Alleviating moxifloxacin-induced biological inhibition in ethanol and Cl⁻ coexisted wastewater: Why UV/H₂O₂ is better than Fenton

Treating pharmaceutical wastewater containing moxifloxacin or other refractory antibiotics is challenging due to the frequent coexistence of high concentrations of ethanol and chloride ions (Cl⁻), which inhibit oxidative degradation and downstream biological processes. Here, we comparatively evaluated the Fenton, UV/Na₂SO₃, and UV/H₂O₂ processes for moxifloxacin removal under certain concentration Cl⁻ and ethanol conditions. The results revealed that the removal of moxifloxacin stagnated for Fenton (16.21%) and UV/Na₂SO₃ (37.72%), while the UV/H₂O₂ maintained a removal rate of >90%. Mechanistic analysis indicated that although Fenton could remove moxifloxacin in the presence of Cl⁻, its dominant radical was ·Cl₂⁻ rather than ·OH, resulting in a lower defluorination rate. Cl⁻ and ethanol consumed Fe²⁺ through reactive species like ·Cl, ·Cl₂⁻, and ·HO₂, which reduced Fe²⁺ concentration and blocked the Fenton reaction. In contrast, UV/H₂O₂ not only sustained direct photolysis but also overcame the reaction inhibition caused by insufficient Fe²⁺ concentration and maintains continuous ·OH production. And moxifloxacin adsorbed H₂O₂ than ethanol, Cl⁻, generating ·OH around the moxifloxacin, leading to more effective removal and defluorination (47.30%). Importantly, UV/H₂O₂ pretreatment restored the nitrification activity of downstream bioprocesses, demonstrating its robustness and practical potential for detoxifying antibiotic-laden wastewater under complex conditions.