Effects Of Medium Pressure UV/Chlorine Oxidation And Coexisting Ions On Disinfection Byproduct Formation Of Five Typical Amino Acids

UV/Chlorine Advanced oxidation process（UV/Cl₂AOP）can introduce high activity HO·and a variety of chlorine radicals in situ on the basis of the existing chlorine disinfection in water plants.In order to effectively remove a variety of organic pollutants in water,such as pharmaceuticals and personal care products,endocrine disruptors and algal toxins,has great application potential in the advanced treatment of drinking water,is the current research focus.However,chlorine free radicals can react with natural organic compounds in water to produce some highly toxic DBPs,which is a potential problem in the application of UV/Cl₂ in water treatment.At the same time,the presence of bromine,ammonium and nitrate ions in water can transform chlorinated disinfection by-products（Cl-DBPs）into bromine（Br-DBPs）or nitrogenous disinfection by-products（N-DBPs）,which are more toxic.Therefore,it is of great theoretical significance to evaluate the effects of UV/Cl₂ AOP and typical ions on DBPs generation and species.In this paper,amino acids which accounted for a large proportion of dissolved organic nitrogen were selected as precursors,including two kinds of aspartic acid and tryptophan with high chlorine activity,and three kinds of asparagine,phenylalanine and proline with relatively low chlorine activity or even inert.Medium pressure UV lamp（MPUV）,the most widely used in water plants,is used as UV light source.The MPUV/Cl₂ advanced oxidation treatment of five amino acids was carried out under the conditions of"high chlorination（3-10 mg/L）and short irradiation time（6-40 s）"in the actual water plant application,and the post-treatment was carried out under the conditions of actual chlorination of pipe network.To evaluate the effect of MPUV/Cl₂AOP on DBPFP of five amino acids in China.In addition,the effects of p H of MPUV/Cl₂ AOP treatment system and typical coexisting ions（bromine,ammonium and nitrate ions）in water on DBPFP were also studied.Compared with short-time dark chlorination,MPUV/Cl₂ AOP reduced DBPFP of the strong active precursor aspartic acid（647.2-957.6μg/mg C）and tryptophan（289.7-398.5μg/mg C）.However,DBPFP of low chlorine active amino acids asparagine（7-12.6μg/mg C）,phenylalanine（66.0-218.9μg/mg C）and proline（25.1-47.8μg/mg C）could be promoted to varying degrees,and the effect was more significant under acidic conditions.In addition,aromatic amino acids（tryptophan and phenylalanine）are more susceptible to MPUV/Cl₂AOP than aliphatic amino acids（aspartic acid,asparagine and proline）.MPUV/Cl₂AOP reduced the cytotoxicity of DBPs produced by high chlorine active amino acids,but increased the cytotoxicity of DBPs produced by inert amino acids.The total DBPs calculated cytotoxicity of aspartic acid and tryptophan decreased by 18.9-30.1%（P<0.05）and 52.9-70.0%（P<0.05）,respectively,mainly due to the reduction of MPUV/Cl₂-induced Haloacetonitriles（HANs）.In contrast,the cytotoxicity of asparagine,phenylalanine and proline showed an opposite trend,and phenylalanine had the most significant effect,which increased the cytotoxicity of DBPs by 19.5-73.0times（P<0.05）.Effects of coexisting ions It is found that the presence of bromine ions can promote the formation of Trihalomethanes（THMs）of three active precursors.However,the decrease of the generation of Haloacetic Acids（HAAs）,Haloacetaldehydes（HALs）,HANs and Trichloronitromethane（TCNM）results in the decrease of the total DBPFP.Meanwhile,the presence of bromine significantly increased the calculated cytotoxicity of DBPs produced by aspartic acid,phenylalanine,proline and asparagine（38.8-499.1%,P<0.05）,while the cytotoxicity of DBPs produced by tryptophan decreased by19.7-46.7%（P<0.05）.The presence of bromine incorporation Factor（BIF）can also affect the degree of bromine substitution in DBPs.For two aromatic amino acids,MPUV/Cl₂ AOP increases the BIFs of THMs,HAAs and HANs,while for the other three aliphatic amino acids,MPUV/Cl₂ AOP increases the BIFs of THMs but slightly decreases the BIFs of HAAs and HANs.In the presence of ammonium ions,the production of HAAs and HANs is reduced to varying degrees due to the rapid reaction between chlorine and amino acids,the production of RNS under UV irradiation and the quenching effect of chloramine on HO·and Cl·,but TCNM generation is promoted.Nitrate ions can promote the generation of HANs and HNM,but reduce the generation of HAAs,HALs and total DBPFP,which may be related to RNS produced by NO₃^-photolysis.In addition,the presence of ammonium and nitrate ions reduced the total DBPs calculated cytotoxicity of the five amino acids.