Enhanced Mineralization of Mixed Antibiotics via Asymmetric Microdroplet-Interface Photocatalysis

Microdroplets present distinct reaction microenvironments for aqueous heterogeneous catalysis due to their miniature reaction domains and the asymmetric gas–liquid interfacial electric fields that they generate. This study reports the development of an integrated system that combines the asymmetric gas–liquid interfaces of microdroplets with sulfur-doped bismuth tungstate (S–Bi₂WO₆) photocatalysis. This novel system demonstrated remarkable efficiency, achieving the removal of 99.8% tetracycline and 81.2% ciprofloxacin, 10 mg L^–1 each, within 20 min, with kinetic constants 6 and 3 times higher than those in bulk solution. The energy consumption per unit TOC removal decreased by 52%, reaching 0.0435 kWh. Furthermore, the measured concentration of interfacial •OH reached 2218 μM, which is 6 times higher than that in microdroplets alone and 2.7 times higher than in bulk photocatalysis. Comprehensive characterizations and density functional theory (DFT) analyses revealed that sulfur doping narrowed the band gap by ∼0.5 eV, improved charge separation and transfer, and enhanced interfacial compatibility and surface hydrophobicity. These modifications strengthened the coupling between S–Bi₂WO₆ and the microdroplets interface, intensifying local electric fields and promoting the enrichment and activation of reactants, thereby enhancing the generation of reactive oxygen species (ROS). Molecular-level simulations (MD, frontier orbitals, Fukui functions), corroborated by LC-MS, further revealed interface-biased orientations that expose electron-rich regions (e.g., CIP quinolone/piperazine; TC dimethylamine/phenolic groups). This orientation drives preferential •OH attack and accelerates defluorination, demethylation, and ring cleavage. The results provide mechanistic insight into asymmetric-interface photocatalysis and establish a microdroplets-enhanced S–Bi₂WO₆ platform for efficient and energy-positive antibiotic mineralization.