Do We Appropriately Detect and Understand Singlet Oxygen Possibly Generated in Advanced Oxidation Processes by...

Area of Science:

• Environmental Chemistry
• Analytical Chemistry
• Advanced Oxidation Processes

Background:

• Electron paramagnetic resonance (EPR) spectroscopy with hindered amines is a common method for detecting singlet oxygen (¹O₂).
• ¹O₂ is crucial in advanced oxidation processes (AOPs) for water treatment and pollutant degradation.
• Previous studies assumed EPR signals directly correlated with ¹O₂ presence in AOPs.

Purpose of the Study:

• To investigate the reliability of EPR spectroscopy in detecting ¹O₂ in various AOPs.
• To identify the source of false positive ¹O₂ signals observed in EPR.
• To clarify the role and reactivity of ¹O₂ compared to other oxidants in AOPs.

Main Methods:

• Utilized EPR spectroscopy with sterically hindered amines to detect ¹O₂.
• Employed 9,10-diphenyl-anthracene quenching and near-infrared phosphorescence to confirm ¹O₂ presence/absence.
• Investigated reactive species in hydrogen peroxide/hypochlorite, Fe(II)/H₂O₂, UV/H₂O₂, and ferrate (Fe(VI)) processes.
• Identified radical intermediates using EPR at 100 K and performed theoretical calculations.

Main Results:

• False ¹O₂ signals were detected by EPR in ¹O₂-absent Fe(II)/H₂O₂, UV/H₂O₂, and Fe(VI) processes.
• These false signals originated from the direct oxidation of hindered amines by reactive species like hydroxyl radicals (^•OH) and high-valence iron species.
• ¹O₂ exhibited lower reactivity and efficiency in degrading organic contaminants compared to ^•OH and high-valence iron species.

Conclusions:

• EPR detection of ¹O₂ can be significantly misled by common oxidative species in AOPs.
• The formation of nitroxide radicals via amine oxidation, not ¹O₂, explains the observed EPR signals in certain AOPs.
• Rethinking the interpretation of EPR signals is crucial for accurate understanding of AOP mechanisms and ¹O₂'s role.