Surface complexation modeling of carbonate effects on the adsorption of Cr(VI), Pb(II), and U(VI) on goethite.

Dissolved carbonate species are known to affect the sorption behavior of trace species. The macroscopic description of these interactions with a thermodynamic approach has been limited by the lack of data on the binary interaction between carbonate and relevant mineral surfaces. This work follows from two detailed studies of carbonate adsorption on goethite (4, 13). It shows that independent triple-layer surface complexation modeling (TLM) of carbonate adsorption allows successful descriptions of carbonate-trace element ternary sorption on this oxide, using relatively simple and optimal stoichiometries. Carbonate adsorption was considerably enhanced in the presence of Pb(II), despite an invariant total Pb(II) sorption to equilibration with up to 1% C02(g). Both the Pb(II)-carbonate system behavior and the anion-like pH adsorption behavior of U(VI) in the presence of C02 were successfully modeled using binary and ternary metal-bound surface complexes. The significant reduction of Cr(VI) adsorption edges to lower pH values in the presence of C02 was accurately simulated and explained via site competition and surface electrostatic repulsion effects on the predicted inner- and outer-sphere Cr(VI) surface complexes formed. The results of this research are highly relevant to modeling of metal transport field data and of potential soil remediation schemes using carbonate.