Study On Distribution,formation And Control Of Nitrosamine Disinfection Byproducts In Drinking Water In A Southern City

Drinking water risk issues has been an international research hotspots.Nitrosamine disinfectant by-products as emerging nitrogen-containing disinfection by-products in drinking water are more toxic and more carcinogenic than conventional disinfection by-products,and have a"triple effect",causing great concern and discussion.Aiming at the nitrosamines disinfection by-products in drinking water,solid-phase extraction combined with LC-MS/MS technology was used to establish and optimize the testing methods for eight nitrosamine disinfection by-products.Based on this,a southern city was investigated the concentration levels and distribution characteristics of eight kinds of nitrosamines disinfection by-products in 7 tap water plants.The influencing factors and the formation mechanismof nitrosamines disinfection byproducts were analyzed.The degradation behavior and reaction kinetics of three typical nitrosamines were studied based on UV/H₂O₂/O₃ technology.The main findings are as follows:1.The optimum solid phase extraction conditions for eight kinds of nitrosamines disinfection byproducts:loading time is 90 min,dichloromethane elution volume is 13 ml,nitrogen blowing time is 60 min,pH of reaction solution is8.The method detection limits of nitrosaminesranged from 0.55-3.12 ng/L,and the relative standard deviations of precision and accuracy were less than 6%,except that the recovery rates of NDPA and NDBA were slightly lower than 70%,and the average recovery rates of the remaining nitrosamines were 77.39-93.78%.2.The results of the survey of the 7 major water plants in a certain city in the south show that although there are many types of N-nitrosamines detected in the effluent of each unit,the concentration of N-nitrosamines is low,and generally lower than 10 ng/L.The average concentration of nitrosamines in the city’s factory water was ranked as follows:NPyr（5.18 ng/L）,NDMA（5.12 ng/L）,NPIP（2.54ng/L）,NMor（1.59 ng/L）,NDEA（1.02 ng/L）,NDBA（0.61 ng/L）,NDPA（0.45 ng/L）and NEMA（0.19 ng/L）.The average concentration of NDMA after tapping the water distribution network was slightly lower than that of the factory water.The average concentration of other seven nitrosamine disinfection by-products was 1-2times higher than the nitrosamine content of the factory water.3.pH,TOC,NH₄⁺,NO₂^-,NO₃^-and disinfection process in water are important factors influencing the formation of N-nitrosamines.There was a moderate correlation between the presence of the three nitrosamines and the TOC concentration.There was no apparent correlation between the UV254 parametersand three nitrosamines.There was a significant positive correlation with the NH₄⁺concentration.And there was also a certain degree of nitrite ion positive correlation;a significant negative correlation with low concentrations of nitrate and humic acid.The conversion of secondary amines is highest at 25°C,and will be suppressed to some extent at lower or higher temperatures.The effect of pH on NAs was significant.The maximum potential of DMA,DEA,and PYR generation was at pH is 7.0 or 8.0,and the corresponding conversion rate also reached the highest.Compared with chlorine disinfection,chloramine disinfection in this experiment will significantly increase the formation of nitrosamines disinfection byproducts,the molar yield of secondary amine precursors is more than a hundred times than chlorine disinfection.4.Using simple different light sources irradiate of three typical nitrosamines NDMA,NDEA and NPyr,the results showed that natural light had no degradation ability to NDMA,NDEA and NPyr within a certain period of time.Under the irradiation of metal halide lamps,three N-nitrosamines have a slower degradation effect.Under the irradiation of 365 nm UV-enhanced xenon lamp,the degradation efficiency of N-nitrosamine was 40-50%within 1 h.Under the irradiation of 254 nm UV lamp,the nitrosamines disinfection by-products in water were rapidly degraded within 10 min,and the removal rate reached about 99.97%.NDMA,NDEA and NPyr all met the pseudo first-order reaction kinetics during the light degradation of the 365 nm xenon lamp.The order of the photolytic activity of the three N-nitrosamines was:NDMA>NDEA>NPyr.As the initial concentration of nitrosamines increases,the degradation rate of the corresponding nitrosamines gradually decreases.5.The degradation processes of NDMA,NDEA and NPyr in UV,UV/H₂O₂和UV/H₂O₂/O₃systems all satisfied the pseudo first-order reaction kinetics equation,and the apparent reaction rate constants of NDMA,NDEA and NPyr were approximately increased by 5.45 times,4.4 times and 4.2 times respectively.The degradation mechanisms of NDMA,NDEA,and NPyr in the UV/H₂O₂/O₃ system,except that the photolysis is the N-N bond cleavage,the·OH starts to attack one of the three nitrosamine molecules.A hydrogen atom is extracted from the NDMA,NDEA and NPyr into a nitroso radical centered on a carbon atom.Then,the nitroso radical becomes a double bond-containing imine by a series of decomposition reactions,and finally is oxidized to produce other products such as methylamine,ethylamine,nitrate,and so on.By analyzing the pH,ammonia nitrogen,and nitrate ion concentrations in samples treated with the UV/H₂O₂/O₃ system,combined with the degradation effects of NDMA,NDEA,and NPyr,it was concluded that the final product of NAs is mainly inorganic nitrogen in the form of NO₃^-.From the perspective of surface removal efficiency and degradation products,UV/H₂O₂/O₃combined process is superior to UV/H₂O₂ combined process compared with 3different treatment processes,and UV/H₂O₂ process is superior to UV irradiation alone.