Chlorination And Nitrogenous Disinfection By-Products Formation Of Short-Chain Aliphatic Amines

The safety of drinking water has always been one of the most serious concerns. Biologic safety of drinking water has been always ensured by chlorine chlorination disinfection due to its great power to inactivate the pathogenic microorganism. Unfortunately, however, some disinfection by-products (DBPs), especially some nitrogenous disinfection by-products (N-DBPs) have been proved to be produced in the process of chlorination disinfection. N-DBPs have been documented as harmful materials which could cause great harm to human health due to the carcinogenesis, genetic toxicity and mutagenicity. In order to ensure the chemical safety of drinking water, it is particularly important to do some researches relating to the source, occurrence, and control of N-DBPs. In this paper, three typical nitrogenous organic matter——short-chain aliphatic amines (methylamine, dimethylamine, ethylamine) have been selected as precursors to carry out the experiments of chlorination disinfection. And then the formation, impact facters, and reaction pathway of N-DBPs (such as halonitromethanes HNMs, halonitronitriles HANs, and dimethylnitrosamine NDMA) were studied.In this paper, the analysis method of trace short-chain aliphatic amines in water by reversed-phase high performance liquid chromatography (RP-HPLC) with precolumn dansylation and fluorimetric detection was optimized and applied in the experiments. The optimized HPLC analysis method for methylamine, ethylamine, and dimethylamine could achieve better results:the detection limit less than 0.1μg/L reaching to the trace level; the recovery was in the range of 92.6%～117.5%; better sensitivity and precision; samples with concentration in the range of 1～200μg/L could be detected precisely. The determination of trihalomethanes (THMs) using the technology of Purge and Trap combined with gas chromatography and electron capture detector (GC-ECD) detection was established. The determination of HNMs using the pretreatment technology of liquid-liquid extraction combined with GC-ECD was also established. And the confirmation of HNMs were determined by GC-MS. The qualitative determination of HANs and NDMA were adopted liquid-liquid extraction-GC-MS-SCAN as well.Methylamine was selected as precursor to conduct the experiments of chlorination. And the formation, influence factors of THMs and HNMs were stuied. The yield amounts of TCM and TCNM gradually increased over time, and were stable around 60 min, with the concentration of 32.6 μg/L and 0.37μg/L, respectively. The generation curve of TCM and TCNM was fited using the first order kinetics model and the degree of fitting are both greater than 0.97, the reaction rate constants were respectively 0.055 and 0.1546 min-1. The rise of temperature could accelerate TCM and TCNM in the range of 10-30℃ and the higher temperature, the more amounts were produced. The TCNM amount increased with the increase of pH value, while the TCM amount was the opposite. It was indicated that the TCNM amount produced under alkali condition was higher than those produced under the neutral and acid conditions, and TCM was also the opposite. What’s more, the influence of pH in the TCNM formation is far greater than that of TCM. It was also found that the TCM and TCNM amount increased with the increase of chlorine addition when the chlorine dosage was in the range of 1-12 mg/L. However, TCNM amount reduced when the chlorine dosage was increased to 8 mg/L, in which the concentration of free chlorine was excessive and MA probably turned into other reactions which could generate formaldehyde. And the dosage of MA could greatly accelerate the formation of TCM and. The presence of bromide ions promoted the HNM formation, HNM formation amount in the 5 mg/L concentration of bromide ions was 2.54 times of that without bromide ions. The bromo-HNMs of all the HNMs took up a large proportion, and the majority ones were TCNM, BDCNM, DBCNM and TBNM. The presence of nitrite could greatly increase the HNM formation. TCNM formation amount in the 5 mg/L concentration of NO2- was 2.27 times of that without NO2-, but NO3- had little impact on the HNM formation.According to the chlorination experiments and theoretical analysis for the generation of N-DBPs with the presusors of MA, DMA, and EA, the probable reaction mechanism and pathway were as follows:1) The products in the chlorination of MA existed THMs and HNMs. The probable pathway for the generation of THMs is as follows:firstly, MA would proceed deamination reaction to generate CH3-OH. Later, on the one hand, CH3-OH could be oxidated by HO-Cl and result in the formation of CH2O. On the other hand, CH3-OH and HO-Cl could proceed substitution reaction, thus generating CH3-Cl, and then generating CHCl3, namely TCM. The probable pathway for the generation of HNMs is as follows:firstly, MA could easily react with HO-Cl thus generating CH3NCl2. CH3NCl2 could proceed reactions with HO-Cl in the presence of OH-and then oxidated to CH3-NO2, CH3-NO2 and HO-Cl could proceed substitution reaction thus generating some HNMs. Moreover, trihalo-nitromethanes could decompose as THM by taking off the-NO2; and in the presence of NO2-, the NO2· generating from NO2- and Cl3C·generating from CHCl3 could proceed addition reaction and then transferred into TCNM.2) The products in the chlorination of EA existed halogenated nitroethane and HANs. It is found that the pathway of halogenated nitroethane generation from EA followed the same pattern as HNMs generation from MA. The probable pathway for the generation of HANs is as follows:firstly, EA could easily react with HO-Cl thus generating CH3-CH2-NCl2. Secondly, CH3-CH2-NCl2 could proceed elimination reaction thus transferring into CH3-C≡N by elimination two H2O molecules. At last, CH3-C≡N and HO-Cl could proceed substitution reaction thus generating some HANs.3) The products in the chlorination of DMA existed HNMs and NDMA. The probable pathway for the generation of HNMs is as follows:firstly, the cleavage of C-N in the CH3-NH-CH3 resulting the formation of CH3-NH-, and then transterred into CH3NC12. Secondly CH3NCl2 turned into HNMs step and step following the same pattern as HNMs generation from MA. The probable pathway for the generation of NDNA is as follows:it is found that in the presence of ON-OH, (CH3)2N-H (DMA) and ON-OH could proceed condensation reaction thus transferring into NDNA by elimination one H2O molecules.