Pilot-scale in situ bioremedation of uranium in a highly contaminated aquifer. 2. Reduction of u(VI) and geochemical control of u(VI) bioavailability.
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 approximately 1 microM 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 corresponding 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 bioavailability of U(VI) for reduction.
Wu
Wei-Min
WM
Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, USA.
Carley
Jack
J
Gentry
Terry
T
Ginder-Vogel
Matthew A
MA
Fienen
Michael
M
Mehlhorn
Tonia
T
Yan
Hui
H
Caroll
Sue
S
Pace
Molly N
MN
Nyman
Jennifer
J
Luo
Jian
J
Gentile
Margaret E
ME
Fields
Matthew W
MW
Hickey
Robert F
RF
Gu
Baohua
B
Watson
David
D
Cirpka
Olaf A
OA
Zhou
Jizhong
J
Fendorf
Scott
S
Kitanidis
Peter K
PK
Jardine
Philip M
PM
Criddle
Craig S
CS
D001419:Bacteria / Q000254:growth & development / Q000378:metabolism; D001639:Bicarbonates / Q000378:metabolism; D001673:Biodegradation, Environmental; D001682:Biological Availability; D003470:Culture Media; D003666:Decontamination / Q000295:instrumentation / Q000379:methods; D004867:Equipment Design; D000431:Ethanol / Q000378:metabolism; D005618:Fresh Water / Q000737:chemistry* / Q000382:microbiology; D019015:Geologic Sediments / Q000737:chemistry* / Q000382:microbiology; D010084:Oxidation-Reduction; D010865:Pilot Projects; D011850:Radioactive Waste; D014501:Uranium / Q000737:chemistry / Q000378:metabolism*; D014875:Water Pollutants, Radioactive / Q000737:chemistry / Q000378:metabolism*; D018508:Water Purification / Q000295:instrumentation / Q000379:methods
D001639:Bicarbonates; D003470:Culture Media; D011850:Radioactive Waste; D014875:Water Pollutants, Radioactive; D000431:Ethanol; D014501:Uranium