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Environmental fate & pathways

Phototransformation in water

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Phototransformation 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, phototransformation is not considered to be relevant.

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Phototransformation 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, phototransformation is not considered to be relevant.

A non-peer-reviewed experimental study by the US Environmental Protection Agency (Hill et al, 1976) concluded that direct photolysis is not a significant environmental process. This result was to be expected, given that in the same study it was determined that VCM dissolved in water does not absorb wavelengths longer than 218 nm (broadly in line with several published measurements of the UV absorption spectrum of VCM in air), while ground-level sunlight is devoid of wavelengths shorter than 290 nm.

In the same study, Hill et al (1976) did not observe any indirect photransformation in natural waters containing humic substances, or in pure water with added commercial humic acid. Furthermore, electronically excited (singlet delta) molecular oxygen, generated using methylene blue as a photosensitiser, did not "readily" degrade VCM. On the other hand, Hill et al (1976) did observe rapid photochemically-induced transformation of VCM in the presence of acetone (a high-energy triplet photosensitiser) or hydrogen peroxide (a source of hydroxyl free radicals). The environmental significance of the experiments conducted with acetone or H2O2, surrogates for the precursors to reactive species potentially present in the environment, cannot however be readily assessed.

Some insight into the reactivity of VCM with respect to indirect photolysis in environmental waters may nevertheless be gained by combining data on (a) rate constants for reactions of VCM with photochemically generated reactive species known to be present in natural waters, and (b) environmental concentrations of such species. This exercise is performed below for the OH radical and the hydrated electron, for which input data are available:

  • VCM + OH: Köster & Asmus (Z. Naturforsch. 26b, 1108-1116, 1971) reported an aqueous-phase rate constant of 7.1 E+9 L mol-1 s-1. A typical range of concentrations of OH radicals in the surface layers of natural waters has been stated as 1.0 E-18 - 1.0 E-14 mol L-1 (Lam et al, Environ. Sci. Technol. 37: 899-907, 2003). Combining the kinetic and concentration data leads to VCM half-lives versus reaction with OH between about 3 hours and 3 years. In some special local situations, concentrations of OH higher by orders of magnitude may nevertheless exist (Allen et al, Environ. Tox. Chem. 15: 107-113, 1996), corresponding to even shorter half-lives.
  • VCM + e-(aq.): Köster & Asmus (loc. cit.) reported an aqueous-phase rate constant of 2.5 E+8 L mol-1 s-1. The steady-state summer surface-water concentration of hydrated electrons in a Swiss lake has been estimated to be 1.2 E-17 mol L-1. Combining the kinetic and concentration data leads to a VCM half-life versus reaction with the hydrated electron of 6.5 years. However, the full range of environmental hydrated electron concentrations has not yet been adequately characterised.

It should be emphasised that the half-lives estimated above for indirect photolysis are only very rough approximations, since the concentrations of OH radicals and hydrated electrons in natural waters are highly dependent on the presence of impurities in the waters (dissolved and suspended organic matter, nitrate, carbonate, etc.), the intensity of sunlight, its penetration into the water bodies, and other factors.