Lower molecular weight fractions of PolyDADMAC coagulants disproportionately contribute to N-nitrosodimethylamine formation during water treatment

Abstract

N-nitrosodimethylamine (NDMA) is a chloramine disinfection by-product, and its formation in drinking waters can increase due to the addition of cationic polydiallyldimethylammonium chloride (polyDADMAC). PolyDADMAC is a cationic polymer added as a coagulant or coagulant aid to enhance turbidity removal during sedimentation and filtration. This paper answers two central questions to understanding the nature of the NDMA precursors in polyDADMAC. First, what is the reactivity of different molecular weight (MW) fractions of polyDADMAC with chloramines? NDMA formation potential (NDMAFP) and kinetic experiments with chloramines were conducted for non-fractionated (raw) and size-excluded fractions (<3K, 3–10K, and >10K Da.) of polyDADMAC. The lower MW fraction (<3K Da.) of polyDADMAC solutions was responsible for forming 64 ± 6% of the NDMA, despite containing only 8.7 and 9.8% of the carbon or nitrogen present in the bulk polymer. The chloramine demand kinetics of the lowest MW fraction were also >2× faster than the higher MW fractions. Therefore, in a water treatment application the lower MW polyDADMAC likely contributes to most of the NDMA attributed to the use of polyDADMAC. The second question was: can 1H and 13C nuclear magnetic resonance spectroscopy (NMR) be used to characterize the molecular structures in polyDADMAC that react with chloramines? A peak for 1H NMR dimethylamine (DMA), a known low MW NDMA precursor, was found in a commercial polyDADMAC solution and decreased upon chloramination. The estimated DMA alone could not account for the observed NDMAFP, indicating the presence of other low MW precursors. Diffusion order spectroscopy (DOSY) NMR also showed multiple lower MW organics in polyDADMAC that change upon chloramination, including a 1.5× decrease in MW, suggesting chloramines cleave C–C or C–N bonds. These reactions may produce intermediates responsible for NDMA formation. Polymer manufacturers could use NMR to synthesize polyDADMAC with less DMA and other low MW compounds that produce NDMA upon chloramination.