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Nucleophilic substitution reactions in hydrothermal degradation of per- and polyfluoroalkyl substances
Brooks, Sean E. Hao, Shilai Strathmann, Timothy J.
Brooks, Sean E.
Hao, Shilai
Strathmann, Timothy J.
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2023-09
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Per- and polyfluoroalkyl substances (PFAS) are a class of fluorinated surfactants used in aqueous film forming foam, plastics production, and semiconductor manufacturing, among other industrial applications. PFAS are highly recalcitrant and have been demonstrated to toxicologically affect wild organisms and humans. Hydrothermal alkaline treatment (HALT), which involves the rection of PFAS in a subcritical aqueous environment with the addition of a strong base, has been demonstrated to destroy two major categories of PFAS, perfluorocarboxylic acids (PFCAs) and perfluorosulfonic acids (PFSAs), in highly-concentrated aqueous matrices. This research investigates the reaction mechanisms underpinning HALT by comparing the reactivity of different nucleophiles. Solutions (1M) of four nucleophiles – sodium bromide (nucleophilicity [n] =3.89), sodium hydroxide (n=4.20), potassium iodide (n=5.04), and sodium hydrosulfide (n=5.10 ) – were combined with a concentrated sample of ultrashort chain PFAS trifluoromethane sulfonate (TFMS) and reacted at 350°C for times ranging from 30 minutes to 180 minutes. Defluorination rates were determined by measuring fluoride in the post-treatment samples with a fluoride ion selective electrode (FISE). For each reaction time, sodium hydroxide promoted the greatest defluorination, which was nearly an order of magnitude higher than the next-best performing nucleophile, sodium hydrosulfide, for the 180 minute reaction. The findings from this study extend prior research into long chain PFSAs by experimentally determining the hydroxide amendment as the most effective nucleophile for defluorination of ultrashort chain PFSAs. This study provides further evidence to support the hypothesis that nucleophilic substitution reactions form the mechanism through which defluorination of PFAS occurs in HALT.
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