Binary MOFs-derived Mn-Co3O4 for efficient peroxymonosulfate activation to remove sulfamethoxazole: Oxygen vacancy-assisted high-valent cobalt-oxo species generation

High-valent cobalt-oxo species show obvious advantages in oxidant utilization efficiency and selective oxidation of organic pollutants, while reducing the energy barrier of O-O bond cleavage of peroxymonosulfate (PMS) to continuously produce high-valent cobalt-oxo species remains challenging. Herein, binary metal-organic frameworks-derived Mn-Co3O4 was synthesized and utilized to activate PMS for sulfamethoxazole removal. Almost 100 % of sulfamethoxazole was removed within 10 min by Mn-Co3O4/PMS system and the reaction rate constant was 0.3148 min-1. Moreover, Mn doping improved oxygen vacancies content in Mn-Co3O4 and reduced the leaching of cobalt ions. The results exhibited that hydroxyl radicals, sulfate radicals, superoxide radicals, singlet oxygen, electron transfer mediated by metal-PMS*, and high-valent cobalt-oxo species resulted in the sulfamethoxazole removal and their contributions were highly dependent on pH values. Under acidic, neutral, and weakly basic conditions, singlet oxygen-dominant non-radical oxidation was the primary pathway. While under strong alkaline conditions, radical oxidation was the primary pathway, in which superoxide radicals and hydroxyl radicals played dominant roles. It was also found that both acidic and alkaline conditions were conducive to high-valent cobalt-oxo species production. The formation process of high-valent cobalt-oxo species was revealed through density functional theory calculations. It has been demonstrated that oxygen vacancies decreased the adsorption energy of PMS onto Mn-Co3O4 and the energy barrier of O-O bond breakage of PMS in Co(II)-PMS*, promoting the generation of high-valent cobalt-oxo. Additionally, the Mn-Co3O4/PMS system showed desirable potential for eliminating refractory organic pollutants from actual waterbodies. This study deepens the understanding of defect engineering-assisted high-valent metal-oxo species formation mechanism.