Unveiling Halogenated and Nitrogenous DBPs in Chloramination-Ozonation Process: DFT-Driven Formation Pathways from Tyrosine and Toxicity Assessment

The prechloramination followed by ozonation (NH₂Cl-O₃) process effectively mitigates bromate (BrO₃⁻) formation in bromide-containing waters but inadvertently enhances the production of halogenated and nitrogenous disinfection byproducts (DBPs). Cytotoxicity assays revealed that the presence of precursors such as L-tyrosine (Tyr), L-tryptophan (Trp) and tannic acid during the NH₂Cl-O₃ process contributed to this increase in nitrogenous and halogenated DBPs and resulted in elevated overall toxicity. Nontargeted ultrahigh performance liquid chromatography-Orbitrap tandem mass spectrometry identified diverse halogenated DBPs, with NH₂Cl-O₃ yielding up to 5-fold more structures and higher abundances than NH₂Cl or O₃ alone. Taking Tyr as an example, proposed DBP (4-bromophenol, 4-hydroxyphenylacetamide, etc.) formation pathways, involving haloamine-mediated N-halogenation, O₃-driven oxidation, decarboxylation, and deamination, were supported by density functional theory (DFT) calculations, confirming both thermodynamic and kinetic feasibility. Matrix-dependent quantitative validation in Suwannee River humic acid (SRHA) and wastewater effluents showed that concentrations of some identified DBPs reached tens of μg/L, dominated by aromatic halo-phenolics and brominated derivatives. In silico quantitative structure-activity relationship (QSAR) models and cytotoxicity assays indicated that these aromatic halo-DBPs exhibited comparable toxicities to conventional DBPs (e.g., dichloroacetonitrile, N-nitrosodimethylamine), but had a higher toxicity index than BrO₃⁻. These findings highlight trade-offs in water disinfection, underscoring the need for optimized strategies to co-control BrO₃⁻ and toxic organic DBPs.