Pitfalls in probing hydroxyl radical by electron paramagnetic resonance spectroscopy in heterogeneous persulfate-based systems

Abstract

Spin-trapping agents-based electron paramagnetic resonance (EPR) is still widely used to detect hydroxyl radicals (•OH) in engineered environmental systems. Conventionally, the “four-line peak” of DMPO/•OH (1:2:2:1) was considered the gold standard for the presence of •OH, and signal intensity was occasionally applied to quantify •OH concentrations. Based on chemical reaction networks and reaction rate constants, we established a network dynamics model to quantitatively determine the concentrations of •OH and SO₄^•−. For example, when persulfate (S₂O₈^2–, 100 mmol/L, final pH = 3.33) was activated by FeS₂ (100 g/L), SO₄^•− concentration was 2.66 × 10⁻¹⁰ mol/L, 6 orders of magnitude higher than that of •OH (2.67 × 10⁻¹⁶ mol/L), while the concentration of DMPO/SO₄^•− (3.14 × 10⁻¹¹ mol/L) was 7 orders of magnitude lower than that of DMPO/•OH (2.34 × 10⁻⁴ mol/L). These results were validated by EPR. Our study revealed that 81.1%−81.5% of DMPO/•OH is derived from DMPO/SO₄^•− hydrolysis and only 18.5%−18.9% is from direct capture of •OH, questioning the reliability of detecting •OH based on the appearance of the “four-line peak”. Our study underscores the necessity of considering all the transformations among radicals and their adducts during EPR analysis, which also provides a direct and effective method for detecting other radicals with extremely short half-lives in other heterogeneous persulfate systems. The high sensitivity of SO₄^•− and •OH to pH also provides an avenue to regulate the generation of reactive species.

Graphical abstract

Keywords

Electron paramagnetic resonance / Hydroxyl radical / Sulfate radical / Kinetic model / Spin-trapping adduct / Persulfate-based advanced oxidation process

Highlight

	● Combining EPR and kinetic model, we achieved the quantification of •OH and SO4•−.
	● DMPO/•OH primarily derived from the hydrolysis of DMPO/SO4•− (~81%).
	● The concentration of SO4•− is 6 orders of magnitude higher than that of •OH.
	● A novel method for detecting other radicals with extremely short half-lives.

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