Study On The Removal Mechanism And Product Toxicity Of Primidone And Caffeine By UV/chlorine Advanced Oxidation System

As new contaminants in water environment,Pharmaceuticals and personal care products(PPCPs)have harmful effects on organisms even at trace levels because of their "pseudo-persensitivity" and biological activity.With the development of social economy and the rapid growth of human activities,this kind of compounds are inevitably released into the natural environment,which may have unpredictable consequences for the ecosystem.Recently,wastewater from wastewater treatment plants has become the main source of PPCPs in the environment because studies found that the traditional water treatment process is difficult to degrade this kind of material,thus,the effective removal of PPCPs from wastewater is very important for human health and reducing environmental risks.UV/chlorine advanced oxidation process(AOP)is considered as a potential alternative process because of its advantages of fast reaction rate,less secondary pollution and the ability to produce non-selective radicals ·OH,which has been favored by scholars at home and abroad.Primidone(PRM)and Caffeine(CAF),common PPCPs in water were selected as target contaminants in this study,the degradation efficiency and reaction mechanism of PRM and CAF in UV/chlorine advanced oxidation process were analysed,and the effects of pH,oxidant dosage,contaminants concentration,common anions(Cl-?HCO3and NO3-)and Effluent Organic Matter(EfOM)on the degradation were investigated.The decay of PRM and CAF in Fenton-like system and UV/NO3-system were studied;Meanwhile,The degradation mechanism of PRM and CAF in UV/chlorine system was clarified,the mineralization of TOC and the formation of small molecular acids in four different conditions were compared;As an important precursor of disinfection by-products(DBPs),the mechanism of EfOM effect on the reaction was analyzed,and the cytotoxicity level of the products under the reaction system was systematically evaluated by synthesizing the production of Adsorbable Organic Chlorine(AOCl)and cell inhibition experiments.The following conclusions were drawn from the above experiments:PRM and CAF could be removed efficiently in UV/chlorine AOPs,and the reactions followed the first order reaction kinetic model(R2>0.97).When initial concentration of contaminants and free chlorine(FC)dosage was 5 ?M and 70 ?M respectively,the removal rates of PRM and CAF at pH 7 were 84%and 99%after 10 min reaction,respectively.The degradation of contaminants were mainly attributed to the formation of ·OH and Reactive Chlorine Species(RCS,Cl·,ClO·,Cl2·-).Radical scavenging experiments showed that ·OH and ClO· play a leading role in PRM degradation,·OH,ClO· and Cl· had strong reactivity with CAF,Cl2·-did not participated in the conversion process of the two pollutants.The results of single factor experiments showed that,pH had great effects on PRM and CAF degradation,showing that acidic conditions facilitated the reaction but with the increase of pH,the scavenging effects of free radicals were gradually enhanced,and the quantum yield of free chlorine decreased.PRM degradation rate increased firstly and then remained constant as the FC dose increased,degradation of CAF accelerated with increasing FC concentration.Second,increasing the concentration of initial contaminants had adverse effects on PRM and CAF degradation.Common anion interference experiments showed that the quasi-first-order kinetic rate constants(kobs)for PRM and CAF degradation are almost unaffected by Cl-,the capture of free radicals by HCO3-inhibits PRM and CAF degradation,while the introduction of NO3-greatly promote the degradation of the two pollutants.To further explore the mechanism of NO3-,the degradation of PRM and CAF in UV/NO3-system were analyzed and free radical identification was carried out.The results showed that the existence of NO3promoted the degradation of PRM and CAF because the concentrations of ·OH were greatly improved in the system,and the reactions were accelerated with the increase of NO3-concentration.The introduction of Fe3+ under pH 3 conditions significantly promoted the degradation of contaminants with the increase of Fe3+ concentration,which was mainly due to the increase of 'OH concentrations in the system.The addition of two EfOM(Hydrophobic acids,HPO;Transphilic acids,TPI)extracted from wastewater had inhibitory effects on PRM and CAFdegradation by removing ClO·.Compared with TPI,HPO had a stronger effect due to its stronger aromaticity.Real water degradation simulation tests found that the kobs of the two contaminants in the collected wastewater was low because of the high TOC contents and alkalinity of real water.UV/chlorine AOP had a certain mineralization effect on PRM and CAF.The intermediate products of PRM and CAF in UV/chlorine AOP were detected by HPLC-MS and GC-MS.It was found that the main degradation mechanism of the two pollutants was the continuous hydroxylation process in the system.PRM could be oxidized to butyric,maleic,acetic,formic acid,oxalic,lactic,formic and acetic,The conversion of CAF produced aromatic intermediates and citric,after that,citric could be further oxidized to maleic,lactic,and decomposed into acetic and formic.Meanwhile,it was found that the nitrogen elements in the PRM structure eventually existed in the form of NO2-and NH4+,and under the action of various active free radicals NH4+ could transform into NO2-,nitrogen elements in CAF structure are continuously transformed into NH4+.FT-ICR-MS results showed that the chemical composition of the two EfOM was different,Compared to TPI,HPO was more aromatic.The aromatic rings in the EfOM broke down into aliphatic compounds in chlorination and UV/chlorine AOP,the constant attack of ·OH and RCS in the UV/chlorine system caused the transformation efficiency of the EfOM more efficient and the products showed less aromaticity,and more Cl-DBPs were formed in UV/chlorine AOP,Which was mainly attributed to the role of Cl' and ClO·.Analysis showed that the main reaction type in UV/chlorine system were substitution reaction.The detection of toxic compounds showed that the amount of AOCl produced in the PRM and CAF degradation systems after 10 min reaction was 28.3 and 14.0 ?g/L,respectively.The addition of EfOM increased the concentrations of AOCl by about 8 times and 17 times,respectively.ClO3-yield in solution increased with the initial FC dose,when the initial concentration of FC was 210 ?M the production of ClO3-reached a maximum of 130 ?M.The addition of different concentrations of Cl-and Br-had little effects on the formation of ClO3-,and there was no BrO3-detected in the system.Cytotoxicity tests showed that except for PRM,the cytotoxicity of contaminants and EfOM treated by UV/chlorine or chlorination process decreased,and the effects increased with the prolongation of reaction time.HPO greatly promoted the formation of reactive oxygen species(ROS),resulting in the inhibition of ATP production,DNA damage and cytotoxicity increase.Samples containing TPI did not show cytotoxicity after the reaction.