Impact Of Prechlorination On Removal And Transformation Of Organophosphorus Pesticides During Conventional Drinking Water Treatment

Prechlorination could sterilize alga,minimize biological growth on filters,destroy the stability of colloidal particles in water to make it easier coagulable,as well as control taste or odor.Thus,it has been commonly used in many water treatment plants.However,previous studies have shown that micropollutants could be oxidized in subsequent chlorination,and consequently leaded to the formation of various oxidation byproducts.Compared with their parent molecules,these oxidation byproducts are more toxic and with higher chlorinated disinfection byproducts formation potential.Thus,the transformation and removal of these toxic substances has aroused wide concern.In this study,organophosphorus pesticides(OPPs)were as the research objects,and firstly,the effects of prechlorination on decomposition and removal of OPPs using a simulated conventional water treatment process of powdered activated carbon assisted coagulation sedimentation-filtration(PAC-CSF)and postchlorination were investigated.Secondly,the degradation kinetics of malation and malaoxon were investigated,as well as the identification of UV photo-oxidation byproducts.The main results are as follows:Firstly,the main chlorination byproducts of malathion,diazinon,chlorpyrifos and tolclofos-methyl were identified based on liquid-liquid microextraction pretreatment and gas chromatography-tandem quadrupole mass spectrometry in full scan mode.It was found that their chlorination byproducts were malaoxon,diazoxon,chlorpyrifos oxon,tolclofos-methyl oxon,respectively.Secondly,the effects of prechlorine dose,initial pesticide concentration and prechlorination contact time on decomposition and removal of OPPs using a simulated conventional water treatment process of powdered activated carbon assisted coagulation-sedimentation-filtration(PAC-CSF)and postchlorination were investigated systematically.The results indicated that amount of OPPs in finished water decreased and amount of oxons increased with prechlorine dose increasing.Further more,the gross removal efficiency(RGross)of each OPP plus its corresponding oxon decreased as the prechlorine dose increased,especially for the more chlorine-reactive and less absorbable OPPs.Molar ratio of OPPs to oxons varied with OPP species,and the average molar ratio of other three OPPs to corresponding oxons was above 90% except for diazinon(only 79.2%).Moreover,initial pesticide concentration and prechlorination contact time also effected the removal of OPPs and oxons.With initial pesticide concentration decreasing,percentage of toxic oxons in finished water increased.The concentration of a specific oxon analog in water increased with the increasing of prechlorination time,but its concentration was almost the same after the PAC-CSF and postchlorination,not correlated with prechlorination time.Finally,the effects of UV photo-oxidation,UV based advanced oxidation processes(UV/H2O2,UV/TiO2 and UV/Fenton)and Fenton process on malathion and malaoxon degradation kinetics and the formation of UV photo-oxidation intermediates were investigated.The results indicated that the degradation of malathion and malaoxon in these oxidation processes followed pseudo first-order kinetics.Under UV/H2O2,UV/TiO2 and UV/Fenton processes,their degradation rate were high and the removal efficiencies were above 85% after 30 minutes reaction.But under Fenton process,both malathion and malaoxon could only remove less than 10% at the same reaction time.Moreover,the degradation rate of malathion was significantly higher than that of malaoxon using the same oxidation process.The identification of UV photo-oxidation intermediates indicated that six and four kinds of photo-degradation intermediates were detected in malathion and malaoxon reaction solution,and only two of them were the same.Thus the differences in their degradation pathways resulted in the significant difference in their degradation rate.