SO4 to affect the adsorption 2– 2– of dicamba on a calcined hydrotalcite more markedly than did HPO4 or CO3 . 2– Finally, it is worth noting the little influence of SO4 on Nicosulfuron relative 2– to other anions, and also the —surprisingly— extremely low interference of CO3 on Mecoprop-P adsorption at an anion/pesticide ratio of 10 (see Fig. 4). 3.1.4. Retention of the pesticides in the presence of various anions In order to assess the stability of the host–guest complex, we measured the retention of the pesticides by the adsorbent in the presence of various anions. Desorption isotherms obtained by adding distilled water (Fig. 2) exhibited a high reversibility. The influence of different anions on the release of Nicosulfuron and Mecoprop-P previously adsorbed on HT500 is illustrated in Fig. 5. Based on the results, both pesticides were desorbed in the presence of the studied anions, albeit to different extent. The influence of increasing concentrations of dissolved anions was also studied. Goswamee et al. [16] found anions stereochemically suitable for inclusion into LDH interlayers to boost the release of existing interlayer anions. Our results suggest that Nicosulfuron and Mecoprop-P adsorbed on HT500 can be desorbed, whether 2– 2– – – 2– partially or fully, into solutions containing HPO4 , CO3 , NO3 , Cl or SO4 ; therefore, the nature of the ionic species present in solution affects the desorption behavior of the pesticides. Also, increasing the anion/adsorbed pesticide mole ratio increased the amount of pesticide removed. With Mecoprop-P, a ratio of 100/1 resulted in nearly quantitative removal of the host–guest complex (viz. 100%, 100%, 98%, 93% and 87% 2– – – 2– for CO3 , NO3 , Cl , SO4 2– and HPO4 , respectively). Removal of the host–guest complex for Nicosulfuron was less efficient and never complete, however; thus, it 2– peaked at only 46% with CO3 . The effect of the anions was different when they competed for adsorption on HT500 (Fig. 4) and when they acted by removing the pesticides from the host–guest 143 Capítulo V complex (Fig. 5). These results testify to the ability of the hydrotalcite to retain Nicosulfuron and Mecoprop-P, and to remove the two pesticides from contaminated 2– – water in the presence of other anions. The high capacity of CO3 and NO3 to recover Mecoprop-P suggests that the adsorbent can be reused. 3.1.5. Adsorbent recyclability In order to assess its recyclability, the adsorbent was subjected to four adsorption–desorption cycles on pristine HT500. As can be seen from Fig. 6, adsorption of the two pesticides decreased after each cycle. A similar trend was previously observed by Bruna et al. [43] in successive adsorption–desorption cycles of carbetamide and metamitron on organohydrotalcites. Our results can be ascribed to erosion of the adsorbent particles as seen in the TEM images after the third adsorption cycle (Fig. 7b) relative to the pristine HT500 (Fig. 7a). Fig. 8. XRD patterns for (a) HT500, (b) HT500-Nicosulfuron and (c)HT500-Mecoprop-P. (*Diffraction peaks due to the Al sample holder) 3.2. Characterization of adsorption products by X-ray diffraction and FT-IR spectroscopies Fig. 8 shows the diffraction patterns for the adsorbent (HT500) and adsorption products, namely: HT500-Nicosulfuron (0.03 g of HT500 and 30 mL of 0.5 144 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability mM Nicosulfuron) and HT500-Mecoprop-P (0.02 g of HT500 and 30 mL of 1 mM Mecoprop-P). The patterns of Fig. 8a correspond to the hydrotalcite upon calcination at 500 °C and exhibit the typical peaks for an Al–Mg mixed oxide [44]. Once Nicosulfuron and Mecoprop-P were adsorbed (Figs 8b and 8c, respectively), the XRD patterns were consistent with regeneration of the hydrotalcite structure; however, d00l exhibited no change with respect to the hydrotalcite with carbonate as interlayer anion (d003 = 7.8 Å). By contrast, Khan et al. [25] observed an increase in interlayer spacing (d003 = 22.54 Å) in a Mecoprop-P intercalated LDH prepared from a very high concentration of Mecoprop dissolved in 50% methanol —where the pesticide is much more readily soluble. Under our experimental conditions, the greatest pesticide adsorption amounted to only 20% of the AEC of HT500 for Mecoprop-P and 10% for Nicosulfuron; the pesticides probably adopt a flat configuration in the interlayer, on the outer surface and at the edges of hydrotalcite particles. Based on the d003 values derived from the X-ray diffraction patterns and FT-IR data (Fig. 9), most interlayer sites 2– - must have been occupied by CO3 or OH ions. Similar results were previously obtained with other pesticides [45]. Adsorption of Nicosulfuron and Mecoprop-P on the LDH was further confirmed by the FT-IR spectra of Fig. 9. The spectrum for the adsorbent, HT500, –1 exhibited a broad band at ∼3500 cm corresponding to vibrations of residual OH – –1 groups in the layers in addition to the bending vibration of water at 1638 cm and a –1 band at 1383 cm that was assigned to stretching vibrations in residual or surface carbonate ions [46]. The spectrum for the product corresponding to the last point in the adsorption isotherm, HT500-Mecoprop-P, exhibited the breathing bands for the –1 benzene ring at 1496 and 1243 cm , thus confirming the presence of the pesticide. –1 The absence of a band at 1700 cm for unionized carboxyl groups in Mecoprop-P from the spectrum for the complex, and the presence of antisymmetric and symmetric -1 carboxylate vibration peaks at 1600 and 1376 cm [47], confirmed that the pesticide was adsorbed in anionic form on HT500, as expected from the LDH regeneration mechanism established from the X-ray diffraction patterns (Fig. 8c). 145 Capítulo V Fig. 9. FT-IR spectra for (A) HT500, (B) Nisoculfuron, (C) HT-Nicosulfuron, (D) Mecoprop-P and (E) HTMecoprop-P The presence of Nicosulfuron among the adsorption products was apparent from the characteristic bands at 1492 and 1375 cm–1 for C=C bonds in aromatic rings, –1 –1 and by that at 1028 cm , due to S=O vibrations. However, the band at 1718 cm for –1 νC=O in secondary amides was absent and a new band at 1608 cm due to νC=O in tertiary amides was observed instead [47]. This suggests that the pesticide may intercalate into the LDH interlayer via the deprotonated nitrogen atom in the secondary amide. 4. Conclusions The adsorption–desorption behavior of the pesticides Nicosulfuron and Mecoprop-P pesticides on calcined hydrotalcite (HT500) was studied. Based on kinetic data, their adsorption involves intraparticle diffusion. Only with Nicosulfuron, however, did the intraparticle diffusion rate constant increase with increasing 146 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability concentration by effect of the dissimilar size of the pesticides. The adsorption isotherms for both pesticides were L-shaped, and application of the Langmuir and Dubini–Radushkevich equations revealed that adsorption was favorable and occurred by ion-exchange. The maximum amount of Mecoprop-P adsorbed was twice than of Nicosulfuron, which is consistent with the structural, size and ionization differences between the two pesticides. 2– Competing anions (particularly HPO4 ) had an adverse effect on Nicosulfuron and Mecoprop-P adsorption. On the other hand, extraction of the pesticides from the 2– 2– host–guest complex was especially effective with CO3 . The ability of CO3 to recover large amounts of Mecoprop-P suggests that the adsorbent can be recycled. Regeneration tests involving four adsorption–desorption cycles showed that HT500 can be reused several times to remove Nicosulfuron and Mecoprop-P, albeit with gradually decreasing efficiency. The fact that the X-ray diffraction patterns were nearly identical with those for hydrotalcite intercalated with carbonate ions suggests that the pesticides adopt a flat arrangement in the interlayer and, also, that they occupy outer surfaces and edges of the particles. Acknowledgments This work was partly supported by Spain’s Ministry of Science and Innovation (Projects AGL2008-04031-CO2-2 and CTM2011-25325), and cofunded by FEDER and the Andalusian Regional Government (Junta de Andalucía, Research Group FQM-214). R. Otero also acknowledges funding from MEC-ERDF. The authors wish to thank the staff at the Electron Microscopy and Elemental Analysis units of the Central Research Support Service (SCAI) of the University of Córdoba for technical assistance. References [1] G. Shukla, A. Kumar, M. Bhanti, P.E. Joseph, A. Taneja, Organochlorine pesticide contamination of ground water in the city of Hyderabad, Environ. Int. 32 (2006) 244–247. 147 Capítulo V [2] J. Cornejo, R. Celis, I. Pavlovic, M. A. Ulibarri, Interactions of pesticides with clays and layered double hydroxides: a review. Clay Minerals. 43 (2008) 155–176. [3] V. Rives, Layered Double Hydroxides: Present and Future, Nova Science Publishers, Inc., New York, 2001. [4] R. Rojas, Layered Double Hydroxides Applications as sorbents for environmental remediation in A. Calixto and D. Analiz Griego (Eds) Hydroxides: Synthesis, Types and Applications. Published by Nova Science Publishers, Inc. 2012, 39-79. [5] U. Costantino, M. Nocchetti, M. Sisani, R. Vivani, Recent progress in the synthesis and application of organically modified hydrotalcites, Z. Kristallogr. 224 (2009) 273 – 281. [6] C. Forano, T. Hibino, F. Leroux, C. Taviot-Guého, Layered double hydroxides, in: F. Bergaya, B.K.G. Theng, G. Lagaly (Eds), Handbook of Clay Science, Published by Elsevier Ltd., 2006, 1021-1095. [7] W. T. Reichle, Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite), Solid State Ionics 22 (1986) 135–141. [8] P. S. Braterman,, Z. P. Xu and F. Yarberry, Layered Double Hydroxides (LDHs) in S. M. Auerbach, K. A. Carrado, and P. K. Dutta,(Eds), Handbook of Layered Materials, Published by CRS Press, 2004, 373-474. [9] X. Duan and D.G. Evans, Layered Double Hydroxides in D.M.P. Mingos (Ed), Structure and Bonding 119, Published by Springer-Verlag Belin Heidelberg, 2006. [10] Y. You, H. Zhao, G. F. Vance, Adsorption of dicamba (3,6-dichloro-2methoxybenzoic acid) in aqueous solution by calcined-layered double hydroxide, Appl. Clay Sci. 21 (2002) 217–226. [11] S. Miyata, Anion-exchange properties of hydrotalcite-like compounds, Clays Clay Min. 31(4) (1983) 305–311. [12] P. K. Dutta, M. Puri, Anion exchange in lithium aluminate hydroxides, J. Phys. Chem. 93 (1989) 376–381. [13] T. Sato, T. Wakabayashi, M. Shimada, Adsorption of various anions by magnesium aluminum oxide (Mg0.7Al0.3O1.15). Ind. Eng. Chem. Prod. Res. Dev. 25 (1986) 89–92. [14] K. Chibwe, W. Jones, Intercalation of organic and inorganic anions into layered double hydroxides, J. Chem. Soc., Chem. Commun. 14 (1989) 926–927. 148 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability 2+ 3+ [15] V.R.L. Constantino, T.J. Pinnavaia, Basic properties of Mg1-x Alx layered double hydroxides intercalated by carbonate, hydroxide, chloride, and sulfate anions. Inorg. Chem. 34 (1995) 883–892. [16] R.L. Goswamee, P. Sengupta, K.G. Bhattacharyya, D.K. Dutta, Adsorption of Cr(VI) in layered double hydroxides. Appl. Clay Sci. 13 (1998) 21–34. [17] M.A. Ulibarri, I. Pavlovic, C. Barriga, M.C. Hermosiın, J. Cornejo, Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity, Appl. Clay Sci. 18 (2001) 17–27. [18] M.C. Hermosin, I. Pavlovic, M.A. Ulibarri, J.Cornejo, Hydrotalcites as sorbent for trinitrophenol: sorption capacity and mechanism, Water Res. 30 (1996) 171–177. [19] J. M. Gonzalez and L. Ukrainczyk , Adsorption and Desorption of Nicosulfuron in Soils. Journal of Environmental Quality 25(6) (1996) 1186-1192. [20] J. Fan; Z. Wei; Z. ZhongMing; X. Xiang; W. JinJun; Q. Zhen; L. Ke; L. LinMing. Desorption character of nicosulfuron and effect of pH on adsoroption of nicosulfuron in soils. Southwest China Journal of Agricultural Sciences 21(3) (2008) 702-708. [21] L. Ukrainczyk , N. Rashid, Irreversible Sorption of Nicosulfuron on Clay Minerals, J. Agric. Food Chem. 43(4) (1995) 855–857. [22] M. Matallo, E. Romero, F. Sánchez‐Rasero, A. Peña and G. Dios . Adsorption of mecoprop and dichlorprop on calcareous and organic matter amended soils: Comparative adsorption of racemic and pure enantiomeric forms. Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes 33(1) (1998) 51-56 [23] P. Ye, A. T. Lemley. Adsorption Effect on the Degradation of Carbaryl, Mecoprop, and Paraquat by Anodic Fenton Treatment in an SWy-2 Montmorillonite Clay Slurry. J. Agric. Food Chem. 56 (21) (2008) 10200–10207. [24] L. Clausen , I. Fabricius, Atrazine, Isoproturon, Mecoprop, 2,4-D, and Bentazone Adsorption onto Iron Oxides. Journal of Enviromental Quality 30(3) (2001) 858869 [25] M.A. Khan, C-L. Choi, D-H. Lee, M. Park, B-K. Limb, J-Y. Leeb, J. Choi, Synthesis and properties of mecoprop-intercalated layered double hydroxide, J. Phys. Chem. Solids 68 (2007) 1591–1597. 149 Capítulo V [26] M.C. Hermosin, I. Pavlovic, M.A.Ulibarri, J. Cornejo, Trichlorophenol adsorption on layered double hydroxide: a potential sorbent, J. Environ. Sci. Health A28 (9) (1993) 1875–1888. [27] R. Otero, J.M. Fernández, M.A. Ulibarri, R. Celis, F.Bruna, Adsorption of non-ionic pesticide S-Metolachlor on layered double hydroxides intercalated with dodecylsulfate and tetradecanedioate anions. Appl. Clay Sci. 65-66 (2012) 72-79. [28] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem. Soc. 40 (1918) 1361–1403. [29] H. Freundlich, Uber die adsorption In Iaosungen, Z. Phys. Chem. 57 (1906) 385– 470. [30] M.M. Dubinin, L.V. Radushkevich, Equation of the characteristic curve of activated charcoal, Proceedings of the Academy of Sciences, Physical Chemistry Section, U.S.S.R. 55 (1947) 331–333. [31]S. Lagergren, Zur theorie der sogenannten adsorption gelöster stoffe, Kungliga Svenska Vetenskapsakademiens, Handlingar 24 (4) (1898) 1–39. [32] A. Özcan, A. S. Özcan, Adsorption of Acid Red 57 from aqueous solutions onto surfactant modified sepiolite, J. Hazard. Mat. B125 (2005) 252–259. [33] W.J. Weber Jr., J.C. Morriss, Kinetics of adsorption on carbon from solution, J. Sanit. Eng. Div. Am. Soc. Civ. Eng. 89 (1963) 31–60. [34] M. Islam, R. Patel, Physicochemical characterization and adsorption behavior of Ca/Al chloride hydrotalcite-like compound towards removal of nitrate, J. Hazard. Mater. 190 (2011) 659–668 [35] M. Islam, R. Patel, Nitrate sorption by thermally activated Mg/Al chloride hydrotalcite-like Compound, J. Hazard. Mat. 169 (2009) 524–531. [36] N. Kannan, M.M. Sundaram, Kinetics and mechanism of removal of methylene blue by adsorption on various carbons — a comparative study, Dyes Pigm. 51(1) (2001) 25–40. [37] J.P. Chen, S. Wu, K.H. Chong, Surface modification of a granular activated carbon by citric acid for enhancement of copper adsorption, Carbon 41 (10) (2003) 1979– 1986. [38] C.H. Giles, A general treatment and classification of the solute adsorption isotherm. I. Theoretical. J. Colloid Interf. Sci., 47(3) (1974) 755–765. 150 Pesticide adsorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability [39] K.R. Hall, L.C. Eagleton, A. Acrivos, T. Vermeulen, Pore and solid-diffusion kinetics in fixed-bed adsorption under constant-pattern conditions, Ind. Eng. Chem. Fund. 5 (1966) 212–223. [40] X. Guo, F. Zhang, Q. Peng, S. Xu, X. Lei, D.G. Evans, X. Duan, Layered double hydroxide/eggshell membrane: An inorganic biocomposite membrane as an efficient adsorbent for Cr(VI) removal, Chem. Eng. J. 166 (2011) 81-87 [41] C. Namasivayam, R.T. Yamuna, Adsorption of direct red 12 B by biogas residual slurry: equilibrium and rate processes, Environ. Pollut. 89 (1995) 1–7. [42] T. Yamaoka, M. Abe, M. Tsuji, Synthesis of Cu–Al hydrotalcite like compound and its ion exchange property. Mat. Res. Bull. 24 (1989) 1183–1199. [43] F. Bruna, I. Pavlovic, C. Barriga, J. Cornejo, M.A. Ulibarri, Adsorption of pesticides Carbetamide and Metamitron on organohydrotalcite, Appl. Clay Sci. 33 (2006) 116–124 [44] F.M. Labajos, V. Rives, M.A. Ulibarri, Effect of hidrothermal and thermal treatments on the physicochemical properties of Mg–Al hydrotalcite like materials. J. Mat. Sci. 27 (1992) 1546–1552. [45] I. Pavlovic, C. Barriga, C. Hermosin, J. Cornejo, M.A. Ulibarri, Adsorption of acidic pesticides 2,4-D, Clopyralid and Picloram on calcined hydrotalcite. Appl. Clay Sci. 30 (2005) 125–133. [46] F. Cavani, F. Trifiro, A. Vaccari, Hydrotalcite-type anionic clays: preparation, properties and applications, Catal. Today 11 (1991) 173-301 [47] L.J. Bellamy, The Infrared Spectra of Complex Molecules, 3rd ed., Chapman and Hall, London, 151 1975 . CAPÍTULO VI. ADSORPTION OF THE HERBICIDE SMETOLACHLOR ORGANOSILICAS ON PERIODIC MESOPOROUS Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas GRAPHICAL ABSTRACT Abstract Due to their hydrophobic pore systems and high surface areas, periodic mesoporous organosilicas (PMOs) offer unique properties for use as adsorbents of organic compounds. Two organosilicas (ethane-PMO and benzene-PMO) and a silica (PMS) were synthesized following a similar procedure. The adsorption of the herbicide S-Metolachlor from an aqueous solution was then compared on the three solids. Both PMOs exhibited a higher adsorption capacity than their silica counterpart. Additionally, both PMOs showed much higher stability than the PMS, whose mesostructure was seriously damaged under aqueous conditions. Moreover, both PMOs exhibited a very different adsorption behavior. The adsorption of S-Metolachlor on each material was described by different isotherms. The amount retained depended on the pH, and was maximum at pH=2 for ethane-PMO and at pH=4 for benzene-PMO. Our results suggest that both hydrophobic and aromatic π-π interactions play an important role in the adsorption of this herbicide. The adsorption capacity of benzene-PMO was very high with a loading of ca. 0.5 g herbicide per gram of adsorbent after successive adsorption cycles. Keywords: S-Metolachlor, herbicides, ethane-PMO, benzene-PMO, adsorption, water treatment. 155 Capítulo VI 1. Introduction The so-called periodic mesoporous organosilicas (PMOs) were firstly reported in 1999 by three different research groups which synthesized them via the condensation of bridged organosilanes of the type (R’O)3Si-R-Si(OR’)3 in the presence of a surfactant. [1-3]. Unlike other hybrid materials prepared from mesoporous silicas by grafting and co-condensation, the organic fragments (-R-) in PMOs materials are abundantly and homogeneously distributed through their structure and are linked by covalent bonds to two silicon units. Various R groups have been successfully incorporated into these materials, including aliphatic (-CH2-, -CH2-CH2-, -CH=CH-), aromatic (-C6H4-, -C12H8-) and even heteroaromatic fragments (-C4H2S-) [4]. Similarly to mesoporous silicas, PMOs have a mesoporous structure with a controlled pore size and high surface area. However, PMOs are more hydrophobic due to the existence of organic fragments in their framework. All these facts make them excellent candidates as adsorbents for organic molecules. In particular, ethylene- (R= -CH2-CH2-) and phenylene- (R= -C6H4-) bridged PMOs, which are very stable under hydrothermal conditions [5], can be very useful for the removal of organic pollutants from aqueous effluents. An interesting environmental application of PMOs has involved their use as adsorbents for heavy metals. It requires the presence of certain organic functionalities which can be present in the PMOs either by using specific organosilanes and/or organodisilanes for their synthesis or by introducing them later through reactions with the organic moieties present in their frameworks. Thus, PMOs with isocyanurate bridging groups were synthesized by co-condensation of TEOS, 3- mercaptopropyltrimethoxysilane and tris[3-(trimethoxysilyl)propyl]isocyanurate and their high adsorption capacity for Hg 2+ ions was reported [6]. Co-condensation between 1,2-bis(triethoxysilyl)ethane and 3-(mercaptopropyl)trimethoxysilane was used to prepare several PMOs with different pore sizes depending on the surfactant added, i.e. Brij-76 and Pluronic P123, in the range of 1.4-3.4 and 4.2-6.4 nm, respectively [7]. The former were quite selective for Hg 2+ 2+ 2+ ions whereas the latter 3+ exhibited a high affinity for Hg , Cd and Cr cations. More recently, different PMOs have been synthesized by co-condensation of TEOS and an organodisilane precursor 156 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas containing disulfide units. They showed excellent efficiencies for the adsorption of Hg 2+ -1 ions (up to 716 mg g ) [8]. A different synthetic strategy involved the functionalization of the double bonds of an ethenylene-bridged PMO with propylthiol groups [9]. This 2+ material combines the Hg adsorption ability of the thiol groups with the high stability of the PMO, thus allowing it to be reusable after multiple regeneration cycles. Very recently, Walcarius and Mercier have extensively revised the application of mesoporous silicas for the removal of organic and inorganic pollutants [10]. The use of PMO materials as adsorbents for toxic organic pollutants is limited to the work by Borghard et al [11] .They synthesized two mesoporous polysilsesquioxanes from 1,2bis(triethoxysilyl)ethane and 1,4-bis(trimethoxysilylethyl)benzene under alkaline conditions. Even though the latter material showed much lower specific surface area than the former one and a non periodic structure, it adsorbed 14 times more 4nitrophenol than the former [12]. The different performance of both materials was explained by the existence of π-π interactions between the aromatic rings of the phenolic compounds and the arylene bridges. S-Metolachlor is a pre- and post-emergence selective herbicide used to control grasses and some broad-leaved weeds in a wide range of crops. Previous papers have reported the adsorption of S-Metolachlor by different adsorbents such as organohydrotalcites (OHTs) [13, 14], soils [15], activated charcoals [16] and clays [17]. This paper reports the adsorption of S-Metolachlor on ethylene- and phenylenebridged PMOs, i.e., ethane-PMO and benzene-PMO, respectively. The influence of different factors, such as pH and contact time, as well as successive adsorptions, adsorption/desorption experiments and regeneration of the adsorbent has been studied. For comparison, a mesoporous silica has also been used as adsorbent. 2. Materials and methods 2.1. Materials A periodic mesoporous silica (PMS) and two organosilicas, i.e., ethane-PMO and benzene-PMOs, were synthesized by using Brij-76 as surfactant under acidic 157 Capítulo VI conditions according to previously reported procedures (see supporting information) [12]. Analytical standard S-Metolachlor (2 Chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2methoxy-1-methylethyl] acetamide) was supplied by Sigma Aldrich (see scheme 1). The water solubility at 25ᵒC is 480 mg/L. 2.2 Characterization. PMOs were characterized by different techniques. X-ray powder diffraction (XRD) patterns were recorded on a Siemens D-5000 powder diffractometer (Cu-Kα radiation) and FT-IR spectra were obtained with a Perkin Elmer Spectrum One spectrophotometer using KBr disc technique. N2 isotherms were determined on a Micromeritics ASAP 2010 analyzer at −196°C. Particle size data were obtained in a 2000F Mastersizer particle size analyzer by laser diffraction technology, with a dry dispersion automatic system Venturi Scirocco 2000 of Malvern Instruments. Microstructural characterization of the materials was carried out using JEOL 1400 TEM and JEOL 2010 TEM equipments. The 29Si MAS NMR spectra were recorded at 79.49 MHz on a Bruker Avance 400 WB spectrometer at room temperature. An overall 5000 free induction decays were accumulated. The recycle time was 15 s. Chemical shifts were measured relative to a tetramethylsilane standard. UV spectroscopy (Perkin Elmer UV-visible spectrophotometer, Model Lambda 11) at 265 nm was used to determine the concentration of S-Metolachlor. 2.3 Adsorption/desorption experiments. S-Metolachlor adsorption on mesoporous materials was studied at different initial pH values, contact times (kinetics), and initial herbicide concentration (isotherms), by suspending duplicated samples of 20 mg of each adsorbent in 30 mL of aqueous herbicide solutions. Samples of sorbent and herbicide in water were shaken in a turn-over shaker at 52 rpm. 158 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas To study the adsorption at different initial pHs [2.0, 3.0, 4.0, 5.0 and 6.6 (pH of herbicide solution)] and contact times (1, 2, 5, 15 and 24 h), an initial herbicide concentration of 350 µM was used. Adsorption isotherms were obtained by the batch equilibration technique. For that, the samples were equilibrated through shaking at 52 rpm for 24 h at room temperature, as described elsewhere [18, 19], using initial -4 -3 herbicide concentrations between 1.0·10 M and 1.4·10 M. Scheme 1. Adsorption of S-Metolachlor on PMOs showing the main interactions between the herbicide and both surfaces (see text for discussion). The supernatant liquid was centrifuged and separated to determine the concentration of S-Metolachlor by UV spectroscopy. The amounts of herbicide adsorbed were determined from the initial and final solution concentrations. According to a previously described procedure [13], desorption was measured from each equilibrium point of the adsorption curve by replacing a small portion of supernatant (7.5 mL) with the same amount of distilled water. After shaking for 24 h, the amount of herbicide desorbed was calculated as indicated above. The pseudo first order [20-24] and the pseudo second order [23] models and intraparticle diffusion equation [25, 26] were used with the goal at evaluating the adsorption kinetic data. log�𝐶𝑠1 − 𝐶𝑠𝑡 � = log 𝐶𝑠1 − 𝑡 𝐶𝑠𝑡 = 1 𝑘2 ·𝐶𝑆2 2 + 1 𝐶𝑠2 ·𝑡 𝑘1 2.303 159 ·𝑡 (1) (2) Capítulo VI 𝐶𝑠𝑡 = 𝑘𝑝 · 𝑡 1/2 + 𝐶 (3) -1 where Cs1 and Cs2 are the maximum amount of pesticide adsorbed (mg·g ) for first -1 and second-kinetic order, respectively. Cst is the amount adsorbed at time t (mg·g ), k1 -1 is the pseudo first order rate constant for the adsorption process (min ), k2 is the rate −1 −1 constant of pseudo second order adsorption (g·mg ·min ), C is the intercept, and kp is -1 the intraparticle diffusion rate constant (mg·g ·min -1/2 ). S-Metolachlor adsorption isotherms were fitted to Langmuir (Eq. (4)), Freundlich (Eq. (5)) and Dubinin-Radushkevich (D-R) (Eq. (6)) equations: 1 𝐶𝑠 = 1 𝐶𝑚 ·𝐿 · 1 𝐶𝑒 + 1 (4) 𝐶𝑚 𝑙𝑜𝑔𝐶𝑠 = 𝑙𝑜𝑔𝐾𝑓 + 𝑛𝑓 𝑙𝑜𝑔𝐶𝑒 -1 𝑙𝑛𝐶𝑠 = 𝑙𝑛𝑞𝑚 − 𝐾𝜀 2 (5) (6) where Cs (µmol g ) is the amount of herbicide adsorbed per unit mass adsorbent at -1 the equilibrium concentration Ce (µmol L ), Cm is the monolayer capacity of the -1 -1 adsorbent (µmol·g ), and L is the Langmuir adsorption constant (L·µmol ), which is related to the adsorption energy. Kf (µmol 1-nf nf -1 L g ) and nf are Freundlich parameters incorporating all factors affecting the adsorption process. qm is the theoretical -1 2 -2 adsorption capacity of the adsorbent (µmol·g ) and K (mmol ·kJ ) is the constant related to adsorption energy. ε is the Polanyi adsorption potential, and equal to RT·ln(1+1/Ce), where R is the universal gas constant and T is the temperature in the Kelvin scale. 2.4 Successive adsorptions They were carried out in the highest equilibrium point of the adsorption isotherm curves, by suspending the solid (adsorbent-herbicide) in 30 mL of a fresh -3 aqueous herbicide solution (1.25·10 M). The contact time was 24 h. After shaking, the suspensions were centrifuged and the herbicide concentration determined by UV-Vis spectroscopy. This process was repeated up to complete saturation of the adsorbent. 160 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas Fig. 1. Powder X-ray diffraction patterns of PMS (left), ethane-PMO (center) and benzene-PMO (right) before, after being in contact with the solution of herbicide at different pHs and after herbicide extraction (inset). 2.5 Regeneration of the adsorbent In order to test the regeneration of the adsorbent, some desorption experiments were performed with either ethanol or water from the highest equilibrium point of the adsorption isotherms. The extracting volume was 30 mL and the contact time was 24 h. After shaking, the suspensions were centrifuged, the supernatant liquid was separated and evaporated (in the case of ethanol), and the residue dissolved in the same volume of distilled water to determine the herbicide concentration by UV-Vis spectroscopy. 3. Results and discussion A periodic mesoporous silica (PMS) and two organosilicas, i.e., ethane-PMO and benzene-PMO, have been tested for the adsorption of the herbicide SMetolachlor. These three materials exhibited a low angle (100) peak and two broad 161 Capítulo VI and short second-order (110) and (200) peaks at higher incidence angles indicative of materials with 2D hexagonal (P6mm) mesostructures (Fig. 1). The integrity of the organic bridges was confirmed by FTIR measurements (see supporting information) [5]. Their N2 adsorption-desorption isotherms were of type IV with a sharp increase in the adsorption at P/P0 between 0.30-0.47 due to capillary condensation, typical of materials with a narrow pore size distribution in the mesopore range (Fig. 2). The N2 adsorption step was shifted to higher relative pressures following the sequence: benzene-PMO < ethane-PMO < PMS, by the effect of the increase in the pore size. The main physical properties of the three adsorbents are summarized in Table 1. Their mesoporous structures were confirmed by TEM images (Fig. 3); the inset in figure 3B shows a similar pore size to that determined by the BJH method. In addition, the structure is very similar for PMS and organosilicas (Fig. 3A-C) On the other hand, both organosilicas showed very similar particle size distribution curves, which were centred at around 12 and 14 µm for benzene-PMO and ethane-PMO, respectively (Fig. 4). Fig. 2. Nitrogen adsorption-desorption isotherms and the corresponding pore size distributions (insets) for PMS, ethane-PMO and benzene-PMO. 162 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas A comparison of the behavior of the three adsorbents as a function of pH (Fig. 5) revealed significant differences among them. FT-IR spectra (not shown) revealed that the herbicide was intact after adsorption. In order to gain some insights into the relationship between pH and adsorption, measurements of the point of zero charge (pzc) were accomplished for the three materials. This value indicates the pH required to give zero net surface charge. Below the pzc the hydroxyl groups at the surface of an oxide become protonated and so positively charged whereas above the pzc they are deprotonated and consequently negatively charged. By following the mass titration method (Fig. 6) [27], pzc values of 3.9-4.0 were obtained for the three samples, close to those reported for silica by other authors [28]. As a consequence, the surfaces of the three materials were positively charged at pH=2-3. This suggest that the main interactions between the adsorbents and S-Metolachlor at pH ≤ 3 were electrostatic, i.e., interactions between positive charges on the surface and electron-rich regions (N, O and Cl atoms) in the adsorbed molecules, as a result of which the values of adsorption for samples PMS and ethane-PMO were maximum (Fig. 5). At pH=4 their surfaces were essentially neutral and so their interactions with S-Metolachlor became weaker. Consequently, their adsorption values decreased. Hydrophobic interactions seem to play an important role, since the amount of adsorbed herbicide was higher for ethane-PMO than for PMS throughout the pH range. This reasoning is based on the assumption that both materials possess an analogous amount of Si-OH groups. 29Si MAS NMR spectra of the three samples were recorded in order to ascertain it (Fig. 7). They allowed determining their condensation degree (Table 2), which decreased in the order PMS > ethane-PMO > benzene-PMO. In addition, the OH/Si ratio for benzenePMO was apparently higher than for PMS and ethane-PMO. However, the organic moieties present in the framework of both organosilicas also contribute to the masses of the corresponding materials. In fact, similar relative contents of silanol groups were found in the three materials when that ratio was corrected taking into account the mass of the Si unit for each material (Table 2). On the other hand, the maximum adsorption for benzene-PMO occurred at pH=4 (Fig. 5). Considering the higher hydrophobic character of ethane-PMO than that of benzene-PMO [29], the aromatic π-π interactions between the aromatic ring of the 163 Capítulo VI herbicide and the phenylene bridges of the hybrid material should be crucial for the adsorption of this molecule onto benzene-PMO. Table 1 Physicochemical properties of periodic mesoporous organosilicas. aoa (nm) SBETb (m2 g-1) Smpc (m2 g-1) Vp d (cm3 g-1) Dpe (nm) Wf (nm) PMS 6.7 885 0 1.17 4.2 2.5 Ethane-PMO 7.0 1059 26 1.00 4.1 2.9 Benzene-PMO 6.2 1237 257 0.94 3.6 2.6 Material Unit-cell dimension calculated from ao= (2d100/√3) BET specific surface area determined in the range of relative pressures from 0.05 to 0.2 c Micropore area, calculated by t-Plot method. d Single-point pore volume e Diameter pore average size, calculated from the adsorption branch according to the BJH method; f Pore wall thickness, estimated from W= ao – Dp a b Both mesoporous organosilicas exhibited a higher adsorption capacity toward the herbicide S-Metolachlor than that of the mesoporous silica, as showed by the adsorption kinetics (Fig. 8). The amount of adsorbed herbicide significantly increased -1 -1 from 53µmol·g on PMS to 127 and 135µmol·g on ethane-PMO and benzene-PMO, respectively. Also, both PMOs presented a higher stability under the conditions of adsorption than their silica counterpart (Fig. 1). The material PMS significantly decreased its mesostructural order as the pH of the herbicide solution increased. This effect was much less pronounced for benzene-PMO and particularly for ethane-PMO. The decrease in the intensity of the (100) peak for PMOs can be attributed to the presence of adsorbate molecules inside the pores which causes a decrease in the contrast for X-rays between the organosilica walls and the pores. In fact, both PMOs recovered their original X-rays patterns upon removing the herbicide (vide infra), what was confirmed before and after the adsorption for benzene-PMO by TEM (Figure 3, C and D), unlike PMS whose mesostructure was destroyed (Figure 1 inset). 164 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas Fig. 3. TEM images for PMS (A), ethane-PMO (B), including a TEM image of high resolution (inset), and benzene-PMO before adsorption of S-Metolachlor (C) and after desorption (D). Fig. 4. Particle size distribution for ethane-PMO and benzene-PMO Fig. 5 Influence of pH on the adsorption of S-Metolachlor on PMS, ethane-PMO and benzene-PMO 165 Capítulo VI The adsorption kinetics of the herbicide S-Metolachlor were elucidated by substituting the initial concentrations into the pseudo first order (Largergren equation) and pseudo second order adsorption rate equations, respectively. Table 3 shows the adsorption rate constants k1 and k2 calculated from the slope of each plot. The 2 correlation coefficients R for the pseudo first order kinetic model were between 0.89 2 and 0.97 and the correlation coefficients R for the pseudo second order kinetic model were 0.99. Therefore, this adsorption system fitted better to a pseudo second order kinetic model [22]. The mechanism behind the adsorption was studied by analyzing the kinetic results according to the Morris–Weber equation for intraparticle diffusion (Eq.(3)). The adsorption rate-determining step could therefore be either adsorption or intraparticle diffusion. Based on the applied model, a plot of the amount of pesticide adsorbed, Cst, 1/2 against the square root of time, t , should be linear if intraparticle diffusion was involved in the process; also, if the lines passed through the origin, then intraparticle diffusion would be the rate-determining step [21, 30]. Fig. 6. Mass titration curves for PMS,ethane-PMO and benzene-PMO 2 The correlation coefficients R of Table 3, obtained at Cst values spanning up to 24 h, suggested that the adsorption of herbicide S-Metolachlor on PMOs may involve intraparticle diffusion; however, none of the curves passed through the origin, so intraparticle diffusion cannot have been the rate-determining step and other factors may have operated simultaneously to govern the adsorption process. Kp and C were 166 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas calculated from the plot of Cst versus t 1/2 for each adsorbent (see Table 3). The values of kp and C for benzene-PMO were higher and lower than those for ethane-PMO, respectively, suggesting a higher contribution of the intraparticle diffusion for benzene-PMO. 29 Fig. 7. Si MAS NMR spectra for PMS, ethane-PMO and benzene-PMO Table 1 29 Si MAS NMR results obtained for mesoporous materials. Adsorption isotherms were measured for both mesoporous organosilicas due to their significantly higher capacity compared to that of the PMS sample (Fig. 8). The adsorption isotherms at the pH of maximum adsorption for each material are depicted in Fig. 9. Ethane-PMO exhibited a Langmuir isotherm, specifically L2 [31]. In this case, the adsorbed molecules are most likely to be adsorbed flat (Scheme 1), according to the abovementioned electrostatic interactions. The plateau of the curve gave 255 -1 µmol g . By contrast, the adsorption of the herbicide on benzene-PMO followed a type L4 isotherm. It exhibited an ill-defined first plateau which suggests a close affinity of SMetolachlor for two different regions in the surface of benzene-PMO. We tentatively 167 Capítulo VI assume that adsorption firstly occurred in hydrophilic regions, i.e., silanol groups, through hydrogen bonds with oxygen and nitrogen atoms of the herbicide, while the second stage would be caused by the adsorption of S-Metolachlor on hydrophobic regions, i.e., phenylene bridges, through aromatic π-π interactions (Scheme 2). However, we cannot ascertain whether a partially flat or a perpendicular orientation occurred at both stages. Also, the interaction of silanol groups (Lewis acid) of benzenePMO and aromatic rings (Lewis base) of S-Metolachlor cannot be completely ruled out [32]. The second plateau would be reached because of the complete saturation of the hydrophobic regions. Fig. 1. Adsorption kinetics of S-Metolachlor on PMS, ethane-PMO and benzene-PMO. The adsorption results show that the amount of S-Metolachlor adsorbed on -1 benzene-PMO (667 µmol g ) exceeded the amount adsorbed by activated charcoals -1 -1 (480-550 µmol g ) [16], organohydrotalcites (OHTs) (356-385µmol g ) [13, 14], clays -1 -1 (200 µmol g ) [17] and soils (50 µmol g ) [15]. The adsorption capacity of the ethanePMO was slightly smaller than that of the organohydrotalcites (OHTs) and similar to that of clays. 168 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas Table 2 Kinetic parameters for the adsorption of S-Metolachlor onto PMS and PMOs The desorption curves almost matched the adsorption ones for ethane-PMO (Figure 9), thus indicating a high reversibility of the adsorption-desorption process. However, the isotherm for benzene-PMO displayed a considerable hysteresis, which must be related to the interactions between the adsorbent and S-Metolachlor. Fig. 2. Adsorption and desorption isotherms of S-Metolachlor on ethane-PMO and benzene-PMO Freundlich and Langmuir equations were tested to model the adsorption isotherms in order to determine the parameters which quantify the adsorption process 2 (Table 4). According to R values, the Langmuir equation was fitted much better to the experimental data than the Freundlich one. 169 Capítulo VI In order to predict the adsorption efficiency, a dimensionless separation factor R was determined using the following equation [33, 34]. 𝑅= 1 (7) 1+𝐿 𝐶𝑖 where Ci is the initial herbicide concentration. Values in the range 0 < R < 1 represent a favorable adsorption. The R values for adsorption on ethane-PMO and benzene-PMO, -1 with initial concentrations of S-Metolachlor 100-1400 µmol L , were found in the range of 0.94-0.51 and 0.95-0.53, respectively. These values indicate a favorable adsorption, and also allow to confirm the higher adsorption at low concentrations in both cases. Scheme 2. Plausible model for the adsorption of S-Metolachlor onto benzene-PMO through adsorption on hydrophilic (a) and hydrophobic regions (b) of the original surface and formation of a new adsorbed layer up to complete filling of the pores (c). The polar part and the aromatic ring of the molecule are represented by a circle and a stick, respectively. The different colors depict different adsorption stages. In order to distinguish between physical and chemical adsorption, parameters K and qm from Dubinin-Radushkevich equation (D-R) were calculated from the slope and intercept in the plot of lnCs versus ε2, respectively [24, 35]. The values of qm (Table 4) agreed with the abovementioned higher adsorption on benzene-PMO than on ethane-PMO (Figures 8 and 9). The constant K is related to the mean free energy of adsorption (E), which can be calculated by Eq. 8 [33, 36]: )-1/2 E = (2K (8) The mean energy provides information about the adsorption mechanism. The -1 -1 adsorption is physical in nature when 0<E<8 kJ mol ; if it is between 8 kJ mol and 16 170 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas kJ mol-1, the adsorption is due to exchange of ions; and if the value of E is between -1 20<E<40 kJ mol , a chemisorption occurs. The values found in the present study were -1 -1 2.7 kJ mol for benzene-PMO and 3.3kJ mol for ethane-PMO, thus pointing to a physical adsorption. Aiming at the saturation of the adsorbents, successive adsorptions were accomplished (Fig. 10). The maximum S-Metolachlor adsorption capacities for both host materials were different. Ethane-PMO was saturated after two adsorptions, being -1 the adsorbed amount of 416.63 µmol g , whereas benzene-PMO required four -1 successive adsorptions, which accounted for 1729.35 µmol g . Table 3 Freundlich, Langmuir and Dubinin-Radushkevich model parameters for the S-Metolachlor adsorption onto ethanePMO and benzene-PMO Probably, this adsorption occurred on the exposed parts of the layer already present (Scheme 2c). Taking into account that the volume of the S-Metolachlor 3 molecule is around 875 Å [37], the resulting volume occupied by the adsorbed 3 -1 herbicide was estimated to be 0.22 and 0.91 cm g for ethane-PMO and benzenePMO, respectively. Accordingly, the pores of benzene-PMO are completely filled in with S-Metolachlor after successive adsorptions (see Table 1). This is not the case for ethane-PMO, thus remarking the key role of the phenylene bridges to the adsorption of S-Metolachlor. The high loading (up to 0.5 g/g) and slow release of S-Metolachlor from benzene-PMO make it an excellent candidate as a delivery system. 171 Capítulo VI Fig. 3. Successive adsorptions of S-Metolachlor from the highest equilibrium point of the adsorption isotherm curve for ethanePMO and benzene-PMO. Finally, some desorption experiments were performed with either ethanol or water from the highest equilibrium point of the adsorption isotherms to test the regeneration of the adsorbent. In this sense, both materials behaved similarly. SMetolachlor completely desorbed from ethane-PMO and benzene-PMO when using ethanol. However, the herbicide was not completely removed in water. Specifically, 74.5% and 84.0% were desorbed from ethane-PMO and benzene-PMO, respectively. As expected, the herbicide showed a higher affinity for the more hydrophobic solvent due to its non-ionic character. 4. Conclusion Periodic mesoporous organosilicas have unique properties to be used as adsorbents of organic molecules in virtue of their high surface area and narrow mesopore distribution, similarly to mesoporous silicas, as well as their bridges of organic nature which can interact with them. We have examined a potentially practical use of these materials with ethylene and phenylene bridges for the adsorption of a relatively complex herbicide molecule (S-Metolachlor). While ethane-PMO behaved 172 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas like a mesoporous silica (PMS) but with an enhanced adsorption capacity due to its more hydrophobic character, benzene-PMO displayed a much higher herbicide retention at a pH close to its point of zero charge. Certainly, aromatic π-π interactions between phenylene bridges and S-Metolachlor should play an important role. In fact, both PMOs exhibited different isotherms, specifically types Langmuir L2 and L4 for ethane- and benzene-PMO, respectively. In addition, the latter isotherm exhibited a desorption hysteresis effect. Both PMOs were saturated by successive adsorptions. -1 Thus, ethane-PMO showed an uptake of 416.63 µmol g whereas it was of 1729.35 -1 µmol g for benzene-PMO. Due to its high aqueous stability, high adsorption capacity (higher than activated charcoals, organohydrotalcites, clays and soils) and easy regenerability, benzene-PMO is a promising adsorbent for other complex aromatic molecules. Acknowledgements The authors would like to thank Spain’s Ministry of Science and Innovation (Projects MAT2010-18778 and CTM2011-25325), Junta de Andalucía (Project P06FQM-01741 and Research Groups FQM-214 and FQM-346) and FEDER funds. R. Otero acknowledges Ministry of Education and Science for a research and teaching fellowship. References [1] S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, Novel mesoporous materials with a uniform distribution of organic groups and inorganic oxide in their frameworks, J Am Chem Soc, 121 (1999) 9611-9614. [2] B.J. Melde, B.T. Holland, C.F. Blanford, A. Stein, Mesoporous sieves with unified hybrid inorganic/organic frameworks, Chem Mater, 11 (1999) 3302-3308. [3] T. Asefa, M.J. MacLachlan, N. Coombs, G.A. Ozin, Periodic mesoporous organosilicas with organic groups inside the channel walls, Nature, 402 (1999) 867871. 173 Capítulo VI [4] F. Hoffmann, M. Cornelius, J. Morell, M. Froba, Silica-based mesoporous organicinorganic hybrid materials, Angew Chem Int Edit, 45 (2006) 3216-3251. [5] D. Esquivel, C. Jimenez-Sanchidrian, F.J. Romero-Salguero, Comparison of the thermal and hydrothermal stabilities of ethylene, ethylidene, phenylene and biphenylene bridged periodic mesoporous organosilicas, Mater Lett, 65 (2011) 14601462. [6] O. Olkhovyk, S. Pikus, M. Jaroniec, Bifunctional periodic mesoporous organosilica with large heterocyclic bridging groups and mercaptopropyl ligands, J Mater Chem, 15 (2005) 1517-1519. [7] Q.H. Yang, J. Liu, J. Yang, L. Zhang, Z.C. Feng, J. Zhang, C. Li, Acid catalyzed synthesis of ordered bifunctionalized mesoporous organosilicas with large pore, Micropor Mesopor Mat, 77 (2005) 257-264. [8] N. Hao, L. Han, Y.X. Yang, H.T. Wang, P.A. Webley, D.Y. Zhao, A metal-ion-assisted assembly approach to synthesize disulfide-bridged periodical mesoporous organosilicas with high sulfide contents and efficient adsorption, Appl Surf Sci, 256 (2010) 5334-5342. [9] E. De Canck, L. Lapeire, J. De Clercq, F. Verpoort, P. Van Der Voort, New Ultrastable Mesoporous Adsorbent for the Removal of Mercury Ions, Langmuir, 26 (2010) 1007610083. [10] A. Walcarius, L. Mercier, Mesoporous organosilica adsorbents: nanoengineered materials for removal of organic and inorganic pollutants, J Mater Chem, 20 (2010) 4478-4511. [11] W.G. Borghard, D.C. Calabro, F.P. DiSanzo, M.M. Disko, J.W. Diehl, J.C. Fried, M.A. Markowitz, M. Zeinali, B.J. Melde, A.E. Riley, Characterization and Testing of Periodic Mesoporous Organosilicas as Potential Selective Benzene Adsorbents, Langmuir, 25 (2009) 12661-12669. [12] M.C. Burleigh, S. Jayasundera, C.W. Thomas, M.S. Spector, M.A. Markowitz, B.P. Gaber, A versatile synthetic approach to periodic mesoporous organosilicas, Colloid Polym Sci, 282 (2004) 728-733. [13] R. Otero, J.M. Fernandez, M.A. Ulibarri, R. Celis, F. Bruna, Adsorption of non-ionic pesticide S-Metolachlor on layered double hydroxides intercalated with dodecylsulfate and tetradecanedioate anions, Appl Clay Sci, 65-66 (2012) 72-79. 174 Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas [14] D. Chaara, F. Bruna, M.A. Ulibarri, K. Draoui, C. Barriga, I. Pavlovic, Organo/layered double hydroxide nanohybrids used to remove non ionic pesticides, J Hazard Mater, 196 (2011) 350-359. [15] Y.B. Si, K. Takagi, A. Iwasaki, D.M. Zhou, Adsorption, desorption and dissipation of metolachlor in surface and subsurface soils, Pest Manag Sci, 65 (2009) 956-962. [16] M. Bosetto, P. Arfaioli, P. Fusi, Adsorption of the Herbicides Alachlor and Metolachlor on 2 Activated Charcoals, Sci Total Environ, 123 (1992) 101-108. [17] A. Nennemann, Y. Mishael, S. Nir, B. Rubin, T. Polubesova, F. Bergaya, H. van Damme, G. Lagaly, Clay-based formulations of metolachlor with reduced leaching, Appl Clay Sci, 18 (2001) 265-275. [18] M.C. Hermosin, I. Pavlovic, M.A. Ulibarri, J. Cornejo, Trichlorophenol Adsorption on Layered Double Hydroxide - a Potential Sorbent, J Environ Sci Heal A, 28 (1993) 1875-1888. [19] I. Pavlovic, M.A. Ulibarri, M.C. Hermosin, J. Cornejo, Sorption of an anionic surfactant from water by a calcined hydrotalcite-like sorbent, Fresen Environ Bull, 6 (1997) 266-271. [20]S. Lagergren, Zur theorie der sogenannten adsorption gelöster stoffe, Kungliga Svenska Vetenskapsakademiens, Handlingar 24 (4) (1898) 1–39. [21] N. Kannan, M.M. Sundaram, Kinetics and mechanism of removal of methylene blue by adsorption on various carbons - a comparative study, Dyes Pigments, 51 (2001) 25-40. [22] A. Özcan, A. S. Özcan, Adsorption of Acid Red 57 from aqueous solutions onto surfactant modified sepiolite, J. Hazard. Mat. B125 (2005) 252–259. [23] Y.S. Ho, G. McKay, Kinetic models for the sorption of dye from aqueous solution by wood, Process Saf Environ, 76 (1998) 183-191. [24] M. Islam, R. Patel, Physicochemical characterization and adsorption behavior of Ca/Al chloride hydrotalcite-like compound towards removal of nitrate, J Hazard Mater, 190 (2011) 659-668. [25] A.K. Yadav, C.P. Kaushik, A.K. Haritash, A. Kansal, N. Rani, Defluoridation of groundwater using brick powder as an adsorbent, J Hazard Mater, 128 (2006) 289-293. [26] W.J. Weber Jr., J.C. Morriss, Kinetics of adsorption on carbon from solution, J. Sanit. Eng. Div. Am. Soc. Civ. Eng. 89 (1963) 31–60. 175 Capítulo VI [27] J.S. Noh, J.A. Schwarz, Estimation of the Point of Zero Charge of Simple Oxides by Mass Titration, J Colloid Interf Sci, 130 (1989) 157-164. [28] J.A. Schwarz, C.T. Driscoll, A.K. Bhanot, The Zero-Point of Charge of Silica-Alumina Oxide Suspensions, J Colloid Interf Sci, 97 (1984) 55-61. [29] P. Van Der Voort, D. Esquivel, E. De Canck, F. Goethals, I. Van Driessche, F.J. Romero-Salguero, Periodic mesoporous organosilicas: from simple to complex bridges; a comprehensive overview of functions, morphologies and applications, Chem Soc Rev 42 (2013) 3913-3955. [30] J.P. Chen, S. Wu, K.H. Chong, Surface modification of a granular activated carbon by citric acid for enhancement of copper adsorption, Carbon 41 (10) (2003) 1979– 1986. [31] C.H. Giles, T.H. Macewan, S.N. Nakhwa, D. Smith, Studies in Adsorption .11. A System of Classification of Solution Adsorption Isotherms, and Its Use in Diagnosis of Adsorption Mechanisms and in Measurement of Specific Surface Areas of Solids, J Chem Soc, (1960) 3973-3993. [32] B. Camarota, Y. Goto, S. Inagaki, B. Onida, Basic Sites on Periodic Mesoporous Organosilicas Investigated by XPS and in Situ FTIR of Adsorbed Pyrrole, Langmuir, 27 (2011) 1181-1185. [33] K.R. Hall, L.C. Eagleton, A. Acrivos, Vermeule.T, Pore- and Solid-Diffusion Kinetics in Fixed-Bed Adsorption under Constant-Pattern Conditions, Ind Eng Chem Fund, 5 (1966) 212-&. [34] X.X. Guo, F.Z. Zhang, Q. Peng, S.L. Xu, X.D. Lei, D.G. Evans, X. Duan, Layered double hydroxide/eggshell membrane: An inorganic biocomposite membrane as an efficient adsorbent for Cr(VI) removal, Chem Eng J, 166 (2011) 81-87. [35] M. Islam, R. Patel, Nitrate sorption by thermally activated Mg/Al chloride hydrotalcite-like compound, J Hazard Mater, 169 (2009) 524-531. [36] C. Namasivayam, R.T. Yamuna, Adsorption of Direct-Red-12-B by Biogas Residual Slurry - Equilibrium and Rate-Processes, Environ Pollut, 89 (1995) 1-7. [37] Hyperchem 7.51; Hypercube, Inc.: Gainesville, Florida, USA. 176 CAPÍTULO VII. MESOPOROUS PHENOLIC RESIN AND MESOPOROUS CARBON FOR THE REMOVAL OF S-METOLACHLOR AND BENTAZON HERBICIDES Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides GRAPHICAL ABSTRACT Abstract Two mesoporous materials, i.e., a mesoporous phenolic resin and a mesoporous carbon, were synthesized following a soft template method to test the removal of two pesticides, Bentazon and S-Metolachlor. The adsorbents were characterized by X-ray diffraction analysis, nitrogen adsorption-desorption isotherms, thermogravimetric analysis, transmission electron microscopy, particle size measurement and XPS spectroscopy. Bentazon and S-Metolachlor adsorption kinetics and isotherm studies were carried out at pH 2 and 4, respectively, on these mesoporous materials as well as on a reference commercial carbon. Freundlich, Langmuir, Dubinin-Raduskevich and Temkin models were applied to describe the adsorption behavior with the Langmuir model showing the best fit. The regeneration of the adsorbents was carried out by calcination under nitrogen atmosphere and the results indicated that the regeneration of the mesoporous carbon was more efficient than that of the mesoporous phenolic resin and the commercial carbon after being reused several times. Another disadvantage of commercial carbon was the great loss of material that it experienced after several adsorption-desorption cycles. Keywords: S-Metolachlor, Bentazon, mesoporous phenolic resin, mesoporous carbon, water treatment. 179 Capítulo VII 1. Introduction Currently, the pollution caused by pesticides has greatly increased due to their widespread use in agriculture. Pesticides are highly noxious, sometimes nonbiodegradable and very mobile throughout the environment [1]. Therefore, many researchers are studying the removal of these toxic pollutants from aqueous solutions. Photocatalysis [2-4], adsorption [5-7] and electrolysis [8-10] are some processes involved in the removal of these pollutants. The main advantages of adsorption compared to other techniques are its low cost, high removal efficiency and easy operation. Activated carbons and silica gels have been considered as excellent adsorbents for many years. These materials are not suitable as adsorbents of large molecules because they are mainly microporous [11]. Recently, new mesoporous materials that show promising results for the adsorption of large pollutants have been developed [12-17]. Ordered mesoporous materials possess tunable and uniform pore sizes, large surface areas and large pore volumes. Among them, the ordered mesoporous polymer resin and mesoporous carbon (MPR and MC) are relatively new materials that combine the high porosity of mesoporous materials with the physicochemical properties of organic polymers and carbons, respectively [18]. MC is an excellent adsorbent and therefore, as carbon is inert, stable, hydrophobic, light and presents a high affinity towards organic pollutants. Ordered mesoporous carbons were firstly synthesized by a hard template method using mesoporous silica [19-21]. However, this synthesis cannot be used for large scale production because it is time-consuming and expensive. Later, a direct synthesis method using phenolic resin as a carbon precursor was developed [22-24]. S-Metolachlor is a selective herbicide used in several crops. It is quite mobile and although it is mainly found in surface water, it can contaminate groundwater. Several authors have investigated the adsorption of this herbicide on different materials, such as activated carbon [25], soil [26], organohydrotalcites [5] or periodic mesoporous organosilicas (PMOs) [6]. Bentazon is a contact post-emergency herbicide widely used in agriculture, which has a great mobility in soil and is moderately persistent in water. That is why previous papers have reported the removal of Bentazon mainly using activated carbon as adsorbent [1, 27, 28]. The maximum 180 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides concentration of S-Metolachlor and Bentazon admitted by the World Health Organization (WHO) in drinking water are 0.01 mg/L and 0.03 mg/L, respectively. Unlike activated carbons, mesoporous phenolic resins and mesoporous carbons have been hardly used as adsorbents of pesticides. Herein we have synthesized two mesoporous materials (MPR and MC) to be tested as regenerable adsorbents for removing S-Metolachlor and Bentazon. MPR has been prepared by the EISA method (evaporation induced self-assembly) [29, 30], using resorcinol and formaldehyde in the presence of surfactant F127 (EO106-PO70-EO106) under acid conditions. MC was synthesized by calcination of MPR. For comparison, a commercially available activated carbon (CC) was also used as adsorbent. The adsorbents were characterized by several structural and surface techniques. Different experiments have been carried out to study the influence of different factors, such as pH and contact time, adsorption/desorption behaviour and regeneration of the adsorbents. Finally, adsorption experiments at different temperatures have been undertaken in order to determine thermodynamic parameters. 2. Experimental section 2.1. Chemicals Pluronic127 (EO106PO70EO106), resorcinol and commercial activated carbon were purchased from Sigma-Aldrich. The pesticides used as adsorbates in the experiments, Bentazon (3-(1-methylethyl)-1H2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) and S-Metolachlor (2-chloro-N-(2-ethyl6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]acetamide) were purchased from Sigma Aldrich. Some of their properties are included in Table 1. 2.2. Synthesis of adsorbents The synthesis of phenolic resin and mesoporous carbon was carried out using a soft-template method [18]. For that, 9 ml of 3 M HCl were added to a solution of 2.2 g of triblock copolymer Pluronic F127 and 2.2 g of resorcinol in 9 ml of ethanol. After stirring for at least 20 min [31], a formaldehyde solution (2 ml, 36 wt%) was slowly added to it. Then, the mixture was stirred for 20min. The resultant solution was 181 Capítulo VII transferred to a plate to evaporate ethanol. Afterwards, the product was cured in an oven at 60˚C overnight. Finally, the solid was calcined at 380˚C under inert atmosphere to remove the template. The resulting material, a mesoporous phenolic resin, was designed as MPR. Amorphous mesoporous carbon, designed as MC, was obtained by calcination of MPR in a tubular furnace under inert atmosphere at 800˚C. 2.3. Characterization Mesoporous phenolic resin and mesoporous carbon were characterized by different techniques. Powder X-ray diffraction (XRD) patterns were recorded with a Thermo Scientific ARL X’TRA Powder X-ray Diffraction System (radiation CuKα generated by 45kV and 44mA, with slits of 2, 4, 1.5 and 0.2 for the divergence, scatter, -1 receiving scatter and receiving slit, respectively, at 0.25° 2θ min ). Nitrogen adsorption-desorption isotherms were obtained on a Belsorp-mini II gas analyser. Prior to the measurements, the samples were degassed at 120ᵒC for 24 h to remove adsorbed water. Thermogravimetric curves were recorded on a Setaram Setsys Evolution 16/18 apparatus undernitrogen at a heating rate of 5ᵒC/min. Microstructural characterization of the materials was carried out using JEOL 1400 TEM and JEOL 2010 TEM equipments. Particle sizes were measured in a Mastersizer S analyser (Malvern Instruments) using ethanol as dispersant. The samples were sonicated for 10 min before the analysis. XPS spectra were recorded with a SPECS Phoibos HAS 3500 150 -9 MCD. The residual pressure in the analysis chamber was 5·10 Pa. The X-ray source was generated by a Mg anode (hm = 1253.6 eV) powered at 12 kV and with an emission current of 25 mA. The powdered sample was pressed and introduced into the spectrometer without previous thermal treatment. They were outgassed overnight and analyzed at room temperature. Accurate binding energies (BE) have been determined with respect to the position of the C 1s peak at 284.9 eV. The surface atomic concentration ratios were calculated using sensitivity factors from the Casa XPS element library. The potential zeta (ζ) has been determined with a Zetasizer Nano ZS (Malvern Instruments), with a laser of 632.8 nm, coupled to a MPT-2 autotitrator. 182 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides Table 1. Properties of pesticides Bentazon and S-Metolachlor. 2.4. Adsorption experiments. All adsorption experiments were carried out in duplicate by using the batch equilibration technique at 25±1ᵒC. Adsorption of Bentazon (25 mg/L to 250 mg/L) and S-Metolachlor (125 mg/L to 400 mg/L) were conducted by mixing 20 mg of adsorbent with 30 mL of pesticide solution and stirring continuously the mixture for 24h. The suspension was centrifuged and the concentration of Bentazon and S-Metolachlor were determined by UV-Vis spectroscopy at 232 nm and 265 nm, respectively, on a Perkin Elmer Lambda 11 UV-Vis spectrophotometer. The adsorption kinetics were investigated by three models, the Lagergren pseudo– first-order model [32], the pseudo-second-order-model [33] and intraparticle diffusion model [34]. log(𝐶𝑠 − 𝐶𝑠𝑡 ) = log 𝐶𝑠 − 𝑡 𝐶𝑠𝑡 = 1 𝑘2 ·𝐶𝑠2 + 1 𝐶𝑠 ·𝑡 𝑘1 2.303 ·𝑡 𝐶𝑠𝑡 = 𝑘𝑝 · 𝑡 1/2 + 𝐶 [1] [2] [3] -1 where k1 is the pseudo first order rate constant for the adsorption process (h ), Cs and -1 Cst (mg·g ) are the amounts of adsorbed pesticide at equilibrium and at time t (h), -1 -1 respectively, k2 is the rate constant of pseudo-second-order adsorption (g·mg ·h ) and -1 kp (mg·g ·h -1/2 ) is the intraparticle diffusion rate constant. 183 Capítulo VII The equilibrium experimental adsorption data were fitted to Freundlich [35], Langmuir [36], Temkin [37] and Dubinin-Raduskevich [38] models and the estimated parameters calculated from the fitting results are reported in Table 4. The Freundlich model assumes that the adsorption occurs on a heterogeneous surface. The logarithmic equilibrium expression of this model is: where Kf (mg [4] log 𝐶𝑠 = log 𝐾𝑓 + 𝑛𝑓 log 𝐶𝑒 1-nf nf -1 L g ) and nf are the Freundlich constants indicative of adsorption capacity and adsorption intensity, respectively. The Langmuir model describes the adsorbate-adsorbent interactions and is based on the assumption that the adsorption energy is constant and independent of the surface coverage. Its expression is: -1 1 𝐶𝑠 = 1 𝐶𝑚 + 1 𝐶𝑚·𝐿·𝐶𝑒 [5] -1 where Cm (mg·g ) is the monolayer capacity of the adsorbent and L (L·mg ) is the Langmuir adsorption constant. Temkin model describes the behavior of adsorption on heterogeneous surfaces. This model is based on the heat of adsorption (due to the interactions between adsorbate and adsorbent). The Temkin isotherm has generally been applied in the following form: [6] 𝐶𝑠 = 𝐵𝑇 𝑙𝑛𝐾𝑇 + 𝐵𝑇 𝑙𝑛𝐶𝑒 where KT and BT are constants related to adsorption capacity and intensity of adsorption, respectively. The isotherm of desorption was measured at each equilibrium point in the adsorption curve by replacing 7.5 mL of supernatant with an identical volume of distilled water. After shaking for 24 h, the amount of herbicide desorbed was calculated as indicated above. This procedure is described in detail elsewhere [5]. The isotherms of adsorption were studied at three constant temperatures: 10, 20 and 30ᵒC for Bentazon and S-Metolachlor. To test the reversibility, the adsorbents were subjected to three adsorption-desorption cycles. The desorption from mesoporous phenolic resin was carried out by calcination under nitrogen at 380ᵒC for 4 hours; mesoporous carbon and commercial carbon were treated under nitrogen at 400ᵒC for 4 h. 184 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides 3. Results and discussion 3.1. Characterization of the materials. The XRD-patterns of the mesoporous phenolic resin (MPR) and the mesoporous carbon (MC) exhibited a low angle (100) peak with d-spacing of 11.6 nm and 10.3 nm, respectively (Fig. 1). Additional peaks at higher incidence angles were illdefined [39]. Their mesoporous structures were confirmed by TEM images (Fig. 2). The pore channels with hexagonal arrangement suggest an ordered mesostructure. The size of these channels was around 10 nm (see insets in figure 2b-c). However, no ordering was observed in the commercial carbon (Fig 2a). Moreover, the N2 adsorption-desorption isotherms for MPR and MC were essentially of type IV (Fig. 3) with a sharp capillary condensation step at P/P0= 0.45-0.57, which is typical of mesoporous materials as defined by IUPAC [40].Nevertheless, the commercial carbon exhibited a type I isotherm, which corresponds to a microporous material. MC had a slightly higher surface area than MPR. This is probably due the formation of micropores as a consequence of the decomposition of the phenolic framework of MPR to obtain the mesoporous carbon as shown in the TGA curve between 500-750ᵒC (Fig.4). The pore size for MPR and MC was around 9 nm (inset in Fig. 3), in agreement with TEM. The main physicochemical properties of the two mesoporous adsorbents (MRP and MC) along with those of the commercial carbon are listed in Table 2. Fig. 1. Powder X-ray diffraction patterns of mesoporous phenolic resin (MPR) and mesoporous carbon (MC) 185 Capítulo VII The particle size distribution curves of MPR and MC are shown in Fig 5. MPR displayed a considerably large particle size with the main peak centered at about 250 µm. Interestingly, the distribution of particle size for MC was wider and clearly shifted to lower values (ca. 50 µm). This revealed the significant reduction of the particle size upon carbonization of the MPR to MC. The curve for CC was very broad and centered between 10 and 30 µm, showing a significantly lower particle size than MPR and MC. Fig. 2. TEM images for comercial carbon (CC) (a), mesoporous phenolic resin (MPR) (b), including a TEM image of high resolution (inset), and mesoporous carbon (MC) (c), including a TEM image of high resolution (inset). Surface analyses obtained by XPS for the three adsorbents are depicted in Table 2. MPR possessed a significant amount of oxygen, as expected from the composition of the polymer. The fraction of oxygen decreased as a result of the transformation of MPR into MC, which involves the deoxygenation of the phenolic 186 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides groups, among other reactions. CC also presented a considerably amount of oxygen which was much higher than that of MC. The C 1s and O 1s XP spectra are represented in Figure 6. The large FWHM (full width at half-maximum) values of the peaks (1.4-2.7 eV for C 1s and 2.9-3.6 eV for O 1s) suggested the existence of different carbon and oxygen species. Obviously, MPR exhibited various types of carbon atoms, namely C-C, C=C and C-O. MC displayed the narrowest C 1s peak due to the major presence of aromatic carbon atoms in its framework whereas CC even showed additional small contributions at high binding energies indicative of the existence of C=O species. Similarly, the O 1s spectra confirmed the contribution of a higher variety of species in CC than in MPR and MR. Fig. 3. Nitrogen adsorptio-desorption isotherms and the corresponding pore size distributions (insets) for mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC) 187 Capítulo VII 3.2. Adsorption Kinetics of Bentazon and S-Metolachlor The adsorption of Bentazon and S-Metolachlor on the mesoporous materials and the commercial carbon was studied at different pH values using 30 mL of either 50 mg/L Bentazon or 100 mg/L S-Metolachlor and 20 mg of adsorbent (Fig.7). An increase in the pH caused a decrease in the adsorption of Bentazon probably due to the enhancement of the electrostatic repulsion between the ionized pesticide and the adsorbent surface [1]. The point of zero charge (pzc) was determined for the three materials and revealed some differences among them. (see supporting information, Figure SI1). The pzc value indicates the pH required to give zero net surface charge. The pHs were 5.3, 3.2 and 2.7 for commercial carbon, mesoporous carbon and phenolic resin, respectively. When the pH is higher than the pzc, the surface is negatively charged. As pH is increased, the surface is more negatively charged. On the other hand, Bentazon is a weak acid with pKa of 3.3; and at pH above the pKa, it exists predominantly in anionic form. As pH is increased, the extent of dissociation of bentazon molecules is increased and so it becomes more negatively charged. As a result, the equilibrium adsorption of bentazon decreased with an increase in the pH of the initial solution. This could be attributed to the increased electrostatic repulsion between bentazon ions and the surface of the three materials [1]. Similar observations were made for the adsorption of bentazon onto AC cloth [41] Fig. 4. Thermogravimetric curves for mesoporous phenolic resin (MPR) and mesoporous carbon (MC) 188 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides Table 2. Physicochemical properties of commercial carbon, mesoporous phenolic resin and mesoporous carbon Given the aromatic structure of bentazon, dispersion forces should be expected between the π cloud of the adsorbents and the aromatic ring of the adsorbate. Also, some specific localized interactions might arise from the polar groups (i.e., sulfoxide and carbonyl-type) [28]. The decrease in the amount adsorbed at higher pH (anionic form predominant in solution) suggests a weaker interaction of the surfaces with deprotonated (anionic) bentazon than with its neutral form, and consequently that the adsorption is dominated by dispersive interactions between the pesticide and the adsorbent surface. Similar results on the pH-dependence of bentazon adsorption have been reported by other authors [42, 43]. In addition, the relative drop in the adsorption was higher when the pH of the point of zero charge was lower (see Figure 7 and Figure SI1). Thus, the phenolic resin was the most affected by changes in the pH. Fig. 5. Particle size distribution curves for mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC) 189 Capítulo VII However, the adsorption of S-Metolachor on both mesoporous materials, MPR and MC, and the commercial carbon, CC, was not affected by the pH. This could be associated to the non-ionizable nature of S-Metolachlor in the pH range under study. Indeed, the adsorption of S-Metolachlor would occur through π-π interactions between the aromatic ring of the pesticide and π-electrons of the adsorbent structure [41]. The effect of the contact time at the pH of maximum adsorption is depicted in Fig. 8. The adsorption of Bentazon and S-Metolachlor on commercial carbon was fast, reaching the equilibrium in 2 h, while their adsorption on the mesoporous materials was slower (ca. 16 h). These differences in the adsorption kinetics might be explained by the higher specific surface area (Table 2) and smaller particle size (Fig. 5) of CC with respect to both mesoporous materials. Fig. 6. XPS analysis for mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC) 190 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides Fig. 7. Influence pf pH on the adsorption of S-Metolachlor and Bentazon on mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC) The fitting parameters for the adsorption kinetic of Bentazon and SMetolachlor on MPR, MC and CC are listed in Table 3. The correlation coefficients R2 for the pseudo-second-order kinetics (0.999) were best fitted than for a pseudo-firstorder kinetics model (0.561-0.995). Similar results were reported by Salman et al. using an activated carbon as adsorbent for two pesticides, Bentazon and Carbofuran [1]. Following the Morris-Weber equation for intraparticle diffusion (Eq. [3]), the plots of Cs versus t1/2 consist of straight lines for the pesticide adsorption on the three adsorbents. The extrapolation of these lines does not pass through the origin, suggesting that intraparticle diffusion cannot be the rate-determining step and other factors may have operated simultaneously to govern the adsorption process [44, 45]. Further information about the adsorption mechanism can be obtained analysing the adsorption isotherm for the systems under study. The isotherms for the adsorption of Bentazon and S-Metolachlor on mesoporous phenolic resin and mesoporous carbon (Fig. 9) were compared with that of commercial carbon (Fig.10). According to Giles classification [46], the isotherms for Bentazon adsorption on MPR, MC and CC corresponded to a L2 type. Also, S-Metolachor on CC exhibited a L2 type isotherm. However, the adsorption of S-Metolachlor on MPR and MC gave a L4 type isotherm. In the case of Bentazon, the plateau was reached at Cs value of 130.74 mg·g191 Capítulo VII 1 -1 -1 , 166.72 mg·g and 218.04 mg·g for MPR, MC and CC, respectively. By contrast, the second plateau for S-Metolachlor on MPR and MC was not reached, indicating that the maximum adsorption capacity of the adsorbent was not achieved under these conditions. Similar behaviour was observed in the adsorption of this pesticide on phenylene-bridged and ethylene-bridged periodic mesoporous organosilicas [6]. The adsorption isotherm of S-Metolachlor on CC exhibited an ill-defined plateau with a -1 value of Cs= 340.17 mg·g . Fig. 8. Adsorption kinetics of S-Metolachlor and Bentazon on mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC) The fitted parameters of adsorption were calculated from the linear regressions of Eqs. [4]-[6]. According to the regression coefficient values, the Langmuir model showed a much better fit than the Freundlich and Temkin models (Table 4). Table 3. Kinetic parameters for the adsorption of Bentazon and S-Metolachlor onto mesoporous phenolic resin and mesoporous carbon compared with commercial carbon 192 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides These models do not give any idea about the mechanism of adsorption. To understand the adsorption type, experimental data were fitted to the DubininRadushkevich (D-R) model: 𝑙𝑛𝐶𝑠 = ln 𝑞𝑚 − 𝐾𝜀 2 [7] -1 2 -2 where qm is the theoretical adsorption capacity of the adsorbent (mg·g ), K (mg kJ ) is the constant related to adsorption energy and ε=RTln(1+1/Ce) is the Polanyi potencial. The adsorption energy (E) can be calculated using the D-R equation and the following relationship has been used. -0.5 E = (2K) [8] -1 The adsorption is physical in nature when E is between 0 and 8 kJ mol ; if it is -1 8<E<16 kJ mol the adsorption is due to exchange of ions; and if the value of E is -1 between 20<E<40kJ mol , a chemisorption occurs. In the case of S-Metolachlor, the values found in the present study for the three adsorbents were around 8 KJ mol -1 -1 whereas in the case of Bentazon were between 11 and 13 kJ mol . Consequently, the adsorption of Bentazon could be explained by exchange of ions. In addition, E values for S-Metolachlor were in the limit between physical adsorption and exchange of ions and so it was not possible to explain precisely the nature of the adsorption. The desorption curves [5] above the adsorption curves evidenced the difficulty to remove S-Metolachlor and Bentazon from the adsorbents. The presence of hysteresis (figs 9-10) indicates the irreversibility of the process in all cases. 3.3. The effect of temperature In order to determine the thermodynamic parameters of the adsorption of Bentazon and S-Metolachlor on the three adsorbents, the effect of the temperature was studied. When the temperature was increased from 10 to 30ºC the amount of adsorbed Bentazon slightly increased, unlike S-Metolachlor whose adsorption behaviour was similar in the whole temperature range. 193 Capítulo VII Fig. 9. Adsorption and desorption isotherms of S-Metolachlor and Bentazon on mesoporous phenolic resin (MPR) and mesoporous carbon (MC). Fig. 4 Adsorption and desorption isotherms of S-Metolachlor and Bentazon on commercial carbon (CC). The change in standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) of adsorption was calculated using the following equations (Eqs. (9) and (10)) [47, 48]: ΔGº = ΔHº−TΔSº [9] lnK = ΔSº/R−ΔHº/RT [10] 194 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides where K is the equilibrium constant, R the molar gas constant and T the absolute temperature. The values of ΔH° and ΔS° were calculated from the slope and intercept of Van't Hoff plots (Ln KC versus 1/T) and are summarized in Table 5. The values of K have been obtained from the values of KL from the Langmuir equation because our isotherms were well fitted to this equation. K has been recalculated to become dimensionless by multiplying it by 106 [49]. ΔG° values were calculated from Eq. (9). The negative ΔG° values indicate that the process is thermodynamically feasible and spontaneous. In the case of Bentazon, the ΔG° values were slighter higher as the temperature increased indicating that the process was chemically controlled. By contrast, the behaviour of S–Metolachlor showed two different tendencies (physical and chemical). Table 4. Freundlich, Langmuir, Dubinin-Raduskevich and Temkin model parameters for S-Metolachlor and Bentazon adsorption onto mesoporous phenolic resin and mesoporous carbon compared with commercial carbon. 3.4. Regeneration of the adsorbents To test their reusability, the adsorbents were regenerated by calcination under nitrogen atmosphere at 380ºC for mesoporous phenolic resin and 400ºC for mesoporous carbon and commercial carbon. The adsorbents were subjected to three adsorption-desorption cycles (Fig. 11). The regeneration efficiencies of the mesoporous materials were compared with the commercial carbon. In the third cycle, the adsorption capacity for S-Metolachlor on MPR, MC and CC were 60.5%; 69.29% and 59.85%, respectively, while for Bentazon were 77.33% for MPR; 90.22% for MC and 88.82% for CC. For both pesticides, the regeneration was better in mesoporous carbon than in mesoporous phenolic resin and commercial carbon. The differences between both mesoporous materials can be associated to the presence of more defects in the mesostructure of the phenolic resin than in that of the mesoporous carbon after their regeneration (see figure SI2 in supporting information). In relation to 195 Capítulo VII CC, the lower decrease in the adsorption capacity for MC could be ascribed to its mesoporous character and its lower fraction of micropores [50, 51]. Similar behaviour was described by Suchithra et al. [52], who studied the regeneration of hybrid mesoporous material compared with commercial carbon for the removal of dyes and other organic compounds. Table 5. Thermodynamic parameters for S-Metolachlor and Bentazon adsorption onto mesoporous phenolic resin and mesoporous carbon compared with commercial carbon. 7 A great disadvantage of the commercial carbon compared to the mesoporous materials in relation to their reusability was its significant loss of material after several adsorption-desorption cycles. After three cycles, the losses of the commercial adsorbent were 48.2% and 47.1% for the adsorption of S-Metolachlor and Bentazon, respectively, while it was less than 5% for the mesoporous materials (Fig. 11). After each adsorption cycle the material was recovered with a filter of 0.45 μm of pore size, which should retain all particles according to the particle size distribution depicted in Fig. 5. However, these results suggest that the commercial carbon undergoes a shear degradation that decreases the particle size, thus allowing the resulting smallest particles to pass through the filter. The loss of this material involves serious concerns for its practical application because it reduces drastically its durability, unlike both mesoporous materials, which proven to be more resistant to friction. 196 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides Fig. 11. Adsorption-desorption cycles for S-Metolachlor and Bentazon. Adsorption after first (1), second (2) and third (3) cycle. Weight lost after three adsorption-desorption cycles (% Loss). 4. Conclusions Mesoporous phenolic resin and mesoporous carbon exhibited good properties to be used as adsorbents in virtue of their high surface area and narrow mesopore distribution. The N2 adsorption-desorption isotherms for MPR and MC were essentially of type IV. The pore size of these materials was around 9-10 nm according to the N2 adsorption and TEM results. We have examined a potentially practical use of these materials for the adsorption of S-Metolachlor and Bentazon. The adsorption of these pollutants varied with the pH. The adsorption of Bentazon decreased when the pH increased, probably due to the increase of the electrostatic repulsion between the pesticide ion and the adsorbent surface. By contrast, the adsorption of S-Metolachor was not affected by the pH. The adsorption kinetics fitted better to a pseudo-second-order kinetics model. Intraparticle diffusion was present in the adsorption process together with other factors. The isotherms for Bentazon on MPR, MC and CC and for S-Metolachor on CC were of L2 type while the adsorption of S-Metolachlor on MPR and MC exhibited a L4 type isotherm. In the case of Bentazon, a plateau was reached for the three adsorbents. By contrast, the second plateau for S-Metolachlor on MPR and MC was not reached, indicating that the maximum capacity of these adsorbents was not 197 Capítulo VII achieved under these conditions. In all cases the Langmuir model showed a much better fit. The adsorption of Bentazon could be explained as exchange of ions according to the Dubinin-Radushkevich model. The presence of hysteresis in the desorption curves indicated irreversibility of the process in all cases. The regeneration efficiency was better in mesoporous carbon than in mesoporous phenolic resin and commercial carbon for both pesticides. Another disadvantage of the commercial carbon compared to the mesoporous materials for their regeneration was the great loss of material after successive adsorption-desorption cycles. Acknowledgements The authors wish to acknowledge funding of this research by Ministerio de Ciencia e Innovación (Project MAT2010-18778) and Junta de Andalucía (Project P06-FQM01741). D.E. is a postdoctoral researcher of the FWO-Vlaanderen (Fund Scientific Research - Flanders), grant number 3E10813W. R. Otero acknowledges Ministry of Education and Science for a research and teaching fellowship. References [1] J.M. Salman, V.O. Njoku, B.H. Hameed, Bentazon and carbofuran adsorption onto date seed activated carbon: Kinetics and equilibrium, Chem Eng J, 173 (2011) 361-368. [2] C.D. Stan, I. Cretescu, C. Pastravanu, I. Poulios, M. Dragan, Treatment of Pesticides in Wastewater by Heterogeneous and Homogeneous Photocatalysis, International Journal of Photoenergy, (2012). [3] S. Rafqah, P. Wong-Wah-Chung, A. Aamili, M. Sarakha, Degradation of metsulfuron methyl by heterogeneous photocatalysis on TiO2 in aqueous suspensions: Kinetic and analytical studies, Journal of Molecular Catalysis A: Chemical, 237 (2005) 50-59. [4] X. Zhu, C. Yuan, Y. Bao, J. Yang, Y. Wu, Photocatalytic degradation of pesticide pyridaben on TiO2 particles, Journal of Molecular Catalysis A: Chemical, 229 (2005) 95105. [5] R. Otero, J.M. Fernandez, M.A. Ulibarri, R. Celis, F. Bruna, Adsorption of non-ionic pesticide S-Metolachlor on layered double hydroxides intercalated with dodecylsulfate and tetradecanedioate anions, Appl Clay Sci, 65-66 (2012) 72-79. 198 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides [6] R. Otero, D. Esquivel, M.A. Ulibarri, C. Jiménez-Sanchidrián, F.J. Romero-Salguero, J.M. Fernández, Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas, Chem Eng J, 228 (2013) 205-213. [7] O.A. Ioannidou, A.A. Zabaniotou, G.G. Stavropoulos, M.A. Islam, T.A. Albanis, Preparation of activated carbons from agricultural residues for pesticide adsorption, Chemosphere, 80 (2010) 1328-1336. [8] R.-h. Chen, L.-y. Chai, Y.-y. Wang, H. Liu, Y.-d. Shu, J. Zhao, Degradation of organic wastewater containing Cu–EDTA by Fe–C micro-electrolysis, Transactions of Nonferrous Metals Society of China, 22 (2012) 983-990. [9] Q. Lu, J. Yu, J. Gao, Degradation of 2,4-dichlorophenol by using glow discharge electrolysis, Journal of Hazardous Materials, 136 (2006) 526-531. [10] P. Jiang, J. Zhou, A. Zhang, Y. Zhong, Electrochemical degradation of p-nitrophenol with different processes, Journal of Environmental Sciences, 22 (2010) 500-506. [11] Z. Wu, D. Zhao, Ordered mesoporous materials as adsorbents, Chem. Commun., 47 (2011) 3332-3338. [12] M.R. Awual, T. Yaita, S.A. El-Safty, H. Shiwaku, S. Suzuki, Y. Okamoto, Copper(II) ions capturing from water using ligand modified a new type mesoporous adsorbent, Chem Eng J, 221 (2013) 322-330. [13] T. Yokoi, Y. Kubota, T. Tatsumi, Amino-functionalized mesoporous silica as base catalyst and adsorbent, Applied Catalysis A: General, 421–422 (2012) 14-37. [14] J. Brown, R. Richer, L. Mercier, One-step synthesis of high capacity mesoporous Hg2+ adsorbents by non-ionic surfactant assembly, Micropor Mesopor Mat, 37 (2000) 41-48. [15] X.-W. Wu, H.-W. Ma, J. Yang, F.-J. Wang, Z.-H. Li, Adsorption of Pb(II) from aqueous solution by a poly-elemental mesoporous adsorbent, Applied Surface Science, 258 (2012) 5516-5521. [16] A.S. Barreto, A. Aquino, S.C.S. Silva, M.E. de Mesquita, M.J. Calhorda, M.S. Saraiva, S. Navickiene, A novel application of mesoporous silica material for extraction of pesticides, Materials Letters, 65 (2011) 1357-1359. 199 Capítulo VII [17] M.Z. Momčilović, M.S. Ranđelović, A.R. Zarubica, A.E. Onjia, M. Kokunešoski, B.Z. Matović, SBA-15 templated mesoporous carbons for 2,4-dichlorophenoxyacetic acid removal, Chem Eng J, 220 (2013) 276-283. [18] I. Muylaert, A. Verberckmoes, J. De Decker, P. Van der Voort, Ordered mesoporous phenolic resins: Highly versatile and ultra stable support materials, Adv. Colloid Interface Sci., 175 (2012) 39-51. [19] R. Ryoo, S.H. Joo, S. Jun, Synthesis of Highly Ordered Carbon Molecular Sieves via Template-Mediated Structural Transformation, The Journal of Physical Chemistry B, 103 (1999) 7743-7746. [20] Q. Liu, A. Wang, X. Wang, T. Zhang, Ordered Crystalline Alumina Molecular Sieves Synthesized via a Nanocasting Route, Chemistry of Materials, 18 (2006) 5153-5155. [21] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles, Nature, 412 (2001) 169-172. [22] D. Zhao, Q. Huo, J. Feng, B. Chmelka, G. Stucky, Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures, Journal of the American Chemical Society, 120 (1998) 6024-6036. [23] A. Thomas, Functional Materials: From Hard to Soft Porous Frameworks, Angewandte Chemie International Edition, 49 (2010) 8328-8344. [24] Y. Wan, Y. Shi, D. Zhao, Supramolecular Aggregates as Templates: Ordered Mesoporous Polymers and Carbons†, Chemistry of Materials, 20 (2007) 932-945. [25] M. Bosetto, P. Arfaioli, P. Fusi, Adsorption of the herbicides alachlor and metolachlor on two activated charcoals, Science of The Total Environment, 123–124 (1992) 101-108. [26] X.M. Wu, M. Li, Y.H. Long, R.X. Liu, Y.L. Yu, H. Fang, S.N. Li, Effects of adsorption on degradation and bioavailability of metolachlor in soil, Journal of soil science and plant nutrition, 11 (2011) 83-97. [27] L. Allievi, C. Gigliotti, C. Salardi, G. Valsecchi, T. Brusa, A. Ferrari, Influence of the herbicide bentazon on soil microbial community, Microbiological Research, 151 (1996) 105-111. 200 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides [28] A. Omri, A. Wali, M. Benzina, Adsorption of bentazon on activated carbon prepared from Lawsonia inermis wood: Equilibrium, kinetic and thermodynamic studies, Arabian Journal of Chemistry. [29] D. Grosso, F. Cagnol, G.J.d.A.A. Soler-Illia, E.L. Crepaldi, H. Amenitsch, A. BrunetBruneau, A. Bourgeois, C. Sanchez, Fundamentals of Mesostructuring Through Evaporation-Induced Self-Assembly, Advanced Functional Materials, 14 (2004) 309322. [30] C.-K. Tsung, J. Fan, N. Zheng, Q. Shi, A.J. Forman, J. Wang, G.D. Stucky, A General Route to Diverse Mesoporous Metal Oxide Submicrospheres with Highly Crystalline Frameworks, Angewandte Chemie International Edition, 47 (2008) 8682-8686. [31] J. Jin, N. Nishiyama, Y. Egashira, K. Ueyama, Pore structure and pore size controls of ordered mesoporous carbons prepared from resorcinol/formaldehyde/triblock polymers, Micropor Mesopor Mat, 118 (2009) 218-223. [32] S. Lagergren, Zur theorie der sogenannten adsorption gel oster stoffe, Kungliga Svenska Vetenskapsakademiens, 24 (1898) 1-39. [33] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochemistry, 34 (1999) 451-465. [34] M.J.C. Weber W.J, Kinetics of adsorption of carbon from solutions J. Sanit. Eng. Div. Am. Soc. Civ. Eng., 89 (1963) 31-63. [35] H.M.F. Freundlich, Ober Die Adsorption in Lösungen, Zeitschrift Fuär Physikalische Chemie, 57 (1906) 385. [36] I. Langmuir, THE ADSORPTION OF GASES ON PLANE SURFACES OF GLASS, MICA AND PLATINUM, J Am Chem Soc, 40 (1918) 1361-1403. [37] O. Çelebi, C. Üzüm, T. Shahwan, H.N. Erten, A radiotracer study of the adsorption behavior of aqueous Ba2+ ions on nanoparticles of zero-valent iron, J Hazard Mater, 148 (2007) 761-767. [38] M.M. Dubinin, L.V. Radushkevich, Zh. Fiz Khim., 55 (1947) 327. [39] U. Ciesla, F. Schüth, Ordered mesoporous materials, Micropor Mesopor Mat, 27 (1999) 131-149. 201 Capítulo VII [40] N. Farzin Nejad, E. Shams, M.K. Amini, J.C. Bennett, Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology, 106 (2013) 376-384. [41] C.O. Ania, F. Béguin, Mechanism of adsorption and electrosorption of bentazone on activated carbon cloth in aqueous solutions, Water Research, 41 (2007) 3372-3380. [42] A. Boivin, R. Cherrier, M. Schiavon, A comparison of five pesticides adsorption and desorption processes in thirteen contrasting field soils, Chemosphere, 61 (2005) 668676. [43] T.L. Grey, G.R. Wehtje, R.H. Walker, Sorption and mobility of bentazone in coastal plain soils., Weed Sci. , 44 (1996) 166-170. [44] M. Islam, R. Patel, Nitrate sorption by thermally activated Mg/Al chloride hydrotalcite-like compound, J Hazard Mater, 169 (2009) 524-531. [45] R. Otero, J.M. Fernández, M.A. González, I. Pavlovic, M.A. Ulibarri, Pesticides adsorption–desorption on Mg–Al mixed oxides. Kinetic modeling, competing factors and recyclability, Chem Eng J, 221 (2013) 214-221. [46] C.H. Giles, T.H. Macewan, S.N. Nakhwa, D. Smith, Studies in Adsorption .11. A System of Classification of Solution Adsorption Isotherms, and Its Use in Diagnosis of Adsorption Mechanisms and in Measurement of Specific Surface Areas of Solids, J Chem Soc, (1960) 3973-3993. [47] A.S. Özcan, Ö. Gök, A. Özcan, Adsorption of lead(II) ions onto 8-hydroxy quinolineimmobilized bentonite, J Hazard Mater, 161 (2009) 499-509. [48] M. Islam, R. Patel, Polyacrylamide thorium (IV) phosphate as an important lead selective fibrous ion exchanger: Synthesis, characterization and removal study, J Hazard Mater, 156 (2008) 509-520. [49] S.K. Milonjic, A consideration of the correct calculation of thermodynamic parameters of adsorption J. Serb. Chem. Soc., 72 (2007) 1363-1367. [50] Muhammad, M.A. Khan, T.S.Y. Choong, T.G. Chuah, R. Yunus, Y.H.T. Yap, Desorption of β-carotene from mesoporous carbon coated monolith: Isotherm, kinetics and regeneration studies, Chem Eng J, 173 (2011) 474-479. [51] K. Misra, S.K. Kapoor, R.C. Bansal, Regeneration of granular activated carbon loaded with explosives, Journal of Environmental Monitoring, 4 (2002) 462-464. 202 Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides [52] P.S. Suchithra, L. Vazhayal, A. Peer Mohamed, S. Ananthakumar, Mesoporous organic–inorganic hybrid aerogels through ultrasonic assisted sol–gel intercalation of silica–PEG in bentonite for effective removal of dyes, volatile organic pollutants and petroleum products from aqueous solution, Chem Eng J, 200–202 (2012) 589-600. 203 CAPÍTULO VIII. CONCLUSIONES GENERALES GENERAL CONCLUSIONS General Conclusions A partir de las consideraciones finales desarrolladas en los correspondientes capítulos se pueden establecer las siguientes conclusiones generales: Hidrotalcitas con aniones orgánicos en la interlámina. 1. Los aniones dodecilsulfato y tetradecanodioato se intercalaron en el espacio interlaminar de la hidrotalcita aumentando el espaciado d(00l) en comparación con la hidrotalcita con carbonato, mejorando así la capacidad de adsorción para S-Metolacloro. 2. Los diagramas de difracción de rayos X de las organohidrotalcitas sugirieron una disposición vertical de aniones dodecilsulfato y una posición inclinada de los aniones tetradecanodioato en el espacio interlaminar. La adsorción de S-Metalocloro originó cambios en el valor d(00l) para ambas organohidrotalcitas. En particular, la adsorción de S-Metolacloro en HT-TDD condujo a un aumento muy importante en el espaciado basal, lo que sugirió cambios en la orientación del anión tetradecanodiato de inclinada a perpendicular. 3. Se ha propuesto un nuevo procedimiento de la rama de desorción comparable al procedimiento empleado en la adsorción. Esto ha permitido alcanzar una interpretación del proceso físico-químico de adsorcióndesorción. 4. La hidrotalcita con tetradecanodiato en la interlámina mostró una mayor adsorción de S-Metolacloro con respecto a la hidrotalcita con dodecilsulfato en la interlámina. Esto se puede asociar a la longitud de la cadena del anión interlaminar, es decir, un anión con una cadena hidrocarbonada de mayor longitud dará lugar a una mayor adsorción. 207 Capítulo VIII 5. El efecto de la temperatura en la adsorción de S-Metolacloro en ambas hidrotalcitas orgánicas mostró un descenso del valor de ΔG⁰ con el aumento de la temperatura. 6. Los resultados sugirieron la posibilidad de utilizar estas organohidrotalcitas como adsorbentes de S-Metolacloro en agua contaminada. La HT-TDD presentó una mayor adsorción de S-Metolacloro, sin embargo la HT-DDS es posiblemente un mejor adsorbente ya que puede ser reutilizada empleando etanol como disolvente. Hidrotalcita calcinada 7. Los pesticidas Nicosulfuron y Mecoprop-P mostraron una buena adsorción en la hidrotalcita calcinada. El Nicosulfuron presentó menor adsorción que el Mecoprop-P, lo que puede ser justificado por las diferencias estructurales, de tamaño y de ionización de ambos pesticidas. 8. Los diagramas de difracción de rayos X fueron casi idénticos a los de la hidrotalcita intercalada con iones carbonato, lo que es consistente con una disposición plana de los pesticidas en la interlámina, en la superficie externa y/o en los bordes de las partículas del adsorbente. 9. Los datos experimentales de las cinéticas de adsorción de Nicosulfuron y Mecoprop-P se ajustaron a la ecuación de Largergren o pseudo primer orden. El análisis de los datos mediante la ecuación de Morris-Weber permitió concluir que la difusión intrapartícula no era el único factor determinante de la adsorción para ambos pesticidas. 10. Las isotermas de adsorción de ambos pesticidas son de tipo L y la aplicación de las ecuaciones de Langmuir y Dubini-Radushkevich a las isotermas de adsorción para los dos pesticidas reveló que la adsorción es favorable y se produce por un proceso de intercambio iónico. 208 General Conclusions 11. La adsorción y la desorción de los dos pesticidas se vieron afectadas por la presencia de otros aniones. Los aniones competidores (especialmente 2- HPO4 ) tuvieron un efecto adverso en la adsorción de Nicosulfuron y Mecoprop-P. Por otro lado, la extracción de los pesticidas del complejo 2- 2- soporte-pesticida fue especialmente eficaz con CO3 . La capacidad del CO3 para recuperar grandes cantidades de Mecoprop-P sugirió que el adsorbente puede ser reciclado. 12. La reciclabilidad de HT500 realizada mediante calcinación en aire mostró una disminución de la adsorcíon después de cada ciclo de adsorcióndesorción. Esto es debido a una deformación de las partículas del adsorbente. PMOs 13. Se sintetizaron dos organosilices (etano-PMO y benceno-PMO) y una sílice (PMS). Ambos PMOs exhibieron una capacidad de adsorción de SMetalocloro superior a su homólogo de sílice (PMS). 14. Los dos PMOs sintetizados exhibieron un comportamiento de adsorción muy diferente. Mientras etano-PMO se comportó de manera similar a una sílice mesoporosa (PMS), aunque con una capacidad de adsorción mejorada debido a su carácter más hidrófobo, el benceno-PMO mostró una adsorción del herbicida mucho mayor. 15. La cantidad de S-Metolacloro adsorbida se vió influida por el pH y fue máxima a pH 2 para etano-PMO y a pH 4 para el benceno-PMO. Los resultados sugirieron que ambas interacciones, hidrofóbicas y aromáticas ππ entre puentes fenilo y S-Metolacloro, deben desempeñar un papel importante en la adsorción de este herbicida. 209 Capítulo VIII 16. La ecuación de Morris-Weber mostró que existe una mayor contribución de la difusión intrapartícula en el benceno-PMO. 17. La isoterma de adsorción de S-Metolacloro en etano-PMO es de tipo L2 -1 alcanzándose el plateau a 255 µmol·g mientras que la isoterma de benceno-PMO para el mismo plaguicida es de tipo L4 existiendo dos plateaus, probablemente el primer plateau es debido a la adsorción en las regiones hidrofílicas mientras que el segundo podría ser causado por la adsorción de S-Metolacloro en las regiones hidrofóbicas. 18. Las adsorciones sucesivas de S-Metolacloro en los PMOs permitieron conocer la capacidad máxima de adsorción. El etano-PMO se saturó -1 después de la segunda adsorción (416.63 µmol·g ) mientras que el -1 benceno-PMO después de cuatro adsorciónes (1729.35 µmol·g ). 19. Teniendo en cuenta la alta estabilidad acuosa, la elevada capacidad de adsorción y la fácil regenerabilidad, el benceno-PMO es un adsorbente prometedor para moléculas aromáticas. Resinas fenólicas y carbones mesoporosos ordenados 20. Se sintetizaron dos materiales mesoporosos: una resina fenólica mesoporosa (MPR) y un carbono mesoporoso (MC), siguiendo un método de “soft template”. Se ha examinado el uso potencialmente práctico de estos materiales para la adsorción de S-Metolacloro y Bentazona. 21. Los estudios de las cinéticas y de las isotermas de adsorción de Bentazona y de S-Metolacloro se llevaron a cabo a pH 2 y 4, respectivamente. La adsorción de Bentazona disminuyó al aumentar el pH, probablemente debido al aumento de la repulsión electrostática entre el ion pesticida y la superficie adsorbente. Por el contrario, la adsorción de S-Metolacloro no se vió afectada por el pH. 210 General Conclusions 22. El equilibrio en la cinética de adsorción de Bentazona y S-Metolacloro en carbón comercial se alcanzó a las 2 h mientras que en el caso de los materiales mesoporosos se necesitaron 16 h. Estas diferencias pueden ser debidas probablemente a una mayor superficie específica del carbon comercial y un tamaño de partícula más pequeño que los presentados por los materiales mesoporosos. 23. Las cinéticas de adsorción se ajustaron mejor a un modelo de pseudosegundo orden y la difusión intrapartícula estuvo presente en los procesos de adsorción junto con otros factores. 24. Las isotermas de adsorción de Bentazona en MPR, MC y CC y de SMetolacloro en CC fueron de tipo L2, mientras que la adsorción de SMetolacloro en MPR y MC exhibió una isoterma tipo L4. En el caso del Bentazona, se alcanzó una meseta para los tres adsorbentes. Por el contrario, no se alcanzó la segunda meseta en la adsorción de SMetolacloro en MPR y MC, lo que indicó que la capacidad máxima de estos adsorbentes no se alcanzó bajo estas condiciones. 25. En todos los casos el modelo de Langmuir mostró un mejor ajuste. La adsorción de Bentazona podría explicarse como intercambio de iones de acuerdo con el modelo de Dubinin-Radushkevich. La presencia de histéresis en las curvas de desorción puso en evidencia la irreversibilidad del proceso en todos los casos. 26. Las isotermas realizadas a diferentes temperaturas mostraron que la adsorción de Bentazona está controlada químicamente, mientras que para el S-Metolacloro se observaron dos tendencias distintas, química para MC y física para MPR. 211 Capítulo VIII 27. La regeneración de los adsorbentes se llevó a cabo por calcinación bajo atmósfera de nitrógeno y los resultados indicaron que la regeneración del MC fue más eficiente que la del MPR y el CC. 28. Los materiales MC y MPR presentan mejores características que el CC para ser utilizados como adsorbentes de pesticidas. En concreto, el CC experimentó una gran pérdida de material después de varios ciclos de adsorción-desorción. Resumiendo, se han sintetizado diferentes materiales para ser utilizados como adsorbentes en la eliminación de plaguicidas, dependiendo del medio en que estos se encuentren y de la naturaleza de los mismos. Para los pesticidas con carácter iónico o polar, la hidrotalcita calcinada (HT500) ha mostrado buena capacidad de adsorción tanto para Nicosulfuron como Mecoprop-P. Así mismo, los carbones mesoporosos permitieron la eliminación del pesticida aniónico Bentazona en disolución acuosa. Por otra parte, los pesticidas no polares tales como S-Metolacloro fueron adsorbidos en organohidrotalcitas a pH básicos y en PMOs y carbones mesoporosos a pHs ácidos. El mejor adsorbente a pH básico fue HT-TDD aunque HT-DDS mostró ser un mejor candidato debido a su mayor reciclabilidad. A pHs ácidos los mejores adsorbentes fueron el benceno-PMOs y el MC, aunque este último exhibió una mayor capacidad de adsorción. 212 General Conclusions The following general conclusions are set from the final considerations developed in the corresponding chapters: Hydrotalcites with organic anions in the interlayer 1- Dodecylsulphate and tetradecanedioate anions were intercalated in the interlayer space of the hydrotalcite increasing of space d(00l) compared with the carbonate hydrotalcite, thus improving the capacity of adsorption for S-Metolachlor. 2- XRD patterns of the organohydrotalcites suggested a vertical order of dodecylsulphate anions and an inclined order of tetradecanedioate anions in the interlayer space. The adsorption of S-Metolachlor originated changes in d(00l) for both organohydrotalcites. Particularly, the adsorption of SMetolachlor in HT-TDD presented an increase in the basal spacing, suggesting changes in the orientation of the tetradecanedioate anion from tilt to perpendicular position. 3- We have proposed a new procedure for the desorption isotherm comparable with the procedure used in the adsorption. This has allowed a physico-chemical interpretation of the adsorption-desorption process. 4- The Hydrotalcite with tetradecanedioate in the interlayer had a better adsorption on S-Metolachlor than the hydrotalcite with dodecylsulphate. This could be associated with the chain length of the interlayer anion (o “the anion introduced in the interlayer”), i.e., an anion with a hydrocarbonated chain will have a greater adsorption. 5- The effect of the temperature in the adsorption on S-Metolachlor showed a decrease in the ΔG⁰ value with increasing temperature in both organohydrotalcites. 213 Capítulo VIII 6- The results suggested the possibility of using organohydrotalcites as adsorbents of S-Metolachlor in contaminated water. The HT-TDD showed a higher adsorption of S-Metholachlor. However, since HT-DDS can be reused using ethanol as a desorbent, it might be a better adsorbent. Calcined hydrotalcite 7- Nicosulfuron and Mecoprop-P were adsorbed on calcined hydrotalcite. Nicosulfuron had a lower adsorption than Mecoprop-P. This could be due to structural differences, size and ionization of pesticides. 8- XRD patterns of Nicosulfuron and Mecoprop-P adsorbed in calcined hydrotalcite were similar to XRD patterns of hydrotalcite with carbonate in the interlayer. This fact is consistent with a flat disposal of the pesticides in the interlayer, on the surface and at the edges of the pesticide particles. 9- The Largergren equation or pseudo first order was used for analysing the experimental data of the adsorption kinetics of Nicosulfuron and Mecoprop-P. Furthermore, the Morris-Weber equation showed that the intraparticle diffusion wasn’t the determining factor for the adsorption for both pesticides. 10- The adsorption isotherms were L type for both pesticides. According to Langmuir and Dubini-Radushkevich equations, the adsorption was favourable and it occurred by ion exchange. 11- Adsorption and desorption of both pesticides were affected by the 2- presence of other anions. Competitor anions (especially HPO4 ) had an adverse effect on the adsorption of Nicosulfuron and Mecoprop-P. 2- Moreover, the removal of pesticides was particularly effective with CO3 . 2- The capacity of CO3 to recover large amounts of Mecoprop-P suggested that the adsorbent can be recycled. 214 General Conclusions 12- HT500 recyclability by calcination in air showed a decrease in the adsorption after each adsorption-desorption cycle. This is due to a deformation of the adsorbent particles. PMOs 13- Two organosilicas (ethane and benzene-PMO) and a silica (PMS) were synthesized. PMOs exhibited adsorption capacity of S-Metalocloro higher than PMS. 14- PMOs exhibited a very different adsorption behaviour. The behaviour of Ethane-PMO was similar to the PMS one. However, benzene-PMO presented a much higher adsorption of the pesticide due to its hydrophobicity. 15- The adsorption of S-Metolachlor was affected by the pH. The study of adsorption was carried out at pH 2 on ethane-PMO and at pH 4 on benzenePMO. The results suggested that the adsorption of S-Metolachlor on benzene-PMO would occur through π-π interactions between phenyl bridges. 16- According to the Morris-Weber equation, the contribution of intraparticle diffusion for benzene-PMO was higher than ethane-PMO. 17- Adsorption isotherm of S-Metolachlor on ethane-PMO is L2 and the plateau -1 was reached at 255 µmol·g . However, the adsorption of the herbicide on benzene-PMO followed a type L4 isotherm. This isotherm presented two plateaus, the first one probably due to the adsorption in the hydrophilic regions and the second one could be associated to the adsorption in the hydrophobic regions. 215 Capítulo VIII 18- The successive adsorptions S-Metholachlor in PMOs allowed to know the maximum adsorption capacity. Ethane-PMO was saturated after the second -1 adsorption (416.63 µmol·g ) while the benzene-PMO was saturated after -1 four adsorptions (1729.35 µmol·g ). 19- Considering the high aqueous stability, high adsorption capacity and easy regenerability, benzene-PMO is a promising adsorbent to remove aromatic pollutants. Ordered mesoporous phenolic resin and carbon 20- Two mesoporous materials were synthesized by a method of "soft template": a mesoporous phenolic resin (MPR) and a mesoporous carbon (MC). The use of these materials for the adsorption of Bentazon and SMetolachlor has been assesed. 21- Kinetic studies and adsorption isotherms for Bentazon and S-Metolachlor were conducted at pH 2 and 4, respectively. Bentazon adsorption decreased when increasing pH, probably due to increased electrostatic repulsion between the pesticide and the ion adsorbent surface. On the contrary, the adsorption of S-Metolachlor was not affected by pH. 22- The equilibrium at the adsorption kinetics of Bentazon and S-Metolachlor in CC was reached at 2 hours, while 16 hours were required in the case of mesoporous materials. Probably, these differences are due to the greater surface area of CC and a smaller particle size of mesoporous materials. 23- Adsorption kinetics were better fitted to a pseudo-second model. MorrisWeber equation showed that the intraparticle diffusion was present in adsorption processes along with other factors. 216 General Conclusions 24- Adsorption isotherms of Bentazon on MPR, MC and CC and S-Metolachlor on CC were L2 type. However, adsorption isotherms of S-Metolachlor on MPR and MC exhibited L4 type. The plateau was reached for Bentazon in all cases. On the contrary, the maximum capacity was not reached for SMetolachlor on MPR and MC. 25- In all cases, the Langmuir model showed a better fit. Adsorption of Bentazon could be explained as ion exchange according to the DubininRadushkevich model. The presence of hysteresis in the desorption curves revealed the irreversibility of the process in all cases studied. 26- The adsorption of Bentazon was chemically controlled, according to the isotherms at differences temperatures, while for S-Metolachlor two different tendencies were found, chemical for MC and physical for MPR. 27- Regeneration of the adsorbents was carried out by calcination under nitrogen. The results indicated that regeneration of MC was more efficient than the regeneration of MPR and CC. 28- MC and MPR materials have better characteristics than CC to be used as adsorbents of pesticides. Specifically, the CC had a great loss of material after several adsorption-desorption cycles. In summary, different materials have been synthesized to use as adsorbents in removing pesticides, depending on the environment in which they are located and their nature. Calcined hydrotalcite (HT500) shows good adsorption capacity for both Nicosulfuron and Mecoprop-P due to their ionic or polar character. Also, the mesoporous carbons allowed the removal of anionic Bentazon pesticide in aqueous solution. Moreover, nonpolar pesticides such as S-Metolachlor were adsorbed at basic pH by organohydrotalcites and at acidic pHs by mesoporous carbons and 217 Capítulo VIII PMOs. HT-TDD was the best adsorbent at basic pH although HT-DDS proved to be a better option because of its greater reusability. The best adsorbents were benzenePMOs and MC at acidic pH, although the latter showed the higher adsorption capacity. 218 CAPÍTULO IX. SUMMARY Resume The probability of producing pesticide-contaminated drainage water, surface water and underground water rises due to the increase of the use of pesticides in agriculture. Pollution of the surface and ground waters is potentially dangerous to human health because of the contents of inorganic and organic compounds and it can persist in the aquatic environment for many years after its application. The removal of these toxic pollutants from aqueous solutions is the main aim of many researches. Photocatalysis, adsorption and electrolysis are some of the processes involved in the removal of these pollutants. Adsorption is the most commonly used technique to remove toxic substances from wastewater due to its low cost, ease of operation and high removal efficiency. Pesticides are frequently detected in groundwater extraction wells all over Europe (EEA, 2004). In addition to this, the concentration of pesticides in drinking water and groundwater should not exceed 0.1 μg/L for a single compound or 0.5 μg/L for the sum of all pesticides (European Parliament and Council, 2006). For these reasons, the goal of this work is the synthesis of new materials in order to reduce the pollution. Layered double hydroxides (LDHs) constitute a large family of materials with x+ n− the general formula [M(II)1−xM(III)x(OH)2] [A x/n].mH2O, where M(II) is a divalent cation such as Mg, Ni, Zn, Cu or Co, M(III) is a trivalent cation such as Al, Cr, Fe or Ga n− and A is an anion of charge n. The x value, generally between 0.2 and 0.4, determines the layer charge density and the ion exchange capacity. The interest of these materials is related to their structure, high anion exchange capacity and simplicity of their synthesis. For this reason, they are suitable for smoothing as well as preventing the environmental impact caused by pesticides, i.e. they can be also used as filters for pesticide-contaminated water. Furthermore, the adsorption efficiency of LDHs is strongly affected by the properties of their interlayer anions. Therefore, LDHs usually have a greater affinity with anions with a high charge density and hence tend to easily adsorb multivalent anions rather than monovalent ions. In addition, LDHs exhibit preferential affinity 2– with CO3 , which usually hinders further ion-exchange. 221 Capítulo IX Similarly to clay minerals, the inorganic anions of HTs can be replaced with organic anions. This simple modification changes the nature of the HT surface from hydrophilic to hydrophobic, increasing its affinity for non-ionic organic compounds, including many non-ionic pesticides. These organoclays and organo/LDHs have applications in a wide range of organic pollution control fields. Recently, and prior to the development of this current Ph. D. Thesis, our group researches the synthesis of organohydrotalcites with sebacate in the interlayer obtained by the coprecipitation method. Our original approach consisted in comparing the adsorption of two adsorbents with the same anion chain length but differing in the nature of the chemical groups bonded to the HT layers. We synthesized the organohydrotalcite with dodecylsulphate and another one with tetradecanedioate in the interlayer. Elemental analysis was used for determining the value of both Mg/Al and S/Al molar ratio among others. The results showed than a partial dissolution of Al 3+ seemed to occur during the synthesis of HT-TDD. Furthermore, the molar ratio S/Al in HT-DDS indicated that most of the layer charge was compensated by DDS anions. According to the scanning electron micrograph, an agglomerate of particles was observed in both organohydrotalcites, although HT-DDS showed a slightly more porosity and less homogeneity. The study of X-ray diffraction and TG analysis were also tackled. S-Metolachlor is the result of a breakthrough in the catalyst system of the manufacturing process for Metolachlor. This pesticide is physically and chemically equivalent to Metolachlor, but is more active at the site of action in susceptible plants and allows for lower use rates. Our group considers the removing of this pestice because it is being employed more than Metolachlor. This is due to the fact that S-Metolachlor is more environmentally friendly than Metolachlor. S-Metolachlor is a pre- and post-emergence selective non polar herbicide used to control grasses and some broad-leaved weeds in a wide range of crops. It is quite mobile and although it is mainly found in surface water, it can contaminate groundwater. Several papers have reported the adsorption of S-Metolachlor by 222 Resume different adsorbents such as organohydrotalcites (OHTs), soils, activated charcoals and clays. Adsorption-desorption of S-Metolachlor pesticide by two organohydrotalcites (HT-DDS and HT-TDD) has been characterized. The organohydrotalcites presented an expansion of the interlayer space compared to the inorganic hydrotalcite with carbonate anion. X-ray diffraction patterns suggested a vertical monolayer arrangement of dodecylsulfate anions and a tilt position of the tetradecanedioate anions in the interlayer space. Adsorption of S-Metolachlor resulted in changes in the d00l-value for both organohydrotalcites. In particular, the adsorption of SMetolachlor on HT-TDD led to a very important increase in the basal spacing, suggesting changes in the orientation of the tetradecanedioate anion from tilt to perpendicular position. The results indicated that the adsorption of S-Metolachlor on the organohydrotalcite HT-TDD was higher than on HT-DDS, but on the contrary, desorption was lower. The amount of adsorbed pesticide increased with the rise of the temperature from 10 to 30°C, especially for HT-TDD. The decomposition of hydrotalcite when calcined at around 500°C leads to mixed metal oxides, which are characterized by high specific surface areas and homogeneous dispersion of metal cations. The mixed metal oxides can rehydrate and combine with anions from aqueous solution to recovery the original structure, as it is expressed by the following two equations: [Mg1−xAlx(OH)2](CO3)x/2·mH2O → Mg1−xAlxO1+x/2 +(x/2)CO2 + (m + 1)H2O − Mg1−xAlxO1+x/2 + (x/n)A + (m+1 + x/2)H2O →[Mg1−xAlx(OH)2]Ax/n·mH2O + xOH - The memory effect (recovery of LDH structure) takes place when a Mg–AlLDH, usually containing carbonate in the interlayer, is put in water or a solution of a given anion. The Mg–Al-LDH is calcined at a temperature around 500°C to eliminate most of the interlayer anion. The mixed oxide obtained by calcination is mostly amorphous, with a high specific surface area and an ability to recover the layered structure with water. The calcination has an anion-exchange capacity (AEC) much –1 higher than that of the original LDH (5.8 vs 3.3 mmol·g ); this allows to increase significantly the adsorption efficiency of an LDH by calcination. In this sense, the 223 Capítulo IX reconstruction in pure water gives rise to the intercalation of hydroxyl anions, while the intercalation of other anions can occur if they are present in the solution. Calcined layered double hydroxides arouses a great interest in the environmental community because of their high anion retention capacity and simple thermal regeneration procedure. Phenoxy acids, including Mecoprop-P (MCPP), are some of the most frequently detected pesticides in groundwater, and they have been used extensively as herbicides in agriculture. MCPP is still used in some European countries, i.e. France, Italy and Austria. Mecoprop-p has high water solubility and poses environmental concern because its low sorption, high mobility and slow degradation in soil make it susceptible to leaching from soil to water. Moreover, the literature about the effects of Mecoprop-P is poorly documented. Sulfonylureas are a class of herbicides used for crop protection and have been widely used to control weeds. However, the sulfonylurea herbicide residue is highly phytotoxic for some rotational plants at only 1% or even less of the originally applied amount. Nicosulfuron is mostly used on maize. Approximately 91,000 Kg of herbicide after the foliar application into soil annually can run off from cropland into rivers, ponds and lakes, causing surface and groundwater contamination. We have studied the adsorption-desorption behavior of these pesticides and the influence of the presence of some anions in water. Additionally, we have carried out the recyclability of the calcined hydrotalcite by several cycles of adsorptioncalcination on the two pesticides, Nicosulfuron and Mecoprop-P. Kinetic tests showed the adsorption of both Nicosulfuron and Mecoprop-P pesticides, ionizable or ionic respectively, involve intraparticle diffusion, and the rate constant of intraparticle diffusion for Nicosulfuron increases with increasing concentration of the compound by effect of its increased molecular size relative to Mecoprop-P. According the Giles classification, adsorption isotherms for both pesticides were L-shaped, and application of the Langmuir and Dubini–Radushkevich equations revealed that adsorption was favorable and occurred by ion-exchange. Moreover, the maximum amount of Mecoprop-P adsorbed was twice the 224 Resume Nicosulfuron one. This result is consistent with the structural, size and ionization differences between the two pesticides. One the other hand, the presence of others anions affected the adsorption and desorption of Nicosulfuron and Mecoprop-P. Competing anions, (particularly 2– HPO4 ), had an adverse effect on Nicosulfuron and Mecoprop-P adsorption, and the extraction of the pesticides from the host–guest complex was especially effective 2– 2– with CO3 . The ability of CO3 to recover large amounts of Mecoprop-P suggests that the adsorbent can be recycled. According to the results of regeneration tests, calcined hydrotalcite is an effective adsorbent for the removal of Nicosulfuron and Mecoprop-P from contaminated water. This adsorbent can be reused several times to remove Nicosulfuron and Mecoprop-P, albeit with gradually decreasing efficiency. Periodic Mesoporous Organosilicas (PMOs) are a class of mesostructured organic–inorganic hybrid nanomaterials wherein the organic moieties are totally incorporated in the walls. The inorganic and organic moieties placed alternately along the mesopore walls make PMOs more hydrophobic and stable in water than pure organized silicas. Due to their hydrophobic mesoporous structure with a controlled pore size and high surface areas, similarly to mesoporous silicas (PMS), these materials offer unique properties for use as adsorbents of organic compounds in virtue of their high surface area and narrow mesopore distribution, as well as their organic nature bridges which can interact with them. Following the search of new materials, we tested the adsorption of benzenePMO, ethane-PMO and PMS on S-Metolachlor with the collaboration of Dr. Francisco Romero Salguero of the group FQM 346 of the University of Cordoba. The influence of different factors, such as the study of acid pH and contact time, as well as successive adsorptions, adsorption/desorption experiments and regeneration of the adsorbent has been studied. For making the comparison, a mesoporous silica has also been used as adsorbent. The results indicated that both PMOs exhibited a higher adsorption capacity than their silica counterpart. Moreover, both PMOs exhibited a very different 225 Capítulo IX adsorption behavior. While ethane-PMO behaved like a mesoporous silica (PMS) but with an enhanced adsorption capacity due to its more hydrophobic character, benzene-PMO displayed a much higher herbicide retention at a pH close to its point of zero charge. Certainly, aromatic π-π interactions between phenylene bridges and S-Metolachlor should play an important role. Furthermore, the adsorption was affected by the pH and it was maximum at pH=2 for ethane-PMO and at pH=4 for benzene-PMO. PMOs exhibited different isotherms, specifically types Langmuir L2 and L4 for ethane- and benzene-PMO, respectively. We decided to do the experiment of successive adsorptions. Ethane-1 -1 PMO showed an uptake of 416.63 µmol g whereas it was of 1729.35 µmol g for benzene-PMO (ca. 0.5 g herbicide per gram of adsorbent after successive adsorption cycles). These materials, ethane-PMO and benzene-PMO, have a high aqueous stability, high adsorption capacity (higher than activated charcoals, organohydrotalcites, clays and soils) and easy regenerability. Therefore, benzenePMO is a promising adsorbent for other complex aromatic molecules. Activated carbon is an adsorbent which is widely used in many fields. This material is not suitable for adsorbents of large molecules since it is mainly microporous. Recently, new mesoporous materials that show promising results for the adsorption of large pollutants have been developed. Ordered mesoporous polymers and carbons are an advanced class of ordered mesoporous materials, combining the high porosity properties of mesoporous materials with the physicochemical characteristics of organic polymers and carbons. Ordered mesoporous carbons were firstly synthesized by a hard template method using mesoporous silica. However, this synthesis cannot be used for large scale production because it is timeconsuming and expensive. Later, a direct synthesis method using phenolic resin as a carbon precursor was developed. Finally, our final work was focused on the preparation of two mesoporous materials ordered mesoporous polymer resin (MPR) and mesoporous carbon (MC) to be tested as regenerable adsorbents. We thank Dr. Pascal Van Der Voort and his research group at the Centre for Ordered Materials, Organometallics and Catalysis 226 Resume (COMOC), University of Ghent, Bélgica for their indispensable collaboration on the realization of this part of the study. The adsorbent MPR has been prepared by the EISA method (evaporation induced self-assembly), using resorcinol and formaldehyde under acid conditions in the presence of surfactant F127 (EO106-PO70-EO106). MC was synthesized by calcination of MPR at high temperature. A commercially available activated carbon (CC) was also used as adsorbent for comparison. The adsorbents were characterized by X-ray diffraction analysis, FT-IR spectroscopy, and nitrogen adsorption-desorption isotherms, thermogravimetric analysis, transmission and scanning electron microscopy, particle size measurement and XPS spectroscopy. We decided to study S-Metolachlor to compare with another previous adsorbents and Bentazon because is moderately toxic by ingestion and slightly toxic by dermal absorption and because it is moderately irritating to the skin, eyes, and respiratory tract as well. Bentazon is a post-emergence contact diazinone herbicide used to control annual weeds in a variety of crops, especially rice. This compound is one of the most commonly used herbicides in developed countries. Bentazon and S-Metolachlor adsorption kinetics and isotherm studies were carried out at pH 2 and 4, respectively, on these mesoporous materials as well as on a reference commercial carbon. The adsorption of Bentazon decreased when the pH increased, probably due to the increase of the electrostatic repulsion between the pesticide ion and the adsorbent surface. By contrast, the adsorption of S-Metolachor was not affected by the pH. The adsorption kinetics have been investigated by three models, the Lagergren pseudo-first-order, the pseudo-second order and intraparticle diffusion. The equilibrium experimental adsorption data were fitted to Freundlich, Langmuir, Temkin and Dubinin–Raduskevich models. The adsorption kinetics fitted better a pseudo-second-order kinetics model. Intraparticle diffusion was present in the adsorption process together with other factors. Furthermore, the isotherms for Bentazon on MPR, MC and CC and for S227 Capítulo IX Metolachor on CC were of L2 type, while the adsorption of S-Metolachlor on MPR and MC exhibited a L4 type isotherm. In the case of Bentazon, a plateau was reached for the three adsorbents. By contrast, the second plateau for S-Metolachlor on MPR and MC was not reached, indicating that the maximum capacity of these adsorbents was not achieved under these conditions. In all cases, the Langmuir model showed a much better fit. The adsorption of Bentazon could be explained as an exchange of ions, according to the Dubinin-Radushkevich model. The presence of hysteresis in desorption curves indicated in all cases irreversibility of the process. The regeneration of the adsorbents was carried out by calcination under nitrogen atmosphere because of the decomposition of these materials under air atmosphere. The results indicated that the regeneration of the mesoporous carbon was more efficient than those of the mesoporous phenolic resin and the commercial carbon after being reused several times. Another disadvantage of commercial carbon was the great loss of material that it experienced after several adsorptiondesorption cycles. In summary, calcined hydrotalcite (HT500) shows good adsorption capacity for both Nicosulfuron and Mecoprop-P due to their ionic or polar character. Besides, the mesoporous carbons allowed the removal of anionic Bentazon pesticide in aqueous solution. Moreover, nonpolar pesticides such as S-Metolachlor were adsorbed at basic pH by organohydrotalcites and at acidic pH by mesoporous carbons and PMOs. HTTDD was the best adsorbent at basic pH although HT-DDS proved to be a better option because of its greater reusability. The best adsorbents were benzene-PMOs and MC at acidic pH, although the latter showed the higher adsorption capacity. 228 CAPÍTULO X ABREVIATURAS 230 Anexos ABREVIATURAS BTEB 1,4-bis(trietoxisilil)benceno BTEE 1,2-bis(trietoxisilil)etano BTME 1,2-bis(trimetoxisilil)etano 13 resonancia magnética nuclear de C CC carbón comercial DDS anión dodecilsulfato DDT dicloro difenil tricloroetano DNOC 2 metil 4,6-nitrofenol DNP 2,4 dinitrofenol FTIR espectroscopía infrarroja por transfromada de Fourier HDLs hidróxidos dobles laminares HT500 hidrotalcita calcinada a 500°C HTDDS hidrotalcita con dodecilsulfato en la interlámina HTTDD hidrotalcita con tetradecanodiato en la interlámina MC carbón mesoporoso ordenado MPR resina mesoporosa fenólica ordenada OHT organohidrotalcita PMOs organosílicas mesoporosas periódicas PMS sílica mesoporosa periódica TDD anión tetradecanodiato C NMR 13 231 Capítulo XII TEM microscopía electrónica de transmisión TGA análisis termogravimétrico 232 CAPÍTULO XI. PUBLICACIONES E INDICIOS DE CALIDAD Publicaciones e Indicios de Calidad PUBLICACIONES QUE COMPONEN ESTA TESIS DOCTORAL E INDICIOS DE CALIDAD Los resultados obtenidos en esta Tesis Doctoral han sido recogidos en cinco artículos científicos de gran relevancia internacional en este campo de investigación (primer cuartil) y en varias comunicaciones nacionales e internacionales a congresos científicos. Las cinco publicaciones son: -Adsorption of non-ionic pesticide S-Metolachlor on layered double hydroxides intercalated with dodecylsulfate and tetradecanedioate anions. R. Otero, J.M. Fernández, M.A. Ulibarri, R. Celis, F. Bruna. Applied Clay Science 65-66 (2012) 7279. Impact Factor: 2.342 Category name: MINERALOGY 5 / 26 Q1 Category name; MATERIALS SCIENCE, MULTIDISCIPLINARY 52 / 241 Q1 Cites: 6 -Pesticides adsorption-desorption on Mg-Al mixed oxides. Kinetic modeling, competing factors and recyclability. R. Otero, J.M. Fernández, M.A. González, I. Pavlovic, M.A. Ulibarri. Chemical Engineering Journal 221 (2013) 214-221. Impact Factor: 4.058 Category name: ENGINEERING, CHEMICAL 8 / 133 Q1 Category name; ENGINEERING, ENVIRONMENTAL 7 / 46 Q1 Cites: 6 -Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas. R. Otero, D. Esquivel, M.A. Ulibarri, C. Jimenez-Sanchidrián, F. J. Romero-Salguero, J. M. Fernández. Chemical Engineering Journal 205 (2013) 205-213 Impact Factor: 4.058 Category name: ENGINEERING, CHEMICAL 8 / 133 Q1 Category name; ENGINEERING, ENVIRONMENTAL 7 / 46 Q1 Cites: 6 235 Capítulo Xi Mesoporous phenolic resin and mesoporous carbon for the removal of SMetolachlor and Bentazon herbicides. R. Otero, D. Esquivel, M.A. Ulibarri, F. J. Romero-Salguero, P. Van Der Voort, J. M. Fernández Chemical Engineering Journal 251 (2014) 92-101. Impact Factor: 4.058 Category name: ENGINEERING, CHEMICAL 8 / 133 Q1 Category name; ENGINEERING, ENVIRONMENTAL 7 / 46 Q1 Cites: 6 -Evaluation of different bridged organosilicas as efficient adsorbents for the herbicide S-metolachlor. María Isabel López, Rocío Otero, Dolores Esquivel, César Jiménez-Sanchidrian, José María Fernández and Francisco José Romero-Salguero. RSC Adv. 5 (2015), 24158–24166, Impact Factor: 3.708 Category name: CHEMISTRY, MULTIDISCIPLINARY 35 / 148 Q1 Cites: 0 Trabajos presentados en congresos nacionales o internacionales: - Adsorption of herbicide Fluometuron on layered double hydroxides intercalated by dodecylsulphate and tetradecanoate anions. Fourth International Conference on Hybrid Materials. Sitges, Spain. 2015. R. Otero, M.A. Ulibarri, J.M. Fernández. - Adsorption of the herbicide Fluometuron on periodic mesoporous organosilicas. Fourth International Conference on Hybrid Materials. Sitges, Spain. 2015. Rocío Otero, Dolores Esquivel; César Jiménez-Sanchidrián; Francisco J. Romero-Salguero; Jose Maria Fernández-Rodríguez. - Adsorption of the herbicide Fluometuron on periodic mesoporous organosilicas .Nanouco V. Encuentro sobre Nanociencia y Nanotecnología de Investigadores Andaluces. Campus de Rabanales. Universidad de Córdoba, Córdoba. 2015. Rocío 236 Publicaciones e Indicios de Calidad Otero, Dolores Esquivel; César Jiménez-Sanchidrián; Francisco J. Romero-Salguero; Jose Maria Fernández-Rodríguez. - Organosílices mesoporosas periódicas (PMOs) como materiales adsorbentes de contaminantes orgánicos. Nanouco IV. Encuentro sobre Nanociencia y Nanotecnología de Investigadores Andaluces. Campus de Rabanales. Universidad de Córdoba, Córdoba. 2013. Rocío Otero, Dolores Esquivel, M. Ángeles Ulibarri, César Jiménez-Sanchidrián, Francisco J. Romero-Salguero and José M. Fernández. - Incorporación de herbicidas en hidrotalcitas modificadas. Nanouco III. Encuentro sobre Nanociencia y Nanotecnología de Investigadores Andaluces. Campus de Rabanales. Universidad de Córdoba, Córdoba. 2011. Rocío Otero Izquierdo; Mª Del Rosario Pérez Pérez; Chaara, Dikra; Rosalia Extremera Jaen; Jose Maria Fernandez Rodriguez; Cristobalina Barriga Carrasco; Maria Angeles Ulibarri Cormenzana; Ivana Pavlovic Milicevic. - Sorption of pesticides nicosulfuron and mecoprop-p by layered double hydroxides. Hybrid Materials 2011. Strasbourg, France, 2011. Rocío Otero Izquierdo; Mª Ángeles González Millán; Ivana Pavlovic Milicevic; Maria Angeles Ulibarri Cormenzana; Cristobalina Barriga Carrasco; Jose Maria Fernandez Rodriguez. - Adsorption of non-polar herbicide s-metolachlor on layered double hydroxides intercalated by dodecylsulphate and tetradecanoate anions. Hybrid Materials. Strasbourg, France, 2011. Rocío Otero Izquierdo; Ivana Pavlovic Milicevic; Maria Ángeles Ulibarri Cormenzana; Cristobalina Barriga Carrasco; Jose Maria Fernandez Rodríguez. - Hidrotalcitas: Aplicaciones en Prevención y Remediación Ambiental. I Jornadas del Campus de Excelencia Internacional Agroalimentario. Córdoba, 2010. Maria Angeles Ulibarri Cormenzana; Cristobalina Barriga Carrasco; Jose Maria Fernandez Rodriguez; Ivana Pavlovic Milicevic; Mª Del Rosario Pérez Pérez; Rocío Otero Izquierdo; Rosalia Extremera Jaen. 237 Capítulo Xi - Hidróxidos Dobles Laminares como Materiales Adsorbentes de Contaminantes en Aguas. Nanouco (II Encuentro sobre Nanociencia y Nanotecnología de Investigadores y Tecnólogos de la Universidad de Córdoba). Córdoba, 2010. Rocío Otero Izquierdo; Rosalia Extremera Jaen; Chaara-Dikra; Felipe Bruna Gonzalez; Mª Del Rosario Pérez Pérez; Ivana Pavlovic Milicevic; Cristobalina Barriga Carrasco; Maria Angeles Ulibarri Cormenzana; Jose Maria Fernandez Rodriguez. - Uptake of Pesticides Fluometuron and Nicosulfuron by Layered Double Hydroxide. 2010 SEA-CSSJ-CMS TRILATERAL MEETING ON CLAYS. SEVILLA (ESPAÑA), 2010. Rocío Otero Izquierdo; Mª Del Rosario Pérez Pérez; Cristobalina Barriga Carrasco; Maria Angeles Ulibarri Cormenzana; Ivana Pavlovic Milicevic; Jose Maria Fernand 238 Applied Clay Science 65-66 (2012) 72–79 Contents lists available at SciVerse ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay Research Paper Adsorption of non-ionic pesticide S-Metolachlor on layered double hydroxides intercalated with dodecylsulfate and tetradecanedioate anions R. Otero a, J.M. Fernández a,⁎, M.A. Ulibarri a, R. Celis b, F. Bruna b a Departamento de Química Inorgánica e Ingeniería Química. Instituto Universitario de Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba, Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain b Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS), CSIC, Avenida Reina Mercedes 10, Apartado 1052, 41080 Sevilla, Spain a r t i c l e i n f o Article history: Received 27 January 2012 Received in revised form 26 April 2012 Accepted 30 April 2012 Available online xxxx Keywords: S-Metolachlor Organohydrotalcite Dodecylsufate anion Tetradecanedioate anion Adsorption Water treatmen a b s t r a c t The objective of the present research was to investigate the adsorption of non-ionic pesticide S-Metolachlor on two different organohydrotalcites (OHTs): one intercalated with dodecylsulfate (HT-DDS) and the other one with tetradecanedioate (HT-TDD) anions, both of them obtained by the coprecipitation method. The adsorbents and the adsorption products were characterized by X-ray diffraction and FT-IR spectroscopy. The results indicated that the adsorption of S-Metolachlor on the organohydrotalcite HT-TDD was higher than that on HT-DDS, but desorption was lower. The amount of S-Metolachlor adsorbed increased with temperature, especially for HT-TDD. S-Metolachlor desorption from HT-DDS and HT-TDD was characterized using two experimental procedures, which indicated the higher reversibility of S-Metolachlor adsorption from HT-DDS compared to HT-TDD. The results suggest the possibility to use both organohydrotalcites uptaking S-Metolachlor from contaminated water. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The widespread use of pesticides in modern agriculture has given rise to an increase about the environmental concern. The contamination of soil and ground water by pesticides applied to soil and swept by transport processes, such as leaching and run-off, is becoming an important environmental problem (Modabber Ahmed et al., 2007). Pesticides are very hazardous pollutants, toxic to many forms of wildlife, especially aquatic organisms, insects and mammals and can persist in the aquatic environment for many years after their application (Shukla et al., 2006). A strategy to reduce the pesticide transport losses after soil application consists of the use of controlled release formulations, in which the pesticide is gradually released over time. This limits the amount of pesticide immediately available for transport processes. Beneficial effects related to the use of controlled release formulations include reduction in rapid losses of pesticide by volatilization, runoff and leaching. The savings in manpower and energy, by reducing the number of applications required in comparison to conventional formulations, increase safety for the pesticide applicator and lead to ⁎ Corresponding author at: Dpto. Química Inorgánica e Ingeniería Química, Campus de Rabanales, Universidad de Córdoba, Córdoba, Spain. Tel.: +34 957 218648; fax: +34 957218621. E-mail address: [email protected] (JM. Fernández). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.04.022 a general decrease in non-target effects (Celis et al., 2002; Gerstl et al., 1998). Previous research has indicated that layered double hydroxides (LDHs), also known as anionic clays or hydrotalcite-like compounds (HTs), could be suitable to smooth and even prevent the environmental impact caused by pesticides, i.e. they can be used either as filters of pesticide-contaminated water, or as supports in pesticide controlled release formulations. LDHs or HTs are brucite-like layered n− ·mH2O, materials with a general formula [M1II − xMxIII(OH)2] x+Xx/n where M II and M III are divalent and trivalent cations respectively, and X n− is the interlayer anion, which balances the positive charge originated by the presence of M III in the layers. The layer charge is determined by the molar ratio x = MIII/ (MIII + MII) and can vary between 0.2 and 0.4 (Cavani et al., 1991; Costantino et al., 2009). The interest of these materials is related to their structure, high anion exchange capacity, and simplicity of their synthesis (Reichle, 1986; Rives, 2001). Similarly to clay minerals, the inorganic anions of HTs can be replaced with organic anions (Clearfield et al., 1991; Costantino et al., 2009; Meyn et al., 1990; Zhao and Vance, 1997). This simple modification changes the nature of the HT surface from hydrophilic to hydrophobic, increasing its affinity for non-ionic organic compounds, including many non-ionic pesticides (Bruna et al., 2006; Celis et al., 2000; Costa et al., 2008; Pavlovic et al., 1996; Villa et al., 1999; Wang et al., 2005). These organoclays and organo/LDHs have applications in a wide range of organic pollution control fields R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79 (Bruna et al., 2009; Celis et al., 2000; Costa et al., 2008; Frost et al., 2008; Laha et al., 2009; Zhou et al., 2008). In the present paper, we report on the adsorption of a non-ionic pesticide, S-Metolachlor, on two different organohydrotalcites (OHTs): one intercalated with dodecylsulfate (HT-DDS) and the other with tetradecanedioate (HT-TDD) anions in the interlayer space, both surfactants with the same carbon chain (twelve –CH2 units) but differing in the nature of the chemical groups bonded to the HT layers. The aim of this paper was to analyze the influence of the surfactant as well as the effect of temperature on the adsorption process of S-Metolachlor to complete the study about the behavior of this pesticide carried out previously (Chaara et al., 2011). A detailed study of the reversibility of the adsorption–desorption process was also conducted using two different experimental approaches. 2. Materials and methods 2.1. Organic anions and pesticide Dodecylsulfate (DDS) was supplied as sodium salt by Fluka, and tetradecanedioic acid (TDD) and analytical standard S-Metolachlor (2Chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl] acetamide) were provided by Sigma-Aldrich. The structure of these materials is included in the following scheme: 73 2.2. Synthesis of organohydrotalcites Two organohydrotalcites (OHTs), [Mg3Al(OH)8][(C12H25SO4)·4H2O] and [Mg3Al(OH)8][(C14H24O4)·4H2O], named HT-DDS and HT-TDD, respectively, were obtained by the coprecipitation method (Reichle, 1986). To prepare hydrotalcite with DDS in the interlayer, a solution containing 0.06 mol Mg(NO3)2·6H2O and 0.02 mol Al(NO3)3·9H2O in 100 mL of distilled water was dropped to an alkaline solution (0.16 mol of NaOH in 500 mL of water) containing 0.05 mol of dodecylsulfate (DDS). The synthesis was carried out without control of pH and at room temperature. To prepare hydrotalcite with TDD in the interlayer, a solution containing 0.06 mol Mg(NO3)2·6H2O and 0.02 mol Al(NO3)3·9H2O in 100 mL of distilled water was dropped to an alkaline solution of NaOH at pH =8, containing 0.01 mol of tetradecanedioic acid (TDD); during the addition the pH was constant and the temperature 75 °C. In both cases, the syntheses were carried out in inert atmosphere (N2 bubbling), to avoid CO2 dissolution and subsequent intercalation of carbonate into the LDH interlayer. The resultant suspensions were hydrothermally treated at 80 °C for 24 h, and the precipitate washed with CO2-free distilled water and dried at 60 °C. 2.3. Characterization of organohydrotalcites Elemental analysis of Mg and Al was performed with an AAS Perkin Elmer 3100 spectrometer after dissolving the samples in concentrate HCl, whereas S, C and N contents were determined with an elemental analyzer Eurovector 3A 2010. The water content was calculated by using a thermogravimetric analyzer SETARAM Setsys Evolution 16/18. For this purpose, 20 mg of sample was submitted to heating with a constant temperature gradient of 5 °C/min in the range of 30 to 1000 °C. Powder X-ray diffraction patterns (PXRD) were obtained on a Siemens D-5000 instrument with CuKα radiation and FT-IR spectra were recorded by using the KBr disk method on a Perkin Elmer Spectrum One spectrophotometer. Specific surface areas were measured from nitrogen adsorption/desorption isotherms on a Micromeritics ASAP 2020 instrument using N2 gas as adsorbate. The surface morphology of both samples was obtained using a scanning electron microscope JEOL JSM 6300. 2.4. Adsorption and desorption experiments S-Metolachlor, a chloroacetamide type compound with a water solubility of 480 mg/L (25 °C), is a selective herbicide applied as preand post-emergence to control grasses and some broad-leaved weeds in a wide range of crops. S-Metolachlor adsorption on OHTs was studied at different initial pH values, contact times (kinetics), and initial pesticide concentration (isotherms), by suspending duplicate samples of 20 mg of each adsorbent in 30 mL of aqueous pesticide solutions. Samples of sorbent and pesticide in water were shaken in a turn-over shaker at 52 rpm and the supernatants were centrifuged and separated to determine the concentration of S-Metolachlor by UV spectroscopy (Perkin Elmer UV-visible spectrophotometer, Model Lambda 11) at 265 nm. The amounts of pesticide adsorbed were determined from the initial and final solution concentrations. To study the adsorption at different initial pHs (5.0, 6.6 and 9.0) and contact times (1, 2, 5, 24 h), an initial pesticide concentration of 0.35 mM was used. Adsorption isotherms were obtained by the batch equilibration technique as described elsewhere (Hermosín et al., 1993; Pavlovic et al., 1997), using initial pesticide concentrations between 0.05 mM and 0.35 mM. Desorption isotherms were obtained using two different procedures: In the first one, described by Bowman (1979) and Bowman and Sans (1985), desorption was carried out immediately after adsorption from the highest equilibrium point of the adsorption isotherm. The procedure consisted of removing 15 mL of the supernatant liquid and replacing it with the same amount of distilled water. After shaking at room temperature for 24 h, the suspensions were centrifuged, and the pesticide concentration in the supernatant was determined by UV–vis spectroscopy. This process was repeated 74 R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79 four times ([Bruna et al., 2008). In the second procedure, which was a modification of that described by Yang (1974), one unique desorption point was measured from each equilibrium point of the adsorption curve by replacing a small portion of supernatant (15 mL) with the same amount of distilled water. After shaking for 24 h, the amount of pesticide desorbed was calculated as describe above. This procedure avoids the difficulties mentioned by Bowman (1979) about the procedure used by Yang (1974). Adsorption and desorption parameters were calculated using the Freundlich equation (Eq. (1)) 1=nf Cs ¼ K f Ce ð1Þ where Cs(μmol/g) is the amount of S-Metolachlor adsorbed at the equilibrium concentration Ce (μmol/L), and Kf and nf are the empirical Freundlich constants (Bowman and Sans, 1985). Hysteresis coefficients, H, were calculated according to H = nf-des/nf-ads, where nf-ads and nf-des are the Freundlich slopes of the adsorption and desorption isotherms, respectively (Barriuso et al., 1994). For the adsorbent regeneration and recyclability study, a preliminary experiment was conducted to obtain the kinetics of SMetolachlor desorption from HT-DDS and HT-TDD using water, methanol, and ethanol as extracting solutions. Desorption kinetic curves were obtained from the highest equilibrium point of the adsorption kinetic curve. The extracting volume was 30 mL and the contact times 1, 2, 5, 24, and 30 h. After shaking, the suspensions were centrifuged, the supernatant liquid was separated and evaporated, and the residue dissolved in distilled water to determine the pesticide concentration by UV–vis spectroscopy. Successive SMetolachlor/methanol adsorption–desorption cycles for HT-DDS were conducted to assess the potential recyclability of this material as an adsorbent of S-Metolachlor. 3. Results and discussion 3.1. Characterization of the organohydrotalcite sorbents 3.1.1. Elemental analysis According to the values of Mg/Al molar ratio (Table 1), a partial dissolution of Al 3 + during the synthesis of HT-DDS and Mg 2 + in HTTDD seemed to occur. This could be attributed to the differences on the synthesis conditions (temperature, pH…), as well as, to the different acid/base properties of the organic anions (pKa = 4.46 for TDD and −3.29 for DSS). The molar ratio S/Al in HT-DDS (Table 1), which corresponds to the fraction of the positive charge of HT compensated by DDS anions, indicated that most of the layer charge (91%) was compensated by DDS anions and about 9% by other anions, probably CO32 − (as it will be seen below). The elemental analysis of C in HT-TDD sample (Table 1) suggests that, if all carbon atoms were as TDD anions, only 45% of the layer charge of HT will be compensated. Therefore, a part of the charge must be balanced by other anions, probably CO32 −, and also a small amount by nitrate anions (from the metal salts) as suggested by the N content of the sample (Table 1). From the elemental analysis and the TG data to obtain the water content (not Fig. 1. XRD patterns of the organohydrotalcites and their adsorption products. (⁎diffraction peaks due to the Al sample holder). included), the formulae for both organohydrotalcites were obtained and included in Table 1. 3.1.2. X-ray diffraction study Powder XRD patterns of the synthesized organohydrotalcites were recorded (Fig. 1). In both cases the diagram corresponds to a hydrotalcite-like structure with an important expansion of the interlayer space with respect to the inorganic hydrotalcite with carbonate anion (d003 = 7.8 Å) (Cavani et al., 1991). X-ray diffraction pattern of HT-DDS sample (Fig. 1A) showed reflections (00 l) at 26.4 Å and 13.1 Å, suggesting a vertical monolayer arrangement of dodecylsulfate anions (Bruna et al., 2006; Clearfield et al., 1991). X-ray diffraction pattern of HT-TDD (Fig. 1B) showed (001) reflections at 17.5 Å and 8.5 Å suggesting a tilt position of the surfactant in the interlayer, since, according to CS Chem 3D 5.0 program, TDD height is 18.3 Å, that added to the hydrotalcite-layer thickness (4.8 Å) would give a value of 23.1 Å for a perpendicular arrangement of TDD anions. 3.1.3. Textural properties The scanning electron micrographs (Fig. 2) showed the surface morphology of HT-DDS and HT-TDD consisting of an agglomerate of particles, although slightly more porosity and less homogeneity in the HT-DDS particle size were observed. These results are in accordance with the low crystallinity observed in the XRD for both samples (Fig. 1) and with the low specific surface area values obtained by nitrogen adsorption/desorption isotherms (not shown), 5.4 m 2/g for HT-DDS and b1 m 2/g for HT-TDD. 3.2. S-Metolachlor adsorption–desorption experiments The adsorption of S-Metolachlor was not affected by the initial pH of the solution (not shown). This was attributed to the fact that the Table 1 Elemental analysis of the adsorbents and proposed formula. Adsorbent HT-DDS HT-TDD wt.% Atomic ratios Proposed formula Mg Al C S N Mg/Al S/Al 15.72 31.39 4.67 14.67 29.38 20.22 4.97 0 0 0.37 3.74 2.38 0.91 0 X = C14H24O42 −, CO32 −. [Mg0.79Al0.21(OH)2](C12H25SO4)0.20(CO3)0.005·0.56H2O [Mg0.71Al0.29(OH)2](NO3)0.01Xn0.28/n·0.75H2O R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79 75 Kf parameters were in the range 40.6–16.3, which indicated strong sorption between the sorbate and sorbent (Liu et al., 2010). The values of nf were less than 1, indicating a non-linear sorption of SMetolachlor on both samples. This behavior is similar to that observed by Bruna et al. (2008) for the adsorption of terbuthylazine on organohydrotalcites and Celis et al. (1999) for adsorption of imazamox on organoclays and organohydrotalcites. The temperature effect on the adsorption of S-Metolachlor by HTDDS (Fig. 5A) and HT-TDD (Fig. 5B) was also investigated. When the temperature was increased from 10 to 30 °C, the amount of pesticide adsorbed at the initial concentration 0.35 mM increased from 45.9% to 50.3% in HT-DDS and from 49.9% to 56.8% in HT-TDD. In the case of HT-DDS, the increase in pesticide adsorption is only observed clearly at high initial concentrations, i.e. at Ce values above 150 μmol/L. These results indicate the endothermic nature of the adsorption process, as confirmed by the calculation of the thermodynamic parameters. The change in standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) of adsorption was calculated using the following equations (Eqs. (2) and (3)) (Islam and Patel, 2007, 2011; Li et al., 2009; Özcan and Safa Özcan, 2005): ∘ ∘ ΔG ¼ ΔH −TΔS ∘ ∘ ð2Þ ∘ lnK ¼ ΔS =R−ΔH =RT Fig. 2. SEM micrographs of samples (A) HT-DDS and (B) HT-TDD. final pH values for the equilibrated suspensions were all between 9 and 10, due to the buffer effect of hydrotalcite-like compounds (Cavani et al., 1991). For that reason, the selected initial pH for all subsequent adsorption experiments was 6.6, i.e. the original pH of the water-dissolved pesticide. The kinetic study of the S-Metolachlor adsorption on HT-DDS (Fig. 3) indicated an almost instantaneous adsorption, reaching the equilibrium in 2 h, while the pesticide adsorption on HT-TDD was slower and the plateau was not achieved. Nevertheless, taking into account that in the last case the amount of pesticide adsorbed at 24 h was only 5% greater than that adsorbed at 6 h, 24 h was assumed to be sufficient to reach an apparent adsorption equilibrium. This different behavior in the adsorption kinetics could be attributed to little changes in the morphology and specific surface area of the sorbents. The adsorption isotherms were obtained by plotting the amount of S-Metolachlor adsorbed (Cs) versus the solute concentration (Ce) in the equilibrium solution and were of L-2 subtype, according to the classification of Giles et al. (1960), on both organohydrotalcites (Fig. 4). The amount of S-Metolachlor adsorbed on HT-DDS and HTTDD was very similar (e.g., between 43% and 45% at an initial herbicide concentration, Ci = 0.35 mM), suggesting a direct relation between the amount of pesticide adsorbed and the length of the surfactant alkyl chain (twelve –CH2 groups in both cases), and agree with the results of a previous work (Chaara et al., 2011) where adsorption of Metolachlor on HT-DDS was higher than that on an organohydrotalcite modified with sebacate anion (HT-SEB), with only eight –CH2 groups. The isotherms shape suggested that the Freundlich equation would be a good model to fitting, and the choice is justified by the high regression coefficients (Table 2). The Freundlich constants (Kf and nf) together with the regression coefficients (R 2) were very similar for both sorbents (Table 2). ð3Þ where K is the equilibrium constant, R the molar gas constant and T the absolute temperature. Values of ΔH° and ΔS° were calculated from the slope and intercept of Van't Hoff plots (Ln KC versus 1/T) and are summarized in Table 3. The values of K have been obtained from value of Kf from the Freundlich equation because our isotherms have been fitted to this equation. K has been recalculated to become dimensionless by multiplying it by 1000 (Milonjic, 2007). ΔG° values were calculated from Eq. (2). ΔH° values were positive in both cases, confirming the endothermic character of the S-Metolachlor adsorption process, but ΔH° for HT-TDD was an order of magnitude higher than that for HT-DDS (Table 3). Changes in ΔS were favorable for both sorbents but the ΔS° value was again higher for HT-TDD compared to HT-DDS. The larger contribution of the entropy than the enthalpy was responsible for the negative value of ΔG° in all cases, thus indicating that the adsorption process was entropically controlled. The most negative ΔG° value was observed for HT-TDD at 30 °C coincident with the greater adsorption constant. Fig. 3. Time evolution of S-Metolachlor adsorption on HT-DDS and HT-TDD (Cinitial = 0.35 mM). 76 R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79 Fig. 4. Adsorption and desorption isotherms of S-Metolachlor on HT-DDS (A, C) and HT-TDD (B, D). Desorption was performed after adsorption from the highest equilibrium point of the adsorption isotherm (A, B). Desorption was performed after each point of the adsorption isotherm (C, D). With respect to the S-Metolachlor desorption study, the desorption isotherms obtained according to the above-described first procedure (the more widely used method (Hermosín et al., 1993; Liu et al., 2010; Pavlovic et al., 2005)), for both HT-DDS and HT-TDD showed high H values, H ≫ 1 (Table 2), indicating a high reversibility of the pesticide adsorption process (Cruz-Guzmán et al., 2004) (Fig. 4A, B). The shape of the curves, with the desorption branch falling below the adsorption isotherm branch, is probably due to the experimental procedure rather than to the adsorption–desorption processes themselves (Koskinen et al., 1979) because that procedure, Table 2 Freundlich parameters for S-Metolachlor sorption (s) and desorption (d) isotherms on organohydrotalcites and hysteresis coefficient H. HT-DDS (s) HT-DDS (d*) HT-DDS (d**) HT-TDD (s) HT-TDD (d*) HT-TDD (d**) Kf nf R2 29.12–16.32 0.07–0.06 9.46–5.81 40.65–27.38 0.017–9.23 × 10− 4 7.55–1.55 0.48 ± 0.06 1.94 ± 0.27 0.61 ± 0.05 0.39 ± 0.02 2.39 ± 0.36 0.90 ± 0.19 0.995 0.973 0.990 0.998 0.967 0.914 *first desorption way. **second desorption way. H 4.04 1.27 6.13 2.31 implying successive treatments with ethanol, probably could modify the solid features. To overcome some of the limitations inherent to the successive desorption procedure, we used a second procedure to study desorption, i.e. a modification of procedure proposed by Yang (1974), where one desorption step was performed from every equilibrium point of the adsorption isotherm. Desorption curves thus obtained were very different from those obtained using the conventional successive desorption procedure (Fig. 4C, D). This procedure tries to reproduce the method carried out when N2 adsorption on any material is performed, where the hysteresis parameter close to 1 indicates high reversibility of the process and H values higher than 1 suggest irreversibility. The H value obtained for S-Metolachlor adsorbed on HT-DDS was very close to 1 (Fig. 4C), according to a high reversibility, indicating that desorption will be favored. However, the hysteresis coefficient (H) for S-Metolachlor on HT-TDD is considerably greater than 1 (2.31) (Fig. 4D), which indicates a certain no reversibility of the adsorption process. Desorption curve above the adsorption one evidences the special difficulty to remove S-Metolachlor from the substrate HT-TDD. This new experimental procedure permits to distinguish between the behavior of S-Metolachlor in both organohydrotalcites, so that we could conclude that the reversibility of S-Metolachlor adsorption is higher for HT-DDS (H = 1.27) than for HT-TDD (2.31). However, the R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79 77 Fig. 5. Adsorption isotherms of S-Metolachlor at different temperatures: (A) on HT-DDS and (B) on HT-TDD. first procedure is still useful to simulate the conditions occurring in soil when, after adsorption, a pesticide is sequentially desorbed by rainfall or irrigation water. 3.3. Sorbent regeneration and recyclability experiments These experiments consisted of successive adsorption–desorption stages of S-Metolachlor on HT-DDS and HT-TDD with the final purpose to analyze the possibility of sorbent regeneration. As a preliminary step, desorption experiments were performed with methanol, ethanol and water from the highest equilibrium point of the adsorption kinetic curves. The process was quite fast in all the cases (Fig. 6), desorption reaching the plateau in a few hours. SMetolachlor adsorbed on HT-DDS (Fig. 6A) was totally removed (100%) with ethanol and almost the same happened with methanol (92.8%) while only 25.4% was desorbed using water. However, for hydrotalcite HT-TDD (Fig. 6B) desorption was not completed with ethanol (62.7%) nor methanol (58.0%) and with water was only 20.7%. As expected, this pesticide showed more affinity for hydrophobic solvents due to its non-ionic character. According to the above results, the study of recyclability was only performed on hydrotalcite HT-DDS using ethanol as solvent, the unique case where desorption was completed. HT-DDS adsorbent allowed only a stage of adsorption–desorption with ethanol removing completely S-Metolachlor. Although the second stage was only slightly less effective in the adsorption (95%) than the first one, only 40% of the pesticide was desorbed after the second adsorption step. These results indicate that the possibility of regeneration of the sorbent with ethanol is limited, and new experiments should be necessary to find a more effective regeneration method. Table 3 Thermodynamic parameters for S-Metolachlor sorption on organohydrotalcites HTDDS and HT-TDD determined at different temperatures. Sample HT-DDS HT-TDD T (°C) 10 20 30 10 20 30 K 11000 11020 12050 14420 20320 38470 ΔS ΔH ΔG (KJ·mol− 1·K− 1) (KJ·mol− 1) (KJ·mol− 1) 0.090 3.53 0.204 35.32 −21.89 −22.67 −23.67 −22.53 −24.16 −26.60 3.4. Characterization of solids after adsorption In order to characterize the adsorption products in the “plateau” of the isotherms, powder XRD patterns and FT-IR spectra were registered to confirm the adsorption of S-Metolachlor in the hydrotalcite interlayers. After adsorption of S-Metolachlor, a change in the d00l-value was observed for both organohydrotalcites (Fig. 1C, D). The adsorption product of S-Metolachlor on HT-DDS showed a small decrease of the d003-value close to 1 Å (from 26.4 to 25.6 Å), (Fig. 1C). In contrast, S-Metolachlor adsorption on HT-TDD (Fig. 1D) led to a very important increase in the d003-value compared with the pristine adsorbent (from 17.5 to 23.7 Å). This suggests a change in the orientation of the tetradecanedioate anion from tilt position in HT-TDD to perpendicular position after the adsorption of the pesticide, thus providing strong evidence for sorbent deformation upon adsorption of S-Metolachlor. The d003-value of the HT-TDD-S-Metolachlor adsorption product was only slightly higher than the sum of the size of TDD and the thickness of the brucite layer (23.12 Å). Taking into account that TDD anions have two negative charges versus only one for DDS, half of TDD anions will be necessary to compensate the layer charge producing a lower “congestion” of the interlayer space in the last case. The higher free space in the interlayer for HT-TDD would permit S-Metolachlor to be introduced between the organic chains causing a change in the orientation of the organic molecules, reflected in the increase in d003 space showed in the Fig. 1C. These results confirm those found by Chaara et al. (2011) for Alachlor adsorption on other organohydrotalcites. Fig. 7 shows FT-IR spectra of the adsorbents, S-Metolachlor, and adsorption products. Characteristic features of the hydrotalcite structure are the broad bands around 3460 cm − 1, corresponding to the νO―H of hydration water molecules and free OH − groups of the brucite layers, and a weak band at 1650 cm − 1 corresponding to δO―H of water molecules (Fig. 7A, B). Strong peaks between 2920 cm − 1 and 2850 cm − 1 correspond to C―H stretching modes (Bruna et al., 2008)]. The most characteristic bands of HT–DDS (Fig. 7A) are those due to the antisymmetric (1211 cm − 1) and symmetric (1071 cm − 1) stretching S O vibration of the sulfate group (Bellamy, 1975; Clearfield et al., 1991). FT-IR of HT-TDD (Fig. 7B) shows characteristic bands at 1561 cm − 1 and 1421 cm − 1 corresponding to vibrations of carboxylate groups (antisymmetrical and symmetrical, respectively). The presence of the pesticide bands (1760 cm − 1 attributed to νC O of the carbonyl group, and 1113 cm − 1 corresponding to νC―O of the ether group and/or δC―H associated to the benzene ring) in the FT-IR spectra of the adsorption products in the “plateau” of the isotherms 78 R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79 Fig. 6. Desorption kinetics of S-Metolachlor with different solvents: (A) HT-DDS and (B) HT-TDD. (Fig. 7D, E) revealed the adsorption of S-Metolachlor in both organohydrotalcites. These results are similar to those observed by Bruna et al. (2006) for the adsorption of Metamitron on an organohydrotalcite with dodecylsulfate in the interlayer. 4. Conclusions Adsorption–desorption of S-Metolachlor pesticide by two organohydrotalcites (HT-DDS and HT-TDD) have been characterized. The organohydrotalcites presented an expansion of the interlayer space compared to the inorganic hydrotalcite with carbonate anion. X-ray diffraction patterns suggested a vertical monolayer arrangement of dodecylsulfate anions and a tilt position of the tetradecanedioate anions in the interlayer space. Adsorption of S-Metolachlor resulted in changes in the d00l-value for both organohydrotalcites. In particular, the adsorption of S-Metolachlor on HT-TDD led to a very important increase in the basal spacing, suggesting changes in the orientation of the tetradecanedioate anion from tilt to perpendicular position. The amount of adsorbed pesticide increased with the rise of the temperature from 10 to 30 °C, especially for HT-TDD. A new experimental procedure has been used to distinguish that the reversibility of S-Metolachlor adsorption is higher for HT-DDS than for HT-TDD. Only S-Metolachlor adsorbed on HT-DDS was possible to be removed with ethanol. Therefore, the study of recyclability was only performed on hydrotalcite HT-DDS using ethanol as solvent. The results indicated that the possibility of regeneration of the sorbent with ethanol was limited to one regeneration step, so that further experiments would be needed to find a more effective way for effective regeneration of the sorbent. Acknowledgements This work has been partially supported by grants AGL2008-04031, AGL2011-23779 and CTM2011-25325 from MICINN, cofinanced with FEDER funds, and Research Groups FQM-214 and AGR-264 of Junta de Andalucía. R. Otero also acknowledges financial support from MECERDF. We appreciate the technical assistance received in the SCAI (Universidad de Córdoba) from Electron Microscopy and Elemental Analysis Units. References Fig. 7. FT-IR spectra of (A) HT-DDS, (B) HT-TDD, (C) S-Metolachlor, (D) adsorption product HT-DDS-S-Metolachlor and (E) adsorption product HT-TDD-S-Metolachlor. Barriuso, E., Laird, D.A., Koskinen, W.C., Dowdy, R.H., 1994. Atrazine desorption from smectites. Soil Science Society of America Journal 58, 1632–1638. Bellamy, J.L., 1975. The Infrared Spectra of Complex Molecules, third ed. Chapman and Hall, New York. Bowman, B.T., 1979. Method of repeated additions for generating pesticide adsorption– desorption isotherm data. Canadian Journal of Soil Science 59, 435–437. Bowman, B.T., Sans, W.W., 1985. Effect of temperature on the water solubility of insecticides. Journal of Environmental Science and Health B20, 625–631. Bruna, F., Pavlovic, I., Barriga, C., Cornejo, J., Ulibarri, M.A., 2006. Adsorption of pesticides Carbetamide and Metamitron on organohydrotalcite. Applied Clay Science 33, 116–124. Bruna, F., Pavlovic, I., Celis, R., Barriga, C., Cornejo, J., Ulibarri, M.A., 2008. Organohydrotalcites as novel supports for the slow release of the pesticide terbuthylazine. Applied Clay Science 42, 194–200. Bruna, F., Celis, R., Pavlovic, I., Barriga, C., Cornejo, J., Ulibarri, M.A., 2009. Layered double hydroxides as adsorbents and carriers of the herbicide (4-chloro-2methylphenoxy)acetic acid (MCPA): systems Mg–Al, Mg–Fe and Mg–Al–Fe. Journal of Hazardous Materials 168, 1476–1481. Cavani, F., Trifiro, F., Vaccari, A., 1991. Hydrotalcite-type anionic clays: preparation, properties and applications. Catalysis Today 11, 173–301. Celis, R., Koskinen, W.C., Cecchi, A.M., Bresnahan, G.A., Carrisoza, M.J., Ulibarri, M.A., Pavlovic, I., Hermosín, M.C., 1999. Sorption of the ionizable pesticide imazamox by organo-clays and organohydrotalcites. Journal of Environmental Science and Health B34, 929–941. R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79 Celis, R., Koskinen, W.C., Hermosín, M.C., Ulibarri, M.A., Cornejo, J., 2000. Tridimefon intercalations with organoclays and organohydrotalcites. Soil Science Society of America Journal 64, 36–43. Celis, R., Hermosín, M.C., Cornejo, J., Carrizosa, M.J., 2002. Clay–pesticide complexes to retard picloram leaching in soil. International Journal of Environmental and Analytical Chemistry 82, 503–517. Chaara, D., Bruna, F., Ulibarri, M.A., Draoui, K., Barriga, C., Pavlovic, I., 2011. Organo/ layered double hydroxide nanohybrids used to remove non ionic pesticides. Journal of Hazardous Materials 196, 350–359. Clearfield, A., Kieke, M., Kwan, J., Colon, J.L., Wang, R.C., 1991. Intercalation of dodecylsulfate into layered double hydroxides. Journal of Inclusion Phenomena and Molecular Recognition in Chemistry 11, 361–378. Costa, F.R., Leuteritz, A., Wagenknecht, U., Jehnichen, D., Haeussler, L., Heinrich, G., 2008. Intercalation of Mg–Al layered double hydroxide by anionic surfactants: preparation and characterization. Applied Clay Science 38, 153–164. Costantino, U., Nocchetti, M., Sisani, M., Vivani, R., 2009. Recent progress in the synthesis and application of organically modified hydrotalcites. Zeitschrift fur Kristallograhie 224, 273–281. Cruz-Guzmán, M., Celis, R., Hermosín, M.C., Cornejo, J., 2004. Adsorption of the pesticide simazine by montmorillonite modified with natural organic cations. Environmental Science and Technology 38, 180–186. Frost, R., Zhou, Q., He, H., Xi, Y., 2008. An infrared study of adsorption of paranitrophenol on mono-, di- and tri-alkyl surfactant intercalated organoclays. Spectrochimica Acta Part A 69, 239–344. Gerstl, Z., Nasser, A., Mingelgrin, U., 1998. Controlled release of pesticides into soils from clay–polymer formulations. Journal of Agricultural and Food Chemistry 46 (9), 3797–3802. Giles, C.H., MacEwans, T.H., Nakhwa, N., Smith.Studies,, D., 1960. Part XI.* A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. Journal of the Chemical Society 3973–3993. Hermosín, M.C., Pavlovic, I., Ulibarri, M.A., Cornejo, J., 1993. Trichlorophenol adsorption on layered double hydroxide: a potential sorbent. Journal of Environmental Science and Health A28 (9), 1875–1888. Islam, M., Patel, R.K., 2007. Evaluation of removal efficiency of fluoride from aqueous solution using quick lime. Journal of Hazardous Materials 143, 303–310. Islam, M., Patel, R., 2011. Physicochemical characterization and adsorption behavior of Ca/Al chloride hydrotalcite-like compound towards removal of nitrate. Journal of Hazardous Materials 190, 659–668. Koskinen, W.C., O'Connor, G.A., Cheng, H.H., 1979. Characterization of hysteresis in the desorption of 2,4,5-T from soils. Soil Science Society of America Journal 43, 871–874. Laha, S., Tansel, B., Ussawarujikulchai, A., 2009. Surfactant–soil interactions during surfactant-amended remediation of contaminated soils by hydrophobic organic compounds: a review. Journal of Environmental Management 90, 95–100. Li, Y., Gaoa, B., Wub, T., Wanga, B., Li, X., 2009. Adsorption properties of aluminum magnesium mixed hydroxide for the model anionic dye Reactive Brilliant Red K-2BP. Journal of Hazardous Materials 164, 1098–1104. 79 Liu, Y., Xu, Z., Wu, X., Gui, W., Zhu, Z., 2010. Adsorption and desorption behavior of pesticide diuron on various Chinese cultivated soils. Journal of Hazardous Materials 178, 462–468. Meyn, M., Beneke, K., Lagaly, G., 1990. Anion exchange reaction of layered double hydroxides. Inorganic Chemistry 29, 5201–5207. Milonjic, S.K., 2007. A consideration of the correct calculation of thermodynamic parameters of adsorption. Journal of the Serbian Chemical Society 72, 136–137. Modabber Ahmed, K., Choong-Lyeal, C., Dong-Hoon, L., Man, P., Bu-Kug, L., Jong-Yoon, L., Jyung‐Choi, 2007. Synthesis and properties of mecoprop-intercalated layered double hydroxide. Journal of Physics and Chemistry of Solids 68, 1591–1597. Özcan, A., Safa Özcan, A., 2005. Adsorption of Acid Red 57 from aqueous solutions onto surfactant-modified sepiolite. Journal of Hazardous Materials B125, 252–259. Pavlovic, I., Ulibarri, M.A., Hermosín, M.C., Cornejo, J., 1996. Pesticide adsorption on organo-hydrotalcites. In: Ortega-Huertas, M., López-Galindo, A., Palomo-Delgado, I. (Eds.), Advances in Clay Minerals. Universidad de Granada, Granada, pp. 42–44. Pavlovic, I., Ulibarri, M.C., Hermosín, M.C., Cornejo, J., 1997. Sorption of an anionic surfactant from water by a calcined hydrotalcite-like sorbent. Fresenius Environment Bulletin 6, 266–271. Pavlovic, I., Barriga, C., Hermosín, M.C., Cornejo, J., Ulibarri, M.A., 2005. Adsorption of acidic pesticides 2,4-D, clopyralid and picloram on calcined hydrotalcite. Applied Clay Science 30, 125–133. Reichle, W.T., 1986. Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite). Solid State Ionics 22, 135–141. Rives, V., 2001. Layered Double Hydroxides: Present and Future. Nova Science Publishers, Inc., New York. Shukla, G., Kumar, A., Bhanti, M., Joseph, P.E., Taneja, A., 2006. Organochlorine pesticide contamination of ground water in the city of Hyderabad. Environmental International 32, 244–247. Villa, M.V., Sánchez-Martín, M.J., Sánchez-Camazano, M., 1999. Hydrotalcites and organohydrotalcites as sorbents for removing pesticides from water. Journal of Environmental Science and Health. Part. B 34, 509–525. Wang, B., Zhang, H., Evans, D., Duan, X., 2005. Surface modification of layered double hydroxides and incorporation of hydrophobic organic compounds. Materials Chemistry and Physics 92, 190–196. Yang, M., 1974. Processes of adsorption, desorption, degradation, volatilization and movement of O, O-diethyl O-p-nitrophenol phosphorothioate (parathion) in soils. Ph.D. Thesis. Univ. of California, Davis. Zhao, H.T., Vance, G.F., 1997. Intercalation of carboxymethyl–cyclodextrin into magnesium aluminium layered double hydroxides. Journal of the Chemical Society Dalton Transactions 11, 1961–1965. Zhou, Q., Xi, Y., He, H., Frost, R.L., 2008. Application of near infrared spectroscopy for the determination of adsorbed p-nitrophenol on HDTMA organoclay-implications for the removal of organic pollutants from water. Spectrochimica Acta Part A 69, 835–841. Chemical Engineering Journal 228 (2013) 205–213 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas Rocío Otero a, Dolores Esquivel b,1, María A. Ulibarri a, César Jiménez-Sanchidrián b, Francisco J. Romero-Salguero b,⇑, José M. Fernández a,⇑ a Departamento de Química Inorgánica e Ingeniería Química, Instituto Universitario de Investigación en Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba, Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain b Departamento de Química Orgánica, Instituto Universitario de Investigación en Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba, Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain h i g h l i g h t s g r a p h i c a l a b s t r a c t PMOs are effective materials for the adsorption of S-Metolachlor. Phenylene bridges in PMOs provide a very high adsorption capacity. Hydrophobic and aromatic interactions play a decisive role in the adsorption. The herbicide can be quantitatively recovered by simple extraction with ethanol. PMOs are more stable than mesoporous silicas in aqueous media. a r t i c l e i n f o Article history: Received 7 March 2013 Received in revised form 23 April 2013 Accepted 26 April 2013 Available online 7 May 2013 Keywords: S-Metolachlor Herbicides Ethane-PMO Benzene-PMO Adsorption Water treatment a b s t r a c t Due to their hydrophobic pore systems and high surface areas, periodic mesoporous organosilicas (PMOs) offer unique properties for use as adsorbents of organic compounds. Two organosilicas (ethane-PMO and benzene-PMO) and a silica (PMS) were synthesized following a similar procedure. The adsorption of the herbicide S-Metolachlor from an aqueous solution was then compared on the three solids. Both PMOs exhibited a higher adsorption capacity than their silica counterpart. Additionally, both PMOs showed much higher stability than the PMS, whose mesostructure was seriously damaged under aqueous conditions. Moreover, both PMOs exhibited a very different adsorption behavior. The adsorption of S-Metolachlor on each material was described by different isotherms. The amount retained depended on the pH, and was maximum at pH = 2 for ethane-PMO and at pH = 4 for benzene-PMO. Our results suggest that both hydrophobic and aromatic p–p interactions play an important role in the adsorption of this herbicide. The adsorption capacity of benzene-PMO was very high with a loading of ca. 0.5 g herbicide per gram of adsorbent after successive adsorption cycles. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction ⇑ Corresponding authors. Tel.: +34 957212065; fax: +34 957212066 (F.J. RomeroSalguero), tel.: +34 957218648; fax: +34 957580644 (J.M. Fernández) E-mail addresses: [email protected] (F.J. Romero-Salguero), [email protected] (J.M. Fernández). 1 Present address: Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281, Building S3, 9000 Ghent, Belgium. 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.04.092 The so-called periodic mesoporous organosilicas (PMOs) were firstly reported in 1999 by three different research groups which synthesized them via the condensation of bridged organosilanes of the type (R0 O)3SiARASi(OR0 )3 in the presence of a surfactant [1–3]. Unlike other hybrid materials prepared from mesoporous silicas by grafting and co-condensation, the organic fragments 206 R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213 (ARA) in PMOs materials are abundantly and homogeneously distributed through their structure and are linked by covalent bonds to two silicon units. Various R groups have been successfully incorporated into these materials, including aliphatic (ACH2A, ACH2ACH2A, ACH@CHA), aromatic (AC6H4A, AC12H8A) and even heteroaromatic fragments (AC4H2SA) [4]. Similarly to mesoporous silicas, PMOs have a mesoporous structure with a controlled pore size and high surface area. However, PMOs are more hydrophobic due to the existence of organic fragments in their framework. All these facts make them excellent candidates as adsorbents for organic molecules. In particular, ethylene- (R = ACH2ACH2A) and phenylene- (R = AC6H4A) bridged PMOs, which are very stable under hydrothermal conditions [5], can be very useful for the removal of organic pollutants from aqueous effluents. An interesting environmental application of PMOs has involved their use as adsorbents for heavy metals. It requires the presence of certain organic functionalities which can be present in the PMOs either by using specific organosilanes and/or organodisilanes for their synthesis or by introducing them later through reactions with the organic moieties present in their frameworks. Thus, PMOs with isocyanurate bridging groups were synthesized by co-condensation of TEOS, 3-mercaptopropyltrimethoxysilane and tris[3-(trimethoxysilyl)propyl]isocyanurate and their high adsorption capacity for Hg2+ ions was reported [6]. Co-condensation between 1,2-bis(triethoxysilyl)ethane and 3-(mercaptopropyl)trimethoxysilane was used to prepare several PMOs with different pore sizes depending on the surfactant added, i.e. Brij-76 and Pluronic P123, in the range of 1.4–3.4 and 4.2–6.4 nm, respectively [7]. The former were quite selective for Hg2+ ions whereas the latter exhibited a high affinity for Hg2+, Cd2+ and Cr3+ cations. More recently, different PMOs have been synthesized by co-condensation of TEOS and an organodisilane precursor containing disulfide units. They showed excellent efficiencies for the adsorption of Hg2+ ions (up to 716 mg g1) [8]. A different synthetic strategy involved the functionalization of the double bonds of an ethenylene-bridged PMO with propylthiol groups [9]. This material combines the Hg2+ adsorption ability of the thiol groups with the high stability of the PMO, thus allowing it to be reusable after multiple regeneration cycles. Very recently, Walcarius and Mercier have extensively revised the application of mesoporous silicas for the removal of organic and inorganic pollutants [10]. The use of PMO materials as adsorbents for toxic organic pollutants is limited to the work by Borghard et al. [11]. They synthesized two mesoporous polysilsesquioxanes from 1,2-bis(triethoxysilyl)ethane and 1,4-bis (trimethoxysilylethyl)benzene under alkaline conditions. Even though the latter material showed much lower specific surface area than the former one and a nonperiodic structure, it adsorbed 14 times more 4-nitrophenol than the former [12]. The different performance of both materials was explained by the existence of p–p interactions between the aromatic rings of the phenolic compounds and the arylene bridges. S-Metolachlor is a pre- and post-emergence selective herbicide used to control grasses and some broad-leaved weeds in a wide range of crops. Previous papers have reported the adsorption of S-Metolachlor by different adsorbents such as organohydrotalcites (OHTs) [13,14], soils [15], activated charcoals [16] and clays [17]. This paper reports the adsorption of S-Metolachlor on ethyleneand phenylene-bridged PMOs, i.e., ethane-PMO and benzene-PMO, respectively. The influence of different factors, such as pH and contact time, as well as successive adsorptions, adsorption/desorption experiments and regeneration of the adsorbent has been studied. For comparison, a mesoporous silica has also been used as adsorbent. 2. Materials and methods 2.1. Materials A periodic mesoporous silica (PMS) and two organosilicas, i.e., ethane-PMO and benzene-PMOs, were synthesized by using Brij-76 as surfactant under acidic conditions according to previously reported procedures (see Supporting information) [12]. Analytical standard S-Metolachlor (2 Chloro-N-(2-ethyl-6acetamide) methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl] was supplied by Sigma Aldrich (see Scheme 1). The water solubility at 25 °C is 480 mg/L. 2.2. Characterization PMOs were characterized by different techniques. X-ray powder diffraction (XRD) patterns were recorded on a Siemens D-5000 powder diffractometer (Cu Ka radiation) and FT-IR spectra were obtained with a Perkin Elmer Spectrum One spectrophotometer using KBr disc technique. N2 isotherms were determined on a Micromeritics ASAP 2010 analyzer at 196 °C. Particle size data were obtained in a 2000 Mastersizer particle size analyzer by laser diffraction technology, with a dry dispersion automatic system Venturi Scirocco 2000 of Malvern Instruments. Microstructural characterization of the materials was carried out using JEOL 1400 TEM and JEOL 2010 TEM equipments. The 29Si MAS NMR spectra were recorded at 79.49 MHz on a Bruker Avance 400 WB spectrometer at room temperature. An overall 5000 free induction decays were accumulated. The recycle time was 15 s. Chemical shifts were measured relative to a tetramethylsilane standard. UV spectroscopy (Perkin Elmer UV–Visible spectrophotometer, Model Lambda 11) at 265 nm was used to determine the concentration of S-Metolachlor. 2.3. Adsorption/desorption experiments S-Metolachlor adsorption on mesoporous materials was studied at different initial pH values, contact times (kinetics), and initial herbicide concentration (isotherms), by suspending duplicated samples of 20 mg of each adsorbent in 30 mL of aqueous herbicide solutions. Samples of sorbent and herbicide in water were shaken in a turn-over shaker at 52 rpm. To study the adsorption at different initial pHs [2.0, 3.0, 4.0, 5.0 and 6.6 (pH of herbicide solution)] and contact times (1, 2, 5, 15 and 24 h), an initial herbicide concentration of 350 lM was used. Adsorption isotherms were obtained by the batch equilibration technique. For that, the samples were equilibrated through shaking at 52 rpm for 24 h at room temperature, as described elsewhere [18,19], using initial herbicide concentrations between 1.0 104 M and 1.4 103 M. Scheme 1. Adsorption of S-Metolachlor on PMOs showing the main interactions between the herbicide and both surfaces (see text for discussion). R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213 The supernatant liquid was centrifuged and separated to determine the concentration of S-Metolachlor by UV spectroscopy. The amounts of herbicide adsorbed were determined from the initial and final solution concentrations. According to a previously described procedure [13], desorption was measured from each equilibrium point of the adsorption curve by replacing a small portion of supernatant (7.5 mL) with the same amount of distilled water. After shaking for 24 h, the amount of herbicide desorbed was calculated as indicated above. The pseudo first order [20–24] and the pseudo second order [23] models and intraparticle diffusion equation [25,26] were used with the goal at evaluating the adsorption kinetic data. log ðC s1 C st Þ ¼ log C s1 k1 t 2:303 ð1Þ t 1 1 ¼ þ t C st k2 C 2S C s2 2 ð2Þ C st ¼ kp t1=2 þ C ð3Þ where Cs1 and Cs2 are the maximum amount of pesticide adsorbed (mg g1) for first and second-kinetic order, respectively. Cst is the amount adsorbed at time t (mg g1), k1 is the pseudo first order rate constant for the adsorption process (min1), k2 is the rate constant of pseudo second order adsorption (g mg1 min1), C is the intercept, and kp is the intraparticle diffusion rate constant (mg g1 min1/2). S-Metolachlor adsorption isotherms were fitted to Langmuir (Eq. (4)), Freundlich (Eq. (5)) and Dubinin–Radushkevich (D–R) (Eq. (6)) equations: 1 1 1 1 ¼ þ Cs Cm L Ce Cm ð4Þ log C s ¼ log K f þ nf log C e ð5Þ ln C s ¼ ln qm K e2 ð6Þ 1 where Cs (lmol g ) is the amount of herbicide adsorbed per unit mass adsorbent at the equilibrium concentration Ce (lmol L1), Cm is the monolayer capacity of the adsorbent (lmol g1), and L is the Langmuir adsorption constant (L lmol1), which is related to the adsorption energy. Kf (lmol1nf Lnf g1) and nf are Freundlich parameters incorporating all factors affecting the adsorption process. qm is the theoretical adsorption capacity of the adsorbent (lmol g1) and K (mmol2 kJ2) is the constant related to adsorption energy. e is the Polanyi adsorption potential, and equal to RTln (1 + 1/Ce), where R is the universal gas constant and T is the temperature in the Kelvin scale. 2.4. Successive adsorptions They were carried out in the highest equilibrium point of the adsorption isotherm curves, by suspending the solid (adsorbentherbicide) in 30 mL of a fresh aqueous herbicide solution (1.25 103 M). The contact time was 24 h. After shaking, the suspensions were centrifuged and the herbicide concentration determined by UV–Vis spectroscopy. This process was repeated up to complete saturation of the adsorbent. 2.5. Regeneration of the adsorbent In order to test the regeneration of the adsorbent, some desorption experiments were performed with either ethanol or water 207 from the highest equilibrium point of the adsorption isotherms. The extracting volume was 30 mL and the contact time was 24 h. After shaking, the suspensions were centrifuged, the supernatant liquid was separated and evaporated (in the case of ethanol), and the residue dissolved in the same volume of distilled water to determine the herbicide concentration by UV–Vis spectroscopy. 3. Results and discussion A periodic mesoporous silica (PMS) and two organosilicas, i.e., ethane-PMO and benzene-PMO, have been tested for the adsorption of the herbicide S-Metolachlor. These three materials exhibited a low angle (1 0 0) peak and two broad and short secondorder (1 1 0) and (2 0 0) peaks at higher incidence angles indicative of materials with 2D hexagonal (P6mm) mesostructures (Fig. 1). The integrity of the organic bridges was confirmed by FTIR measurements (see Supporting information) [5]. Their N2 adsorption– desorption isotherms were of type IV with a sharp increase in the adsorption at P/P0 between 0.30 and 0.47 due to capillary condensation, typical of materials with a narrow pore size distribution in the mesopore range (Fig. 2). The N2 adsorption step was shifted to higher relative pressures following the sequence: benzenePMO < ethane-PMO < PMS, by the effect of the increase in the pore size. The main physical properties of the three adsorbents are summarized in Table 1. Their mesoporous structures were confirmed by TEM images (Fig. 3); the inset in Fig. 3B shows a similar pore size to that determined by the BJH method. In addition, the structure is very similar for PMS and organosilicas (Fig. 3A–C). On the other hand, both organosilicas showed very similar particle size distribution curves, which were centered at around 12 and 14 lm for benzene-PMO and ethane-PMO, respectively (Fig. 4). A comparison of the behavior of the three adsorbents as a function of pH (Fig. 5) revealed significant differences among them. FTIR spectra (not shown) revealed that the herbicide was intact after adsorption. In order to gain some insights into the relationship between pH and adsorption, measurements of the point of zero charge (pzc) were accomplished for the three materials. This value indicates the pH required to give zero net surface charge. Below the pzc the hydroxyl groups at the surface of an oxide become protonated and so positively charged whereas above the pzc they are deprotonated and consequently negatively charged. By following the mass titration method (Fig. 6) [27], pzc values of 3.9–4.0 were obtained for the three samples, close to those reported for silica by other authors [28]. As a consequence, the surfaces of the three materials were positively charged at pH = 2–3. This suggest that the main interactions between the adsorbents and S-Metolachlor at pH 6 3 were electrostatic, i.e., interactions between positive charges on the surface and electron-rich regions (N, O and Cl atoms) in the adsorbed molecules, as a result of which the values of adsorption for samples PMS and ethane-PMO were maximum (Fig. 5). At pH = 4 their surfaces were essentially neutral and so their interactions with S-Metolachlor became weaker. Consequently, their adsorption values decreased. Hydrophobic interactions seem to play an important role, since the amount of adsorbed herbicide was higher for ethane-PMO than for PMS throughout the pH range. This reasoning is based on the assumption that both materials possess an analogous amount of SiAOH groups. 29Si MAS NMR spectra of the three samples were recorded in order to ascertain it (Fig. 7). They allowed determining their condensation degree (Table 2), which decreased in the order PMS > ethane-PMO > benzene-PMO. In addition, the OH/Si ratio for benzene-PMO was apparently higher than for PMS and ethane-PMO. However, the organic moieties present in the framework of both organosilicas also contribute to the masses of the corresponding materials. In fact, similar relative contents of silanol 208 R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213 Fig. 1. Powder X-ray diffraction patterns of PMS (left), ethane-PMO (center) and benzene-PMO (right) before, after being in contact with the solution of herbicide at different pHs and after herbicide extraction (inset). Table 1 Physicochemical properties of periodic mesoporous organosilicas. Material aoa (nm) SBETb (m2 g1) PMS EthanePMO BenzenePMO 6.7 7.0 885 1059 6.2 1237 Vp d (cm3 g1) Dpe (nm) Wf (nm) 0 26 1.17 1.00 4.2 4.1 2.5 2.9 257 0.94 3.6 2.6 Smpc (m2 g1) p Unit-cell dimension calculated from ao = (2d100/ 3). BET specific surface area determined in the range of relative pressures from 0.05 to 0.2. c Micropore area, calculated by t-Plot method. d Single-point pore volume. e Diameter pore average size, calculated from the adsorption branch according to the BJH method. f Pore wall thickness, estimated from W = ao Dp. a b Fig. 2. Nitrogen adsorption–desorption isotherms and the corresponding pore size distributions (insets) for PMS, ethane-PMO and benzene-PMO. groups were found in the three materials when that ratio was corrected taking into account the mass of the Si unit for each material (Table 2). On the other hand, the maximum adsorption for benzene-PMO occurred at pH = 4 (Fig. 5). Considering the higher hydrophobic character of ethane-PMO than that of benzene-PMO [29], the aromatic p–p interactions between the aromatic ring of the herbicide and the phenylene bridges of the hybrid material should be crucial for the adsorption of this molecule onto benzene-PMO. Both mesoporous organosilicas exhibited a higher adsorption capacity toward the herbicide S-Metolachlor than that of the mesoporous silica, as showed by the adsorption kinetics (Fig. 8). The amount of adsorbed herbicide significantly increased from 53 lmol g1 on PMS to 127 and 135 lmol g1 on ethane-PMO and benzene-PMO, respectively. Also, both PMOs presented a higher stability under the conditions of adsorption than their silica counterpart (Fig. 1). The material PMS significantly decreased its mesostructural order as the pH of the herbicide solution increased. This effect was much less pronounced for benzene-PMO and particularly for ethane-PMO. The decrease in the intensity of the (1 0 0) peak for PMOs can be attributed to the presence of adsorbate molecules inside the pores which causes a decrease in the contrast for X-rays between the organosilica walls and the pores. In fact, both PMOs recovered their original X-rays patterns upon removing the herbicide (vide infra), what was confirmed before and after the adsorption for benzene-PMO by TEM (Fig. 3C and D), unlike PMS whose mesostructure was destroyed (Fig. 1 inset). The adsorption kinetics of the herbicide S-Metolachlor were elucidated by substituting the initial concentrations into the pseudo first order (Largergren equation) and pseudo second order adsorption rate equations, respectively. Table 3 shows the adsorption rate constants k1 and k2 calculated from the slope of each plot. The correlation coefficients R2 for the pseudo first order kinetic model were between 0.89 and 0.97 and the correlation coefficients R2 for the pseudo second order kinetic model were 0.99. Therefore, this adsorption system fitted better to a pseudo second order kinetic model [22]. R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213 209 Fig. 3. TEM images for PMS (A), ethane-PMO (B), including a TEM image of high resolution (inset), and benzene-PMO before adsorption of S-Metolachlor (C) and after desorption (D). Fig. 6. Mass titration curves for PMS, ethane-PMO and benzene-PMO. Fig. 4. Particle size distribution curves for ethane-PMO and benzene-PMO. Fig. 5. Influence of pH on the adsorption of S-Metolachlor on PMS, ethane-PMO and benzene-PMO. The mechanism behind the adsorption was studied by analyzing the kinetic results according to the Morris–Weber equation for intraparticle diffusion (Eq. (3)). The adsorption rate-determining step could therefore be either adsorption or intraparticle diffusion. Based on the applied model, a plot of the amount of pesticide adsorbed, Cst, against the square root of time, t1/2, should be linear if intraparticle diffusion was involved in the process; also, if the lines passed through the origin, then intraparticle diffusion would be the rate-determining step [21,30]. The correlation coefficients R2 of Table 3, obtained at Cst values spanning up to 24 h, suggested that the adsorption of herbicide S-Metolachlor on PMOs may involve intraparticle diffusion; however, none of the curves passed through the origin, so intraparticle diffusion cannot have been the rate-determining step and other factors may have operated simultaneously to govern the adsorption process. Kp and C were calculated from the plot of Cst versus t1/2 for each adsorbent (see Table 3). The values of kp and C for benzene-PMO were higher and lower than those for ethane-PMO, respectively, suggesting a higher contribution of the intraparticle diffusion for benzene-PMO. Adsorption isotherms were measured for both mesoporous organosilicas due to their significantly higher capacity compared 210 R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213 Fig. 7. 29 Si MAS NMR spectra for PMS, ethane-PMO and benzene-PMO. Table 2 Si MAS NMR results obtained for mesoporous materials. 29 a b c Material T1 (%) T2 (%) T3 (%) Q2 (%) Q3 (%) Q4 (%) Condensation degree (%)a OH/Sib OH/mass of the Si unitc PMS Ethane-PMO Benzene-PMO – 5.6 13.4 – 43.69 56.7 – 50.65 29.9 5.2 – – 40.8 – – 54.0 – – 87 82 72 0.51 0.55 0.83 8.5 103 8.3 103 9.2 103 P P P P Condensation degree calculated from equation D = [ (nTn) + (nQn)]/[(3 Tn) + (4 Qn)]. Content of silanol groups per silicon atom calculated from equation OH/Si = (2T1 + T2 + 2Q2 + Q3)/(T1 + T2 + T3 + Q2 + Q3 + Q4). The masses of the Si units are 60.1 (SiO2), 66.1 (SiO1.5CH2) and 90.1 (SiO1.5C3H2) for materials PMS, ethane-PMO and benzene-PMO, respectively. Fig. 8. Adsorption kinetics of S-Metolachlor on PMS, ethane-PMO and benzenePMO. to that of the PMS sample (Fig. 8). The adsorption isotherms at the pH of maximum adsorption for each material are depicted in Fig. 9. Ethane-PMO exhibited a Langmuir isotherm, specifically L2 [31]. In this case, the adsorbed molecules are most likely to be adsorbed flat (Scheme 1), according to the abovementioned electrostatic interactions. The plateau of the curve gave 255 lmol g1. By contrast, the adsorption of the herbicide on benzene-PMO followed a type L4 isotherm. It exhibited an ill-defined first plateau which suggests a close affinity of S-Metolachlor for two different regions in the surface of benzene-PMO. We tentatively assume that adsorption firstly occurred in hydrophilic regions, i.e., silanol groups, through hydrogen bonds with oxygen and nitrogen atoms of the herbicide, while the second stage would be caused by the adsorption of S-Metolachlor on hydrophobic regions, i.e., phenylene bridges, through aromatic p–p interactions (Scheme 2). However, we cannot ascertain whether a partially flat or a perpendicular orientation occurred at both stages. Also, the interaction of silanol groups (Lewis acid) of benzene-PMO and aromatic rings (Lewis base) of S-Metolachlor cannot be completely ruled out [32]. The second plateau would be reached because of the complete saturation of the hydrophobic regions. The adsorption results show that the amount of S-Metolachlor adsorbed on benzene-PMO (667 lmol g1) exceeded the amount adsorbed by activated charcoals (480–550 lmol g1) [16], organohydrotalcites (OHTs) (356–385 lmol g1) [13,14], clays (200 lmol g1) [17] and soils (50 lmol g1) [15]. The adsorption capacity of the ethane-PMO was slightly smaller than that of the organohydrotalcites (OHTs) and similar to that of clays. The desorption curves almost matched the adsorption ones for ethane-PMO (Fig. 9), thus indicating a high reversibility of the adsorption–desorption process. However, the isotherm for benzene-PMO displayed a considerable hysteresis, which must be related to the interactions between the adsorbent and S-Metolachlor. Table 3 Kinetic parameters for the adsorption of S-Metolachlor onto PMS and PMOs. Pseudo first order model k1 (min1) PMS Ethane-PMO Benzene-PMO 3 3.6 10 3.22 103 4.38 103 Cs1 (mg g1) 7.39 11.68 27.10 Pseudo second order model R2 0.89 0.97 0.97 k2 (g mg1 min1) 4 8.5 10 7.5 104 2.6 104 Intraparticle diffusion Cs2 (mg g1) R2 kp (mg g1 min1/2) C (mg g1) R2 15.80 36.76 40.98 0.99 0.99 0.99 0.25 0.35 0.60 6.48 23.75 17.31 0.78 0.90 0.96 211 R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213 Fig. 9. Adsorption and desorption isotherms of S-Metolachlor on ethane-PMO and benzene-PMO. Fig. 10. Successive adsorptions of S-Metolachlor from the highest equilibrium point of the adsorption isotherm curve for ethane-PMO and benzene-PMO. Freundlich and Langmuir equations were tested to model the adsorption isotherms in order to determine the parameters which quantify the adsorption process (Table 4). According to R2 values, the Langmuir equation was fitted much better to the experimental data than the Freundlich one. In order to predict the adsorption efficiency, a dimensionless separation factor R was determined using the following equation [33,34]: agreed with the above mentioned higher adsorption on benzenePMO than on ethane-PMO (Figs. 8 and 9). The constant K is related to the mean free energy of adsorption (E), which can be calculated by the following equation [33,36]: R¼ 1 1 þ LC i E ¼ ð2KÞ1=2 ð8Þ The mean energy provides information about the adsorption mechanism. The adsorption is physical in nature when 0 < E < 8 kJ mol1; if it is between 8 kJ mol1 and 16 kJ mol1, the adsorption is due to exchange of ions; and if the value of E is between 20 < E < 40 kJ mol1, a chemisorption occurs. The values found in the present study were 2.7 kJ mol1 for benzene-PMO and 3.3 kJ mol1 for ethane-PMO, thus pointing to a physical adsorption. Aiming at the saturation of the adsorbents, successive adsorptions were accomplished (Fig. 10). The maximum S-Metolachlor adsorption capacities for both host materials were different. Ethane-PMO was saturated after two adsorptions, being the adsorbed amount of 416.63 lmol g1, whereas benzene-PMO required four successive adsorptions, which accounted for 1729.35 lmol g1. Probably, this adsorption occurred on the exposed parts of the layer already present (Scheme 2c). Taking into account that the ð7Þ where Ci is the initial herbicide concentration. Values in the range 0 < R < 1 represent a favorable adsorption. The R values for adsorption on ethane-PMO and benzene-PMO, with initial concentrations of S-Metolachlor 100–1400 lmol L1, were found in the range of 0.94–0.51 and 0.95–0.53, respectively. These values indicate a favorable adsorption, and also allow to confirm the higher adsorption at low concentrations in both cases. In order to distinguish between physical and chemical adsorption, parameters K and qm from Dubinin–Radushkevich equation (D–R) were calculated from the slope and intercept in the plot of ln Cs versus e2, respectively [24,35]. The values of qm (Table 4) Scheme 2. Plausible model for the adsorption of S-Metolachlor onto benzene-PMO through adsorption on hydrophilic (a) and hydrophobic regions (b) of the original surface and formation of a new adsorbed layer up to complete filling of the pores (c). The polar part and the aromatic ring of the molecule are represented by a circle and a stick, respectively. The different colors depict different adsorption stages. Table 4 Freundlich, Langmuir and Dubinin–Radushkevich model parameters for the S-Metolachlor adsorption onto ethane-PMO and benzene-PMO. Freundlich Ethane-PMO Benzene-PMO Langmuir Dubinin–Radushkevich nf Kf R2 Cm (lmol g1) L R2 qm (lmol g1) K (lmol2 kJ2) R2 0.89 ± 0.11 1.63 ± 0.18 1.06–0.26 0.032–0.0036 0.95 0.95 283.3 ± 0.2 746.27 ± 0.14 6.82 104 ± 5.43 105 5.42 104 ± 2.81 105 0.98 0.98 440.26 786.83 4.6 1010 7.1 1010 0.98 0.96 212 R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213 volume of the S-Metolachlor molecule is around 875 Å3 [37], the resulting volume occupied by the adsorbed herbicide was estimated to be 0.22 and 0.91 cm3 g1 for ethane-PMO and benzenePMO, respectively. Accordingly, the pores of benzene-PMO are completely filled in with S-Metolachlor after successive adsorptions (see Table 1). This is not the case for ethane-PMO, thus remarking the key role of the phenylene bridges to the adsorption of S-Metolachlor. The high loading (up to 0.5 g/g) and slow release of S-Metolachlor from benzene-PMO make it an excellent candidate as a delivery system. Finally, some desorption experiments were performed with either ethanol or water from the highest equilibrium point of the adsorption isotherms to test the regeneration of the adsorbent. In this sense, both materials behaved similarly. S-Metolachlor completely desorbed from ethane-PMO and benzene-PMO when using ethanol. However, the herbicide was not completely removed in water. Specifically, 74.5% and 84.0% were desorbed from ethanePMO and benzene-PMO, respectively. As expected, the herbicide showed a higher affinity for the more hydrophobic solvent due to its non-ionic character. 4. Conclusion Periodic mesoporous organosilicas have unique properties to be used as adsorbents of organic molecules in virtue of their high surface area and narrow mesopore distribution, similarly to mesoporous silicas, as well as their bridges of organic nature which can interact with them. We have examined a potentially practical use of these materials with ethylene and phenylene bridges for the adsorption of a relatively complex herbicide molecule (S-Metolachlor). While ethane-PMO behaved like a mesoporous silica (PMS) but with an enhanced adsorption capacity due to its more hydrophobic character, benzene-PMO displayed a much higher herbicide retention at a pH close to its point of zero charge. Certainly, aromatic p–p interactions between phenylene bridges and S-Metolachlor should play an important role. In fact, both PMOs exhibited different isotherms, specifically types Langmuir L2 and L4 for ethane- and benzene-PMO, respectively. In addition, the latter isotherm exhibited a desorption hysteresis effect. Both PMOs were saturated by successive adsorptions. Thus, ethane-PMO showed an uptake of 416.63 lmol g1 whereas it was of 1729.35 lmol g1 for benzene-PMO. Due to its high aqueous stability, high adsorption capacity (higher than activated charcoals, organohydrotalcites, clays and soils) and easy regenerability, benzene-PMO is a promising adsorbent for other complex aromatic molecules. Acknowledgments The authors would like to thank Spain’s Ministry of Science and Innovation (Projects MAT2010-18778 and CTM2011-25325), Junta de Andalucía (Project P06-FQM-01741 and Research Groups FQM214 and FQM-346) and FEDER funds. R. Otero acknowledges Ministry of Education and Science for a research and teaching fellowship. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2013.04.092. References [1] S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, Novel mesoporous materials with a uniform distribution of organic groups and inorganic oxide in their frameworks, J. Am. Chem. Soc. 121 (1999) 9611–9614. [2] B.J. Melde, B.T. Holland, C.F. Blanford, A. Stein, Mesoporous sieves with unified hybrid inorganic/organic frameworks, Chem. Mater. 11 (1999) 3302–3308. [3] T. Asefa, M.J. MacLachlan, N. Coombs, G.A. Ozin, Periodic mesoporous organosilicas with organic groups inside the channel walls, Nature 402 (1999) 867–871. [4] F. Hoffmann, M. Cornelius, J. Morell, M. Froba, Silica-based mesoporous organic–inorganic hybrid materials, Angew. Chem. Int. Ed. 45 (2006) 3216– 3251. [5] D. Esquivel, C. Jimenez-Sanchidrian, F.J. Romero-Salguero, Comparison of the thermal and hydrothermal stabilities of ethylene, ethylidene, phenylene and biphenylene bridged periodic mesoporous organosilicas, Mater. Lett. 65 (2011) 1460–1462. [6] O. Olkhovyk, S. Pikus, M. Jaroniec, Bifunctional periodic mesoporous organosilica with large heterocyclic bridging groups and mercaptopropyl ligands, J. Mater. Chem. 15 (2005) 1517–1519. [7] Q.H. Yang, J. Liu, J. Yang, L. Zhang, Z.C. Feng, J. Zhang, C. Li, Acid catalyzed synthesis of ordered bifunctionalized mesoporous organosilicas with large pore, Micropor. Mesopor. Mater. 77 (2005) 257–264. [8] N. Hao, L. Han, Y.X. Yang, H.T. Wang, P.A. Webley, D.Y. Zhao, A metal-ionassisted assembly approach to synthesize disulfide-bridged periodical mesoporous organosilicas with high sulfide contents and efficient adsorption, Appl. Surf. Sci. 256 (2010) 5334–5342. [9] E. De Canck, L. Lapeire, J. De Clercq, F. Verpoort, P. Van Der Voort, New ultrastable mesoporous adsorbent for the removal of mercury ions, Langmuir 26 (2010) 10076–10083. [10] A. Walcarius, L. Mercier, Mesoporous organosilica adsorbents: nanoengineered materials for removal of organic and inorganic pollutants, J. Mater. Chem. 20 (2010) 4478–4511. [11] W.G. Borghard, D.C. Calabro, F.P. DiSanzo, M.M. Disko, J.W. Diehl, J.C. Fried, M.A. Markowitz, M. Zeinali, B.J. Melde, A.E. Riley, Characterization and testing of periodic mesoporous organosilicas as potential selective benzene adsorbents, Langmuir 25 (2009) 12661–12669. [12] M.C. Burleigh, S. Jayasundera, C.W. Thomas, M.S. Spector, M.A. Markowitz, B.P. Gaber, A versatile synthetic approach to periodic mesoporous organosilicas, Colloids Polym. Sci. 282 (2004) 728–733. [13] R. Otero, J.M. Fernandez, M.A. Ulibarri, R. Celis, F. Bruna, Adsorption of nonionic pesticide S-Metolachlor on layered double hydroxides intercalated with dodecylsulfate and tetradecanedioate anions, Appl. Clay Sci. 65–66 (2012) 72– 79. [14] D. Chaara, F. Bruna, M.A. Ulibarri, K. Draoui, C. Barriga, I. Pavlovic, Organo/ layered double hydroxide nanohybrids used to remove non ionic pesticides, J. Hazard. Mater. 196 (2011) 350–359. [15] Y.B. Si, K. Takagi, A. Iwasaki, D.M. Zhou, Adsorption, desorption and dissipation of metolachlor in surface and subsurface soils, Pest Manage. Sci 65 (2009) 956–962. [16] M. Bosetto, P. Arfaioli, P. Fusi, Adsorption of the herbicides alachlor and Metolachlor on 2 activated charcoals, Sci. Total Environ. 123 (1992) 101–108. [17] A. Nennemann, Y. Mishael, S. Nir, B. Rubin, T. Polubesova, F. Bergaya, H. van Damme, G. Lagaly, Clay-based formulations of metolachlor with reduced leaching, Appl. Clay Sci. 18 (2001) 265–275. [18] M.C. Hermosin, I. Pavlovic, M.A. Ulibarri, J. Cornejo, Trichlorophenol adsorption on layered double hydroxide – a potential sorbent, J. Environ. Sci. Health A 28 (1993) 1875–1888. [19] I. Pavlovic, M.A. Ulibarri, M.C. Hermosin, J. Cornejo, Sorption of an anionic surfactant from water by a calcined hydrotalcite-like sorbent, Fresen. Environ. Bull. 6 (1997) 266–271. [20] S. Lagergren, Zur theorie der sogenannten adsorption gelöster stoffe, Kungliga Svenska Vetenskapsakademiens, Handlingar 24 (4) (1898) 1–39. [21] N. Kannan, M.M. Sundaram, Kinetics and mechanism of removal of methylene blue by adsorption on various carbons – a comparative study, Dyes Pigm. 51 (2001) 25–40. [22] A. Özcan, A.S. Özcan, Adsorption of Acid Red 57 from aqueous solutions onto surfactant modified sepiolite, J. Hazard. Mater. B125 (2005) 252–259. [23] Y.S. Ho, G. McKay, Kinetic models for the sorption of dye from aqueous solution by wood, Process. Saf. Environ. 76 (1998) 183–191. [24] M. Islam, R. Patel, Physicochemical characterization and adsorption behavior of Ca/Al chloride hydrotalcite-like compound towards removal of nitrate, J. Hazard. Mater. 190 (2011) 659–668. [25] A.K. Yadav, C.P. Kaushik, A.K. Haritash, A. Kansal, N. Rani, Defluoridation of groundwater using brick powder as an adsorbent, J. Hazard. Mater. 128 (2006) 289–293. [26] W.J. Weber Jr., J.C. Morriss, Kinetics of adsorption on carbon from solution, J. Sanit. Eng. Div. Am. Soc. Civ. Eng. 89 (1963) 31–60. [27] J.S. Noh, J.A. Schwarz, Estimation of the point of zero charge of simple oxides by mass titration, J. Colloid Interface Sci. 130 (1989) 157–164. [28] J.A. Schwarz, C.T. Driscoll, A.K. Bhanot, The zero-point of charge of silicaalumina oxide suspensions, J. Colloid Interface Sci 97 (1984) 55–61. [29] P. Van Der Voort, D. Esquivel, E. De Canck, F. Goethals, I. Van Driessche, F.J. Romero-Salguero, Periodic mesoporous organosilicas: from simple to complex bridges; a comprehensive overview of functions, morphologies and applications, Chem. Soc. Rev. 42 (2013) 3913–3955. [30] J.P. Chen, S. Wu, K.H. Chong, Surface modification of a granular activated carbon by citric acid for enhancement of copper adsorption, Carbon 41 (10) (2003) 1979–1986. [31] C.H. Giles, T.H. Macewan, S.N. Nakhwa, D. Smith, Studies in adsorption. 11. A system of classification of solution adsorption isotherms, and its use in R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213 diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids, J. Chem. Soc. (1960) 3973–3993. [32] B. Camarota, Y. Goto, S. Inagaki, B. Onida, Basic sites on periodic mesoporous organosilicas investigated by XPS and in situ FTIR of adsorbed pyrrole, Langmuir 27 (2011) 1181–1185. [33] K.R. Hall, L.C. Eagleton, A. Acrivos, T. Vermeule, Pore- and solid-diffusion kinetics in fixed-bed adsorption under constant-pattern conditions, Ind. Eng. Chem. Fund. 5 (1966) 212. 213 [34] X.X. Guo, F.Z. Zhang, Q. Peng, S.L. Xu, X.D. Lei, D.G. Evans, X. Duan, Layered double hydroxide/eggshell membrane: an inorganic biocomposite membrane as an efficient adsorbent for Cr(VI) removal, Chem. Eng. J. 166 (2011) 81–87. [35] M. Islam, R. Patel, Nitrate sorption by thermally activated Mg/Al chloride hydrotalcite-like compound, J. Hazard. Mater. 169 (2009) 524–531. [36] C. Namasivayam, R.T. Yamuna, Adsorption of Direct-Red-12-B by biogas residual slurry – equilibrium and rate-processes, Environ. Pollut. 89 (1995) 1–7. [37] Hyperchem 7.51, Hypercube, Inc., Gainesville, Florida, USA. Chemical Engineering Journal 221 (2013) 214–221 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Pesticides adsorption–desorption on Mg–Al mixed oxides. Kinetic modeling, competing factors and recyclability R. Otero, J.M. Fernández ⇑, M.A. González, I. Pavlovic, M.A. Ulibarri Departamento de Química Inorgánica e Ingeniería Química and Instituto Universitario de Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba and Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain h i g h l i g h t s g r a p h i c a l a b s t r a c t " Adsorption of Nicosulfuron and " " " " Mecoprop-P on calcined hydrotalcite (HT500). Kinetic data showed that their adsorption involves intraparticle diffusion. The adsorption is favorable and occurs via an exchange of ions. The adsorption is affected by the presence of ions such as 2 2 PO3 4 ; SO4 ; and CO3 . The extraction of pesticides with CO2 3 suggests that the adsorbent can be recycled. a r t i c l e i n f o Article history: Received 27 December 2012 Received in revised form 5 February 2013 Accepted 5 February 2013 Available online 13 February 2013 Keywords: Nicosulfuron Mecoprop-P Adsorption Hydrotalcite Water treatment a b s t r a c t The adsorption of the ionizable pesticide Nicosulfuron and the ionic pesticide Mecoprop-P on a calcined Mg–Al hydrotalcite (HT-500) in the presence and absence of various anions was studied with a view to assessing the potential of regenerating the adsorbent. Kinetic tests showed the adsorption of both pesticides to involve intraparticle diffusion, and the rate constant of intraparticle diffusion for Nicosulfuron to increase with increasing concentration of the compound by effect of its increased molecular size relative to Mecoprop-P. Application of the Langmuir and Dubini–Radushkevich equations to the adsorption isotherms for the two pesticides revealed easy adsorption of both via an ion-exchange process. Adsorption and desorption of the two pesticides were both affected by the presence of other anions. The adsorbents and their adsorption products were characterized by X-ray diffraction and FT-IR spectroscopies. Based on the results of regeneration tests, calcined hydrotalcite is an effective adsorbent for the removal of Nicosulfuron and Mecoprop-P from contaminated water. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Pesticides are very hazardous pollutants that can persist in the aquatic environment for many years [1]. Contamination of soil and ground water by pesticides applied to soil and swept by transport ⇑ Corresponding author. Tel.: +34 957 218648; fax: +34 957 218621. E-mail address: [email protected] (J.M. Fernández). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.02.007 processes such as leaching or runoff is a posing an increasingly serious environmental problem. Some researchers have suggested that the use of layered double hydroxides (LDHs), also known as ‘‘anionic clays’’ or ‘‘hydrotalcitelike compounds’’ (HTs), as filters for pesticide-contaminated water or additives in controlled-release pesticide formulations might be effective toward partially or completely avoiding the environmental impact of pesticides [2–4]. LDHs are brucite-like layered matexþ n II rials of general formula ½M II1x M III x ðOHÞ2 X x=n mH2 O, where M 215 R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221 and MIII are a divalent and a trivalent cation, respectively, and Xn is an interlayer anion countering the positive charge arising from the presence of MIII in the layers. Layer charge in an LDH is dictated by the mole ratio MIII/(MIII + MII), which typically ranges from 0.2 to 0.4 [5,6]. The main interest of these materials lies in their structure, high anion-exchange capacity and straightforward synthesis [3,7– 9]. The adsorption efficiency of LDHs is strongly affected by the properties of their interlayer anions [10]. Thus, LDHs usually have a greater affinity for anions with a high charge density and hence tend to adsorb multivalent anions easily in relation to monovalent ions [11,12]. In addition, LDHs exhibit preferential affinity for CO2 3 , which usually hinders further ion-exchange ions can be removed by heating [13]. However, interlayer CO2 3 LDH at about 500 °C to obtained calcined LDH (HT500), which can regain the original LDH structure in the presence of dissolved anions [14–16]: NICOSULFURON H 3C O N H3C O O O O N O N H CH3 N S N H CH3 N MECOPROP-P CH3 Cl O CO2H Mg3 AlO4 ðOHÞ þ 4H2 O þ ð1=nÞX n ! ½Mg3 AlðOHÞ8 X n 1=n þ OH where Xn is the anionic species present in solution. The anion-exchange capacity (AEC) of HT500 was much higher than that of the original LDH (5.8 vs 3.3 mmol g1) [17]; this allows the adsorption efficiency of an LDH to be significantly increased by calcination [16,18]. The influence of various common inorganic anions on pesticide adsorption is one other major factor to be considered since some such anions are often encountered in pesticide-contaminated waters [10]. Several author have studied different properties of Mecropop and Nicosulfuron pesticides such as their degradation or its effect in the grown of different vegetables. It had been studied adsorption and desorption of Nicosulfuron in soils [19,20] and clay minerals [21]. Furthermore, it had been studied the adsorption of Mecoprop on calcareous and organic matter amended soils [22], in clays [23] or onto Iron Oxides [24]. Also, Khan et al. [25] have characterized the Mecoprop-LDH hybrid synthetized by the cooprecipitation method. In this work, we examined the adsorption of two widely used, ionizable pesticides (Nicosulfuron and Mecoprop-P) on a calcined hydrotalcite (HT500) with a view to elucidating their adsorption– desorption behavior and the way it is influenced by the presence of some anions. In addition, we subjected the hydrotalcite to repeated adsorption–calcination cycles in order to assess its reusability in the removal of Nicosulfuron and Mecoprop-P from contaminated water. CH3 2.2. Synthesis of the hydrotalcite, and characterization of the adsorbent and adsorption products A hydrotalcite intercalated with carbonate anions and designated MgAl-LDH was prepared according to Reichle [7]. To this end, a solution containing 0.75 mol of Mg(NO3)26H2O and 0.25 mol of Al(NO3)39H2O in 250 mL of distilled water was dropped over 500 mL of another containing 1.7 mol of NaOH and 0.5 mol of Na2CO3 under vigorous stirring for 2 h. The resulting suspension was hydrothermally treated at 80 °C for 24 h, and the precipitate thus formed washed with distilled water and dried at 60 °C to obtain the solid MgAl-LDH, calcination of which at 500 °C for 3 h gave an Mg–Al mixed oxide that was named HT500. The adsorbent (HT500) and adsorption products were characterized from their XRD patterns as obtained on a Siemens D5000 diffractometer using Cu Ka radiation (k = 1.54050 Å) and their FT-IR spectra as recorded by using the KBr disc technique on a Perkin Elmer spectrophotometer. The materials were also characterized microstructurally with a JEOL 200CX TEM instrument. Elemental analyses (Mg and Al) were performed by atomic absorption spectrometry on a Perkin Elmer AA-3100 instrument, and C and N contents determined with a Eurovector 3A 2010 elemental analyzer. 2.3. Adsorption–desorption tests 2. Materials and methods 2.1. Pesticides Nicosulfuron (2-[(4,6-dimethoxypyrimidin-2-ylcarbamoyl)sulfamoyl]-N,N-dimethylnicotinamide) and Mecoprop-P [(R)-2-(4chloro-o-tolyloxy)propionic acid] were both supplied as high-purity products by Sigma–Aldrich. These pesticides are typically used to control weeds in post-emergence treatments. Nicosulfuron is a sulfonylurea of molecular weight 410.41 g mol1, a high solubility in water (7500 mg L1 at 20 °C) and pKa = 4.78. Mecoprop-P is an aryloxyalkanoic acid pesticide of molecular weight 214.65 g mol 1 , water solubility 860 mg L 1 and pKa = 3.86. The structural formula of these pesticides is as follows: Kinetic adsorption and adsorption–desorption isotherms for Nicosulfuron and Mecoprop-P were obtained by using the batch equilibration method. For kinetic adsorption, 30 mg of adsorbent for Nicosulfuron and 20 mg for Mecoprop-P were equilibrated in duplicate by shaking with 30 mL of pesticide solutions containing 0.25–1.0 mM Nicosulfuron and 1.0–3.0 mM Mecoprop-P at room temperature in a turn-over shaker operating at 52 rpm for variable lengths of time. For adsorption–desorption isotherms, duplicate samples containing 30 mg of adsorbent for Nicosulfuron and 20 mg for Mecoprop-P were equilibrated by shaking with 30 mL of 0.5–1.5 mM Nicosulfuron and 0.5–3.0 mM Mecoprop-P at room temperature for 24 h [26]. The resulting supernatant was centrifuged and separated to determine the concentrations of 216 R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221 Nicosulfuron and Mecoprop-P by UV–Vis spectroscopy at 239 and 285 nm, respectively, on a Perkin Elmer Lambda 11 UV–Vis spectrophotometer. Desorption was measured at each equilibrium point in the adsorption curve by replacing a small portion of supernatant (15 mL for Nicosulfuron and 7.5 mL for Mecoprop-P) with an identical volume of distilled water. After shaking for 24 h, the amount of pesticide desorbed was calculated as described above. This procedure is described in detail elsewhere [27]. Nicosulfuron and Mecoprop-P adsorption–desorption isotherms were fitted to the Langmuir [28], Freundlich [29] and Dubinin–Radushkevich (D–R) [30] equations: Ce 1 Ce ¼ þ Cs Cm L Cm ð1Þ C s ¼ K f C s1=nf ð2Þ ln C s ¼ ln qm K e2 ð3Þ where Cs (mmol g1) is the amount of pesticide adsorbed per unit mass of adsorbent at the equilibrium concentration Ce (mmol L1); Cm is the adsorbent monolayer capacity (mmol g1); L is the Langmuir adsorption constant (L mmol1), which is related to the adsorption energy; Kf (mmol1nf Lnf g1) and nf are the Freundlich parameters incorporating all factors affecting adsorption; qm is the theoretical adsorption capacity of the adsorbent (mmol g1); K (mmol2 kJ2) is a constant related to the adsorption energy; and e, which is equal to RTln [1+(1/Ce)], is the Polanyi adsorption potential, R being the universal gas constant and T (K) the temperature. on the results, the adsorption of Nicosulfuron was not influenced by pH and that of Mecoprop-P only slightly. This led us to adopt pH 5 (i.e. that of the pesticide solution) for subsequent adsorption tests on Nicosulfuron and pH 7 (i.e. that of maximum adsorption) for those on Mecoprop-P. Adsorption measurements of the pesticides at different initial concentrations made after a variable contact time were used to calculate their adsorption rate constant, Kad, and kinetic order. In all cases, more than 50% of the adsorbed pesticide amount occurs within 5 h, and the equilibrium was reached within 24 h in both pesticides (see Fig. 1). Both exhibited increasing adsorption with increase in their initial concentration. Adsorption of Nicosulfuron and Mecoprop-P was fast at the start and slowed down as equilibrium was approached. The kinetics of adsorption of the pesticides on HT500 was examined via the kinetic order and an intraparticle diffusion kinetic model based on the following equations was used to fit the experimental results. The adsorption kinetics of the pesticides was elucidated by substituting the initial concentrations of Fig. 1 into the pseudo first-order rate equation of Largergren [31,32]: logðC se C s Þ ¼ log C se K ad t 2:303 ð4Þ where Cse is the maximum amount of pesticide adsorbed and Cs that adsorbed at time t, both in mmol g1. Plots of log (Cse–Cs) versus t spanning different time intervals were all almost linear – the fit being much better for Mecoprop-P, however-, which testifies to the applicability of the Largergren first-order rate equation here. Table 1 shows the adsorption rate constant, Kad, for the two pesticides as calculated from the slope of each plot. 2.4. Effects of competing anions 2 2 The effects of competing anions ðNO 3 ; Cl ;SO4 ; CO3 and 2 HPO4 Þ on the adsorption behavior of Nicosulfuron or MecopropP were examined by mixing variable volumes of 0.5 M solutions of the sodium salts of the anions with 50 mL of a 1 mM pesticide solution in a total volume of 100 mL. The anion/pesticide mole ratio ranged from 0 to 100. Duplicate volumes of 30 mL of these solutions were added to 30 mg of adsorbent. For desorption tests, duplicate volumes of 30 mL of the anion solutions were added to 30 mg of adsorbent containing Nicosulfuron or Mecoprop-P in proportions such that the resulting anion/adsorbed pesticide mole ratio ranged from 0 to 100. The solutions were shaken at 52 rpm for 24 h, and the pesticide concentrations in the supernatant then determined by UV–Vis spectroscopy. 2.5. Adsorbent regeneration and recyclability Duplicate amounts of 30 mg of adsorbent for Nicosulfuron and 20 mg for Mecoprop-P were equilibrated by shaking in 30 mL of solutions containing an initial pesticide concentration of 0.5 mM at room temperature for 24 h. Then, the supernatant was centrifuged and separated to determine the concentration of Nicosulfuron and Mecoprop-P by UV–Vis spectroscopy at 239 and 285 nm, respectively, the LDH-pesticide complex being calcined at 500 °C for 2 h to recover the adsorbent: HT500. The adsorbent was subjected to a total of four successive adsorption–calcination cycles. Fig. 1. Adsorption kinetics of 0.5 mM Nicosulfuron (A) and 1 mM Mecoprop-P (B) on HT500. Table 1 Rate constants (Kad) obtained from adsorption curve for HT500 with different initial concentrations of pesticide. Pesticide Initial concentration (mmol/L) Slope Intercept Kad (h1) R2 Nicosulfuron 0.25 0.50 1.00 0.139 0.130 0.137 2.396 2.384 2.722 0.320 0.300 0.317 0.982 0.972 0.992 Mecoprop-P 1.00 2.00 3.00 0.125 0.121 0.124 3.015 2.930 2.968 0.288 0.278 0.286 0.990 0.998 0.995 3. Results and discussion 3.1. Adsorption–desorption tests 3.1.1. Effect of pH and adsorption kinetics The influence of pH on Nicosulfuron and Mecoprop-P adsorption on HT500 was studied at an initial value of 5, 7 or 9. Based 217 R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221 Table 2 Intraparticle diffusion rate obtained from Weber–Morris equation for different initial concentrations of pesticides. Pesticide Initial concentration (mmol/L) Slope Intercept Kp (h1) R2 Nicosulfuron 0.25 0.50 1.00 45.242 53.211 105.721 41.696 34.395 91.729 45.242 53.211 105.721 0.998 0.960 0.970 Mecoprop-P 1.00 2.00 3.00 219.323 202.396 217.94 6.911 234.62 373.53 219.323 202.396 217.94 0.974 0.914 0.914 Fig. 3. D–R adsorption isotherms for Nicosulfuron and Mecoprop-P on HT500. Fig. 2. Adsorption and desorption isotherms for Nicosulfuron (A) and Mecoprop-P (B) on calcined hydrotalcite. The mechanism behind the adsorption of the two pesticides was elucidated by analyzing the kinetic results in the light of the Morris–Weber equation for intraparticle diffusion [32–34]: C s ¼ K p t 1=2 þ C ð5Þ where Cs is the amount of pesticide adsorbed at time t (mmol g1), Kp the intraparticle diffusion rate constant (mmol g1 h1/2), t the agitation time (h) and C the intercept. At least two different types of adsorption mechanisms were to be expected from the structural characteristics of the adsorbent and the use of stirring. One was transfer of the pesticides from the bulk solution into HT500 pores and the other adsorption at the outer surface of the hydrotalcite. The adsorption rate-determining step could therefore be either adsorption or intraparticle diffusion [35]. Based on the applied model, a plot of the amount of pesticide adsorbed, Cs, against the square root of time, t1/2, should be linear if intraparticle diffusion was involved in the process; also, if the lines passed through the origin, then intraparticle diffusion would be the rate-determining step [36,37]. The correlation coefficients R2 of Table 2, obtained at Cs values spanning up to 16 h, suggest that the adsorption of Nicosulfuron and Mecoprop-P on calcined hydrotalcite (HT500) may involve intraparticle diffusion; however, none of the curves passed through the origin, so intraparticle diffusion cannot have been the rate-determining step and other factors may have operated simultaneously to govern the adsorption process. Increasing the contact time to 24 h led to poor correlation coefficients for the intraparticle diffusion model. Therefore, adsorption of Nicosulfuron and Mecoprop-P on calcined hydrotalcite (HT500) may be followed by intraparticle diffusion for up to 16 h. Kp was calculated from the slope of a plot of Cs versus t1/2 for each pesticide (see Table 2). Whereas the values for Mecoprop-P were all very similar, those for Nicosulfuron increased with increasing pesticide concentration. Removing nitrate with a Ca/Al chloride hydrotalcite [34] was previously found to result in a similar increase in Kp with increasing concentration. The dissimilar behavior of Nicosulfuron and Mecoprop-P in this respect can be ascribed to the greater steric hindrance in the former – a result of its being a bulkier molecule. Table 3 Freundlich and Langmuir model parameters for Nicosulfuron and Mecoprop-P adsorption onto HT500. Freundlich HT500-Nicosulfuron HT500-Mecoprop-P Langmuir Dubini–Radushkevich nf Kf R2 Cm (mmol/g1) L R2 qm(mmol g1) K(mmol2 kJ2) R2 0.23 ± 0.08 0.36 ± 0.09 0.08–0.24 0.05–0.15 0.77 0.85 0.69 ± 0.01 1.4 ± 0.1 1.7 ± 2.3 4±9 0.99 0.99 2.74 3.48 3.64 103 4.07 103 0.98 0.94 218 R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221 Fig. 6. Recyclability of HT500 containing adsorbed Nicosulfuron (A) and MecopropP (B). Fig. 4. Adsorption of 0.5 mM Nicosulfuron (A) and Mecoprop-P (B) by HT500 as a function of the anion/pesticide mole ratio. thermodynamic parameters such as the energy of adsorption and the nature of host–guest interactions. Both isotherms in Fig. 2 are of the L-type in the classification of Giles [38], therefore HT500 has a high affinity for the two pesticides. An L-shaped isotherm suggests that, as substrate sites are filled, it becomes increasingly difficult for a solute molecule to find a vacant site. Based on the adsorption data, the uptake of pesticide per gram of adsorbent, Cs, was much greater for Mecoprop-P (1.21 mmol g1) than it was for Nicosulfuron (0.64 mmol g1), probably as a result of structural, size and ionic differences between the two pesticides. The Freundlich and Langmuir equations were used to model the adsorption isotherms in order to identify the specific factors governing adsorption of the two pesticides. Based on the R2 values of Table 3, the Langmuir equation fitted the experimental data much better than the Freundlich equation (see inset of Fig. 2). For this reason, the adsorption isotherms are discussed in terms of the Langmuir equation (Eq. (1)) here. The adsorption efficiency was assessed via a dimensionless separation factor R [39,40] that was calculated from: R¼ Fig. 5. Release of Nicosulfuron (A) or Mecoprop-P (B) adsorbed on HT500 by various anions at variable anion/adsorbed pesticide ratios (0.36 and 0.47 mmol g1 Nicosulfuron or Mecoprop-P adsorbed per gram of HT500). 3.1.2. Adsorption–desorption isotherms Fig. 2 shows the adsorption–desorption isotherms for Nicosulfuron and Mecoprop-P adsorbed on HT500. Adsorption isotherms are useful to determine the adsorption capabilities of adsorbents, 1 1 þ LC i ð6Þ where Ci is the initial pesticide concentration. R values from 0 to 1 denote favorable adsorption. The R values obtained at Nicosulfuron initial concentrations of 0.25, 0.5, 0.75, 1 and 1.5 mmol L1 fell in the range (31.0–7.02) 102; likewise, those for Mecoprop-P at an initial concentration of 0.5, 1, 1.5 2 or 3 mmol L–1 spanned the range (20.0–7.69) 102. These values are suggestive of favorable adsorption, especially at low adsorbate concentrations. We used the Dubinin–Radushkevich (D–R) equation (Eq. (3)) to discriminate between physical and chemical adsorption. Parameters K and qm were calculated from the slope and intercept, respectively, of a plot of ln Cs against e2 (Fig. 3). The qm values thus obtained (Table 3) were consistent with the increased adsorption of Mecoprop-P relative Nicosulfuron noted earlier (Fig. 2). Constant K provides a measure of the mean free energy of adsorption, E, which can be calculated from [39,41] E ¼ ð2KÞ1=2 ð7Þ R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221 219 presence. The effect was more marked on Mecoprop-P than on Nicosulfuron. The adverse effect on Nicosulfuron adsorption de 2 2 creased in the sequence HPO2 and 4 > CO3 NO3 Cl > SO4 2 that on Mecoprop-P in the sequence HPO4 > CO2 3 2 both with anion/pesticide ratios higher than NO 3 Cl SO4 influenced the adsorption of both 20. Therefore, HPO2 4 pesticides more markedly than did CO3 2 and SO2 4 . This is consistent with the sequences proposed by Forano et al. [6] 2 2 2 and CO2 (HPO2 4 > CrO4 > SO4 3 > SO4 > OH > F > Cl > Br > NO3 > I) from previous results of Miyata [11] and Yamaoka et al. [42]. However, You et al. [10] found SO2 4 to affect the adsorption of dicamba on a calcined hydrotalcite more markedly than did 2 HPO2 4 or CO3 . Finally, it is worth noting the little influence of SO2 4 on Nicosulfuron relative to other anions, and also the – surprisingly – extreon Mecoprop-P adsorption at an mely low interference of CO2 3 anion/pesticide ratio of 10 (see Fig. 4). Fig. 7. TEM images of HT500 before cycling (a) and after the 3rd cycle (b). The mean free energy provides information about adsorption mechanisms. Thus, adsorption is physical when 0 < E < 8 kJ/mol, due to ion-exchange if 8 < E < 16 kJ mol1 and chemical if 20 < E < 40 kJ/mol. Our values (11.72 kJ mol1 for Nicosulfuron and 11.08 kJ mol1 for Mecoprop-P) suggest that, as confirmed by the IR spectra for the adsorption products, the two pesticides were adsorbed by ion-exchange. Finally, the desorption curves closely matched the adsorption curves for both pesticides (Fig. 2), thus testifying to the high reversibility of the adsorption–desorption process. 3.1.3. Effects of competing anions on Nicosulfuron and Mecoprop-P adsorption The effect of competing anions on the adsorption of Nicosulfuron and Mecoprop-P on HT500 is illustrated in Fig. 4. With an initial concentration of 0.5 mM of both pesticides, HT500 adsorbed 0.36 mmol g1 Nicosulfuron and 0.47 mmol g–1 Mecoprop-P in the absence of competing anions, but smaller amounts in their 3.1.4. Retention of the pesticides in the presence of various anions In order to assess the stability of the host–guest complex, we measured the retention of the pesticides by the adsorbent in the presence of various anions. Desorption isotherms obtained by adding distilled water (Fig. 2) exhibited a high reversibility. The influence of different anions on the release of Nicosulfuron and Mecoprop-P previously adsorbed on HT500 is illustrated in Fig. 5. Based on the results, both pesticides were desorbed in the presence of the studied anions, albeit to different extent. The influence of increasing concentrations of dissolved anions was also studied. Goswamee et al. [16] found anions stereochemically suitable for inclusion into LDH interlayers to boost the release of existing interlayer anions. Our results suggest that Nicosulfuron and MecopropP adsorbed on HT500 can be desorbed, whether partially or fully, 2 2 into solutions containing HPO2 4 , CO3 , NO3 , Cl or SO4 ; therefore, the nature of the ionic species present in solution affects the desorption behavior of the pesticides. Also, increasing the anion/ adsorbed pesticide mole ratio increased the amount of pesticide removed. With Mecoprop-P, a ratio of 100/1 resulted in nearly quantitative removal of the host–guest complex (viz. 100%, 100%, 2 2 98%, 93% and 87% for CO2 3 ; NO3 ; Cl ; SO4 and HPO4 , respectively). Removal of the host–guest complex for Nicosulfuron was less efficient and never complete, however; thus, it peaked at only 46% with CO2 3 . The effect of the anions was different when they competed for adsorption on HT500 (Fig. 4) and when they acted by removing the pesticides from the host–guest complex (Fig. 5). These results testify to the ability of the hydrotalcite to retain Nicosulfuron and Mecoprop-P, and to remove the two pesticides from contaminated water in the presence of other anions. The high capacity of CO2 3 and NO3 to recover Mecoprop-P suggests that the adsorbent can be reused. 3.1.5. Adsorbent recyclability In order to assess its recyclability, the adsorbent was subjected to four adsorption–desorption cycles on pristine HT500. As can be seen from Fig. 6, adsorption of the two pesticides decreased after each cycle. A similar trend was previously observed by Bruna et al. [43] in successive adsorption–desorption cycles of carbetamide and metamitron on organohydrotalcites. Our results can be ascribed to erosion of the adsorbent particles as seen in the TEM images after the third adsorption cycle (Fig. 7b) relative to the pristine HT500 (Fig. 7a). 3.2. Characterization of adsorption products by X-ray diffraction and FT-IR spectroscopies Fig. 8 shows the diffraction patterns for the adsorbent (HT500) and adsorption products, namely: HT500-Nicosulfuron (0.03 g of 220 R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221 Fig. 8. XRD patterns for (a) HT500, (b) HT500-Nicosulfuron and (c) HT500-Mecoprop-P. (Diffraction peaks due to the Al sample holder). HT500 and 30 mL of 0.5 mM Nicosulfuron) and HT500-Mecoprop-P (0.02 g of HT500 and 30 mL of 1 mM Mecoprop-P). The patterns of Fig. 8a correspond to the hydrotalcite upon calcination at 500 °C and exhibit the typical peaks for an Al–Mg mixed oxide [44]. Once Nicosulfuron and Mecoprop-P were adsorbed (Fig. 8b and c, respectively), the XRD patterns were consistent with regeneration of the hydrotalcite structure; however, d00l exhibited no change with respect to the hydrotalcite with carbonate as interlayer anion (d003 = 7.8 Å). By contrast, Khan et al. [25] observed an increase in interlayer spacing (d003 = 22.54 Å) in a Mecoprop-P intercalated LDH prepared from a very high concentration of Mecoprop dissolved in 50% methanol – where the pesticide is much more readily soluble. Under our experimental conditions, the greatest pesticide adsorption amounted to only 20% of the AEC of HT500 for Mecoprop-P and 10% for Nicosulfuron; the pesticides probably adopt a flat configuration in the interlayer, on the outer surface and at the edges of hydrotalcite particles. Based on the d003 values derived from the X-ray diffraction patterns and FT-IR data (Fig. 9), most interlayer sites must have been occupied by CO2 3 or OH ions. Similar results were previously obtained with other pesticides [45]. Adsorption of Nicosulfuron and Mecoprop-P on the LDH was further confirmed by the FT-IR spectra of Fig. 9. The spectrum for the adsorbent, HT500, exhibited a broad band at 3500 cm1 corresponding to vibrations of residual OH groups in the layers in addition to the bending vibration of water at 1638 cm1 and a band at 1383 cm1 that was assigned to stretching vibrations in residual or surface carbonate ions [46]. The spectrum for the product corresponding to the last point in the adsorption isotherm, HT500-Mecoprop-P, exhibited the breathing bands for the benzene ring at 1496 and 1243 cm1, thus confirming the presence of the pesticide. The absence of a band at 1700 cm1 for unionized carboxyl groups in Mecoprop-P from the spectrum for the complex, and the presence of antisymmetric and symmetric carboxylate vibration peaks at 1600 and 1376 cm1 [47], confirmed that the pesticide was adsorbed in anionic form on HT500, as expected from the LDH regeneration mechanism established from the X-ray diffraction patterns (Fig. 8c). The presence of Nicosulfuron among the adsorption products was apparent from the characteristic bands at 1492 and Fig. 9. FT-IR spectra for (A) HT500, (B) Nicosulfuron, (C) HT-Nicosulfuron, (D) Mecoprop-P and (E) HT-Mecoprop-P. 1375 cm1 for C@C bonds in aromatic rings, and by that at 1028 cm1, due to S@O vibrations. However, the band at 1718 cm1 for mC@O in secondary amides was absent and a new band at 1608 cm–1 due to mC@O in tertiary amides was observed instead [47]. This suggests that the pesticide may intercalate into the LDH interlayer via the deprotonated nitrogen atom in the secondary amide. 4. Conclusions The adsorption–desorption behavior of the pesticides Nicosulfuron and Mecoprop-P pesticides on calcined hydrotalcite 221 R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221 (HT500) was studied. Based on kinetic data, their adsorption involves intraparticle diffusion. Only with Nicosulfuron, however, did the intraparticle diffusion rate constant increase with increasing concentration by effect of the dissimilar size of the pesticides. The adsorption isotherms for both pesticides were L-shaped, and application of the Langmuir and Dubini–Radushkevich equations revealed that adsorption was favorable and occurred by ion-exchange. The maximum amount of Mecoprop-P adsorbed was twice than of Nicosulfuron, which is consistent with the structural, size and ionization differences between the two pesticides. Competing anions (particularly HPO2 4 ) had an adverse effect on Nicosulfuron and Mecoprop-P adsorption. On the other hand, extraction of the pesticides from the host–guest complex was 2 especially effective with CO2 3 . The ability of CO3 to recover large amounts of Mecoprop-P suggests that the adsorbent can be recycled. Regeneration tests involving four adsorption–desorption cycles showed that HT500 can be reused several times to remove Nicosulfuron and Mecoprop-P, albeit with gradually decreasing efficiency. The fact that the X-ray diffraction patterns were nearly identical with those for hydrotalcite intercalated with carbonate ions suggests that the pesticides adopt a flat arrangement in the interlayer and, also, that they occupy outer surfaces and edges of the particles. Acknowledgments This work was partly supported by Spain’s Ministry of Science and Innovation (Projects AGL2008-04031-CO2-2 and CTM201125325), and cofunded by FEDER and the Andalusian Regional Government (Junta de Andalucía, Research Group FQM-214). R. Otero also acknowledges funding from MEC-ERDF. The authors wish to thank the staff at the Electron Microscopy and Elemental Analysis units of the Central Research Support Service (SCAI) of the University of Córdoba for technical assistance. References [1] G. Shukla, A. Kumar, M. Bhanti, P.E. Joseph, A. Taneja, Organochlorine pesticide contamination of ground water in the city of Hyderabad, Environ. Int. 32 (2006) 244–247. [2] J. Cornejo, R. Celis, I. Pavlovic, M.A. Ulibarri, Interactions of pesticides with clays and layered double hydroxides: a review, Clay Miner. 43 (2008) 155–176. [3] V. Rives, Layered Double Hydroxides: Present and Future, Nova Science Publishers, Inc., New York, 2001. [4] R. Rojas, Layered double hydroxides applications as sorbents for environmental remediation, in: A. Calixto, D. Analiz Griego (Eds.), Hydroxides: Synthesis, Types and Applications, Nova Science Publishers, Inc., 2012, pp. 39–79. [5] U. Costantino, M. Nocchetti, M. Sisani, R. Vivani, Recent progress in the synthesis and application of organically modified hydrotalcites, Z. Kristallogr. 224 (2009) 273–281. [6] C. Forano, T. Hibino, F. Leroux, C. Taviot-Guého, Layered double hydroxides, in: F. Bergaya, B.K.G. Theng, G. Lagaly (Eds.), Handbook of Clay Science, Elsevier Ltd., 2006, pp. 1021–1095. [7] W.T. Reichle, Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite), Solid State Ionics 22 (1986) 135–141. [8] P.S. Braterman, Z.P. Xu, F. Yarberry, Layered double hydroxides (LDHs), in: S.M. Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Layered Materials, CRC Press, 2004, pp. 373–474. [9] X. Duan, D.G. Evans, Layered double hydroxides, in: D.M.P. Mingos (Ed.), Structure and Bonding, vol. 119, Springer-Verlag, Belin Heidelberg, 2006. [10] Y. You, H. Zhao, G.F. Vance, Adsorption of dicamba (3,6-dichloro-2methoxybenzoic acid) in aqueous solution by calcined-layered double hydroxide, Appl. Clay Sci. 21 (2002) 217–226. [11] S. Miyata, Anion-exchange properties of hydrotalcite-like compounds, Clays Clay Min. 31 (4) (1983) 305–311. [12] P.K. Dutta, M. Puri, Anion exchange in lithium aluminate hydroxides, J. Phys. Chem. 93 (1989) 376–381. [13] T. Sato, T. Wakabayashi, M. Shimada, Adsorption of various anions by magnesium aluminum oxide (Mg0.7Al0.3O1.15), Ind. Eng. Chem. Prod. Res. Dev. 25 (1986) 89–92. [14] K. Chibwe, W. Jones, Intercalation of organic and inorganic anions into layered double hydroxides, J. Chem. Soc. Chem. Commun. 14 (1989) 926–927. 3þ [15] V.R.L. Constantino, T.J. Pinnavaia, Basic properties of Mg2þ 1x Alx layered double hydroxides intercalated by carbonate, hydroxide, chloride, and sulfate anions, Inorg. Chem. 34 (1995) 883–892. [16] R.L. Goswamee, P. Sengupta, K.G. Bhattacharyya, D.K. Dutta, Adsorption of Cr(VI) in layered double hydroxides, Appl. Clay Sci. 13 (1998) 21–34. [17] M.A. Ulibarri, I. Pavlovic, C. Barriga, M.C. Hermosiın, J. Cornejo, Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity, Appl. Clay Sci. 18 (2001) 17–27. [18] M.C. Hermosin, I. Pavlovic, M.A. Ulibarri, J. Cornejo, Hydrotalcites as sorbent for trinitrophenol: sorption capacity and mechanism, Water Res. 30 (1996) 171–177. [19] J.M. Gonzalez, L. Ukrainczyk, Adsorption and desorption of nicosulfuron in soils, J. Environ. Qual. 25 (6) (1996) 1186–1192. [20] J. Fan, Z. Wei, Z. ZhongMing, X. Xiang, W. JinJun, Q. Zhen, L. Ke, L. LinMing, Desorption character of nicosulfuron and effect of pH on adsoroption of nicosulfuron in soils, Southwest China J. Agric. Sci. 21 (3) (2008) 702–708. [21] L. Ukrainczyk, N. Rashid, Irreversible sorption of nicosulfuron on clay minerals, J. Agric. Food Chem. 43 (4) (1995) 855–857. [22] M. Matallo, E. Romero, F. Sánchez-Rasero, A. Peña, G. Dios, Adsorption of mecoprop and dichlorprop on calcareous and organic matter amended soils: comparative adsorption of racemic and pure enantiomeric forms, J. Environ. Sci. Health, Part B: Pesticides, Food Contam., Agric. Wastes 33 (1) (1998) 51–56. [23] P. Ye, A.T. Lemley, Adsorption effect on the degradation of carbaryl, mecoprop, and paraquat by anodic Fenton treatment in an SWy-2 montmorillonite clay slurry, J. Agric. Food Chem. 56 (21) (2008) 10200–10207. [24] L. Clausen, I. Fabricius, Atrazine, isoproturon, mecoprop, 2,4-D, and bentazone adsorption onto iron oxides, J. Environ. Qual. 30 (3) (2001) 858–869. [25] M.A. Khan, C.-L. Choi, D.-H. Lee, M. Park, B.-K. Limb, J.-Y. Leeb, J. Choi, Synthesis and properties of mecoprop-intercalated layered double hydroxide, J. Phys. Chem. Solids 68 (2007) 1591–1597. [26] M.C. Hermosin, I. Pavlovic, M.A. Ulibarri, J. Cornejo, Trichlorophenol adsorption on layered double hydroxide: a potential sorbent, J. Environ. Sci. Health A28 (9) (1993) 1875–1888. [27] R. Otero, J.M. Fernández, M.A. Ulibarri, R. Celis, F. Bruna, Adsorption of nonionic pesticide S-Metolachlor on layered double hydroxides intercalated with dodecylsulfate and tetradecanedioate anions, Appl. Clay Sci. 65–66 (2012) 72– 79. [28] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem. Soc. 40 (1918) 1361–1403. [29] H. Freundlich, Uber die adsorption in Iaosungen, Z. Phys. Chem. 57 (1906) 385–470. [30] M.M. Dubinin, L.V. Radushkevich, Equation of the characteristic curve of activated charcoal, Proceedings of the Academy of Sciences, Physical Chemistry Section, U.S.S.R. 55 (1947) 331–333. [31] S. Lagergren, Zur theorie der sogenannten adsorption gelöster stoffe, Kungliga Svenska Vetenskapsakademiens, Handlingar 24 (4) (1898) 1–39. [32] A. Özcan, A.S. Özcan, Adsorption of Acid Red 57 from aqueous solutions onto surfactant modified sepiolite, J. Hazard. Mater. B125 (2005) 252–259. [33] W.J. Weber Jr., J.C. Morriss, Kinetics of adsorption on carbon from solution, J. Sanit. Eng. Div. Am. Soc. Civ. Eng. 89 (1963) 31–60. [34] M. Islam, R. Patel, Physicochemical characterization and adsorption behavior of Ca/Al chloride hydrotalcite-like compound towards removal of nitrate, J. Hazard. Mater. 190 (2011) 659–668. [35] M. Islam, R. Patel, Nitrate sorption by thermally activated Mg/Al chloride hydrotalcite-like compound, J. Hazard. Mater. 169 (2009) 524–531. [36] N. Kannan, M.M. Sundaram, Kinetics and mechanism of removal of methylene blue by adsorption on various carbons – a comparative study, Dyes Pigm. 51 (1) (2001) 25–40. [37] J.P. Chen, S. Wu, K.H. Chong, Surface modification of a granular activated carbon by citric acid for enhancement of copper adsorption, Carbon 41 (10) (2003) 1979–1986. [38] C.H. Giles, A general treatment and classification of the solute adsorption isotherm. I. Theoretical, J. Colloid Interface Sci. 47 (3) (1974) 755–765. [39] K.R. Hall, L.C. Eagleton, A. Acrivos, T. Vermeulen, Pore and solid-diffusion kinetics in fixed-bed adsorption under constant-pattern conditions, Ind. Eng. Chem. Fund. 5 (1966) 212–223. [40] X. Guo, F. Zhang, Q. Peng, S. Xu, X. Lei, D.G. Evans, X. Duan, Layered double hydroxide/eggshell membrane: an inorganic biocomposite membrane as an efficient adsorbent for Cr(VI) removal, Chem. Eng. J. 166 (2011) 81–87. [41] C. Namasivayam, R.T. Yamuna, Adsorption of direct red 12 B by biogas residual slurry: equilibrium and rate processes, Environ. Pollut. 89 (1995) 1–7. [42] T. Yamaoka, M. Abe, M. Tsuji, Synthesis of Cu–Al hydrotalcite like compound and its ion exchange property, Mater. Res. Bull. 24 (1989) 1183–1199. [43] F. Bruna, I. Pavlovic, C. Barriga, J. Cornejo, M.A. Ulibarri, Adsorption of pesticides Carbetamide and Metamitron on organohydrotalcite, Appl. Clay Sci. 33 (2006) 116–124. [44] F.M. Labajos, V. Rives, M.A. Ulibarri, Effect of hidrothermal and thermal treatments on the physicochemical properties of Mg–Al hydrotalcite like materials, J. Mater. Sci. 27 (1992) 1546–1552. [45] I. Pavlovic, C. Barriga, C. Hermosin, J. Cornejo, M.A. Ulibarri, Adsorption of acidic pesticides 2,4-D, Clopyralid and Picloram on calcined hydrotalcite, Appl. Clay Sci. 30 (2005) 125–133. [46] F. Cavani, F. Trifiro, A. Vaccari, Hydrotalcite-type anionic clays: preparation, properties and applications, Catal. Today 11 (1991) 173–301. [47] L.J. Bellamy, The Infrared Spectra of Complex Molecules, third ed., Chapman and Hall, London, 1975. Chemical Engineering Journal 251 (2014) 92–101 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Mesoporous phenolic resin and mesoporous carbon for the removal of S-Metolachlor and Bentazon herbicides Rocío Otero a, Dolores Esquivel b, María A. Ulibarri a, Francisco J. Romero-Salguero c, Pascal Van Der Voort b,⇑, José M. Fernández a,⇑ a Departamento de Química Inorgánica e Ingeniería Química, Instituto Universitario de Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba, Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain b Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281, Building S3, 9000 Ghent, Belgium c Departamento de Química Orgánica, Instituto Universitario de Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba, Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain h i g h l i g h t s g r a p h i c a l a b s t r a c t Mesoporous carbon is an efficient adsorbent of S-Metolachlor and Bentazon. Mesoporous carbon can be reused with a small loss of adsorption capacity. Commercial carbon undergoes the highest loss of material during the regeneration. a r t i c l e i n f o Article history: Received 8 February 2014 Received in revised form 5 April 2014 Accepted 10 April 2014 Available online 24 April 2014 Keywords: S-Metolachlor Bentazon Mesoporous phenolic resin Mesoporous carbon Water treatment a b s t r a c t Two mesoporous materials, i.e., a mesoporous phenolic resin and a mesoporous carbon, were synthesized following a soft template method to test the removal of two pesticides, Bentazon and S-Metolachlor. The adsorbents were characterized by X-ray diffraction analysis, nitrogen adsorption–desorption isotherms, thermogravimetric analysis, transmission electron microscopy, particle size measurement and XPS spectroscopy. Bentazon and S-Metolachlor adsorption kinetics and isotherm studies were carried out at pH 2 and 4, respectively, on these mesoporous materials as well as on a reference commercial carbon. Freundlich, Langmuir, Dubinin–Raduskevich and Temkin models were applied to describe the adsorption behavior with the Langmuir model showing the best fit. The regeneration of the adsorbents was carried out by calcination under nitrogen atmosphere and the results indicated that the regeneration of the mesoporous carbon was more efficient than that of the mesoporous phenolic resin and the commercial carbon after being reused several times. Another disadvantage of commercial carbon was the great loss of material that it experienced after several adsorption–desorption cycles. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction ⇑ Corresponding authors. Tel.: +32 92644442; fax: +32 92644983 (P. Van Der Voort). Tel.: +34 957218648; fax: +34 957580644 (J.M. Fernández). E-mail addresses: [email protected] (P. Van Der Voort), um1feroj@ uco.es (J.M. Fernández). http://dx.doi.org/10.1016/j.cej.2014.04.038 1385-8947/Ó 2014 Elsevier B.V. All rights reserved. Currently, the pollution caused by pesticides has greatly increased due to their widespread use in agriculture. Pesticides are highly noxious, sometimes non-biodegradable and very mobile 93 R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 throughout the environment [1]. Therefore, many researchers are studying the removal of these toxic pollutants from aqueous solutions. Photocatalysis [2–4], adsorption [5–7] and electrolysis [8– 10] are some processes involved in the removal of these pollutants. The main advantages of adsorption compared to other techniques are its low cost, high removal efficiency and easy operation. Activated carbons and silica gels have been considered as excellent adsorbents for many years. These materials are not suitable as adsorbents of large molecules because they are mainly microporous [11]. Recently, new mesoporous materials that show promising results for the adsorption of large pollutants have been developed [12–17]. Ordered mesoporous materials possess tunable and uniform pore sizes, large surface areas and large pore volumes. Among them, the ordered mesoporous polymer resin and mesoporous carbon (MPR and MC) are relatively new materials that combine the high porosity of mesoporous materials with the physicochemical properties of organic polymers and carbons, respectively [18]. MC is an excellent adsorbent and therefore, as carbon is inert, stable, hydrophobic, light and presents a high affinity towards organic pollutants. Ordered mesoporous carbons were firstly synthesized by a hard template method using mesoporous silica [19–21]. However, this synthesis cannot be used for large scale production because it is time-consuming and expensive. Later, a direct synthesis method using phenolic resin as a carbon precursor was developed [22–24]. S-Metolachlor is a selective herbicide used in several crops. It is quite mobile and although it is mainly found in surface water, it can contaminate groundwater. Several authors have investigated the adsorption of this herbicide on different materials, such as activated carbon [25], soil [26], organohydrotalcites [5] or periodic mesoporous organosilicas (PMOs) [6]. Bentazon is a contact postemergency herbicide widely used in agriculture, which has a great mobility in soil and is moderately persistent in water. That is why previous papers have reported the removal of Bentazon mainly using activated carbon as adsorbent [1,27,28]. The maximum concentration of S-Metolachlor and Bentazon admitted by the World Health Organization (WHO) in drinking water are 0.01 mg/L and 0.03 mg/L, respectively. Unlike activated carbons, mesoporous phenolic resins and mesoporous carbons have been hardly used as adsorbents of pesticides. Herein we have synthesized two mesoporous materials (MPR and MC) to be tested as regenerable adsorbents for removing S-Metolachlor and Bentazon. MPR has been prepared by the EISA method (evaporation induced self-assembly) [29,30], using resorcinol and formaldehyde in the presence of surfactant F127 (EO106–PO70– EO106) under acid conditions. MC was synthesized by calcination of MPR. For comparison, a commercially available activated carbon (CC) was also used as adsorbent. The adsorbents were characterized by several structural and surface techniques. Different experiments have been carried out to study the influence of different factors, such as pH and contact time, adsorption/desorption behavior and regeneration of the adsorbents. Finally, adsorption experiments at different temperatures have been undertaken in order to determine thermodynamic parameters. [(1S)-2-methoxy-1-methylethyl]acetamide) were purchased from Sigma Aldrich. Some of their properties are included in Table 1. 2.2. Synthesis of adsorbents The synthesis of phenolic resin and mesoporous carbon was carried out using a soft-template method [18]. For that, 9 ml of 3 M HCl were added to a solution of 2.2 g of triblock copolymer Pluronic F127 and 2.2 g of resorcinol in 9 ml of ethanol. After stirring for at least 20 min [31], a formaldehyde solution (2 ml, 36 wt%) was slowly added to it. Then, the mixture was stirred for 20 min. The resultant solution was transferred to a plate to evaporate ethanol. Afterwards, the product was cured in an oven at 60 °C overnight. Finally, the solid was calcined at 380 °C under inert atmosphere to remove the template. The resulting material, a mesoporous phenolic resin, was designed as MPR. Amorphous mesoporous carbon, designed as MC, was obtained by calcination of MPR in a tubular furnace under inert atmosphere at 800 °C. 2.3. Characterization Mesoporous phenolic resin and mesoporous carbon were characterized by different techniques. Powder X-ray diffraction (XRD) patterns were recorded with a Thermo Scientific ARL X’TRA Powder X-ray Diffraction System (radiation Cu Ka generated by 45 kV and 44 mA, with slits of 2, 4, 1.5 and 0.2 for the divergence, scatter, receiving scatter and receiving slit, respectively, at 0.25° 2h min1). Nitrogen adsorption–desorption isotherms were obtained on a Belsorp-mini II gas analyser. Prior to the measurements, the samples were degassed at 120 °C for 24 h to remove adsorbed water. Thermogravimetric curves were recorded on a Setaram Setsys Evolution 16/18 apparatus undernitrogen at a heating rate of 5 °C/min. Microstructural characterization of the materials was carried out using JEOL 1400 TEM and JEOL 2010 TEM equipments. Particle sizes were measured in a Mastersizer S analyser (Malvern Instruments) using ethanol as dispersant. The samples were sonicated for 10 min before the analysis. XPS spectra were recorded with a SPECS Phoibos HAS 3500 150 MCD. The residual pressure in the analysis chamber was 5 109 Pa. The X-ray source was generated by a Mg anode (hm = 1253.6 eV) powered at 12 kV and with an emission current of 25 mA. The powdered sample was pressed and introduced into the spectrometer without previous thermal treatment. They were outgassed overnight and analyzed at room temperature. Accurate binding energies (BE) have been determined with respect to the position of the C 1s peak at 284.9 eV. The surface atomic concentration ratios were calculated using sensitivity factors from the Casa XPS element library. The potential zeta (f) has been determined with a Zetasizer Nano ZS (Malvern Instruments), with a laser of 632.8 nm, coupled to a MPT-2 autotitrator. Table 1 Properties of pesticides Bentazon and S-Metolachlor. Pesticide MW (g/mol) S (mg/L) pKa Bentazon Structure 240.3 570 3.3 S-Metolachlor 283.79 480 – 2. Experimental section 2.1. Chemicals Pluronic127 (EO106PO70EO106), resorcinol and commercial activated carbon were purchased from Sigma–Aldrich. The pesticides used as adsorbates in the experiments, Bentazon (3-(1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) and S-Metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N- 94 R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 2.4. Adsorption experiments 380 °C for 4 h; mesoporous carbon and commercial carbon were treated under nitrogen at 400 °C for 4 h. All adsorption experiments were carried out in duplicate by using the batch equilibration technique at 25 ± 1 °C. Adsorption of Bentazon (25–250 mg/L) and S-Metolachlor (125–400 mg/L) were conducted by mixing 20 mg of adsorbent with 30 mL of pesticide solution and stirring continuously the mixture for 24 h. The suspension was centrifuged and the concentration of Bentazon and S-Metolachlor were determined by UV–Vis spectroscopy at 232 nm and 265 nm, respectively, on a Perkin Elmer Lambda 11 UV–Vis spectrophotometer. The adsorption kinetics were investigated by three models, the Lagergren pseudo-first-order model [32], the pseudo-secondorder-model [33] and intraparticle diffusion model [34]. logðC s C st Þ ¼ log C s k1 t 2:303 ð1Þ t 1 1 ¼ þ t C st k2 C 2s C s ð2Þ C st ¼ kp t1=2 þ C ð3Þ where k1 is the pseudo first order rate constant for the adsorption process (h1), Cs and Cst (mg g1) are the amounts of adsorbed pesticide at equilibrium and at time t (h), respectively, k2 is the rate constant of pseudo-second-order adsorption (g mg1 h1) and kp (mg g1 h1/2) is the intraparticle diffusion rate constant. The equilibrium experimental adsorption data were fitted to Freundlich [35], Langmuir [36], Temkin [37] and Dubinin– Raduskevich [38] models and the estimated parameters calculated from the fitting results are reported in Table 4. The Freundlich model assumes that the adsorption occurs on a heterogeneous surface. The logarithmic equilibrium expression of this model is: log C s ¼ log K f þ nf log C e 1nf nf ð4Þ 1 where Kf (mg L g ) and nf are the Freundlich constants indicative of adsorption capacity and adsorption intensity, respectively. The Langmuir model describes the adsorbate–adsorbent interactions and is based on the assumption that the adsorption energy is constant and independent of the surface coverage. Its expression is: 1 1 1 ¼ þ Cs Cm Cm L Ce ð5Þ where Cm (mg g1) is the monolayer capacity of the adsorbent and L (L mg1) is the Langmuir adsorption constant. Temkin model describes the behavior of adsorption on heterogeneous surfaces. This model is based on the heat of adsorption (due to the interactions between adsorbate and adsorbent). The Temkin isotherm has generally been applied in the following form: C s ¼ BT ln K T þ BT ln C e 3. Results and discussion 3.1. Characterization of the materials The XRD-patterns of the mesoporous phenolic resin (MPR) and the mesoporous carbon (MC) exhibited a low angle (1 0 0) peak with d-spacing of 11.6 nm and 10.3 nm, respectively (Fig. 1). Additional peaks at higher incidence angles were ill-defined [39]. Their mesoporous structures were confirmed by TEM images (Fig. 2). The pore channels with hexagonal arrangement suggest an ordered mesostructure. The size of these channels was around 10 nm (see insets in Fig. 2b and c). However, no ordering was observed in the commercial carbon (Fig. 2a). Moreover, the N2 adsorption– desorption isotherms for MPR and MC were essentially of type IV (Fig. 3) with a sharp capillary condensation step at P/P0 = 0.45– 0.57, which is typical of mesoporous materials as defined by IUPAC [40]. Nevertheless, the commercial carbon exhibited a type I isotherm, which corresponds to a microporous material. MC had a slightly higher surface area than MPR. This is probably due the formation of micropores as a consequence of the decomposition of the phenolic framework of MPR to obtain the mesoporous carbon as shown in the TGA curve between 500 and 750 °C (Fig. 4). The pore size for MPR and MC was around 9 nm (inset in Fig. 3), in agreement with TEM. The main physicochemical properties of the two mesoporous adsorbents (MPR and MC) along with those of the commercial carbon are listed in Table 2. The particle size distribution curves of MPR and MC are shown in Fig. 5. MPR displayed a considerably large particle size with the main peak centered at about 250 lm. Interestingly, the distribution of particle size for MC was wider and clearly shifted to lower values (ca. 50 lm). This revealed the significant reduction of the particle size upon carbonization of the MPR to MC. The curve for CC was very broad and centered between 10 and 30 lm, showing a significantly lower particle size than MPR and MC. Surface analyses obtained by XPS for the three adsorbents are depicted in Table 2. MPR possessed a significant amount of oxygen, as expected from the composition of the polymer. The fraction of oxygen decreased as a result of the transformation of MPR into MC, which involves the deoxygenation of the phenolic groups, among other reactions. CC also presented a considerably amount of oxygen which was much higher than that of MC. The C 1s and O 1s XP spectra are represented in Fig. 6. The large FWHM (full ð6Þ where KT and BT are constants related to adsorption capacity and intensity of adsorption, respectively. The isotherm of desorption was measured at each equilibrium point in the adsorption curve by replacing 7.5 mL of supernatant with an identical volume of distilled water. After shaking for 24 h, the amount of herbicide desorbed was calculated as indicated above. This procedure is described in detail elsewhere [5]. The isotherms of adsorption were studied at three constant temperatures: 10, 20 and 30 °C for Bentazon and S-Metolachlor. To test the reversibility, the adsorbents were subjected to three adsorption–desorption cycles. The desorption from mesoporous phenolic resin was carried out by calcination under nitrogen at Fig. 1. Powder X-ray diffraction patterns of mesoporous phenolic resin (MPR) and mesoporous carbon (MC). 95 R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 Fig. 3. Nitrogen adsorption–desorption isotherms and the corresponding pore size distributions (insets) for mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC). Fig. 2. TEM images for commercial carbon (CC) (a), mesoporous phenolic resin (MPR) (b), including a TEM image of high resolution (inset), and mesoporous carbon (MC) (c), including a TEM image of high resolution (inset). Fig. 4. Thermogravimetric curves for mesoporous phenolic resin (MPR) and mesoporous carbon (MC). width at half-maximum) values of the peaks (1.4–2.7 eV for C 1s and 2.9–3.6 eV for O 1s) suggested the existence of different carbon and oxygen species. Obviously, MPR exhibited various types of carbon atoms, namely CAC, C@C and CAO. MC displayed the narrowest C 1s peak due to the major presence of aromatic carbon atoms in its framework whereas CC even showed additional small contributions at high binding energies indicative of the existence of C@O species. Similarly, the O 1s spectra confirmed the contribution of a higher variety of species in CC than in MPR and MR. Table 2 Physicochemical properties of commercial carbon, mesoporous phenolic resin and mesoporous carbon. 3.2. Adsorption kinetics of Bentazon and S-Metolachlor The adsorption of Bentazon and S-Metolachlor on the mesoporous materials and the commercial carbon was studied at different a b Sample SBET (m2 g1) Smicropores (m2 g1) Vp a (cm3 g1) d100 (A) Cb (at.%) Ob (at.%) CC MPR MC 688 503 526 406 160 294 0.41 0.63 0.49 – 116 103 85.93 82.92 95.10 14.07 14.08 4.90 Single point total adsorption. C and O determined by XPS. 96 R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 Fig. 5. Particle size distribution curves for mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC). pH values using 30 mL of either 50 mg/L Bentazon or 100 mg/L SMetolachlor and 20 mg of adsorbent (Fig. 7). An increase in the pH caused a decrease in the adsorption of Bentazon probably due to the enhancement of the electrostatic repulsion between the ionized pesticide and the adsorbent surface [1]. The point of zero charge (pzc) was determined for the three materials and revealed some differences among them (see Supporting information, Fig. SI1). The pzc value indicates the pH required to give zero net surface charge. The pHs were 5.3, 3.2 and 2.7 for commercial carbon, mesoporous carbon and phenolic resin, respectively. When the pH is higher than the pzc, the surface is negatively charged. As pH is increased, the surface is more negatively charged. On the other hand, Bentazon is a weak acid with pKa of 3.3; and at pH above the pKa, it exists predominantly in anionic form. As pH is increased, the extent of dissociation of Bentazon molecules is increased and so it becomes more negatively charged. As a result, the equilibrium adsorption of Bentazon decreased with an increase in the pH of the initial solution. This could be attributed to the increased electrostatic repulsion between Bentazon ions and the surface of the three materials [1]. Similar observations were made for the adsorption of Bentazon onto AC cloth [41]. Given the aromatic structure of Bentazon, dispersion forces should be expected between the p cloud of the adsorbents and the aromatic ring of the adsorbate. Also, some specific localized interactions might arise from the polar groups (i.e., sulfoxide and carbonyl-type) [28]. The decrease in the amount adsorbed at higher pH (anionic form predominant in solution) suggests a weaker interaction of the surfaces with deprotonated (anionic) Bentazon than with its neutral form, and consequently that the Fig. 6. XPS analysis for mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC). Fig. 7. Influence of pH on the adsorption of S-Metolachlor and Bentazon on mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC). 97 R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 Fig. 8. Adsorption kinetics of S-Metolachlor and Bentazon on mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC). adsorption is dominated by dispersive interactions between the pesticide and the adsorbent surface. Similar results on the pHdependence of Bentazon adsorption have been reported by other authors [42,43]. In addition, the relative drop in the adsorption was higher when the pH of the point of zero charge was lower (see Figs. 7 and SI1). Thus, the phenolic resin was the most affected by changes in the pH. However, the adsorption of S-Metolachor on both mesoporous materials, MPR and MC, and the commercial carbon, CC, was not affected by the pH. This could be associated to the non-ionizable nature of S-Metolachlor in the pH range under study. Indeed, the adsorption of S-Metolachlor would occur through p–p interactions between the aromatic ring of the pesticide and p-electrons of the adsorbent structure [41]. The effect of the contact time at the pH of maximum adsorption is depicted in Fig. 8. The adsorption of Bentazon and S-Metolachlor on commercial carbon was fast, reaching the equilibrium in 2 h, while their adsorption on the mesoporous materials was slower (ca. 16 h). These differences in the adsorption kinetics might be explained by the higher specific surface area (Table 2) and smaller particle size (Fig. 5) of CC with respect to both mesoporous materials. The fitting parameters for the adsorption kinetic of Bentazon and S-Metolachlor on MPR, MC and CC are listed in Table 3. The correlation coefficients R2 for the pseudo-second-order kinetics (0.999) were best fitted than for a pseudo-first-order kinetics model (0.561–0.995). Similar results were reported by Salman et al. using an activated carbon as adsorbent for two pesticides, Bentazon and Carbofuran [1]. Following the Morris–Weber equation for intraparticle diffusion (Eq. (3)), the plots of Cs versus t1/2 consist of straight lines for the pesticide adsorption on the three adsorbents. The extrapolation of these lines does not pass through the origin, suggesting that intraparticle diffusion cannot be the rate-determining step and other factors may have operated simultaneously to govern the adsorption process [44,45]. Further information about the adsorption mechanism can be obtained analyzing the adsorption isotherm for the systems under study. The isotherms for the adsorption of Bentazon and S-Metolachlor on mesoporous phenolic resin and mesoporous carbon (Fig. 9) were compared with that of commercial carbon (Fig. 10). According to Giles classification [46], the isotherms for Bentazon adsorption on MPR, MC and CC corresponded to a L2 type. Also, S-Metolachor on CC exhibited a L2 type isotherm. However, the adsorption of S-Metolachlor on MPR and MC gave a L4 type isotherm. In the case of Bentazon, the plateau was reached at Cs value of 130.74 mg g1, 166.72 mg g1 and 218.04 mg g1 for MPR, MC and CC, respectively. By contrast, the second plateau for S-Metolachlor on MPR and MC was not reached, indicating that the maximum adsorption capacity of the adsorbent was not achieved under these conditions. Similar behavior was observed in the adsorption of this pesticide on phenylene-bridged and ethylenebridged periodic mesoporous organosilicas [6]. The adsorption isotherm of S-Metolachlor on CC exhibited an ill-defined plateau with a value of Cs = 340.17 mg g1. The fitted parameters of adsorption were calculated from the linear regressions of Eqs. (4)–(6). According to the regression coefficient values, the Langmuir model showed a much better fit than the Freundlich and Temkin models (Table 4). These models do not give any idea about the mechanism of adsorption. To understand the adsorption type, experimental data were fitted to the Dubinin–Radushkevich (D–R) model: ln C s ¼ ln qm K e2 ð7Þ where qm is the theoretical adsorption capacity of the adsorbent (mg g1), K (mg2 kJ2) is the constant related to adsorption energy and e = RT ln(1 + 1/Ce) is the Polanyi potential. The adsorption energy (E) can be calculated using the D–R equation and the following relationship has been used. E ¼ ð2KÞ0:5 ð8Þ The adsorption is physical in nature when E is between 0 and 8 kJ mol1; if it is 8 < E < 16 kJ mol1 the adsorption is due to exchange of ions; and if the value of E is between 20 < E < 40 kJ mol1, a chemisorption occurs. In the case of S-Metolachlor, the values found in the present study for the three adsorbents were around 8 kJ mol1 whereas in the case of Bentazon were between 11 and 13 kJ mol1. Consequently, the adsorption of Bentazon could be explained by exchange of ions. In addition, E values for S-Metolachlor were in the limit between physical adsorption and exchange of ions and so it was not possible to explain precisely the nature of the adsorption. The desorption curves [5] above the adsorption curves evidenced the difficulty to remove S-Metolachlor and Bentazon from Table 3 Kinetic parameters for the adsorption of Bentazon and S-Metolachlor onto mesoporous phenolic resin and mesoporous carbon compared with commercial carbon. Pesticide Adsorbent Cs exp Pseudo first order model 1 K1 (h ) C S1 (mg g 1 ) Pseudo second order model 2 R K2 (g mg 1 Bentazon MPR MC CC 53.20 67.82 71.23 0.39 0.34 0.23 48.97 43.71 0.287 0.992 0.991 0.561 0.018 0.017 4.9 S-Metolachlor MPR MC CC 94.28 108.4 117.9 0.21 0.32 1.22 55.40 47.28 18.28 0.983 0.995 0.941 8.4 103 0.019 0.24 h 1 ) Intraparticle diffusion 1 2 Kp (mg g1 h1/2) 54.64 69.44 71.43 0.999 0.999 0.999 4.05 5.43 0.06 30.62 38.29 70.95 0.780 0.652 0.617 97.08 109.89 117.65 0.999 0.999 0.999 6.91 5.82 0.85 54.77 76.56 113.36 0.829 0.691 0.413 ) C (mg g1) R2 R C S2 (mg g 98 R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 Fig. 9. Adsorption and desorption isotherms of S-Metolachlor and Bentazon on mesoporous phenolic resin (MPR) and mesoporous carbon (MC). Fig. 10. Adsorption and desorption isotherms of S-Metolachlor and Bentazon on commercial carbon (CC). Table 4 Freundlich, Langmuir, Dubinin–Raduskevich and Temkin model parameters for S-Metolachlor and Bentazon adsorption onto mesoporous phenolic resin and mesoporous carbon compared with commercial carbon. Pesticide Adsorbent Freundlich Kf Langmuir nf R2 Cm Dubinin–Raduskevich L R2 qm 3 Temkin R2 K 3 BT KT R2 Bentazon MPR MC CC 15.19–13.31 42.56–33.96 105.86–76.35 0.44 ± 0.03 0.32 ± 0.03 0.26 ± 0.06 0.980 0.964 0.774 129.20 174.82 219.30 0.05 0.11 1.04 0.994 0.998 0.998 2.1 10 1.8 103 2.6 103 3.9 10 2.6 103 2.0 103 0.986 0.968 0.815 28.39 27.56 30.17 0.45 3.09 39.84 0.984 0.947 0.893 S-Metolachlor MPR MC CC 4.12–0.81 9.34–4.33 13.83–4.87 0.90 ± 0.17 0.74 ± 0.08 0.72 ± 0.12 0.900 0.921 0.922 467.28 666.67 1434.7 3.3 103 4.2 103 2.1 103 0.922 0.989 0.978 0.015 0.012 0.014 9.2 103 7.4 103 7.3 103 0.884 0.958 0.937 133.97 133.04 138.02 0.03 0.05 0.06 0.748 0.887 0.960 99 R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 Table 5 Thermodynamic parameters for S-Metolachlor and Bentazon adsorption onto mesoporous phenolic resin and mesoporous carbon compared with commercial carbon. Pesticide Adsorbent T (°C) S-Metolachlor MPR 10 20 30 10 20 30 10 20 30 10 20 30 MC Bentazon MPR MC DS (kJ mol1 K1) DH (kJ mol1) DG (kJ mol1) 979 623 542 11494 10042 9728 0.018 21.16 0.056 6.10 16.20 15.67 15.86 22.00 22.44 23.13 82782 69192 60241 36003 131520 208029 0.054 11.42 K the adsorbents. The presence of hysteresis (Figs. 9 and 10) indicates the irreversibility of the process in all cases. 3.3. The effect of temperature In order to determine the thermodynamic parameters of the adsorption of Bentazon and S-Metolachlor on the three adsorbents, the effect of the temperature was studied. When the temperature was increased from 10 to 30 °C the amount of adsorbed Bentazon slightly increased, unlike S-Metolachlor whose adsorption behavior was similar in the whole temperature range. The change in standard free energy (DG°), enthalpy (DH°), and entropy (DS°) of adsorption was calculated using the following equations (Eqs. (9) and (10)) [47,48]: DG ¼ DH T DS ln K ¼ DS =R DH =RT ð9Þ ð10Þ where K is the equilibrium constant, R the molar gas constant and T the absolute temperature. The values of DH° and DS° were calculated from the slope and intercept of Van’t Hoff plots (ln KC versus 1/T) and are summarized in Table 5. The values of K have been obtained from the values of KL from the Langmuir equation because our isotherms were well fitted to this equation. K has been recalculated to become dimensionless by multiplying it by 106 [49]. DG° values were calculated from Eq. (9). The negative DG° values indicate that the process is thermodynamically feasible and spontaneous. In the case of Bentazon, the DG° values were slighter higher as the temperature increased indicating that the process was chemically controlled. By contrast, the behavior of S-Metolachlor showed two different tendencies (physical and chemical). 0.31 62.87 26.64 27.15 27.72 24.68 28.71 30.85 information). In relation to CC, the lower decrease in the adsorption capacity for MC could be ascribed to its mesoporous character and its lower fraction of micropores [50,51]. Similar behavior was described by Suchithra et al. [52], who studied the regeneration of hybrid mesoporous material compared with commercial carbon for the removal of dyes and other organic compounds. A great disadvantage of the commercial carbon compared to the mesoporous materials in relation to their reusability was its significant loss of material after several adsorption–desorption cycles. After three cycles, the losses of the commercial adsorbent were 48.2% and 47.1% for the adsorption of S-Metolachlor and Bentazon, respectively, while it was less than 5% for the mesoporous materials (Fig. 11). After each adsorption cycle the material was recovered with a filter of 0.45 lm of pore size, which should retain all particles according to the particle size distribution depicted in Fig. 5. However, these results suggest that the commercial carbon undergoes a shear degradation that decreases the particle size, thus allowing the resulting smallest particles to pass through the filter. The loss of this material involves serious concerns for its practical application because it reduces drastically its durability, unlike both mesoporous materials, which proven to be more resistant to friction. 3.4. Regeneration of the adsorbents To test their reusability, the adsorbents were regenerated by calcination under nitrogen atmosphere at 380 °C for mesoporous phenolic resin and 400 °C for mesoporous carbon and commercial carbon. The adsorbents were subjected to three adsorption– desorption cycles (Fig. 11). The regeneration efficiencies of the mesoporous materials were compared with the commercial carbon. In the third cycle, the adsorption capacity for S-Metolachlor on MPR, MC and CC were 60.5%; 69.29% and 59.85%, respectively, while for Bentazon were 77.33% for MPR; 90.22% for MC and 88.82% for CC. For both pesticides, the regeneration was better in mesoporous carbon than in mesoporous phenolic resin and commercial carbon. The differences between both mesoporous materials can be associated to the presence of more defects in the mesostructure of the phenolic resin than in that of the mesoporous carbon after their regeneration (see Fig. SI2 in Supporting Fig. 11. Adsorption–desorption cycles for S-Metolachlor and Bentazon. Adsorption after first (1), second (2) and third (3) cycle. Weight lost after three adsorption– desorption cycles (% loss). 100 R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 4. Conclusions Mesoporous phenolic resin and mesoporous carbon exhibited good properties to be used as adsorbents in virtue of their high surface area and narrow mesopore distribution. The N2 adsorption– desorption isotherms for MPR and MC were essentially of type IV. The pore size of these materials was around 9–10 nm according to the N2 adsorption and TEM results. We have examined a potentially practical use of these materials for the adsorption of S-Metolachlor and Bentazon. The adsorption of these pollutants varied with the pH. The adsorption of Bentazon decreased when the pH increased, probably due to the increase of the electrostatic repulsion between the pesticide ion and the adsorbent surface. By contrast, the adsorption of S-Metolachor was not affected by the pH. The adsorption kinetics fitted better to a pseudo-second-order kinetics model. Intraparticle diffusion was present in the adsorption process together with other factors. The isotherms for Bentazon on MPR, MC and CC and for S-Metolachor on CC were of L2 type while the adsorption of S-Metolachlor on MPR and MC exhibited a L4 type isotherm. In the case of Bentazon, a plateau was reached for the three adsorbents. By contrast, the second plateau for S-Metolachlor on MPR and MC was not reached, indicating that the maximum capacity of these adsorbents was not achieved under these conditions. In all cases the Langmuir model showed a much better fit. The adsorption of Bentazon could be explained as exchange of ions according to the Dubinin–Radushkevich model. The presence of hysteresis in the desorption curves indicated irreversibility of the process in all cases. The regeneration efficiency was better in mesoporous carbon than in mesoporous phenolic resin and commercial carbon for both pesticides. Another disadvantage of the commercial carbon compared to the mesoporous materials for their regeneration was the great loss of material after successive adsorption–desorption cycles. Acknowledgements The authors wish to acknowledge funding of this research by Ministerio de Ciencia e Innovación (Project MAT2010-18778) and Junta de Andalucía (Project P06-FQM-01741). D.E. is a postdoctoral researcher of the FWO-Vlaanderen (Fund Scientific Research – Flanders), Grant Number 3E10813W. R. Otero acknowledges Ministry of Education and Science for a research and teaching fellowship. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.04.038. References [1] J.M. Salman, V.O. Njoku, B.H. Hameed, Bentazon and carbofuran adsorption onto date seed activated carbon: kinetics and equilibrium, Chem. Eng. J. 173 (2011) 361–368. [2] C.D. Stan, I. Cretescu, C. Pastravanu, I. Poulios, M. Dragan, Treatment of pesticides in wastewater by heterogeneous and homogeneous photocatalysis, Int. J. Photoenergy (2012), http://dx.doi.org/10.1155/2012/194823. [3] S. Rafqah, P. Wong-Wah-Chung, A. Aamili, M. Sarakha, Degradation of metsulfuron methyl by heterogeneous photocatalysis on TiO2 in aqueous suspensions: kinetic and analytical studies, J. Mol. Catal. A: Chem. 237 (2005) 50–59. [4] X. Zhu, C. Yuan, Y. Bao, J. Yang, Y. Wu, Photocatalytic degradation of pesticide pyridaben on TiO2 particles, J. Mol. Catal. A: Chem. 229 (2005) 95–105. [5] R. Otero, J.M. Fernandez, M.A. Ulibarri, R. Celis, F. Bruna, Adsorption of nonionic pesticide S-Metolachlor on layered double hydroxides intercalated with dodecylsulfate and tetradecanedioate anions, Appl. Clay Sci. 65–66 (2012) 72– 79. [6] R. Otero, D. Esquivel, M.A. Ulibarri, C. Jiménez-Sanchidrián, F.J. RomeroSalguero, J.M. Fernández, Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas, Chem. Eng. J. 228 (2013) 205–213. [7] O.A. Ioannidou, A.A. Zabaniotou, G.G. Stavropoulos, M.A. Islam, T.A. Albanis, Preparation of activated carbons from agricultural residues for pesticide adsorption, Chemosphere 80 (2010) 1328–1336. [8] R.-H. Chen, L.-Y. Chai, Y.-Y. Wang, H. Liu, Y.-D. Shu, J. Zhao, Degradation of organic wastewater containing Cu–EDTA by Fe–C micro-electrolysis, Trans. Nonferr. Met. Soc. China 22 (2012) 983–990. [9] Q. Lu, J. Yu, J. Gao, Degradation of 2,4-dichlorophenol by using glow discharge electrolysis, J. Hazard. Mater. 136 (2006) 526–531. [10] P. Jiang, J. Zhou, A. Zhang, Y. Zhong, Electrochemical degradation of pnitrophenol with different processes, J. Environ. Sci. 22 (2010) 500–506. [11] Z. Wu, D. Zhao, Ordered mesoporous materials as adsorbents, Chem. Commun. 47 (2011) 3332–3338. [12] M.R. Awual, T. Yaita, S.A. El-Safty, H. Shiwaku, S. Suzuki, Y. Okamoto, Copper(II) ions capturing from water using ligand modified a new type mesoporous adsorbent, Chem. Eng. J. 221 (2013) 322–330. [13] T. Yokoi, Y. Kubota, T. Tatsumi, Amino-functionalized mesoporous silica as base catalyst and adsorbent, Appl. Catal. A 421–422 (2012) 14–37. [14] J. Brown, R. Richer, L. Mercier, One-step synthesis of high capacity mesoporous Hg2+ adsorbents by non-ionic surfactant assembly, Micropor. Mesopor. Mater. 37 (2000) 41–48. [15] X.-W. Wu, H.-W. Ma, J. Yang, F.-J. Wang, Z.-H. Li, Adsorption of Pb(II) from aqueous solution by a poly-elemental mesoporous adsorbent, Appl. Surf. Sci. 258 (2012) 5516–5521. [16] A.S. Barreto, A. Aquino, S.C.S. Silva, M.E. de Mesquita, M.J. Calhorda, M.S. Saraiva, S. Navickiene, A novel application of mesoporous silica material for extraction of pesticides, Mater. Lett. 65 (2011) 1357–1359. [17] M.Z. Momčilović, M.S. Randelović, A.R. Zarubica, A.E. Onjia, M. Kokunešoski, B.Z. Matović, SBA-15 templated mesoporous carbons for 2,4dichlorophenoxyacetic acid removal, Chem. Eng. J. 220 (2013) 276–283. [18] I. Muylaert, A. Verberckmoes, J. De Decker, P. Van der Voort, Ordered mesoporous phenolic resins: highly versatile and ultra stable support materials, Adv. Colloid Interf. Sci. 175 (2012) 39–51. [19] R. Ryoo, S.H. Joo, S. Jun, Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation, J. Phys. Chem. B 103 (1999) 7743–7746. [20] Q. Liu, A. Wang, X. Wang, T. Zhang, Ordered crystalline alumina molecular sieves synthesized via a nanocasting route, Chem. Mater. 18 (2006) 5153– 5155. [21] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles, Nature 412 (2001) 169–172. [22] D. Zhao, Q. Huo, J. Feng, B. Chmelka, G. Stucky, Nonionic triblock and star diblock copolymer and oligomeric surfactant syntheses of highly ordered, hydrothermally stable, mesoporous silica structures, J. Am. Chem. Soc. 120 (1998) 6024–6036. [23] A. Thomas, Functional materials: from hard to soft porous frameworks, Angew. Chem. Int. Ed. 49 (2010) 8328–8344. [24] Y. Wan, Y. Shi, D. Zhao, Supramolecular aggregates as templates: ordered mesoporous polymers and carbons, Chem. Mater. 20 (2007) 932–945. [25] M. Bosetto, P. Arfaioli, P. Fusi, Adsorption of the herbicides alachlor and metolachlor on two activated charcoals, Sci. Total Environ. 123–124 (1992) 101–108. [26] X.M. Wu, M. Li, Y.H. Long, R.X. Liu, Y.L. Yu, H. Fang, S.N. Li, Effects of adsorption on degradation and bioavailability of metolachlor in soil, J. Soil Sci. Plant Nutr. 11 (2011) 83–97. [27] L. Allievi, C. Gigliotti, C. Salardi, G. Valsecchi, T. Brusa, A. Ferrari, Influence of the herbicide bentazon on soil microbial community, Microbiol. Res. 151 (1996) 105–111. [28] A. Omri, A. Wali, M. Benzina, Adsorption of bentazon on activated carbon prepared from Lawsonia inermis wood: equilibrium, kinetic and thermodynamic studies, Arab. J. Chem. http://dx.doi.org/10.1016/ j.arabjc.2012.04.047 [29] D. Grosso, F. Cagnol, G.J.d.A.A. Soler-Illia, E.L. Crepaldi, H. Amenitsch, A. BrunetBruneau, A. Bourgeois, C. Sanchez, Fundamentals of mesostructuring through evaporation-induced self-assembly, Adv. Funct. Mater. 14 (2004) 309–322. [30] C.-K. Tsung, J. Fan, N. Zheng, Q. Shi, A.J. Forman, J. Wang, G.D. Stucky, A general route to diverse mesoporous metal oxide submicrospheres with highly crystalline frameworks, Angew. Chem. Int. Ed. 47 (2008) 8682–8686. [31] J. Jin, N. Nishiyama, Y. Egashira, K. Ueyama, Pore structure and pore size controls of ordered mesoporous carbons prepared from resorcinol/ formaldehyde/triblock polymers, Micropor. Mesopor. Mater. 118 (2009) 218–223. [32] S. Lagergren, Zur theorie der sogenannten adsorption gel oster stoffe, Kungliga Svenska Vetenskapsakademiens 24 (1898) 1–39. [33] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem. 34 (1999) 451–465. [34] J.C. Morris, W.J. Weber, Kinetics of adsorption of carbon from solutions, J. Sanit. Eng. Div. Am. Soc. Civ. Eng. 89 (1963) 31–63. [35] H.M.F. Freundlich, Ober Die Adsorption in Lösungen, Zeitschrift Fuär Physikalische Chemie 57 (1906) 385. [36] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem. Soc. 40 (1918) 1361–1403. R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101 [37] O. Çelebi, C. Üzüm, T. Shahwan, H.N. Erten, A radiotracer study of the adsorption behavior of aqueous Ba2+ ions on nanoparticles of zero-valent iron, J. Hazard. Mater. 148 (2007) 761–767. [38] M.M. Dubinin, L.V. Radushkevich, Zh. Fiz. Khim. 55 (1947) 327. [39] U. Ciesla, F. Schüth, Ordered mesoporous materials, Micropor. Mesopor. Mater. 27 (1999) 131–149. [40] N. Farzin Nejad, E. Shams, M.K. Amini, J.C. Bennett, Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Process. Technol. 106 (2013) 376–384. [41] C.O. Ania, F. Béguin, Mechanism of adsorption and electrosorption of bentazone on activated carbon cloth in aqueous solutions, Water Res. 41 (2007) 3372–3380. [42] A. Boivin, R. Cherrier, M. Schiavon, A comparison of five pesticides adsorption and desorption processes in thirteen contrasting field soils, Chemosphere 61 (2005) 668–676. [43] T.L. Grey, G.R. Wehtje, R.H. Walker, Sorption and mobility of bentazone in coastal plain soils, Weed Sci. 44 (1996) 166–170. [44] M. Islam, R. Patel, Nitrate sorption by thermally activated Mg/Al chloride hydrotalcite-like compound, J. Hazard. Mater. 169 (2009) 524–531. [45] R. Otero, J.M. Fernández, M.A. González, I. Pavlovic, M.A. Ulibarri, Pesticides adsorption–desorption on Mg–Al mixed oxides. Kinetic modeling, competing factors and recyclability, Chem. Eng. J. 221 (2013) 214–221. 101 [46] C.H. Giles, T.H. Macewan, S.N. Nakhwa, D. Smith, Studies in adsorption. 11. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids, J. Chem. Soc. (1960) 3973–3993. [47] A.S. Özcan, Ö. Gök, A. Özcan, Adsorption of lead(II) ions onto 8-hydroxy quinoline-immobilized bentonite, J. Hazard. Mater. 161 (2009) 499–509. [48] M. Islam, R. Patel, Polyacrylamide thorium (IV) phosphate as an important lead selective fibrous ion exchanger: synthesis, characterization and removal study, J. Hazard. Mater. 156 (2008) 509–520. [49] S.K. Milonjic, A consideration of the correct calculation of thermodynamic parameters of adsorption, J. Serb. Chem. Soc. 72 (2007) 1363–1367. [50] Muhammad, M.A. Khan, T.S.Y. Choong, T.G. Chuah, R. Yunus, Y.H.T. Yap, Desorption of b-carotene from mesoporous carbon coated monolith: isotherm, kinetics and regeneration studies, Chem. Eng. J. 173 (2011) 474–479. [51] K. Misra, S.K. Kapoor, R.C. Bansal, Regeneration of granular activated carbon loaded with explosives, J. Environ. Monit. 4 (2002) 462–464. [52] P.S. Suchithra, L. Vazhayal, A. Peer Mohamed, S. Ananthakumar, Mesoporous organic–inorganic hybrid aerogels through ultrasonic assisted sol–gel intercalation of silica–PEG in bentonite for effective removal of dyes, volatile organic pollutants and petroleum products from aqueous solution, Chem. Eng. J. 200-202 (2012) 589–600. RSC Advances PAPER Cite this: RSC Adv., 2015, 5, 24158 Evaluation of different bridged organosilicas as efficient adsorbents for the herbicide Smetolachlor† Marı́a Isabel López,a Rocı́o Otero,b Dolores Esquivel,a César Jiménez-Sanchidrián,a José Marı́a Fernández*b and Francisco José Romero-Salguero*a Three groups of porous non-ordered bridged organosilicas and silicas have been synthesized using three different surfactants of the Tween family as porogens. One group consisted of silica materials while the other two were organosilicas, i.e., ethanesilicas and benzenesilicas, but differing within each group in their textural properties. The adsorption of the herbicide S-metolachlor from an aqueous solution was then compared on all solids. The composition of the walls, i.e., the bridging group, was the main factor governing the adsorption capacity. Broadly, the uptake of the herbicide follows the sequence: benzenesilica > ethanesilica > silica. However, pore volume also has a significant influence on adsorption. Thus, higher capacities result from materials with larger pore volumes and with a larger size of the mesopores. Moreover, surface area and pore size distributions also affect the adsorption behavior, even though their influence seems to be less pronounced. Finally, a comparison between the porous non-ordered (organo)- Received 11th February 2015 Accepted 26th February 2015 silicas and the corresponding periodic mesoporous (organo)silicas has revealed some interesting features for the use of these materials as adsorbents. Benzenesilicas with a large pore volume and pores in the DOI: 10.1039/c5ra02711j micropore and/or large mesopore regions seemed to be the best adsorbents for adsorption in a single step www.rsc.org/advances whereas periodic mesoporous benzenesilicas were more appropriate for repetitive adsorptions. 1. Introduction Bridged silsesquioxanes consist of organosilica networks prepared by hydrolysis and condensation of bis-silanes of the type (R0 O)3Si–R–Si(OR0 )3. Usually, the bridge R is a relatively simple hydrocarbon unit such as a methylene, an ethylene or a phenylene group. Nevertheless, these bridges can also be of a high complexity.1 Advantageously, all these groups are homogenously distributed along their structure. A particular case of these materials are periodic mesoporous organosilicas (PMOs).2–4 They are synthesized in the presence of a surfactant template, thus giving rise to materials with long-distance ordering and pores in the meso range.5,6 Both periodic mesoporous silicas and organosilicas have narrow pore size distributions in the mesoporous region (2–50 nm) and high surface areas. These properties make them a Departamento de Quı́mica Orgánica, Instituto de Quı́mica Fina y Nanoquı́mica (IUIQFN), Campus de Rabanales, Universidad de Córdoba, Campus de Excelencia Internacional Agroalimentario (ceiA3), Córdoba, Spain. E-mail: [email protected]; Fax: +34 957212066; Tel: +34 957212065 b Departamento de Quı́mica Inorgánica e Ingenierı́a Quı́mica, Instituto de Quı́mica Fina y Nanoquı́mica (IUIQFN), Campus de Rabanales, Universidad de Córdoba, Campus de Excelencia Internacional Agroalimentario (ceiA3), Córdoba, Spain. E-mail: um1feroj@ uco.es; Fax: +34 957580644; Tel: +34 957218648 † Electronic supplementary 10.1039/c5ra02711j information 24158 | RSC Adv., 2015, 5, 24158–24166 (ESI) available. See DOI: good candidates for the adsorption of different molecules. Castricum et al.7 described the application of microporous hybrid silica materials with alkylene and aromatic bridging groups of different sizes in membranes for the separation of gas mixtures such as H2/N2, CO2/H2 and CO2/CH4. The improved hydrothermal stability of hybrid structures has also provided membranes useful for water desalination.8 In this sense, PMOs have been proven to be excellent adsorbents for organic compounds due to their improved hydrophobicity. Phenol derivatives,9,10 trinitrotoluene11 and aromatic and polyaromatic hydrocarbons12,13 have been adsorbed on aliphatic and aromatic bridged PMOs. The combination of inorganic and organic moieties in their framework represented a real advantage even for the immobilization of enzymes.14 The presence of phenylene bridges is particularly advantageous when the adsorption of aromatic pollutants is concerned due to the pi–pi interactions between these compounds and the organosilica. Thus, a phenylene bridge led to a higher uptake of several phenol derivatives than an ethylene unit.10 Similarly, the herbicide S-metolachlor was retained in a higher extent by a phenylene-bridged PMO than an ethylene-bridged PMO and a mesoporous silica.15 The favorable presence of aromatic units in the pore walls was also proven for the relatively bulky pesticide mesosulfuron-methyl.16 In this case, the introduction of sulfonic acid groups in the phenylene-bridged PMO enhanced the adsorption capacity via acid–base interactions with the NH This journal is © The Royal Society of Chemistry 2015 Paper RSC Advances group of the sulfonamide moiety in the pesticide. Very recently, a divinylaniline bridged PMO was employed for the removal of hexanal through a chemisorptive process.17 S-metolachlor is a pre- and post-emergence selective herbicide used to control grasses and some broad-leaved weeds in a wide range of crops. It is widely distributed in the environment and, in fact, 50.3% surface water sources and 2% of groundwater sources are contaminated with metolachlor in United States.18,19 Metolachlor is also one of the most relevant compounds contributing to Spanish groundwater contamination with concentrations above the environmental quality standard dened in the Directives 2006/118/EC and 2008/105/ EC.20 A similar situation has been found in other countries such as Greece21 and Italy22 in surface water and groundwater. Recent studies suggest that exposure to metolachlor results in decreased cell proliferation, growth and reproductive ability of non-target organisms.23 The WHO Water Quality Criteria limits its concentration in 10.0 mg L1. Herein, we report the synthesis of various silicas and organosilicas with different composition, i.e., nature of bridges, and textural properties using surfactants of the Tween family, which were later used as adsorbents for the herbicide S-metolachlor. The relationship between the physicochemical properties of these materials and the pesticide uptake was analyzed and compared to that of the corresponding periodic mesoporous (organo)silicas. 2. Materials and methods 2.1. Materials Three ordered materials, i.e., a periodic mesoporous silica (PMS) and two organosilicas, namely ethane-PMO (E-PMO) and benzene-PMO (Ph-PMO), were synthesized by using Brij-76 as surfactant under acidic conditions according to previously reported procedures.24 Non-ordered (organo)silicas were synthesized using tetraethyl orthosilicate (TEOS) (98%), 1,2-bis(triethoxysilyl)ethane (97%) and 1,4-bis(triethoxysilyl)benzene (96%), which were purchased from Acros, ABCR and Aldrich, respectively, as silica sources. The surfactants employed, all commercially available from Aldrich, were Tween 20 [polyoxyethylene(20)sorbitan monolaurate], Tween 40 [polyoxyethylene(20)sorbitan monopalmitate] and Tween 60 [polyoxyethylene(20)sorbitan monostearate]. HCl (37%) and ethanol (96% v/v), used to remove the surfactant, were provided by Panreac. NH4F ($98.0%), used as catalyst, was acquired from Sigma-Aldrich. All these chemicals were used without further purication. The synthesis of the non-ordered materials was performed following a similar procedure to that described in a previous work for silicas.25 In brief, 184 mmol of TEOS or 92 mmol of organodisilane, i.e., 1,2-bis(triethoxysilyl)ethane or 1,4-bis(triethoxysilyl)benzene, were added dropwise to 184 mL of a solution that was 0.1 M in the corresponding surfactant (Tween 20, 40 or 60) and 0.02 M in the catalyst (NH4F). Aer stirring for 24 h at room temperature, the formed precipitate was aged for 5 days at room temperature. Finally, the resulting This journal is © The Royal Society of Chemistry 2015 solid was ltered, washed with 300 mL of distilled water and dried in the air. The surfactant was removed by reuxing the as-synthesized material in an HCl solution (1 mL of 37% HCl in 50 mL of ethanol, per gram of solid) for 12 h. Subsequently, the solid was ltered and washed with ethanol. This process was repeated twice. The nal material was then dried in a drying chamber at 100 C under vacuum. The non-ordered materials, i.e., silica, ethanesilica and benzenesilica, were named as S, EOS and PhOS, followed by a number denoting the surfactant used, that is, 20, 40 and 60 for Tween 20, Tween 40 and Tween 60, respectively. Analytical standard S-metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]acetamide) was supplied by Sigma Aldrich. Its water solubility at 25 C is 480 mg L1. 2.2. Characterization All hybrid materials were characterized by different techniques. X-ray powder diffraction (XRD) patterns were recorded on a Siemens D-5000 powder diffractometer (Cu-Ka radiation). N2 isotherms were determined on a Micromeritics ASAP 2010 analyzer at 196 C. The specic surface area of each solid was determined using the BET method over a relative pressure (P/P0) range of 0.05–0.20 and the pore size distribution was obtained by analysis of the adsorption branch of the isotherms using the Barrett–Joyner–Halenda (BJH) method. Prior to the measurements, the samples were outgassed at 120 C for 24 h. A 3Flex equipment from Micromeritics was used for two samples. In this case, the porosity distribution was determined by a density functional theory (DFT) method using a cylindrical model for N2 on an oxide surface. Particle size data were obtained in a Mastersizer S particle size analyzer by laser diffraction technology with a small volume dispersion unit of Malvern Instruments. Before the measurements, the samples were dispersed in distilled water and sonicated for 10 min. TEM micrographs were taken using a JEOL JEM 1400 instrument on samples supported on copper grids with carbon coating. 29Si MAS NMR spectra were recorded at 79.49 MHz on a Bruker Avance 400 WB dual channel spectrometer at room temperature. An overall of 1000 free induction decays were accumulated. The excitation pulse and recycle time were 6 ms and 60 s, respectively. Chemical shis were measured relative to tetramethylsilane standard. Before analysis, the samples were dried at 150 C for 24 h. Deconvolution of the spectra were performed with PeakFit soware. The variation of zeta potential with pH was determined using Laser Doppler Velocimetry implemented in a Malvern Zetasizer Nano ZS equipped with a MPT-2 autotitrator. UV spectroscopy (Perkin Elmer UV-visible spectrophotometer, Model Lambda 11) at 265 nm was used to determine the concentration of S-metolachlor. 2.3. Adsorption experiments S-Metolachlor adsorption on mesoporous materials was studied suspending in triplicate samples of 20 mg of each adsorbent in 30 mL of aqueous herbicide solution (100.0 mg L1) with an initial pH value of 2 (for the silica and ethanesilica samples) or 4 RSC Adv., 2015, 5, 24158–24166 | 24159 RSC Advances Paper (for the benzenesilica samples).15 Samples of adsorbent and herbicide in water were shaken in a turn-over shaker at 52 rpm for 24 h. Adsorption kinetics was carried out at different contact times (Fig. S1†). Then, the supernatant liquid was centrifuged and separated to determine the concentration of S-metolachlor by UV-vis spectroscopy (Fig. S2†). The amounts of adsorbed herbicide were determined from the initial and nal solution concentrations. Scheme 1 Schematic representation of a pore for the different materials synthesized. R ¼ –O–, –C2H4– or –C6H4– for silicas, ethanesilicas and benzenesilicas, respectively. The shaded area represents the wall of the material. Table 1 Physicochemical properties of non-ordered porous (organo)silicas and periodic mesoporous (organo)silicas Material SBETa (m2 g1) Vpb (cm3 g1) Smicroc (m2 g1) S-20 S-40 S-60 PMS EOS-20 EOS-40 EOS-60 E-PMO PhOS-20 PhOS-40 PhOS-60 Ph-PMO 723 721 746 885 715 994 697 1059 942 1160 824 986 0.81 1.39 1.67 1.17 0.68 1.44 1.19 1.00 0.58 1.69 0.81 0.72 47 52 92 0 84 84 21 26 250 111 48 161 a BET specic surface area determined in the range of relative pressures from 0.05 to 0.20. b Single point total pore volume of pores. c Micropore area determined by t-plot method. Fig. 1 Successive adsorptions were carried out by suspending the solid (adsorbent–herbicide) in 30 mL of a fresh aqueous herbicide solution (100.0 mg L1). The contact time was 24 h. Aer shaking, the suspensions were centrifuged and the herbicide concentration determined by UV-vis spectroscopy. The charged material was used again for a new adsorption cycle up to completion of 4 successive adsorptions. The regeneration of the adsorbent was performed in ethanol with 30 mL of extracting volume and 24 h of contact time. 3. Results and discussion 3.1. (Organo)silicas as adsorbents for S-metolachlor Different silicas, ethanesilicas and benzenesilicas have been synthesized using three different Tween surfactants as porogens (Scheme 1). The integrity of the organic bridges was veried by FT-IR measurements (Fig. S3†).26 Thus, bands at 1277, 1414, 2899 and 2986 cm1 corresponded to vibrations of the CH2 groups in ethane bridges. Benzene bridges showed the C–H and C–C vibrations at 3069, and 1461 and 1636 cm1, respectively. Furthermore, intense bands between 1000 and 1110 cm1 assigned to Si–O vibrations could be observed in all (organo)silicas. XRD analyses (Fig. S4†) revealed that all these materials were non-periodic, that is, they lacked long distance ordering. Nevertheless, all of them exhibited high specic surface areas and pore volumes (Table 1). Their N2 adsorption–desorption isotherms were of type IV with hysteresis loops exhibiting different shapes (Fig. 1). Therefore, even though all these materials had pores in the meso range, their porosity showed signicant differences. In fact, all these adsorbents contained pores over a wide range of sizes. Samples S-20 and S-40 displayed a step at a relative pressure of 0.4–0.6 and a hysteresis loop of type H2, typical of disordered mesoporous systems.26 Their pore size distribution curves indicated the existence of pores in the mesopore range at ca. 4 and 5 nm, respectively (Fig. 2). Material EOS-60 also had pores around 4 nm, although the step was hardly observed. In some cases, the adsorption step was shied to high relative pressures (>0.6) by the effect of the increase in pore size, which was the case for solid S-60 with mesopores around 7 nm. On the contrary, materials EOS-20 and Nitrogen adsorption–desorption isotherms for non-ordered porous (organo)silicas. 24160 | RSC Adv., 2015, 5, 24158–24166 This journal is © The Royal Society of Chemistry 2015 Paper RSC Advances Fig. 2 Pore size distribution curves for non-ordered porous (organo)silicas. Fig. 3 TEM images for non-ordered porous (organo)silicas. This journal is © The Royal Society of Chemistry 2015 RSC Adv., 2015, 5, 24158–24166 | 24161 RSC Advances Paper EOS-40 did not show any hysteresis loop at P/P0 < 0.8 because the step occurred at quite low relative pressure, thus resulting in pores in the low mesopore range (<3 nm). Furthermore, material PhOS-20 exhibited a type H4 loop, which is usually ascribed to slit-shaped pores in the micropore range.27 The presence of a separate hysteresis loop above P/P0 ¼ 0.8 for several samples (i.e., S-40 and EOS-60) was suggestive of textural porosity, which represents the void space between randomly packed elementary particles, in the large meso and macropore range. As can be observed, several samples (i.e., S-60 and PhOS-40) exhibited different overlapped hysteresis loops, thus suggesting a combination of different porous systems. Clearly, the use of different surfactants of the Tween family gave rise to important differences within each group. Most of the samples exhibited mesopores with diameters above 10 nm and/or macropores, even though those materials prepared from Tween 20 mainly had small mesopores and/or micropores. Periodic mesoporous silica (PMS) and organosilicas (ethanePMO and benzene-PMO) were previously shown to exhibit isotherms of type IV typical of mesoporous materials with a narrow pore size distribution.15 Among the non-ordered porous materials, only those synthesized in the presence of surfactant Tween 20 presented an analogous pore size distribution to PMS and PMOs, even though it was shied to smaller pore sizes for PhOS-20 than for Ph-PMO. The transmission electron micrographs (Fig. 3) clearly reected the existence of small elementary particles forming larger porous aggregates, which is consistent with the textural porosity apparent from the nitrogen adsorption–desorption isotherms. Also, in agreement with their isotherms, silica materials exhibited highly disordered pores in the meso range (4–5 nm). Although less evident, pores with sizes around 3–4 nm were also observable in ethanesilicas. Higher magnication images were in good agreement with the pore sizes determined by N2 adsorption (Fig. 2). The porous network of fused particles, particularly for silica samples, packed more densely in the following order of surfactants: Tween 20 > Tween 40 $ Tween 60, thus resulting in more uniform pore systems when using the former. In the case of benzenesilicas, the pores in sample PhOS-20 were hardly observable because they fell in the microporous range. The different (organo)silicas exhibited the particle size distribution curves depicted in Fig. 4. In general, wide distributions in particle size were observed in all cases, even though silicas presented narrower size distributions than organosilicas. Within each group, the particle size seemed to be dependent on the surfactant used as porogen. Thus, it increased in the following order: Tween 20 < Tween 60 < Tween 40. Moreover, those (organo)silicas synthesized in the presence of Tween 20 surfactant displayed a narrower particle size distribution. In this case, the particle size decreased in the order PhOS-20 < EOS-20 < S-20. Also, the particle size distribution curves for the corresponding PMOs are given as a reference. All the silicas and organosilicas were used as adsorbents for the removal of S-metolachlor from water. The adsorption experiments for silicas and ethane-PMOs were carried out at pH ¼ 2, whereas those for benzenesilicas were done at pH ¼ 4, according to the behavior previously observed in PMOs.15 The amount of adsorbed herbicide in the three groups of materials Fig. 4 Particle size distribution curves for non-ordered porous (organo)silicas and periodic mesoporous (organo)silicas. Fig. 5 Adsorption of S-metolachlor on non-ordered porous (organo)silicas and periodic mesoporous (organo)silicas. 24162 | RSC Adv., 2015, 5, 24158–24166 This journal is © The Royal Society of Chemistry 2015 Paper RSC Advances Adsorption capacities for S-metolachlor on different adsorbents Table 2 Adsorbent C0a (mg g1) Csb (mg g1) Adsorption (%) Ref. Soils Charcoals Clays Organohydrotalcites (OHTs) PMS E-PMO 0.08 400.0 158.9 149.0 149.0 149.0 596.0 532.1 (4 times)c 149.0 596.0 150.0 (4 times)c 532.1 (4 times)c 150.0 150.0 (3 times)c 150.0 600.0 150.0 600.0 150.0 600.0 0.04 136.2–156.0 56.8 79.7 15.0 36.0 72.4 118.2 38.3 189.3 133.6 490.8 76.1 90.4 117.9 334.2 94.3 312.0 108.4 347.1 50.0 36.5 37.8 53.5 10.1 24.2 12.2 22.2 25.7 31.8 89.1 92.2 50.7 60.3 78.6 55.7 62.9 52.0 72.3 57.9 29 30 31 32 15 15 15 15 15 15 This work 15 This work This work 33 33 33 33 33 33 Ph-PMO PhOS-40 Commercial carbon Mesoporous phenolic resin Mesoporous carbon a Milligrams of herbicide in the starting solution per gram of adsorbent. b Milligrams of herbicide adsorbed per gram of adsorbent. c Number of adsorption cycles performed with the same material. revealed signicant differences among them (Fig. 5). Broadly, as expected, the composition of the adsorbent, that is, the nature of the bridge, was decisive. The adsorption capacity toward the herbicide S-metolachlor was higher for benzenesilicas, followed by ethanesilicas and nally by silicas. Unlike silicas and ethanesilicas, all benzenesilicas adsorbed herbicide. The importance of the aromaticity in the adsorption of this non-ionic herbicide has also been evidenced for other very different adsorbents, as was the case for maize mulch residues exposed to microbial decomposition, whose capacity corresponded to the lignin fraction.28 However, signicant differences could be observed for different materials within the same group, that is, with the same composition. Obviously, within each group, an increase in the surface area of the materials would benet to the uptake of S-metolachlor because the area of interaction between the (organo)silicas and the herbicide would be more extensive. However, although the materials with the largest surface gave the highest uptakes, some results could not be properly explained. Thus, sample EOS-60 had a slightly lower surface area than sample EOS-20, but the former adsorbed much more herbicide than the latter. Apparently, the main factor governing the adsorption capacity within each group was the pore volume. Thus, the larger the pore volume, the higher the adsorption capacity was for both silica and organosilicas. This explained the differences between samples EOS-60 and EOS-20. By comparing the herbicide uptake for S-60, EOS-40 and PhOS-40, which were those materials with the highest pore volumes within each group, a higher efficiency of the organosilicas, particularly the benzenesilica, than the silica was observed. However, the distribution of pore size must affect the herbicide uptake because otherwise it is not possible to explain the signicant differences among them. Material PhOS-60 gave a lower herbicide uptake than sample PhOS-20, even though the former exhibited a higher pore Table 3 Fig. 6 29 29 Si MAS NMR results obtained for two benzenesilicas Material T1 (%) T2 (%) T3 (%) Da (%) OH/Sib PhOS-40 Ph-PMO 17.1 13.3 50.3 57.1 32.6 29.6 72 72 0.85 0.84 P a degree calculated from equation D ¼ [ (nTn)]/[(3 PCondensation Tn)] 100. b Content of silanol groups per silicon atom calculated from equation OH/Si ¼ (2T1 + T2)/(T1 + T2 + T3). Si MAS NMR spectra for two benzenesilicas. This journal is © The Royal Society of Chemistry 2015 RSC Adv., 2015, 5, 24158–24166 | 24163 RSC Advances volume. The signicant difference between these two benzenesilicas seemed to be caused by the much higher micropore area in PhOS-20 than in PhOS-60. A comparison between samples S60 and S-40 as well as between EOS-40 and EOS-60 suggested that a larger micropore area also favored the adsorption capacity of the adsorbent. Moreover, the best adsorbents within the silica and ethanesilica families, i.e., samples S-60 and EOS-40, respectively, displayed pores in the upper limit of the mesopores (18–30 nm) rather than macropores (>50 nm), apart from small mesopores. Indeed, sample PhOS-40 also exhibited an important fraction of pores at ca. 40 nm. Benzenesilica PhOS-40 can be considered as an efficient adsorbent for S-metolachlor, with an uptake in the same range as some organohydrotalcites and charcoals, although lower than some microporous and mesoporous carbons (Table 2). Furthermore, the herbicide S-metolachlor can be completely desorbed from this benzenesilica by simple extraction with ethanol. 3.2. Comparison of a non-ordered porous benzenesilica with a periodic mesoporous benzenesilica as adsorbents for Smetolachlor As previously stated, periodic mesoporous silica (PMS) and organosilicas (E-PMO and Ph-PMO) exhibited a good adsorption capacity within each family. Taking into account that the best adsorption behavior corresponded to benzenesilicas, we focused our study on materials PhOS-40 and Ph-PMO in order to ascertain the key factors inuencing their differences in adsorption. A previous study revealed that the aromatic p–p interactions prevailed for the adsorption of S-metolachlor on benzenesilica,15 whereas electrostatic and hydrophobic interactions played an important role in silica and ethanesilica. Although PhOS-40 and Ph-PMO were synthesized starting from the same precursor, both materials were obtained under rather different conditions. Therefore, 29Si MAS NMR spectra were registered in order to determine the hydroxylation degree in both samples (Fig. 6). Both materials exhibited practically identical composition of Si–OH groups. In fact, their condensation degrees and their number of OH groups per silicon atom were analogous (Table 3). Consequently, silanol groups cannot determine the different adsorption behavior of these materials. Fig. 7 Variation of zeta potential as a function of pH for two benzenesilicas. 24164 | RSC Adv., 2015, 5, 24158–24166 Paper Fig. 8 Pore size distribution curves for two benzenesilicas. Also, measurements of the variation of zeta potential with pH were carried out (Fig. 7). As observed, both materials showed very similar curves, thus indicating a similar surface charge, even though they were synthesized under different conditions. The point of zero charge (pzc) values for both materials were ca. 2.4, close to those reported for silica by other authors but lower than the value calculated by us following the mass titration method.15,34,35 Consequently, the disparities in the adsorption behavior for Ph-PMO and PhOS-40 cannot be attributed to differences in composition or surface charge. Since the surface chemistry of both benzenesilicas was similar, the different adsorption capacity of these materials should be ascribed to their different textural properties. Again, the pore volume, which was much higher for PhOS-40 than for Ph-PMO, seemed to be decisive for the adsorption of the herbicide. A comparison of their pore size distribution curves (Fig. 8) determined by a DFT method showed clear differences between both solids. Ph-PMO mainly exhibited mesopores with a narrow distribution centered at ca. 4 nm as well as some micropores, whereas PhOS-40 had pores distributed in the whole size range with an important contribution of large mesopores. A previously observed characteristic of Ph-PMO as adsorbent of S-metolachlor was its high capacity aer successive adsorptions cycles (Table 2).15 In fact, this material was able to retain herbicide up to saturation of its pore volume. This was attributed to the existence of phenylene bridges in its walls because ethylene bridges resulted in much less uptake and incomplete Fig. 9 Successive benezenesilicas. adsorptions of S-metolachlor for two This journal is © The Royal Society of Chemistry 2015 Paper lling of pores. Thus, a comparison between Ph-PMO and PhOS-40 was done by subjecting them to four adsorption cycles (Fig. 9). Interestingly, PhOS-40 was saturated aer only two successive adsorptions, whereas Ph-PMO was able to retain herbicide even aer four adsorption cycles. At that point, Ph-PMO showed an uptake of 133.6 mg g1 of S-metolachlor, thus still far from the maximum capacity reported for this material (490.8 mg g1).15 Therefore, the high adsorption capacity of Ph-PMO has to be ascribed not only to the existence of aromatic bridges but also to its mesoporous character that allows to ll its whole pore volume. 4. Conclusion Various non-ordered porous (organo)silicas have been synthesized using Tween surfactants as porogens. They have been characterized and used as adsorbents for the herbicide S-metolachlor. A combination of several factors has been shown to determine the adsorption capacity of these materials. The main one was the chemical nature of the bridge. Thus, benzenesilicas gave the higher uptake, followed by ethanesilicas and nally by silicas. The surface area and particularly the pore volume were also proven to be essential for having a high uptake. Concerning the pore size distribution, those pores in the micropore and/or the large mesopore range seemed to be more favorable to adsorption. Clearly, the adsorption capacities of materials with different textural properties for organic molecules must be compared with caution. The non-ordered materials were compared to the corresponding periodic mesoporous (organo)silicas. The use of the latter assured a good uptake in all cases unlike some of the nonordered (organo)silicas. Interestingly, the phenylene-bridged periodic mesoporous organosilica exhibited a higher capacity than the non-ordered benzenesilica aer several successive adsorption cycles and therefore it would be the material of choice for multiple-pass adsorption processes. Acknowledgements The authors would like to thank Spanish Ministry of Economy and Competitiveness (Projects MAT2013-44463-R and CTM2011-25325), Junta de Andalucı́a (Project P10-FQM-6181 and Research Groups FQM-214 and FQM-346) and FEDER funds. M.I.L. and R.O. acknowledge Spanish Ministry of Education, Culture and Sport for their FPU research and teaching fellowships. The authors are greatly indebted to Micromeritics for measurements of the textural properties of samples PhOS-40 and Ph-PMO in a 3Flex equipment. References 1 L. C. Hu and K. J. Shea, Chem. Soc. Rev., 2011, 40, 688. 2 S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc., 1999, 121, 9611. 3 B. J. Melde, B. T. Holland, C. F. Blanford and A. Stein, Chem. Mater., 1999, 11, 3302. This journal is © The Royal Society of Chemistry 2015 RSC Advances 4 T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 1999, 402, 867. 5 F. Hoffmann, M. Cornelius, J. Morell and M. Fröba, Angew. Chem., Int. Ed., 2006, 45, 3216. 6 P. Van Der Voort, D. Esquivel, E. De Canck, F. Goethals, I. Van Driessche and F. J. Romero-Salguero, Chem. Soc. Rev., 2013, 42, 3913. 7 H. L. Castricum, G. G. Paradis, M. C. Mittelmeijer-Hazeleger, R. Kreiter, J. F. Vente and J. E. Ten Elshof, Adv. Funct. Mater., 2011, 21, 2319. 8 S. M. Ibrahim, R. Xu, H. Nagasawa, A. Naka, J. Ohshita, T. Yoshioka, M. Kanezashi and T. Tsuru, RSC Adv., 2014, 4, 23759. 9 M. C. Burleigh, S. Jayasundera, M. S. Spector, C. W. Thomas, M. A. Markowitz and B. P. Gaber, Chem. Mater., 2004, 16, 3. 10 M. C. Burleigh, M. A. Markowitz, M. S. Spector and B. P. Gaber, Environ. Sci. Technol., 2002, 36, 2515. 11 S. A. Trammell, M. Zeinali, B. J. Melde, P. T. Charles, F. L. Velez, M. A. Dinderman, A. Kusterbeck and M. A. Markowitz, Anal. Chem., 2008, 80, 4627. 12 C. P. Moura, C. B. Vidal, A. L. Barros, L. S. Costa, L. C. Vasconcellos, F. S. Dias and R. F. Nascimento, J. Colloid Interface Sci., 2011, 363, 626. 13 C. B. Vidal, A. L. Barros, C. P. Moura, A. C. A. de Lima, F. S. Dias, L. C. G. Vasconcellos, P. B. A. Fechine and R. F. Nascimento, J. Colloid Interface Sci., 2011, 357, 466. 14 V. Gascón, I. Diaz, R. M. Blanco and C. Marquez-Alvarez, RSC Adv., 2014, 4, 34356. 15 R. Otero, D. Esquivel, M. A. Ulibarri, C. Jiménez-Sanchidrián, F. J. Romero-Salguero and J. M. Fernández, Chem. Eng. J., 2013, 228, 205. 16 S. O. Ganiyu, C. Bispo, N. Bion, P. Ferreira and I. BatonneauGener, Microporous Mesoporous Mater., 2014, 200, 117. 17 S. Martens, R. Ortmann, F. J. Brieler, C. Pasel, Y. Joo Lee, D. Bathen and M. Fröba, Z. Anorg. Allg. Chem., 2014, 640, 632. 18 Guidelines for drinking-water quality, Second edition, volume 2, Health criteria and other supporting information, World Health Organization, Geneva, Switzerland, 1996, http://www.who.int/water_sanitation_ health/dwq/2edvol2p1.pdf. 19 Guidelines for drinking-water quality, Second edition, volume 2, Health criteria and other supporting information, Addendum, World Health Organization, Geneva, Switzerland, 1996, http://www.who.int/water_ sanitation_health/dwq/2edaddvol2a.pdf. 20 A. Jurado, E. Vàzquez-Suñé, J. Carrera, M. López de Alda, E. Pujades and D. Barceló, Sci. Total Environ., 2012, 440, 82. 21 T. A. Albanis, T. G. Danis and M. K. Kourgia, Sci. Total Environ., 1994, 156, 11. 22 R. Meffe and I. de Bustamante, Sci. Total Environ., 2014, 481, 280. 23 S. Hartnett, S. Musah and K. R. Dhanwada, Chemosphere, 2013, 90, 1258. 24 M. C. Burleigh, S. Jayasundera, C. W. Thomas, M. S. Spector, M. A. Markowitz and B. P. Gaber, Colloid Polym. Sci., 2004, 282, 728. RSC Adv., 2015, 5, 24158–24166 | 24165 RSC Advances 25 M. A. Aramendı́a, V. Borau, C. Jiménez, J. M. Marinas and F. J. Romero, J. Colloid Interface Sci., 2004, 269, 394. 26 J. E. S. S. Lowell, M. A. Thomas and M. Thommes, Characterization of porous solids and powders: surface are, pore size and density, Kluwer Academic Publishers, Dordrecht (The Netherlands), 2004. 27 J. R. F. Rouquerol, K. S. W. Sing, P. Llewellyn and G. Maurin, Adsorption by powders and porous solids, Academic Press, Oxford, 2nd edn, 2004. 28 S. Aslam, P. Garnier, C. Rumpel, S. E. Parent and P. Benoit, Chemosphere, 2013, 91, 1447. 29 Y. Si, K. Takagi, A. Iwasaki and D. Zhou, Pest Manage. Sci., 2009, 65, 956. 24166 | RSC Adv., 2015, 5, 24158–24166 Paper 30 M. Bosetto, P. Arfaioli and P. Fusi, Sci. Total Environ., 1992, 123, 101. 31 A. Nennemann, Y. Mishael, S. Nir, B. Rubin, T. Polubesova, F. Bergaya, H. Van Damme and G. Lagaly, Appl. Clay Sci., 2001, 18, 265. 32 R. Otero, J. Fernández, M. Ulibarri, R. Celis and F. Bruna, Appl. Clay Sci., 2012, 65, 72. 33 R. Otero, D. Esquivel, M. A. Ulibarri, F. J. Romero-Salguero, P. Van Der Voort and J. M. Fernández, Chem. Eng. J., 2014, 251, 92. 34 J. S. Noh and J. A. Schwarz, J. Colloid Interface Sci., 1989, 130, 157. 35 J. Schwarz, C. Driscoll and A. Bhanot, J. Colloid Interface Sci., 1984, 97, 55. This journal is © The Royal Society of Chemistry 2015
