Studies On The Photochemical Degradation Kinetics Of Two Fungicides With Humic Substance Obtained In Agricultural Wetland

Along with the extensive use of fungicides in agricultural wetlands, the accumulation of organic pesticide in soil and surface water, becomes a non-point pollution source.Generally, photochemical degradation initiated by reactive oxygen species(ROSs) is an important naturally occurred degradation pathway of organic pesticides in wetland and reactive oxygen species(ROSs) was mainly induced by the photochemical reactions of dissolved organic matter(DOM). Reactive oxygen species(ROSs) induced by dissolved organic matter(DOM) in agricultural wetland which was severely influenced by the anthropic activities.The structure and concentration of DOM would be significantly different with the natural environment, and finally result in the different species and concentrations of ROSs as well as the degradation kinetics of pesticide in agricultural wetland.The photochemical initiated by humic substance was studied based on the degradation kinetics and influencing factors of two kinds of fungicides(tricyclazole and prochloraz).The humic substance stand for typical agricultural wetland was isolated from the rice paddy and lotus pond in Fuhe river irrigation area. The influence of pH, concentration of humic was examined, and the role of ROSs towards specifics fungicides was further discussed. Detailed results and conclusions were summarized as follow:(1) In direct photolysis experiment, prochloraz was stable, and tricyclazole was weakly decomposed. The degradation of prochloraz and tricyclazole in humic substance solution followed pseudo first order kinetics. The prochloraz degradation rate in paddy field humic substance is faster than in lotus pond humic substance, while the tricyclazole degradation rate in lotus pond humic substance is faster than in paddy field humic sustance.Moreover, the fulvic acid rather than humic acid show higher photochemical reactivity towards two fungicides(2) For paddy fields humus, high concentrations were found inhibit the degradation of prochloraz and tricyclazole, possiblely due to effects of DOM towards of ROSs and reduction of excited state pesticide by humic substance. In the lotus pond humic substance,the degradation rate of prochloraz was inhibited by high concentrations of the lotus pond humic substance, and the degradation rate of tricyclazole were increased with the rises of concentration of lotus pond humic substance.(3)The bimolecular reaction rate constant of prochloraz and tricyclazole with HO·are3.79±0.07×109M-1·s-1and 4.25±0.30×109M-1·s-1. and the bimolecular reaction rate constant of prochloraz and tricyclazole with1O2 are 7.31±0.40×107M-1s-1and 5.06±2.3×107M-1s-1.The photolysis pathway of prochloraz in humic substance is hydroxyl radicals and singlet oxygen. For humic substance obtained from paddy field, singlet oxygen contribute greaterthan hydroxyl radicals to reaction, while in the lotus pond sources of humus, hydroxyl radicals contribute greater than the singlet oxygen. Hydroxyl radicals was the predominant ROS involved in the photochemical degradation of tricyclazole.(4) Relatively high levels of reactive oxygen species generation was achieved in paddy field humus compared with lotus pond humus. Generally, The hydroxyl radicals is the major ROS in the photochemical reactions of humic substance obtained in rice paddy fields,while the singlet oxygen was the predominant ROS in lotus pond humus solutions.The ability of humus to generate ROSs associated with humus concentration and pH.