Exchange between Dissolved U(VI) and Adsorbed and Precipitated Forms of Solid-Associated U

The mobility of uranium(VI) in subsurface environments can be limited by its adsorption to clay minerals and by chemical reduction to less mobile U(IV) solid phases. Even for adsorbed and precipitated U species that control the net release of U(VI) to the aqueous phase, dynamic exchange between phases at equilibrium conditions can occur with a pool of exchangeable solid-associated uranium considered as the labile fraction. In this study, the lability of uranium adsorbed to montmorillonite and uranium present in UO_2(s) was examined by an isotope exchange technique. Crystalline UO_2(s) containing depleted uranium (predominantly ²³⁸U and with small amounts of ²³⁵U) was contacted with a U(VI) aqueous solution enriched in ²³⁶U under oxygen-free (95% N_2(g) + 5% H_2(g)) conditions that altered the aqueous phase isotopic composition while minimally affecting the overall aqueous U(VI) concentration. Complete isotope exchange between U(VI) adsorbed onto montmorillonite and aqueous U(VI) was observed within 3.5 h. In contrast, no isotope exchange of aqueous U(VI) with UO_2(s) was observed even after 47 days of contact. X-ray photoelectron spectroscopy of the UO_2(s), both before and after its reaction with aqueous U(VI), showed the presence of U(VI), U(V), and U(IV) on the surface. Although U(VI) was present on the UO_2(s) surface, it was nonlabile over a timescale of weeks. This study highlights the difference in lability between adsorbed and precipitated U-containing species, and it illustrates that U(VI) at the surface of a UO₂ precipitate does not behave the same as U(VI) adsorbed to minerals not composed of U.