Effects of Nitrate on the Stability of Uranium in a Bioreduced Region of the Subsurface
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- 作者：Wei-Min Wu ; Jack Carley ; Stefan J. Green ; Jian Luo ; Shelly D. Kelly ; Joy Van Nostrand ; Kenneth Lowe ; Tonia Mehlhorn ; Sue Carroll ; Benjaporn Boonchayanant ; Frank E. Lfller ; David Watson ; Kenneth M. Kem
- 刊名：Environmental Science & Technology
- 出版年：2010
- 出版时间：July 1, 2010
- 年：2010
- 卷：44
- 期：13
- 页码：5104-5111
- 全文大小：500K
- 年卷期：v.44,no.13(July 1, 2010)
- ISSN：1520-5851
文摘
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, H2S, 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 μM.