Partial and Complete Dechlorination of Tetrachloroethylene (PCE) and Molecular Characterization of PCE-dechlorinating Bacteria

Tetrachloroethylene (PCE) is a volatile, chlorinated organic hydrocarbon that is widely used as a solvent in the dry-cleaning and textile-processing industries and as an agent for degreasing metal parts. It is an environmental contaminant that has been detected in the air, groundwater, surface waters, and soil. PCE becomes a groundwater contaminant due to leaks and improper disposal practices; it can persist in groundwater for years because it has little contact with air. The U.S. Environmental Protection Agency (EPA) has classified tetrachloroethylene as a hazardous air pollutant under the Clean Air Act, a toxic pollutant under the Clean Water Act, a contaminant under the Safe Drinking Water Act, a hazardous waste under the Resource Conservation and Recovery Act, and a hazardous substance under the Comprehensive Environmental Response, Compensation, and Liability Act. EPA classified tetrachloroethylene as “likely to be carcinogenic to humans.” PCE is among the most prevalent groundwater pollutants. Its frequent occurrence at contaminated sites is due to its widespread use as an industrial solvent. PCE and its incomplete degradation products are known or suspected carcinogens. Therefore, the treatment of PCE-bearing wastes and the remediation of PCE-contaminated soils and aquifers are global priority in environmental pollution control. PCE can be reductively dechlorinated by anaerobic microorganisms to trichloroethylene (TCE) and cis-1,2-dichloroethylene (cis-DCE). However, to make anaerobic bioremediation useful, PCE must be degraded to nonchlorinated, environmentally acceptable harmless products. Complete dechlorination of PCE to ethylene or ethane has been reported in mixed cultures and some pure cultures belonging to Dehalococcoides spp. However, the consensus is that although complete microbial reductive dechlorination is possible in anaerobic aquifers, complete reductive dechlorination to ethene is rarely observed due to low electron donor availability, unfavorable redox conditions, or the absence of appropriate microorganisms.

This review shows the degradation, biochemical and molecular characterization of the DPH-1 strain, a well-known example of partial dechlorinating of PCE. In addition, the author reviewed the dechlorination of PCE using the Propionibacterium genus, not Dehalococcoides spp., which is known to dechlorinate PCE completely. Finally, the author outlined the complete degradation of PCE by a two-step treatment of zerovalent ion and strain DPH-1. More importantly, the author hopes this book chapter will be valuable when conducting PCE remediation procedures and designing treatment processes.