Groundwater contaminants such as trichloroethene (TCE) and tetrachloroethene (PCE) are mobile and refractory in aquatic environments. This project’s demonstration objectives were to validate palladium (Pd) catalytic treatment efficacy for destruction of chlorinated ethenes in groundwater, optimize operational treatment efficiency, and develop cost and performance data for full-scale technology application.
Palladium-catalyzed reductive dechlorination transforms chlorinated ethenes and other volatile organic compounds into their respective saturated hydrocarbons or lesser chlorinated analogues. With hydrogen gas as the reductant, the process is selective requiring only small quantities of hydrogen to remove contaminants to below regulatory limits. The process can be used to efficiently treat water contaminated with reactive chlorinated contaminants. A one-pass catalytic process has many advantages, mainly that contaminants are destroyed instead of being transferred to another medium (e.g., air or activated carbon), avoiding the generation of a secondary waste stream. The technology is particularly favorable for treating water contaminated with high concentrations (greater than 1 mg/L) of chlorinated ethenes and may therefore be suited for source control.
TCE treatment via Pd catalytic reduction was demonstrated for contaminated groundwater at Edwards Air Force Base, California. Although the site water had a tendency to turn sulfidic, treatment goals were achieved. As initially proposed, the reactors were to be mounted below grade within horizontal flow treatment wells, thus qualifying as an in-situ technology; however, this in-situ, dual-column configuration was not feasible within the constraints of the pilot-scale demonstration due to the long and difficult start-up phase. Subsurface operation of the reactors would have added costs that might be out of proportion to the benefits gained. Once operational issues were resolved and a catalyst regeneration protocol was optimized, the aboveground catalyst reactor successfully reduced the TCE concentrations in groundwater by two to three orders of magnitude (more than 99%) consistently and without significant loss of catalyst activity.
This aboveground Pd catalysis technology can destroy TCE below its maximum contaminant level even in anaerobic but not sulfidic groundwater, will not generate hazardous byproducts, and might be appropriate at sites contaminated with other chlorinated ethenes (PCE, dichloroethene isomers, and vinyl chloride). For some groundwater contaminants such as 1,1,1-trichloroethane, 1,2-dichloroethane, and methylene chloride, the process is not effective, and for others (e.g., chloroform), it is slower than for TCE and PCE. Nonetheless, unique groundwater matrices and the opportunity for single-pass, multi-contaminant remediation makes Pd reductive catalysis competitive with other remediation schemes such as air stripping, granular activated carbon, and permeable reactive barriers. (Project Completed – 2008)