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 Cl₂ 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.