Photocatalytic activation of peroxyacetic acid by MnFe₂O₄ for efficient caffeine degradation: Efficacy, mechanisms and reusability

Caffeine (CAF), a compound with stimulant and antimicrobial properties, exhibits widespread environmental distribution due to incomplete wastewater treatment. To address this issue, this study systematically evaluated the removal efficiency and mechanisms of CAF using a vacuum ultraviolet/peracetic acid/MnFe₂O₄ (VUV/PAA/MnFe₂O₄) system. The magnetic MnFe₂O₄ catalyst, synthesized via a hydrothermal method, enabled near-complete CAF removal (>99 % within 20 min) in VUV/PAA/MnFe₂O₄ system under neutral conditions (PAA: 2.0 mM; MnFe₂O₄: 0.25 g∙L −1 ). The assessment of water quality effects indicated that humic acid (HA), CO₃²⁻, Cl⁻, and SO 4 ²⁻ inhibited CAF degradation, while NO₃⁻ demonstrated a slight concentration-dependent influence. Mechanistic studies identified hydroxyl radical (•OH) as the primary reactive species (95 % contribution), with secondary roles played by organic radical (R-C•) and direct VUV photolysis. The sustained •OH generation was facilitated by oxygen vacancies (V O ) and cyclic redox conversion of Mn²⁺/Mn³ ⁺ and Fe²⁺/Fe³ ⁺ on the MnFe₂O₄ surface under VUV irradiation. The system exhibited excellent regenerability, maintaining high removal efficiencies over five consecutive cycles. Importantly, toxicity assessment revealed that the degradation process preferentially transformed CAF into less toxic end products, demonstrating the inherent ecological safety of the system. This study provides insights for designing efficient catalytic systems for CAF-contaminated wastewater treatment.