Influence of Magnetite Stoichiometry on UVI Reduction

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• 作者：Drew E. Latta ; Christopher A. Gorski ; Maxim I. Boyanov ; Edward J. O鈥橪oughlin ; Kenneth M. Kemner ; Michelle M. Scherer
• 刊名：Environmental Science & Technology (ES&T)
• 出版年：2012
• 出版时间：January 17, 2012
• 年：2012
• 卷：46
• 期：2
• 页码：778-786
• 全文大小：392K
• 年卷期：v.46,no.2(January 17, 2012)
• ISSN：1520-5851

文摘

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 鈮?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 鈮?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 M枚ssbauer 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.