Pb(II) and Zn(II) adsorption onto Na- and Ca-montmorillonites in acetic acid/acetate medium: experimental approach and geochemical modeling.

Smectites are usually used as a clay barrier at the bottom of subsurface waste landfills due to their low permeability and their capacity to retain pollutants. The Na- and Ca-saturated SWy2 montmorillonites were interacted with initial Zn(NO(3))(2) or Pb(NO(3))(2) concentrations ranging from 10(-6) to 10(-2)M with a solid/liquid ratio of 10 g L(-1) and using acetic acid/acetate as buffer at pH 5 in order to reproduce a biodegradable leachate of a young landfill. These experiments revealed that Zn and Pb sorption onto Na-SWy2 is higher than that onto Ca-SWy2 in the whole range of concentrations. Metal retention into both montmorillonites increases with the decrease in acetic acid/acetate concentration. The two-site protolysis model with no electrostatic term (2SPNE model) was used to model these experiments. As the experimental data of Zn sorption were well fitted, this model was validated and has been improved by taking into account the metal-acetate complexation in solution. In order to validate the model for Pb sorption, new selectivity coefficients have been determined, namely logK(c)(PbNa)=0.5 for Na-montmorillonite and logK(c)(PbCa)=0.3 for Ca-montmorillonite.