| Summary: | Palladium has been demonstrated to catalyze the reduction of halogenated hydrocarbons to alkanes by hydrogen. Batch studies with palladium indicate a rate of reaction several orders of magnitude higher than in the zero-valent iron system (another method currently under study). Chlorinated ethenes, including PCE and vinyl chloride, were completely removed from tap water within ten minutes at room temperature by 0.5% palladium on alumina at 0.1 atm of hydrogen pressure. (Schreier and Reinhard, Chemosphere, 31(6) pp 3475-3487, 1995.) This quick reaction time demonstrates palladium's great potential for application in pump-and-treat groundwater remediation. Although palladium has been used as a reduction catalyst for years in organic chemistry applications, the system has not yet been well characterized for groundwater treatment. This project entails construction and testing of bench-scale continuous-flow reactors. For testing, we have been using artificially contaminated deionized (DI) water, groundwater from Livermore, California, and groundwater from the California Central Valley. Two bench-scale systems have been constructed, one which is operated in the flow-through mode and one which is operated in a rotating basket-mode. Base-line tests with deionized (DI) water have indicated 99% TCE removal at inlet concentrations ranging from 1.5 to 25 mg/L with no noticeable decrease in catalyst activity over 5 months. When actual ground waters is used, the catalyst is deactivated at rates that depend on water quality. The approach to identify deactivating water quality parameters has been to correlate the water quality with catalyst deactivation time and then to study the effect of the hypothesized parameters in separate experiments. Efforts are directed towards establishing the reactivities of different VOCs and getting a better understanding of the catalyst performance under groundwater treatment conditions, specifically (1) quantifying the competing effects of oxygen and TCE reduction, (2) improving the methodology to characterize catalyst deactivation using spectroscopic methods, and (3) characterizing the effects of the water matrix on the process. We developed the methodology to evaluate samples of catalyst in column and rotating basket reactors and different water matrices. A model was evaluated for examining the column performance in the presence of different catalyst poisoning substances and competing electron accepting substances. The rates of reductive dehalogenation of the most prevalent VOCs were measured in pure (Milli-Q) water. Tetrachloroethylene, trichloroethylene, the DCE isomers, carbon tetrachloride and 1,2-dibromo-3-chloropropane appear to react at diffusion limited rates with no evidence for catalyst deactivation. Freon 113 and chloroform react markedly slower, by approximately 80 and 90%, respectively, and the rates of 1,1- and 1,2-dichloroethane were too small to measure. In groundwater, rates were generally slower. Possible deactivating agents are carbonic acid, bicarbonate and carbonate ions. The field-scale test conducted by the Lawrence Livermore National Laboratory to treat TCE and tritium contaminated groundwater in a treatment well using a submersed Pd reactor is ongoing.
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