Uranium and technetium bio-immobilization in intermediate-scale permeable reactive scale barriers

Graduation date: 2004Groundwater at Oak Ridge National Laboratory's Field Research Center\ud (FRC) is contaminated with U(VI) and Tc(VII), has pH values as low as 3.3, and\ud nitrate concentrations as high as 120 mM. The objective of this research was to\ud determine if in-situ bio-immobilization is a viable treatment alternative for this water.\ud A laboratory column packed with crushed limestone and bicarbonate was used\ud to model in-situ pH adjustment. Denitrification and metal reduction were modeled in\ud columns packed with FRC sediment with ethanol as the electron donor. Two\ud intermediate-scale physical models deployed in the field were packed with limestone\ud and sediment and were stimulated with ethanol to support denitrification, U(VI)\ud reduction, and Tc(VII) reduction of FRC groundwater.\ud The limestone/bicarbonate column maintained a pH of above 5 for nearly one\ud hundred pore volumes without significant loss in hydraulic conductivity. The high-nitrate\ud (~120 mM) column study provided rates of denitrification (~15.25 mM/day),\ud ethanol utilization (~13 mM/day), and technetium reduction (~120 pM/day) by\ud sediment microorganisms, but no uranium reduction was detected. Results of the low\ud nitrate (3 mM) column study indicate that once the pH of FRC water is adjusted to pH\ud ~7 and nitrate is removed, uranium (~3 μM) and technetium (~500 pM) reduction\ud occurred with ethanol as the electron donor at rates of 0.5 μM/day and 57 pM/day.\ud Similar results were obtained in two intermediate-scale (~3 m long) physical\ud models. Data from the high-nitrate, low-pH model indicate that the pH was increased\ud and nitrate and technetium reduction were occurring. Decreased U(VI) concentrations\ud were measured in the presence of high nitrate concentrations. Thus, U(VI) precipitates\ud may form or sorption of U(VI) may occur near the inlet in the pH adjustment region.\ud The maximum pseudo-first order rates of reduction measured during the seventh week\ud of model operation were: nitrate at 0.76 day⁻¹, Tc(VII) at 0.28 day⁻¹, and U(VI) at 0.12\ud day⁻¹. Ethanol concentrations were reduced from ~180 mM to zero in ~10 days\ud during the seventh week of model operation. No Fe(II) production was measured.\ud Concentration data collected from the low nitrate, neutral pH model indicate\ud that nitrate, uranium, and technetium reduction were occurring, though the model had\ud been operational for only ~6 weeks. No Fe(II) production was detected but sulfate\ud reduction was occurring.\ud The results of the laboratory experiments and the performance of the\ud intermediate-scale physical models suggest that bio-immobilization is a viable\ud treatment alternative for the contaminated groundwater at the FRC