Exploring N-nitrosodimethylamine formation induced from water flocculants by isotope tracking and spectral analysis

Poly-diallyldimethylammonium chloride (polyDADMAC) and polyacrylamide (PAM) are widely used flocculants in drinking water treatment and may interact with chloramine to form the carcinogenic disinfection byproduct N-nitrosodimethylamine (NDMA). However, the molecular pathways governing NDMA formation from these polymers are not well characterized. In this study, the reaction mechanisms of NDMA formation from polyDADMAC and PAM, two representative flocculants, were investigated using isotopically labeled ¹⁵NH₂Cl, Fourier-transform ion cyclotron resonance mass spectrometry, and two-dimensional correlation spectroscopy. The nitroso-group nitrogen in NDMA was traced to be formed primarily from monochloramine, while the amino-group nitrogen originated from the nitrogen inherently present in the flocculant compounds. Nitrogen-containing compounds in both flocculants exhibited similar mass spectral patterns, with even-numbered nitrogen species dominating among unsaturated compounds. In polyDADMAC, NCH₃ groups on quaternary amines were the dominant reactive sites compared with heterocyclic NC bonds, whereas in PAM, chlorimanation was initiated at the amide carbonyl (-CONH₂), followed by sequential NH and CN bond cleavage. Understanding the molecular pathways of NDMA formation from polymeric flocculants during chloramination is essential for developing treatment strategies that reduce NDMA occurrence in drinking water.