Dechlorination of trichloroethene to ethene by Dehalococcoides -like microorganisms: Kinetics, growth characteristics and substrate limitations

A potential remediation strategy for the groundwater contaminants, tetrachloroethene (PCE) and trichloroethene (TCE), is biological anaerobic reductive dechlorination. However, incomplete dechlorination to cis-1, 2-dichloroethene (DCE) and vinyl chloride (VC), the latter a known human carcinogen, is a significant problem. To investigate VC and DCE dechlorination in a mixed culture, a quantitative PCR technique was developed and used to confirm that a Dehalococcoides -like microorganism (called bacterium VS) in this culture could couple growth to VC and DCE dehalogenation. Determined yield (Y) and maximum utilization coefficient (qˆ) values agreed well with values found previously for other dechlorinating cultures. This is the first demonstration of microorganism growth through VC reductive dechlorination.; The dechlorinating ability of bacterium VS was compared with two mixed cultures commonly used for bioaugmentation (KB-1 and Pinellas), also containing Dehalococcoides-like microorganisms. Growth rates on VC were similar (0.43, 0.40, and 0.33 d−1 for VS, KB-1 and Pinellas, respectively). The three cultures failed to dechlorinate PCE or did so very slowly. However, unexpectedly all three experienced growth on TCE, and did so with similar growth rates (0.39 d−1). All three mixed cultures also exhibited similar growth rates on DCE (0.36 ± 0.06 d −1), which is about the same as that with TCE and VC. Obtaining energy from each step in the dehalogenation of TCE to ethene will be an important advantage for the use of these cultures in bioaugmentation.; The problem of reaction kinetic limitations on DCE and VC removal was addressed by investigating the effect of limiting substrate concentrations on dechlorination kinetics and microorganism growth. For this, a model based on Monod kinetics but also accounting for competition between DCE and VC and the effect of low electron donor and acceptor (dual-substrate kinetics) was examined. Both the model and experimental data were used to determine substrate concentrations at which the dechlorinating population would be in net decay. The model indicates net decay will result if the total electron acceptor concentration (DCE plus VC) is below 0.7 μM, regardless of electron donor levels. The ability to achieve sustainable bioremediation to acceptable levels can be greatly influenced by this threshold level.