Pentavalent Uranium Enriched Mineral Surface under Electrochemically Controlled Reducing Environments

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.