Graduate Thesis Or Dissertation
 

Uranium and technetium bio-immobilization in intermediate-scale physical models of an in situ bio-barrier

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  • Metal and hydrogen ion acidity and extreme nitrate concentrations typical of Department of Energy (DOE) legacy waste sites pose formidable challenges to successful implementation of in situ bio-immobilization. Intermediate-scale (~ 2.5 m), flow through models of an in situ bio-barrier were constructed to investigate U and Tc removal from groundwater at a contaminated site in Oak Ridge, TN. In one study, ethanol additions to pH-neutral contaminated site groundwater, flowing through a mixture of site sediment and crushed limestone, effectively stimulated iron- and sulfatereducing conditions and sustained U and Tc removal for 20 months. In a related study, ethanol additions to nitric acid contaminated site groundwater, flowing through a mixture of site sediment and crushed limestone, effectively promoted denitrification for over 20 months. A combined signature lipid and nucleic acid-based approach was used to spatially characterize microbial communities at relatively small spatial scales (<67 cm) in sediment along the groundwater flow paths in both experimental systems. The results showed that ethanol additions stimulated growth of a distinct microbial community in both the pH-neutral and nitric acid-contaminated systems, and that shifts in community composition were spatially correlated with geochemistry along the groundwater flow paths. Collectively, these results suggest that an in situ bio-barrier could be potentially effective for U and Tc removal from nitric-acid contaminated groundwater at the FRC. Hydraulic conductivity decreased by an order of magnitude in both experimental systems due primarily to solids deposition. Preferential flow path formation, due to biomass accumulation or solids deposition, could ultimately reduce residence time within the treatment zone and decrease U and Tc removal efficiency.
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