Emerging heterocyclic nitrogenous disinfection byproducts: Occurrence, precursors, and mechanisms of halogenated pyridines

Identifying emerging disinfection byproducts (DBPs) and elucidating their formation mechanisms are essential for improving water safety assessments. This study reports the first detection of halogenated pyridines and pyridinecarboxylic acids in chlorinated and chloraminated surface water, simulating drinking water conditions. Chloramination resulted in higher concentrations of these N-DBPs (7.99 ng/L) compared to chlorination (3.63 ng/L), suggesting that chloramination is more conducive to their formation. Precursor experiments identified phenols and chlorophenols as key contributors, with molar yields ranging from 0.19% to 4.32%, influenced by chlorine substitution patterns. Mechanistic studies, combining high-resolution mass spectrometry and DFT-based thermodynamic calculations, revealed two primary formation pathways: (i) post-cyclization stepwise halogenation, where pyridine intermediates are chlorinated sequentially, and (ii) pre-cyclization multihalogenation, which introduces multiple halogens before ring closure. This study expands the chemical diversity of N-DBPs, offers new insights into their structure-dependent formation mechanisms, and highlights the transformation of phenolic precursors into potentially toxic DBPs in water treatment. • Eight halogenated pyridines were identified as novel heterocyclic N-DBPs. • Chlorophenols were confirmed as key precursors of heterocyclic N-DBPs. • Chlorine substitution governs formation yields and pathways of heterocyclic N-DBPs. • Two previously unrecognized mechanistic routes were revealed.