Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242, United States.
Environ Sci Technol. 2012 Jan 17;46(2):778-86. doi: 10.1021/es2024912. Epub 2011 Dec 22.
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.
六价铀（U(VI)）可以被各种微生物酶促还原，也可以被磁铁矿等含亚铁矿物非酶促还原，这很有趣，因为它是由 Fe(3+)（氧）氢氧化物通过异化铁还原形成的。磁铁矿也是亚缺氧和缺氧条件下铁金属的腐蚀产物，并且可能在含有铀的乏核燃料的钢废料容器的腐蚀过程中形成。以前的工作表明磁铁矿还原 U(VI)的程度存在差异。在这里，我们证明磁铁矿的化学计量（总体 Fe(2+)/Fe(3+) 比，x）部分可以解释观察到的差异。在我们的研究中，磁铁矿的化学计量极大地影响了磁铁矿还原 U(VI)的程度。化学计量和部分氧化的磁铁矿（x≥0.38）将 U(VI)还原为 UO2（铀矿）纳米颗粒中的 U(IV)，而对于更氧化的磁铁矿（x<0.38）和磁赤铁矿（x=0），吸附的 U(VI)是观察到的主要相。此外，与我们合成的磁铁矿（x≥0.38）一样，纳米级 UO2 是由生物磁铁矿（x=0.43）热灭活悬浮液中 U(VI)的还原形成的。X 射线吸收和穆斯堡尔光谱结果表明，U(VI)还原与磁铁矿中 Fe(2+)的氧化耦合。将水合 Fe(2+)添加到氧化磁铁矿悬浮液中会导致 U(VI)还原为 UO2，这与我们之前的发现一致，即从溶液中吸收的 Fe(2+)增加了磁铁矿的化学计量。我们的结果表明，磁铁矿的化学计量和水合 Fe(2+)补充磁铁矿的能力是地下环境中 U(VI)还原的重要因素。
