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Environmental fate & pathways

Phototransformation in water

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Description of key information

Minimum  DT50: 245 min

Key value for chemical safety assessment

Half-life in water:
245 min

Additional information

The results indicate that photochemical processes induced by sunlight are effective in degrading the test substance; in natural waters these processes may be their main degradation pathway since biodegradation in natural waters is practically nonexistent.

The isomerization process appears to play a major role in the photochemical behaviour. A constant isomer mixture is maintained during photochemical degradation processes. Since the photoisomerization process is temperature - as well as wavelength- dependent, the isomer distribution changes in sunlit natural waters with season (temperature) and depth (wavelength).

Fluorescence lifetimes decrease and Z/E-isomer ratios increase with increasing temperature, which can be interpreted in terms of an activated rate process in the first excited singlet state of the E-isomer having activation energy of 20-26 kJ/mol.

The photodegradation rates of the substance in natural sunlight are fast enough for photodegradation to be significant in the photic zone of lakes and rivers. On clear summer days the half lives range from about to 4 to 5 hours for the test substance near the surface of natural waters.

The importance of the photodegradation 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. 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. The particles may however reduce the availability of light since they adsorb light themselves.

In suspensions containing larger amounts of particles the presence of particles may lead to a shift in the photostationary state of the isomers.

In conclusion photodegradation is expected to be an important elimination process for the substance to be registered in the upper layer of surface waters.

As a consequence, the photodegradation process will lead to the formation of photodegradation products. Based on the studies performed on CAS 16090-02-1 the chemical constitution of the degradation products can be hypothesized (see attachment).

It is important to underline that most of the aquatic toxicity tests are conducted under normal light conditions, therefore water toxicity results will represent the realistic case in which the parent and degradation products are present as would be the case if this substances were directly released into the environment. Two fish acute toxicity tests are available (Ciba-Geigy, 1994 and 1998) on CAS 16090-02-1 pre-treated with light in order to obtain a high percentage of photodegraded products and verify their impact on aquatic toxicity. Both tests resulted in a LC50 > 100 mg/l, demonstrating that those degradation products are no more dangerous for the environment than the parent compounds.