Remediation of Technetium in Vadose Zone Sediments Using Ammonia and Hydrogen Sulfide Gases
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
The feasibility of applying a combination of H2S and NH3 gases for Tc-99 remediation in vadose zone sediments evaluated in laboratory experiments showed a significant mobility decrease from 75 to 95% leachable Tc-99 in untreated sediments to 15 to 48% leachable Tc-99 after gas treatment. Individual H2S or NH3 gas treatment of sediments had little lasting effect. For the combined gas treatment, the H2S gas created reducing conditions at the pore water/mineral interface that pertechnetate was temporarily reduced/precipitated. The NH3 gas created alkaline pore water, which induces some mineral dissolution and subsequent aluminosilicate precipitation as the pH neutralizes, which may coat Tc-99 surface phases and render them less mobile. Surface phase analysis showed Tc-99 associated with weathered basalt clasts and sulfur, possibly from the precipitation of TcSx. Treatment performance was nearly the same at different Tc-99 concentration (34 to 6500 pCi/g), water content (1 to 8%), and gas injection rate, but was sensitive to gas concentrations. Low gas concentrations (< 3%) had insufficient reductant or slower mineral dissolution. High gas concentrations (>30%) formed NH4SH precipitate. The 15 - 23% mobile Tc-99 remaining after gas treatment may be caused by limited time for aluminosilicates to precipitate in experiments. Nitrogen species concentrations showed no nitrate or nitrite production and a significant decrease in the sediment microbial population over three months, suggesting nitrification of added ammonia was inhibited. Overall, this study showed that combined H2S and NH3 gas treatment of low water content sediments was a robust technology to significantly decrease Tc-99 mobility.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1212240
- Report Number(s):
- PNNL-SA-107578; 830403000
- Journal Information:
- Vadose Zone Journal, Vol. 14, Issue 7; ISSN 1539-1663
- Publisher:
- Soil Science Society of America
- Country of Publication:
- United States
- Language:
- English
Similar Records
Ammonia Gas Transport and Reactions in Unsaturated Sediments: Implications for Use as an Amendment to Immobilize Inorganic Contaminants
Characterization of U(VI)-Bearing Precipitates Produced by Ammonia Gas Injection Technology for Unsaturated Sediments - 16600