Hexachlorobenzene reductive dechlorination

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. 

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. 

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

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]. 

Influence of Nonionic Surfactants on the Bioavailability of Hexachlorobenzene for Microbial Reductive Dechlorination... 

Finally, halorespiration has been observed with highly chlorinated benzenes such as hexachlorobenzene, pentachlorobenzene, tetrachlorobenzene, and trichlorobenzene (Hol-liger et al, 1992 Ramanand et al, 1993 Suflita and Townsend, 1995). In anaerobic conditions, highly chlorinated benzenes can be easily reductively dechlorinated and product lower chlorinated benzenes. Bacteria from the genus Dehalococcoides are known for having the main role in this process. Conversely, lower chlorinated benzenes have a lower tendency for reductive dechlorination, and finally mono-chlorinated benzene is the most resistant in this anaerobic process (Field and Sierra-Alvarez, 2008). 

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... 

Experimental and mathematical modeling studies were performed to evaluate the potential benefits and limitations associated with the use of nonionic surfactants to enhance the microbial transformation of hexachlorobenzene (HCB) by a dechlorinating mixed culture enriched from a contaminated sediment. In general. Tween series surfactants were shown to have little impact on methanogenesis, whereas, polyoxyethylene (POE) alcohols, Triton X-100 and SDS were found to strongly inhibit methanogenesis and HCB dechlorination. Subsequent experiments conducted with Tween 80 illustrated the ability of this surfactant to enhance the solubility of HCB and to reduce the HCB-soil distribution coefficient. Model simulations demonstrated, however, that the aqueous phase mass fraction of HCB was substantially reduced in micellar solutions, which corresponded with observed reductions in HCB dechlorination. These results indicate that the impacts of surfactants on both biological activity and contaminant phase distributions should be evaluated in order to accurately assess the potential for biotransformation of hydrophobic contaminants in the presence of surfactants. 

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