Photochemical Degradation of PFAS: Mechanistic Insights and Design Strategies for Homogeneous and Heterogeneous Systems.

Poly/perfluoroalkyl substances (PFAS) have emerged as contaminants of global concern due to their extreme persistence, bioaccumulation, and toxicity. Conventional wastewater treatment technologies are ineffective for PFAS removal, prompting increasing interest in photochemical degradation as a promising alternative. Among these, homogeneous ultraviolet (UV) based systems and heterogeneous photocatalysis have attracted significant attention, while a comprehensive mechanistic discussion and comparison of these approaches remains fragmented. This review critically assesses recent advances in PFAS photodegradation via homogeneous and heterogeneous processes, with emphasis on the roles of reactive species such as oxidative radicals, hydrated electrons, and photoexcited charges in governing degradation pathways. Moreover, key factors, including system configuration, PFAS structure, chemical dosage, catalyst properties, and water matrix composition, are systematically analyzed. By comparing reactivity trends and degradation pathways across systems, the review identifies essential mechanistic insights and bottlenecks that hinder complete defluorination. Finally, the design strategies to improve selectivity, efficiency, and scalability are discussed, along with perspectives on translating these insights into practical treatment systems for complex aqueous environments, especially those containing low concentrations of PFAS in real water matrices. This review aims to inform the rational development of next-generation PFAS remediation technologies that are both mechanistically grounded and practically applicable.