Photochemical decomposition of 1H,1H,2H,2H-perfluorooctane sulfonate (6:2FTS) induced by ferric ions

Abstract

Perfluorooctane sulfonate (PFOS) had wide applications, such as in the electroplating industry, but its use was restricted in 2009 by the Stockholm Convention, due to its environmental persistence and potential hazards. As the most common PFOS alternative, 1H,1H,2H,2H-perfluorooctane sulfonic acid (6:2FTS) and its salts have been increasingly used. However, little is known about its photochemical decomposition. This paper reports the ferric ion-induced efficient decomposition and defluorination of 6:2FTS under 254nm ultraviolet (UV) irradiation; the underlying mechanisms were also investigated. In the presence of 100μmol/L ferric ion and at pH3.0, the first-order decomposition rate constant of 6:2FTS (10mg/L) was 1.59/hr, which was 6 times higher than for direct UV photolysis. The effects of the ferric ion concentration and the solution pH on the 6:2FTS photodecomposition were investigated and the optimal reaction conditions were determined. In addition to fluoride and sulfate ions, shorter-chain PFCAs (C2–C7) were detected as major intermediates. The addition of hydrogen peroxide or oxalic acid accelerated the decomposition of 6:2FTS under UV irradiation, but not its defluorination, indicating that hydroxyl radicals can directly react with 6:2FTS but not with the shorter-chain PFCAs. Accordingly, a mechanism for 6:2FTS photochemical decomposition in the presence of ferric ion was proposed, which comprises two reaction pathways. First, hydroxyl radicals can directly attack 6:2FTS, leading to CC bond cleavage. Alternatively, 6:2FTS coordinates with ferric ion to form Fe(III)-6:2FTS complexes, which can undergo ligand-to-metal charge transfer under UV irradiation, causing CS bond cleavage.