Cation exchange and surface complexation of lead on montmorillonite and illite including competitive adsorption effects

The adsorption of divalent lead on the homoionic Na forms of illite (Na-IdP) and montmorillonite (Na-SWy) was investigated by batch type adsorption experiments. Ion equilibrium experiments between Pb2+ and Na+ yielded selectivity coefficients for cation exchange on the planar sites of both clay minerals. Pb adsorption edges (pH 2-11) and isotherms (pH 3, 6 and 7 and at 10-(12) M < [Pb-eql] < 10(-3) M) were measured and modelled with the two site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model. The surface complexation constants derived from an iterative fitting of the edges and isotherms could describe all of the Pb adsorption data sets satisfactorily. The anomalous adsorption behaviour of trace Pb on Na-IdP in the acidic region required the consideration of an additional site with a low capacity and high affinity and an associated pH independent surface complexation reaction. Adsorption experiments with Pb/Ni, Pb/Co, Pb/Zn and Pb/Eu on Na-IdP indicated, depending on the experimental set-up, partially and/or non-competitive behaviour. The competitive adsorption experiments could be quantitatively described with a modified 2SPNE SC/CE model. In the case of non-competitive adsorption additional metal specific sets of strong sites are required. In the case of partial competitiveness, a certain fraction of the strong site capacity is assumed to be competitive whereas the remaining fraction is non-competitive.