Influence of magnetite stoichiometry on U ^VI reduction

Hexavalent uranium (U ^VI) can be reduced enzymatically by various microbes and abiotically by Fe ²⁺-bearing minerals, including magnetite, of interest because of its formation from Fe ³⁺ (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 Fe ²⁺/Fe ³⁺ 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 UO ₂ (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 UO ₂ was formed from reduction of U ^VI in a heat-killed suspension of biogenic magnetite (x = 0.43). X-ray absorption and Mössbauer spectroscopy results indicate that reduction of U ^VI to U ^IV is coupled to oxidation of Fe ²⁺ in magnetite. The addition of aqueous Fe ²⁺ to suspensions of oxidized magnetite resulted in reduction of U ^VI to UO ₂, consistent with our previous finding that Fe ²⁺ taken up from solution increased the magnetite stoichiometry. Our results suggest that magnetite stoichiometry and the ability of aqueous Fe ²⁺ to recharge magnetite are important factors in reduction of U ^VI in the subsurface.