Identification and quantification of reactive species in aqueous medium during application of advanced oxidation processes: A critical review.

[Display omitted] • Lack of pre-evaluation of applicability of additives and methods is the main issue. • Photoluminescence and chemiluminescence are the most promising methods. • EPR, HPLC and the chemical probe are suitable for most AOPs. • Steady-state concentration of reactive species in AOPs ranges from 10-16 to 10-8 M. • Environmental applications benefit disclosing mechanisms and pollutants degradation. Advanced oxidation processes (AOPs) are increasingly being used to efficiently degrade recalcitrant contaminants with in situ generated reactive species. Despite their high efficiency, there is a lack of critical and comprehensive review of robust methods for the identification and quantification of reactive species, thus hampering efforts to disclose the mechanisms of AOPs and to promote their application in the environmental field. This review systematically summarizes detection methods (e.g., qualitative, quantitative, in situ detection techniques) of reactive species, points out problems in identification and quantification, proposes potential strategies and directions for future research. Reactive species involve radicals (e.g., SO 4 •-, •OH, O 2 •-) and non-radical pathways (e.g., 1O 2 , high-valent metal species, activated peroxo-species, photogenerated holes and electron transfers). Current research lacks selective additives and methods, pre-evaluation of the applicability of additives, stability of products, and quantitative relationship between products and reactive species, resulting in misleading. Although photoluminescence and chemiluminescence are the most promising methods, EPR for identification and HPLC and the chemical probe for quantification are suitable for most AOPs. The steady-state concentration of reactive species normally ranged from 10-16 to 10-8 M. The main environmental applications of qualitative and quantitative methods for determining reactive species in AOPs include determining contribution, calculating steady-state concentration, kinetic modeling, optimizing processes, revealing mechanisms and inferring the degradation pathways of contaminants. Developing sensitive and specific methods, without additives, and in situ detection technologies would contribute to disclosing mechanisms, while applying these technologies to more environmental applications would benefit the development of efficient AOPs for treating wastewater containing persistent pollutants. This review provides practical methodological guidance for mechanistic analysis and to constitute a fundamental benchmark for the environmental application of AOPs. [ABSTRACT FROM AUTHOR]