Forever no more: Complete mineralization of per- and polyfluoroalkyl substances (PFAS) using an optimized UV/sulfite/iodide system

As the global issue of PFAS contamination in water continues to grow there exists a need for technologies capable of fully mineralizing PFAS in water, with destruction being measured as both a loss of the initial PFAS and a quantitative recovery of the resultant fluoride ions. This study investigates the use of sulfite and iodide in a bicarbonate-buffered alkaline system activated with ultraviolet (UV) light to destroy PFAS. The UV/sulfite/iodide system creates a reductive environment through the generation of aqueous electrons, which can degrade PFAS. The extent of degradation and defluorination was explored for perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), 6:2 fluorotelomer sulfonic acid (6:2 FTS), and perfluorobutane sulfonic acid (PFBS). An initial UV/sulfite/iodide system achieved 100 % degradation and > 90 % defluorination for PFOS, PFOA, and 6:2 FTS, but was not capable of completely degrading PFBS. Transformation product elucidation experiments were performed for PFOS under different UV systems, and 6:2 FtSaB using the initial UV/sulfite/iodide system. Several transformation products were identified including -nF/+nH PFOS (n = 1-13), -F/+H shorter-chain PFSAs, 6:2 fluorotelomer sulfonamidoamine (6:2 FtSaAm), 6:2 fluorotelomer sulfonamide, and 6:2 fluorotelomer unsaturated sulfonamide. Novel identification of -F/ +H perfluoropropane sulfonic acid (PFPS) and -F/+H perfluoroethane sulfonic acid (PFES) following degradation of PFOS confirms C\\C bond cleavage, and different isomers of -F/+H PFOS confirms the potential for C\\F bond cleavage to occur throughout the perfluoroalkyl chain. Additional optimization experiments were performed aiming to fully degrade PFBS. The optimal protocol found in this study involved an elevated initial sulfite concentration and adding additional sulfite at regular intervals during UV-activation, achieving >99.9 % destruction and complete quantitative defluorination of PFBS.

Automated summary

With the increasing global issue of PFAS contamination in water, there is a need for technologies that can completely mineralize PFAS in water while measuring the destruction through loss of initial PFAS and quantitative recovery of fluoride ions. This study examines the use of sulfite and iodide in a bicarbonate-buffered alkaline system activated with UV light to destroy PFAS. The UV/sulfite/iodide system creates a reductive environment that can degrade PFAS, although complete degradation of PFBS was not achieved. Transformation products of PFOS and 6:2 FtSaB were identified, confirming carbon-carbon bond cleavage and the potential for cleavage of carbon-fluorine bonds. The study also found an optimized protocol for fully degrading PFBS.