Critical Role of Trichloramine Interaction with Dichloramine for -Nitrosamine Formation during Breakpoint Chlorination.

Breakpoint chlorination is prevalent in drinking water and potable reuse water treatment. Breakpoint chlorination enhances the formation of -nitrosamines through reactions that form nitrosating agents. The most recent study suggests that nitroxyl (HNO) can react with free chlorine (HOCl) to form the nitrosyl chloride (ClNO) nitrosating agent but has not experimentally verified its importance in breakpoint chlorination. This study first assessed the formation of -nitrosamines from model -chloro-alkylamine precursors when they were added to a mixture of HOCl and HNO-derived nitrosating agents generated by chlorinating hydroxyurea. Results demonstrated negligible -nitrosamine formation. Instead, we observed that the interaction of NCl with NHCl (total Cl/total N molar ratio = 2.4-3:1) produced an intermediate capable of nitrosating -chloro-alkylamines to -nitrosamines at yields 8-fold higher to those observed in NHCl treatment alone, within a very short timescale (<3 min). We examined the stoichiometry of the reaction of NCl with NHCl using a UV-spectrum-based approach. Nitrosyl chloride was proposed as the key intermediate, likely formed alongside the reformation of NHCl. Further isotopic experiments, byproduct measurements, and kinetic modeling supported the hypotheses. Modeling indicated that the reaction of NCl with NHCl explained ∼75% of NDMA formation during breakpoint chlorination. Because NCl is mainly derived from the reaction of HOCl with NHCl, controlling NHCl (e.g., with additional treatment) is critical for minimizing nitrosamine formation in waters where breakpoint chlorination occurs.