Influence of Magnetite Stoichiometry on UVI Reduction

Hexavalent uranium (U-VI) can be reduced enzymatically by various microbes and abiotically by Fe2+-bearing minerals, including magnetite, of interest because of its formation from Fe3+ (oxy)hydroxides via dissimilatory iron reduction. Magnetite is also a corrosion product of iron metal in suboxic and anoxic conditions and is likely to form during corrosion of steel waste containers holding uranium-containing spent nuclear fuel. Previous work indicated discrepancies in the extent of U-VI reduction by magnetite. Here, we demonstrate that the stoichiometry (the bulk Fe2+/Fe3+ ratio, x) of magnetite can, in part, explain the observed discrepancies. In our studies, magnetite stoichiometry significantly influenced the extent of U-VI reduction by magnetite. Stoichiometric and partially oxidized magnetites with x >= 0.38 reduced U-VI to U-IV in UO2 (uraninite) nanoparticles, whereas with more oxidized magnetites (x < 0.38) and maghemite (x = 0), sorbed U-VI was the dominant phase observed. Furthermore, as with our chemically synthesized magnetites (x >= 0.38), nanoparticulate UO2 was formed from reduction of U-VI in a heat-killed suspension of biogenic magnetite (x = 0.43). X-ray absorption and Mossbauer spectroscopy results indicate that reduction of U-VI to U-IV is coupled to oxidation of Fe2+ in magnetite. The addition of aqueous Fe2+ to suspensions of oxidized magnetite resulted in reduction of U-VI to UO2, consistent with our previous finding that Fe2+ taken up from solution increased the magnetite stoichiometry. Our results suggest that magnetite stoichiometry and the ability of aqueous Fe2+ to recharge magnetite are important factors in reduction of U-VI in the subsurface.