The Efficiency And Mechanism Of Microcontaminants And Byproducts Control In UV/chlor(am)ine Combined Process

With the rapid development of industry,agriculture,and service industry in China,more and more contaminants of emerging concern（CECs）have been discharged into water.As some CECs cannot be effectively removed through the traditional drinking water treatment process,resulting in their entry into the disinfection process to generate disinfection by-products（DBPs）and into the distribution system,causing a threat to the safety of drinking water.Therefore,considering the current complex water quality conditions,there is an urgent need to develop more effective new processes and technologies for water treatment.This study proposed a combined process,UV/chlorine,UV/chloramine,and vacuum UV-chlorine,which can efficiently remove contaminants in water including diatrizoate（DTA）,ranitidine（RAN）,bisphenol A（BPA）,etc.,and proposed a control method for disinfection byproducts in the combined process.The conversion and contribution of free radicals in the combined process were investigated by free radical quenching,radical electron paramagnetic resonance（EPR）tests,and radical probe compounds.Nitrobenzene（NB）and benzoic acid（BZA）were selected as radical probe compounds to quantify the steady-state concentration of radicals and the contribution of each radical to the degradation of contaminants in the combined process.In the UV/chlorine combined process,the reactive chlorine species（RCS）played a major contribution to the degradation of DTA,followed by HO^·;in the UV/monochloramine process,RCS and HO^·contributed approximately equally to the degradation of DTA,and RCS,HO^·and reactive nitrogen species（RNS）all contributed to the degradation of RAN,of which HO·played a major role;in the UV/organic chloramine process,HO^·was characterized for the first time by EPR,and HO^·and Cl^·were the main contributors to the degradation of BPA,and RNS was almost absent in the system.The effects of typical water quality parameters（solution pH,chloride ions,bicarbonate,and natural organic matter）on the degradation efficiency of contaminants in the combined process were investigated.At pH 6.0,the degradation efficiency of contaminants in the combined process was higher than at pH 7.0 and pH 8.0.The efficiency of contaminant degradation in the combined process decreased with increasing concentrations of chloride,bicarbonate,and natural organic matter by reacting with radicals in the combined process to generate selective weak oxidation radicals and absorb irradiation.Through the competitive kinetics method,the second-order reaction rate constant for CO₃^·-and RAN was obtained for the first as 8.05×10⁶M^-1 s^-1.The degradation pathway of contaminant and the generation mechanism of disinfection by-product precursor in the combined process was investigated.The reaction of the UV/monochloramine process to degrade RAN included C-S bond breaking,hydroxyl addition,chlorine substitution,electron transfer,dimethylamine group dropping,etc.The degradation products include amino-containing products and N-nitrosodimethylamine（NDMA）proving that RNS contributed to the degradation of RAN.The reaction of the UV/organic chloramine process to degrade BPA can be divided into the attack on the benzene ring,the attack on the isopropyl group,and the cleavage of the bridged isopropyl group.The involvement of HO·,Cl·,and（CH₃）₂N·in the degradation of BPA was demonstrated by the hydroxylated products,chlorinated products,and products containing dimethylamine groups detected in the degradation products.In addition,a quantitative structure-activity relationships（QSAR）model between the concentration of formed trichloronitromethane（TCNM）and the Hammett constant of the model compound was developed in this study.The model in the UV-based process showed that benzoic acid compounds with smaller Hammett constants produced more TCNM;the model in the VUV process showed that phenolic compounds with smaller Hammett constants produced more TCNM,and benzoic acid compounds were affected by hydration electrons in the vacuum UV system,resulting in more TCNM production for benzoic acid compounds with larger Hammett constants.Gaussian quantum chemistry software calculations showed that NO₂·was more likely to attack the ortho and para positions on phenol and undergo electrophilic substitution reactions to produce the corresponding TCNM precursors.The toxicity of the solutions and DBPs after the treatment of the combined process was calculated using luminescent bacteria,ECOSAR software,and Chinese hamster Ovary cell data.The results showed that although the toxicity of some products was reduced after the treatment of UV/monochloramine and UV/organic chloramine processes,the acute toxicity of the overall solution was elevated.The TCNM generated in the combined process has high genotoxicity,resulting in the majority of the genotoxicity of the generated disinfection by-products from TCNM.The control method of disinfection by-products in the combined process has been proposed.For the UV/chlorine process,dichloroacetonitrile（DCAN）and trichloromethane（TCM）can be reduced by reducing chlorine dosage;for the UV/chloramine process,DCAN and TCM can be reduced by increasing the dosage of monochloramine,and TCNM can be reduced by controlling the pH as acidic and using monochloramine disinfection,and NDMA can be reduced by controlling the pH as alkaline and using chlorine disinfection.For the VUV-chlorine process,nitrate and natural organic matter should be removed from the water,thus reducing the production of TCNM.In conclusion,the UV/chlorine,UV/chloramine,and VUV-chlorine combination disinfection process proposed in this study can not only efficiently remove CECs represented by DTA,RAN,BPA,etc.in water,but also control DBPs formation by optimizing the combination disinfection process conditions.Therefore,the combined process in the drinking water treatment has a bright future for practical applications.