When high-frequency cavitation rewrites photocatalysis: reactor-engineered synergy unlocking new oxidation pathways

The search for energy-efficient water treatment technologies is accelerating interest in advanced oxidation processes (AOPs), particularly photocatalysis. However, conventional photocatalytic systems remain limited by rapid charge-carrier recombination and catalyst deactivation [1]. High-frequency cavitation, generated by ultrasound in the kilohertz range, offers a pathway to overcome these constraints: at hundreds of kilohertz, cavitation bubbles are smaller, more uniform, and more numerous, producing dense oxidative microdomains that fundamentally reshape liquid–solid interfacial chemistry [2]. Unlike low-frequency sonochemical regimes, high-frequency acoustic fields promote chemically dominated cavitation with reduced mechanical effects, making them intrinsically compatible with semiconductor catalysts. When coupled with light excitation, this controlled acoustic environment creates reaction conditions unattainable by either process alone [2], enabling a level of tunability rarely accessible in conventional AOP configurations. In this study, ibuprofen degradation over bismuth oxybromide (BiOBr) was investigated under photocatalysis, sonocatalysis, and their combined application in a broadband high-frequency reactor operating between 584 and 1148 kHz. The effects of catalyst dosage, pollutant concentration, water matrix composition, and radical scavengers were systematically examined to elucidate the interaction between cavitation-induced chemistry and photogenerated charge carriers. Under all tested conditions, the combined process outperformed the individual ones, exhibiting a synergy index of 53%. This enhancement arises from intensified reactive oxygen species generation, improved mass transfer, and continuous catalyst surface renewal. Crucially, these effects are directly linked to the broadband frequency distribution and reactor geometry, demonstrating that the observed synergy is the result of deliberate reactor–material co-design rather than an intrinsic material property. Ultra-high-performance liquid chromatography coupled with tandem mass spectrometry revealed that high-frequency cavitation does not simply accelerate photocatalytic reactions but actively redirects their chemistry. While photocatalysis and sonocatalysis produced distinct and relatively narrow transformation product profiles, the combined process generated a broader and more oxidized spectrum of intermediates, unveiling reaction pathways otherwise inaccessible. Several low-abundance transformation products were detected exclusively under combined conditions, indicating that acoustic fields influence not only reaction kinetics but also radical speciation and electron–hole dynamics, with direct implications for reaction selectivity in complex water matrices. Preliminary assessments of energy efficiency and cost-of-ownership indicate that broadband high-frequency reactors can operate within feasible power ranges, supporting their potential for scale-up. By aligning mechanistic understanding with practical feasibility, this work positions high-frequency sonophotocatalysis as a reactor-engineered evolution of photocatalytic water treatment, capable of enhancing pollutant removal while unlocking new oxidation pathways for faster and greener AOPs. Keywords: High-frequency sonophotocatalysis; cavitation-driven oxidation; broadband acoustic reactors. References [1] Galloni, M.G. et al., Curr. Opin. Chem. Eng., 48, 101129 (2025). [2] Falletta, E. et al., ACS Photonics, 10, 3929–3943 (2023).