Undiscovered Spin Trapping Artifacts in Persulfate Oxidation Processes: Implications for Identification of Hydroxyl or Sulfate Radicals in Water

Abstract

Electron paramagnetic resonance (EPR) coupled with the spin trapping technique is widely used to identify radicals and has become a tool that is being heavily relied on to probe the reaction mechanism in advanced oxidation processes (AOPs). However, the unexpected spin trapping artifacts in a highly oxidizing environment may mislead the mechanistic interpretations of AOPs. Here, we report previously unrevealed reactions between peroxydisulfate (PDS) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) or N-tert-butyl-α-phenylnitrone (PBN). The mainly detectable adducts are attributed to DMPO-OH and PBN-OH. Through radical quenching experiments, the nonradical generation pathways of DMPO-OH and PBN-OH are validated, and the 17O incorporation experiment demonstrates that H2O plays a critical role in DMPO-OH production. The formation routes of the primary artifact signals (DMPO-OH, DMPO-SO4, PBN-OH, and PBN-SO4) are further proposed by applying diverse solvents. PDS may directly oxidize DMPO to generate DMPO-SO4 in acetonitrile or acetone solvent, while DMPO-OH is yielded using H2O as the solvent. The generation of PBN-OH or PBN-SO4 follows the inverted spin trapping mechanism (a combination of one-electron oxidation and nucleophilic addition). These findings manifest the necessity of selecting the proper spin traps and performing careful control tests during exploration of the catalytic oxidation mechanism under highly oxidizing conditions.