The ineffectiveness of conventional water treatment processes for the remediation of per- and polyfluoroalkyl substances (PFAS) in water has triggered the investigation into the alternative advanced and innovative technologies. In this study, we utilized ultrasound for the degradation of PFAS with a specific focus on the extend of mineralization to fluoride ion. Ultrasonic irradiation can result in the generation of micro bubbles, which collapse with production of extreme temperatures and pressures. We studied ultrasound-induced degradation of perfluorocarboxylic acids (PFCAs, including perfluorooctanoic acid, PFOA, C8HF15O2; perfluoropentanoic acid, PFPeA, C5HF9O2; perfluorobutyric acid, PFBA, C4HF7O2), perfluorooctanesulfonate (PFOS, as potassium salt C8F17KSO3), and 6:2 fluorotelomer sulfonic acid (6:2 FTS, C8H5F13SO3). The defluorination of these compounds followed zero-order kinetics. Higher [PFAS]0 (10 – 50 µM) led to a higher fluoride formation rate (k, µM min–1) and percentage defluorination rate (k, % min–1) regardless of the PFAS type. Shorter chain length did not necessarily result in a lower k for the three studied PFCAs. The terminal functional group and side chain both influenced the percentage defluorination of PFAS with a k (% min–1) order of PFOA &gt; 6:2 FTS &gt; PFOS, suggesting the advantage of ultrasound in remediating PFAS especially PFCA contaminated water. Presence of an alcohol (isopropyl alcohol, iPrOH, or tert-butyl alcohol, t-BuOH) or pre-purging with a different type gas (Ar/O2) resulted in diverse influence on the defluorination of PFOA/6:2 FTS/PFOS. Overall, pyrolysis in the gas and gas-liquid cavitation phases was proposed as the major route of defluorination. Ultrasonic irradiation of PFAS contaminated field leachate samples led to the defluorination of the spiked 10 µM PFOA with the zero-order k (R2 ≥ 0.99) that decreased linearly with the increasing leachate percentage (0 – 99%). This study provides an excellent set of information on the effective sonolytic defluorination of PFAS.

Ultrasound-induced defluorination of per- and polyfluoroalkyl substances (PFAS) with different chain lengths and terminal groups

ACS Fall 2021

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