U(VI) exists as the soluble uranyl ion (UO22+) in aqueous environments, which can be reduced to UO2+ and to insoluble U(IV) species. The change in oxidation state has a significant influence on the solubility and mobility of uranium. Solid U(IV) species have been found on magnetite (Fe3O4) surfaces, which were ascribed to the reduction of U(VI) to U(IV) through gaining electrons from the structure Fe(II) or from electrons shuttled by the magnetite from electron donors. The favorable reduction pathway is that U(VI) is first reduced to U(V), followed by the formation of U(IV) through the rapid disproportionation of U(V), which is generally treated as an unstable intermediate product. However, stabilized U(V) has been identified in abiotic and bio reduction experiments. Electrochemical combined with spectroscopic methods provide a unique approach that allows one to investigate reduction precipitation and oxidation dissolution of uranium on magnetite. The redox condition close to the magnetite electrode can be controlled electrochemically through the potentiostat. Thus, uranium species containing the whole spectrum of oxidation states from U(VI), U(V) to U(IV) can be selectively prepared at different voltages. The redox reaction between U(VI) and U(V) was confirmed to be a one-electron step using electrochemical methods. Mixed-valence state uranium precipitates were identified by electrochemical AFM and XPS with inhomogeneous distribution of Fe and U on the AES element mapping on the surface of the magnetite electrode. 

Uranium precipitation/dissolution reactions on magnetite investigated by electrochemical AFM, XPS, and AES

248th National Meeting (2014)

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