Critical review on iron photoreduction (Fe3+ to Fe2+): Mechanisms, applications, and boosting strategies

Iron photoreduction, governed by light-driven Fe 3+ /Fe 2+ redox cycling, is emerging as a unifying mechanism for sustainable photocatalytic and advanced oxidation processes (AOPs) in environmental and energy applications. This review critically consolidates recent advances across homogeneous and heterogeneous systems, highlighting how iron photoreduction enables efficient reactive oxygen species (ROS) generation under visible light. Particular emphasis is placed on its roles in photocatalysis, photo-Fenton, and persulfate-based systems, where distinct radical pathways and operational windows define performance. Beyond mechanistic understanding, we present an integrated perspective spanning material design, process optimization, and system engineering. Key strategies, including visible-light harvesting, ligand-assisted photoreduction, and hybrid process coupling, are evaluated in terms of scalability, energy efficiency, and environmental compatibility. Importantly, we identify the mismatch between laboratory efficiency and real-world operation as a central challenge, driven by catalyst instability, iron leaching, and matrix complexity. Looking forward, this review outlines sustainable pathways based on green ligands, hybrid materials, and solar-driven systems, positioning iron photoreduction as a viable platform for low-energy water treatment and resource recovery. By bridging fundamental chemistry with engineering considerations, this work provides actionable insights to accelerate the deployment of iron-based photochemical technologies in a circular and sustainable context.