Townsend, and 10 co-authors, reported in 2019 that uranium is a risk-driving radionuclide in both radioactive waste-disposal and contaminated-land scenarios. In these environments, a range of biogeochemical processes can occur, including sulfate reduction, which can induce sulfidation of iron (oxyhydr)oxide mineral phases. During sulfidation, labile U(VI) is known to reduce to relatively immobile U(IV); however, the detailed mechanisms of the changes in U speciation during these biogeochemical reactions are poorly constrained. The authors performed carefully controlled sulfidation experiments at pH 7 and pH 9.5 on U(VI) adsorbed to ferrihydrite and investigated the system using geochemical analyses, X-ray absorption spectroscopy (XAS), and computational modeling.
Analysis of the XAS data indicated the formation of a novel, transient U(VI)−persulfide complex as an intermediate species during the sulfidation reaction, concomitant with the transient release of uranium to the solution. Extended X-ray absorption fine structure (EXAFS) modeling showed that a persulfide ligand was coordinated in the equatorial plane of the uranyl moiety, and formation of this species was supported by computational modeling. The final speciation of U was nanoparticulate U(IV) uraninite, and this phase was evident at 2 days at pH 7 and 1 year at pH 9.5. Our identification of a new, labile U(VI)-persulfide species under environmentally relevant conditions may have implications for U mobility in sulfidic environments pertinent to radioactive waste disposal and contaminated land scenarios.
The authors concluded that by combining our controlled experimental methodology with quantum chemical calculations, we provide strong evidence for the formation of a novel U(VI)−persulfide complex during iron (oxyhydr)oxide sulfidation, which facilitates the transient release of U into the aqueous solution. This speciation is newly observed under aqueous conditions and provides new insight into U behavior in environments where sulfide is present. Furthermore, the existence of a U(VI)−persulfide under environmentally relevant conditions provides the intriguing potential for U−S complexes to be used in a range of applications, from actinide separations in nuclear fuel reprocessing to U materials chemistry and potential catalytic reactions.
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