Registration Dossier

Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

Abiotic degradation of HHCB takes place by photodegradation. A hydrolysis study is waived as the molecule does not contain any hydrolysable structures.

The photodegradation of HHCB in air was studied by Aschmann et al. (2001) under laboratory conditions using blacklamps for irradiation (l> 300 nm) at 25°C and 740 mm Hg (0.986 bar) total pressure of purified air at ~5% relative humidity. Analysis for HHCB was by adsorption to SPME fibre with subsequent thermal desorption and GC-FID. Rate constants were measured for the gas phase reactions of OH radicals, using methylvinylketone as a reference substance: k1= 2.6±0.6.10-11cm3molecule-1s-1. The reaction of OH-radicals with HHCB is predicted to proceed mainly by initial addition to the aromatic ring and by H-atom abstraction from the C-H bonds of the –CH2-groups adjacent to the ether-O atom. The rate constant estimated from the structure by EPIWEB/AOP (Suracuse Research Corporation), 3.8.10-11cm3molecule-1s-1, agrees very well with the empirical value. Combined with estimated ambient atmospheric concentrations of OH radicals (a 12-h daylight average concentration of 2.0*106molecule cm-3), an atmospheric lifetime of 5.3 hours is calculated (t½= 3.7 h).These data indicate that the atmospheric lifetime of HHCB is sufficiently short that it will not undergo long-range transport to any significant extent. Assuming a daylight period of 12 h and 1.5*106OH cm-3, t½ based on the empirical rate is 4.9 h or 0.41 d.

In another study (Buerge et al. 2003) HHCB was incubated in lake water from the Zürichsee (CH) or in distilled water at 20 ± 1 ºC and illuminated with actinic lamps (mercury-vapor fluorescent lamps, between 300 and 460 nm with a maximum at 365 nm, comparable to that of 24h-averaged sunlight at 50 ºN in July under clear sky conditions). The rates of photodegradation were 0.15 and 0.12 d-1 in lake water and distilled water, respectively (half-lives of 109 and 135 h). The minimal differences between the photolysis rate constants determined in lake water and distilled water indicate that HHCB is degraded primarily via direct photolysis and that indirect photochemical degradation by reactive oxygen species is of minor importance. The photodegradation may explain the decreased concentrations in the epilimnion of the lake in summer.

It is concluded that photodegradation is an important degradation pathway in the environment.