Use of halogenated benzoates and other halogenated aromatic compounds to stimulate the microbial dechlorination of PCBs

We recently reported that certain polychlorinated biphenyl (PCB) congeners and bromobiphenyl congeners activate or "prime" dechlorination of the Aroclor 1260 in Housatonic River sediments. In this study, we tested the ability of halogenated benzoates and other halogenated aromatic compounds to prime PCB dechlorination in the same sediments. We found that none of the fluorinated or chlorinated benzoates primed PCB dechlorination, but several brominated and iodinated benzoates initiated this activity. Of the halogenated benzoates tested, 4-bromobenzoate, 4-iodohenzoate, and 2,5-dibromobenzoate primed the most extensive PCB dechlorination, decreasing the hexa-through nonachlorobiphenyl fraction of Aroclor 1260 by 40- 70%, 10-50%, and 10-50%, respectively. None of the halogenated benzoates were as effective at priming PCB dechlorination as 2,6-dibromobiphenyl, which primed a 60- 80% decrease of the hexa- through nonachlorobiphenyl fraction of Aroclor 1260. Several other brominated aromatic compounds were also tested for their ability to prime PCB dechlorination. Monobrominated isomers of acetophenone, phenol, or toluene did not prime PCB dechlorination, but all monobrominated isomers of benzonitrile, 2-bromo-, 4-bromo-, and 2,5-dibromonitrobenzene, 4-bromobenzamide, 4-bromobenzophenon 4-bromobenzoic hydrazide, 4-bromobenzoic methyl ester, and 2,5-dibromobenzene sulfonate primed PCB dechlorination in Housatonic River sediments. All of the compounds primed PCB Dechlorination Process N (primarily flanked meta dechlorination) except 4-bromonitrobenzene, which primed Dechlorination Process P (flanked para dechlorination). These results indicate that halogenated aromatic compounds that are not structural analogues to PCBs can prime PCB dechlorination.