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

This study investigated the influence of H 2 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 micro‐organism. Both VC formation and dechlorination showed a definite increase in rate with increasing H 2 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 H 2 for VC formation (1.5 ± 1.4 µmol H 2 L −1 ) was more than six times lower than that for VC dechlorination (9.1 ± 5.1 µmol H 2 L −1 ), confirming that the last step of PCE dechlorination (i.e. conversion of VC to ethene) was much more H 2 ‐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 H 2 L −1 . Experiments at different temperatures indicated the presence of severe internal (diffusional) mass transfer limitations and, in turn, of steep H 2 concentration gradients through the biofilm, which strongly influenced the estimated apparent half‐velocity coefficients for H 2 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 H 2 in natural or engineered bioremediation systems. Copyright © 2006 Society of Chemical Industry