Kinetics of the sequential microbial reductive dechlorination of hexachlorobenzene

The reductive dechlorination kinetics of hexachlorobenzene and all other chlorinated benzene congeners (CBs) were evaluated using a mixed enrichment culture derived from a contaminated estuarine sediment. The predominant, sequential hexachlorobenzene dechlorination pathway was hexachlorobenzene --> pentachlorobenzene --> 1,2,3,5-tetrachlorobenzene --> 1,3,5-trichlorobenzene --> 1,3-dichlorobenzene. Very low levels of 1,2,4,5-tetrachlorobenzene, 1,2,4-trichlorobenzene, and 1,4-dichlorobenzene were also detected. Batch dechlorination assays were performed separately for all CBs with an initial biomass concentration of 330 mg/L as volatile suspended solids, initial concentrations of CBs of 180 nM, and electron donor in excess. The maximum dechlorination rate (K) and the half-saturation coefficient (K-S) as well as the pseudo-first-order rate constant (k(obs)) for eight dechlorination reactions were estimated by nonlinear regression of CBs progress curves data based on a Michaelis-Menten-type and the pseudo-first-order model, respectively. The K and K-S values (+/- standard error) ranged from 4.6 +/- 2 to 208 +/- 2 nM/d and from 5 +/- 2 to 122 +/- 10 nM, respectively. The k(obs) values (+/- standard error) ranged from 0.035 +/- 0.002 to 1.455 +/- 0.124 d(-1). On the basis of the Michaelis-Menten model and the parameter values estimated in independent batch assays, an overall model was developed that successfully described the batch sequential reductive dechlorination of hexachlorobenzene and other polychlorinated benzene congeners to predominantly 1,3-dichlorobenzene within 25 d of incubation. Instead, when the pseudo-first-order model and the estimated k(obs) values were used, a predicted incubation period of about 100 d was required for the conversion of hexachlorobenzene to dichlorobenzene. On the basis of the results of this study, Michaelis-Menten kinetics are recommended over first-order kinetics for the modeling of the reductive dechlorination of CBs.