Cadmium Adsorption on Mixed Mineral Assemblages of Kaolinite, Silica and Hydrous Ferric Oxide

The bioavailability and mobility of trace metals in near-surface environments are greatly affected by metals binding to minerals via sorption reactions. Because Cd^II is highly toxic, having adverse affects on the respiratory and reproductive systems, its presence in aqueous environments at elevated levels is a source of concern. The adsorption of Cd^II on kaolinite, hydrous ferric oxide and silica, solids which are abundant in near-surface soils and sediments, together with mixtures of the three solids, was investigated as a function of pH, sorbate/sorbent ratios and background electrolyte concentrations. The experimental Cd^II adsorption data for the single-solid systems were used to obtain thermodynamic stability constants within a double layer surface complexation model framework. The thermodynamic parameters optimized for the single-solid systems were then used to predict adsorption for binary and ternary mixtures of the three solids based on the component additivity model proposed by Davis et al. (1998, ES&T). The statistical goodness-of-fit of the model predictions for the mixed systems was compared to the goodness-of-fit for the single solid systems. For most systems, the goodness-of-fit was equivalent, within 95% confidence intervals, in the parameterized single and predicted mixed solid systems. This suggests that mineral-mineral interactions, at least in these relative short time-scale experiments, can be neglected.