Assessing Cd, Co, Cu, Ni, and Pb Sorption on montmorillonite using surface complexation models

Highlights: → Cd, Co, Cu, Ni, and Pb sorption on montmorillonite is measured at 6 distinct conditions. → The data are used to develop a diffuse layer model with variable charge and fixed charge sites. → The DLM lacked the robustness to accurately predict adsorption at all conditions. - Abstract: Cadmium, Co, Cu, Ni and Pb adsorption is measured on montmorillonite as a function of pH (3-11), ionic strength (0.001-0.1 M NaNO3), and sorbate concentration (0.1-10 μM metal on 0.5 g/L solid). Sorption of all metals shows strong dependence on ionic strength and sorbate concentration, as well as a break in the slope of the edge, indicative of a 2-site interaction with montmorillonite. The resulting adsorption edges are used to parameterize diffuse layer surface complexation models (DLMs) for each metal. A 2-site DLM with a bidentate variable charge surface hydroxyl site and a bidentate permanent charge exchange site produced good fits for the individual experiments, but lacked the robustness to accurately predict adsorption across the entire experimental range. Other models, such as CCM, TLM, or CD-MUSIC may be required for more accurate predictions across broad ranges of solution conditions.