Enhancement of zinc-mediated dechlorination by cobalt porphyrin complexes

Chlorinated ethylenes, such as trichloroethylene, are common groundwater contaminants. Zinc has been shown to dechlorinate these compounds, and has been used in permeable reactive barrier (PRB) systems for groundwater remediation. Zinc-mediated dechlorination rates can be increased when a catalyst is bound to the zinc surface. Cobalt compounds such as Vitamin B12 and 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin cobalt have been shown to catalyze dechlorination. In this study [Co(TCPP)] was used to determine the degree to which it binds to zinc and how it changes the rate of dechlorination compared to a non-catlyzed system. A column of sand and zinc was used to mimic a PRB. Saturated buffered solutions of TCE were run through the column, and the dechlorination products were analyzed by GC-MS. The cis-dichloroethylene to trans-dichloroethylene ratio was monitored and used as a diagnostic of the mechanism to determine the site of dechlorination, since zinc and Co(TCPP) dechlorination mechanisms are different and produce different product ratios. In the non-catalyzed system, the average concentration of cDCE and tDCE produced was 46 μM, whereas when Co(TCPP) was added, the product concentration more than doubled to 112 μM. In the catalyzed system, the average ratio of cDCE:tDCE was 1.12:1, which increased from 0.39:1 in the non-catalyzed system. Thus, the increased dechlorination rate and changed product ratio give evidence that Co(TCPP) is participating directly in the reaction and enhancing the dechlorination of chlorinated ethylenes by zinc metal.