Kinetic estimation of standard reduction potentials of polyhalogenated biphenyls

Two anaerobic polychlorinated biphenyl (PCB)-dechlorinating enrichments with distinct substrate specificities were obtained: a 2,3,4,6-tetrachlorobiphenyl (2346-CB) para-dechlorinating enrichment derived from Aroclor 1260-contaminated Woods Pond (Lenox, Mass.) sediment and a 2,4,6-trichlorobiphenyl (246-CB) unflanked para-dechlorinating enrichment derived from PCB-free Sandy Creek Nature Center (Athens, Ga.) sediment. The enrichments have been successfully transferred to autoclaved soil slurries over 20 times by using 300 to 350 M 2346-CB or 246-CB. Both enrichments required soil for successful transfer of dechlorination activity. The 2346-CB enrichment para dehalogenated, in the absence or presence of 2346-CB, only 4 of 25 tested para halogen-containing congeners: 234-CB, 2345-CB, 2346-CB, and 2,4,6-tribromobiphenyl (246-BrB). In the presence of 246-CB, the 246-CB enrichment para dehalogenated 23 of the 25 tested congeners. However, only three congeners (34-CB, 2346-CB, and 246-BrB) were dehalogenated in the absence of 246-CB, indicating that these specific congeners initiate dehalogenation in this enrichment culture. The addition of the 2346-CB (para)-dechlorinating enrichment did not further stimulate the 2346-CB-primed dechlorination of the Aroclor 1260 residue in Woods Pond sediment samples. Compared to the addition of the primer 246-CB or the 246-CB unflanked para-dechlorinating enrichment alone, the addition of both 246-CB (300 M) and the 246-CB enrichment stimulated the unflanked para dechlorination of the Aroclor 1260 residue in Woods Pond sediments. These results indicate that the two enrichments contain different PCB-dechlorinating organisms, each with high substrate specificities. Furthermore, bioaugmentation with the enrichment alone did not stimulate the desired dechlorination in PCB-contaminated Woods Pond sediment.

Section: Discussionmentioning

confidence: 99%

Two anaerobic polychlorinated biphenyl-dehalogenating enrichments that exhibit different para-dechlorination specificities

Wiegel

1997

“…Whether the mechanism of aromatic reductive dechlorination is similar to that of aliphatic transformations is unknown. Electrochemical studies in organic solvents indicate that the intermediate of PCB dechlorination is also a carbon radical (9,19). No pure cultures of bacteria capable of dechlorinating PCBs have been isolated, and no studies with cell-free enzymes and aromatic compounds have been de-scribed; hence, the mechanism of microbially mediated PCB dechlorination is unknown.…”

mentioning

confidence: 99%

Identification of the Proton Source for the Microbial Reductive Dechlorination of 2,3,4,5,6-Pentachlorobiphenyl

Nies

Vogel

1991

Microbially mediated reductive dechlorination of polychlorinated biphenyls (PCBs) in anaerobic sediments has been observed during laboratory experiments. Reductive dechlorination is a two-electron transfer reaction which involves the release of chlorine as a chloride ion and its replacement on the aromatic ring by hydrogen. The exact mechanism of the electron transfer for PCBs is unknown; however, this work shows that the source of the hydrogen atom is the proton (H+) from water.

“…Anaerobic reductive dechlorination involves the loss of a chlorine substituent and its replacement by a hydrogen. The reduction potential of PCBs increases with increasing chlorine number (6,12). However, the positions of the chlorine substituents also influence ease of reductive dechlorination, which occurs primarily at para-and meta-positions, with the subsequent accumulation of lightly chlorinated orthosubstituted products (11).…”

mentioning

confidence: 99%

Effects of Organic Substrates on Dechlorination of Aroclor 1242 in Anaerobic Sediments

Nies

Vogel

1990

121

The effects of different organic substrates on the abilities of anaerobic sediment enrichments to reductively dechlorinate polychlorinated biphenyls (PCBs) were studied. Sediments collected from a site previously contaminated with PCBs were dosed with additional PCBs (Aroclor 1242; approximately 300 ppm [300 ,ug/g], sediment dry weight) and incubated anaerobically with acetate, acetone, methanol, or glucose. The pattern of dechlorination was similar for each substrate-fed batch; however, the extents and rates of dechlorination were different. Significant dechlorination over time was observed, with the relative rates and extents of dechlorination being greatest for methanol-, glucose-, and acetone-fed batches and least for acetate-fed batches. Dechlorination occurred primarily on the meta-and para-positions of the highly chlorinated congeners, resulting in the accumulation of less-chlorinated, primarily ortho-substituted products. No significant dechlorination was observed in incubation batches receiving no additional organic substrate, even though identical inorganic nutrients were added to all incubation batches. In addition, dechlorination was not observed in autoclaved controls that received substrate and nutrients. * Corresponding author. incubations without high concentrations of solvents was designed. MATERIALS AND METHODS Chemicals. Aroclor 1242 was provided by the General Electric Co. (Schenectady, N.Y.). All mineral medium chemicals were obtained from Fisher Scientific Co. (Fairlawn, N.J.). Pesticide-free hexane and acetone were obtained from J. T. Baker Chemical Co. (Phillipsburg, N.J.). Methanol was obtained from EM Science (Cherry Hill, N.J.). Octachloronaphthalene was purchased from Ultra Scientific (North Kingstown, R.I.). Mineral medium. The anaerobic mineral medium con