Worldwide detection of per- and poly-fluoroalkyl substances (PFAS), as well as their toxicity and recalcitrant nature pose serious environmental challenges. Photochemical processes are viable options that have exhibited promising potentials for effective decomposition of PFAS. Among photochemical processes, vacuum-UV (VUV) irradiation covering the spectral range of 100 to 200 nm of UV spectrum has attracted increasing attention due to several advantages like emission of high-energy photons and its chemical-free nature. VUV irradiation could serve as a promising adjustable tool to generate either oxidative or reductive environment by photolysis of water and/or mediators. In the present study, the degradability, intermediate evolution, and decomposition pathways of perfluorooctane sulfonate (PFOS) and its common alternatives (perfluorobutane sulfonate (PFBS), perfluorohexane sulfonate (PFHxS), and 6:2 fluorotelomer sulfonic acid (6:2 FTSA)) were investigated using quasi-collimated beam of UV and VUV irradiations. Results showed that PFOS and its alternatives are all poor absorbers of 185 nm irradiation, implying the minor contribution of direct photolysis in their decomposition. Given their structural differences, different reactivities with ●OH were observed among PFOS alternatives where PFOS, PFHxS, and PFBS with fully fluorinated alkyl chain were persistent against ●OH attack, whereas 6:2 FTSA was readily degraded with the rate of 4.51 J-1 cm2. Shorter chain perfluoroalkyl carboxylic acids (PFCAs) and fluorotelomer sulfonic/carboxylic acids were determined as the main intermediates of 6:2 FTSA decomposition upon the reaction with ●OH. On the other hand, eaq- generated in VUV/sulfite process was able to degrade different structures of PFAS where 66%, 55%, 39%, and 14% of PFOS, PFHxS, 6:2 FTSA, and PFBS were degraded within 15.7 J1 cm-2, respectively. Shorter chain length or presence of C-H bond in PFOS alternatives led to their lower degradability with eaq-. H/F-exchanged by-products and PFCAs were the main intermediates generated in photo-reductive decomposition of PFOS and its alternatives. Further, we assessed the synergistic effect of adding iodide and high-temperature conditions to VUV/sulfite process for decomposition of persistent PFOS substituents. PFBS decomposition efficiencies in VUV/sulfite (10 mM)/iodide (10 mM) and VUV/sulfite (10 mM)/85 °C were respectively 2.8 and 4.0 times greater than that obtained in VUV/sulfite (10 mM)/25 °C process.

All we need is just a little bit of 185 nm: Application of VUV irradiation in photo-oxidative/reductive decomposition of PFAS

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