Unveiling the Critical Pathways of Hydroxyl Radical Formation in Breakpoint Chlorination: The Role of Trichloramine and Dichloramine Interactions

Chlorination of ammonia or chloramine-containing waters induces breakpoint chlorination reactions, producing a hydroxyl radical (•OH), but enhances the formation of undesirable N-nitrosamines. The prevailing view attributes •OH formation to a nitrosyl intermediate derived from the hydrolysis of dichloramine, but this pathway is unlikely at neutral or acidic pH. This study reveals a novel mechanism where •OH is generated via interactions between trichloramine (NCl3) and dichloramine (NHCl2), which also form nitrosation agents. Our experiments demonstrated that the NCl3–NHCl2 interaction degrades micropollutants with kinetics 2–3 times faster than breakpoint chlorination. Using electron paramagnetic resonance, we detected •OH in the NCl3–NHCl2 reaction. Micropollutant removal was unimpaired under low dissolved oxygen (O2(aq)) conditions, aligning with negligible O2(aq) changes during the NCl3–NHCl2 reaction and suggesting O2(aq) does not participate in •OH formation. Using benzene as a probe in 18O-labeled H2O, we confirmed water contributes to the oxygen source of •OH in NCl3–NHCl2 interactions, through which parallel reactions occur, leading to the formation of one mole of •OH alongside 1.92 mol of N2. A kinetic model developed in this study accurately predicted •OH and N2 and demonstrated the NCl3–NHCl2 interaction as the primary pathway for •OH formation in breakpoint chlorination, providing new insights into breakpoint chemistry.