Do We Appropriately Detect and Understand Singlet Oxygen Possibly Generated in Advanced Oxidation Processes by Electron Paramagnetic Resonance Spectroscopy?

Abstract

Electron paramagnetic resonance (EPR) spectroscopy using sterically hindered amine is extensively applied to detect singlet oxygen (¹O₂) possibly generated in advanced oxidation processes. However, EPR-detectable ¹O₂ signals were observed in not only the ¹O₂-dominated hydrogen peroxide (H₂O₂)/hypochlorite (NaClO) reaction but surprisingly also the ¹O₂-absent Fe(II)/H₂O₂, UV/H₂O₂, and ferrate [Fe(VI)] process with even stronger intensities. By taking advantage of the characteristic reaction between ¹O₂ and 9,10-diphenyl-anthracene and near-infrared phosphorescent emission of ¹O₂, ¹O₂ was excluded in the Fe(II)/H₂O₂, UV/H₂O₂, and Fe(VI) process. The false detection of ¹O₂ was ascribed to the direct oxidation of hindered amine to piperidyl radical by reactive species [e.g., ^•OH and Fe(VI)/Fe(V)/Fe(IV)] via hydrogen transfer, followed by molecular oxygen addition (forming a piperidylperoxyl radical) and back reaction with piperidyl radical to generate a nitroxide radical, as evidenced by the successful identification of a piperidyl radical intermediate at 100 K and theoretical calculations. Moreover, compared to the highly oxidative species (e.g., ^•OH and high-valence Fe), the much lower reactivity of ¹O₂ and the profound nonradiative relaxation of ¹O₂ in H₂O resulted it too selective and inefficient in organic contaminant destruction. This study demonstrated that EPR-based ¹O₂ detection could be remarkably misled by common oxidative species and thereby jeopardize the understandings on ¹O₂.