Scanning Electrochemical Microscopy for Quantifying Reactive Intermediate Species in Advanced Oxidation and Reduction Processes (AOPs and ARPs)

Abstract Radical-driven contaminant degradation plays an increasingly important role in environmental applications, demanding novel techniques to quantify them in situ. This review summarizes recent progress in applying scanning electrochemical microscopy (SECM) for the detection and quantification of reactive intermediate species (RIS) critical in advanced water treatment processes such as advanced oxidation and advanced reduction processes (AOPs/ARPs). Two SECM strategies, including direct detection and indirect detection, are discussed. Direct detection can capture the current signal from the short-lived RIS reaction but requires nanoscale precision in positioning the ultramicroelectrodes (UME) to ensure that the diffusion time is shorter than the lifetime of the RIS of interest, limiting broader applicability in real-world aqueous systems. Indirect detection uses spin traps to convert transient RIS into more stable adducts or uses redox mediators to amplify their current signal. However, it faces challenges of poor selectivity and redox mediator selection complexity. In AOPs/ARPs, the presence of RIS quenchers in the water matrix, such as background anions and natural organic matter (NOM), can markedly complicate the interpretation of SECM outputs. In this review, the fundamental processes of RIS detection by SECM, as well as the potential uses and possible challenges of applying SECM for RIS detection in AOPs/ARPs, are discussed. The insights presented provide a conceptual framework to extend the RIS characterization by SECM to the field of advanced water treatment technologies.