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Environmental fate & pathways

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The tested substance is estimated as hydrolytically stable at pH 4, 7 and 9 at 50 °C: it has a half-life period longer than one year at 25 °C at pH 4, 7 and 9. The substance has not hydrolysable chemical groups within the chemical structure.

Based on calculations according to AOPWIN v1.92, the substance is indirectly photodegraded by reaction with hydroxyl radicals in the atmosphere with half-lives of about 0.74 hours taking into account a 12 -h day and a mean OH radical concentration of 1.6E-12 cm³/molecules*s.

In dilute solutions and in presence of sunlight, the substance undergoes reversible isomerisation of the stilbene moiety.In this process, two isomeric forms occur. The E- and Z-isomers are under environmental conditions in equilibrium within a few minutes. The parent substance consists of the E-isomer, while isomerisation to the Z-form leads to complete loss of fluorescence. The isomerisation process appears to play a major role in the photochemical behaviour. A constant isomer mixture is maintained during photochemical degradation processes. Since the photoisomerisation process is temperature - as well as wavelength- dependent, the isomer distribution changes in sunlit natural waters with season (temperature) and depth (wavelength).

Two studies (for 2 -A and 3a-MSA) demonstrate that at solar latitude 60 °, at 25 °C, and on surface layer depths of 0 to 5 m the remaining E-isomer fraction is 13.9 to 9.5 and 17.8 to 13.3, respectively. The preceding isomer equilibrium influences the photo-degradation rate. The half-life for photo-oxidation in natural water was measured for the same substances and under the same light conditions on the surface layer: 278 min (2 -A) and 313 min (3a-MSA). Based on measured kinetic parameters and the quantum yield, photochemical half-lives in three Swiss lakes are calculated according to GCSOLAR as a function of surface layer depth and time of the year (Kramer, 1996). Both substances degrade with nearly identical half-lives. Therefore, it is assumed that all substances of the category follow the same photolytic processes and are effectively degraded in surface waters.

The photochemical processes induced by sunlight are effective in degrading the substance. The photodegradation importance in water does however strongly depend on the light screening of the water constituents. The presence of suspended solids or sediments in natural waters does not seem to have a major impact on the photoreactivity of the substance. The extent of adsorption at environmental relevant pHs is rather small. Thus, the small amounts of particles in the photic zone of natural surface waters are not capable of adsorbing larger amounts of the substance tested. The particles may however reduce the availability of light for the substance tested since they adsorb light themselves.