The roles of reactive nitrogen species in chloramination of phenolic compounds: Insights into kinetics and by-product formation mechanisms

Monochloramine (NH 2 Cl) is widely applied in water disinfection, yet its effectiveness is compromised by co-existing substances that deplete NH 2 Cl and promote by-product formation. This study reveals that the simultaneous presence of bromide (Br − ) and nitrite (NO 2 − ) significantly enhances the transformation of phenolic compounds into halogenated and nitrogenous by-products during chloramination. Reactive nitrogen species (RNS), including nitryl halides (NO 2 X, e.g., halogen nitrite (ONOX), where X represents a halogen) and nitrogen dioxide (NO 2 , including ONO), contribute to over 50 % of model compound conversion, underscoring their role as key intermediates. These findings shift the focus from traditional halogenating agents (HOX, NHClX) to RNS as major drivers of by-product formation. For the first time, second-order rate constants between key reactive species (NHBrCl, NO 2 X (ONOX), and NO 2 (ONO)) and model compounds are determined through batch kinetic experiments, kinetic modeling, and further validation via density functional theory (DFT) calculations. Results uncover two primary reaction pathways: electrophilic halogenation mediated by ONOX, and nitrogen incorporation facilitated by NO 2 X and NO 2 . Compared to traditional halogenating agents, RNS exhibit higher reactivity and kinetic efficiency in forming halogenated and nitrogenous by-products. These insights highlight the need to recognize RNS-driven pathways in chloramination to better mitigate harmful by-product formation. • Br − and NO 2 − synergy accelerates chloramination of phenolic compounds. • Coexisting Br − and NO 2 − increase reactive species and by-product diversity. • Reactive nitrogen species (RNS) drive the formation of halogenated by-products. • Second-order rate constants for RNS reactions with phenols are first determined. • New mechanisms of RNS-driven by-product formation are revealed.