Influence of Mn oxides on the reduction of uranium(VI) by the metal-reducing bacterium Shewanella putrefaciens

Authors

James K. Fredrickson, Pacific Northwest National LaboratoryFollow
John M. Zachara, Pacific Northwest National LaboratoryFollow
David Kennedy, Pacific Northwest National LaboratoryFollow
Chongxuan Liu, Pacific Northwest National LaboratoryFollow
Martine Duff, Savannah River Technology Center
Douglas Hunter, Savannah River Technology Center
Alice Dohnalkova, Pacific Northwest National LaboratoryFollow

Date of this Version

2002

Citation

Geochimica et Cosmochimica Acta, Vol. 66, No. 18, pp. 3247–3262, 2002

Abstract

The potential for Mn oxides to modify the biogeochemical behavior of U during reduction by the subsurface bacterium Shewanella putrefaciens strain CN32 was investigated using synthetic Mn(III/IV) oxides (pyrolusite [β-MnO₂], bixbyite [Mn₂O₃] and K⁺-birnessite [K₄Mn₁₄O₂₇ • 8H₂O]). In the absence of bacteria, pyrolusite and bixbyite oxidized biogenic uraninite (UO₂[s]) to soluble U(VI) species, with bixbyite being the most rapid oxidant. The Mn(III/IV) oxides lowered the bioreduction rate of U(VI) relative to rates in their absence or in the presence of gibbsite (Al[OH]₃) added as a non-redox-reactive surface. Evolved Mn(II) increased with increasing initial U(VI) concentration in the biotic experiments, indicating that valence cycling of U facilitated the reduction of Mn(III/IV). Despite an excess of the Mn oxide, 43 to 100% of the initial U was bioreduced after extended incubation. Analysis of thin sections of bacterial Mn oxide suspensions revealed that the reduced U resided in the periplasmic space of the bacterial cells. However, in the absence of Mn(III/IV) oxides, UO₂(s) accumulated as copious fine-grained particles external to the cell. These results indicate that the presence of Mn(III/IV) oxides may impede the biological reduction of U(VI) in subsoils and sediments. However, the accumulation of U(IV) in the cell periplasm may physically protect reduced U from oxidation, promoting at least a temporal state of redox disequilibria.