Reductive dehalogenation of chlorinated aromatic and aliphatic hydrocarbons under anaerobic conditions

The process of reductive dehalogenation involves the removal of a halogen substituent from a molecule with the concurrent addition of electrons to the molecule, resulting in a more reduced and often less toxic product. Anaerobic bacteria have the potential to utilize hazardous chlorinated aromatic and aliphatic hydrocarbons as electron acceptors in metabolic reductive dehalogenation processes. In experiments performed with chlorinated aromatic compounds a highly enriched anaerobic culture stoichiometrically converted 2,4,6-trichlorophenol (2,4,6-TCP) to 4-monochlorophenol. Dehalogenation occurred only in alkaline media (pH 8-9) at concentrations of 2,4,6-TCP up to 1 mM. Data indicated that the dehalogenating organism did not fit into any of the typical metabolic classifications of anaerobic bacteria: methanogenic, sulfidogenic, nitrate-reducing, metal-reducing, or fermentative. Data suggested that dehalogenation was linked to growth and proceeded as a respiratory process. The organism was capable of utilizing a number of supplementary chlorinated compounds as electron acceptors, in addition to the 2,4,6-trichlorophenol. Experiments performed with chlorinated aliphatic compounds involved soil microcosms from a perchloroethylene (PCE) contaminated site. The approach was to provide slowly fermentable compounds, which are not widely used by bacteria, as a source of low potential electrons. The data obtained show that N-Z-Soy Peptone, xanthan gum, polyethylene glycol-60, Tween-80, xanthine, crude DNA, and a volatile fatty acid mix were all able to support dehalogenation as far as cis-dichloroethylene. Additionally, the data show that xanthan gum was able to carry the dehalogenation process past dichloroethylene to vinyl chloride, with no perchloroethylene or trichloroethylene remaining. The active population was able to dehalogenate up to 250 μM PCE (about 40 ppm). Inhibitor experiments performed with molybdate and bromo-ethane sulfonic acid suggested that part of the active population consisted of sulfidogenic bacteria, while methanogens did not play a significant role in the dehalogenation activity. Taken together the results of these studies investigating the reductive dehalogenation of chlorinated aromatic and aliphatic compounds under anaerobic conditions demonstrate that bacteria which play significant roles in the dehalogenation processes come from diverse metabolic backgrounds which include fermentative, sulfidogenic, and actual chlorinated-compound-respiring organisms