Reconsideration of the role of hydrogen peroxide in peroxymonocarbonate-based oxidation system for pollutant control

Advanced oxidation processes that utilize peroxymonocarbonate (HCO₄^-), generated in-situ through the reaction of HCO₃^- and H₂O₂, are employed for the removal of pollutants in water. Nevertheless, the precise role of H₂O₂ in these processes remains a subject of debate. This study established a HCO₄^--based oxidation system using NaHCO₃ and H₂O₂ for the degradation of acetaminophen and investigated the activation mechanisms of coexisting oxidants. Under thermal activation conditions, the O-O bond in HCO₄^- (HO-OCOO^-) was more readily cleaved than the O-O bond in the co-existing oxidant H₂O₂ (HO-OH), leading to the generation of reactive oxygen species (ROS). Based on kinetics and ROS evaluation, H₂O₂ primarily served to form HCO₄^- rather than converting to ·OH or O₂, with HCO₄^- acting as the primary oxidant for degradation through the formation of ${\text{CO}}_3^{·-}$and ·OH. In this oxidation system, H₂O₂ utilization efficiency for ·OH production reached 27.34%, ·OH yield reached 24.15% and acetaminophen degradation efficiency realized 83% at 60 °C with 20 mM HCO₃^- and 20 mM H₂O₂. The apparent activation energy of acetaminophen degradation and HCO₄^- activation were calculated as 90.83 kJ mol^-1 and 18.81 kJ mol^-1, respectively. Moreover, a novel CO₂-derived HCO₄^--based system led to a comparable acetaminophen degradation efficiency of 82% and a higher k_obs of 0.028 min^-1. The system optimization and ROS evaluation suggest that high concentration of H₂O₂ inhibited the degradation and quenched ${\text{CO}}_3^{·-}$ and ·OH to yield ·O₂^- and ¹O₂. Furthermore, EPR analysis and quenching experiments indicate that ${\text{CO}}_3^{·-}$ was mainly responsible for acetaminophen degradation. This work provides fundamental understanding of the HCO₄^--based oxidation system.