Mechanistic Insights into Per- and Polyfluoroalkyl Substance (PFAS) Photolysis under Intensified Simulated Solar Light

Per- and polyfluoroalkyl substances (PFAS) are widely detected in the water environment at levels posing significant risks to the ecosystem and human health. While these “forever chemicals” are considered highly resistant to natural photolysis, this study demonstrates the unexpected decomposition of perfluoroalkyl carboxylic acids (PFCAs) and hexafluoropropylene oxide dimer acid (GenX) under simulated solar light in a catalyst-free environment, with GenX exhibiting up to 49.1% degradation and 21.2% defluorination within 5 h. The probe experiment and electron spin resonance spectroscopy, together with a subpicosecond transient absorption spectrometer, confirm the production of hydrogen radicals from the photoexcitation of water at acidic and neutral pH. The hydrogen radical-driven reactions are inconsistent with the widely proposed hydrated electron-dominated defluorination, and we have theoretically elaborated the PFAS decomposition pathways supported by the identified decarboxylated shorter-chain PFCAs and hydrodefluorinated intermediates in mass spectrometry analysis. It is also concluded that light above 300 nm may degrade PFCAs through a long, continuous photolysis treatment (e.g., >24 h) but is still insufficient for defluorination; instead, the hydrogen radical-driven defluorination is primarily attributed to UV wavelengths below 300 nm. This study contributes a comprehensive and fundamental perspective on PFAS photolysis, and the obtained results will inform new strategies by applying simulated solar light for sustainable PFAS remediation.