Advanced oxidation processes that utilize peroxymonocarbonate (HCO
4-), generated in-situ through the reaction of HCO
3- and H
2O
2, are employed for the removal of pollutants in water. Nevertheless, the precise role of H
2O
2 in these processes remains a subject of debate. This study established a HCO
4--based oxidation system using NaHCO
3 and H
2O
2 for the degradation of acetaminophen and investigated the activation mechanisms of coexisting oxidants. Under thermal activation conditions, the O
O bond in HCO
4- (HO
OCOO
-) was more readily cleaved than the O
O bond in the co-existing oxidant H
2O
2 (HO
OH), leading to the generation of reactive oxygen species (ROS). Based on kinetics and ROS evaluation, H
2O
2 primarily served to form HCO
4- rather than converting to ·OH or O
2, with HCO
4- acting as the primary oxidant for degradation through the formation of
and
·OH. In this oxidation system, H
2O
2 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 HCO
3- and 20 mM H
2O
2. The apparent activation energy of acetaminophen degradation and HCO
4- activation were calculated as 90.83 kJ mol
-1 and 18.81 kJ mol
-1, respectively. Moreover, a novel CO
2-derived HCO
4--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 H
2O
2 inhibited the degradation and quenched
and
·OH to yield
·O
2- and
1O
2. Furthermore, EPR analysis and quenching experiments indicate that
was mainly responsible for acetaminophen degradation. This work provides fundamental understanding of the HCO
4--based oxidation system.