Semiquinone radical as a key mediator for the formation of SO4− and OH in quinones activated peroxydisulfate system under alkaline conditions

Quinones, widely considered as environmental pollutants in wastewater, have been identified as redox-active substances capable of activating persulfate for decontamination purpose. However, the mechanism of peroxydisulfate (PDS) activation by quinones under alkaline conditions remains elusive. In this study, methyl- p -benzoquinone (MBQ) was chosen as a model quinone to elucidate the intrinsic mechanism of activating PDS. The results showed that MBQ/PDS system can effectively degrade atrazine (ATZ) under alkaline conditions (especially at pH 10.5) and achieve simultaneous removal of MBQ. Derivatization analysis, quenching experiments and electron spin resonance tests revealed that the self-transformation product of MBQ, methyl-semiquinone radical (MSQ − ), played a major role in activating PDS while alkali served as an auxiliary activator. Specifically, MSQ − functioned as a mediator in PDS activation, regulating the generation of sulfate radical (SO 4 − ) and hydroxyl radical ( OH), thereby inducing the degradation of ATZ through free radical pathways. Two degradation pathways of ATZ were proposed based on UPLC-Q-TOF-MS analysis and DFT calculation. ECOSAR evaluation indicated MBQ/PDS system can detoxify ATZ, yet it remains imperative to concern the formation of more toxic by-products. Furthermore, MBQ/PDS system exhibited strong resistance to water matrixes (excluding Cl − ) and favorable degradation capabilities towards diverse contaminants. This work broadened the possibility of applying the persulfate system in actual water bodies with alkaline environments, achieving the dual benefit of degrading target contaminants and simultaneously removing the activator quinones themselves, providing a novel “treating pollution with waste” water treatment strategy.