Role of surface alteration in determining the mobility of U(VI) in the presence of citrate: implications for extraction of U(VI) from soils

In the present study, the adsorption of U(VI) by a natural iron-rich sand in the presence of citrate was studied over a range of citrate concentrations and pH values. Adsorption of U(VI) on the iron-rich sand decreased in the presence of increasing concentrations of citrate. Adsorption of citrate to the sand was weak under most conditions studied. Several explanations for the adsorption behavior of U(VI) and citrate were investigated, including aqueous complexation of U(VI) by citrate, competition of U(VI) and citrate for adsorption sites, and extraction of Fe and Al from the sorbent surface by citrate (surface alteration). Although aqueous complexation of U(VI) by citrate may still play a significant role, both competitive adsorption and aqueous complexation proved to be inadequate explanations of the adsorption behavior. Both physical surface alteration (i.e., loss of surface area) and chemical surface alteration (i.e., change in the chemical composition of the sand surface) were investigated, with chemical surface alteration controlling the bulk of U(VI) adsorption. Considering these results, remediation schemes that involve organic complexing agents should address the possibility of surface alteration affecting radionuclide adsorption and mobility.