Dual-frequency ultrasound superposition modes: Persulfate activation and PFOA defluorination in water and soil

In the dual-frequency ultrasound (DFUS) coupling persulfate (PS) (DFUS/PS) system, acoustic-wave superposition can synergistically activate PS and promote the defluorination of perfluorooctanoic acid (PFOA). However, the influence of different superposition modes on PFOA defluorination remains unclear. In this study, four superposition modes—probe-bath, orthogonal, opposite, and dual-probe—were systematically evaluated in both aqueous and soil matrices. The probe-bath mode achieved the highest PFOA defluorination rate, reaching 69.4% at a power allocation ratio of 1/4, while maintaining soil integrity. Sound-field characterization revealed that the probe-bath mode enhanced cavitation intensity by increasing the amplitude of the synthesized wave, which intensified the chemical effects that served as the primary driver of PFOA defluorination, whereas mechanical effects provided auxiliary support. This mode also promoted constructive interference and exhibited the highest sound intensity with the largest coverage area, and achieved superior energy utilization efficiency. These findings provide mechanistic insights into mode-dependent DFUS/PS activation and offer practical guidance for optimizing ultrasound-assisted remediation of PFAS-contaminated water and soil.