Reduction and Reoxidation of Soils During & After Uranium Bioremediation; Implications for Long-Term Uraninite Stability & Bioremediation Scheme Implementation Page: 1 of 2
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Reduction and Reoxidation of Soils During & After Uranium Bioremediation;
Implications for Long-Term Uraninite Stability & Bioremediation Scheme
Jaffe, Peter R.
RESULTS TO DATE: 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 S042-. 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. Bromide was added as a tracer after 1
month of column operation in order to document hydraulic retention times and diffusion limitations. The
hydraulic retention time for each column ranged from 3.6 h to 8.8 h. Trends were observed regarding
column hydrodynamics, Fe(II) production and U(VI) removal from each column. The longer retention time
resulted in higher Fe(II) concentrations at the effluent of the column and more uranium removal. Sulfate
was present in the influent media as part of the trace vitamin and mineral solution added with the 30mm
bicarbonate. Sulfate reduction was also measured in the columns resulting in ~ 50% - 70% of the initial
0.7 mg/L sulfate removed in each column. Most of the phosphate, present in the influent media at a
concentration of 1.2 mg/L, was also removed from the system. One of the four columns was taken offline
after 104 days of column operation and destructively sampled to analyze geochemical parameters such
as Fe(II), total HCI extractable iron, U(VI), total uranium, and acid extractable sulfide. The analysis of
these samples is not complete, but preliminary results indicate that all or most of the Fe present in the
sediment (after 1 h extraction in 0.5N HCI) is in the form of Fe(II) with the Fe(II) concentration on the
surface of the sediment throughout the column measured to be 9.4 (+/-3.9) umol/g dry sediment. In
addition to the analysis performed in Dr. Jaffe?s laboratory, samples were packed anaerobically, frozen
and provided to Dr. Myneni and Dr. Zachara for additional analysis. Dr. Myneni is currently analyzing the
samples using X-ray absorption spectroscopy to determine the uranium speciation throughout the column
and Dr. Zachara is using Mossbauer spectroscopy to determine the Fe phases throughout the column.
Both Dr. Myneni and Dr. Zachara will compare their analysis of the reduced samples to the pristine
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Jaffe, Peter R. Reduction and Reoxidation of Soils During & After Uranium Bioremediation; Implications for Long-Term Uraninite Stability & Bioremediation Scheme Implementation, report, June 1, 2005; Princeton, New Jersey. (digital.library.unt.edu/ark:/67531/metadc883180/m1/1/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.