Remediation of technetium in vadose zone sediments using ammonia and hydrogen sulfide gases

Technetium-99 is a mobile, long-lived radionuclide and environmental risk driver at some nuclear waste sites. The feasibility of decreasing (super 99) Tc mobility in vadose zone sediments using H (sub 2) S and NH (sub 3) gases was evaluated in laboratory experiments. In untreated sediments, 75 to 95% of the (super 99) Tc was leachable. Using combinations of H (sub 2) S and NH (sub 3) gases, the (super 99) Tc mobility was reduced to 14 to 48%. Individual H (sub 2) S or NH (sub 3) gas treatment of sediments had little lasting effect. For the combined gas treatment, the H (sub 2) S gas created reducing conditions at the pore water-mineral interface, which temporarily reduced and precipitated (super 99) Tc, while the NH (sub 3) gas created alkaline pore water that caused mineral dissolution. As the pH neutralized, subsequent aluminosilicate precipitation probably coated (super 99) Tc precipitates and rendered them less mobile. Surface phase analysis showed that (super 99) Tc was associated with weathered basalt clasts and S, possibly from the precipitation of TcS (sub x) . Treatment performance was nearly the same at different (super 99) Tc concentrations (1.3-240.5 Bq g (super -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 (sub 4) SH precipitate. The 14 to 48% mobile (super 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 (sub 3) was not observed during the 3-mo experiment. Overall, this study showed that combined H (sub 2) S and NH (sub 3) gas treatment of low-water-content sediments can be applied to significantly decrease (super 99) Tc mobility.