Kinetic And Mechanism For The Oxidation Degradation Of Pesticiedes

China is the biggest pesticide producer and consumer.Pesticides pollution has been detected in source water which might threaten drinking water safety.In traditional drinking water process,pesticides are removed ineffectively in Coagulation,sedimentation and filtration process,but some pesticides can be degraded in oxidation and disinfection process under the action of oxidants.However,pesticides can not be mineralized during oxidation,which might even produce more toxic oxidation products.Therefore,it is necessary to study the pesticide oxidation degradation efficiency,kinetic and mechanism.Targeting commonly used pesticides---isoprothiolane,fenobucarb,imidacloprid,alachlor and simazine,which were detected in surface water with high concentration and frequency but lacked related investigation in China,the SPE-LC-MS/MS determination method,the occurrence in drinking water treatment plant,and the degradation efficiency,kinetic and mechanism by Cl₂,O₃,ClO₂ and KMnO₄ were investigated in this research.The method detection limits of SPE-LC-MS/MS for the target pesticides were all less than 3 ng/L and the recoveris was between 85%and 120%.The investigation of the water treatment plant indicated that isoprothiolane,alachlor and simazine were detected in all samples,where isoprothiolane was detected with the highest concentration of 135.4ng/L,alachlor and simazine at the range of 5.1-41.1 and 4.3-41.5 ng/L respectively.The concentration of imidacloprid and fenobucarb were less than 10 ng/L.The UF-NF membrane process removed isoprothiolane and alachlor effectively,simazine modestly but imidacloprid poorly.Isoprothiolane was the most degradable one in the five target pesticides,it could be fully degraded by Cl₂ and O3,but couldn't by ClO₂ and KMnO₄.The other four pesticides showed no reaction with Cl₂,ClO₂ and KMnO₄,but reached the removal efficiency of20%-40%in ozonation process by OH radical formed by ozone decomposition.The second order rate constant for the reaction of hypochloric acid with isoprothiolane was73.34 M^-1s^-1 at 25?,while the raction of hypochlorite were negligible.That's the reason why the apparent rate constant decreased with the increasing pH value.As for the ozonation of isoprothiolane,the observed rate constant was 1419.6 M^-1s^-1 under the combined reaction of ozone and OH radical,while the direct ozone reaction rate constant is 326.4 M^-1s^-1,which indicating that the oxidation of OH radicl was dominated.Therefore,the degradation efficiency will be affected by some OH radical scavenger in natural water such as CO₃^2-.The thioether bond and the carbon-carbon double bond on isoprothioane could be easily attacked by both Cl₂ and O₃.Isoprothiolane sulfoxide and sulfone were form when Cl₂ attacked the thioether bond,while Isoprothiolane sulfoxide and disulfoxide formed when O₃ attacked.The reaction on the double bond with Cl₂ or O₃ resulted in the formation of diisopropyl ester of malonic acid,diisopropyl ester of tartronic acid and diisopropyl ester of ketomalonic acid monohydrate.Compared with ozonation,chlorination processes were more complicated and produced more intermediats,including chlorinated and hydroxylated products.Isoprothiolane sulfone-Cl and Diisopropyl ester of dichloromalonic were identified as the chlorinated products.The hydroxylation resulted in the formation of ring-opening products which contained carboxyl and sulfonate acid groups.The fenobucarb ozonation products contained aldehydes,demethylation products and quinone.