Characterization and Optimization of an Intermediate-Scale Sonochemical Reactor Design

Per- and polyfluoroalkyl substances (PFASs) are pollutants that have demonstrated a high level of environmental persistence and are very difficult to remediate. Sonochemical processes have shown considerable potential for PFAS destruction, but the relationships between ultrasound operating conditions, cavitation behavior, and reactor performance remain insufficiently understood. Ultrasonic cavitation is a key mechanism in sonochemical processes, yet the coupled effects of ultrasound frequency, cavitation activity, energy efficiency, and bubble characteristics remain insufficiently understood. This study systematically investigated these interactions in an intermediate scale sonochemical reactor operated at 680, 850, and 950 kHz under different power densities. Cavitation was quantified using potassium iodide (KI) dosimetry, while energy transfer and efficiency were evaluated by calorimetric analysis. Bubble size, bubble concentration and zeta potential were measured to characterize cavitation-generated nanobubbles and added argon nanobubbles, and their interfacial properties. The results showed that ultrasound frequency strongly influenced cavitation behavior and energy utilization. The 850 kHz system exhibited the highest cavitation activity, whereas the 950 kHz system showed the highest calorimetric efficiency. Increased power density enhanced cavitation intensity but also increased thermal losses. Higher frequencies produced smaller bubbles, while power density and solution properties had weaker effects on bubble size and surface charge. Zeta potential measurements revealed consistently negative surface charges, with markedly greater negative values in the presence of perfluorooctanoic acid (PFOA) and argon nanobubbles, consistent with possible interfacial association of PFAS rather than direct confirmation of adsorption. Nanobubble concentration and size were also highly dependent on ultrasound and solution properties, with ultrasound promoting nanobubble generation and PFAS contributing to their stability and growth. Overall, the findings demonstrate that cavitation activity, energy efficiency, and bubble interfacial properties are strongly interdependent and primarily governed by ultrasonic frequency. This work provides new insights into sonochemical reactor characterization and optimization and identifies operating conditions that may be beneficial for future PFAS degradation studies and other advanced oxidation processes.