Modeling Boron Adsorption on Kaolinite

Autor: Shivi P. N. Singh, S. V. Mattigod
Rok vydání: 1992
Předmět:
Zdroj: Clays and Clay Minerals. 40:192-205
ISSN: 0009-8604
DOI: 10.1346/ccmn.1992.0400209
Popis: Boron adsorption at constant ionic strength (0.09 _+ 0.01 moles/liter of KC10, or Ca(C104)2) on 0.2-2 #m clay fraction of pretreated kaolinite was modeled using both phenomenological equations and surface complexation reactions. Phenomenological equations were expressed as linear relationships between the distribution coefficient and adsorption density or equilibrium concentration. The normalized form of the isotherms allowed the distribution coefficient to be predicted over a wide range of adsorption densities or equilibrium concentrations and pH. The Langmuir isotherm revealed a weak two-part linear trend supported by a similar behavior of the van Bemmelen-Freundlich isotherm. Potential adsorption mechanisms were assessed from these isotherms. The bases for the inner-sphere (surface coordination) and outer-sphere (ion-pair) surface reactions were postulated, and equations were developed and incor- porated into the generalized triple-layer surface-complexation model (TL(g)-SCM). Boron adsorption was best modeled using the inner-sphere complexes. The results confirm that the generalized triple-layer surface-complexation model can provide information regarding plausible reactions at the substrate/aque- ous interface. Intrinsic constants for postulated surface reactions were derived as fitting parameters over a range of pH and initial boron concentrations. Equilibrium adsorption studies conducted in labo- ratory model systems have provided the basis for the understanding of the surface chemistry of minerals. The bulk of the thermodynamic data extracted from these equilibrium adsorption studies has been based on experimental observations of metal-ion adsorption onto well-characterized substrates, consisting predom- inantly of metal-oxides. Models using highly detailed laboratory information have been widely applied for the oxide-solution interfacial reactions. Extensive studies of boron adsorption and fixation on clay min- erals have stemmed from boron's importance as an essential micronutrient, paleosalinity indicator, and trace component of fossil fuel wastes. There is, how- ever, limited evidence of the application of these mod- els to boron adsorption on kaolinite. This paper will attempt to address the application of these detailed modeling techniques to boron adsorption on kaolinite using the published experimental data of Mattigod et al. (1985). The application of the concepts and mathematical formalism of coordination chemistry to ion-particle interaction forms the basis of the surface-coordination or site-binding models. These models incorporate ex- plicit solution speciation and reaction stoichiometry, and are therefore useful devices for understanding the physico-chemical nature of the surface-complexation reactions. The constant capacitance surface complex- ation model (CC-SCM) has been used by Goldberg and Glaubig (1985, 1986) to predict boron adsorption be- havior on oxides and clay minerals including kaolinite.
Databáze: OpenAIRE
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