Detection Of Pesticides In Water And Investigation Of Photo-oxidation Degradation Of Pesticides And The Influence On Disinfection By-product Formation Following Chlorination

Although the concentrations of pesticides and disinfection by-products(DBPs)in drinking water are usually low(?g/L to ng/L),these subatances could cause potentially adverse effects to human health.Thus,developing fast,accurate and low cost detection methods of these substances is becoming one of the important research topics in water quality mangagement.This thesis is composed of two parts.In part I,simple and rapid detection methods were developed using gas chromatography-electron impact ionization mode-triple quadrupole mass spectrometry(GC-EI-MS/MS)and ultra performance liquid chromatography-electrospray ionization mode-triple quadrupole mass spectrometry(UPLC-ESI-MS/MS)for the determination of the most important volatile disinfection by-products and pesticides in drinking water.In part II,photo-oxidation degradation of organophosphorus and organochlorine pesticides were investigated,and the influence on post-chlorination and DBPs formation were was analysed.Previous studies have shown that conventional drinking water treatment processes are not effective for removing the pesticide residues from water,and a proportion of pesticide residues can be oxidized in the subsequent chlorination.However,chlorination can result in incomplete destruction of pesticide molecules,and consequently leading to formation of more toxic oxidation products.In addation,chlorine can also react with pesticides and the resulting degradation intermediates may alter or even enhance DBP formation potentials.UV photo-oxidation processes have received considerable attention for their effective destruction and mineralization of organic micro-pollutants in water,and have been considered under certain condition for pesticide residues removal in water treatment.In this part,the effects of photo-oxidation process types and reaction conditions on degradation kinetics,intermediates formation and degradation pathways of the two common pesticides(diazinon and atrazine)were investigated systematically.The influence of photo-oxidation pretreatment and the intermittent species on the formation of DBPs following chlorination was also studied.The main results of this study are summarized as follows:(1)A detection method for 13 volatile disinfection by-products(include THMs,HANs,HKs,and CHP)in water has been developed based on liquid-liquid extraction(LLE)and GC-EI-MS/MS in multiple reactions monitoring(MRM)mode.The precursor ions and product ions for the 13 analytes were identified based on the MS/MS,and optimal collision energy in MRM mode was determined.The effect of the addition of sodium sulfate in LLE on the extraction efficiency of the 13 analytes was analyzed,and the optimum addition amount of sodium sulfate for the LLE process was determined.On the basis of the above research results,the standard calibration curves of the 13 analytes were established.The precision,the accuracy and the sensitivity of the method are better than those of the US EPA method 551 and US EPA method 551.1.(2)Through systematic experimental research,the reason for the loss of detection signal of the 15 pesticides in direct injection liquid chromatography-tandem mass spectrometric method(DI-LC-MS/MS)analysis were analysed and determined.A simple,feasible and effective method to alleviate or even eliminate the above negative effect on signal intensity for pesticides was proposed and based on which a rapid detection method for 15 pesticides in water has been established.The research shows that the addition of methanol into water sample could change the polarity of the solution and the van der Waals attraction force between hydrophobic organic analytes in solution and hydrophobic surfaces of the system.This can substantially alleviate or even eliminate the losses of pesticide caused by surface adsorption in detection system.It is found that an addition of methanol into standard solution of 15 pesticides could effectively alleviate or eliminate the negative effect caused by the adsorption.In addition,the study also found that addition of methanol into real water samples(TSW)as a modifier was shown to be effective in alleviating or even eliminating the negative effect on signal intensity caused by the adsorption effects for the late-eluting analytes,and at the same time being able to reduce or eliminate the detection signal suppression for the early-eluting pesticides caused by certain matrix effects.Based on the above research results,TSW has been chosen as a model water sample matrix,analytes-spiked mixed standard solutions of 30%Me OH-TSW have been used for method validation experiments.The linear calibration range extended from 0.05 to 10?g/L and low LODs were obtained under without sample pre-concentration,and has good precision and accuracy.The proposed method can meet the requirements of fast detection 15pesticides in water.(3)The effects of UV irradiation time and solution p H as well as H₂O₂ dose on diazinon degradation kinetics,formation of intermediates,degradation pathways,and the effects of the intermediates on DBPs formation following chlorination were investigated.The results indicated that the direct UV photolysis of diazinon in water followed pseudo first-order reaction kinetics,and that the rate of diazinon degradation was lower at acidic condition than at neutral and alkaline conditions.The differences of UV-vis absorption or light quantum yield because of the different molecular forms of diazinon at different pHs affect its degradation rate.The speciation and formation of photolysis intermediates were also found to vary with solution p H.There are 5,8 and 6major species of photolysis intermediates after 60 minutes of UV irradiation at p H of4.0,7.0 and 10.0,respectively.At p H 7.0,the degradation rate of diazinon in the UV/H₂O₂ process was higher than that in UV irradiation alone,and the reaction rate constant(k_obs)increased as H₂O₂ concentration increased.There are six kinds of main photo-degradation intermediates for diazinon after UV/H₂O₂ oxidation,and the speciation of intermediates remained unchanged as the H₂O₂ dose increased,though the amounts of these intermediates were different under different H₂O₂ dose conditions.The possible photo-degradation pathways of diazinon at different solution p H(4.0,7.0and 10.0)and in the presence of H₂O₂ were proposed based on the MS/MS mass-spectra results of the photo-oxidation intermediates and the formation of these products.The main reaction scheme and routes of degradation include the cleavage of the diethyl phosphorothioate moiety or diethyl phosphate moiety from pyrimidine group(cleavage of the pyrimidine ester bond),fracture opening of the pyrimidine ring,oxidation of isopropyl group and substitution of sulfur by oxygen on the P=S bond of diazinon.Six kinds of DBPs(include MCAA,DCAA,TCAA,TCM,1,1,1-TCP,and DCAN)were detected in chlorinated diazinon solutions treated by UV or UV/H₂O₂ process.The UV irradiation time,solution p H and H₂O₂ dose had significant effects on the DBPs formation of such treated diazinon solutions depending on the individual DBPs species during subsequent chlorination.The disinfection by-product formation potential(DBPFP)was found increased in the diazinon solutions as UV irradiation time increased.The increase in total DBPs formation was found to be attributable mainly to oxidation product IMP and its secondary oxidation products.Moreover,its oxidation fragment diazoxon intensified the formation of HAAs during UV/H₂O₂ pre-oxidation condition.(4)The effects of reaction conditions and the processes(UV,UV/H₂O₂,UV/TiO₂,and UV/H₂O₂/TiO₂)on atrazine degradation kinetics,the formation of intermediates and the degradation pathways were investigated.The effect of pre-oxidation on the DBPs formation in the treated atrazine solutions following chlorination were also sudied.The effect of H₂O₂,TiO₂ and pH condition on degradation efficiency of atrazine at low atrazine concentration(?100?g/L)was evaluated.The results indicated that the photo-degradation rate(k_obs)at different UV photo-oxidation process followed pseudo first-order degradation kinetics.Under direct UV photolysis conditions,k_obsdecreased with the increase of initial atrazine concentration,no significant impact of initial p H was found on the direct photolysis of atrazine.Under UV/H₂O₂ process,the k_obs of atrazine increased as the H₂O₂ dose increased from 0 to 10 mg/L,but it decreased with further increase of the H₂O₂ concentration.The k_obs under UV/Ti O₂ process was lower than that under the condition of UV irradiation alone and it decreased monotonically with the increase of TiO₂ addation.In addition,the k_obs of high initial concentration(5mg/L)was lower significantly than that of low initial concentration(100?g/L)for atrazine,and the effect of solution p H and H₂O₂ or TiO₂ dose on degradation rate of atrazine at high concentration was different from that at low concentration.A systematic qualitative and quantitative analysis of the photo-degradation intermediates under different photo-oxidation process were conducted.The molecular structure of the intermediates was identified based on the information of mass spectrometry.The results showed that the photo-oxidation process and solution p H can affect significantly on the speciation and the formation of photo-degradation intermediates of areazine.The concentration of H₂O₂ and TiO₂ influenced the formation of the intermediates of atrazine,but not change the speciation of intermediates.Based on the results,the possible photo-degradation pathways of atrazine under different p H conditions in different photo-oxidation process were proposed.This study found that the photo-oxidation pretreatment will increase chlorine demand and DBPs formation of the atrazine solutions following chlorination,depending on the type of process,the catalyst concentration and solution p H.Five kinds of DBPs(include DCAA,TCAA,TCM,CHP,and 1,1,1-TCP),were detected for pre-treated atrazine solutions after chlorination.The DBPFP of atrazine solutions increased under treatment conditions of low solution p H and high H₂O₂ or TiO₂ concention.