Computational Insights Into The Formation Mechanisms Of Disinfection By-products Tobacco-specific Nitrosamine And Trichloronitromethane During Drinking Water Disinfection

With the increasing pollution of drinking water and the improvement of disinfection process,many countries gradually use chloramine disinfectant instead of traditional chlorine disinfectant to reduce the concentrations of typical carbonaceous disinfection by-products(C-DBPs),such as trihalomethanes(THMs),haloacetic acids(HAAs)and mutagen X(MX).However,this leads to the emergence of nitrogenous disinfection by-products(N-DBPs)with higher toxicity,such as haloacetonitriles(HANs),nitrosamines(NMs),haloacetamides(HAcAms),and halonitromethanes(HNMs).These new N-DBPs are formed by the reaction of disinfectants with dissolved organic matter in water,especially dissolved organic nitrogen compounds.Among the N-DBPs,NMs and HNMs are of great concern due to their extensive detection in water and potential high threat to human health.Tobacco-specific nitrosamines(TSNAs)is a special NMs mainly existing in tobacco products and tobacco smoke,which can cause the tumors of lung,nasal cavity and esophagus in human bodies.The recent studies have found that TSNAs can also be formed as N-DBPs in the process of drinking water disinfection,and their precursors are identified to be tobacco alkaloids,such as nicotine,neonicotine and nornicotine.Due to the high cytogenetic toxicity and mutagenicity,HNMs has been listed as the priority controlled DBPs by national environmental protection agency.Trichloronitromethane(TCNM)is the first HNMs found in drinking water,and itsformation potential is higher than other HNMs in water.Moreover,the amino acids,nitromethane,aniline,nitrophenol and algae have been identified as the main precursors of TCNM.Although the precursors of TSNAs and TCNMs have been partially identified,the studies of their formation potential and reaction path are very limited,which is not conducive to the effective control of the precursors or formation potential of these DBPs.In recent years,theoretical calculation methods are playing an increasingly important role in the studies of physical and chemical properties,transformation mechanism and toxicology of environmental pollutants.Therefore,the quantum chemical calculations were applied to simulate the transformation mechanisms of tobacco alkaloids and p-toluidine into the respective TSNAs and TCNM in the process of water disinfection.The mechanisitc details and the related kinetics for the reaction pathways were clarified,and the dominant reaction pathway and products were revealed.These results may provide a solid theoretical basis for further studies on the formation potential and mechanism of these DBPs.The main contents of this thesis are as follows:(1)Based on density functional theory(DFT)calculations,we explored the formation mechanisms of TSNAs 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanol(NNAL),N'-nitrosoanabasine(NAB)and N'-nitrosonornicotine(NNN)from respective tobacco alkaloids nicotine(NIC),nicotine(ANA)and nornicotine(NOR)during the water disinfection with chloramine.The results show that the pyrrolidine moiety of NIC will react with NH2 Cl to generate an amino intermediate,followed by the two hydroxylations of amino group by HClO to form a dihydroxylated intermediate.This intermediate subsequent undergoes amino dehydration,oxidative ring opening and hydrolysis to produce the product NNAL.The NNAL formation from NIC was identified as the major pathway of TSNA formations with the rate-determining barrier of 18.79 kcal/mol.Besides,the molecular structures of NOR and ANA are similar with that of NIC,and they can also undergo amination,hydroxylation and dehydration steps to generate NNN and NAB as the minor products.This study revealed the molecular mechanisms of TSNA formations from tobaccoalkaloids at the molecular level,which could provide theoretical basis for improving the experimental detection and formation mechanism of these DBPs.(2)Halonitromethanes(HNMs)have recently attracted strong interest due to its extensive detection in environment and potential toxicological effects on humans.In this work,the density functional theory(DFT)calculations were performed to unravel the mechanistic details of trichloronitromethane(TCNM)formation from p-toluidine during the water disinfection with chlorine.The results show that protonated p-toluidine has higher reactivity than the neutral form,and the reaction first undergo?-hydroxylation to generate the exothermic dihydroxy intermediate,followed by the dehydrogenation to form ?-carbonyl intermediate.The subsequent ClO~– attack on?-carbonyl and ring-opening of the six-membered ring intermediate further facilitate the formation of product TCNM.This study for the first time reveals the complexity of reaction mechanism and the relevant kinetics at the molecular level,which may provide meaningful guidance to the further study of the formation potential and reaction mechanism of HNMs from different precursors during the disinfection process in drinking water.