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

Electron paramagnetic resonance (EPR) spectroscopy usingstericallyhindered amine is extensively applied to detect singlet oxygen (O-1(2)) possibly generated in advanced oxidation processes.However, EPR-detectable O-1(2) signals were observedin not only the O-1(2)-dominated hydrogen peroxide(H2O2)/hypochlorite (NaClO) reaction but surprisinglyalso the O-1(2)-absent Fe(II)/H2O2, UV/H2O2, and ferrate [Fe(VI)] processwith even stronger intensities. By taking advantage of the characteristicreaction between O-1(2) and 9,10-diphenyl-anthraceneand near-infrared phosphorescent emission of O-1(2), O-1(2) was excluded in the Fe(II)/H2O2, UV/H2O2, and Fe(VI) process.The false detection of O-1(2) was ascribed to thedirect oxidation of hindered amine to piperidyl radical by reactivespecies [e.g., (OH)-O-center dot and Fe(VI)/Fe(V)/Fe(IV)] viahydrogen transfer, followed by molecular oxygen addition (forminga piperidylperoxyl radical) and back reaction with piperidyl radicalto generate a nitroxide radical, as evidenced by the successful identificationof a piperidyl radical intermediate at 100 K and theoretical calculations.Moreover, compared to the highly oxidative species (e.g., (OH)-O-center dot and high-valence Fe), the much lower reactivity of O-1(2) and the profound nonradiative relaxation of O-1(2) in H2O resulted it too selective and inefficientin organic contaminant destruction. This study demonstrated that EPR-based O-1(2) detection could be remarkably misled by commonoxidative species and thereby jeopardize the understandings on O-1(2). Hinderedamine-based electron paramagnetic resonance spectroscopymay mislead O-1(2) detection because of the interferenceof oxidative species such as center dot OH and high-valence Fe.