Study On N-nitrosamine Formation From Amine-Based Organic Flocculants During Chloramintion

N-nitrosamine is a new kind of nitrogen-containing disinfection by-product,and its potential carcinogenic,teratogenic and mutagenicity has attracted extensive attention from scholars at home and abroad.Recent studies confirmed that poly(epichlorohydrin dimethylamine)(polyamine)and polydimethylenediallyl ammonium chloride(polyDADMAC)were important precursors of N-nitrosodimethylamine(NDMA)formation.Polyacrylamide,another kind of amine organic flocculant widely used in water treatment and sewage treatment,has also been found to generate a variety of N-nitrosamine disinfection by-products including NDMA.Given the high carcinogenic risk of N-nitrosamines and the extensive use of amine-based polymers in water treatment,it is of great significance to study the main factors affecting N-nitrosamines formation in related water treatment processes and the mechanism of N-nitrosamines formation.In this paper,single factor experiments and orthogonal experiments were used to evaluate N-nitrosamines formation potential from amine-based polymers.GC-MS and GC-FID were used to analyze N-nitrosamines and dimethylamine,respectively.And the Mulliken population analysis,calculation by Materials Studio,was used to analyze the mechanism of N-nitrosamine formation by amine-based polymers.Finally,we evaluated the risk of N-nitrosamines formation from amine-based polymers through coagulation-sedimentation-filter-disinfection processThe main contents and conclusions of this research are as follows:? The significance of factors affecting NDMA formation potential from poly amine and polyDADMAC during chloramination was investigated.The results showed that the significance of factors influencing polyDADMAC and polyamine forming NDMA was from highest to lowest:pH,bromide concertation,NOM,ammonia concertation and the dose of monochloramine.The highest NDMA formation potential from polyamine and polyDADMAC occurred at pH near 7?8.In the presence of excess bromide,the NDMA formation potential was enhanced significantly at all different pH values owing to bromochloramine which has a higher electronegativity of the brominated nitrogen atom than monochloramine or dichloramine.Polyamine and polyDADMAC might formed NDMA through Br-UDMH.The NDMA formation potential from polyDADMAC and polyamine in the presence of NOM was lower than NDMA formation in the absence of NOM.The formation of dichloramine,from ammonia reacted with hypochlorous acid,enhanced the NDMA formation potential.If polyamine and polyDADMAC are used as flocculants in drinking water treatment,the removal of bromide before disinfection and the avoidance of chlorine disinfection in water samples containing higher concentrations of ammonia are suggested.?The effect of chemical structure on N-nitrosamines formation potential from polyacrylamide was investigated by cationic polyacrylamides(CPAMs).The results showed that the formation of five N-nitrosamines,including NDMA,was detected in the reaction between CP AM and chloramine.There were no significant correlations between the molecular weight and type cationic monomers of CPAMs with N-nitrosamines formation potential.However,corresponding cationic monomers have much higher N-nitrosamines formation than CPAMs.N-nitrosamine formation increased with the increase of charge density.FTIR experiments showed that both amide groups and quaternary ammonium groups of CPAMs could reacted with chloramine,but the quaternary ammonium group reacted more easily with chloramine than the amide group.If CPAMs are used in drinking water treatment in the future,it is recommended to evaluate the N-nitrosamines formation in practical treatment and select CPAMs with lower charge density.? Based on the results of Mulliken population analysis of monomers and CP AM,the three cationic monomers are more likely to generate other N-nitrosamines than NDMA due to the chargeability of their functional groups and specific atoms and their molecular structure.The copolymerization of acrylamide and cationic monomers enhanced the charge of dimethylamine functional group and the bond length of N—C linked dimethylamine functional group.CPAMs become more likely to generate NDMA than the cationic monomer.The charge carried by the dimethylamine functional group increased with increase of charge density of CPAMs.It became more susceptible to attack by nucleophiles,resulting in an increase of NDMA formation from CPAMs.? The results of NDMA formation potential of polymer showed that the polymers'NDMA formation potentials during chloramination are in the order of Mannich polyacrylamide>polyamine>polyDADMAC>CP AM>nonionic polyacrylamide(NPAM)>anionic polyacrylamide,which is mainly related to dimethylamine concertation of polymers solution and chemical structure of polymers.Based on the toxicity of the flocculant itself and the risk of producing N-nitrosamines,it is recommended to use polyDADMAC,NPAM and APAM as coagulants.?Based on the Mulliken population to analyze the molecular structure of polymers,the molecular structure of polymers mainly influences N-nitrosamines formation from two aspects:the absolute value of the charge in the DMA functional group and the bond length of the N—C of dimethylamine functional group.The higher the absolute value of the charged charge,the more activity of polymers and the longer the bond length of the N—C bond connecting the dimethylamine functional group.It become more easily to break and degrade to a lower amine.?We simulated the process of coagulation-precipitation-filtration-disinfection with organic flocculants and evaluated the risk of organic flocculants forming N-nitrosamines.Under relevant water treatment condition,most coagulation with either polyamine or polyDADMAC yielded NDMA at levels lower than the current advisory guidelines.However,with the presence of bromide and the increasing of polymers dosage,the NDMA formation is close to the value of the current water quality standard.In addition,when CP AM and nonionic polyacrylamide(NPAM)reacted with residual chlorine in several days,it still potential to form over amount of NDMA.Therefore,the risk of polyacrylamide could not completely excluded.Our work studied the influencing factors,formation risks and N-nitrosamines mechanisms of polymers.It provides a theoretical basis to control N-nitrosamines formation.This work has significant meaning for drinking water safety and human health.