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Instruments / Technical Area	Publication Details	Published	Actions
B18-Core EXAFS I20-Scanning-X-ray spectroscopy (XAS/XES)	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. Neil , Jonathan R. Lloyd , Sarah Heath , Rosemary Hibberd , Katherine Morris Environmental Science & Technology DOI: 10.1021/acs.est.9b03180 Diamond Proposal Number(s): [13559, 17376, 17243] Open Access Show Abstract ▼ Abstract: 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.	Dec 2019

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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)

Formation of a U(VI)–persulfide complex during environmentally relevant sulfidation of iron (oxyhydr)oxides