Studies On N-nitrosamines Formation Characteristics And Controlling Technology Of Sequential Chlorination

N-nitrosamines are a class of emerging nitrogenous disinfection by-products,which have been detected in drinking water around the world and are confirmed asstrong carcinogens. In this paper, nine kinds of N-nitrosamines are simultaneouslytested using solid phase extraction (SPE) coupled with liquid chromatography tandemmass spectrometry (LC-MS/MS), and the detection limits were0.5-5.0ng/L. On thisbasis, occurrence and formation characteristics of N-nitrosamines in drinking water inChina were evaluated; N-nitrosodimethylamine (NDMA) formation during sequentialchlorination was investigated.N-nitrosamines formation potential of fifty nine raw water samples wasinvestigated. The results showed that eight kinds of N-nitrosamines were detected withthe rates of3.4%-91.5%, including NDMA, NMEA, NPyr, NDEA, NPip, NMor, NDPA,and NDBA; NDPhA was not detected; the detection rate of NDMA was66.1%with thehighest concentrations of5.1-757ng/L. Furthermore, the concentrations ofN-nitrosamines of thirty five finished water samples and fifty six tap water sampleswere monitored. Besides NDPhA, eight other kinds of N-nitrosamines were detectedwith the rates of2.9%-82.9%in finished water samples, and the detection rate ofNDMA was34.3%with the highest concentrations of4.9-163ng/L. In tap watersamples, all nine kinds of N-nitrosamines were detected with the rates of1.8%-89.3%,and the detection rate of NDMA was28.6%with the highest concentrations of3.7-189ng/L.The NDMA and TONA (total N-nitrosamines) formation potential of raw watersamples had no significant correlation with the water quality indexes of DOC, TDN,UV254, SUVA254, and molecular weight distribution, but they had certain correlationswith the fluorescence intensity of aromatic proteins and soluble microbialbyproducts-like organic matters. The concentration of NDMA in finished wateraccounted for up to20percent of the NDMA formation potential of the correspondingraw water. Pre-chlorination with free chlorine could significantly improve the removalof NDMA precursors in conventional water treatment processes.Sequential chlorination process could control NDMA formation fromdimethylamine (DMA) and poly diallyl dimethyl ammonium chloride (polyDADMAC), in which the formation of NDMA decreased by60%and90%, respectively. But it couldnot control NDMA formation from chlordimeform and chlorotoluron, while it couldpromote pirimicarb generate NDMA.This paper analyzed possible reaction pathways of NDMA precursors andproposed a generalized model concerning the control of NDMA formation in sequentialchlorination process. This model contained three reaction pathways. Firstly, sequentialchlorination process was capable of controlling NDMA formation from the precursorswhich could react with free chlorine and result in intermediates which was no longerprecursors of NDMA. Secondly, sequential chlorination process had no controllingeffect on the precursors which could hardly react with free chlorine. Thirdly, sequentialchlorination process was not capable of controlling NDMA formation from theprecursors which could react with free chlorine and result in even more precursors ofNDMA. With this model, NDMA precursors could be classified, the main reactionpathway of NDMA precursors in natural waters could be determined, and thendisinfection processes of drinking water plants could be guided.