Fate of Adsorbed U(VI) during Sulfidization of Lepidocrocite and Hematite

The impact on U(VI) adsorbed to lepidocrocite (gamma-FeOOH) and hematite (alpha-Fe2O3) was assessed when exposed to aqueous sulfide (S(II)(aq)) at pH 8.0. With both minerals, competition between S(-II) and U(VI) for surface sites caused instantaneous release of adsorbed U(VI). Compared to lepidocrocite, consumption of S(-II)(aq) proceeded slower with hematite, but yielded maximum dissolved U concentrations that were more than 10 times higher, representing about one-third of the initially adsorbed U. Prolonged presence of S(-II)(aq) in experiments with hematite in combination with a larger release of adsorbed U(VI), enhanced the reduction of U(VI): after 24 h of reaction about 60-70% of U was in the form of U(W), much higher than the 2S% detected in the lepidocrocite suspensions. X-ray absorption spectra indicated that U(IV) in both hematite and lepidocrocite suspensions was not in the form of uraninite (UO2). Upon exposure to oxygen only part of U(IV) reoxidized, suggesting that monomeric U(W) might have become incorporated newly formed iron precipitates. sulfidization of Fe oxides can have diverse consequences for U mobility: in short-term, desorption of U(VI) increases U mobility, while reduction to U(W) and its possible incorporation in Fe transformation products may lead to long-term U immobilization.European Union's European Atomic Energy Community's (Euratom) Seventh Framework Programme FP7 [212287

Sulfidization of lepidocrocite and its effect on uranium phase distribution and reduction

Sulfidization of iron oxyhydroxides can be accompanied by a release of adsorbed uranium, thus enhancing the mobility of uranium in systems undergoing a shift in redox conditions. We investigated the phase distribution and redox state of uranium in batch experiments, in which lepidocrocite with adsorbed U(VI) was reacted with sulfide. The amount of added sulfide was varied in the experiments performed, at pH 8 and ionic strength of 0.1 M. Sulfide, when not added in excess, was removed from solution within less than 1 h of reaction time. Consumption of dissolved sulfide was accompanied by reduction of Fe(III) and formation of iron sulfide. Each addition of sulfide led to an instantaneous release of uranium into solution. This release is most likely caused by the exchange of hydroxide groups at the lepidocrocite surface by thiol groups which have a lower tendency to bind uranium. Along with the consumption of dissolved sulfide, part of the released uranium became reassociated with the solid phase. This can be explained by a reversal of the ligand exchange process at the solid surfaces. However, steady state concentrations of dissolved uranium remained higher than before sulfide addition, indicating that the product of lepidocrocite sulfidization has a lower affinity for uranium than the starting material. Reduction of U(VI) also contributed to the transfer of dissolved uranium back to the solid phase. X-ray absorption spectroscopy revealed that reduction of U(VI) occurred in all experiments. The extent of U(VI) reduction depended on sulfide addition, however, formation of UO2 occurred within a period of 48 h only when sulfide was added in excess. This suggests that the presence of dissolved sulfide is a prerequisite for fast reduction of U(VI) and formation of UO2. This would imply that the fast reaction of lepidocrocite with sulfide outcompetes reduction of U(VI) and, by this, kinetically inhibits the thermodynamically more favorable reduction of U(VI) to uraninite. Our results demonstrate that the transition from oxic to sulfidic conditions can lead to intermittent mobilization of uranium which is not expected based on equilibrium thermodynamics