Studies On The Photochemical Degradation Of Pesticides In Different Medium

Photochemical degradation of pesticides is an important transformation pathway in the environment. The processes of pesticides photodegradation play a key role in assessment of their ecological risks. With low pressure mercury lamp(LPML), xenon lamp and sunlight as light sources, through comparing and analyzing the photolysis characteristics of four pesticides (bioallethrin, chlorpyrifos-methyl, triazophos and gamma-HCH) in water, the correlation of different photolysis methods were studied. At the same time, the important factors affecting photodegradation of pesticides were studied, including temperature, medium and chemical structure. The major degradation products of four pesticides in petroleum ether were tentatively identified with a gas chromatography-mass spectrometer. The possible degradation pathways of four pesticides in petroleum ether were proposed. The main results were summarized as following:1. The photolysis of four pesticides was studied under different illuminating conditions. The photolysis correlativity of simulation condition and sunlight was analyzed and the relativity of different photolysis methods was deduced. The study can supply basic data for perfecting photolysis technique and estimate standard.2. The results showed that the photolysis of all the four pesticides in water followed the first-order decay model. The half-life of bioallethrin, chlorpyrifos-methyl, triazophos and gamma-HCH were 1.49h, 9.90h, 9.90d, 13.86d under sunlight, 0.85h, 3.39h, 6.56h, 15.68h under xenon lamp and 0.39h, 2.77h, 3.85h, 38.51h under mercury lamp respectively. The photolysis rates of four pesticides under three different lamp-houses were as follows: bioallethrin > chlorpyrifos-methyl > triazophos > gamma-HCH.But The results showed that the ratios of photolysis rates were different under different lamps for four pesticides. It was concluded that the photolysis characteristic of pesticide in simulation condition can reflect that of under sunlight on the whole. But the sensitivities of different pesticides to different light sources were dissimilar. The photolysis rates of pesticide were not evident correlative under the different illuminating conditions. The photolysis half-lives of pesticide in simulation condition can' t deduced that of under sunlight on the whole, and their correlation need further study.3. The photolysis of pesticide was greatly affect under different temperature within a certain temperature range. When the temperature were 20℃, 25℃, 35℃, the half life of triazophos were 7.70h, 6.56h , 3.65h and the half life of gamma-HCH were 23.10 h, 15.68 h, 11.46 h. The photolysis rate was increased with the rise of temperature. The same temperature grads had different effect for different pesticides.4. The different solvent had outstanding effect on the photolysis of pesticides. Under xenon lamp, the half life of triazophos extended to 13.59h, 41.01h and 61.89h in petroleum ether, toluene or acetone from 6.56h in pure water. The half life of gamma-HCH extended to 115.52, 138.63 and 231.05h in petroleum ether, n-Hexane or ethyl acetate respectively from 15.68h in pure water. The study with xenon lamp illuminating showed that acetone photoquened the photolysis of bioallethrin and its photoquenching rate was found distinctly related to its concentration. The photolytic half life of bioallethrin was 49.51min in pure water, when added 10% acetone, the photodegradation of bioallethrin was delayed and the half-life was 86.64min. The difference of UV-Vis spectra of pesticide in solvents maybe resulted in the difference of photolysis results.5. The study with a gas chromatography-mass spectrometer showed that the photolysis mechanisms of bioallethrin in petroleum ether as follows: one being photoisomerization and intramolecular rearrangement, and the other photochemical hydrolysis. There were three kinds of photodegradation pathways of chlorpyrifos-methyl: drop chlorine, photooxidation and photochemical hydrolysis. For major degradation products of triazophos, 1-Phenyl-3-hydroxy-1,2,4-triazole and O,O-Diethyl phosphorothioic acid were identified with a gas chromatography-mass spectrometer. The results reveal that the two degradation products are relatively unstable and could be easily degraded. Gamma-HCH photodegraded by dropping a hydrogen chloride in petroleum ether under xenon lamp.