| Summary: | Batch studies with supported palladium catalysts have demonstrated the potential of the palladium/hydrogen process for treating groundwaters or effluent streams that are contaminated with halogenated compounds. These studies yielded virtually complete reductive dehalogenation of chlorinated ethylenes to ethane at room temperature in short contact times, with reaction rates that are orders of magnitude higher than zero-valent iron. Other batch studies have shown the ability of palladium to catalyze the reaction of a range of compounds: all 6 species of chlorinated ethylenes, carbon tetrachloride, chloroform, 1,2-dibromo-3-chloropropane, Freon 113, chlorobenzene, naphthalene and lindane. However, laboratory column studies and field tests have indicated that catalyst activity may decline under some conditions, thereby potentially affecting the economic competitiveness of this process. Research is needed to optimize the catalyst and operating parameters for the field, by determining the causes of activity loss and preventing or minimizing such effects. Advanced surface spectroscopic techniques will be used for characterizing processes that occur at the catalyst surface. The surface of the fresh catalyst will be compared to that of samples taken throughout the duration of column tests run with natural and synthetic groundwaters. The surface of catalyst that was subjected to different treatments will be characterized by chemical and crystal analysis, and the long-term effects of biological growth on catalyst activity will be evaluated. With this understanding of catalyst deactivation mechanisms, custom catalyst supports will be designed to circumvent the competition, inhibition, fouling, and poisoning effects of naturally found groundwater solutes. Finally, convenient methods for regenerating the catalyst beds in situ will be developed and evaluated. Samples of dispersed Pd/alumina were exposed to synthetic and natural groundwater in column experiments and were then analyzed using XPS (x-ray photoelectron spectroscopy). All water-exposed catalysts showed elevated levels of carbon and reduced nitrogen; this indicates an accumulation of organics or biofilms. In addition, the Pd metal was found to oxidize from its zero-valent state to a Pd(II) state, with larger percentages of Pd(II) correlating to lower catalytic activity. No effect was seen with pH changes (among carbonate, bicarbonate and carbonic acid). In addition to the work with dispersed catalysts, a model catalyst was developed and consists of a 0.5" X 0.5" square, flat, polycrystalline, a-alumina plate coated with approximately 500 ? of Pd. The catalyst showed activity for TCE removal from water. A continuously stirred tank reactor (CSTR) was designed and constructed to hold the model catalyst plates. A bromide tracer test indicated that the flow characteristics and residence time distribution match theoretical expectations. TCE sorption to the Pd will be investigated, then the catalyst will be exposed to a continuous flow of water and the surface will be periodically spectroscopically analyzed for changes.
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