Amide heterocyclic disinfection byproducts with overlooked disinfectant capacity during chlorination

Nitrogenous heterocyclic disinfection byproducts (NH-DBPs) are an emerging group of N-DBPs, with several species detected in treated waters and some exhibiting notable toxicity ( e.g. , halopyrroles, halopyridines). However, the structural diversity and reaction behaviors of other NH-DBPs remain insufficiently characterized. In this study, we investigate a series of amide heterocyclic DBPs (AH-DBPs) formed during chlorination. These compounds possess dual identities as both NH-DBPs and organic chloramines, including chloro‑cyanuric acid, chloro‑2-methyl-4-hydroxypyrimidine, chloro‑uracil, chloro‑6-methyluracil, chloro‑isocytosine, chloro‑5,5-dimethylhydantoin, and chloro‑hydantoin. Unlike conventional organic chloramines, these AH-DBPs exhibit higher stability in the presence of free chlorine, maintaining 45–90% residue after 24 h. These AH-DBPs exhibit three distinct kinetic behaviors based on the profiles of chlorine speciation ( i.e. , total chlorine, free chlorine, and organic chloramines) across environmentally relevant conditions. We further observe comparable bactericidal performance between freshly prepared chlorine and AH-DBPs, while after 48-h storing, AH-DBPs achieve 1.5–2.0 times higher log reductions against Escherichia coli than chlorine. The residues of free chlorine and AH-DBPs are 1.0 and 0.2∼0.5 mg L -1 as Cl 2 after 48-h storing. This enhancement correlates with slower self-decomposition kinetics of AH-DBPs than free chlorine. Furthermore, using 1,3,5-trimethoxybenzene (TMB) as a probe, we demonstrate that AH-DBPs exhibit higher oxidative reactivity than conventional organic chloramines like chloro‑ n -propylamine. Theoretical calculations quantitatively corroborate the experimental phenomenon, revealing activation energies of 17.98 (chlorine), ∼21 (AH-DBPs), and 31.39 (chloro‑ n -propylamine) kcal mol -1 for TMB oxidation. These findings deepen our understanding of AH-DBPs as a class of organic chloramines exhibiting unexpected oxidative reactivity, which may influence disinfection performance in chlorinated water.