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Environmental fate & pathways

Phototransformation in water

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The experimentally determined phototransformation half-life of NP in water is between 15 and 20 h corresponding to a rate constant of ca. 0.9 h-1. No photodegradation of NPE-1 was observed, and correspondingly it is not expected to occur for NPEO. Overall, aquatic phototransformation is not expected to represent a relevant route of degradation of NPEO in the environment.

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A study was conducted to assess the rates of photochemical transformation of NP (a mixture of differently branched isomers) and nonyl phenol, branched 1 EO (NPE-1) (isolated from a commercial mixture) in natural waters. The phototransformation experiments were conducted both in the presence of sunlight and artificial light. In the sunlight test, quartz tubes containing the solutions of NP or NPE-1 in filtered lake water were exposed to sunlight (average sun irradiation intensities were 0.705 kW/m2 and 0.760 kW/m2) at a depth of 20-25 cm (in Chriesbach Creek, Switzerland) or at the surface (in shallow flat-bottomed container filled with tap water) of water. In the laboratory experiment using a merry-go-round reactor (MGRR), the solutions of NP and NPE-1 in distilled water (for direct photolysis) or filtered lake water (for sensitized photolysis) were exposed to the mercury lamp (700 W) light (10 times more light intensity than sunlight during a sunny summer day). Quantitative determinations of the analytes were performed by normal-phase HPLC after a simple extraction of the water samples with n-hexane.

The first-order rate constant of sunlight photolysis (kp) for NP was estimated to be 0.09 h-1. This corresponds to a half-life of 10-15 h under continuous clear sky, noon and summer sunlight in the surface layer of natural waters. The photolysis rate in the deeper layers was strongly attenuated, being approximately 1.5 times slower (kp = 0.06-0.05 h-1) at depths of 20-25 cm than at the surface. The photochemical degradation of NPE-1 was found to be insignificant. The laboratory experiments using artificial light showed that the photochemical degradation of both NP and NPE-1 was due mainly to sensitized photolysis whilst direct photolysis was comparatively slow. The data generated with artificial light can only be considered to be of qualitative nature since the chosen conditions are different from natural sunlight and are not recommended for standard testing (Ahel M et al., 1994).