Formation of a U(VI)–Persulfide Complex during Environmentally Relevant Sulfidation of Iron (Oxyhydr)oxides

Luke T. Townsend, Samuel Shaw, Naomi E. R. Ofili, Nikolas Kaltsoyannis, Alex S. Walton, J. Frederick W. Mosselmans, Thomas S. Neill, Jonathan R. Lloyd, Sarah Heath, Rosemary Hibberd, Katherine Morris

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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.
Original languageEnglish
Pages (from-to)129-136
Number of pages8
JournalEnvironmental Science & Technology
Issue number1
Publication statusPublished - 16 Dec 2019

Research Beacons, Institutes and Platforms

  • Dalton Nuclear Institute


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