Undiscovered roles of aminopolycarboxylates as reversible masking agents for the DMPO-OH artifact in persulfate oxidation systems

Although electron paramagnetic resonance (EPR) spin trapping is widely used technique for characterizing reactive radicals in advanced oxidation processes (AOPs), the presence of EPR artifact signals can significantly undermine the accuracy of results, particularly in highly oxidative environments. Previous studies have indicated that the peroxydisulfate (PDS)/5,5-dimethyl-1-pyrroline-N-oxide (DMPO) system generates a time-dependent DMPO-OH artifact signal. Herein, we report for the first time the reversible masking of these artifacts by aminopolycarboxylates (APCs) chelators such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA). The efficacy of this masking was found to be positively correlated with the chelation strength of the APCs and remained largely unaffected by variations in pH. Notably, the masking effect was reversed upon the introduction of redox-inactive metal ions, thereby underscoring the pivotal role of unbound APCs. Solvent-controlled experiments revealed that APCs suppress artifact formation via three pathways: accelerating DMPO-OH adduct decomposition, blocking nucleophilic substitution of DMPO-SO₄ by water, and inhibiting direct oxidation of DMPO by PDS. Radical validation and UV–Vis spectral analysis further suggest that APCs may also form protective complexes with DMPO, thereby averting its oxidation. These findings highlight the necessity of considering APCs-DMPO interactions in AOPs systems involving chelators, as failure to do it may lead to an underestimation of reactive species and misinterpretation of EPR results.