Stimulation of Microbial para-Dechlorination of Polychlorinated Biphenyls That Have Persisted in Housatonic River Sediment for Decades

We added 2,5,3‘,4‘-tetrachlorobiphenyl (25-34-CB) (350 μM) to slurries of Aroclor 1260-contaminated sediment from Woods Pond (Lenox, MA) in an attempt to stimulate dechlorination of the PCBs. The slurries were incubated under methanogenic conditions at 23−25 °C and analyzed by GC/ECD and GC/MS. The 25-34-CB was stoichiometrically converted to 25-3-CB and stimulated a selective para -dechlorination, which we designate Process P, that decreased the penta- through heptachlorobiphenyls containing 2,3,4- (234-), 245-, or 2345-chlorophenyl groups by up to 83% in 12 weeks. The products were tetra- and pentachlorobiphenyls containing 23-, 25-, and 235-chlorophenyl groups, especially 25-25-CB, 23-25-CB, 24-25-CB, 235-25-CB, and 235-23-CB. Mass balances for key parent congeners and their dechlorination products ranged from 80 to 115%. No dechlorination was detected in autoclaved controls or in live controls that were not amended with 25-34-CB. We propose that the 25-34-CB selectively enriched a population of PCB-dechlorinating microorganisms that can use it as an electron acceptor and thus “primed” the PCB dechlorination. This research demonstrates for the first time a successful strategy for stimulating rapid microbial dechlorination of PCBs that have persisted in sediments for decades and lays a foundation for the development of in situ methods for bioremediation of PCBs.