From batch to flow-through multi-frequency sonicator for PFAS degradation.

The destruction of per- and polyfluoroalkyl substances (PFAS) remains a significant environmental challenge. Acoustic cavitation has shown promising potential for PFAS degradation; however, its broader application is limited by the low treatment capacity and insufficient energy efficiency of conventional sonication systems. This study investigates the transition from batch to flow-through multi-frequency sonication. A high-frequency unit was numerically designed and acoustically optimized to enhance acoustic pressure focusing and cavitation intensity. The optimized unit was implemented in a single-frequency flow-through configuration (135 kHz, 0.2 kWh/L), achieving 66% PFOS and 55% PFOA removal. In addition, conventional batch and hybrid batch/flow-through configurations were evaluated as intermediate steps toward the development of a flow-through sonication system. The dual frequency batch system (21 and 31 kHz, 0.8 kWh/L) achieved 40% PFOS and 45% PFOA degradation, whereas the triple-frequency hybrid sonicator (21, 31, and 135 kHz, 0.8 kWh/L) achieved up to 77% PFOS and 81% PFOA removal. The progressive formation of short-chain PFAS indicated sustained chain scission during sonication. To further investigate treatment capacity and energy efficiency under larger-volume operation, a dual-frequency flowthrough reactor (21 and 38 kHz) was proposed and evaluated, achieving 74% PFOS and 36% PFOA degradation at an energy density of 0.10 kWh/L. These findings demonstrate the influence of frequency and reactor configuration on PFAS degradation mechanism and highlight the potential of multifrequency flow-through sonication to improve the energy efficiency of PFAS degradation.