U(VI) adsorption to heterogeneous subsurface media: Application of a surface complexation model

The pH-dependent adsorption of U(VI) onto three heterogeneous, subsurface media from the Department of Energy (DOE) Oak Ridge Reservation, Savannah River Site, and Hanford Reservation was investigated. The three materials contained significant quantities of iron and manganese oxides with nearly identical extractable iron oxide contents (25.3-25.8 g/kg). A model independently developed for the adsorption of U(VI)to synthetic ferrihydrite (Waite, T. D.; Davis, J. A.; Payne, T. E.; Waychunas, G. A.; Xu, N. Geochim. Cosmochim. Acta 1994, 58, 5465-5478) was able to predict the major features of the pH-dependent U(VI) adsorption to the materials under the assumption that all the dithionite-citrate-bicarbonate extractable iron oxide was present as ferrihydrite. Further experiments with the Oak Ridge soil as a function of carbonate and U(VI) concentration indicated that the model could predict pH-dependent U(VI) adsorption to within a root mean square error of 0.163-0.408, even under conditions outside of those for which the model was developed. These results indicate that this model could be used as a first approximation in predicting U(VI) adsorption and transport in the subsurface. U(VI) adsorption also decreased at pH >10, even in the absence of carbonate, which is of potential importance to U(VI) mobility in extreme environments present in the subsurface at some DOE facilities. The pH-dependent adsorption of U(VI) was fundamentally different in systems with a constant CO₂ partial pressure as compared to a constant total carbonate concentration. Experiments at constant CO₂ partial pressure may not be representative of the conditions present in the subsurface, and a constant carbonate concentration does not always result in decreased U(VI) adsorption at higher pH values.