Dimethyl sulfoxide as a qualitative EPR indicator of sulfate radical in advanced oxidation processes

The identification of sulfate radical (SO₄^•−) by electron paramagnetic resonance (EPR) in water is challenging owing to its rapid conversion to hydroxyl radicals (^•OH). Here we report for the first time that dimethyl sulfoxide (DMSO), a typical quencher for ^•OH, can be applied as an indicator for SO₄^•− by analyzing the type and intensity variations of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) adducts. The introduction of DMSO into the UV/peroxydisulfate (PDS) system unexpectedly increased the intensity of the DMPO-^•OH signal, and accelerated the rates of pollutants degradation and PDS decomposition. This paradox that adding a radical quencher leads to enhanced radical formation is attributable to DMSO-assisted SO₄^•−(subsequent DMPO-^•OH) formation in the UV/PDS system. SO₄^•−generated from PDS photolysis reacts with DMSO to produce ^•CH₂SOCH₃ that is able to activate another PDS molecule to yield SO₄^•−, as evidenced by the detection of DMPO-^•CH₂SOCH₃ in mass spectrometry and predicted by density functional theory (DFT) calculations. The contrasting EPR responses of DMSO toward ^•OH (quenching DMPO-^•OH and generating DMPO-^•CH₃) and SO₄^•− (increasing DMPO-^•OH) have thus established it as a reliable marker for the presence of SO₄^•−, a finding that has been successfully validated across twelve advance oxidation processes (AOPs) that potentially involve SO₄^•−.