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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.

A 73-day field study of in situ aerobic biodegradation of polychlorinated biphenyls (PCBs) in the Hudson River shows that indigenous aerobic microorganisms can degrade the lightly chlorinated PCBs present in these sediments. Addition of inorganic nutrients, biphenyl, and oxygen enhanced PCB biodegradation, as indicated both by a 37 to 55 percent loss of PCBs and by the production of chlorobenzoates, intermediates in the PCB biodegradation pathway. Repeated inoculation with a purified PCB-degrading bacterium failed to improve biodegradative activity. Biodegradation was also observed under mixed but unamended conditions, which suggests that this process may occur commonly in river sediments, with implications for PCB fate models and risk assessments.

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.