title: Influence of magnetite stoichiometry on U(VI) reduction.
abstract: 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.
authors.affiliation: Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242, United States. dlatta@anl.gov
authors.forename: Drew E
authors.initials: DE
authors.lastname: Latta
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Gorski
authors.forename: Maxim I
authors.initials: MI
authors.lastname: Boyanov
authors.forename: Edward J
authors.initials: EJ
authors.lastname: O'Loughlin
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Kemner
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Scherer
mesh_terms: D004784:Environmental Monitoring; D052203:Ferrosoferric Oxide / Q000737:chemistry*; D007501:Iron / Q000737:chemistry; D010084:Oxidation-Reduction; D012990:Soil Pollutants, Radioactive / Q000737:chemistry*; D014501:Uranium / Q000737:chemistry*
chemical_list: D012990:Soil Pollutants, Radioactive; D014501:Uranium; D007501:Iron; D052203:Ferrosoferric Oxide