Destruction of Per- and Polyfluoroalkyl Substances (PFASs) in Aqueous Film-Forming Foam (AFFF) with UV-Sulfite Photoreductive Treatment

Abstract

Ultraviolet photochemical reaction of sulfite (SO₃²⁻) photosensitizer generates strongly reducing hydrated electrons (e_aq⁻; NHE = −2.9 V) that have been shown to effectively degrade individual per- and polyfluoroalkyl substances (PFASs), including perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). However, treatment of complex PFAS mixtures in aqueous film-forming foam (AFFF) remains largely unknown. Here, UV-sulfite was applied to a diluted AFFF to characterize e_aq⁻ reactions with 15 PFASs identified by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) targeted analysis. Results show that reactivity varies widely among PFASs, but reaction rates observed for individual PFASs in AFFF are similar to rates observed in single-solute experiments. While some structures, including long-chain perfluoroalkyl sulfonic acids (PFSAs) and perfluoroalkyl carboxylic acids (PFCAs) were readily degraded, other structures, most notably short-chain PFSAs and fluorotelomer sulfonic acids (FTSs) were more recalcitrant. This finding is consistent with results showing incomplete fluoride ion release (up to 53% of the F content in AFFF) during reactions. Furthermore, results show that selected PFSAs, PFCAs, and FTSs can form as transient intermediates or unreactive end-products via e_aq⁻ reactions with precursor structures in AFFF. These results indicate that while UV-sulfite treatment can be effective for treating PFOS and PFOA to meet health advisory levels, remediation of the wider range of PFASs in AFFF will prove more challenging.