Enantioselective Study On Fate Of The Enantiomers Of Imazethapyr In Soils

Nowadays, most of the herbicides appeared in the market have a chiral structure. The chiral herbicides have incomparable advantages on biological effectiveness and environmental safety, therefore its application in agriculture has attracted widespread attention. Imidazolinone herbicides (IMI), the key herbicides in our country, are typical chiral pesticides. It has been reported that the imidazolinones enantioselectively inhibited the acetolactate synthase (ALS). And there also exist enantioselectivity on its degradation in the soil after entering into the soil environment. However, whether the adsorption-desorption behavior of the enantiomers of this herbicide in soils exists enantioselectivity, and how environmental conditions affect its degradation enantioselectively have not been reported. In this study, imazethapyr (IM), one of the imidazolinones, which is chiral and is the most popular among all of the imidazolinones herbicides in sales, was choosed in this article as a model herbicide. Enantiomeric pure IM was separated by High Performance Liquid Chromatography (HPLC). And liquid-liquid extraction method can effectively extract imazethapyr from the tested soils, which lay a solid foundation for the further research. On these basis, enantioselectivity on the adsorption-desorption process and degradation in different environmental factors in two soils were studied. The main findings of the paper are as follows:The IM enantiomers were baseline separated by HPLC on chiral column Chiralpak OJ-H with the mobile phase of hexane/ethanol/acetic acid solution (75/25/0.1by volume) and254nm UV detector wavelength.Imazethapyr can be effectively extracted from the tested soils by liquid-liquid extraction method, and the target peak of chromatography by HPLC can be distinguished easily without interference created by other substances. In the optimized method, the recoveries of S-(+)-IM, R-(-)-IM in both soils were greater than86.7%and the lowest detection limit was0.10mg/kg. Therefore, this method was successfully used in the study on enantioselective fate of the enantiomers of this herbicide in field soils. In the soils, the adsorption-desorption behaviors of S-(+)-IM,R-(-)-IM were in line with the Freundlich equation. From the correlation coefficient, the capacity of adsorption for S-(+)-IM in the soil was higher, and the capacity of desorption is smaller. It is asserted that the soils may have a strong chemical adsorption to S-(+)-IM. However, when the adsorption-desorption process reached a equilibrium state, the Enantiomer fraction (EF) is approximately0.500in the aqueous solution. We can think about the enantioselectivity does not exist in the adsorption-desorption process.The degradation of S-(+)-IM、R-(-)-IM in different soils and circumstances were studied. The degradation rate of S-(+)-IM、R-(-)-IM in soils can be described as C=Co·e-kt. The degradation rates changed with different factors such as temperature, moisture content, microbial activities. Compered with high temperature, lower temperature in upland soil affects largely the degradation rate of the S-(+)-IM、 R-(-)-IM by prolonging the half-time obviously. On the contrary, and high temperature is apparent in paddy soil. Under the condition of70%moisture content, the degradation rate of imazethapyr in soils is the fastest. It is proved that microbial metabolism is the most important way to the degradation of imazethapyr in soil. Nevertheless, the fact that enantiomer fraction(EF) is greater a little than0.500suggested that the enantioselectively of imazethapyr exists in two soils in the steriled conditions. It is inferred that microbial degradation is not the only pathway for S-(+)-IM、R-(-)-IM in field soils. In spite of this, the conclusion still need be verified through further experiments. Because of the dark-incubation, other degradation pathways may exist enantioselectivity, and the transformation of two enantiomers may be the reason which induces enantioselectivity.