Synergistic degradation of PFAS in water and soil by dual-frequency ultrasonic activated persulfate

Ultrasound (US) can degrade per- and polyfluoroalkyl substances (PFAS) by the cavitation or pyrolysis effect. In this study, we find that the US equipment typically used in a common laboratory, such as 20 kHz and 43 kHz for cleaning or homogenisation, can degrade PFAS, although the process is slow (with approximately 14.6% and 20.1% defluorination after 6 h, respectively). To accelerate the degradation process, a dual-frequency US is combined with persulfate (PS) to synergistically degrade PFAS in water and soil. Typical PFAS including perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS) and 1H, 1H, 2H, 2H-perfluorooctanesulfonic acid (6:2 FTS) are used to evaluate the degradation performance of dual-frequency US/PS in water. After 6 h of degradation, the defluorination percentage is, from highest to lowest, PFOA (100 ± 1.2%) > 6:2 FTS (86.9 ± 0.9%) > PFOS (46.5 ± 1.0%). Interestingly, the dual-frequency US/PS process can also efficiently degrade PFAS in the contaminated soil, with a 62–71% degradation of 14 PFAS (28 is the maximum number of PFAS that can be quantitatively monitored), representing an encouraging progress. Finally, the degradation pathways and kinetics of PFOA are studied by monitoring the degradation intermediates, and by indicating the combination of the dual-frequency US/PS is not simply addition with each other, but with a synergistic effect of degradation.