Degradation Pathways and Rates of Chlorinated Ethanes

Chlorinated ethanes, specifically, 1,1,1-trichloroethane, 1,1,2-trichloroethane, and 1,2-dichloroethane are the three of twelve volatile organic compounds (VOCs) specified by Japanese Soil Contamination Countermeasures Act as contaminants. However, the number of the sites contaminated with chlorinated ethanes are much less than that of the sites contaminated with chlorinated ethenes represented by tetrachloroethene (PCE), trichloroethene (TCE) and their biodegradation products such as cis-1,2-dichloroethene and vinyl chloride. Therefore, the studies and information on degradation of chlorinated ethanes are very limited in Japan compared with those on chlorinated ethanes, especially on anaerobic biodegradation.

In this presentation, we summarize the results based on literature surveys on potential degradation pathways as well as degradation rates of chlorinated ethanes. The types of degradation of chlorinated ethanes can be abiotic and biotic. A variety of mechanisms such as hydrolysis, dehydrohalogenation, reductive dichlorination and co-metabolism may contribute to degradation of 1,1,1-trichloroethane and 1,1,2-trichloroethane, and reductive dichlorination, anaerobic oxidation and catabolic reaction may contribute to degradation of 1,2-dichloroethane. Degradation rates of chlorinated ethanes are dependent on many factors such as the substance itself and temperature, and may vary over two to three orders of magnitude under different conditions. Converted half-life assumed with first order reaction varies from several months to several years or even longer. It would be very difficult to predict the fate of chlorinated ethanes in geo-environments, such as soil and groundwater, without detailed investigations about the hydrogeological, chemical and biological conditions in a specific site.