Catalytic hydrodehalogenation of chlorinated ethylenes using palladium and hydrogen for the treatment of contaminated water

A kinetic model is presented for the catalytic hydrodehalogenation of chlorinated ethylenes using Pd and H2 under water treatment conditions. All five chlorinated ethylenes, including tetrachloroethylene (PCE) and vinyl chloride, were completely removed from tap water within 10 minutes at room temperature by 0.5 g of 0.5% Pd on alumina and 0.1 atm H2. Ethane accounted for 55–85% of the mass balance in these systems. Ethene was a reactive intermediate whose maximum concentration accounted for less than about 5% of the initial substrate. Palladium on granular carbon was also an effective catalyst, although ethane yield for PCE was somewhat lower than with Pd-alumina (55% versus 85%). The transformation of PCE was first order with respect to both substrate and amount of metal, with a half-life of t12 = 9 min for 0.055 μmole Pd (583 μg of 1% Pd on powdered activated carbon). Addition of ∼10 mg/L of nitrite to the water decreased the rate constant by about 50%. The nitrite concentration decreased by about 25% over the course of the reaction. Addition of nitrate or sulfate had smaller effect on the rate of PCE transformation; chloride had no effect. The presence of oxygen greatly reduced the amount of ethane produced regardless of the catalyst support. Bisulf1de poisoned the catalyst.