Decreased bromate formation but inadvertently increased toxicity in the chloramination-ozonation process: Essential roles of halamines in generating halogenated and nitrogenous byproducts

The prechloramination followed by post-ozonation (NH₂Cl-O₃) process has been widely recognized for its effectiveness in suppressing bromate (BrO₃^-) formation. However, the presence of bromide (Br^-) during NH₂Cl treatment results in the formation of various halamines. This study reveals that while the NH₂Cl-O₃ process reduces BrO₃^- formation, it leads to a substantial increase in overall cytotoxicity (from 2.01-4.10 to 4.30-12.05 mg-Phenol/L) and genotoxicity (from 0.29 to 0.88 µg-4-NQO/L) to mammalian cells at the condition of 3 mg/L NH₂Cl and 1 mg-O₃/mg-C. Total organic halogen, especially total organic bromine, markedly increases during the NH₂Cl-O₃ process, with TOBr rising from 7.2 μg/L in SE to 72.5 μg/L with 3 mg/L NH₂Cl, and to 75.7 μg/L with 5 mg/L NH₂Cl. By employing Ultra-Performance Liquid Chromatography-Orbitrap Mass Spectrometry (UPLC-Orbitrap MS) combined with halogenated isotopic feature detection and ¹⁵N-labeled isotope techniques, we precisely identified the molecular formulas of these disinfection byproducts (DBPs), demonstrating that the NH₂Cl-O₃ process promotes the formation of a broader range of both halogenated DBPs and nitrogenous byproducts (N-DBPs). In the presence of Br^-, after prechloramination, various halamines such as bromochloramine (NHBrCl), monobromamine (NH₂Br), and dibromamine (NHBr₂) were formed. Prechloramination inadvertently enhances a synergistic halamines/O₃ process, amplifying DBP formation. These halamines can all contribute to an increase in toxicity, particularly when combined with O₃ treatment. While effectively controlling BrO₃^-, our findings highlight the need to consider overall toxicity and evaluate the formation of other potentially toxic DBPs.