In situ bio-immobilization of Uranium and Technetium via indigenous soil microbes: preliminary study results

Groundwater at Oak Ridge National Laboratory's Field Research Center (FRC) is contaminated with uranium and technetium, has pH as low as 3.3, and nitrate concentrations as high as 120 mM. The objective of this research was to determine if in-situ bio-immobilization is a viable treatment alternative for this water. A laboratory column packed with crushed limestone and bicarbonate was used to model in-situ pH adjustment. Denitrification and metal reduction were modeled with column experiments as well, using FRC sediment and ethanol as the electron donor. The limestone column maintained a pH of above 5 for nearly one hundred pore volumes without significant loss in hydraulic conductivity. The high nitrate (120 mM) column study provided rates of denitrification ( ∼ 15.25 mM/day), ethanol utilization ( ∼ 13 mM/day), and technetium reduction (120 pM/day) by indigenous microorganisms, while no uranium reduction was detected. Results of the low nitrate (3 mM) column experiment indicate that once the pH of FRC water is adjusted and nitrate is removed, uranium and technetium reduction, ( ∼ 0.5 uM/day) and ( ∼ 57 pM/day) respectively, proceed with ethanol as the electron donor. These preliminary findings demonstrate that microorganisms indigenous to FRC sediment are capable of reducing, and thus immobilizing uranium and technetium in small scale reactors. Intermediate scale models (3m long) were designed to simulate in-situ pH adjustment, nitrate removal, and radionuclide reduction, all in one continuous reactor. Preliminary data from the intermediate scale, high nitrate model indicates that nitrate, uranium, and technetium reduction are occurring. Large scale, in-situ bio-immobilization zones have the potential to prevent further radionuclide contamination of groundwater world wide.