Oxidative Transformation of Nafion-Related Fluorinated Ether Sulfonates: Comparison with Legacy PFAS Structures and Opportunities of Acidic Persulfate Digestion for PFAS Precursor Analysis

The total oxidizable precursor (TOP) assay has been extensively used for detecting PFAS pollutants without analytical standards. It uses hydroxyl radical (HO•) from the heat-activation of persulfate under alkaline pH to convert H-containing precursors to perfluorocarboxylates (PFCAs) for target analysis. However, the current TOP assay oxidation method does not apply to emerging PFAS because (i) many structures do not contain C−H bonds for HO• attack, and (ii) the transformation products are not necessarily PFCAs. In this study, we explored the use of classic acidic persulfate digestion, which generates sulfate radical (SO4−•), to extend the capability of TOP assay. We examined the oxidation of Nafion-related ether sulfonates that contain C−H or −COO−, characterized the oxidation products, and quantified the F atom balance. The SO4−• oxidation greatly expanded the scope of oxidizable precursors. The transformation was initiated by decarboxylation, followed by various spontaneous steps such as HF elimination and ester hydrolysis. We further compared the oxidation of legacy fluorotelomers using SO4−• versus HO•. The results suggest novel product distribution patterns depending on the functional group and oxidant dose. Therefore, (1) the combined use of SO4−• and HO• oxidation, (2) the expanded list of standard chemicals, and (3) further elucidation of SO4−• oxidation mechanisms will provide more critical information to probe PFAS precursor structures.