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. Here, we performed highly 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.

Naturally occurring minerals, such as the iron sulfide mackinawite, play a key role in the remediation of uranium from groundwater systems. Here, density functional theory (DFT) is used to investigate the interaction of uranium with the most stable surface of stoichiometric mackinawite, {001}-S. The high reactivity of the mineral toward oxygen may affect its ability to sequester uranium; therefore, two models of oxidized mackinawite are also used to study the effect of surface oxidation on adsorption. Weak adsorption of mononuclear uranyl­(VI) complexes is found on stoichiometric mackinawite; however, equivalent adsorption modes on the oxidized mackinawite models generally exhibit stronger adsorption. Some of the most energetically stable DFT structures are found to match well with experimental extended X-ray absorption fine structure (EXAFS) data. The implications for the proposed use of mackinawite as a scavenger material for uranium in groundwater systems are discussed.