Frequency-dependent sonochemical degradation of perfluoroalkyl substances and numerical analysis of cavity dynamics

Ultrasonic breakdown of perfluorooctanoic acid and perfluorooctane sulfonic acid were evaluated at various ultrasonic frequency (575 kHz, 860 kHz, and 1140 kHz), pH (3 - 12), bulk water temperature (14.5 - 30 oC), and gases (Helium, Nitrogen, Oxygen, Ozone, Argon). Contrary to the result reported in the literature, we observed an increase in the rate kinetics of PFOA and PFOS decomposition in an air environment (i.e., without sparging any gases), at higher pH, and higher bulk water temperature. The rate kinetics of PFOA degradation in gases follows the order as Helium>Nitrogen>Argon>Oxygen>Ozone. The present work concludes that the presence of known/unknown chemical compounds, formed during sonolysis, influence the interaction of PFOA and PFOS with the cavity-water interface. The cavity collapse simulation using Gilmore equation showed that an increase in acoustic pressure increases the compression ratio and bubble radial velocity of the collapsible cavity. This study suggests that the lower degradation rate of PFOS as compared to PFOA, over a range of ultrasound frequencies, is due to the lower number of active cavities collapsing at higher temperatures. The radius of active collapsible cavities, with maximum compression ratio and bubble radial velocity, was 3.2 µm, 2 µm, and 1.7 µm at an ultrasonic frequency of 575 kHz, 860 kHz, and 1140 kHz, respectively.