Ferric ion mediated photodecomposition of aqueous perfluorooctane sulfonate (PFOS) under UV irradiation and its mechanism

•Photodecomposition of PFOS under UV increased 50 times by ferric ions.•Addition of ferric ion results in reduction of PFOS signal in UPLC–MS/MS.•Excitation of PFOS–Fe3+ complex by UV leads to PFOS decomposition.•The main intermediates, i.e. perfluorocarboxylic acids were further decomposed. Perfluorooctane sulfonate (PFOS) recently has received much attention due to its global distribution, environmental persistence and bioaccumulation. The methods for PFOS decomposition are very limited due to its inertness. In this report we first found the photodecomposition of PFOS under UV was greatly accelerated by addition of ferric ions. In the presence of ferric ion (100μM), PFOS (20μM) decreased to below the detection limit within 48h, with the rate constant of 1.67d−1, which was 50 times higher than that by direct photolysis (0.033d−1). Besides fluoride and sulfate ions, C2–C8 perfluorocarboxylic acids (PFCAs) were identified as the main intermediates. It was found that addition of PFOS into the FeCl3 aqueous solution led to reduction of UV absorption, and the presence of ferric ion reduced the response of PFOS as analyzed by UPLC–MS/MS, which indicated that PFOS formed a complex with ferric ion. The ESR detection indicated that the electronic state of Fe3+–PFOS complex changed during reaction. And the role of oxygen and hydroxyl radical on the defluorination of PFOS was investigated. Accordingly the mechanism for PFOS photodecomposition in the presence of ferric ion was proposed.