Doping of zinc with silver, palladium, and gold was found to increase reactivity towards carbon tetrachloride in water. Commercial zinc dust, cryochemically prepared zinc metal particles (SMAD nanoparticles), and zinc dust pressed into pellets (mechanically activated zinc) were employed. Reduction products detected were methane, ethylene, acetylene, and other hydrocarbons along with products of partial dechlorination such as chloroform, methylene chloride, and methyl chloride. Dichloroethylenes (DCEs) and long-term reactions traces of trichloroethylene (TCE) were also detected. The use of zinc dust doped with palladium, gold, and silver resulted in 4-10 fold increases in carbon tetrachloride degradation rate and conversion into methane. Up to 30 % of carbon tetrachloride was converted into methane by the Zn dust / 2 mol % Ag bimetallic system after the first six hours of reaction. Doping of activated forms of zinc, both cryoparticle and pellets, caused a further increase in methane formation and decrease in the concentration of methylene chloride. The data show that bimetallic enhancement with Pd, Ag, Au, as well as cryo and mechanical activation of zinc, enhances the metal surface reactivity and changes the priority of reaction pathways such that fully reduced products are favored. The non-catalytic gold metal was especially effective and this suggests that electron transfer, not catalytic hydrogenation, is rate determining.Key words: zinc, bimetallic, silver, palladium, gold, carbon tetrachloride, echlorination, water
INTRODUCTIONUse of zero-valent metals, in particular iron, for reduction in water of chlorinated solvents, such as carbon tetrachloride (CT), trichloroethylene (TCE), and tetrachloroethylene (PCE), has been an area of significant interest, as the prospects for field application look quite promising (Appleton, 1996; Orth and Gillham, 1994;Matheson and Tratnyek, 1994;Starr and Cherry, 1994). Nevertheless, implementation of zero-valent iron technology still faces several problems. One of them is the production and accumulation of chlorinated products due to the low reactivity of iron towards lightly chlorinated hydrocarbons. Dechlorination of CT by iron was shown to produce persistent methylene chloride (Johnson, Scherer, and Tratnyek, 1996;Matheson and Tratnyek, 1994). Products of perchloroethylene (PCE) and trichloroethylene (TCE) dechlorination, 1,2 -dichloroethylenes (DCEs), and vinyl chloride (VC) react only slowly with iron (Orth and Gillham, 1994;Hardy and Gillham, 1996;Roberts et al., 1996) and results are only marginally better in the presence of zinc Roberts et al., 1996). On the other hand, zinc converts chlorinated methanes into gaseous methyl chloride and methane, though methylene chloride degradation is still about two orders in magnitude slower than CT under the same experimental conditions . (We speculate that the relatively low reactivity with methylene chloride, DCE, and VC is due to the lessened electron demand due to fewer chlorines, thereby electron transfer from the metals...