Chlorinated Ethene Half-Velocity Coefficients (K S) for Reductive Dehalogenation

Biological reduction of the chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE) completely to ethene is of interest for engineered or intrinsic destruction of these prevalent groundwater contaminants. However, the transformations are frequently not complete, leading to the production of vinyl chloride (VC), a more hazardous compound. Factors affecting the relative rates of destruction of the solvents and their intermediate products are thus of interest. The maximum degradation rates (kX) and half-velocity coefficients (K S) for these chlorinated ethenes used as electron acceptors in reductive dehalogenation with hydrogen in excess were examined using an enrichment culture grown on benzoate, hydrogen, and PCE. Initial dehalogenation rates were measured at various chlorinated ethene concentrations in batch studies. With 38 mg/L volatile suspended solids of this culture, the kX and 95% confidence intervals for PCE, TCE, cis-dichloroethene (cDCE), and VC at 25 °C were found to be 77 ± 5, 59 ± 11, 14 ± 3, and 13 ± 3 μM/day with K S values of 0.11 ± 0.04, 1.4 ± 0.9, 3.3 ± 2.2, and 2.6 ± 1.9 μM, respectively. The lower maximum transformation rates and higher K S values for cDCE and VC partly explain why incomplete transformation of PCE and TCE often occurs in the field.