Carbonyl-Induced Reduction from Co(III) to Co(II) in CoxSy Enables Sulfate Radical-Dominated Peroxymonosulfate Activation

Abstract

Cobalt sulfide (CoxSy) shows promise for activating peroxymonosulfate (PMS) in advanced oxidation processes (AOPs), but its application faces key challenges, including low sulfate radical (SO4•–) generation, inefficient PMS decomposition, and limited mineralization efficiency for antibiotic removal. This study addresses these limitations by coordinating carbonyl groups (C=O) with cobalt to stabilize low-valence cobalt species and enhance electron transfer through defect engineering. A lignite-activated coke-supported cobalt sulfide (LAC@CoxSy-V) was synthesized, exhibiting a nearly complete conversion to low-valence cobalt (Co2+) and abundant sulfur vacancies. Compared to CoxSy/PMS system, the Kobs of sulfamethoxazole (SMX) degradation and the mineralization efficiency of SMX in the LAC@CoxSy-V/PMS system are 18-fold and 2-fold higher, respectively. Mechanistic studies revealed that CoxSy complexed on the LAC surface enables cobalt-carbonyl coordination, facilitating electron transfer from the electronegative C=O to Co3+, reducing it to Co2+. The introduction of sulfur vacancies further increases the proportion of low-valence cobalt, promoting PMS activation to generate SO4•–. These radicals act as the primary reactive oxygen species (ROS), while sulfur vacancies simultaneously promote singlet oxygen (1O2) generation, synergistically enhancing SMX degradation. The applicability of this system was validated under various conditions, including matrix interference tests, cyclic stability assessment, and continuous-flow fixed-bed reactor experiments simulating medical wastewater treatment. This study provides valuable insight into defect and ligand engineering strategies for efficient antibiotic removal via PMS-AOPs.