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 H2. This study compared the kinetics of H2 utilization between the two types of organisms at 35 °C under conditions of continuous agitation. Limiting levels of H2 were administered to 160-mL serum bottles seeded with a PCE/butyric acid enrichment culture; H2, methane, and vinyl chloride were tracked over time using headspace samples. Measured half-velocity constants with respect to H2 - Ks(H2) values ±95% Cl - for methanogenesis and dechlorination were 960±180 and 100±50 nM, respectively. Mass-transfer equations were used to calculate aqueous H2 concentrations at the half-velocity point from headspace measurements. The possible effect on Ks(H2) values arising from interconversion between H2 and formate through an active formate/H2 lyase system was examined by comparing results from formate-fed and H2-fed bottles. Only methanogens in the culture were apparently capable of using formate; hence, the measured methanogenic Ks(H2) was dependent on which electron donor was administered. The nearly 10-fold difference in Ks(H2) 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 H2 over time could maximize dechlorination potential while minimizing methanogenic competition for H2.