Oxidative and reductive mechanisms for the photocatalytic degradation of per- and polyfluoroalkyl substances (PFAS): A critical review

Per- and polyfluoroalkyl substances (PFAS) are highly persistent environmental contaminants whose stability originates from strong C-F bonds, electron-deficient fluorinated chains, and resistance to conventional degradation processes. Separation-based technologies such as adsorption, ion exchange, and membrane filtration can remove PFAS from water but do not achieve chemical destruction and often generate secondary waste streams. Photocatalysis has emerged as a promising destruction-oriented strategy because it enables light-driven oxidative and reductive reactions under relatively mild conditions. This review critically examines oxidative and reductive photocatalytic mechanisms governing PFAS degradation, with emphasis on C-F bond activation, defluorination, mineralization, and transformation-product formation. Oxidative pathways driven by photogenerated holes and reactive oxygen species mainly initiate headgroup oxidation, decarboxylation, desulfonation, and chain shortening, but frequently result in persistent short-chain intermediates. In contrast, reductive pathways involving conduction-band electrons and surface-trapped electrons can directly activate the fluorinated backbone through dissociative electron transfer, enabling deeper defluorination, whereas homogeneous advanced reduction processes are discussed separately as mechanistic references for hydrated-electron-mediated PFAS reduction. The review further discusses how PFAS molecular structure, headgroup chemistry, chain length, adsorption behavior, semiconductor band alignment, heterojunction design, surface functionalization, and reactor conditions influence photocatalytic performance. Recent advances in defect engineering, Z-scheme and S-scheme systems, MOF-based photocatalysts, photoelectrocatalysis, and 3D-printed structured photocatalytic reactors are highlighted as promising directions. Finally, key challenges are identified, including incomplete mineralization, unclear oxidative–reductive pathway contributions, limited fluorine mass balance, poor scalability, and insufficient validation in realistic water matrices. This review provides a mechanistic and design-oriented framework for developing next-generation photocatalytic systems capable of sustainable PFAS destruction.