Unveiling the Critical Pathways of Hydroxyl Radical Formation in Breakpoint Chlorination: The Role of Trichloramine and Dichloramine Interactions
Yi-Hsueh Chuang, Chia-Shun Chou, Yi-Lin Chu
Abstract
High Resolution Image Download MS PowerPoint Slide 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 (NCl 3 ) and dichloramine (NHCl 2 ), which also form nitrosation agents. Our experiments demonstrated that the NCl 3 –NHCl 2 interaction degrades micropollutants with kinetics 2–3 times faster than breakpoint chlorination. Using electron paramagnetic resonance, we detected •OH in the NCl 3 –NHCl 2 reaction. Micropollutant removal was unimpaired under low dissolved oxygen (O 2(aq) ) conditions, aligning with negligible O 2(aq) changes during the NCl 3 –NHCl 2 reaction and suggesting O 2(aq) does not participate in •OH formation. Using benzene as a probe in 18 O-labeled H 2 O, we confirmed water contributes to the oxygen source of •OH in NCl 3 –NHCl 2 interactions, through which parallel reactions occur, leading to the formation of one mole of •OH alongside 1.92 mol of N 2 . A kinetic model developed in this study accurately predicted •OH and N 2 and demonstrated the NCl 3 –NHCl 2 interaction as the primary pathway for •OH formation in breakpoint chlorination, providing new insights into breakpoint chemistry.