Hexachlorobenzene, reductive

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). 

Fathepure BZ, JM Tiedje, SA Boyd (1988) Reductive dechlorination of hexachlorobenzene to tri- and dichlorobenzenes in anaerobic sewage sludge. Appl Environ Microbiol 54 327-330. 

Jackson, W.A. and Pardue, J.H.S.U., Assessment of metal inhibition of reductive dechlorination of hexachlorobenzene at a Superfund site, Environ Toxicol Chem, 17 (8), 1441-1446, 1998. 

Pavloststhis, S.G. and Prytula, M.T. Kinetics of the sequential microbial reductive dechlorination of hexachlorobenzene. Environ. Sci TechnoL., 34(18) 4001-4009, 2000. 

Assaf-Anid, N.,Nies, L. Vogel, T. M. (1992). Reductive dechlorination cf a polychlorinated biphenyl congener and hexachlorobenzene by vitamin B12. Applied and Environmental Microbiology, 58, 1057-60. 

Yeh, D.H., Pennell, K.D., and Pavlostathis, S.G. (1999). Effect of Tween surfactants on methanogenesis and microbial reductive dechlorination of hexachlorobenzene. Environ. Toxicol. Chem., 18, 1408-1416. 

M. Monagheddu, S. Doppiu, G. Cocco, MSR Reduction of Hexachlorobenzene, j. Mater. Synthesis Processing,... 

Competitive Substrate Utilization. Various experiments with phenanthrene mineralization demonstrated partial inhibition with nonionic surfactants at doses less than that resulting in micellization. Such data suggest an alternative explanation for inhibition, other than surfactant effects on cell membranes and proteins. Possibly PAH-degrading microorganisms, or their competitors, utilize the surfactant as preferential substrate or carbon source. Jalvert et al. (66) made a similar conclusion about the effect of C12E4 on reductive dechlorination of hexachlorobenzene. 

Recent work with N-acetyl-S-(pentachlorophenyl)-cystelne also implicates a reductlve defuntlonallzation process in the metabolism of hexachlorobenzene and pentachloronltrobenzene (54) as shown in Equation 9. In addition to pentachlorobenzene, other reduction... 

Figure 2. Proposed pathway for reductive defunctionalization (see text) and methylthio-group turnover observed in hexachlorobenzene metabolism.

It is of great importance to know the biological degradability of pesticides [86], as studied, for instance, in the biotransformation reactions of pentachlorobenzene-based fungicides. In another study [87], the biotransformation reactions of pentachloronitrobenzene with hexachlorobenzene are compared. The mechanism of reduction denitrification is discussed by the same authors elsewhere [88]. Similar studies are presented in the papers [89, 90]. 

Reductive dehalogenation is a mechanism for the anaerobic biotransformation of chlorinated hydrocarbons such as hexachlorobenzene (HCB). In reductive dehalogenation, the halogenated compound serves as the electron acceptor rather than the donor that requires a separate carbon source. In a microbially catalyzed reaction, a halide ion is replaced by a hydrogen ion (Figure 13.7). The removal of halide ions results in compounds that are generally easier to degrade, and, in some instances, are completely mineralized. 

Yeh DH. (2000). Influence of Nonionic Surfactants on the BioavailabiUty and Microbial Reductive Dechlorination of Hexachlorobenzene. PhD Dissertation. Georgia Institute of Technology, Atlanta, GA. 

Direct reductions of organohalide pollutants have been done in solutions containing ionic and nonionic surfactants [41], but often with low current efficiencies. One interesting approach involved the use of an acid-labile nonionic surfactant, 1% oil, and water for the dechlorination of hexachlorobenzene. This allowed facile... 

Reductive dechlorination of HCB can be achieved with the combination of electrokinetics with the appropriate catalysts such as nanoscale zero valent iron (NZVl). The electric field can be used as a mechanism for the delivery of NZVl into the soil [10]. Reductive dechlorination can be achieved in the soil or even upon the cathode if the contaminant reaches the cathode compartment. Other metallic catalysts such as Cu/Fe or Pd/Fe bimetal microscale particles were satisfactorily used with the same purpose. Dechlorination of hexachlorobenzene up to 98 % was achieved with Cu/Fe [27] and only 60 % with Pd/Fe [20]. 

Highly reactive metal complexes serve as electrocatalysts for a number of detoxification reactions. An example is the use of Co(II) N, N -bis(salicylidene)ethylene (CoSalen) to facilitate the electrochemical reductive elimination of chlorine from hexachlorobenzene (HCB) [15]. Even though more environmentally friendly conditions have yet to be developed to scale up this process, the CV voltammogram in Fig. 6 shows separate dehalogenation electrochemical steps from the sequence below. 

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