Web Release Date: May 13,
Pilot-Scale in Situ Bioremedation of Uranium in a Highly Contaminated Aquifer. 2. Reduction of U(VI) and Geochemical Control of U(VI) Bioavailability
and
Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, Department of Microbiology, Miami University, Oxford, Ohio 45056, Ecovation Inc., Victor, New York 14564, and Swiss Federal Institute of Aquatic Science and Technology (EAWAG), P.O. Box 611, Ueberlandstrasse 133, CH-8600 Duebendorf, Switzerland
Received for review October 3, 2005
Revised manuscript received March 30, 2006
Accepted March 31, 2006
Abstract:
In situ microbial reduction of soluble U(VI) to sparingly
soluble U(IV) was evaluated at the site of the former S-3
Ponds in Area 3 of the U.S. Department of Energy Natural
and Accelerated Bioremediation Research Field Research
Center, Oak Ridge, TN. After establishing conditions favorable
for bioremediation (Wu, et al. Environ. Sci. Technol.
2006, 40, 3988-3995), intermittent additions of ethanol
were initiated within the conditioned inner loop of a nested
well recirculation system. These additions initially
stimulated denitrification of matrix-entrapped nitrate, but
after 2 months, aqueous U levels fell from 5 to ~1 
M and
sulfate reduction ensued. Continued additions sustained
U(VI) reduction over 13 months. X-ray near-edge absorption
spectroscopy (XANES) confirmed U(VI) reduction to
U(IV) within the inner loop wells, with up to 51%, 35%,
and 28% solid-phase U(IV) in sediment samples from the
injection well, a monitoring well, and the extraction well,
respectively. Microbial analyses confirmed the presence of
denitrifying, sulfate-reducing, and iron-reducing bacteria
in groundwater and sediments. System pH was generally
maintained at less than 6.2 with low bicarbonate level (0.75-1.5 mM) and residual sulfate to suppress methanogenesis
and minimize uranium mobilization. The bioavailability
of sorbed U(VI) was manipulated by addition of low-level
carbonate (<5 mM) followed by ethanol (1-1.5 mM). Addition
of low levels of carbonate increased the concentration
of aqueous U, indicating an increased rate of U desorption
due to formation of uranyl carbonate complexes. Upon
ethanol addition, aqueous U(VI) levels fell, indicating that
the rate of microbial reduction exceeded the rate of desorption.
Sulfate levels simultaneously decreased, with a corre
sponding increase in sulfide. When ethanol addition ended
but carbonate addition continued, soluble U levels
increased, indicating faster desorption than reduction.
When bicarbonate addition stopped, aqueous U levels
decreased, indicating adsorption to sediments. Changes
in the sequence of carbonate and ethanol addition confirmed
that carbonate-controlled desorption increased bioavail
ability of U(VI) for reduction.
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