Theoretical Study On The Atmospheric Degradation Mechanism Of Typical Organic Pesticides

Pesticides is a class of drugs of application of insecticides, sterilization and killing harmful animals (or weeds), which is for protecting and promoting the growth of plants and crops in agriculture production. In this paper two categories of commomly used pesticides were studied:organochlorine pesticides and organophosphorus pesticides. These two types of pesticides are highly toxic, easily into the atmosphere during spraying and very easy to human aspiration so as to endanger human health. Pollution and environmental behavior of pesticides in the atmosphere has attracted a lot of attention.Three representative organochlorine and organophosphorus pesticide DDT, methamidophos, dimethoate as the research objects were selected. Density Functional Theory(DFT) was employed to study their degradation mechanism in atmosphere at the MPWB1K6-31+G(d,p)//MPWB1K6-311+G(3df,2p) level systematically. Results are listed as follows:1. The degradation mechanism of DDT in atmosphereFor the H abstraction reactions, H in the alkyl group is more active than the other four hydrogen; for the addition pathways, addition to C3, C5, C6in the benzene ring is easier and the main secondary products are nitrate compounds and HO2; at298K, the overall reaction rate constant is4.01x10-15cm-3molecule-1s-1and the atmospheric lifetime of DDT determined by NO3radical is52.5days.DDT is easily transformed into DDE in atmosphere. The degradation mechanism of DDE was also studied. The calculations show that the main oxidation reactions of DDE is addition pathways and the main products are nitrate compounds and HO22. The degradation mechanism of methamidophos in atmosphereFor substitution and addition-elimination pathways of OH radical,(P2) is the main product; For H abstraction pathways, the hydrogen on SCH3group is the most active; Suming up H abstraction pathways and secondary pathways, the main products are and formaldehyde.3. The degradation mechanism of dimethoate in atmosphereThere are many products in the degradation process of dimethoate. For H abstraction pathways, the main products of secondary reactions are For substitution and addition-elimination pathways of OH radical, the main products are...