Study On The Formation Of Brominated Halonitromethanes From Amino Acids Under UV/Chlorine Disinfection

Brominated halonitromethanes(HNMs)are typical nitrogen-containing disinfection by-product,which have the risk of carcinogenicity,teratogenicity and mutagenicity.The cytotoxicity and genotoxicity of brominated HNMs are at the forefront of HNMs,which pose a great risk to water safety and public health.Therefore,this paper selected threonine,tryptophan and aspartic acid as precursors,and studied the formation rules and generation pathways of brominated HNMs during UV/chlorine disinfection.Meanwhile,the photodegradation rules and pathways of bromochloronitromethane(BCNM)and tribromonitromethane(TBNM)were investigated under UV irradiation.Finally,the raw water samples were used to verify the formation rules of brominated HNMs during the UV/chlorine disinfection in laboratory.This study can provide the scientific evidence and fundamental data for the control of brominated HNMs formation during UV/chlorine disinfection in water treatment.Firstly,this study investigated the effects of UV light intensity,initial concentration,pH,free chlorine and bromide ions concentration on the photodegradation of BCNM and TBNM.The results showed that the photodegradation rates of BCNM and TBNM were more than 75% after 5 min under UV irradiation.The photodegradation of BCNM and TBNM under UV irradiation followed the quasi-first-order reaction kinetics equation when the concentrations of them were in the concentration range of this study.Besides,the photodegradation rates of BCNM and TBNM increased with the increase of UV light intensity,pH,free chlorine concentration concentration,and slightly increased with the increasing bromide ions concentration,while decreased with the increase of initial concentration.The photodegradation pathways of BCNM and TBNM were deduced based on the experimental results and related literatures.The experimental results in this chapter could help to control the formation of brominated HNMs in water,and provide an explanation for the increasing and decreasing trend of brominated HNMs formation from amino acids in the process of UV/chlorine disinfection.Secondly,this paper selected threonine,tryptophan and aspartic acid as precursors,and investigated the effects of free chlorine concentration,bromide ions concentration,UV light intensity,precursor concentration and pH on the formation of brominated HNMs from amino acids,and the changes of ammonia nitrogen,nitrate nitrogen,total nitrogen,free chlorine and total chlorine were analyzed.Besides,the reaction pathways of brominated HNMs from amino acids were deduced based on the experimental results and related literatures.The results showed that when bromide ions were present,threonine,tryptophan and aspartic acid had the potential to generate brominated HNMs under UV/chlorine disinfection,and the rank of generation potential was aspartic acid> threonine> tryptophan.During UV/chlorine disinfection,bromonitromethane(BNM),dibromonitromethane(DBNM),TBNM and bromodichloronitromethane(BDCNM)were generated in the threonine and trypotphan reaction system,while BNM,DBNM,TBNM,BCNM,BDCNM and dibromochloronitrome-thane(DBCNM)were generated in the aspartic acid reaction system,and the productions of them increasd initially and then decreased with the reaction time.In the experiments,the productions of brominated HNMs generated from threonine,tryptophan and aspartic acid increased with the increase of free chlorine concentration.Additionally,with the increasing concentration of bromide ions,the productions of BNM and BDCNM generated from threonine and tryptophan,as well as BNM,BCNM,BDCNM and DBCNM generated from aspartic acid increased initially and then decreasd,while the productions of DBNM and TBNM generated from each amino acid continuously increased.Besides,the productions of brominated HNMs generated from each amino acid increased with the increase of UV light intensity and precursor concentration.Moreover,the productions of brominated HNMs generated from threonine and tryptophan decreased with increasing pH,while aspartic acid generated more brominated HNMs at pH=7 than at pH=6 and 8.Finally,this study investigated the formation of HNMs in raw water under chlorine and UV/chlorine disinfection,and the effects of bromide ions concentration and amino acids on the production of brominated HNMs in raw water were investigated.Meanwhile,the formation rules of brominated HNMs obtained in the laboratory were verified.The results showed that some HNMs were generated in raw water during UV/chlorine disinfection,and the formation trends were similar to that of the laboratory.When bromide ions were added,the productions of brominated HNMs generated in raw water during UV/chlorine disinfection increased with the increase of bromide ions concentration.Besides,the formation trends of brominated HNMs generated in raw water containing amino acid were similar to that in laboratory,but there were differences in the production of brominated HNMs and the time point when the peak appeared.