In Situ Bioreduction of Uranium (VI) to Submicromolar Levels and Reoxidation by Dissolved Oxygen

Abstract

Groundwater within Area 3 of the U.S. Department of Energy (DOE) Environmental Remediation Sciences Program (ERSP) Field Research Center at Oak Ridge, TN (ORFRC) contains up to 135 microM uranium as U(VI). Through a series of experiments at a pilot scale test facility, we explored the lower limits of groundwater U(VI) that can be achieved by in-situ biostimulation and the effects of dissolved oxygen on immobilized uranium. Weekly 2 day additions of ethanol over a 2-year period stimulated growth of denitrifying, Fe(III)-reducing, and sulfate-reducing bacteria, and immobilization of uranium as U(IV), with dissolved uranium concentrations decreasing to low levels. Following sulfite addition to remove dissolved oxygen, aqueous U(VI) concentrations fell below the U.S. Environmental Protection Agengy maximum contaminant limit (MCL) for drinking water (< 30/microg L(-1) or 0.126 microM). Under anaerobic conditions, these low concentrations were stable, even in the absence of added ethanol. However, when sulfite additions stopped, and dissolved oxygen (4.0-5.5 mg L(-1)) entered the injection well, spatially variable changes in aqueous U(VI) occurred over a 60 day period, with concentrations increasing rapidly from < 0.13 to 2.0 microM at a multilevel sampling (MLS) well located close to the injection well, but changing little at an MLS well located further away. Resumption of ethanol addition restored reduction of Fe(III), sulfate, and U(VI) within 36 h. After 2 years of ethanol addition, X-ray absorption near-edge structure spectroscopy (XANES) analyses indicated that U(IV) comprised 60-80% of the total uranium in sediment samples. Atthe completion of the project (day 1260), U concentrations in MLS wells were less than 0.1 microM. The microbial community at MLS wells with low U(VI) contained bacteria that are known to reduce uranium, including Desulfovibrio spp. and Geobacter spp., in both sediment and groundwater. The dominant Fe(III)-reducing species were Geothrix spp.

Bacterial community succession during in situ uranium bioremediation: spatial similarities along controlled flow paths

“…Additional clean water was injected at well FW024 at 0.7-0.9 l min À1 to minimize entry of ambient groundwater, increase the pH, and remove nitrate. Detailed descriptions are available elsewhere Luo et al, 2006;Wu et al, 2006aWu et al, , 2007, http:// www.esd.ornl.gov/orifrc/). Stimulation of bacterial growth with ethanol began on day 137 (7 January 2004) and ended on day 754 (15 September 2005) (Wu et al, 2006b.…”

Stochasticity, succession, and environmental perturbations in a fluidic ecosystem

“…Third, the shift in community composition and structure by nutrient input could be substrate dependent. A simple substrate, such as the ethanol used for U bioremediation (58,59), could have less effect in driving communities from determinism to stochasticity. Fourth, under natural settings, an ecosystem is always under multiple and continuous perturbations.…”

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).…”