Publikationsdatum:
2015-07-21
Beschreibung:
Technetium-99 is a mobile, long-lived radionuclide and environmental risk driver at some nuclear waste sites. The feasibility of decreasing 99 Tc mobility in vadose zone sediments using H 2 S and NH 3 gases was evaluated in laboratory experiments. In untreated sediments, 75 to 95% of the 99 Tc was leachable. Using combinations of H 2 S and NH 3 gases, the 99 Tc mobility was reduced to 14 to 48%. Individual H 2 S or NH 3 gas treatment of sediments had little lasting effect. For the combined gas treatment, the H 2 S gas created reducing conditions at the pore water–mineral interface, which temporarily reduced and precipitated 99 Tc, while the NH 3 gas created alkaline pore water that caused mineral dissolution. As the pH neutralized, subsequent aluminosilicate precipitation probably coated 99 Tc precipitates and rendered them less mobile. Surface phase analysis showed that 99 Tc was associated with weathered basalt clasts and S, possibly from the precipitation of TcS x . Treatment performance was nearly the same at different 99 Tc concentrations (1.3–240.5 Bq g –1 ), water contents (1–8%), and gas injection rates but was sensitive to gas concentrations. Low gas concentrations (〈3%) had insufficient reductant or slower mineral dissolution. High gas concentrations (〉30%) formed an NH 4 SH precipitate. The 14 to 48% mobile 99 Tc remaining after gas treatment may have been caused by the limited time for aluminosilicates to precipitate in our experiments. Degradation of added NH 3 was not observed during the 3-mo experiment. Overall, this study showed that combined H 2 S and NH 3 gas treatment of low-water-content sediments can be applied to significantly decrease 99 Tc mobility.
Digitale ISSN:
1539-1663
Thema:
Geologie und Paläontologie
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Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft
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