Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic chemicals persistent in diverse environment compartments. More stringent guidelines and increasing PFAS health concerns have led to the manufacture of PFAS alternatives. With ultrasound demonstrated effective in the defluorination of legacy PFAS, we expanded in this study the application of ultrasound irradiation (USI) for the mineralization of emerging PFAS, i.e., hexafluoropropylene oxide homologues GenX (ammonium salt of the hexafluoropropylene oxide dimer acid, HFPO-DA), HFPO-DA, hexafluoropropylene oxide trimer acid (HFPO-TA), and hexafluoropropylene oxide tetramer acid (HFPO-TeA). Relative to perfluorooctanoic acid (PFOA), HFPO-DA was more prone to ultrasound-induced defluorination. While the percentage defluorination rate (k, % min–1) followed HFPO-DA ≥ HFPO-TA &gt; HFPO-TeA, the rates of fluoride ion generation (k, µM min–1) were in the order of HFPO-TA &gt; HFPO-TeA &gt; HFPO-DA at an [PFAS]0 of 10 µM. Pyrolysis was proposed as the major ultrasonic defluorination route. Initial product studies by the ultra high performance liquid chromatography Orbitrap high resolution mass spectrometry (UHPLC-Orbitrap-HRMS) showed perfluoropropionic acid (PFA, C3HF5O2) and flupropanate (C3H2F4O2) as the major HFPO-DA degradation products. In accordance with the calculated molecular bond strength, these two compounds were probably generated via the cleavage of the weakest C-C bond CF3CF2CF2-C3F4O3–. Small residual percentages (&lt; 3 %) along with the predominant defluorination suggested, however, that an analogous bond cleavage was not the rate determining step for the sonolytic defluorination of HFPO-DA. In addition, PFA was much less effectively defluorinated than HFPO-DA. Absence of trifluoroacetic acid (TFA, C2HF3O2) implied that the defluorination was likely a more immediate step of ultrasonic induced degradation. The results demonstrate the strong potential of ultrasound in the remediation of emerging FPAS hexafluoropropylene oxide homologues.

Kinetic and mechanistic studies of the ultrasound-induced remediation of hexafluoropropylene oxide homologues, as emerging per- and polyfluoroalkyl substances (PFAS)

ACS Fall 2021

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