Influence of Dynamical Conditions on the Reduction of UVI at the Magnetite-Solution Interface

The heterogeneous reduction of U-VI to U-IV by ferrous iron is believed to be a key process influencing the fate and transport of U in the environment The reactivity of both sorbed and structural Fe-II has been studied for numerous substrates, including magnetite. Published results from U-VI-magnetite experiments have been variable, ranging from no reduction to clear evidence for the formation of U-IV. In this contribution, we used XAS and high resolution (+/- cryogenic) XPS to study the interaction of U-VI with nanoparticulate magnetite. The results indicated that U-VI was partially reduced to U-V with no evidence of U-IV. However, thermodynamic calculations indicated that U phases with average oxidation states below (V) should have been stable, indicating that the system was not in redox equilibrium. A reaction pathway that involves incorporation and stabilization of U-V and U-VI into secondary phases is invoked to explain the observations. The results suggest an important and previously unappreciated role of U-V in the fate and transport of uranium in the environment