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Bioreduction of Hydrogen Uranyl Phosphate: Mechanisms and U(IV) Products
Rui, Xue,Kwon, Man Jae,O’Loughlin, Edward J.,Dunham-Cheatham, Sarrah,Fein, Jeremy B.,Bunker, Bruce,Kemner, Kenneth M.,Boyanov, Maxim I. American Chemical Society 2013 Environmental science & technology Vol.47 No.11
<P>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 <I>Bacillus subtilis</I>, a non-U<SUP>VI</SUP>-reducing, Gram-positive bacterium. The current study examined the reduction of this biogenic, cell-associated HUP mineral by three dissimilatory metal-reducing bacteria, <I>Anaeromyxobacter dehalogenans</I> strain K, <I>Geobacter sulfurreducens</I> strain PCA, and <I>Shewanella putrefaciens</I> 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<SUP>VI</SUP> reduction to U<SUP>IV</SUP>. The reduction extent of the same mass of HUP to U<SUP>IV</SUP> 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<SUP>VI</SUP> reduction is controlled by dissolution of the HUP phase, suggesting that the metal-reducing bacteria transfer electrons to the dissolved or bacterially adsorbed U<SUP>VI</SUP> species formed after HUP dissolution, rather than to solid-phase U<SUP>VI</SUP> in the HUP mineral. Interestingly, the bioreduced U<SUP>IV</SUP> atoms were not immediately coordinated to other U<SUP>IV</SUP> atoms (as in uraninite, UO<SUB>2</SUB>) but were similar in structure to the phosphate-complexed U<SUP>IV</SUP> species found in ningyoite [CaU(PO<SUB>4</SUB>)<SUB>2</SUB>·H<SUB>2</SUB>O]. This indicates a strong control by phosphate on the speciation of bioreduced U<SUP>IV</SUP>, expressed as inhibition of the typical formation of uraninite under phosphate-free conditions.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/esthag/2013/esthag.2013.47.issue-11/es305258p/production/images/medium/es-2012-05258p_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/es305258p'>ACS Electronic Supporting Info</A></P>
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