title: Electrochemical and Spectroscopic Evidence on the One-Electron Reduction of U(VI) to U(V) on Magnetite.
abstract: Reduction of U(VI) to U(IV) on mineral surfaces is often considered a one-step two-electron process. However, stabilized U(V), with no evidence of U(IV), found in recent studies indicates U(VI) can undergo a one-electron reduction to U(V) without further progression to U(IV). We investigated reduction pathways of uranium by reducing U(VI) electrochemically on a magnetite electrode at pH 3.4. Cyclic voltammetry confirms the one-electron reduction of U(VI) to U(V). Formation of nanosize uranium precipitates on the magnetite surface at reducing potentials and dissolution of the solids at oxidizing potentials are observed by in situ electrochemical atomic force microscopy. XPS analysis of the magnetite electrodes polarized in uranium solutions at voltages from -0.1 to -0.9 V (E(0)(U(VI)/U(V))= -0.135 V vs Ag/AgCl) show the presence of only U(V) and U(VI). The sample with the highest U(V)/U(VI) ratio was prepared at -0.7 V, where the longest average U-O(axial) distance of 2.05 ± 0.01 Å was evident in the same sample revealed by extended X-ray absorption fine structure analysis. The results demonstrate that the electrochemical reduction of U(VI) on magnetite only yields U(V), even at a potential of -0.9 V, which favors the one-electron reduction mechanism. U(V) does not disproportionate but stabilizes on magnetite through precipitation of mixed-valence state U(V)/U(VI) solids.
authors.affiliation: †Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Ke
authors.initials: K
authors.lastname: Yuan
authors.affiliation: ‡Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
authors.forename: Eugene S
authors.initials: ES
authors.lastname: Ilton
authors.affiliation: §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Antonio
authors.affiliation: †Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Zhongrui
authors.initials: Z
authors.lastname: Li
authors.affiliation: †Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Cook
authors.affiliation: †Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Udo
authors.initials: U
authors.lastname: Becker
mesh_terms: D011232:Chemical Precipitation; D004563:Electrochemistry / Q000379:methods*; D004566:Electrodes; D004573:Electrolytes / Q000737:chemistry; D004583:Electrons; D052203:Ferrosoferric Oxide / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D010084:Oxidation-Reduction; D056951:Photoelectron Spectroscopy; D012996:Solutions; D014501:Uranium / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D004573:Electrolytes; D012996:Solutions; D014501:Uranium; D052203:Ferrosoferric Oxide