Degradation of acrylamide during chlorination as a precursor of haloacetonitriles and haloacetamides

摘要

AbstractAcrylamide is a monomer of polyacrylamide, which is widely used in the water treatment process as a flocculant. The degradation kinetics and formation of disinfection by-products (DBPs) during acrylamide chlorination were investigated in this study. The reaction between chlorine and acrylamide followed a pseudo-first-order kinetics. A kinetic model regarding acrylamide chlorination was established and the rate constants of each predominant elementary reaction (i.e., the base-catalyzed reaction of acrylamide with ClO−as well as the reactions of acrylamide with HOCl and ClO−) were calculated as 7.89×107M−2h−1, 7.72×101M−1h−1, and 1.65×103M−1h−1, respectively. The presence of Br−in water led to the formation of HOBr and accelerated the rate of acrylamide degradation by chlorine. The reaction rate constant of acrylamide with HOBr was calculated as 1.33×103M−1h−1. The degradation pathways of acrylamide chlorination were proposed according to the intermediates identified using ultra-performance liquid chromatography and electrospray ionization-quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS). Five chlorinated DBPs including chloroform (CF), dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), dichloroacetamide (DCAcAm), and trichloroacetamide (TCAcAm) were identified during acrylamide chlorination. The formation of CF, DCAN, DCAcAm, and TCAcAm kept increasing, while that of TCAN increased and then decreased with increasing reaction time. As the chlorine dosage increased from 0.75 to 4.5mM, DCAN became the dominant DBP. Large amounts of CF, DCAN, and TCAN were formed at basic pHs. The hydrolysis of DCAN and TCAN led to the formation of DCAcAm and TCAcAm, respectively. The results of this study elucidated that acrylamide can be a precursor for the formation of haloacetonitriles (HANs) and haloacetamides (HAcAms) during drinking water treatment.Graphical abstractDisplay OmittedHighlights•The kinetics of acrylamide chlorination can be well described by a second-order model.•The degradation of acrylamide during chlorination is favorable at basic pHs.•Hofmann degradation between acrylamide and ClO−was the dominant degradation pathway.•High concentration of DCAN was generated during acrylamide chlorination.•HAcAm formation favors basic pHs during acrylamide chlorination.