Study On The Formation Of Halonitromethanes From Amines In The Process Of UV/Chlorine Disinfection

Halonitromethanes(HNMs),a typical nitrogen-containing disinfection by-products(N-DBPs),has attracted more and more attention due to its higher cytotoxicity and genotoxicity than other carbon-containing disinfection by-products(C-DBPs).Traditionally,scholars at home and abroad have carried out large numbers of researches related to this topic,including precursor identification,formation mechanism and control method.However,these studies were mainly limited to chlorine disinfection or chloramine disinfection,and there were few studies on emerging UV/chlorine disinfection.Besides,it is worth to mentioning that current research object was mainly concentrated on trichloronitromethane(TCNM)and there still lack systematic studies of HNMs,especially brominated HNMs which was much more toxic than chlorinated HNMs.By choosing amines as precursors,this paper deeply studied the formation and pathways of HNMs during the process of UV/chlorine disinfection.Additionally,this paper also investigated the photodegradation of dibromonitromethane(DBNM)and dibromonitrochloromethane(DBCNM)and their reaction pathways under UV light irradiation.The results can provide theoretical basis for controlling the generation of HNMs during the process of UV/chlorine disinfection.This paper firstly discussed the photodegradation characteristics of HNMs under UV irradiation.In the experiment,taking DBNM and DBCNM as the research objects,the effects of initial concentration,ultraviolet light intensity,p H,free chlorine and bromine ion on their photodegradation were investigated.Meanwhile,the photodegradation kinetics rules and reaction pathway of DBNM and DBCNM were carefully analyzed.The results demonstrated that the photodegradation of DBNM and BDCNM actually followed the law of first-order kinetics.To illustrate,while negatively related to the initial concentration,the photodegradation rate of DBNM and DBCNM were positively related to with the external light intensity and the p H of solution.Besides,the existence of Br~—could slightly raise the rate of photodegradation.Due to the contribution of free chlorine,the rate of photodegradation was significantly accelerated and other HNMs could be generated during the process.Secondly,this paper studied the formation rules and mechanism of HNMs from amines during the process of UV/chlorine disinfection.By choosing methylamine(MA),dimethylamine(DMA)and polydiallyldimethylammonium(poly DADMAC)as precursors,the influence factors on the formation of HNMs(e.g.the concentration of precursor,free chloride and Br~—,p H,light intensity)were investigated in the experiments,and the changes of free chlorine,combined chlorine,total chlorine,ammonia nitrogen,nitrate nitrogen and total nitrogen were analyzed.Besides,the reaction pathway of HNMs from amines was also deduced based on the experimental results and related literature.The results indicated that when Br~—presented,MA,DMA and poly DADMAC all had potential for generating HNMs during the process of UV/chlorine disinfection,and the order of their potential was MA>DMA>poly DADMAC.In the experiments,the formation of HNMs from MA,DMA and poly DADMAC were enhanced at the beginning,then gradually decreased with increasing the reaction time.On the other hand,with the concentration of Br~—increased,the production of chlorinated HNMs would correspondingly decrease and the production of mixed chlorinated/brominated HNMs would increase initially but gradually decrease,while brominated HNMs would continuously grow in number.What’s more,as the concentration of free chlorine increases,the production of chlorinated HNMs and mixed chlorinated/brominated HNMs would correspondingly increase,while brominated HNMs would increase initially and then gradually decrease.It is worth to mentioning that the productions of nine HNMs increased with the increasing of light intensity and precursor concentration,while decreased with the increase of p H value.Finally,the actual water samples of sewage and water supply plants(before disinfection)and after adding amines were selected to study the formation law of HNMs in the process of UV/chlorine disinfection.The results showed that the formation trend of HNMs from actual water under UV/chlorine disinfection was similar to that in laboratory when amines were used as precursors.Besides,the formation rules of HNMs from amines in actual water were basically the same as that in ultra-pure water.However,due to the complexity of the components in the actual water samples,the amount of HNMs generation,the time point when the maximum generation and the final remaining amount were different from the results of the laboratory ultra-pure water simulation experiment.