Activation of Peroxymonosulfate by Oxygen Vacancies-enriched Cobalt-doped Black TiO_2 Nanotubes for Removal of Organic Pollutants

Cobalt-mediated activation of peroxymonosulfate (PMS) has been widely investigated for the oxidation of organic pollutants. Herein, we employ cobalt-doped Black TiO_2 nanotubes (Co-Black TNT) for the efficient, stable, and reusable activator of PMS for the degradation of organic pollutants. Co-Black TNTs induce the activation of PMS by itself and stabilized oxygen vacancies that enhance the bonding with PMS and provide catalytic active sites for PMS activation. A relatively high electronic conductivity associated with the coexistence of Ti^(4+) and Ti^(3+) in Co-Black TNT enables an efficient electron transfer between PMS and the catalyst. As a result, Co-Black TNT is an effective catalyst for PMS activation, leading to the degradation of selected organic pollutants when compared to other TNTs (TNT, Co-TNT, and Black TNT) and other Co-based materials (Co_3O_4, Co-TiO_2, CoFe_2O_4, and Co_3O_4/rGO). The observed organic compound degradation kinetics are retarded in the presence of methanol and natural organic matter as sulfate radical scavengers. These results demonstrate that sulfate radical is the primary oxidant generated via PMS activation on Co-Black TNT. The strong interaction between Co and TiO_2 through Co–O–Ti bonds and rapid redox cycle of Co^(2+)/Co^(3+) in Co-Black TNT prevents cobalt leaching and enhances catalyst stability over a wide pH range and repetitive uses of the catalyst. Electrode-supported Co-Black TNT facilitates the recovery of the catalyst from the treated water.