Identification and Quantification of Reactive Oxygen Species in Photocatalytic Membrane Reactors: Reassessment of the Validity of Methods.

Reactive oxygen species (ROS), such as hydroxyl radicals (·OH), superoxide radicals (·O2-), and singlet oxygen (1O2), play a critical role in sustainable micropollutant remediation using a photocatalytic membrane reactor (PMR) where the reactants are spatially confined within the membrane structure. However, the understanding of its reaction mechanisms regarding ROS generation remains contentious, with varying and often contradictory reported findings obtained using different detection methodologies. This review comprehensively surveys prevalent methods for detecting ROS, including scavenging, chemical-probing, and electron spin resonance (ESR) techniques. It focuses on the uncertainties and possible misinterpretations encountered in recent studies. The resulting insights are applied to reassess the reliability of these techniques in accurately detecting ROS within a PMR by comparing outcomes from diverse methods. The literature review and experimental findings presented reveal that the foundational assumptions for ROS measurement in many approaches are often unrealistic in PMR systems due to the extremely short hydraulic residence time and interference from complex coexisting radicals and light. Therefore, caution is necessary when applying these methods to interpret the mechanisms of photocatalytic degradation. Employing a combination of detection techniques can enhance the accuracy of ROS identification, while many validation challenges remain and are highly dependent on experimental conditions and solution chemistry.