| Abstract
| - The bioreduction of soluble uranium [U(VI)] to sparinglysoluble U(IV) species is an attractive remedial technologyfor contaminated soil and groundwater due to thepotential for immobilizing uranium and impeding its migrationin subsurface environments. This manuscript describes acolumn study designed to simulate a three-step strategyproposed for the remediation of a heavily contaminated siteat the U.S. Department of Energy's NABIR Field ResearchCenter in Oak Ridge, TN. The soil is contaminated withhigh concentrations of uranium, aluminum, and nitrate andhas a low, highly buffered pH (∼3.5). Steps proposed forremediation are (i) flushing to remove nitrate and aluminum,(ii) neutralization to establish pH conditions favorable forbiostimulation, and (iii) biostimulation for U(VI) reduction. Wesimulated this sequence using a packed soil columncontaining undisturbed aggregates of U(VI)-contaminatedsaprolite that was flushed with an acidified salt solution (pH4.0), neutralized with bicarbonate (60 mM), and thenbiostimulated by adding ethanol. The column was operatedanaerobically in a closed-loop recirculation setup.However, during the initial month of biostimulation, ethanolwas not utilized, and U(VI) was not reduced. A bacterialculture enriched from the site groundwater was subsequentlyadded, and the consumption of ethanol coupled withsulfate reduction immediately ensued. The aqueousconcentration of U(VI) initially increased, evidently becauseof the biological production of carbonate, a ligandknown to solubilize uranyl. After ∼50 days, aqueous U(VI)concentrations rapidly decreased from ∼17 to <1 mg/L.At the conclusion of the experiment, the presence of reducedsolid phase U(IV) was confirmed using X-ray absorptionnear edge structure spectroscopy. The results indicate thatbioreduction to immobilize uranium is potentially feasibleat this site; however, the stability of the reduced U(IV) andits potential reoxidation will require further investigation,as do the effects of groundwater chemistry and competitivemicrobial processes, such as methanogenesis.
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