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Title: Remediation of Technetium in Vadose Zone Sediments Using Ammonia and Hydrogen Sulfide Gases

The feasibility of applying a combination of H 2S and NH 3 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 H 2S or NH 3 gas treatment of sediments had little lasting effect. For the combined gas treatment, the H 2S gas created reducing conditions at the pore water/mineral interface that pertechnetate was temporarily reduced/precipitated. The NH 3 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 TcS x. 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 NH 4SH precipitate. The 15 - 23% mobile Tc-99 remaining after gas treatment may be caused by limited time formore » 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 H 2S and NH 3 gas treatment of low water content sediments was a robust technology to significantly decrease Tc-99 mobility.« less
Authors:
 [1] ;  [1] ;  [1] ;  [1] ;  [1] ;  [1] ;  [1]
  1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Publication Date:
OSTI Identifier:
1212240
Report Number(s):
PNNL-SA-107578
Journal ID: ISSN 1539-1663; 830403000
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Vadose Zone Journal; Journal Volume: 14; Journal Issue: 7
Publisher:
Soil Science Society of America
Research Org:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES