Intensification of triphasic photocatalytic microreactor performance via high-frequency ultrasound for enhanced mass transfer and dye degradation

This study demonstrates that high-frequency ultrasound can significantly enhance mass transfer efficiency in a three-phase (gas–liquid–solid) photocatalytic microreactor, improving dye degradation performance. A custom-designed microreactor, consisting of four diamond-shaped mixing chambers, was equipped with piezoelectric plate (PZT) transducers operating at 1.7 MHz, with a nominal input power of 5.1–5.2 W per transducer. The effective acoustic power delivered to the reaction medium was determined using a calorimetric method based on the temperature rise of the liquid, and was found to correspond to 51.3–61.6% of the nominal electrical input power depending on the number of activated PZT transducers. To investigate the synergistic effect between ultrasound and photocatalysis, methylene blue (MB) degradation was evaluated under three operational modes: photocatalysis alone (UV), sonolysis (US), and combined sonophotocatalysis (UV + US). A magnetically recoverable Fe 2 O 3 /TiO 2 photocatalyst was synthesized and used together with oxygen as the gas phase. Under optimal conditions, the sonophotocatalytic process removed 44.3% of MB, representing a 13% improvement over UV photocatalysis alone and a 55% increase compared with sonolysis. This enhancement was associated with substantial improvements in key performance indicators, including an apparent overall removal coefficient (k ORC ) of 2470 min −1 and a significantly elevated reaction rate constant under mixed conditions. Statistical analyses using multiple linear regression (MLR) and Pearson correlation confirmed that catalyst dosage had the strongest positive correlation with removal efficiency (r = 0.47, p < 0.01), while flow rate and initial dye concentration negatively influenced performance. The PZT transducer arrangement was also critical: simultaneous activation of three transducers yielded 37.83% removal efficiency, more than doubling the performance of single-transducer configurations. These results demonstrate that high-frequency, plate-type ultrasound integrated into microreactors enhances cavitation, microstreaming, and mass transfer, offering a powerful approach to intensifying triphasic photocatalytic processes and advancing wastewater treatment technologies.