Cavitation-regime-controlled synergy in high-frequency sonophotocatalysis

High-frequency ultrasound is proposed as an intensification tool for advanced oxidation processes, yet its role is often treated as a simple energy input and not as a regime-dependent process variable. This work investigated reactor-driven synergy in sonophotocatalysis using a plate-type ultrasonic system operating at 584 kHz for ibuprofen (IBU) degradation. Cavitation efficiency was quantified by potassium iodide dosimetry, identifying 584 kHz in continuous mode as the most effective and stable radical-generating condition. The individual and combined effects of ultrasound and simulated solar irradiation were evaluated using BiOCl, BiOBr, and TiO₂ P25. True synergy emerged only for BiOBr. The kinetic constant of the coupled process (4.55·10^-2 min^-1) exceeded the sum of the individual photocatalytic and sonocatalytic contributions by a factor of 2.23, corresponding to ca. 55% synergy. Reactive-species trapping indicated a cooperative mechanism involving photogenerated holes together with cavitation-derived hydroxyl and superoxide radicals. The synergistic contribution was retained in simulated drinking water and at reduced catalyst loading. Sonophotocatalysis also enhanced TOC removal relative to the individual processes, although mineralization remained partial, indicating sequential oxidation under the investigated conditions. Transformation products were identified by UHPLC-MS/MS and their potential ecotoxicological relevance was assessed through ECOSAR modelling. A preliminary process-oriented assessment yielded a cost of ownership of ca. 2.20 €·m^-3 and an electrical energy per order of 0.49 kWh·m^-3·order^-1 for 90% IBU abatement. These findings demonstrate that synergy is not intrinsic to process coupling, but depends on the cavitation regime and photocatalyst properties, supporting reactor-oriented development of high-frequency sonophotocatalytic systems.