Hydroxyl Radical Transformations of Perfluoroalkyl Acid (PFAA) Precursors in Aqueous Film Forming Foams (AFFFs).

Abstract

Historical releases of aqueous film forming foam (AFFF) are significant sources of poly- and perfluoroalkyl substances (PFASs), including perfluoroalkyl acids (PFAAs) and their precursors, to the environment. While several studies have focused on microbial biotransformation of polyfluorinated precursors to PFAAs, the role of abiotic transformations at AFFF-impacted sites is less clear. Herein, we use photochemically generated hydroxyl radical to demonstrate that environmentally relevant concentrations of hydroxyl radical (•OH) can play a significant role in these transformations. High-resolution mass spectrometry (HRMS) was used to perform targeted analysis, suspect screening, and nontargeted analyses, which were used to identify the major products of AFFF-derived PFASs as perfluorocarboxylic acids, though several potentially semi-stable intermediates were also observed. Using competition kinetics in a UV/H2O2 system, hydroxyl radical rate constants (kOH) for 24 AFFF-derived polyfluoroalkyl precursors were measured to be 0.28 to 3.4 × 109 M-1 s-1. Differences in kOH were observed for compounds with differing headgroups and perfluoroalkyl chain lengths. Also, differences in kOH measured for the only relevant precursor standard available, n-[3-propyl]tridecafluorohexanesulphonamide (AmPr-FHxSA), as compared to AmPr-FHxSA present in AFFF suggest that intermolecular associations in the AFFF matrix may affect kOH. Considering environmentally relevant [•OH]ss, polyfluoroalkyl precursors are expected to exhibit half-lives of ∼8 days in sunlit surface waters and possibly as short as ∼2 h during oxygenation of Fe(II)-rich subsurface systems.