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Environmental fate & pathways

Phototransformation in air

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Description of key information

Phototransformation in air: Measured rate constant for reaction with OH radicles 1.4 E-12 cm3 molecule-1 second-1 at 24°C (half-life 11.5 days)

Key value for chemical safety assessment

Half-life in air:
11.5 d
Degradation rate constant with OH radicals:
0 cm³ molecule-1 s-1

Additional information

Hexamethyldisiloxane contains no chromophores that would absorb visible or UV radiation so direct photolysis is not likely to be significant.

An experimental relative rates study (Atkinson, 1991) found that the NO3 radical and O3 reactions are of no importance as tropospheric removal processes for this compound. The dominant gas-phase chemical loss process is by reaction with the OH radical, with measured rate constant of 1.4 E-12 cm³ molecule-1 second-1 at 24°C (calculated half-life 11.5 days, using tropospheric concentration of OH radicals 5 E05 molecule/cm3 over 24 -h period (EC, 2003)).

A further experimental relative rates study (Sommerlade et al., 1993) found a reaction with the OH radical rate constant of 1.19 E-12 cm³ molecule-1 second-1 at 24°C (calculated half-life 13.5 days, using tropospheric concentration of OH radicals 5 E05 molecule/cm3 over 24 -h period (EC, 2003)).

In another study (Parlar, 1992) a reaction with OH radicles resulted in a rate constant of 3.13 E-12 cm³ molecule-1 second-1

(calculated half-life 5 days, using tropospheric concentration of OH radicals 5 E05 molecule/ cm³ over 24 -h period (EC, 2003))

A reaction with OH radical rate constant of 0.90 E-12 cm³ molecule-1 second-1 (calculated half-life 17.8 days, using tropospheric concentration of OH radicals 5 E05 molecule/ cm³ over 24 -h period (EC, 2003)) was obtained using an accepted calculation method (USEPA SRC). The result is considered to be reliable.

Reaction rate constants of 8.8 days (Stewart 1997) and 12 days (Kirk-Othmer 1982) are reported in studies to which a reliability could not be assigned.

European Communities, 2003. European Union Technical Guidance Document on Risk Assessment for New and Existing Substances, Part II, Section 2.3.6.3.