Ultrasonic degradation of GenX (HFPO-DA) - Performance comparison to PFOA and PFOS at high frequencies

Sonolytic degradation kinetics of hexafluoropropylene oxide-dimer acid (HFPO-DA or GenX) were studied for the first time at four high ultrasonic frequencies (375, 580, 860 and 1,140 kHz) and three power densities (200, 300 and 400 W/L), and compared to the degradation of previously studied perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). The frequency of 580 kHz displayed the highest degradation of all three PFAS tested. Within each frequency, the degradation performance increased consistently with increasing power density. Degradation rates were highest for GenX over PFOA to PFOS (kGenX = 0.0501 min-1, kPFOA = 0.0444 min-1, kPFOS 0.0153 min-1), which is in direct agreement with their reported thermal stability (PFOS > PFOA > GenX). No known by-product formation as expected from the literature on truncation mechanism was detected in the samples (i.e. shorter chain carboxylic acid PFAS). Fluorine mass balance experiments at 580 kHz and 400 W/L confirmed that GenX defluorinated fastest among the three tested PFAS and had lowest loss of fluorine in the mass balance. Degradation experiments with a mixture of all three PFAS displayed lower degradation rates than the individual experiments, where PFOA exhibited the largest reduction in degradation rate (by 31%), followed by GenX (by 19%), and finally by PFOS (by 17%). Overall, our study demonstrates that ultrasound can provide effective destruction of different PFAS (70-90% fluorine from mitigated PFAS was detected as inorganic fluoride) with a similar energy demand to alternative PFAS treatment methods reported in literature.