Product analysis and insight into the mechanochemical destruction of anionic PFAS with potassium hydroxide

Abstract Per- and polyfluoroalkyl substances (PFAS) are persistent, bioaccumulative, and potentially toxic chemicals that contaminate many water sources. Removing PFAS from water and ultimately degrading them into products that lack carbon-fluorine bonds will be important for decontaminating water resources. Among numerous emerging degradation methods, mechanochemical degradation of PFAS by ball milling has proven to be a simple and efficient method. However, the reaction pathway and final products after defluorination of PFAS have not been rigorously identified. Herein, we study anionic PFAS degradation by ball milling with alkali hydroxides and identify its degradation byproducts in the final powders for the first time. The degradation of PFAS occurred when KOH was used as a co-reagent, and no degradation is observed with LiOH, NaOH, and CsOH. Results showed degradation of both carboxylic and sulfonated PFAS within 24 h and best KOH:PFAS mass ratio was found to be approximately 20:1. Ion chromatography, thermogravimetric analysis, and 1H, 13C, and 19F NMR spectroscopy revealed that milled powders obtained after perfluorocarboxylate degradation consisted of KF, K2CO3, and potassium formate. Perfluorinated sulfonates also produced K2SO4. Further, we characterized a byproduct by solid-state 19F NMR spectroscopy that contains CxFx functional groups, which was formed during the degradation of all tested PFAS. The results and underlying methodology in this study will inform decisions on the scalability of ball milling processes for PFAS destruction, as well as the safe disposal of end products.