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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

Fluorescent whitening agents such as OB 1 -MSA have the ability to absorb part of the terrestrial UV-sunlight (λ = 300 – 400 nm) and transform it into visible, blue fluorescence light. For that reason they are potentially photodegradable substances. In an experiment on the elimination of OB 3a-MSA, a similar substance of the Stilbene Fluorescent Whitening Agents category, in an eutrophic lake water irradiated by sunlight, rapid direct photolysis was reported (t1/2 < 6 hours).

Several extensive studies have been performed on the similar substances OB 3a-MSA, the analogous dihydroxyethyl derivative tetrasulphonated and OB 2 -A, the morpholino analogous of the substance under registration. The differences in substitution seem not to have a significative impact on the stilbene bound energy and little influence on the process of photoisomerisation, as can be deduced by the similar results in the photodegradation study.

The results indicate that photochemical processes induced by sunlight are sufficient to degrade the test substance. In natural waters these processes may be their main degradation pathway since biodegradation is practically not existent.

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 sunlight 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.

Photoisomerization quantum yields and isomer distributions at the photo stationary state affect the rates of photodegradation of the Fluorescent Brighteners. With an equation containing these values one can successfully calculate photodegradation rate coefficients under natural sunlight conditions. The photoisomerizaion is caused by an activated rate process in the first exited singlet state. Additional radiationless deactivation channels could be promoted by a limited torsion of the excited singlet E-isomer about the C-C double bond, which would be allowed since the bulky substituents at the double bond are highly flexible. The activation energy for photoisomerizaion is low and somewhat influenced by the  viscous drag on the twisting group about the C-C double bond caused by the bulky substituents. The bulkiness of the substituents is very similar within the category.  As a consequence of these mechanisms, all substances of the category behave very similar with respect to photoisomerizion and direct photolysis in sunlit aqueous solutions.

In conclusion photodegradation is expected to be an important elimination process for the substance under registration 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 OB 2 -A 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 Novartis Services AG ,1998) on OB 2 -A 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.