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 <i>N</i>-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 (O_2(aq)) conditions, aligning with negligible O_2(aq) changes during the NCl₃-NHCl₂ reaction and suggesting O_2(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.