Sediment studies of the biological factors controlling the reduction of U(VI).
Studies were conducted primarily with sediments, both in laboratory incubations and in a field experiment, with supporting studies with pure cultures. To our knowledge the sediment studies were the first on microbial U(VI) reduction in actual uranium-contaminated subsurface sediments, under conditions that mimic those found in situ. Important findings included: (1) U(VI) reduction is a biotic process in subsurface sediments. (2) U(VI) reduction can be stimulated most effectively with the addition of acetate. Although it had been speculated that microbial U(VI) reduction might be capable of this type of environmental remediation ever since the discovery of microbial U(VI) reduction, this had not been previously demonstrated under environmentally relevant conditions. (3) U(VI) is reduced concurrently with Fe(III) and prior to sulfate reduction. U(VI) and Fe(III) reduction proceeded concurrently, accompanied by a dramatic enrichment in organisms in the Geobacteraceae. Sulfate-reducing microorganisms do not appear to be important components of the microbial community reducing U(VI) in these subsurface sediments. (4) Nitrate has important influences on U(VI) reduction. Nitrate inhibits the reduction of metals until nitrate is depleted. Fe(III)-reducing microorganisms such as Geobacter metallireducens and Desulfitobacterium species can oxidize Fe(II) with the reduction of nitrate which is an important consideration because our previous studies have demonstrated that freshly precipitated Fe(III) oxides can reoxidize U(IV) to U(VI). The discovery that G. metallireducens can ''run backwards'' and oxidize U(IV) when nitrate is present reveals another mechanism preventing precipitation of U(IV) in the presence of nitrate as well as potential novel strategy for removing uranium from the subsurface after a site has been remediated. (5) Importance of understanding Fe(III) forms available for microbial reduction. Fe(III) is orders of magnitude more abundant than U(VI) as an electron acceptor to support microbial growth. It was demonstrated that poorly crystalline Fe(III) oxides and structural Fe(III) in clays are the predominant forms of microbially reducible Fe(III). Such findings are important for the development of models of Fe(III) reduction in similar aquifer environments, such as those found at many UMTRA sites. (6) Mechanisms for Fe(III) oxide reduction. It was discovered that phylogenetically distinct Fe(III) reducer have different strategies for reducing Fe(III) and the fact that Geobacter species must directly contact Fe(III) in order to reduce it may help explain its predominance over other Fe(III) reducers in the subsurface. (7) Transfer of laboratory results to the field. Results from laboratory studies were used to design a field experiment in which U(VI) reduction was successfully precipitated from the contaminated water with the injection of acetate.
- Research Organization:
- University of Massachusetts, Amherst, MA (US)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER) (US)
- DOE Contract Number:
- FG02-00ER62985
- OSTI ID:
- 827480
- Report Number(s):
- DOE/ER/62985-3; TRN: US0500462
- Resource Relation:
- Other Information: PBD: 4 Aug 2004
- Country of Publication:
- United States
- Language:
- English
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