An investigation of factors limiting the reductive dechlorination of polychlorinated biphenyls

A study was conducted to determine whether the apparent limitation of dechlorination of Aroclor 1248 was due to bioavailability of polychlorinated biphenyls (PCBs) or an accumulation of metabolic products. After 15 weeks of laboratory incubation, sediment microorganisms from the St. Lawrence River reduced the total number of chlorines in Aroclor 1248-spiked sediments by 33%. However, dechlorination apparently leveled off with a significant number of meta- and para-chlorines still remaining, showing no further change out to 30 weeks. When these sediments were incubated an additional 18 weeks with either fresh medium or the original supernatant, no additional dechlorination was found in either treatment. Similarly, dechlorination was not inhibited in freshly spiked Aroclor 1248 sediment slurries made with the old supernatant. Addition of the nonionic surfactant Tween 20, at a concentration below the critical micelle concentration that increased PCB desorption, also failed to enhance dechlorination of the plateau sediments. The extent and final congener pattern in all treatments were nearly identical. Therefore, the termination of dechlorination at the plateau level was not due to PCB bioavailability or accumulation of inhibitory metabolic products. These results strongly suggest that the cessation of dechlorination at the plateau was due to the accumulation of daughter congeners with chlorine substitution patterns that were not amenable to further dechlorination by the present microbial consortium.