Hydroxyl radical in anodic oxidation-based electrochemical advanced oxidation processes: Critical understanding and a new perspective

The rapid expansion of industrial activities and urbanization induced the widespread presence of recalcitrant organic pollutants in aquatic systems. Anodic oxidation (AO)-based electrochemical advanced oxidation processes (EAOPs) have emerged as an intriguing strategy, utilizing reactive oxygen species (ROS) to achieve near-complete mineralization of contaminants. Among ROS, hydroxyl radicals (•OH) stand out as the most potent oxidant (E⁰ = 2.73 V vs standard hydrogen electrode, SHE), with their generation mechanisms, detection methodologies, and regulation strategies being central to enhancing EAOPs efficiency. This review systematically examines the reaction mechanisms of •OH (electron transfer, hydrogen abstraction, hydroxyl addition), detection techniques (spin-trapping, fluorescence probes, electrochemical sensors), and generation dynamics in EAOPs. In addition, surface-bound hydroxyl species (OH*) on transition metal oxide electrodes reveal novel pathways for efficient pollutant degradation at lower potentials was also reviewed. This review shows that the key challenges facing anodic oxidation technology are related to (1) developing electrode materials with balanced oxygen evolution reaction (OER) overpotential and rapid charge transfer, (2) evaluating the state-of-the-art detection techniques, highlighting their merits and limitations in •OH detection, (3) decoupling the •OH generation kinetics and discussing the conflict and intrinsic connection of •OH and surface bonding OH*, and (4) designing strategies adaptable to complex wastewater matrices. By bridging electrochemistry, materials science, and environmental engineering, EAOPs hold transformative potential for transitioning from benchtop innovations to scalable water remediation technologies.