Zinc-Silver, Zinc-Palladium, and Zinc-Gold as Bimetallic Systems for Carbon Tetrachloride Dechlorination in Water

Abstract

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...

Metal-Mediated Reductive Hydrodehalogenation of Organic Halides

“…An important contribution to this field was done by Boronina et al, who demonstrated that metallic zinc is an ideal candidate for the practical destruction of carbon tetrachloride in water . In a preliminary work, zinc dust was reported to cause deep sequential dechlorination of carbon tetrachloride to give methane as the main final organic product, chloroform, methylene chloride, and methyl chloride being intermediate reaction compounds 472a.…”

“…An important contribution to this field was done by Boronina et al, who demonstrated that metallic zinc is an ideal candidate for the practical destruction of carbon tetrachloride in water . In a preliminary work, zinc dust was reported to cause deep sequential dechlorination of carbon tetrachloride to give methane as the main final organic product, chloroform, methylene chloride, and methyl chloride being intermediate reaction compounds 472a. In general, superior and different behavior was observed for zinc in comparison with magnesium or tin, the most active samples being prepared by a metal vapor−solvent co-deposition method (cryoparticles) or by mechanical treatment. 472a-e Doping zinc with silver, palladium, or gold resulted in a 4−10-fold increase in the carbon tetrachloride degradation rate and conversion to methane 472f.…”

The Removal of Metals and Release of Edta From Pulp Wash Water

“…12 The drawbacks of this technique include the rate of reaction, the selectivity of the reaction and the poisoning of electrode surfaces. 16,17 The following series of reactions is considered to represent a reasonable mechanistic scheme that can account for the liberation of EDTA and the removal of copper, iron and manganese from aqueous media by cementation and/or precipitation. 13 The use of zero-valent metals has been investigated 14,15 as a mean of removing metal ions from aqueous streams by cementation/precipitation.…”

Chloride Effect on TNT Degradation by Zerovalent Iron or Zinc during Water Treatment

“…Zerovalent zinc (ZVZn) has also been evaluated for treating organics-contaminated waters ( − ). This metal is more thermodynamically active and reactive than iron.…”

“…This metal is more thermodynamically active and reactive than iron. Experi ments conducted to examine the degradation of chlorinated solvents using zerovalent zinc have shown faster degradation kinetics compared to iron ( − ). This metal has not been used for the degradation of nitroaromatic compounds in environmental media.…”