In situ bioreduction of uranium (VI) to submicromolar levels and reoxidation by dissolved oxygen.
Groundwater within Area 3 of the U.S. Department of Energy (DOE) Environmental Remediation Sciences Program (ERSP) Field Research Center at Oak Ridge, TN (ORFRC) contains up to 135 microM uranium as U(VI). Through a series of experiments at a pilot scale test facility, we explored the lower limits of groundwater U(VI) that can be achieved by in-situ biostimulation and the effects of dissolved oxygen on immobilized uranium. Weekly 2 day additions of ethanol over a 2-year period stimulated growth of denitrifying, Fe(III)-reducing, and sulfate-reducing bacteria, and immobilization of uranium as U(IV), with dissolved uranium concentrations decreasing to low levels. Following sulfite addition to remove dissolved oxygen, aqueous U(VI) concentrations fell below the U.S. Environmental Protection Agengy maximum contaminant limit (MCL) for drinking water (< 30/microg L(-1) or 0.126 microM). Under anaerobic conditions, these low concentrations were stable, even in the absence of added ethanol. However, when sulfite additions stopped, and dissolved oxygen (4.0-5.5 mg L(-1)) entered the injection well, spatially variable changes in aqueous U(VI) occurred over a 60 day period, with concentrations increasing rapidly from < 0.13 to 2.0 microM at a multilevel sampling (MLS) well located close to the injection well, but changing little at an MLS well located further away. Resumption of ethanol addition restored reduction of Fe(III), sulfate, and U(VI) within 36 h. After 2 years of ethanol addition, X-ray absorption near-edge structure spectroscopy (XANES) analyses indicated that U(IV) comprised 60-80% of the total uranium in sediment samples. Atthe completion of the project (day 1260), U concentrations in MLS wells were less than 0.1 microM. The microbial community at MLS wells with low U(VI) contained bacteria that are known to reduce uranium, including Desulfovibrio spp. and Geobacter spp., in both sediment and groundwater. The dominant Fe(III)-reducing species were Geothrix spp.
Wu
Wei-Min
WM
Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, USA.
Carley
Jack
J
Luo
Jian
J
Ginder-Vogel
Matthew A
MA
Cardenas
Erick
E
Leigh
Mary Beth
MB
Hwang
Chiachi
C
Kelly
Shelly D
SD
Ruan
Chuanmin
C
Wu
Liyou
L
Van Nostrand
Joy
J
Gentry
Terry
T
Lowe
Kenneth
K
Mehlhorn
Tonia
T
Carroll
Sue
S
Luo
Wensui
W
Fields
Matthew W
MW
Gu
Baohua
B
Watson
David
D
Kemner
Kenneth M
KM
Marsh
Terence
T
Tiedje
James
J
Zhou
Jizhong
J
Fendorf
Scott
S
Kitanidis
Peter K
PK
Jardine
Philip M
PM
Criddle
Craig S
CS
D001419:Bacteria / Q000378:metabolism; D001673:Biodegradation, Environmental; D000431:Ethanol; D005618:Fresh Water / Q000737:chemistry; D019015:Geologic Sediments / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D012987:Soil; D012995:Solubility; D013057:Spectrum Analysis; D014481:United States; D014484:United States Environmental Protection Agency; D014501:Uranium / Q000302:isolation & purification* / Q000378:metabolism; D014875:Water Pollutants, Radioactive / Q000302:isolation & purification / Q000378:metabolism
D012987:Soil; D014875:Water Pollutants, Radioactive; D000431:Ethanol; D014501:Uranium; D010100:Oxygen