Ultrasound for degradation of complex matrices of PFAS mixtures

This study evaluates the effectiveness of ultrasound technology for degrading per- and polyfluoroalkyl substances (PFAS) in three complex environmental matrices: groundwater (GW), still bottom (SB), and aqueous film-forming foam (AFFF). A 10-L ultrasonic reactor, equipped with multi-frequency piezoelectric elements (850 kHz and 950 kHz), was used to treat PFAS-contaminated samples for 6 to 12 h. Degradation efficiency was measured using liquid chromatography-mass spectrometry (LC-MS/MS), fluoride ion-selective electrode (F-ISE), suppressed conductivity ion chromatography (IC), nuclear magnetic resonance (NMR) spectroscopy, total organically bound fluorine (TOF) analysis, and inductively coupled plasma mass spectrometry (ICP-MS). LC-MS/MS confirmed PFAS degradation, while F-ISE quantified fluoride release, indicating defluorination. IC analysis measured changes in anion concentrations, particularly sulfate and chloride, to assess transformation pathways. NMR and TOF provided structural insights into PFAS breakdown, and ICP-MS tracked variations in metal concentrations, highlighting potential interactions with degradation byproducts. In SB samples, fluoride concentration increased from 0 to 8.71 mg/L after 12 h, indicating successful defluorination of PFAS compounds. For GW samples, fluoride levels rose moderately from 0.54 to 1.78 mg/L, demonstrating that sonolysis can degrade PFAS in lower-concentration matrices. However, AFFF samples, dominated by perfluorooctanesulfonic acid (PFOS), showed only a slight increase in fluoride concentration (0.75 to 1.37 mg/L), indicating resistance to sonolytic degradation due to strong carbon-fluorine bonds. Anion and metal analysis revealed matrix-specific interactions influencing sonolysis outcomes, with energy distribution analysis highlighting the competitive role of chemical oxygen demand (COD) in scavenging reactive radicals. This research demonstrates ultrasound as a promising technology for PFAS degradation in complex matrices. However, the test results for AFFF suggest that with high surfactant concentrations, modifications may be necessary for complete mineralization of PFAS compounds.