Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States.
Environ Sci Technol. 2013 Jun 4;47(11):5668-78. doi: 10.1021/es305258p. Epub 2013 May 21.
The mobility of uranium (U) in subsurface environments is controlled by interrelated adsorption, redox, and precipitation reactions. Previous work demonstrated the formation of nanometer-sized hydrogen uranyl phosphate (abbreviated as HUP) crystals on the cell walls of Bacillus subtilis, a non-U(VI)-reducing, Gram-positive bacterium. The current study examined the reduction of this biogenic, cell-associated HUP mineral by three dissimilatory metal-reducing bacteria, Anaeromyxobacter dehalogenans strain K, Geobacter sulfurreducens strain PCA, and Shewanella putrefaciens strain CN-32, and compared it to the bioreduction of abiotically formed and freely suspended HUP of larger particle size. Uranium speciation in the solid phase was followed over a 10- to 20-day reaction period by X-ray absorption fine structure spectroscopy (XANES and EXAFS) and showed varying extents of U(VI) reduction to U(IV). The reduction extent of the same mass of HUP to U(IV) was consistently greater with the biogenic than with the abiotic material under the same experimental conditions. A greater extent of HUP reduction was observed in the presence of bicarbonate in solution, whereas a decreased extent of HUP reduction was observed with the addition of dissolved phosphate. These results indicate that the extent of U(VI) reduction is controlled by dissolution of the HUP phase, suggesting that the metal-reducing bacteria transfer electrons to the dissolved or bacterially adsorbed U(VI) species formed after HUP dissolution, rather than to solid-phase U(VI) in the HUP mineral. Interestingly, the bioreduced U(IV) atoms were not immediately coordinated to other U(IV) atoms (as in uraninite, UO2) but were similar in structure to the phosphate-complexed U(IV) species found in ningyoite [CaU(PO4)2·H2O]. This indicates a strong control by phosphate on the speciation of bioreduced U(IV), expressed as inhibition of the typical formation of uraninite under phosphate-free conditions.
铀（U）在地下环境中的迁移性受到相互关联的吸附、氧化还原和沉淀反应的控制。之前的研究表明，在非 U(VI)-还原的革兰氏阳性细菌枯草芽孢杆菌的细胞壁上形成了纳米级的氢铀磷酸盐（简称 HUP）晶体。本研究检测了三种异化金属还原菌对这种生物形成的、与细胞相关的 HUP 矿物的还原作用，这三种菌分别是脱硫弧菌 Anaeromyxobacter dehalogenans 菌株 K、脱硫杆菌 Geobacter sulfurreducens 菌株 PCA 和腐败希瓦氏菌 Shewanella putrefaciens 菌株 CN-32，并将其与无生命形成的和自由悬浮的较大粒径的 HUP 的生物还原作用进行了比较。通过 X 射线吸收精细结构光谱（XANES 和 EXAFS）在 10-20 天的反应期内对固相中的铀形态进行了跟踪，结果表明 U(VI)被还原为 U(IV)的程度不同。在相同的实验条件下，与无生命形成的 HUP 相比，生物形成的 HUP 还原到 U(IV)的同一质量的程度更大。在溶液中存在碳酸氢盐时，HUP 的还原程度更大，而添加溶解的磷酸盐时，HUP 的还原程度降低。这些结果表明，U(VI)的还原程度受 HUP 相的溶解控制，这表明金属还原菌将电子转移到 HUP 溶解后形成的溶解或细菌吸附的 U(VI)物种，而不是 HUP 矿物中的固相 U(VI)。有趣的是，生物还原的 U(IV)原子并没有立即与其他 U(IV)原子配位（如在铀矿中），而是与在人形石[CaU(PO4)2·H2O]中发现的与磷酸盐配位的 U(IV)物种结构相似。这表明磷酸盐对生物还原的 U(IV)的形态有很强的控制作用，表现为在无磷酸盐条件下抑制典型的铀矿的形成。
