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Environmental fate & pathways

Phototransformation in water

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Description of key information

In a key study (Huntingdon Life Sciences, JHW0006, 2014) according to OECD 316 (2008) the test substance was photolytically degradable in both pH 7 buffer and natural water at 25°C. The DT50 values were 28 (pH 7 buffer) and 12 equivalent days (natural water) of latitude 40°N summer sunlight, respectively, or 78 (pH 7 buffer) and 34 equivalent days (natural water) of Tokyo spring sunlight.

Key value for chemical safety assessment

Half-life in water:
12 d

Additional information

In a key study (Huntingdon Life Sciences, JHW0006, 2014) according to OECD 316 (2008) the aqueous photolysis of the test substance was investigated in sterile pH 7 aqueous buffer and sterile natural water. Water samples were treated with radiolabelled test substance, at a nominal initial concentration of 2.0 mg/L. Treated samples were irradiated with a xenon arc lamp (simulated sunlight source) continuously for up to 8 days. The samples were irradiated (and control samples incubated in darkness) at 25 ± 1 °C. Aliquots were analysed at 0, 1, 2, 3, 4, 6 and 8 days.

 

The recoveries of the applied radioactivity (AR) from irradiated pH 7 buffer samples treated with radiolabelled test substance were 90.3% to 96.3% AR and were 98.1% to 105.3% AR for irradiated natural water samples. For the non-irradiated control samples, recoveries of radioactivity were 91.4% to 98.6% for pH 7 buffer and 100.9% to 104.9% AR for natural water. Volatile radioactivity (14CO2) accounted for a maximum of 0.5% in irradiated pH 7 buffer samples and 0.8% AR in irradiated natural water samples and was not collected for the dark control samples.

 

The DT50 for the test substance in pH 7 buffer was 28.0 equivalent days for latitude 40°N summer sunlight or 77.7 equivalent days for Tokyo spring sunlight. In natural water, the DT50 for ME5343 was 12.2 equivalent days of latitude 40°N summer sunlight or 33.8 equivalent days of Tokyo spring sunlight. There was no significant degradation of the test substance in the dark control samples therefore no DT50 was determined.

 

One primary HPLC region of radioactivity was observed to form that represented a maximum of 12.8% AR in the pH 7 buffer and 37.8% AR in natural water. The region was shown by TLC to consist of at least four components, one of which represented a maximum of 8.4% AR in buffer and 25.8% AR in natural water, which was identified by co-chromatography with the authentic reference substance as nicotinic acid. All other degradation products (total of 13), including M440I007, were less than 8.0% AR. The degradation product M440I007 was detected in all but one of the irradiated samples at levels between 0.5% and 5.8% AR.

 

The quantum yield for the test substance in pH 7 buffer was determined to be 4.83 × 10E-5. From the quantum yield the theoretical lifetime was determined for each season at 10° longitude and 40 °N latitude close to the surface of water and the values ranged from 4.3 days in summer to 15.7 days in winter.  The results indicate that photolysis would be one of the potential degradation routes for the test substance in the aquatic environment.

 

In addition, in a supporting study (The Institute of Environmental Toxicology, IET11-8010, 2015) according to OECD 316 (2008) the aqueous photolytic fate of the radiolabelled test substance was investigated with simulated sunlight at a nominal concentration of 2 mg/L in sterilized pH 7 buffer solution and sterilized river water at 25°C. Under the study conditions, the first-order DT50 was calculated using linear regression to be 16 days for pH 7 buffer solution and 9 days for river water. The equivalent half-lives at 40°N latitude were 17 days in pH 7 buffer and 10 days in natural water. A few minor photodegradation products were detected in the test solutions during the irradiation duration, but none exceeded 7% AR. Based on the results of the study, it was concluded that photolysis would be one of the potential factors that contribute to the degradation of the test substance in the aquatic environment.