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Environmental fate & pathways

Phototransformation in water

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Description of key information

Ahel et al (1994) assessed the rates of photochemical transformation of nonylphenol in natural waters by exposing the solutions in filtered lake water (DOC=4 mg/L) to sunlight. Additionally, laboratory experiments were conducted by using a merry-go-round reactor (MGRR).  Two additional studies (Martinez-Zapata et al (2013) and Dulov et al (2013)) also assessed the photo transformation of 4n-nonylphenol in laboratory prepared water and considered both direct and indirect transformation.  Martinez-Zapata et al (2013) also considered photodegradation in two different natural reservoir waters.

Overall, photodegradation is likely to provide an important transformation route for nonylphenol in water.

Key value for chemical safety assessment

Additional information

The study by Ahel et al (1994) demonstrated that first-order rate constant of sunlight photolysis (kp) for nonylphenol was estimated at 0.09 m²/(kWh). This corresponds to a half-life of 10-15 hours under continuous clear sky, noon, summer sunlight in the surface layer of natural waters. The photolysis rate in the deeper layers is strongly attenuated, being approximately 1.5 times slower at depths of 20-25 cm than at the surface. Additional laboratory experiments using a merry-go-round reactor (MGRR) have shown that the photochemical degradation of nonylphenol was due mainly to sensitized photolysis whilst direct photolysis was comparatively slow.

Although sunlight photolysis rates of nonylphenol were found to be much slower than previously reported for some other alkylphenols (Faust and Hoigné, 1987, as cited in Ahel et al., 1994), the results suggest that a significant portion (30%) of nonylphenol could be photochemically degraded in the surface layer of natural waters within one day.

The study by Martinez-Zapata et al (2013) considered photo degradation of 4-n-nonylphenol, rather than 4-nonyphenol, and investigated both direct and indirect photolysis, with humic acids and Fe (III) as sensitizers, in addition to the effects of pH on transformation rates. Using a multivariate analysis of variance, pH was indicated to be the most important factor in the degradation of 4n- nonylphenol. There was a positive synergistic effect between the concentration of Fe (III) and humic acids at pH9. Humic acids without the influence of other variables had a negative effect by inhibiting photolysis in all other conditions. Up to approximately 70% photodegradation occurred in the natural waters after 5 hours and was thought to be dependent upon the environmental conditions. A first order reaction kinetics was adjusted to describe the photodegradation of 4n- nonylphenol with a half-life of 2.3 hours.

Dulov et al (2013), also considered indirect phototransformation of 4-n-nonylphenol with H202and Fe2+as sensitizers and the influence of pH on these transformation rates. Nonylphenol removal by the direct and hydrogen peroxide photolysis was shown to be pH dependent. The addition of H202to the UV system substantially improved the degradation of nonylphenol. The increase in the H202dose from 50 to 500 μmol/L intensified the degradation of the target compound in the H202/UV process. The application of the hydrogen peroxide photolysis at pH 11 and at H202dose of 250 μmol/L demonstrated the highest nonylphenol removal rate ((31.7 ± 0.55) ×10-4sec-1).

Overall, photodegradation is likely to provide an important transformation route for nonylphenol in water.