Biotransformation of mixtures of chlorinated aliphatic hydrocarbons by an acetate-grown methanogenic enrichment culture

Biotransformation of chlorinated aliphatic hydrocarbons under anaerobic conditions has received considerable attention due to the prevalence of these compounds as groundwater contaminants. However, information concerning the impact of mixtures of chlorinated compounds on their own transformation is limited. The focus of this research was to investigate the effect of combinations of chlorinated aliphatics on transformation rates and the role of toxicity to a methanogenic enrichment culture. Batch studies using combinations of carbon tetrachloride (CT), perchloroethene (PCE) and 1,1,1-trichloroethane (1,1,1-TCA) were performed with acetate as the electron donor. CT was transformed most rapidly, followed by 1,1,1-TCA and PCE. Rate coefficients for the transformation of 1,1,1-TCA and CT were significantly lower when the concentration of either compound was increased. PCE transformation was limited, and its presence did not significantly impact degradation of CT or 1,1,1-TCA. In once-fed batch reactors used to assess the potential for toxicity, a transformation limit was observed for both PCE (1.91±0.21 μg of PCE per mg of cells) and 1,1,1-TCA (7.84±0.34 μg of 1,1,1-TCA per mg of cells) but not CT. The low observed limit of transformation for PCE, combined with its lack of effect on the transformation of the other two compounds, suggests that the number of PCE-degrading organisms in the mixed culture is low. Transformation of repeated CT spikes has been maintained for over 400 days without supplementation with acetate, indicating that inhibition, rather than potential inactivation, was responsible for the negative impact observed when CT is present in mixtures.