Increasing attention has been paid to the activation of the ideal oxidant O-2 to reactive oxygen species for environmental pollutants transformation and complete degradation. In this work, the oxidation of o-aminophenol (OAP), a poor biodegradable intermediate, by simple Co2+ ions in HCO3- aqueous solution with O-2 under ambient conditions was investigated. The results reveal that OAP is efficiently transformed to a less harmful compound 2-aminophenoxazine-3-one (APZ) by the CO2+-HCO3- system. HCO3- is necessary for the reaction by promoting the protonation of the two-electron reduction species of O-2 to produce H2O2. The metal-bound hydroxyl radical ((OH)-O-center dot) generated from the intermediacy of H2O2 contributes to APZ formation. Based on the analysis of electron spin resonance spin-trapping technologies, radical scavenging measurements and kinetics of APZ formation under different conditions, a possible pathway for (OH)-O-center dot radical formation and APZ production is proposed. This study can provide new insight on the mechanism of the molecular oxygen activation by simple metal complexes and their application on pollutant removal under mil reaction conditions. (C) 2014 Elsevier B.V. All rights reserved.