Adsorption of lead and cadmium ions from aqueous solution to the montmorillonite/water interface

In order to prevent contamination of subsoil and groundwater by leachates containing heavy metals, montmorillonite linings are used on landfill bottoms. It is therefore important to understand ion uptake by this clay. The sorption of Pb(II) and Cd(II) from aqueous solution to the montmorillonite/water interface has been studied as a function of pH, for three concentrations. Two montmorillonites have been used: an industrial bentonite and a reference clay. Clay acidity constants have been determined, indicating that both clays seem to have characteristics similiar to each other. Adsorption increases with pH. Whatever the clay or pH, lead is retained more than cadmium. A model that assumes two kinds of binding sites can describe metallic ion uptake: XNa groups, responsible of ion exchange, and ampholitic surface hydroxyls SOH, responsible of surface complexation.