Date of this Version
Environ. Sci. Technol. 2011, 45, 8336–8344; dx.doi.org/10.1021/es2014049
Reduction of UVI to UIV as the result of direct or indirect microbial activity is currently being explored for in situ remediation of subsurface U plumes, under the assumption that UIV solubility is controlled by the low solubility mineral uraninite (UIV-dioxide). However, recent characterizations of U in sediments from biostimulated field sites, as well as laboratory UVI bioreduction studies, report on the formation of UIV species that lack the U=O2=U coordination of uraninite, suggesting that phases other than uraninite may be controlling UIV solubility in environments with complexing surfaces and ligands. To determine the controls on the formation of such nonuraninite UIV species, the current work studied the reduction of carbonate-complexed UVI by (1) five Gram-positive Desulfitobacterium strains, (2) the Gram-negative bacteria Anaeromyxobacter dehalogenans 2CP-C and Shewanella putrefaciens CN32, and (3) chemically reduced 9,10-anthrahydroquinone-2,6-disulfonate (AH2QDS, a soluble reductant). Further, the effects of 0.3 mM dissolved phosphate on UIV species formation were explored. Extended X-ray absorption fine structure (EXAFS) spectroscopy analysis demonstrated that the addition of phosphate causes the formation of a nonuraninite, phosphate-complexed UIV species, independent of the biological or abiotic mode of UVI reduction. In phosphate-free medium, UVI reduction by Desulfitobacterium spp. and by AH2QDS resulted in nonuraninite, carbonate-complexed UIV species, whereas reduction by Anaeromyxobacter or Shewanella yielded nanoparticulate uraninite. These findings suggest that the Grampositive Desulfitobacterium strains and the Gram-negative Anaeromyxobacter and Shewanella species use distinct mechanisms to reduce UVI.