Degradation And Mechanism Of Chemical Oxidation Of Pesticide Residues In Freshwater Bodies

In order to improve the yield and quality of crops and to prevent the occurrence of diseases or insect pests,agricultural producers usually employed a large number of pesticides every year.However,most of the pesticides directly infiltrated into the environment,resulting in the severe pollution to water and soil.Therefore,the efficient degradation and remove of pesticide residues in water are of great significance to the environment.Chemical oxidation is a method for degrading pesticides by mainly using oxidant to change the properties of pesticides.In this work,the reactivities of 12 pesticides with three oxidants NaClO,KMnO₄ and K₂FeO₄ at different concentrations were studied,and the dissociation constant of acephate was determined.The oxidative degradation of acephate by ferrate and degradation of imidacloprid by permanganate were also studied.The conclusions were obtained as follows:?1?Under the conditions of 25±2^o C and pH=7.5,the studied oxidants cannot degrade the phenoxycarboxylic pesticides including 2,4-D,4-CPA and the aromatic pesticides including clopyralid and dicamba.The studied oxidants can degrade phenylurea pesticides including diuron and tebuthiuron,among which NaClO is found to be the most effective oxidant.K₂FeO₄ has the highest oxidative degradation ability to acephate,and the degradation abilities of three oxidants to chlorpyrifos were low.The oxidative degradation ability of the studied oxidants to imidacloprid,acetamiprid and thiacloprid in neonicotinoid pesticides sorted from strong to weak is NaClO>KMnO₄>K₂FeO₄.?2?The dissociation constant of acephate was determined by UV spectrophotometry combined with Sigmoidal mathematical model.Three wavelengths were selected to calculate the dissociation constants of acephate.The obtained pK_a11 and pK_a22 were found to be 7.46 and 12.78,respectively.?3?The degradation rate of acephate increased by increasing the added K₂FeO₄ dosage.The optimal molar ratio was 50:1.At 25^oC,K₂FeO₄/acephate=50:1,pH=4⁷,t=20 min,K₂FeO₄ had the best performance for the degradation of acephate.No degradation products were detected in the sample after the reaction,which may be due to limitations of experimental instrument and method,requiring further research.?4?The degradation of imidacloprid by KMnO₄ increased with the increase of KMnO₄ dosage,and the optimal molar ratio was 300:1.The optimal molar ratio was 300:1.In the range of pH=5⁹,the oxidative degradation effect did not change significantly with pH.The product analysis showed that the oxidation products were C₉H₁₀ClN₅O₃ and C₉H₁₀ClN₅O₄.The reaction process is mainly the oxidation of the C-H bond,that is,a hydroxyl group is added to the imidacloprid molecule by oxidizing the C-H bond.