Effects of nitrate on the stability of uranium in a bioreduced region of the subsurface.
The effects of nitrate on the stability of reduced, immobilized uranium were evaluated in field experiments at a U.S. Department of Energy site in Oak Ridge, TN. Nitrate (2.0 mM) was injected into a reduced region of the subsurface containing high levels of previously immobilized U(IV). The nitrate was reduced to nitrite, ammonium, and nitrogen gas; sulfide levels decreased; and Fe(II) levels increased then deceased. Uranium remobilization occurred concomitant with nitrite formation, suggesting nitrate-dependent, iron-accelerated oxidation of U(IV). Bromide tracer results indicated changes in subsurface flowpaths likely due to gas formation and/or precipitate. Desorption-adsorption of uranium by the iron-rich sediment impacted uranium mobilization and sequestration. After rereduction of the subsurface through ethanol additions, background groundwater containing high levels of nitrate was allowed to enter the reduced test zone. Aqueous uranium concentrations increased then decreased. Clone library analyses of sediment samples revealed the presence of denitrifying bacteria that can oxidize elemental sulfur, H(2)S, Fe(II), and U(IV) (e.g., Thiobacillus spp.), and a decrease in relative abundance of bacteria that can reduce Fe(III) and sulfate. XANES analyses of sediment samples confirmed changes in uranium oxidation state. Addition of ethanol restored reduced conditions and triggered a short-term increase in Fe(II) and aqueous uranium, likely due to reductive dissolution of Fe(III) oxides and release of sorbed U(VI). After two months of intermittent ethanol addition, sulfide levels increased, and aqueous uranium concentrations gradually decreased to <0.1 microM.
Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, USA.
Wei-Min
WM
Wu
Jack
J
Carley
Stefan J
SJ
Green
Jian
J
Luo
Shelly D
SD
Kelly
Joy
J
Van Nostrand
Kenneth
K
Lowe
Tonia
T
Mehlhorn
Sue
S
Carroll
Benjaporn
B
Boonchayanant
Frank E
FE
Löfller
David
D
Watson
Kenneth M
KM
Kemner
Jizhong
J
Zhou
Peter K
PK
Kitanidis
Joel E
JE
Kostka
Philip M
PM
Jardine
Craig S
CS
Criddle
D001673:Biodegradation, Environmental; D005740:Gases; D019015:Geologic Sediments / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D009566:Nitrates / Q000737:chemistry*; D009573:Nitrites / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry; D012336:RNA, Ribosomal, 16S / Q000378:metabolism; D013440:Sulfides / Q000737:chemistry; D013455:Sulfur / Q000737:chemistry; D013499:Surface Properties; D014501:Uranium / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis*
D005740:Gases; D009566:Nitrates; D009573:Nitrites; D012336:RNA, Ribosomal, 16S; D013440:Sulfides; D014874:Water Pollutants, Chemical; D014501:Uranium; D013455:Sulfur; D009584:Nitrogen; D010100:Oxygen