Chapter 11 Biogeochemical Uranium Redox Transformations: Potential Oxidants of Uraninite

In aerobic environments, uranium is generally found in the hexavalent oxidation state, is quite soluble, and readily forms complexes with calcium and carbonate. However, under anaerobic conditions, common metal respiring bacteria can enzymatically reduce U(VI) to U(IV), resulting in the formation of sparingly soluble UO 2 (uraninite). Uranium(VI) reduction, therefore, has a prominent role in natural attenuation of uranium and is being explored as a potential remediation option for this hazardous element. The stability of biologically precipitated uraninite is critical for determining the long-term fate of uranium and is not well characterized within soils and sediments. Given their environmental predominance, Fe(III) (hydr)oxides, which act as both U(VI) reductants and U(IV) oxidants, exert a prominent role in determining uranium chemical fate and associated mobility. Here, we examine several factors controlling the extent of uraninite oxidation by Fe(III) (hydr)oxides, including iron oxide and bicarbonate concentration, in addition to iron oxide type. Our analysis reveals that the extent of uraninite oxidation increases under conditions that increase the thermodynamic probability of the reaction; however, recrystallization of poorly crystalline Fe(III) (hydr)oxides to more crystalline forms may ultimately limit the uraninite oxidation reaction. Nevertheless, uraninite oxidation by Fe(III) (hydr)oxides may be a limiting factor on uraninite stability in the environment.