Nonradical degradation of antibiotics in seawater by Cl⁻/Br⁻-activated peracetic acid: Dominant contribution of ¹O₂ and reactive halogens

Natural seawater contains abundant halide ions that remain largely untapped, while peracetic acid often requires activation to enhance its oxidative properties. The interaction mechanisms between these components have not been thoroughly investigated. This study investigated the activation effect of high concentrations of Cl⁻ and Br⁻ coexisting in seawater on PAA by adding PAA to seawater. Quenching experiments, along with Electron Paramagnetic Resonance (EPR) spectroscopy and UPLC-TQ-MS, were conducted to determine the reaction mechanism and degradation process of SMX. Seawater systems containing 35 ‰ salinity or Br⁻-PAA systems (0.5 mM Br⁻) degraded 100 % SMX within 10 min or 12 min, respectively ( k _obs=0.4873 min⁻¹ or 0.3581 min⁻¹). While the Cl⁻-PAA system, though slower, still degraded 97.7 % within 180 min (300 mM Cl⁻, k _obs=0.0173 min⁻¹). SMX degradation rates remained above 95 % across the pH range of 3–9, with optimal performance at pH= 3. However, the presence of HA inhibited SMX degradation efficiency by over 40 %. Results indicate that SMX degradation efficiency correlates positively with Cl⁻/Br⁻ content and PAA concentration. The superior removal efficiency stems from activated PAA not only generating HOCl and HOBr via oxygen atom transfer but also further producing ¹O₂, which dominates the non-selective degradation process. This approach adapts to a wide pH range but is consumed by coexisting HA. The superiority of the Br⁻-PAA system over Cl⁻-PAA stems from the strong oxidizing power of HOBr. The oxidation pathways include halogenation, bond cleavage, and hydroxylation. ECOSAR predictions indicated that most transformation products exhibited higher toxicity than the parent SMX compound.