Surface Speciation Modeling of Heavy Metals in Kaolin: Implications for Electrokinetic Soil Remediation Processes
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Surface Speciation Modeling of Heavy Metals in Kaolin: Implications for Electrokinetic Soil Remediation Processes ASHRAF Z. AL-HAMDAN Department of Civil and Environmental Engineering, University of Central Florida, P.O. Box 162450, Orlando, Florida 32816, USA [email protected]
KRISHNA R. REDDY∗ Department of Civil and Materials Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, Illinois 60607, USA [email protected]
Received September 29, 2003; Revised March 1, 2005; Accepted April 6, 2005
Abstract. The driving mechanisms for flux occur at interface between the soil particles and solution during electrokinetic soil remediation and the nature of this interface affects the electrokinetic response of the system. The pH-dependent adsorption of heavy metal contaminants by kaolin and the sensitivity of kaolin zeta potential to the aqueous phase properties are two important aspects that complicate the metal movement during electrokinteic soil remediation. This paper addresses these aspects and presents an electrostatic adsorption model that describes the behavior of kaolin surface for Cr(VI), Cr(III), Ni(II), and Cd(II) under various chemical conditions. This study showed that the aqueous properties: pH, ionic strength and the presence of the heavy metals Cr(VI), Cr(III), Ni(II), and Cd(II) in the system affect the zeta potential of kaolin surface. The zeta potential of kaolin shifts to a more negative value if the system pH increases. However, it shifts to a more positive value if the system ionic strength or metal concentration increases. It was found that the amount of the heavy metal adsorbed by kaolin has a pronounced sensitivity to the pH. As a result of the adsorption modeling, the constant capacitance protonation-dissociation int intrinsic constants of kaolin are: pKint + = −3.8 ± 0.5 and pK − = 9.4 ± 0.5. The ion-kaolin surface complexation int constants (pK ) of Cr(VI), Cr(III), Ni(II), and Cd(II) are: −12.5 ± 0.5, −5.0 ± 1.0, 2.6 ± 0.6, and 3.3 ± 1.0, respectively. Keywords: kaolin, electrophoresis experiments, equilibrium constants, electrostatic adsorption models, electrokinetic remediation Introduction The mobility and reactivity of heavy metals in soils are critically dependent on the aqueous speciation, controlling their solubility and adsorption behavior. Evaluation of remediation strategies for heavy metalcontaminated soils requires that the partitioning of con∗ To
whom correspondence should be addressed.
taminants between the pore water and the soil solids is understood. Modeling of the surface speciation can be used to predict the trends in adsorption behavior of heavy metals in soils with master variables such as pH and redox status. Application of sound thermodynamic principles to adsorption reactions provides a modeling framework that is theoretically rigorous, which can be applied for even complex adsorption reactions (Grenthe and Puigdomenech, 1997).
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Adsorption/desorption process is one of the important physico-chemical processes affect
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