Kinetics and Mechanism of the Sonolytic Conversion of the Aqueous Perfluorinated Surfactants, Perfluorooctanoate (PFOA), and Perfluorooctane Sulfonate (PFOS) into Inorganic Products

TLDR

PFOS and PFOA are pervasive environmental contaminants that resist most conventional water treatment methods. This study demonstrates that high‑frequency ultrasound‑induced acoustic cavitation effectively degrades aqueous PFOS and PFOA across concentrations from 10 nM to 10 µM. Degradation initiates at the bubble‑water interface via pyrolysis that removes the ionic group, forming 1H‑fluoroalkanes or perfluoroolefins, which then undergo further vapor‑phase pyrolysis to C1 fluoro‑radicals that react bimolecularly and hydrolyze to CO, CO₂, HF, and fluoride. Mass‑balance analyses reveal near‑complete mineralization to CO, CO₂, F⁻, and SO₄²⁻ immediately after interfacial pyrolysis, with sonochemical half‑lives matching high‑temperature gas‑phase kinetics.

Abstract

The perfluorinated surfactants perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) are recognized as widespread in the environment as well as recalcitrant toward most conventional water treatment technologies. In this study, acoustic cavitation as driven by high-frequency ultrasound is shown to be effective in the degradation of aqueous solutions of PFOS and PFOA and effective over a wide range of concentrations from 10 nM to 10 muM for a given compound. Sulfur, fluorine, and carbon mass balances indicate that mineralization occurs immediately following the degradation of the initial perfluorinated surfactant. Near complete conversion of PFOS and PFOA to CO, CO2, F-, and SO42- occurs due to pyrolytic reactions at the surface and vapor phase of transiently collapsing cavitation bubbles. The initial PFOS or PFOA pyrolytic degradation occurs at the bubble-water interface and involves the loss of the ionic functional group leading to the formation of the corresponding 1H-fluoroalkane or perfluoroolefin. The fluorochemical intermediates undergo a series of pyrolytic reactions in the bubble vapor leading to C1 fluoro-radicals. Secondary vapor-phase bimolecular reactions coupled with concomitant hydrolysis converts the C1 fluoro-radicals to carbon monoxide, carbon dioxide, and HF, forming a proton and fluoride upon dissolution. Sonochemical half-lives, which are calculated from high-temperature gas-phase kinetics, are consistent with kinetic observations and suggest that mineralization occurs shortly after initial perfluorinated surfactant interfacial pyrolysis.