Reductive dechlorination chloroform

Ifi-Ditritiodihenzo- -dioxin, prepared by reductive dechlorination of 1,6-dichlorodihenzo-p-dioxin with tritium gas, is chlorinated in chloroform solution, containing catalytic amounts of iodine and ferric chloride, to produce 1,6-ditritio-2,3,7,8-tefrachlorodibenzo-p-dioxin. 

The dehalogenation process has been approved by the EPA s Office of Toxic Substances for PCB treatment and has been experimentally implemented for the cleanup of PCB-contaminated soil at the following three Superfund sites Wide Beach in Erie County, New York (1985) Re-Solve in Massachusetts (1987) and Sol Lynn in Texas (1988). The glycolate process has been used to successfully treat contaminant concentrations of PCBs from less than 2 ppm to reportedly as high as 45,000 ppm. Using this technology, Helland et al. (1995) investigated reductive dechlorination of carbon tetrachloride with elemental iron and found that the rate of dechlorination to chloroform and methylene chloride was a fast first-order process. 

The reaction proceeds until each chlorine ion is removed. For example, carbon tetrachloride would be reduced to chloroform, then to methylene chloride, and finally to methane (the reduction of methylene chloride takes several months, however). No degradation products other than the parent compounds were found therefore, degradation is simple, reductive dechlorination, with the zero-valent iron serving as an electron donor. The reaction was pseudo first-order and the reaction constant, k, decreased with each additional dehalogenation step (Gillham and O Hannesin, 1994). 

Hori, Y., Murata, K. and Oku, T. (2003) Electrochemical dechlorination of chlorinated hydrocarbons - electrochemical reduction of chloroform in acetonitrile/water mixtures at high current density. Chem. Lett. 32, 230-231. 

Chloroform was dechlorinated by ptme cultures of Methanosarcina sp. strain DCM and Methanosarcina mazei S6. The initial dechlorination product of chloroform was dichloromethane, accounting for about 70 percent of added chloroform. The product of dichloromethane, chloromethane was detected consistently at trace levels but could not be accurately quantified. The production of C02 from [ H1] - chloroform and the absence of CH imply that reaction mechanisms other than reductive dechlorination alone are important [92]. Degradation of dichloromethane by oxidation via phosgene or alcohols was proposed previously, for methanogenic systems [13]. 

FIGURE 13. Reductive dechlorination of carbon tetrachloride and subsequent oxidation of chloroform to reactive phosgene. 

Halomethyl compounds are subdivided into monohalomethyls, which are alkylating agents, and polyhalomethyls, which must be metabolized to an ultimate species. Reductive dechlorination of carbon tetrachloride 353) to chloroform by rabbit liver microsomes parallels the concentration of cytochrome P-450 in the microsomes but requires anaerobic conditions and NADPH. The identification of hexachloroethane after incubation of NADPH-reduced microsomes with carbon tetrachloride is indicative of homolytic formation of the free radicals of chlorine and trichloromethyl and supports the hypothesis that such species initiate an autocatalytic peroxidation of lipid membranes that results in the observed hepatotoxicity. A similar scheme for radical formation and lipid destruction has been described by Reynolds and Moslen for halothane. In contrast to the reductive dechlorination of carbon tetrachloride, the metabolism of chloroform to carbon dioxide in vitro requires oxygen and produces carbonyl chloride (phosgene) as an intermediate. That this also... 

Initially, the facility operated for four hours per day, followed by purges with three pore volumes of nonhydrogenated water and then drainage this is equivalent to approximately 50 pore volumes/day. The system successfully dechlorinated PCE (>99%), TCE (>99%), 1,1-DCE (>99%), carbon tetrachloride (>98%), and chloroform (>91%) however, 1,2-DCA concentrations were not significantly changed. Chlorinated intermediates were not observed in the effluent, except for low levels of cis-DCE, which may have been present in the influent. Analysis of effluent from the first reactor indicated that almost all removal occurred in the first reactor only CF showed further reductions in concentration across the second column. (McNab etal. 2000)... 

Figure 4.3f shows a way of contact of the reactants on the catalytic membrane based on the forced flow of both reactants through the catalytic layer directly from one membrane side. Tliis configuration with respect to traditional reactors can offer an important improvement in the contact of reactants with the catalytic sites. Mass transfer from the gas phase to the liquid phase will occur in the same way as traditional reactors the liquid phase needs to be previously saturated with the gas reactant. The features of this last feeding configuration have been reported by Reif and Dittmeyer (2003) for both the catalytic nitrite reduction and the dechlorination of chloroform. 

Dechlorination rates and byproducts for most environmentally relevant chlorocarbons have been determined under a wide range of experimental conditions (7-75, 16). Chlorinated alkane reduction by zerovalent iron has been found to proceed sequentially, and produces chlorinated products (5). For example, dechlorination of carbon tetrachloride (CT) produces primarily chloroform as the first stable product (5), In contrast, chlorinated alkene reduction yields primarily completely dechlorinated species as the first stable products. For example, dechlorination of perchloroethylene (PCE) and trichloroethylene (TCE) has been hypothesized to involve a -elimination mechanism that yields chloroacetylene as the first detectable product 4, 16). However, the chloroacetylene is unstable and is rapidly reduced to acetylene and ethene, and has therefore not been reported by most investigators 4, 5, 16). 

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