Abstract
Redox reactions of uranium (U) in aqueous environments have important impacts on the mobility and isotopic fractionation of U in the geosphere. Pentavalent U as the cationic uranyl ion, UO2+, is rarely observed in naturally occurring samples because of its limited lifetime, but it may be an important intermediate state controlling the redox kinetics between hexavalent and tetravalent U. Increasing evidence has indicated that U(V) can be stabilized under laboratory conditions. Here, we showed that U(V) is the dominant species on the magnetite (Fe3O4) surface under reducing conditions controlled by electrochemical methods. Cyclic voltammetry reveals coupled redox peaks corresponding to the U(VI)O22+/U(V)O2+ one-electron redox reaction. Magnetite electrodes polarized at a series of potentials to reduce U(VI)O22+ were characterized by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and Auger electron mapping. The results showed that up to twice the amount of U(V) to U(VI) was present on the magnetite surface. U(V) adopted a typical uranyl-type structure, and the U coverage on the magnetite surface increased with decreasing potentials. The formation of mixed-valence U(V)/U(VI) species on the surface of magnetite may hinder the U(V) disproportionation reaction, thereby eliminating the presence of tetravalent U. These results show that U(V) can exist over short time scales as the dominant U species on mineral surfaces under selected reducing conditions by the controlled polarization of a mineral electrode.
Original language | English |
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Pages (from-to) | 1204-1212 |
Number of pages | 9 |
Journal | ACS Earth and Space Chemistry |
Volume | 6 |
Issue number | 5 |
DOIs | |
State | Published - May 19 2022 |
Funding
This work is supported by the U.S. Office of Science, Basic Energy Sciences (Grant No. DE-FG02-06ER15783). Manuscript preparation/revision and analysis of XAS and XPS data by K.Y. was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Work performed by E.S.I. was supported by the Geosciences Research Program at PNNL sponsored by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for the DOE by Battelle. The authors acknowledge the assistance from the beamline scientists Benjamin Reinhart and Sungsik Lee at 12-BM-B, APS. The work at Argonne National Laboratory including the use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.
Keywords
- Uranium
- electrochemical
- fractionation
- magnetite
- pentavalent
- redox