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

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•–.