Hydroxyl radical chemistry in advanced oxidation processes: a cross-system mechanistic framework and catalyst design...

Hydroxyl radicals (˙OH) are widely recognized as the most powerful oxidizing species in advanced oxidation processes (AOPs) and play a central role in the degradation of persistent organic contaminants. However, current understanding of ˙OH chemistry remains fragmented because radical generation is typically interpreted within technology-specific frameworks, limiting cross-system mechanistic understanding and catalyst development. This review establishes a unified framework for ˙OH generation across AOPs and demonstrates that diverse technologies can be interpreted through three recurring mechanistic pathways: oxidant activation, ˙OH generation from water through oxidation or dissociation, and metal redox cycling. Furthermore, we introduce the concept of radical convergence, highlighting that reactive oxygen species (ROS), including SO₄˙^-, O₂˙^-, ¹O₂, , etc., generated from different oxidants can participate in interconnected ROS reaction networks, in which ˙OH represents an important reactive species under appropriate conditions. Based on this framework, the relative contribution of ˙OH across representative AOPs is systematically compared, together with the relationships among radical generation, radical utilization, competing ROS, and water-matrix effects. A key contribution of this review is the introduction of ˙OH utilization efficiency (η _˙OH) as a conceptual descriptor linking radical generation with effective pollutant oxidation. Building upon this perspective, catalyst design principles are proposed, including interfacial confinement of ˙OH, local pollutant enrichment, pathway optimization, and matrix-tolerant catalyst development. Future opportunities in ˙OH engineering are also discussed, with emphasis on quantitative radical-flux analysis and data-driven, mechanism-guided catalyst discovery. By shifting the focus from technology-specific classifications to ˙OH-centered reaction networks, this review provides a unified mechanistic perspective and mechanism-guided design principles for advancing AOP catalyst development.