Constructing zinc single-atom catalysts for the direct electron-transfer mechanism in peroxymonosulfate activation to degrade sulfamethoxazole efficiently

Direct electron-transfer dominated organic pollutant removal technology is considered an economical and promising method for selective water and wastewater treatment. However, in the heterogeneous catalysis of activating peroxymonosulfate (PMS) to generate the surface-bound PMS*, reactive oxygen species such as sulfate radical, hydroxyl radical, and singlet oxygen are easily produced at the same time, resulting in waste of PMS. Herein, we reported an efficient zinc single-atom catalyst (Zn-N@C) that could activate PMS to induce an electron-transfer mechanism and degrade 95.7% sulfamethoxazole (SMX) within 20 min, which was superior to most of the advanced oxidation systems that have been reported for the removal of SMX. The negligible effect of anions and humic acid in water on Zn-N@C/PMS systems made it potential for practical application. Experiments and density functional theory calculations revealed that ZnN4 as the active site for PMS activation, and the enhanced redox potential of Zn-N@C/PMS* complexes improved the removal efficiency of SMX by demonstrating the increased work function and enlarged electron density near the Fermi level of Zn-N@C after PMS adsorption. SMX was degraded predominately via SO2 extrusion, hydroxylation, and cleavage of the S-N and S-C bonds. The diminished ecotoxicity of transformation products suggested a controlled risk of SMX degradation during the Zn-N@C/PMS treatment process. This study expands the research scope of transitional metal-based single-atom catalysts to zinc on PMS activation and deepens the understanding of electron-transfer oxidation pathways.