Gas-Phase Treatment of Technetium in the Vadose Zone at the Hanford Site Central Plateau

Abstract

Technetium-99 (Tc-99) is present in the vadose zone of the Hanford Central Plateau and is a concern with respect to the protection of groundwater. The persistence, limited natural attenuation mechanisms, and geochemical behavior of Tc-99 in oxic vadose zone environments must be considered in developing effective alternatives for remediation. This report describes a new in situ geochemical manipulation technique for decreasing Tc-99 mobility using a combination of geochemical Tc-99 reduction with hydrogen sulfide gas and induced sediment mineral dissolution with ammonia vapor, which create conditions for deposition of stable precipitates that decrease the mobility of Tc-99. Laboratory experiments were conducted to examine changes in Tc-99 mobility in vadose zone sediment samples to evaluate the effectiveness of the treatment under a variety of operational and sediment conditions. The laboratory experiments demonstrated a robust treatment approach to decrease the mobility of Tc-99 in the vadose zone at the Hanford Central Plateau using gas-phase amendments. The gas-phase Tc-99 treatment was shown to be effective for a variety of operational and site conditions, including 1) sequential and parallel addition of hydrogen sulfide and ammonia gases over a range of treatment gas concentrations, 2) Tc-99 concentrations ranging from 34 to 3800 pCi/g, 3) soil moisture contents of 1, 4, and 8 wt%, and 4) gas delivery times into the sample ranging from minutes to hours. A large fraction of Tc-99 in treated sediments was also shown be resistant to leaching in saturated soil column tests. These laboratory results suggest that the combined hydrogen sulfide and ammonia gas treatment is a viable candidate for treatment to decrease the mobility of Tc-99 in the vadose zone. The combined-gas Tc-99 treatment technique is an extension of the ammonia gas treatment for uranium. Previous efforts have examined many aspects of the ammonia treatment process, including key information needed to scale this process to field application. Much of this scale-up information is applicable to field application of the combined hydrogen sulfide and ammonia gas treatment demonstrated in this study to decrease Tc-99 mobility. The information presented herein suggests that, with moderate additional scale-up investigation, the combined-gas Tc-99 treatment could be field tested. Key scale-up elements would include evaluating the fate and transport of hydrogen sulfide gas in the subsurface in the presence of ammonia gas to support determining safe operational conditions for use of hydrogen sulfide. Some additional information on the reaction rates and processes in relation to subsurface pH conditions and Tc-99 precipitate stability and whether microbial processes interact with these reactions will also be important. For health and safety reasons, it may be important to minimize field collection of gas samples for treatment monitoring. Thus, extension of the electrical resistivity tomography monitoring previously applied for soil desiccation and researched fo...