【作者】 杨磊；

【导师】 陈忠林；

【作者基本信息】 哈尔滨工业大学 ， 市政工程， 2010， 博士

【摘要】 近年来的研究结果显示,亚硝胺类物质以消毒副产物的形式被检出于水处理构筑物之中。鉴于该物质的强致癌风险性,有必要研究其在水处理过程中的生成机理,并采取相应的措施控制其生成。本论文即从上述两方面的问题入手,研究了几种模拟前质在氯消毒和臭氧氧化过程中亚硝胺的生成规律,并考察了复合氧化工艺和吸附工艺对水中的亚硝胺生成的控制效能。本文首先探讨了几种我国北方水体中常见的仲胺(二甲胺,二乙胺,甲基乙胺)在氯和氯胺消毒过程中生成相应亚硝胺的规律并考察了几种重要的影响因素。在次氯酸消毒过程中,三种亚硝胺的生成量较低且无明显规律,但在氨氮加入后其产量明显增加,且生成量随氨氮浓度的增加呈先增加后减少的趋势。氯胺的种类对亚硝胺的生成量有重要影响,水体的pH值和氯氮比均会影响氯胺的存在形式,二氯胺相比一氯胺能更高效地将前质转化为相应的亚硝胺。另外,水体的pH值也会通过影响前质的质子化状态影响亚硝胺的形成。实验结果显示,在氯胺消毒过程中,这三种亚硝胺可能是通过相同的反应途径生成的,即三种脂肪胺均先与氯胺反应生成不对称仲肼或氯化不对称仲肼,然后该中间产物进一步被氧化生成相应的亚硝胺。氯胺消毒过程中,水中共存的硝酸根、亚硝酸根和天然有机物均对亚硝胺的生成有不同程度的抑制作用,而溴离子可通过生成卤胺促进亚硝胺的生成。两种叔胺(三甲胺,3-二甲氨甲基吲哚)在氯胺消毒过程中生成N-亚硝基二甲胺(NDMA)的规律截然不同,但二者均可在与氯胺的反应中分解为直接的NDMA前质——二甲胺(DMA)。在相同条件下,同物质量的3-二甲氨甲基吲哚(DMAI)在氯胺消毒过程中会分解为更多的DMA,消毒结束后生成的NDMA量也最大。值得注意的是,在二氯胺消毒过程中,3-二甲氨甲基吲哚生成的NDMA量甚至高于等物质量的DMA生成的NDMA量,这说明一些叔胺可能通过其他未知高效途径生成相应的亚硝胺。硝酸根离子、亚硝酸根离子、溴离子及天然有机物对叔胺向亚硝胺的转化过程均有一定抑制作用。经实验研究发现,臭氧氧化含DMA水时也会导致亚硝胺的生成。国外学者提出的甲醛催化理论,并不能完全解释在中性及碱性条件下臭氧氧化生成亚硝胺的规律。羟胺是脂肪胺氧化后的常见产物,其可与DMA生成不对称二甲肼,该中间产物经进一步氧化后可以生成NDMA。另外,亚硝酸根可与羟基自由基经多步反应生成四氧化二氮,该产物是一种高效的亚硝化试剂也可与前质反应生成亚硝胺。经实验证明,甲基乙胺和二乙胺也可与羟胺反应生成相应的亚硝胺。在总结臭氧氧化生成亚硝胺机理的过程中,发现若在水中加入其他氧化剂,可在去除有机前质的同时去除活性的中间产物,从而达到控制亚硝胺生成的目的。本实验采用的两种复合氧化工艺(O3/H2O2和O3/KMnO4)均可在20min内有效地去除DMA([DMA]0=0.01mmol/L,去除率约80%)。两种复合工艺对DMA的去除率均在碱性条件下达到最高值,中性时次之,酸性条件下效果最差。其中,O3/H2O2工艺是通过生成强氧化性的羟基自由基促进DMA的降解,而O3/KMnO4则主要通过协同氧化作用达到强化去除DMA的目的。通过对复合氧化后的产物分析可知,复合工艺可以有效地减少活性中间产物(羟胺,甲醛等),从而使得NDMA的生成量显著降低。但是为达到理想的DMA去除效果并有效控制NDMA的生成,加入的氧化剂的剂量很高,在实际水处理中难以应用。同时,水中残留的H2O2和超标的锰离子可能带来新的水质问题。因此,亟待研发一种安全高效的除去水中DMA的方法。在复合氧化的实验中,发现反应过程中生成的二氧化锰对DMA有一定的吸附去除效果。因此,首先考察了市售二氧化锰对DMA的去除效果,但结果并不理想。通过多种不同方法在实验室制备了不同形态的二氧化锰,考察其去除DMA的效果。其中,通过高锰酸钾和硫代硫酸钠预制备的二氧化锰不但对DMA的去除率高且沉降效果好。但该材料强度不高,连续震荡反应24h后水体中会出现棕黄色的悬浊物。于是,在制备过程中加入活化步骤(48h)并加入硅酸钠以提高所制备二氧化锰的强度和稳定性。改进后的二氧化锰材料(MnO2,ksSi)对DMA仍有较好的去除效果,且沉降效果好、不易碎裂。MnO2,ksSi主要通过静电吸附和离子交换机理吸附DMA,其吸附等温线符合Freundlich模型。吸附后的DMA中的N被吸附基团保护,所以只有接近吸附剂表面的甲基被氧化为羧基离子(-COO-)。DMA一旦吸附之后,就不会再度出现在水体当中,所以这种吸附工艺是一种安全高效的去除水中DMA的方法。另外,该吸附剂对甲基乙胺也有一定的去除效果,但对二乙胺并没有明显吸附。同时也考察了几种共存物质对吸附效果的影响,其中金属阳离子可明显抑制吸附的进行,而腐殖酸对吸附没有明显影响。更多还原

【Abstract】 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.更多还原