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Environmental fate & pathways

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Hydrolysis is not expected to be an important removal process for tetrachloroethylene. Half-lives in the range from 8.8 months to several million years have been reported (Dilling et al., 1975; Jeffers et al 1989). Photolysis is not likely to be a significant removal process for tetrachloroethylene. Tetrachloroethylene (1 mg/ml) in water was degraded by 59 -65% after one year in the dark and by 75 -76% in the presence of sunlight (Dilling et al., 1975).

Tetrachloroethylene undergoes reactions with hydroxyl radicals in the atmosphere. The calculated half life of tetrachloroethylene due to this reaction is 50 days with a OH radical concentration of 1.5E6 OH/cm3 (AOPWIN, 2000), with an overall OH-rate constant of 2.14E-13cm3/molecule.sec. Tetrachloroethylene also reacts with ozone, nitrate radicals and hydroperoxy radicals in the atmosphere but are thought to be insignificant atmospheric degradation processes. Tetrachloroethylene can also react with chlorine atoms in the atmosphere. Overall, tetrachloroethylene is degraded in the atmosphere.

A number of studies have been reported on the biodegradation of tetrachloroethylene. The substance was not biodegraded under the stringent conditions of the modified shake flask closed bottle biodegradation test after 21 days (Mudder, 1982). However, data from simulation and screening tests demonstrated that tetrachloroethylene degrades well under specific anaerobic conditions. More than 99% of the tetrachloroethylene was dechlorinated in less than 200 days of incubation under either sulfate-reducing or methanogenic conditions in soil (Pavlostathis SG and Zhuang P, 1993). The conditions and inocula used appear to be important. Tetrachloroethylene does not appear to undergo aerobic biodegradation.

The rate constants used in the assessment are listed in the following table:

 

Degradation rates

Degradation for hydrolysis

0 d-1

Degradation for photolysis

0 d-1

Degradation rate in air

0.01386 d-1

Degradation rate in water

1.39·10-6d-1

Degradation rate in sediment

6.937·10-7d-1

Degradation rate in soil

6.937·10-7d-1

Degradation in the STP

0 d-1

 

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 49 to 77.1. Also studies with algae were available. The flow through study with concentration monitoring, which is highly similar to OECD guideline 305 (Barrows, 1980), is taken as the key study.

 

The available measured values for adsorption/desorption range from 40.7 to 525. 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 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.

 

The Henry’s Law constant for tetrachloroethylene and the air-water partitioning coefficient are calculated as 2,110 Pa.m3/mol and 0.893 m3/m3 respectively using EUSES with a vapour pressure of 1,900 Pa and a water solubility of 149 mg/l.

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