Research On N-nitrosamines Formation During Disinfection And Removal Methods Of Their Precursors

As reported in the recent studies, N-nitrosamines have been detected as new types of disinfection by products in the water treatment process. According to their high cancinogen risk, it is necessary to study the formation mechanisms of N- nitrosamines, and to develop some effective technologies for N-nitrosamines formation control. The two aspects mentioned above were both discussed in this study. Several direct precursors were used as the model precursors during chorination disinfection and ozonation treatment process to discuss some important influencing factors for N-nitrosamines formation. Furthermore, combined oxidation and adsorption process were both used for N-nitrosamines precursors removal and N-nitrosamines formation control.At the beginning of this study, the influencing factors and formation mechanisms of three different secondary N-nitrosamines from their relevant precursors (dimethylamine, methylethylamine, dimethylamine) during chlorination and chloramination was discussed. Low yields of N-nitrosamines were detected when chlorine was used as the disinfectant, and no significant relationship was found between the N-nitrosamines yields and chlorine dosage. After the addition of NH4+, the yield of N-nitrosamines was much higher than that without the presence of NH4+. And the N-nitrosamines increased first then decreased with the increasing dosage of NH4+. The species of chloramine might be one of the most important influencing fators for N-nitrosamines formation. The results showed that dichloramine could transfer presursors into N-nitrosamines more effectively than monochloramine. The species of chloramine can be influenced by the solution pH value and the Cl/N raitio of disinfection reagent, while the pH value can also change the protonation state of precursors which is also an important factor for N-nitrosamines formation. The results indicated that these three N-nitrosamines could be formed via similar pathways,i.e., the secondary aliphatic amines first react with chloramines into reactive intermediates (secondary unsymmetrical hydrazine or chlorinated secondary unsymmetrical hydrazine), which could be further oxidized into relevant N-nitrosamines. With the coexistence of nitrate ion, nitrite ion and natrual organic matter, the formation of N-nitrosamines during chloramination would be limited. However, the bromide ion co-existed in the water would increase the yield of N-nitrosamines by leading the formation of halamines.Different phenomenon appeared when two tertiary amines, i.e. trimethylamine (TMA) and 3-(dimethylaminomethyl) indole (DMAI), were used as the model precursors of N-nitrosodimethylamine (NDMA) during chloramination. But dimethylamine (DMA) , the most direct precursor of NDMA, was both detected when these two tertiary amines were treated by chloramines. With the same expriment condition, more DMA and NDMA can be formed from DMAI than that form TMA of the same mole concentration. It should be noticed that DMAI can transfer into NDMA even more effectively than DMA during disinfection by dichloramine. It indicates that some tertiary amines can form N-nitrosamines via some effectively pathways which haven't been reported before, rather than that including the DMA formed from its degradation during chlorination/chloramination. The co-existed nitrate ion, nitrite ion,bromide ion and natural organic matter all restrain the formation of NDMA from tertiary amines during chloramination.As shown in the results of this study, N-nitrosamines can also be formed during ozonation. The formaldehyde-catalyzed nitrosation pathway proposed by some abroad researches can not explain the formation of N-nitrosamines at netural and alkaline pH completely. Hydroxylamine is a common intermediate during oxidation of aliphatic amines, and it can react with DMA into unsymmetrical dimethylhydrazine which could be oxidized further into NDMA. Furthermore, dinitrogen tetroxide (N2O4), a reactive nitrosating reagent, can be formed by a multi-step reaction between nitrite ions and hydroxyl radicals. N2O4 can act as the inorganic precursors of N-nitrosamines during ozonation. Relevant N-nitrosamines can also be formed from methylethylamine (MEA) and diethylamine (DEA) during ozonation.By studying the formation mechanisms of N-nitrosamines during ozonation, it was found that both organic precursors and reactive intermediate can be removed at the same time by adding another oxidant. The method will be also helpful to control N-nitrosamines formation during oxidation treatment process. In this study, two combined ozonation technologies, i.e. O3/H2O2 and O3/KMnO4, were introduced to eliminate DMA from water. Both these two technologies can remove nearly 80% of DMA within 20min ([DMA]0=0.01mmol/L). The removal ratio of DMA at alkaline pH was the highest, followed by that of netural pH and acidic pH. The hydroxyl radicals formed during O3/H2O2 can promote the degradation of DMA, while collaborative oxidation might be the mechanism of DMA removal during O3/KMnO4 process. By analysing the products after these two combined ozonation process, the decreasing of NDMA formation should be explained by the lower yield of reactive intermediates (hydroxylamine, formaldehyde, etc.). Unfortunately, the dosage of oxidants in these two technologies were both much higher than pratical water treatment process in order to get higer removal ratios. The residual H2O2 and manganese exceeding the standard of drinking water might do harm to human health. Therefore, it is necessary to develop a new technology which is more safe and effective for DMA removal.In the experiments of combined ozonation, it was found that MnO2 can remove certain amount of DMA from the water. The commercial MnO2 showed poor removal capacity on DMA, therefore several MnO2 adsorbents were synthesized in this study. The MnO2 formed via the reaction of KMnO4 and Na2S2O3 (MnO2,ksP) has the best settleability and removal effect on DMA, but has poor strength. MnO2,ksP could break into small MnO2 suspensions after adsorption for 24h, leading the water appears brown-yellow colour. Herein preparation methods were improved by adding Na2SiO3 after the formation of MnO2, then aging for 48h. The improved MnO2, i.e. MnO2,ksSi, still showed good removal efficiency on DMA and good settleability, but better strength. The adsorption mechanisms of DMA on MnO2,ksSi can be contributed by both electrostatic interaction and ion exchange, and the adsorption isotherms can be fitted well using Freundlich isotherm model. After adsorbed on the surface of MnO2,ksSi, the N-H in DMA was protected by the adsorption sites while the methyl groups closed to the surface of MnO2,ksSi were oxidized into -COO-. Since DMA is consumed by the oxidization, DMA can not dissolve back into water during the desorption process, and no NDMA can be formed in the following disinfection. Therefore, this asdorption method seems to be a safety and effective solution for DMA removal. In addition, MnO2,ksSi can also remove certain amount of MEA from the water, but it has no effect on DEA removal. Some influencing factors on DMA adsorption by MnO2,ksSi were also discussed. The coexisted metal cations can restrain the adsorption significantly while no effect was caused by humic acid in the water.