Introducción Conceptos fundamentales What is environmental geochemistry? • If you breathe air, drink water, or eat food, then you already have some experience with the environmental geochemical cycles of virtually every natural chemical element. Environmental geochemistry is that branch of geochemistry which seeks to quantify the natural geochemical cycles of elements at the surface of the Earth, and the effects of human activities upon these fluxes. Environmental Geochemistry is a relatively young branch of Geochemistry. Modern Geochemistry seeks to elucidate the natural cycling of chemical elements within the Earth. "Environment" as used here refers to the Biosphere which, as defined by Vernadsky, is "that region of the Earth containing Life." Environmental Geochemistry, therefore, considers all processes which directly or indirectly affect or influence living beings (both plants and animals, including humans). Environmental geochemistry today is very much process oriented, and includes not only element sources and sinks, but the distribution and transport of chemical elements and chemical substances (including e.g. xenobiotic organic chemicals) in the Atmosphere, Pedosphere, Hydrosphere and Biosphere (from microorganisms, through plants and animals, to Man). • When you breathe air, you inhale respirable organic and inorganic aerosols and gases of both natural and anthropogenic origin. Those of anthropogenic origin (such as fossil fuel burning and automobile exhausts) may easily be less than a micron (0.001 mm) in diameter and penetrate deep into the respiratory system. These particles are very much of concern as they may be highly enriched in potentially toxic "heavy metals" such as lead, as well as organic molecules such as polycyclic aromatic hydrocarbons (PAHs). Natural inorganic particles are generally larger, less soluble mineral materials such as clays supplied by windblown soil dust. PAHs are also produced naturally, however, for example during forest fires. Even halogenated organic compounds may be produced naturally, via redox reactions in soils and sediments. Trying to distinguish between natural and anthropogenic sources of metals and organic compounds to air, soil, and water is one of our greatest challenges. What is environmental geochemistry? • Similarly, the water you drink contains a variety of anions and cations of natural origin derived from the physical and chemical breakdown of minerals in rocks, soil, and sediments. Bicarbonate is the single most important anion in most natural waters; ultimately derived from atmospheric carbon dioxide, it provides a quantitative index of the amount of mineral dissolution which has taken place in a natural water system. More and more waters today, however, are enriched in contaminants such as nitrate from agricultural fertilizers and animal manures, as well as pesticides. Mineral weathering reactions at the surface of the Earth buffer the pH of natural waters, and it is the pH of these waters which largely controls the solubility, speciation, and therefore biological availability of trace metals. The breakdown of primary minerals consumes protons and dissolved CO2, yielding dissolved salts; this process regulates the pH and chemical composition of natural waters. However, addition of strong acids (from acid rain created by anthropogenic atmospheric emissions of sulphur and nitrogen oxides) can lower the pH and increase the concentrations of a wide range of trace metals, making them more available to organisms. Aluminum is of particular concern in poorly buffered watersheds of crystalline rocks because a small decrease in pH will not only enhance the solubility of Al, but promote the formation of potentially toxic inorganic, monomeric species. • The food plants we eat contain a wide range of metallic elements such as Cu and Zn which are essential to all living organisms and are taken up via the roots from the soil solution. However, plants may become contaminated with heavy metals, either from direct atmospheric deposition (from polluted air masses), or via plant uptake from contaminated soils. In this case cadmium is of special concern because it has no biological role, its natural abundance is very low, it is commonly enriched in phosphate fertilizers and sewage sludge, and is taken up by plants in direct proportion to the concentration in soils. A wide range of organic contaminants is now ubiquitous in the environment, and their effects on living organisms is of great interest to human and ecosystem health. What is environmental geochemistry? • The changes to the natural geochemical cycles of many elements effected by human activities have a very long history, and are remarkably well preserved in a wide range of geological archives such as ice cores from alpine and polar regions, peat bogs, and sediments from both marine and lacustrine environments. Careful analyses of these archives using sophisticated analytical instruments for isotopic, elemental, and molecular abundances, coupled with state-of-the-art techniques for age dating, can provide extremely detailed, high resolution reconstructions of both natural and anthropogenic environmental change. • We can use equilibrium chemical thermodynamics is used to predict the direction of chemical reactions. Specifically, the Gibbs Free Energy change of a reaction and the equilibrium constant Keq which is obtained from this term indicates whether or not a reaction will proceed spontaneously from left to right as written. However, this information offers no insight into the rate of the reaction. The rates of chemical reactions between mineral particles and aqueous solutions, for example, are governed not only by structural properties of the mineral surface such as steps and defects in the crystal lattice, but also by adsorption and desorption reactions taking place at the mineral-water interface. Here, the surface concentrations of protons and ligands, are especially important, but the size and molecular structure of ligands also plays an important role. Oxidation-reduction reactions are notoriously slow, e.g. those involving C, N, S, Fe, Mn, but they are often mediated and catalyzed by microorganisms or enzymes; thus, environmental geochemical studies of many elements must fully considered the relevant biological processes. In addition to the chemical and biological processing of materials, physical transport phenomena (diffusion and advection) are critical to understanding dispersion processes. Conceptos Fundamentales Biogeoquímica • Geoquímica de la biosfera, geoquímica orgánica, geoquímica ambiental • Ciencia interdisciplinar que integra el estudio de las reacciones químicas en la biosfera (concepto de Vernadsky: parte de la tierra capaz de sustentar la vida) • Tierra = sistema complejo irreproducible en el que interaccionan litosfera, hidrosfrera, atmósfera y biosfera. Propiedades emergentes. • Interacción recíproca organismos y su entorno, evolución continua • Objetivo: entender la conexión entre litosfera, hidrosfrera, atmósfera, biosfera a través de los procesos biogeoquímicos y la influencia de la actividad antrópica. • Niveles de estudio: • • • – – – Estático: composición, estructura,. .. Dinámico: reactividad, afinidad, procesos,… Sistémico: interacción, retroalimentación,.. Medios de estudio – Observación, archivos ambientales – Capacidad analítica – Experimentación – Modelización Dinámica de sistemas, Modelización global Procesos/Condiciones de la Tierra: resultado de flujos de energía y ciclos de materia dentro/entre los subsistemas Conceptos Fundamentales • Procesos/Condiciones de la Tierra: resultado de flujos de energía y ciclos de materia dentro/entre los subsistemas Ciclos biogeoquímicos Ciclo Biogeoquímico • • Transferencia de elementos químicos/compuestos entre diversos subsistemas terrestres a escalas diversas de tiempo/espacio. Diagramas causales. Características básicas de un ciclo: contenido en reservorios y flujo entre ellos Reservorio (caja, compartimento). M (masa, moles) Flujo (F) (dM/dt). Densidad de flujo (dM/dS). Fuente (Q). Sumidero (S). S=kM, (proceso de primer orden); S=K (proceso de orden cero),.. Estado estacionario. ΣQ=ΣS, i.e. M no cambia con el tiempo. Tiempo medio de residencia: TMR=M/ΣQ ó M/ΣS Ciclo • • • Ciclos generales (ciclo del agua y ciclo de las rocas), ciclos de los elementos Componente abiótico y biótico Organismos vivos: crecen en condiciones muy variables (temperatura, presión, salinidad, EhpH,…), gran importancia (C, N, S, P, especialmente). Ciclos biogeoquímicos Ciclo Biogeoquímico Relaciones casuales Relaciones simples: positivas, negativas, encadenadas Relaciones complejas: bucles de realimentación (+/-) Retroalimentación +: Acción de refuerzo Retroalimentación -: Equilibrio dinámico Conceptos Fundamentales Ciclo Biogeoquímico Ciclo geoquímico. En geología y geoquímica, los reservorios y flujos de la figura. Conceptos Fundamentales Ciclo Biogeoquímico Ciclo biogeoquímico Ciclo biogeoquímico. Ciclos globales y regionales de los elementos de la vida C, O, N, S y P, con reservorios que incluyen el todo o una parte de la atmósfera, los océanos, los sedimentos y los organismos vivos. El término puede ser aplicado a los ciclos correspondientes de otros elementos o compuestos.
