Solution and microbial controls on the formation of reduced U(IV) species

Reduction of U ^VI to U ^IV 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 U ^IV solubility is controlled by the low-solubility mineral uraninite (U ^IV-dioxide). However, recent characterizations of U in sediments from biostimulated field sites, as well as laboratory U ^VI bioreduction studies, report on the formation of U ^IV species that lack the U=O ₂=U coordination of uraninite, suggesting that phases other than uraninite may be controlling U ^IV solubility in environments with complexing surfaces and ligands. To determine the controls on the formation of such nonuraninite U ^IV species, the current work studied the reduction of carbonate-complexed U ^VI 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 (AH ₂QDS, a soluble reductant). Further, the effects of 0.3 mM dissolved phosphate on U ^IV 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 U ^IV species, independent of the biological or abiotic mode of U ^VI reduction. In phosphate-free medium, U ^VI reduction by Desulfitobacterium spp. and by AH ₂QDS resulted in nonuraninite, carbonate-complexed U ^IV species, whereas reduction by Anaeromyxobacter or Shewanella yielded nanoparticulate uraninite. These findings suggest that the Gram-positive Desulfitobacterium strains and the Gram-negative Anaeromyxobacter and Shewanella species use distinct mechanisms to reduce U ^VI.