The Effect Of Ozonation On Nitrosamines Precursors And Its Mechanism

More and more water plants use ozone as a pre-oxidation process to deal with the growing problem of organic pollution in water.However,ozone can not completely mineralize organic matter,will react with some organic matter directly generated N-Nitrosodimethylamine(NDMA)or precursors with higher NDMA molar conversion,which can bring a great threat to the subsequent disinfection process.In this paper,10 different precursors with dimethylamine((CH3)2N-)group were selected to study NDMA formation from ozonation of those precursors and the effect of pre-ozonation during the subsequence disinfection.It was found that all the selected 10 precursors with dimethylamine group were able to react with ozone to form NDMA.It was also found that the ozonation of precursors with a structure of(CH3)2N-R-N-generated a higher NDMA molar conversion than that of compounds only with a structure of(CH3)2N-.Moreover,precursors with a structure of(CH3)2N-N-or(CH3)2N-R-N-,where R was a good leaving group or an electron withdrawing group,had a high likehood of forming NDMA with ozonation.The NDMA formation from some precursors can be reduced by ozonation prior to disinfection.Taking ranitidine as an example,the effect of ozonation prior to chloramination disinfection on NDMA formation and the reaction pathway were determined.The results demonstrated that low dosages of ozone(0.5mg/L)reduced the NDMA molar conversion sharply.The NDMA molar conversion was influenced by pH of the pre-ozonation with the highest value at pH 8.The scavenging of hydroxyl radical during pre-ozonation resulted in increasing NDMA formation.NOM enhanced NDMA generation from ranitidine during chloramination,especially with pre-ozonation.Without ozone dosage,the presence of bromide ion reduced the NDMA molar conversion sharply.However,with higher ozone dosage,the NDMA molar conversion of ranitidine with bromide ion was higher than that in the absence of bromide ion.Combined with Q-TOF spectra and GC-MS can be found that ozonation preceding chloramination led to the formation of DMA,which had much lower NDMA molar conversion compared with ranitidine.The NDMA formation from some other precursors can be enhanced by ozonation prior to disinfection.Taking daminozide as an example,the effect of ozonation prior to chlorination and chloramination on NDMA formation and the reaction pathway were determined.It was found that pre-ozonation of daminozide and followed by chlorination and chloramination could cause NDMA formation.TheNDMA molar conversion was influenced by pH of the pre-ozonation with the highest value at pH 8.The scavenging of hydroxyl radical during pre-ozonation resulted in increasing NDMA formation.NOM and bromide ion reduced NDMA generation from daminozide.Combined with Q-TOF spectra and literature can be found that ozonation preceding chloramination led to the formation of N-Oxide,which had higher NDMA molar conversion compared with daminozide.According to the characteristics of precursors and water sources,rational optimization of pre-oxidation process can reduce NDMA generation.The results provide theoretical basis and technical support for the use of ozone.