Behaviors Of Pyrazosulfuron-ethyl In The Soil Of Rice-Tobacco Rotation Field

Pyrazosulfuron-ethyl(ethyl5-(4,6-dimethoxypyrimidin-2-ylcarbamoylsulfamoyl)-1-m ethylpyrazole-4-carboxylate) is one of the sulfonylureas widely used for selective pre-and postmergence control of annual and perennial broadleaf and cyperaceous weeds in transplanted and direct-seeded rice in China. However, the widespread use of pyrazosulfuron-ethyl have raised increasing concerns about phytotoxicity problems in direct seeded rice fields, which gave severe economic loss to agricultural production, the risk of contaminating water and environments, especially for human. The main work and results of the dissertation are presented as follows:A simplified quick, easy, cheap, effective, rugged and safe (QuEChERS) approach was described for determination of pyrazosulfuron-ethyl from soil samples by high performance liquid chromatography/ultraviolet (HPLC/UV) detector. Optimized results were obtained dispersing soil in water, followed by the addition of1%acetic acid in acetonitrile, anhydrous magnesium sulfate and sodium acetate tri-hydrate, which was a modification of QuEChERS method without primary and secondary amine (PSA) and C18sorbent. The results showed the recovery ranged from70.8%to99.0%and83.5%to86.4%with the relative standard deviations of2.1%to7.9%and7.0%to10.0%in Changsha and Nanning soil samples, respectively. The limit of detection (LOD) of the method was0.05mg/L. The limit of quantification (LOQ) was0.1mg/kg.The adsorption-desorption behaviors of pyrazosulfuron-ethyl were investigated in seven Rice-Tobacco Rotation soils of China. The effect of system pH was also studied. The results showed that the Freundlich equation fitted its adsorption and desorption well, and the Freundlich constant values (Kd) are3.29,2.14,1.95,2.22,2.66,2.61,1.74, respectively. The adsorption Kf values indicated the adsorption of pyrazosulfuron-ethyl in the seven soils was in the sequence of S1>S5>S6>S4>S2>S3>S7. The adsorption of pyrazosulfuron-ethyl was mainly a physical process, because its free energy (AG) in seven soils was less than40kJ/mol. The desorption hysteresis of pyrazosulfuron-ethyl were observed in all of the tested soils. Soil pH and organic matter (OM) were the main factors influencing its adsorption and desorption.The half lives of pyrazosulfuron-ethyl were20.3,24.5,26.3and23.4, days in soils of Zhuzhou, Hengyang, Chenzhou and Liuyang in China, respectively. The dissipation rate of pyrazosulfuron-ethyl on soil was increased with temperature and moisture increasing. The transports of pyrazosulfuron-ethyl were investigated in soils of Liuyang, Hengyang, Zhuzhou, and Chenzhou in China using thin layer chromatography (TLC) method. The Rf values were0.78,0.61,0.36,0.48in soils of Liuyang, Hengyang, Zhuzhou. and Chenzhou in China, respectively. The Rf values indicated the transports of pyrazosulfuron-ethyl in the four soils was different, the sequence of Liuyang>Hengyang> Zhuzhou>Chenzhou.The hydrolytic transformation kinetics of pyrazosulfuron ethyl was investigated as a function of pH and temperature. The results showed that the hydrolysis rate was pH-dependent and increased with increasing temperature. The results showed that hydrolysis of pyrazosulfuron-ethyl was much faster in acidic or basic media than under neutral conditions. Three hydrolytic products were detected by HPLC/MS/MS. Based on products; the hydrolytic pathways of pyrazosulfuron ethyl were subject to cleavage and contraction of the sulfonylurea bridge.Photodegradation of pyrazosulfuron-ethyl has been investigated in aqueous solution at different irradiation wavelength ranges and pH. Intermediate products and Kinetics were identified using HPLC-UV and HPLC-MS/MS analyses. Kinetics measurements have shown that photodegradation was faster in acidic medium but more efficient in basic medium, when the sulfonylureas were in their ionic form. The half-lives of pyrazosulfuron-ethyl under UV light and simulated sunlight were found to be27and152min, respectively. The senven products were identified by LC/MS/MS, GC/MS, and HPLC. Degradation pathway has been proposed:carbon-sulfur cleavage:4,6-dimetoxypyrimidine-2-yl aminocarbonylsulphamic acid, nitrogen-sulfur:2-ureido-4,6-dimetoxypyrimidine, and photohydrolysis of the sulfonylurea bridge:2-amino-4,6-dimethoxypyrimidine.The pyrazosulfuron-ethyl-degrading bacterium, designated as CW17, was isolated from contaminated soil near the warehouse of the factory producing pyrazosulfuron-ethyl in Changsha city, China. The strain CW17was identified as Acinetobacter sp. based on analyses of94carbon source utilization or chemical sensitivity in Biolog microplates, conventional phenotypic characteristics and16S rDNA gene sequencing. When pyrazosulfuron-ethyl was provided as the sole carbon source, the degradation rates of pyrazosulfuron-ethyl at initial concentrations of20.0mg/L was77.0%after inoculation for7days. Two metabolites of degradation were analyzed by liquid chromatography-mass spectroscopy (LC/MS). Based on the identified products, strain CW17seemed to be able to degrade pyrazosulfuron-ethyl by cleavage of the sulfonylurea bridge.