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Indirect photochemical degradation of di-isodecyl phthalate (DIDP) as mediated by OH- attack is estimated to have a half-life of 0.41 days or 4.9 hours based on a 12 -hour sunlight day, a rate of 2.62E-11 cm3/molecule*sec, and an average OH- concentration of 1.5E6 OH-/cm3. A 12-hour day half-life value normalizes degradation to a standard day light period during which hydroxyl radicals needed for photolysis are generated in the atmosphere. Although DIDP has the potential to degrade rapidly by OH- attack, multimedia distribution modeling indicates DIDP is predicted to partition negligibly (0.1%) to the air compartment because it has a low vapor pressure (0.000051 Pa). Although DIDP has a relatively short atmospheric oxidation half-life (4.9 hours), this process is unlikely to contribute significantly to the loss of DIDP from the environment.

Results from the EQC (Equilibrium Criterion) Levels I and III distribution models (Mackay, 2001) show that because of its low water solubility, DIDP is not expected to partition to the water compartment (see Section 4.2.3). However, abiotic degradation of any trace amounts of DIDP which may be present in aquatic environments is unlikely to occur at a significant rate based on modeled data. The HYDROWIN model, a subroutine within the USEPA (2000) computer program, estimates a hydrolysis half-life for DIDP of 3.4 years at pH 7 (25°C) and 125.2 days at pH 8 (25°C).

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 DIDP to undergo direct photochemical degradation is to assume that degradation will occur in proportion to the amount of light wavelengths >290 nm absorbed by DIDP molecules. DIDP does not absorb light within a range of 290 to 750 nm. Therefore, direct photolysis will not contribute to the degradation of DIDP in the aquatic environment because it does not absorb light at wavelengths in the range that contributes to this process.