Successful dechlorinations

Cultures of a number of anaerobic bacteria are able to dechlorinate tetrachloromethane and Acetobacterium woodii formed dichloromethane as the final chlorinated metabolite by successive dechlorination, although CO2 was also produced by an unknown mechanism (Egli et al. 1988). 

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)... 

With each successive dechlorination, the reaction proceeds more slowly, and dichloromethane is not reduced by ZVI under ambient conditions (Matheson and Tratnyek, 1994). 

Recent studies indicate that non-ortho congeners are produced de novo during incineration processes [17, 18]. Production of toxic CBs through successive dechlorination by sunlight irradiation has been shown under laboratory conditions [114]. 

Chlorophenols are a class of pesticide substances, e.g. fungicides, used for wood preservation, in pulp production and other miscellaneous applications. The substances were introduced in the 1930s and have been used in very large amounts. Today, the consumption has decreased and the substances are banned in many countries. The main active substance in chlorophenol products is pentachloro-phenol (PCP Figure 3.10). The substance is moderately lipophilic and persistent, yet readily absorbed and accumulated in biota and expresses a rather high acute toxicity. The metabolism and breakdown of this envirotoxicant in biota and in the environment are rather slow, resulting in successively dechlorinated metabolites. 

The tu c-dichloride (1), the adduct of benzyne and cis-3,4-dichlorocyclobutene, was successfully dechlorinated by disodium phenanthrene to give (2) in 27% yield.8 Other reagents give mixtures of tetralin and 1,4-dihydronaphthalene. The product (2) is correctly named as benzobicyclo 2.2.0]hexa-2,5-diene, but can be called hemi Dewar naphthalene. Similar dehalogenations of n c-dihalides to give substituted cyclobutene derivatives have been carried out using naphthalene-sodium. ... 

A comparable reductive dechlorination is used by a pentachlo-rophenol-degrading Flavobacterium sp. and the enzyme has been purified the successive dechlorinations to form 2-chloro-l,4-dihy-droxybenzene are probably mediated by a glutathione S-trans-ferase system (Xun et al. 1992c). It should be noted that the evidence so far shows that the enzyme involved in this reaction is distinct from that involved in the anaerobic dechlorination of 3-chloroben-zoate by Desulfomonile tiedjei DCB-l(Ni et al. 1995) that is discussed further in Section 6.6. 

Photolysis has had limited application for treatment of hazardous waste or detoxification of chemically contaminated materials. The susceptibility of chlorinated aromatics, including herbicides such as 2,4-D and 2,4,5-T, to UV-induced decomposition is well established (7,8). Photodecomposition of such compounds leads to successive dechlorination followed by condensation reactions to form phenolic polymers (7,8). Other research has demonstrated that CDD and CDF decompose in the presence of UV light (8,9,10) Development of a photochemical process for destroying 2,3,7,8-TCDD in a waste tar indicated similar dechlorination and condensation reactions and products (8). The high-molecular weight end products, which are similar in structure to humic acids, would be expected to have low toxicity and mobility. Therefore, essentially complete... 

The new DBF segment was prepared as shown in Scheme 25. The coupling reaction between the two synthons (98 and 122) were attempted in the presence of a host of bases. Fortunately, the reaction was effected smoothly in the presence of LDA. The adduct thus obtained was transformed into an unstable 1,3-diketone derivative 123 which was immediately subjected to the reaction with Mn(III) acetate, copper acetate and chloroform in acetic acid. The corresponding chloro derivative 124 was successively dechlorinated (125) and subsequent removal of substituents should finally form fredericamycin A [89]. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

Experiments have been carried out to mimic the reactions of model systems for coenzyme F430 that is involved in the terminal step in the biosynthesis of methane, and that is able to dechlorinate CCI4 successively to CHCI3 and CH2CI2 (Krone et al. 1989). Nickel(I) isobacteriochlorin anion was generated electrolytically and used to examine the reactions with alkyl halides in dimethylformamide (Helvenston and Castro 1992). The three classes of reaction were the same as those observed with Fe(II) deuteroporphyrin IX that have already been noted. 

Analysis of chlorobenzoates in sediments, which had been contaminated with PCBs, was used to demonstrate that the lower PCB congeners that had initially been produced by anaerobic dechlorination were subsequently degraded under aerobic conditions. The chlorobenzoates were transient metabolites and their concentrations were extremely low since bacteria that could successfully degrade them were present in the sediment samples (Elanagan and May 1993). 

In anaerobic microcosms, l,l,2-trichloro-l,2,2-trifiuoroethane (CFC-113) was transformed by successive reductive dechlorination to l,2-dichloro-l,2,2-trifiuoroethane (HCFC-123a), and under methanogenic conditions to l-chloro-l,2,2-trifiuoroethane (HCFC-133) and l-chloro-l,l,2-triflnoroethane (HCFC-133b) without evidence for the reductive replacement of fluorine (Fignre 7.70b) (Lesage et al. 1992). 

Attention has been directed to the dechlorination of polychlorinated benzenes by strains that use them as an energy source by dehalorespiration. Investigations using Dahalococcoides sp. strain CBDBl have shown its ability to dechlorinate congeners with three or more chlorine substituents (Holscher et al. 2003). Although there are minor pathways, the major one for hexachlorobenzene was successive reductive dechlorination to pentachlorobenzene, 1,2,4,5-tetrachlorobenzene, 1,2,4-trichlorobenzene, and 1,4-dichlorobenzene (Jayachandran et al. 2003). The electron transport system has been examined by the use of specific inhibitors. lonophores had no effect on dechlorination, whereas the ATP-synthase inhibitor A,A -dicyclohexylcarbodiimide (DCCD) was strongly inhibitory (Jayachandran et al. 2004). 

The anaerobic dechlorination of hexachlorobenzene has been described in anaerobic mixed cultures supplemented with electron donors including lactate, ethanol, or glucose (Holliger et al. 1992) successive and partial dechlorination produced 1,2,4- and 1,3,5-trichlorobenzenes, while the 1,2,3-trichlorobenzene was further dechlorinated. The partial dechlorination of 1,2,3,4-tetra-, 1,2,3,5-tetra-, and pentachlorobenzene has been examined in a methanogenic mixed culture using lactate as electron donor (Middeldorp et al. 1997), and sterile Rhine River sand was needed to maintain dechlorination activity for unresolved reasons. 

With the (n-Bu)jSnH reduction of PVC successfully simulated via the kinetic studies of DCP and TCH reduction, it remains to explain the mechanisms (14) of this reductive dechlorination. We need only consider the mechanisms of the reduction of DCP and TCH, because we have demonstrated that the kinetics of their reduction are the same as those observed for PVC when also reduced by (n-Bu)3SnH. 

Reductive dechlorination or reductive hydrogenolysis is a common transformation of 1- and 2-carbon chlorinated aliphatics under methanogenic conditions [373,374]. 1,1,1-Trichloroethane (l,l,l-TCA),for example,is converted to 1,1-dichloroethane (1,1-DCA) [375], and Perchloroethylene (PCE) is successively converted to TCE, cDCE, VC, and ethane [274]. Each reductive dechlorination is a two-electron transfer reaction. 

The addition of fluorine is successfully executed on various deactivated 7t-centers as well. Fluorine was used to prepare perfluorocyclopentadiene and its anion 6 for the first time by addition of fluorine to perchlorocyclopentadiene followed by total dechlorination.68 Fluorine undergoes addition to enones 7,65 including dioxine derivatives 9,69 chloroenone 1070 and even to the very deactivated double bond in maleic anhydride (11) forming difluorosuccinic acids.71... 

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. 

T (2,4,5-trichlorophenoxyacetic acid) ester formulation to a complex mixture of successively less-chlorinated phenoxyacetic esters, and Que Hee, et al. (16) reported a similar dechlorination of 2,4-D (2,4-dichlorophenoxyacetic acid). The toxic 2,4,5-T impurity, TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), was dechlo-rinated in a herbicide ester deposit on a sunlit leaf surface within a few hours (Figure 5) (17). 

---