Comparative Kinetics of Hydrogen Utilization for Reductive Dechlorination of Tetrachloroethene and Methanogenesis in an Anaerobic Enrichment Culture

Anaerobic microorganisms that reductively dechlorinate tetrachloroethene (PCE) must often compete with methanogens for H 2 . This study compared the kinetics of H 2 utilization between the two types of organisms at 35 °C under conditions of continuous agitation. Limiting levels of H 2 were administered to 160-mL serum bottles seeded with a PCE/butyric acid enrichment culture; H 2, methane, and vinyl chloride were tracked over time using headspace samples. Measured half-velocity constants with respect to H 2 K s (H 2 ) values ± 95% CI for methanogenesis and dechlorination were 960 ± 180 and 100 ± 50 nM, respectively. Mass-transfer equations were used to calculate aqueous H 2 concentrations at the half-velocity point from headspace measurements. The possible effect on K s (H 2 ) values arising from interconversion between H 2 and formate through an active formate/H 2 lyase system was examined by comparing results from formate-fed and H 2 -fed bottles. Only methanogens in the culture were apparently capable of using formate; hence, the measured methanogenic K s (H 2 ) was dependent on which electron donor was administered. The nearly 10-fold difference in K s (H 2 ) between methanogens and dechlorinators suggests that the deliberate choice of an electron donor whose fermentation results in a slow, steady, and low-level release of H 2 over time could maximize dechlorination potential while minimizing methanogenic competition for H 2 .