UV-driven •OH oxidation releases perfluorocarboxylic acids from surface-fluorinated high-density polyethylene microplastics

Surface-fluorinated high-density polyethylene (F-HDPE) is an overlooked source of perfluoroalkyl carboxylic acids (PFCAs), yet the mechanisms governing PFCA release remain unclear. Here, we quantify PFCA (C₄-C₈) release from F-HDPE microplastics (MPs) under UV₃₆₅ irradiation and provide mechanistic evidence for •OH-driven release. Industrial F-HDPE MPs showed a 10.4-44.4% higher release rate than personal care MPs, consistent with greater defluorination of industrial MPs. The release rate of small MPs was 1.3-2.4 times as fast as that of large MPs. No PFCA release was observed at 25 and 35 °C, whereas concentrations reached 4.2 ± 0.03 μg·L⁻¹ at 50 °C. Humic acid promoted PFCA release at 10 mg·L^-1 via photosensitization but decreased release by a factor of 1.5-2.3 at 50 mg·L^-1 due to light shielding and •OH scavenging. PFCA release also occurred in natural waters, likely caused by DOM-mediated photosensitization. Short-chain PFCAs dominated because of their preferential formation, faster diffusion and desorption. Mechanistically, photoexcitation of carbonyls promotes adjacent C-C bond cleavage, generating unstable radicals (•C(=O)-(CF₂)_y-CHF-CH₂- and -CF₂CF₂CF₂•) that further undergo H-abstraction and/or •OH oxidation, ultimately yielding PFCAs. These findings identify F-HDPE as an unrecognized PFCA source and highlight associated environmental risks.