Enhancement of microbial PCB dechlorination by chlorobenzoates, chlorophenols and chlorobenzenes

Abstract We investigated the effects of chlorobenzoates (3-, 2,3-, 2,4-, 2,5-, 2,3,5- and 2,4,6-chlorobenzoate), chlorophenols (2,3-, 3,4-, 2,5-, 2,3,6- and penta-chlorophenol), and chlorobenzenes (1,2-, 1,2,3-, 1,2,4- and penta-chlorobenzene) on polychlorinated biphenyl (PCB) dechlorination and on the enrichment of PCB-dechlorinating microorganisms. When the natural microbial populations eluted from St. Lawrence River sediments were enriched with each of the 15 haloaromatic compounds (HACs) in PCB-free sediments, PCB-dechlorinating microorganisms were found in all but pentachlorophenol-amended sediments. Similarly, dechlorinating microorganisms were also found in PCB-spiked sediments amended with all HACs, except for those with pentachlorophenol. In HAC-amended PCB sediments there was a long lag in PCB dechlorination until the HACs were reduced to a plateau level. Despite this lag, once PCB dechlorination started it was faster in the HAC-amended sediments compared to the unamended controls. The overall extent of PCB dechlorination was significantly enhanced by all HACs except pentachlorophenol and pentachlorobenzene, but the extent as well as the pattern of the enhancement varied. Of the 13 effective HACs, six (2,3-, 2,4- and 2,4,6-chlorobenzoates; 3,4- and 2,3,6-chlorophenols; and 1,2,3-chlorobenzene) enhanced only meta-dechlorination, whereas five (3-chlorobenzoate; 2,3- and 2,5-chlorophenols; and 1,2- and 1,2,4-chlorobenzenes) increased both meta- and para-dechlorination, and two (2,5- and 2,3,5-chlorobenzoates) promoted overall, substitution non-specific dechlorination. When the maximum extent of dechlorination was plotted against the highest number of PCB-dechlorinating microorganisms for each HAC, there was a linear relationship (P<0.01), suggesting that dechlorination enhancement was related to the increase in their population size. However, there was also evidence to suggest that different dechlorinating microorganisms were selected.