Pilot-Scale in Situ Bioremedation of Uranium in a Highly Contaminated Aquifer. 2. Reduction of U(VI) and Geochemical Control of U(VI) Bioavailability

Abstract

In situ microbial reduction of soluble U(VI) to sparingly soluble U(IV) was evaluated at the site of the former S-3 Ponds in Area 3 of the U.S. Department of Energy Natural and Accelerated Bioremediation Research Field Research Center, Oak Ridge, TN. After establishing conditions favorable for bioremediation (Wu, et al. Environ. Sci. Technol. 2006, 40, 3988-3995), intermittent additions of ethanol were initiated within the conditioned inner loop of a nested well recirculation system. These additions initially stimulated denitrification of matrix-entrapped nitrate, but after 2 months, aqueous U levels fell from 5 to approximately 1 microM and sulfate reduction ensued. Continued additions sustained U(VI) reduction over 13 months. X-ray near-edge absorption spectroscopy (XANES) confirmed U(VI) reduction to U(IV) within the inner loop wells, with up to 51%, 35%, and 28% solid-phase U(IV) in sediment samples from the injection well, a monitoring well, and the extraction well, respectively. Microbial analyses confirmed the presence of denitrifying, sulfate-reducing, and iron-reducing bacteria in groundwater and sediments. System pH was generally maintained at less than 6.2 with low bicarbonate level (0.75-1.5 mM) and residual sulfate to suppress methanogenesis and minimize uranium mobilization. The bioavailability of sorbed U(VI) was manipulated by addition of low-level carbonate (< 5 mM) followed by ethanol (1-1.5 mM). Addition of low levels of carbonate increased the concentration of aqueous U, indicating an increased rate of U desorption due to formation of uranyl carbonate complexes. Upon ethanol addition, aqueous U(VI) levels fell, indicating that the rate of microbial reduction exceeded the rate of desorption. Sulfate levels simultaneously decreased, with a corresponding increase in sulfide. When ethanol addition ended but carbonate addition continued, soluble U levels increased, indicating faster desorption than reduction. When bicarbonate addition stopped, aqueous U levels decreased, indicating adsorption to sediments. Changes in the sequence of carbonate and ethanol addition confirmed that carbonate-controlled desorption increased bioavailability of U(VI) for reduction.

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

“…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. The ethanol solution (industrial grade, 88.1% ethanol, 4.7% methanol and 7.2% water w/w) had a chemical oxygen Bacterial succession during uranium bioreduction C Hwang et al demand (COD)-to-weight ratio of 2.1.…”

GeoChip: a comprehensive microarray for investigating biogeochemical, ecological and environmental processes

“…Labeling, array hybridization and scanning were conducted as described previously . The analysis of uranium concentration in the groundwater samples was carried out as described by Wu et al (2006c).…”

“…and sulfatereducing bacteria (SRB), such as Desulfovibrio spp. are the two major groups of microorganisms capable of U(VI) reduction through both direct enzymatic (Lovley et al, 1993a, b;Lovley, 1995;Truex et al, 1996;Tebo and Obraztsova, 1998;Petrie et al, 2003;Wu et al, 2006c) and/or indirect chemical mechanisms (Mohagheghi et al, 1985;Liger et al, 1999), we have focused on the analysis of the dynamics of FeRB and SRB. During the uranium reduction period, both FeRB ( Figure 5) and SRB (data not shown) populations reached their highest levels at day 212, followed by a gradual decrease over 500 days.…”

Geophysical Monitoring of Coupled Microbial and Geochemical Processes During Stimulated Subsurface Bioremediation

“…Much of the research has focused on microorganisms capable of either iron or sulfate reduction, and both classes have been shown to reductively immobilize such contaminants under field conditions (2,3). Given the range of geochemical interactions accompanying biostimulation, the development of monitoring approaches sensitive to both the distribution of subsurface metabolic processes and the precipitation of mineralized end products is sorely needed.…”