Reoxidation of bioreduced uranium under reducing conditions.
Nuclear weapons and fuel production have left many soils and sediments contaminated with toxic levels of uranium (U). Although previous short-term experiments on microbially mediated U(VI) reduction have supported the prospect of immobilizing the toxic metal through formation of insoluble U(IV) minerals, our longer-term (17 months) laboratory study showed that microbial reduction of U can be transient, even under sustained reducing conditions. Uranium was reduced during the first 80 days, but later (100-500 days) reoxidized and solubilized, even though a microbial community capable of reducing U(VI) was sustained. Microbial respiration caused increases in (bi)-carbonate concentrations and formation of very stable uranyl carbonate complexes, thereby increasing the thermodynamic favorability of U(IV) oxidation. We propose that kinetic limitations including restricted mass transfer allowed Fe-(III) and possibly Mn(IV) to persist as terminal electron acceptors (TEAs) for U reoxidation. These results show that in-situ U remediation by organic carbon-based reductive precipitation can be problematic in sediments and groundwaters with neutral to alkaline pH, where uranyl carbonates are most stable.
Wan
Jiamin
J
Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. jmwan@lbl.gov
Tokunaga
Tetsu K
TK
Brodie
Eoin
E
Wang
Zheming
Z
Zheng
Zuoping
Z
Herman
Don
D
Hazen
Terry C
TC
Firestone
Mary K
MK
Sutton
Stephen R
SR
D001673:Biodegradation, Environmental; D011232:Chemical Precipitation; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry; D007700:Kinetics; D010084:Oxidation-Reduction; D012988:Soil Microbiology; D012990:Soil Pollutants, Radioactive / Q000378:metabolism*; D012995:Solubility; D014501:Uranium / Q000737:chemistry* / Q000378:metabolism*
D012990:Soil Pollutants, Radioactive; D014501:Uranium; D007501:Iron