Mechanisms and pathways of PFAS degradation by advanced oxidation and reduction processes: A critical review

Abstract

Numerous studies have demonstrated the degradation of perfluoroalkyl substances (PFAS) by advanced oxidation or reduction processes. Howbeit, the current literature lacks a comprehensive and comparative overview of the various degradation pathways and mechanisms. This systematic review compiles and analyzes the mechanisms and degradation pathways of PFAS by persulfate activation, photocatalysis, UV/sulfite, electron beam, electrochemical oxidation, plasma, ozonation, and sonochemical oxidation. We found that the degradation pathways of perfluorocarboxylic acids (PFCAs) and Perfluoroalkanesulfonic acids (PFSAs) were investigated more than any other PFAS. This review implies that the defluorination and degradation pathways of PFAS are reliant on the type of dominant reactive oxidant or reductive species. For instance, holes usually oxidize PFAS to unstable perfluoroalkyl radicals accompanied by losing head groups, whereas hydroxyl and superoxide radicals cleave CC and C-F bonds. On the other hand, hydrated electrons directly eliminate the head group followed by releasing CF2 units. Furthermore, the imposed crystal facets and functional groups of the introduced catalytic surfaces in the case of heterogeneous catalysis (e.g., photocatalysts) affected the degradation pathways and defluorination rates by anchoring PFAS molecules from the head groups (e.g., carboxylate and sulfonate group). This study lays the groundwork for future research to analyze and elucidate the transformation products and pathways of PFAS in various treatment systems.