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 tr ace species. The macroscopic description of these interactions with a therm odynamic approach has been limited by the lack of data on the binary intera ction between carbonate and relevant mineral surfaces. This work follows fr om two detailed studies of carbonate adsorption on goethite (4, 13). It sho ws that independent triple-layer surface complexation modeling (TLM) of car bonate adsorption allows successful descriptions of carbonate-trace element ternary sorption on this oxide, using relatively simple and optimal stoich iometries. Carbonate adsorption was considerably enhanced in the presence o f Pb(II), despite an invariant total Pb(II) sorption to equilibration with UP to 1% CO2(g). Both the Pb(II)carbonate system behavior and the anion-lik e PH adsorption behavior of U(VI) in the presence Of CO2 were successfully modeled using binary and ternary metal-bound surface complexes. The signifi cant reduction of Cr(VI) adsorption edges to lower PH values in the presenc e of CO2 was accurately simulated and explained via site competition and su rface electrostatic repulsion effects on the predicted inner- and outer-sph ere Cr(VI) surface complexes formed. The results of this research are highl y relevant to modeling of metal transport field data and of potential soil remediation schemes using carbonate.