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 positio ns of chlorinated benzoates (CBa's) were present in anoxic slurries fr om a polluted freshwater marsh sediment. The first detectable reductiv e dechlorination was obtained after a reproducible time lapse. Model c alculations based on measured dechlorination rates suggested that this could be accounted for by CBa-dependent growth of a small starter pop ulation of dechlorinating bacteria. Complete reductive dechlorination of CBa's depended on the specific sequence of chlorine removal. In una cclimated slurries, dechlorination started from the ortho rather than the meta or para position. Thus, 2,3,6-triCBa was completely dechlorin ated via 2,5-diCBa and 3-CBa; 2,3,5-triCBa was completely dechlorinate d via 3,5-diCBa and 3-CBa. Acclimation to 3-CBa induced meta-dechlorin ation of 2,3,6- and 2,3,5-triCBa. This in turn prevented complete dech lorination 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 popu lation that first removed chlorine from the ortho position. In this wa y formation of ortho-substituted ''dead-end'' products was avoided, re sulting in complete dechlorination of di- and tri-CBa's with chlorines at ortho and meta positions.