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Environmental fate & pathways

Phototransformation in water

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Description of key information

In surface waters, stilbene fluorescent whitening agents will be effectively degraded by photochemical processes.

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In dilute solutions and in presence of sunlight, stilbene fluorescent whitening agents undergo reversible isomerization 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 substances used as fluorescent whitening agent consist of the E-isomer, while isomerization to the Z-form leads to complete loss of fluorescence. Two studies demonstrate that at solar latitude 60°, at 25°C, and on surface layer depths of 0 to 5m 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 (Lake Greifensee) was measured for the same substances and under the same light conditions on the surface layer: 278 min and 313 min. The experimental kinetic data are used to calculate photochemical half-lives as a function of surface layer depth, optical density of the water, and time of the year.

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 photolytical processes and areeffectively degraded in surface waters.