Reduction and Reoxidation of Soils During & After Uranium Bioremediation; Implications for Long-Term Uraninite Stability & Bioremediation Scheme Implementation

This research focuses on the conditions and rates under which uranium will be remobilized after it has been precipitated biologically, and what alterations can be implemented to increase its long-term stability in groundwater after the injection of an electron donor has been discontinued. Furthermore, this research addresses short-term iron reoxidation as a mechanism to enhance/extend uranium bioremediation under iron reduction, without its remobilization. The research to date has focused on long term column experiments involving the biological removal of uranium from groundwater under iron and sulfate reducing conditions. Aquifer sediment was collected from the background area of the Old Rifle UMTRA site and dried and sieved (<2 mm) before being packed into four 15 cm long x 5 cm diameter glass columns. The initial porosity of each column ranged from 0.33 to 0.40. Prior to biostimulation of the columns, 30 mM bicarbonate (purged with CO2/N2 gas, 20:80 ratio) was pumped through the columns to flush out the natural uranium present in the sediment. After the natural uranium was flushed out of the system, 20 uM of uranyl acetate was added to the 30 mM bicarbonate influent media. The column was operated for 11 days to ensure that the effluent U(VI) concentration was equal to the influent U(VI) concentration (no removal of U(VI) occurred before biostimulation). The start of the biostimulation experiment was facilitated by the addition of one pore volume of a growth culture containing the Fe(III) and U(VI) reducing microorganism, Geobacter metallireducens. Flow to the columns was suspended for 24 hours, after which pumping was resumed with acetate (2.8-3.0 mM), as well as trace vitamins and minerals, supplied to the feed media. The columns were operated at 22 +/- 1 degrees C, upright and under up-flow conditions at a rate of 0.2 ml/min (equivalent to a linear groundwater travel time of approximately 135 m/yr). Water samples from column inlets and outlets were collected and analyzed for acetate, U(VI), Fe(II), Br-, NO3- and SO42-. Iron reduction and U(VI) removal was detected in all four columns after three days of column operation with acetate in the inflow. The Fe(II) concentration at the effluent of the columns increased at a rate of 16.6 (+/-1.9) uM/d until leveling off after 10 days of column operation. The pseudo steady-state Fe(II) concentration at the effluent for each column ranged 130 uM to 170 uM. Uranium removal reached steady-state conditions after approximately 23 days of column operation with removal of between 58% to 77% of the initial 20 uM U(VI) added at the influent of the column