Challenging the contamination of per- and polyfluoroalkyl substances in water: advanced oxidation or reduction?

Nowadays, the increasing discharge of persistent per- and polyfluoroalkyl substances (PFASs) caused serious environmental issues. In addition to advanced oxidation processes (AOPs), advanced reduction processes (ARPs) based on reducing radicals, e.g., hydrated electron and superoxide anions have attracted great attentions as promising methods for remediation of PFASs pollution. This review, based on 128 cited references, provides a critical overview on the performance of different AOPs and ARPs. The unique properties of reactive species, e.g., SO₄^•- and e_aq⁻ and their generation mechanisms in different systems were discussed. Moreover, the efficiencies of different systems were further compared from several aspects, e.g., PFASs decomposition rate, reaction time and energy consumption. More specially, for some model compounds of PFASs, such as perfluorooctanoic acid, perfluorooctane sulfonate and perfluoroalkyl ether carboxylic acids, we systematically discussed their degradation and defluorination pathways in both AOPs and ARPs. The reported literatures showed that the degradation pathways of these PFASs are closely related to their head groups in either AOPs or ARPs. Finally, some key conclusions were summarized, and the implications of the state-of-the-art knowledge on practical PFASs remediation in water treatments were summarized and the future priority research directions were proposed.