Anaerobic Dechlorination

Particnlarly extensive effort has been directed to the anaerobic dechlorination of alkenes and halogenated phenols that are discussed in Chapter 7, Part 3 and Chapter 9, Part 2. As a resnlt of this, the range of organisms that are able to carry this ont is considerable and inclndes... 

Jagnow G, K Haider, P-C Ellwardt (1977) Anaerobic dechlorination and degradation of hexachlorocyclo-hexane by anaerobic and facultatively anaerobic bacteria. Arch Microbiol 115 285-292. 

The extensive use in a series of experiments on the anaerobic dechlorination of chlorocatechols (references in Allard et al. 1994). 

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). 

Although quite extensive use of has been made in studies on the degradation of alkyl sulfonates (Hales et al. 1986), C1 has achieved only limited application on account of technical difficulties resulting from the low specific activities and the synthetic inaccessibility of appropriately labeled substrates. One of the few examples of its application to the degradation of xenobiotics is provided by a study of the anaerobic dechlorination of hexachlorocyclohexane isomers (Jagnow et al. 1977), the results of which are discussed in Chapter 7, Part 3. 

Collectively, there is therefore extensive evidence for the anaerobic dechlorination of PCBs but the extent and specificity of this depends on both the organism and the degree and pattern of substituents in the congener. 

Ingenious experiments to which reference has already been made have used addition of specific PCB congeners that are more readily dechlorinated to prime dechlorination at specific positions (Bedard and Quensen 1995). They have been extended to the use of dibrominated biphenyls in the presence of malate to stimulate dechlorination of the hexachloro- to nanochlorobiphenyls (Bedard and Quensen 1995). Results from these experiments provide valuable evidence of important differences between anaerobic dechlorination and anaerobic debromination, and the greater facility of the latter. 

Other componnds have also been examined as primers for the dechlorination of hexachloro to nonachloro PCB congeners (DeWeerd and Bedard 1999) a nnmber of snbstituted brominated monocyclic aromatic componnds were examined, and 4-bromobenzoate was effective—though less so than 2,6-dibromobiphenyl. In contrast, the chlorobenzoates that are metabolites of aerobic degradation were ineffective. The positive effect of brominated biphenyls in priming the anaerobic dechlorination of CBs has also been encountered in the dechlorination of octachlorodibenzo[l,4] dioxin to the 2,3,7,8 congener indnced by 2-bromodibenzo[l,4]dioxin in the presence of (Albrecht... 

FIGURE 9.26 Partial anaerobic dechlorination of (a) hexachlorobenzene and (b) 2,3,4,5-tetrachloroaniline. 

The partial anaerobic dechlorination of pentachloropyridine to a trichloropyridine of unestablished orientation by the dehalorespiring Desulfitobacterium hafniense (frappieri) PCP-1 has been demonstrated (Dennie et al. 1998). 

Partial anaerobic dechlorination of chlorinated dibenzo[l,4]dioxins has been observed in sediment slurries. The 1,2,3,4-tetrachloro compound produced predominantly l,3-dichlorodibenzo[l,4]-dioxin (Beurskens et al. 1995 Ballerstedt et al. 1997), and for substrates with five to seven chlorine substituents, chlorine was removed from both the peri and the lateral positions (Barkovskii and... 

Mere alteration of the strnctnre of the contaminant by biotransformation may not necessarily be acceptable. For example, although anaerobic dechlorination of PCBs is desirable, microbial redaction of 3,3 -dichlorobenzidine, which is an intermediate in the manufacture of dyes, produced benzidine that is both more toxic and more snsceptible to dissemination in the aqnatic phase (Nyman et al. 1997). 

For aerobic degradation by Pseudomonas sp. strain P51, which carried out degradation by dioxygenation, the valnes were not significant. In contrast, the isotope enrichment factor (e) for anaerobic dechlorination by Dehalococcoides sp. strain CDBl, which produced 1,3-dichlorobenzene from 1,2,3-trichlorobenzene, was -3.4 ppm, and for 1,2,4-trichlorobenzaene, which produced 1,4-dichlorobenzene, was -3.2 ppm. 

Three anaerobic dechlorinating consortia were used to examine fractionation during dechlorination of tetrachloroethene and TCE to cii-dichloroethene (Numata et al. 2002). Fractionation factors (a) for the first reaction ranged from 0.987 to 0.991 for the three consortia, and for the second reaction were 0.9944 for all consortia. Some important limitations were pointed out (a) the chlorinated ethenes were not separated so that the isotopic... 

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. 

Anaerobic dechlorination has been demonstrated under a number of conditions, and a complex pattern of dechlorination among the congeners has been observed (Bedard and Quensen 1995). There are also varying, and sometimes conflicting, views on the role of sulfate. 

The anaerobic dechlorination of PCBs has been extensively studied both in microcosms and in field samples from heavily contaminated sites in the United States. Three main patterns have been fonnd—N that removed flanked meto-chlorines, P that removed para-chlorines, and LP that removed nnflanked para-chlorines (Bedard et al. 1998). By contrast, ort/to-chlorines were more recalcitrant. These experiments, which have been discussed in Chapter 9, Part 2, laid the fonndation for analysis of a field situation and an appreciation of the effect of long-term exposnre of contaminated lake sediment (Magar et al. 2005a,b). Substantial dechlorination took place in buried sediment cores (35-40 cm) compared with the snrface sediment cores (0-5 cm). Although there were some variations among the cores, the di- and trichlorinated biphenyls were produced at the expense of the... 

Rossetti S, LL Blackall, M Majone, P Hugenholtz, JJ Plumb, V Tandoi (2003) Kinetic and phylogenetic characterization of an anaerobic dechlorinating microbial community. Microbiology (UK) 149 459-469. 

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