No abstract

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.

2,3,4,5,6-Pentachlorobiphenyl (23456-CB) primes a broad specificity meta-dechlorination (Process N) of Aroclor 1260 in Housatonic River sediments. We hypothesized that 23456-CB acts as an electron acceptor and could be used to selectively enrich PCB-dechlorinating microorganisms. Here we sought to (a) enrich the microbial population responsible for Process N dechlorination and (b) determine whether bioaugmentation with the enriched culture would stimulate effective PCB dechlorination. We made four serial transfers of actively dechlorinating slurries (10−20% of the final volume) plus 23456-CB to autoclaved PCB-contaminated sediment. The enrichment procedure greatly enhanced Process N dechlorination, reducing the hexa- through nonachlorobiphenyls in the sediment from 66.3 mol % initially to only 16.7 mol % in the fourth generation. The enrichment also fostered a new para-dechlorination activity (Process LP) that caused further conversion of Process N products to tri- and tetrachlorobiphenyls. Bioaugmentation without the primer resulted in only modest dechlorination. Our data demonstrate that enrichment with 23456-CB results in an inoculum that, when primed, rapidly and extensively dechlorinates weathered PCBs, even when excess oil is present. The identification of effective environmentally acceptable non-PCB agents to prime and enrich PCB dechlorinators could lead to a technology for in situ treatment of PCB-contaminated sediments.

We recently demonstrated a strategy for stimulating or "priming" the indigenous microorganisms to dechlorinate weathered PCBs in sediments, but the dechlorination exhibited a very narrow para-dechlorination specificity. We tested the priming activity of various PCB congeners to discover those that would prime broader and more extensive PCB dechlorination activity. 2,3,4,5,6-Pentachlorobiphenyl (23456-CB), 2346-CB, and 236-CB primed extensive and sustained meta-dechlorination (Process N) of the Aroclor 1260 residue in Housatonic River sediment. The dechlorination targeted most of the hexa-, hepta-, and octachlorobiphenyls and converted them to tetra-and pentachlorobiphenyls containing mostly orthoand para-chlorines. Both 234-CB and 2345-CB also primed Process N dechlorination, but the dechlorination ceased at 7 weeks and was much less extensive. 245-CB primed a narrow specificity paradechlorination (Process P). 2356-CB, 235-CB, and 23-CB did not prime PCB dechlorination. The results indicate that the 236-substitution pattern is important for maximal priming of dechlorination Process N. A chlorine at position 5 suppressed the priming activity, but the suppression was overcome by a chlorine at position 4. The discovery of non-PCB primers that are both effective and environmentally acceptable could lead to the development of practical methods for in situ PCB bioremediation.