Peroxymonosulfate activation by graphitic carbon nitride co-doped with manganese, cobalt, and oxygen for degradation of trichloroethylene: Effect of oxygen precursors, kinetics, and mechanism

Graphitic carbon nitride (g-C3N4) doped with metals and nonmetals has been widely applied to activating peroxymonosulfate (PMS) for removing organic pollutants. Herein, g-C3N4 co-doped with manganese, cobalt, and oxygen (MCOCN) was prepared via a facile sol–gel calcination protocol for PMS activation to eliminate trichloroethylene (TCE). The influence of different oxygen precursors, namely citric acid, oxalic acid, and barbituric acid, on the catalytic capability of the MCOCN was investigated. Superior performance was found when the MCOCN was synthesized by the citric acid as the oxygen precursor; 100% TCE removal was achieved within 10 min by adding a 0.5 g/L catalyst and 1 mM of the PMS without a pH adjustment. The difference in the activity of the resultant catalysts could be ascribed to the carbon-to-nitrogen ratio and the content of metal on the surface of prepared catalysts. The impacts of parameters such as the concentration of the PMS, the initial pH, and the dose of the catalyst on the kinetics of trichloroethylene degradation were also investigated. A competitive kinetic model was employed to confirm that contribution of the sulfate radicals in TCE degradation was underestimated when the concentration of the quencher was set at 100 times the concentration of the oxidant. The quenching experiments demonstrated that OH∙ and SO4∙- were the main reactive species, while 1O2 and O2∙- played a minor role in the degradation of the TCE by the MCOCN–PMS system. Furthermore, the reusability and ion leaching of the prepared catalysts were evaluated. Our results provide fundamental insights into applying the MCOCN to PMS activation for eliminating trichloroethylene in groundwater.