Cage-like ulva biochar confined synthesis of Fe₃O₄/ZnO heterojunction nanoparticles for synergistic adsorption and photocatalytic degradation of PFOA

Abstract Perfluorooctanoic acid (PFOA) has emerged as a new urgent pollutant in aquatic environments due to its high persistence and ecotoxicity. In photocatalytic degradation systems, challenges such as rapid recombination of electron–hole pairs (e⁻/h⁺), short lifespans of reactive oxygen species (ROS), and insufficient ROS generation hinder the efficient degradation of PFOA. This study presents a novel "scallop cage" architecture, constructed using Ulva biochar to create confined spaces that encapsulate the Fe₃O₄/ZnO heterojunction. This approach not only controls the crystal size of the Fe₃O₄/ZnO heterojunction but also confines the degradation reactions to a specific space, significantly shortening the mass transfer distance for ROS and effectively mitigating their rapid deactivation in aqueous-phase degradation processes. Furthermore, the confinement effect enhances the generation of multiple reactive species (·O₂⁻, ·OH, 1 O₂, and h⁺). The optimized FZS@UBC-2 composite photocatalyst achieved a PFOA removal efficiency of 97.53%. In practical applications, FZS@UBC-2 efficiently decomposes PFOA in complex aqueous matrices and can be easily recovered using an external magnetic field. This work not only expands the application of algae-derived biochar in advanced oxidation processes but also offers a sustainable strategy for addressing persistent organic pollutants in aquatic environments. Graphical Abstract