Potential Measurements Used To Study The Bentonite (1)

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Potential Measurements Used To Study The Bentonite-Pollutant and KaolinPollutant Interphase
Conference Paper in ECS Transactions · January 2011
DOI: 10.1149/1.3660628
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Emigdia Guadalupe Sumbarda Ramos
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Autonomous University of Baja California
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Benemérita Universidad Autónoma de Puebla
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ECS Transactions, 36 (1) 341-347 (2011)
10.1149/1.3660628 © The Electrochemical Society
Potential Measurements Used To Study The Bentonite-Pollutant and KaolinPollutant Interphase.
E.G.Sumbarda-Ramos a, M.T. Oropeza-Guzmán a, R. Salgado-Rodríguez a, M.M.M. Teutli-León b
a
Centro de Graduados e Investigación del Instituto Tecnológico de Tijuana. Blvd.
Industrial, s/n, Mesa de Otay, Tijuana B.C., C.P. 22500.
b
Facultad de Ingeniería, Benemérita Universidad Autónoma de Puebla, Edif. 123, Ciudad
Universitaria, Av. San Claudio y Blvd. Valsequillo, Puebla, Pue., México.
To support the application of an electrokinetic treatment to clay
soils polluted with metals, a study of the interphase soilpollutant-soil solution was carried out based on Zeta Potential
(]) measurements. The physicochemical and transport
phenomena that occur during electrokinetic treatment due to
the soil-pollutant interaction at different electrolytes can be
deduced from the ] values. The experimental work begins with
the physicochemical characterization of two clays: Bentonite
(B) and Kaolin (K). Both samples were converted to homoionic
form in order to observe the superficial adsorption of cations.
1M solutions of (NH4)2SO4, NH4Cl and CuSO4, CuCl2 were
chosen for this purpose. Copper concentration, cation exchange
capacity (CEC) and ] were analyzed in the homoionic samples
to study the soil-pollutant interaction. Results show that the
adsorption/desorption process can be studied by the ] values;
these values reflect the chemical changes at the soil-pollutantsoil solution interphase.
Introduction
Electrokinetic treatment is a physicochemical method in which it is necessary to generate
an electric field in order to orientate the displacement of the pollutants. It is known that
during the electrokinetic process three principal transport phenomena occur: electromigration (movement of ions), electrophoresis (movement of colloidal particles in
suspension in soil solution) and electro-osmosis (water movement) (1-2). These transport
phenomena relate to the soil physicochemical characteristics, as well as to those of the
pollutant and the interactions between these. When the diagnosis of an electrokinetic
treatment begins, it is necessary to study the physicochemical soil-pollutant interactions
and the effect of the solution properties; this to understand the transport phenomena that
occur. Also the factors that define the extraction of pollutants when the electrokinetic
treatment is applied are identified as: soil texture, pollutant type and concentration, soil
solution pH, buffer capacity of the soil, Zeta Potential (]), Electroosmotic flow (EOF)
direction, operation parameters (electrodes, cell potential and current), addition of
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ECS Transactions, 36 (1) 341-347 (2011)
surfactants or chelating agents to increase the removal efficiency (3). During the
electrokinetic process the pollutant extraction efficiency depends on the knowledge of the
physicochemical properties of the soil, the pollutant and the fluid medium being
transported from one end to another of an electrokinetic cell. Thus, in this work the
polluted soil system is simplified as a series of particles surrounded by an ionic solution
that gives them surface charge and developing a space interfacial electrical potential
different from each phase. Such a situation will trigger a series of surface phenomena, in
the case of the study systems are directly related to soil-pollutant-soil solution
interactions, and therefore to the value of electrical potential at the shear plane
(hypothetical plane from which the fluid phase it is deformed), commonly known as Zeta
Potential (]).
Experimental Methodology
Physicochemical Characterization
The characterization of the soil samples included the determination of these
physicochemical properties: pH (4), electrical conductivity (4), buffer capacity (5),
moisture (4), texture (4), Cation Exchange Capacity (CEC) (6) and Zeta Potential (])(7).
Clay samples Homoionization.
The natural clay samples used in this work are reagent grade Bentonite (B) Sigma
Aldrich and Kaolin (K) Sigma Aldrich, so on its surface the possible exchangeable ions
are the proton as cation and hidroxyl as anion, depending on the solution conditions. To
prepare the homoionic samples and observe the surface adsorption of cations, four
solutions 1M were used: (NH4)2SO4 FagaLab, CuSO4 FagaLab, NH4Cl FagaLab y CuCl2
FagaLab. The natural soil is mixed and stirred into the solution for 6 days. After this time
the clay is separated from the solution by centrifugation. The solid was washed with
deionized water, them is mixed and stirred in deionized water for 30 minutes and
separated by centrifugation at the conclusion of this time. One more wash is done
following the same procedure.
Clay-Pollutant interaction study.
To evaluate the Clay-pollutant interaction the prepared modified samples are chosen
to be tests and compare the soil surface properties with the natural sample. To analyze the
copper adsorption on the soil surface the procedures DG-EN-13 and DG- EN-14 from the
MILESTONE microwave equipment manual were followed to prepare the homoionic soil
samples for the determination of copper concentration by Atomic Absorption
Spectroscopy (AAS) analysis. The ammonium concentration will be determined
following the CEC-C5-B-1 procedure (8). The EPA 9081 procedure was followed to
determine the Cation Exchange Capacity (CEC) for the natural B and K samples, the
homoionic samples and the washed samples. Finally ] it was determine for all the
samples, following the reported procedure (7) which consist on the dispersion of 25 mg
of sample in 25 mL of deionized water during 30 min on ultrasonic bath. The equipment
used for this measurement was the Brookhaven zeta potential/particle size analyzer.
Particle size was measured to the same sample used to measure ], only the top of the cell
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ECS Transactions, 36 (1) 341-347 (2011)
change. For the particle size a plastic top was used and for the ] measurement the top was
removed to insert the electrode top.
Results and discussion
Table 1 shows the results of the physicochemical characterization performed using the
methods mentioned in the experimental section to the soil Bentonite (B) and Kaolin (K).
The results show that even when both samples are clay they have different properties that
will induce a specific behavior at the clay-pollutant interaction. For example, the CEC
values gives an idea of the sites for possible interaction with a pollutant, considering this,
B will interact with the cation almost three times more than the C soil. Table 2 presents
the description of the modified samples (homoionic). In general it is observed that the
value of the CEC values for the homoionic samples increases compared with natural B
and K samples. This change can be attributed to the creation of new exchangeable sites,
this at the interphase between clay and electrolytic solution, the permeable surface is
increased depending on the chemical composition of the solution, these means that the
interfacial area is increased depending on the chemical composition of the electrolyte.
For these reason the exchangeable sites are created by the chemical conditions of the
solution that will change the ] value and the hydraulic conductivity (HC). The different
ionic radius of cations added in the solution 1 M may increase the HC (9), suggesting an
increase of absorb ions in the clay through the opening of nanopores and micropores. The
hydraulic conductivity is the easiness with which water can move through pore spaces or
fractures, in fact it is an effect reflected on the double layer thickness because of the
cationic radius may open or close the soil structure layers. In all cases there is greater the
value of CEC for B compared with C sample derived from the expandable structure of
bentonite. It is observed that the CEC value is greater for SO42- than Cl-. Moreover when
the cation is Cu2+ the CEC values for both soil samples are lower compared to values
obtained from CEC when the cation is NH4+. These results indicate that the retention of
Cu2+ is greater than NH4+ for both B and C (homoionic) clays. The relationship between
adsorption of a cation on the soil surface and CEC values are shown in Figure 1. The
CEC and the ] value are mutually dependent since both are modify by the chemical
conditions of the solution (pH, concentration, ionic strength). The 1M solution modifies
the adsorption equilibrium of the cation specie in the soil/solution system. The original
cation in B and C soil is H+, so it is exchanged with Cu2+ or NH4+ and then released into
the solution. This physicochemical effect will change the ] value, then the double layer
composition and its thickness. Also in Figure 2 it is observed for all samples that the Zeta
Potential (]) values are less negative while the CEC increase indicate that the cations are
not only retained but adsorbed. By Adsorb we mean the specie that is place in the
interchangeable sites at the soil surface. This help to determine in a real site if the
metallic pollutant is absorbed or also adsorbed. By Absorb we mean the total amount of
specie that interact with the soil and is retained by the soil (The occluded specie). Finally
if the Zeta Potential (]) value is less negative the concentration present of the cation is
higher (Figure 3). The variation of the DL thickness its result of the chemical
composition of the aqueous and solid interphase, so the ] value also reflects the
adsorption/desorption process.
Since the system is idealized the only
adsorption/desorption process that occur is: H+ (natural soil) whit the NH4+ or Cu2+ (1M
solution) at the homoionization stage. To measure CEC the adsorption/desorption process
that occur are: NH4+ or Cu2+ (homoionic soil) whit Na+ (1M NaCH3COO solution), and
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ECS Transactions, 36 (1) 341-347 (2011)
Na+ (homoionic soil) whit the NH4+ (1M NH4CH3COO solution). The relationship
between the CEC, Copper concentration and the Zeta Potential (]) values proved that the
measurement of Zeta Potential (]) can be used as a parameter to study, in a polluted soil,
the adsorption/desorption process and its thermodynamic predictions like the Change of
the Free Gibbs adsorption energy (ΔGads) (10-11).
.
TABLE 1. Physicochemical Characterization Results for the B and K Soils.
Soil
EC
pH
Moisture (%
Zeta
Particle size
-1
(mScm )
Field strength
Potential
capacity)
(mV) / H2O
Buffer capacity
-1
C.E.C.
(nm)
(mMpH )
(mol/kg)
B
13.0
9.09
31.05
-34.8
47.9
0.54
0.911
C
2.4
6.01
25.20
-35.6
32.5
0.92
0.261
TABLE 2. Modified Samples (Homoionic) for B and K Clays After the Second Wash.
Label
Cation Exchange
BS
Cu2+
KS
2+
Cu
BCl
Cu2+
KCl
2+
Cu
BN
NH4+
KN
NH4
+
BNCl
NH4+
KNCl
+
NH4
Electrolyte solution
CuSO4
CuCl2
(NH4)2SO4
NH4Cl
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ECS Transactions, 36 (1) 341-347 (2011)
Figure 1. Copper concentration and CEC values obtained for B and K samples.
Figure 2. CEC and Zeta Potential (]) values obtained for B, K, BN, KN, BS, KS, BCl, KCl, BNCl
y KNCl samples.
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ECS Transactions, 36 (1) 341-347 (2011)
Figure 3. Copper concentration and Zeta Potential (]) values obtained for B and K
samples.
Conclusions
The results of determining the Zeta Potential (]) value shows that it is a useful
parameter to study the soil-cation physicochemical interactions. Zeta Potential (]) values
show a direct relationship with the adsorption/desorption process of cations from the soil
surface; it also reflects the equilibrium of this cations with the soil solution. If the Zeta
Potential (]) values are less negative indicates that a greater proportion of a cation will
absorb more then adsorb. To infer if a cation is adsorbed or absorbed it is necessary to
prepare samples by the procedures DG-EN-13, DG- EN-14 and EPA 9081and relate the
copper concentration measured by these digestion procedures with CEC and Zeta
Potential (]) values that represent the adsorbed copper .
Acknowledgments
The authors are grateful with CONACYT for the financial support given to the
development of this investigation.
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ECS Transactions, 36 (1) 341-347 (2011)
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