Kinetic model for sonolytic degradation of non-volatile surfactants: Perfluoroalkyl substances

Sonolytic degradation kinetics of non-volatile surfactant perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) were investigated over a range of concentration, considering active cavity as a catalyst. The Michaelis-Menten type kinetic model was developed to empirically estimate the concentration of active cavity sites during reactions. Sonolytic degradation of PFOA and PFOS, as well as the formation of its inorganic constituents, fluoride, and sulfate, follows saturation kinetics of pseudo-first order at lower concentration (<2.34 µM) and zero order at higher concentration (>23.60 µM). Nitrate and hydrogen peroxide formations were 0.53 ± 0.14 µM/min and 0.95 ± 0.11 µM/min, respectively. At a power density of 77 W/L and frequency of 575 kHz, the empirically estimated maximum number of active cavity sites that could lead to the sonolytic reaction were 89.25 and 8.8 mM for PFOA and PFOS, respectively. This study suggests that a lower number of active cavity sites with higher temperature needed to degrade PFOS might be the reason for lower degradation rate of PFOS compared to that of PFOA. Diffusion of non-volatile surfactants at the cavity-water interface is found to be the rate-limiting step for the mineralization of perfluoroalkyl substances.