Trichloroethylene

Endpoint summary

Additional information

Trichloroethylene is mainly used as chemical intermediate. Use as solvent e.g. for metal degreasing is mainly occurring in closed systems. If released to the environment, the major release is likely to be to the atmosphere where trichloroethylene has a lifetime of about one week and so is not likely to enter the stratosphere. There will be a smaller release to the aquatic environment.

Trichloroethylene is very slowly hydrolysed under normal conditions. Half-lives in the range of 10.7 months to 1.3 million years have been reported (Dilling, 1975, Jeffers et al., 1989).

Photolysis is not likely to be a significant removal process for trichloroethylene. Trichloroethylene in water (1 mg/l) was degraded by 52 -56% after one year in the dark and by 70-79% in the presence of sunlight (Dilling et al., 1975).

 

Trichloroethylene undergoes reactions with hydroxyl radicals in the atmosphere. The calculated half-life of trichloroethylene due to this reaction is 13.3 days with a OH radical concentration of 1.5E6 OH/cm3 (AOPWIN, 2000), with an overall OH-rate constant of 8.048E-13cm3/molecule.sec. Trichloroethylene also reacts with ozone, nitrogen oxide and chlorine atoms in the atmosphere but this is thought to be an insignificant atmospheric degradation process. Overall, trichloroethylene is degraded in the atmosphere.

 

A number of studies have been reported on the biodegradation of trichloroethylene, and the results of these biodegradation tests are variable. Trichloroethylene is not readily biodegradable under the stringent conditions of the OECD (301D) and is only slightly degraded in aerobic studies.

However, trichloroethylene can be degraded under aerobic conditions by a process of co-oxidation when other suitable co-substrates (methane, propane, toluene, phenol) are also present to support growth of the microorganisms and induce the formation of enzymes which due to their broad substrate specificity, can also degrade trichloroethylene.

There is extensive work which shows that under anaerobic conditions, trichloroethylene degrades by a process of reductive dehalogenation, resulting in the formation of lower chlorinated homologues as reaction products. Although metabolites are known to occur, the terminal product of reductive dehalogenation is ethylene.

 

Bioaccumulation does not appear to occur to a significant extent. Several studies measured the whole-body bioaccumulation factors measured for fish, the values ranged from 17 to 90. Studies with algae were also available. The flow through study with concentration monitoring, which is highly similar to OECD guideline 305 (Barrows, 1980) is taken as key value for assessment.

 

The available measured Koc values range from 30 to 921. Rather than to try to derive a single value from the available data or select one of these values as the key parameter, the Koc value is calculated from the octanol-water partitioning coefficient (log Kow = 2.53) using the equation from the TGD (hydrophobics). This results in a Koc value of 141 l/kg (log value is 2.15), which is taken as the key parameter for assessment.

 

The Henry’s Law constant calculated from the ratio of vapour pressure to solubility is 1.03E3 Pa.m3/mol, and this value has been used in the EUSES calculations.