title: Solution and microbial controls on the formation of reduced U(IV) species.
abstract: 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(2)QDS, 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(2)QDS 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).
authors.affiliation: Biosciences Division, Argonne National Laboratory , Argonne, Illinois 60439, United States. mboyanov@anl.gov
authors.forename: Maxim I
authors.initials: MI
authors.lastname: Boyanov
authors.forename: Kelly E
authors.initials: KE
authors.lastname: Fletcher
authors.forename: Man Jae
authors.initials: MJ
authors.lastname: Kwon
authors.forename: Xue
authors.initials: X
authors.lastname: Rui
authors.forename: Edward J
authors.initials: EJ
authors.lastname: O'Loughlin
authors.forename: Frank E
authors.initials: FE
authors.lastname: Löffler
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Kemner
mesh_terms: D001419:Bacteria / Q000378:metabolism*; D001673:Biodegradation, Environmental; D045858:Desulfitobacterium / Q000378:metabolism; D005583:Fourier Analysis; D010084:Oxidation-Reduction; D010710:Phosphates / Q000032:analysis; D020592:Shewanella / Q000378:metabolism; D012995:Solubility; D012996:Solutions; D014501:Uranium / Q000302:isolation & purification*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D010710:Phosphates; D012996:Solutions; D014501:Uranium