Pathway and Rate of Chlorophenol Transformation in Anaerobic Estuarine Sediment

The dechlorination of chlorophenols (CPs) in anaerobic estuarine sediment was studied. The sediment used was fairly contaminated with various anthropogenic chemicals from the surrounding industries. The sulfate content of the interstitial and overlying waters was approximately 20 mmol/L, and the sediment was apparently sulfidogenic. All 13 CPs tested transformed following a first-order reaction kinetics. The rate constants for CP disappearance ranged between 0.010 and 0.38 day-1 or had half-lives between 1.8 and 70 days. The intermediates detected in the experiment indicated that the ortho-dechlorination was the preferred pathway. The disappearance of the parent compound and the accumulation and disappearance of dechlorinated intermediates were simulated using a branched chain first-order irreversible reaction kinetics, and the contributions of different pathways were estimated. A comparison of the estimated rate constants with the redox potential of the reaction showed that the hypothesis of microbial selection of most thermodynamically favorable pathways observed for chlorobenzene dechlorination was not applicable for CPs.