Stimulating the In Situ Activity of Geobacter Species To Remove Uranium from the Groundwater of a Uranium-Contaminated Aquifer

Abstract

The potential for removing uranium from contaminated groundwater by stimulating the in situ activity of dissimilatory metal-reducing microorganisms was evaluated in a uranium-contaminated aquifer located in Rifle, Colo. Acetate (1 to 3 mM) was injected into the subsurface over a 3-month period via an injection gallery composed of 20 injection wells, which was installed upgradient from a series of 15 monitoring wells. U(VI) concentrations decreased in as little as 9 days after acetate injection was initiated, and within 50 days uranium had declined below the prescribed treatment level of 0.18 M in some of the monitoring wells. Analysis of 16S ribosomal DNA (rDNA) sequences and phospholipid fatty acid profiles demonstrated that the initial loss of uranium from the groundwater was associated with an enrichment of Geobacter species in the treatment zone. Fe(II) in the groundwater also increased during this period, suggesting that U(VI) reduction was coincident with Fe(III) reduction. As the acetate injection continued over 50 days there was a loss of sulfate from the groundwater and an accumulation of sulfide and the composition of the microbial community changed. Organisms with 16S rDNA sequences most closely related to those of sulfate reducers became predominant, and Geobacter species became a minor component of the community. This apparent switch from Fe(III) reduction to sulfate reduction as the terminal electron accepting process for the oxidation of the injected acetate was associated with an increase in uranium concentration in the groundwater. These results demonstrate that in situ bioremediation of uranium-contaminated groundwater is feasible but suggest that the strategy should be optimized to better maintain long-term activity of Geobacter species.

Structural Similarities between Biogenic Uraninites Produced by Phylogenetically and Metabolically Diverse Bacteria

“…Reduction from a valence of six to four can be mediated by a variety of dissimilatory metal-and sulfate-reducing bacteria (DMRB and DSRB) in liquid batch, including strains of the genera Anaeromyxobacter, Geobacter, Shewanella, and Desulfovibrio (1)(2)(3)(4)(5). Soil columns amended with DMRB, denitrifying bacteria, or DSRB have confirmed that reductive immobilization can occur in simplified soil systems (6,7), and analogous field-scale pilot biostimulation studies have successfully immobilized uranium in the subsurface (8,9).…”

“…However, Shewanella has not been found to be a prevalent DMRB during biostimulation at contaminated sites (9,17,18). For example, subsurface ethanol amendments enriched populations of DMRB and DSRB at Oak Ridge, TN.…”

Field-Based Detection and Monitoring of Uranium in Contaminated Groundwater using Two Immunosensors

“…Both the site and the field bioremediation research have been described extensively elsewhere [5][6][7] and a more complete description of the ongoing work at the Old Rifle UMTRA site may be found at http://www.pnl.gov/nabir-umtra/intro.stm. In the summer of 2008, much of the research activity was focused upon the collection of samples for proteomic analyses [5,[8][9][10][11], and timely information about the extent of uranium immobilization or remobilization in groundwater samples was useful to the research team in the timing of sample collection.…”

Reoxidation of Bioreduced Uranium under Reducing Conditions

“…Calculations based on measured elevated PCO 2 and PCH 4 yielded even lower pe = -4.2. Effluents also contained high levels of CH 4 and CO 2 (forming gas bubbles), and depleted levels of OC. The soil U(IV) content declined to 58% (±22%) of the total U when measured on day 356.…”