Prediction of Cadmium Complexation in Solution and Adsorption to Montmorillonite

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Abstract
Effects of inorganic complexation in solution, ionic strength, and clay concentration on Cd adsorption to montmorillonite were investigated. Increased ionic strength decreases Cd adsorption. The presence of a complexing anion such as Cl‐ inhibits Cd adsorption. When the complexing anion was HCO‐3, disappearance of Cd from equilibrium solution was enhanced compared to the case where the anion was ClO‐4, which does not form complexes with Cd. The explanation of the latter phenomenon is still incomplete, the two possibilities being either strong binding to the clay of the complex CdHCO+3, or precipitation of otavite (CdCO3). Raising clay concentration enhances Cd adsorption. A model that predicts cation binding and cation exchange in clay suspensions and calculates the surface potential, the concentrations of the species bound to the clay surface, in the double layer and in the equilibrium solution, was coupled with a subroutine to calculate complexation reactions in solution. Agreement between observed and calculated values of adsorbed Cd was fairly good. The binding coefficients that best fit the data are 1, 10, and 30 for Na+, Cd2+, and CdCl+, respectively.