Effect of PCB concentration on reductive dechlorination and dechlorination potential in natural sediments

The effect of PCB concentration on microbial reductive dechlorination was examined using two individual congeners, 2,3,4- and 2,3,4,2′,4′,5′-chlorobiphenyl (CBP), at four concentrations ranging from 4 to 35 ppm. Dechlorination potential in PCB-contaminated river sediments was also determined as the length of the lag period in a single congener dechlorination assay using contaminated sediments as inocula. Dechlorination of 2,3,4-CBP was concentration-dependent and occurred at all concentrations by 14 weeks and the residual concentration of the parent congener appeared to be the same regardless of the initial concentrations. By contrast, 2,3,4,2′,4′,5′-CBP showed dechlorination only at the highest concentration after 23 weeks. The concentration dependence and the lag period thus were congener-specific. Dechlorination potential, measured as the length of the lag before dechlorination of 3,4,5-CBP, seemed to be longer with inocula prepared from sediments with lower ambient PCB concentrations. Taken together, these results imply that in situ dechlorination may not be able to occur in areas with low ambient PCB levels because both dechlorination rate and dechlorination potential are concentration-dependent. In sediments which are no longer submerged, dechlorinating activity was not found.