Kinetics of Hypohalous Acid Intermediates Governing Disinfection Byproduct Formation in Peracetic Acid-Treated Halide-Containing Waters

Peracetic acid (PAA) has emerged as an alternative disinfectant because of its lower reactivity with natural organic matter (NOM) and minimal halogenation. However, PAA may react with halides (Br^-, I^-) in water to form hypohalous acids, which can contribute to halogenated disinfection byproduct (DBP) formation. Meanwhile, coexisting hydrogen peroxide (H₂O₂) can reduce HOBr/HOI back to halides. To assess and mitigate the DBP risks of PAA in halide-containing waters, this study investigated the oxidant change and DBP formation in PAA/halide/NOM systems. A refined kinetic model accurately simulates PAA/Br^- and PAA/Br^-/NOM reactions, revealing that a small fraction of NOM is highly reactive with HOBr and drives brominated DBP formation. Dibromoacetic acid dominated the identified DBPs, while total organic bromine (TOBr) analysis suggested a majority of the unidentified byproducts. In PAA/I^- and PAA/I^-/NOM reactions, newly determined rate constants enabled good model and experimental data agreement. With NOM, total organic iodine (TOI) formation was the dominant iodide sink, with limited iodate formation. Triiodomethane and triiodoacetic acid were the major identified DBPs, along with more unidentified byproducts. Some unknown iodine reactive species were observed to persist in PAA/I^-/NOM systems for hours. Importantly, applying a PAA/H₂O₂ molar ratio of 1:2 substantially mitigated DBP formation, decreasing known Br- or I-DBPs by 85-90% and the TOBr/TOI by 33-44%.