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Competitive ion-exchange reactions of Pb(II) (Pb2+/PbCl+) and Ra(II) (Ra2+) on smectites: Experiments, modeling, and implication for 226Ra(II)/210Pb(II) disequilibrium in the environment.

In this study, new experimental data for the adsorption of lead onto a swelling clay mineral with a tetrahedral charge (beidellite) at the ultratrace level (<10-10 mol/L) are presented. The data were interpreted using an ion-exchange multisite model that considers the sorption of major cations (including H+), which always compete with trace elements for sorption onto mineral surfaces in natural environments. The ability of the proposed model to predict experimental Kd values under various conditions of ionic strength (fixed by NaCl solutions) and aqueous cation compositions (including Pb2+ and PbCl+) was tested. The proposed model was applied to experimental data previously published for other types of swelling clay minerals, and the results were compared with the results obtained using previously published models. The preferential adsorption of chloride ion pairs, as well as the effect of the swelling clay crystal chemistry on lead adsorption, were assessed. Finally, the selective adsorption behavior of 226Ra compared to 210Pb was demonstrated, which has implications for the study of many environmental processes using isotope partitioning.