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Environmental fate & pathways

Hydrolysis

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Hydrolysis is not expected to be a relevant dissipation route for vinyl chloride in water. Due to the fact that vinyl chloride is a gas, rapid volatilisation is expected and no significant concentrations are present in the fresh water and seawater compartment. This is supported by exposure assessment with EUSES. A regional PEC in fresh surface water of 4.18E-06 mg/L and a regional PEC in seawater of 3.44E-07 mg/L were calculated. The maximal local PEC value that was calculated for fresh water was 3.66E-02 mg/L (for S-PVC production). The maximum local PEC value that was calculated for seawater was 3.66E-03 mg/L (for S-PVC production). Since these concentrations are very low, hydrolysis is not considered to be relevant.

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Hydrolysis is not expected to be a relevant dissipation route for vinyl chloride in water. Due to the fact that vinyl chloride is a gas, rapid volatilisation is expected and no significant concentrations are present in the fresh water and seawater compartment. This is supported by exposure assessment with EUSES. A regional PEC in fresh surface water of 4.18E-06 mg/L and a regional PEC in seawater of 3.44E-07 mg/L were calculated. The maximal local PEC value that was calculated for fresh water was 3.66E-02 mg/L (for S-PVC production). The maximum local PEC value that was calculated for seawater was 3.66E-03 mg/L (for S-PVC production). Since these concentrations are very low, hydrolysis is not considered to be relevant.

The only experimental investigation leading to a useful conclusion on the hydrolytic stability of VCM at ambient temperature (half-life > 10 years) is the non-peer-reviewed study of Hill et al (1976), reinterpreted by Kollig et al (1990). However, these authors did not describe the exact procedure by which the absence of any observable hydrolysis at the study temperature of 85 °C was translated into an estimate of the upper limit for the hydrolysis rate constant (and hence a lower limit for the half-life) at ambient temperature.

The results of Hill et al (1976) and the further conclusions of Kollig et al (1990) have also been reinterpreted by Washington JW (Hydrolysis rates of dissolved volatile organic compounds: Principles, temperature effects and literature review, Ground Water, 33: 415-424, 1995). Washington assumes a "typical" activation energy of 110 kJ mol-1 to derive, from the upper-limit rate constants at 25 °C reported by Kollig et al, upper-limit pre-exponential factors and hence upper-limit rate constants at 10 °C. This analysis leads to the conclusion that the hydrolysis half-life of VCM is at least 100 years at 10 °C (that at 25 °C being > 10 years, in line with the conclusion of Kollig et al). These half-lives are independent of pH, since the contribution of the base-catalysed hydrolysis is shown to be negligible and the potential acid-catalysed process is not considered.

The poorly characterised hydrolysis study by the US Environmental Protection Agency (1974), carried out on an aqueous effluent from a VCM plant, does not lead to any useful conclusions, since the observed (but not quantified) decrease in VCM concentration was ascribed to physical loss from the system, rather than hydrolysis - a conclusion that may be erroneous.

Jeffers PM & Wolfe NL (Homogeneous hydrolysis rate constants-Part II: Additions, corrections and halogen effects, Environ. Tox. Chem. 15: 1066-1070, 1996) attempted to study the hydrolysis of VCM, generated in situ by dehydrochlorination of 1,2-dichloroethane in alkaline solution at 140-160 °C, but only polymerisation was observed under these environmentally non-relevant conditions.