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Environmental fate & pathways

Phototransformation in water

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Description of key information

Phototransformation half-life in water cannot be determined because the study substance does not absorbed light in the wavelength >290 nm and therefore, direct photolysis is not expected to contribute to the degradation of the study substance in water or aquatic environment.  Hence, half-life in water is not available for chemical safety assessment.  

Key value for chemical safety assessment

Additional information

Direct photochemical degradation in water occurs through the absorbance of solar radiation by a chemical substance. If the absorbed energy is high enough, then, in the resultant excited state, the chemical may undergo a transformation. A prerequisite for direct photodegradation is the ability of one or more bonds within a molecule to absorb ultraviolet (UV)/visible light in the 290 to 750 nm range. Light wavelengths longer than 750 nm do not contain sufficient energy to break chemical bonds, and wavelengths below 290 nm are shielded from the earth by the stratospheric ozone layer. An approach to assessing the potential for the study substance to undergo direct photochemical degradation is to assume that degradation will occur in proportion to the amount of light wavelengths >290 nm absorbed by the study substance molecules. The study substance which is a long-chain alkylated naphthalene does not absorb sufficient light energy above 290 nm to cause photolysis (see JC Harris, “ Rate of aqueous photolysis”, Chapter 8, in Handbook of Chemical Property Estimation Methods (WJ Lyman et al eds.), mcGraw-Hill,(1982). Therefore, direct photolysis will not contribute to the degradation of the study substance in the aquatic environment because it does not absorb light at wavelengths >290 nm, i.e., in the range that is a requisite to this process.