Influence of hydrogen on the reductive dechlorination of tetrachloroethene to ethene in a biofilm reactor: role of mass transport phenomena

This study investigated the influence of H2 bulk liquid concentration in the range 2-120 μmol L-1 on the kinetics of vinyl chloride (VC) formation from tetrachloroethene (PCE) and VC dechlorination to ethene in a methanogenic biofilm reactor containing Dehalococcoides spp. as the putative dechlorinating microorganism. Both VC formation and dechlorination showed a definite increase in rate with increasing H2 bulk liquid concentration, following a pattern typical of Michaelis-Menten kinetics. The estimated maximum VC formation rate (81.7 ± 9.4 μmol L-1 h-1 mean value ± 90% confidence interval) was about ten times higher than the estimated maximum VC dechlorination rate (8.2 ± 1.0 μmol L-1 h-1), while the estimated apparent half-velocity coefficient for H2 for VC formation (1.5 ± 1.4 μmol H2 L-1) was more than six times lower than that for VC dechlorination (9.1 ± 5.1 μmol H2 L-1), confirming that the last step of PCE dechlorination (i.e. conversion of VC to ethene) was much more H2-sensitive than the previous ones. The estimated maximum methane formation rate was 462.1 ± 213.5 μmol L-1 h-1 and the estimated apparent half-velocity coefficient was 104.7 ± 89.4 μmol H2 L-1. Experiments at different temperatures indicated the presence of severe internal (diffusional) mass transfer limitations and, in turn, of steep H2 concentration gradients through the biofilm, which strongly influenced the estimated apparent half-velocity coefficients for H2 use. The results of this study emphasise the importance of considering mass transfer phenomena when predicting the rate of PCE dechlorination and the outcome of competition for H2 in natural or engineered bioremediation systems.