An activated peroxymonocarbonate (PMC) system is expected to be developed as a chemical oxidation technology for water remediation, especially for the karstic regions with naturally occurring bicarbonates. In the present study, CoxMn(3-x)O(4) (cobalt manganese oxides, where x= 1.08, 1.5, 1.77) catalysts with micron-sized spherical structures were prepared via a solvothermal method followed by an annealing process. Characterization results showed that all the prepared CoxMn(3-x)O(4) catalysts were well-crystallized spinels with porous micro-nano hierarchical structures. Compared to Co3O4 and Mn2O3 catalysts with similar microstructures, the CoxMn(3-x)O(4) catalysts had lower oxidation potentials and more redox-active sites, therefore demonstrating improved PMC activation for the degradation of 2,4-dichlorophenol (2,4-DCP). The activated PMC process (with 5mM bicarbonate) showed a moderate H2O2 consumption rate, but it was more efficient for the degradation of 2,4-DCP than the heterogeneous Fenton (H-Fenton) process (H2O2 + CoxMn(3-x)O(4) catalysts). In a mixture of H2O2, bicarbonate and CoxMn3-xO4 catalyst, the activated PMC process was the dominant route for the degradation of 2,4-DCP, in which the generated hydroxyl radicals (% OH) mostly combined with bicarbonate ions, resulting in a more complex oxidizing mechanism involving various free radicals, e. g. % O2 -, % OH and % CO3 -. Moreover, it was observed that bicarbonate dosage played a more significant role than H2O2 dosage in the degradation of 2,4-DCP, further validating that the activated PMC process was the major contributing route for decontamination of the organics. Based on the experimental results of the PMC system, the functioning mechanism of CoxMn(3-x)O(4) catalysts and the oxidizing routes of 2,4-DCP were depicted. With the use of high-performance Co-Mn binary oxides, the activated PMC method can be engineered for groundwater remediation in karstic regions.