Effects of electron donors and inhibitors on reductive dechlorination of 2,4,6-trichlorophenol

Reductive dechlorination of 2,4,6-trichlorophenol (2,4,6-TCP) to 2,4-dichlorophenol and to 4-chlorophenol is found in mixed methanogenic cultures enriched from municipal sludge digesters. In this study, electron donors, including hydrogen, acetate and fructose, were effectively converted to methane and supported this dehalogenation in periods of 2 to 4 weeks. When bromoethanesulfonic acid (BESA) and vancomycin were used, dechlorination and methanogenesis were strongly affected. With hydrogen, BESA nearly completely inhibited lithotrophic methanogenesis; acetate accumulated from lithotrophic acetogenesis and dehalogenation proceeded. Vancomycin, however, inhibited dehalogenation while methanogenesis was not affected. With acetate, BESA completely inhibited methanogenesis and substantially slowed dehalogenation; the acetate was not used. Vancomycin, on the other hand, stopped dehalogenation, while methanogenesis proceeded. With fructose plus titanium citrate, BESA inhibited methanogenesis; acetate and propionate accumulated, and dehalogenation proceeded; vancomycin inhibited dehalogenation. Autoclaved controls had neither dehalogenation nor methanogenesis. These results point to biological dehalogenation by eubacteria, rather than methanogens. In the case of acetate, where both inhibitors effectively stopped dehalogenation, they also indicate a syntrophic relation between acetoclastic methanogens and the dehalogenating bacteria. In further studies with 2,4,6-TCP concentrations ranging up to 160 mg/l there was increasing toxicity to methanogens and changes in accumulation of fatty acids, but dehalogenation persisted at the highest concentrations.