Donor‐dependent Extent of Uranium Reduction for Bioremediation of Contaminated Sediment Microcosms

Abstract

Bioremediation of uranium was investigated in microcosm experiments containing contaminated sediments from Oak Ridge, Tennessee to explore the importance of electron donor selection for uranium reduction rate and extent. In these experiments, all of the electron donors, including ethanol, glucose, methanol, and methanol with added humic acids, stimulated the reduction and immobilization of aqueous uranium by the indigenous microbial community. Uranium loss from solution began after the completion of nitrate reduction but essentially concurrent with sulfate reduction. When electron donor concentrations were normalized for their equivalent electron donor potential yield, the rates of uranium reduction were nearly equivalent for all treatments (0.55-0.95 micromol L(-1) d(-1)). Uranium reduction with methanol proceeded after a 15-d longer lag time relative to that of ethanol or glucose. Significant differences were not found with the inclusion of humic acids. The extent of U reduction in sediment slurries measured by XANES at various time periods after the start of the experiment increased in the order of ethanol (5-7% reduced at 77 and 153 d), glucose (49% reduced at 53 d), and methanol (93% reduced at 90 d). The microbial diversity of ethanol- and methanol-amended microcosms in their late stage of U reduction was analyzed with 16S rRNA gene amplification. Members of the Geobacteraceae were found in all microcosms as well as other potential uranium-reducing organisms, such as Clostridium and Desulfosporosinus. The effectiveness of methanol relative to ethanol at reducing aqueous and sediment-hosted uranium suggests that bioremediation strategies that encourage fermentative poising of the subsurface to a lower redox potential may be more effective for long-term uranium immobilization as compared with selecting an electron donor that is efficiently metabolized by known uranium-reducing microorganisms.

Uranium reduction and microbial community development in response to stimulation with different electron donors

“…One possible mechanism for uranium reduction in the lactate-amended columns was through organic acid release by Sporomusa, which stimulated uraniumreducing Geobacter, whose presence was supported by the PLFA results. Similar mechanisms have been reported using other electron donors (Edwards et al 2007;Madden et al 2008).…”

“…Luo et al (2007) reported that ethanol resulted in higher uranium reduction than acetate. The extent of U(VI) reduction in Oak Ridge sediments was also found to be higher with methanol than glucose and much higher with glucose as compared to ethanol (Madden et al 2008).…”

“…In addition to Geobacter, unclassified Desulfuromonales were enriched by ethanol, and Desulfovibrio were enriched by acetate ). Ethanol and methanol stimulated acetogens such as Clostridium and Desulfosporosinus, as well as acetate-metabolizing Deltaproteobacteria such as Geobacter (Madden et al 2008). Sometimes stimulation can have negative implications for U(VI) reduction, such as the growth of Desulfobacter spp., which are unable to reduce U(VI) but utilize acetate, often compete with U(VI)-reducing bacteria for electron donors supplied at limiting concentrations (Lovley et al 1993;Williams et al 2011).…”

The Response ofDesulfotomaculum reducensMI-1 to U(VI) Exposure: A Transcriptomic Study

“…Stimulating indigenous microbial communities to carry out the in situ reduction of soluble U(VI) to insoluble U(IV) has been proposed as a potential bioremediation strategy for these contaminated sites (Anderson et al 2003;Madden et al 2009;N'Guessan et al 2008;Wu et al 2007;Yabusaki et al 2007). However, effective bioremediation requires an understanding of the mechanisms involved in bacterial metal reduction and of the response of microorganisms to metal toxicity.…”

Microbial Community Changes in Response to Ethanol or Methanol Amendments for U(VI) Reduction

“…Typically, an organic substrate is added to stimulate microbial growth and promote the development of anaerobic conditions, under which the reduction of U(VI) is favored (67). Various substrates (e.g., acetate, ethanol, glucose, and methanol) have been used either in the field or in microcosm studies, and most were capable of stimulating microbial U(VI) reduction (1,8,42,43,47,60); however, the addition of methanol did not always result in U(VI) reduction (49). Many microorganisms are known to reduce U(VI) in pure culture, including a hyperthermophilic archaeon (28), a thermophilic bacterium Thermoterrabacterium ferrireducens (29), the mesophilic dissimilatory metal-reducing bacteria Geobacter and Shewanella (67) and Anaeromyxobacter dehalogenans (71), the sulfate-reducing bacterium Desulfovibrio sp.…”