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Phototransformation in air

Atmospheric photodegradation occurs with a half-life of approximately 30 days.

The photodegradation of 1,2,4-TCB by hydroxyl radicals in the atmosphere is estimated to be in the order of a month. However, the values should been considered as the upper limit of stability since other degradation modes are not considered (EU Risk Assessment, 2003).



Based on the experimental data (Korte and Freitag, 1986) on hydrolysis at 50°C,1,2,4-TCB is not expected to hydrolyse under normal environmental conditions (Howard, 1989; Schmidt-Bleek et al., 1982, EU RAR 2003, page: 24).

The hydrolysis was studied in buffer solutions at pH 3, 7 and 9 using radioactive 1,2,4-TCB at the initial concentration of 17.7 mg/l. During a study period of 5 days, the half-lives of 1,2,4 TCB at 50°C were estimated to be 1,806 hours (75 days) at pH 3, 4,577 hours (190 days) at pH 7 and 6,889 hours (287 days) at pH 9 (Korte and Freitag, 1986). The large half-lives for hydrolysis which indicate hydrolytic stability mean that it has not been taken into account in the risk assessment (EU RAR 2003, page: 8).


Phototransformation in water

Degradation by direct photolysis is not expected to be essential because the maximum absorption value is 286 nm (Bayer spectral data). The half-life for sunlight photolysis in pure surface water at 40° latitude in summer was 450 years (Dulin et al., 1986). The recovery of 1,2,4-TCB from isopropanol solution in Pyrex glass tubing (with a cut-off at 285 nm) irradiated with 300 and 310 nm fluorescent lamps for 30 minutes was 89.4% under anaerobic conditions where O2 was replaced with N2 and 8.1% under aerobic conditions. The products of photodegradation were 1,3- and 1,4-dichlorobenzene (Akermark et al., 1976).

In a laboratory study using artificial light (high pressure mercury vapour lamp at wavelengths > 290 nm), the photodegradation was studied at a concentration of 2-5 ppm. 1,2,4-TCB was dissolved in distilled water and filtered river water. The photolytic half-life was estimated to be 16.7 hours in distilled water and 12.2 hours in river water. The photolysis products in river water were 1,4-dichlorophenol and 4-chlorophenol (Mansour et al., 1989).

The photolysis was studied by exposing 5 ml 1,2,4-TCB at 4µg/ml at a distance of 30 cm from artificial light (Fluorochemical lamps 20W.2) at 25°C. After 144 hours, 0% was degraded (Kondo et al., 1988).

In a test where 1,2,4-TCB was adsorbed on silica gel and irradiated with light at wavelengths > 290 nm for 17 hours, 9.8% of the applied amount was degraded to CO2 (Freitag et al., 1985).

In addition to the direct photolysis, the photodegradation may also follow an indirect photolysis by sensitisation by secondary reactions with OH- and O2-radicals. The photolysis was studied in water solutions 600 ml at 4-20 mg/l irradiated for 3 hours. The photoreactivity in solutions in the presence of nitrite was observed to increase the photodegradation rate. The rate constant in pure water was 1.5*10-4s-1resulting in T½ to be 1.2 hours. In water added nitrite, the rate constant k was 3.1*10-4s-1and the resulting T½ 0.4 hours (Kotzias et al., 1982). The study was conducted under artificial conditions but indicates that photolysis may be affected by the contents in water. In the study, salts were used but other organic substances and organic matter may also affect the photodegradation rate (EU Risk Assessment 2003).

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