The reductive dechlorination of chlorobenzenes (CBs) in an anaerobic estuarine sediment was examined. The sediment was preexposed to various anthropogenic chemicals from the surrounding industries. The sulfate content (20 mmoles/l) in the interstitial waters indicated that the sediment was sulfidogenic. All the CBs were transformed following a first-order reaction kinetics with rate constants ranging from 0.0016 to 0.0389 day−1 or half-lives between 17 and 433 days. Disappearance of CBs in autoclaved sediment were observed but to a much less extent than in the test sediment. Detected intermediates indicated that the removal of chlorine atom occurred at all possible positions on the aromatic ring. The contribution of different dechlorination pathways were estimated using branched chain first-order irreversible reaction kinetics. The results showed that the preferential dechlorination was in the following order: two chlorine atoms on both sides > one of the adjacent chlorine atoms > chlorine with no adjacent chlorine atoms. The major pathway observed for CBs followed a thermodynamically most favorable step that gave the highest energy for dechlorination.

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