First Direct and Unequivocal Electron Spin Resonance Spin-Trapping Evidence for pH-Dependent Production of Hydroxyl Radicals from Sulfate Radicals

• Published in: Environmental science & technology Vol. 54; no. 21; pp. 14046 - 14056
• Main Authors: Gao, Hui-Ying, Huang, Chun-Hua, Mao, Li, Shao, Bo, Shao, Jie, Yan, Zhu-Ying, Tang, Miao, Zhu, Ben-Zhan
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 03-11-2020
• Subjects: Comparative studies; Coupling; Cyclic N-Oxides; Dimethyl sulfoxide; Electron paramagnetic resonance; Electron spin; Electron spin resonance; Electron Spin Resonance Spectroscopy; Electrons; Environmental science; Free Radicals; Hydrogen-Ion Concentration; Hydroxyl Radical; Hydroxyl radicals; pH effects; Resonance; Scavenging; Spin Labels; Spin resonance; Sulfates; Trapping; Treatment and Resource Recovery
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.0c04410

Recently, the sulfate radical (SO4 • –) has been found to exhibit broad application prospects in various research fields such as chemical, biomedical, and environmental sciences. It has been suggested that SO4 • – could be transformed into a more reactive hydroxyl radical (•OH); however, no direct and unequivocal experimental evidence has been reported yet. In this study, using an electron spin resonance (ESR) secondary radical spin-trapping method coupled with the classic spin-trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and the typical •OH-scavenging agent dimethyl sulfoxide (DMSO), we found that •OH can be produced from three SO4 • –-generating systems from weakly acidic (pH = 5.5) to alkaline conditions (optimal at pH = 13.0), while SO4 • – is the predominant radical species at pH < 5.5. A comparative study with three typical •OH-generating systems strongly supports the above conclusion. This is the first direct and unequivocal ESR spin-trapping evidence for •OH formation from SO4 • – over a wide pH range, which is of great significance to understand and study the mechanism of many SO4 • –-related reactions and processes. This study also provides an effective and direct method for unequivocally distinguishing •OH from SO4 • –.