Formation Mechanism Of Halonitromethanes And The Effect Of Ozonation In Drinking Water

Halonitromethanes(HNMs)are typical nitrogenous disinfection byproducts(DBPs)formed during chlorination.Due to its high genotoxicity,HNMs attract more and more concern.Currently,there are few studies on the formation mechanisms of HNMs,especially the effect of ozonation on halonitromethane formation potentials(HNMFPs)and its mechanisms.In this study,typical amines,azo compounds and nitro-compounds in drinking water were selected as model compounds.The formation characteristics and mechanisms of HNMs for these model compounds during chlorination and the combination of ozonation and chlorination processes were studied.Moreover,the mechanism of the effect of bromide on HNMFPs of amines after ozonation was studied.This study provides a theoretical basis for ensuring the safety of drinking water.Their formation kinetic models of trichloronitromethane(TCNM)for typical nitrogenous organic compounds and natural water bodies during chlorination were established.Results showed that the TCNM formation kinetics of nitrogenous compounds and natural water bodies include a fast reaction phase and a slow reaction phase.The formation of TCNM at two days reached more than 90% of the highest TCNM formation.The two-compartment first-order model fit the TCNM formation kinetics well.The molar conversions of HNMFPs for azo compounds(0.1%-15%)and nitrocompounds(1%-53%)were high,because nitro-compounds are the direct precursors of HNMs,and chlorine can easily oxidize the azo bond into nitro group.When a methyl group was attached to an azo bond in an aliphatic compound,or there is a functional group(such as: hydroxyl radical,alkyl,amino,amide,etc.)that can promote the opening of the benzene ring which linked to azo bonds,their HNMFPs will be higher.Moreover,because chlorine can hardly oxidize amino group into nitro group,the molar conversions of HNMs for amines were low(mostly less than 0.8‰).Since ozone can oxidize the amino group to nitro group,the HNMFPs of the amines significantly increased(2-2000 times)after ozonation.When the amino group contains a methyl group,or the ?-carbon contains a leaving group that promotes the cleavage of the C-C bond,the HNMFPs increases at greater magnification after ozonation.While the HNMFPs of aromatic azo compounds decreased(by 60%-98%)after ozonation.That's because ozone can oxidize the azo bond in the aromatic azo compounds into nitro compounds,and the hydroxyl radical decomposed by ozone can break the C-N bond,and produce nitrate,which leads to the reduction of HNMFPs.During ozonation of amines,bromide can react with ozone and forms hypobromous acid,which reacts with primary amines and secondary amines and produces brominated organic amines.The brominated organic amines can be oxidized into nitro-organic compounds by ozone.In this way,bromide increased the efficiency of the oxidation of organic amines into nitro-organic compounds.As nitro-organic compounds are important precursors of HNMs,compared with the absence of bromide,the increasement of HNMFPs in the presence of bromide during ozonation was promoted(mostly more than 1 time),and the bromide incorporation factors of HNMs were higher than that of trihalomethanes.