Extent of deductive dechlorination of chlorobenzoates in anoxic sediment slurries depends on the sequence of chlorine removal

Distinct capacities for reductive dechlorination from all ring positions of chlorinated benzoates (CBa's) were present in anoxic slurries from a polluted freshwater marsh sediment. The first detectable reductive dechlorination was obtained after a reproducible time lapse. Model calculations based on measured dechlorination rates suggested that this could be accounted for by CBa-dependent growth of a small starter population of dechlorinating bacteria. Complete reductive dechlorination of CBa's depended on the specific sequence of chlorine removal. In unacclimated slurries, dechlorination started from the ortho rather than the meta or para position. Thus, 2,3,6-triCBa was completely dechlorinated via 2,5-diCBa and 3-CBa; 2,3,5-triCBa was completely dechlorinated via 3,5-diCBa and 3-CBa. Acclimation to 3-CBa induced meta-dechlorination of 2,3,6- and 2,3,5-triCBa. This in turn prevented complete dechlorination as indicated by the accumulation of 2,6-diCBa and 2-CBa as end products. In contrast, acclimation to 2,5-diCBa resulted in a population that first removed chlorine from the ortho position. In this way formation of ortho-substituted ''dead-end'' products was avoided, resulting in complete dechlorination of di- and tri-CBa's with chlorines at ortho and meta positions