Systematic assessment of methylene blue removal by Co-N/C nanofibers: key factors, degradation pathways, DFT study and toxicity evaluation

The practical application of metal-organic framework (MOF)-derived catalysts in advanced oxidation processes (AOPs) is often hindered by challenges in recoverability and stability in complex water matrices. To overcome these limitations, we developed a scalable strategy to fabricate N-doped carbon nanofibers with coexisting atomic Co sites and metallic Co nanoparticles (Co-N/C) by electrospinning a ZIF-67/polyacrylonitrile (PAN) composite followed by controlled pyrolysis. This integrated design yields a catalyst with a hierarchical porous structure and a high specific surface area (317.55 m² g^-1), which synergistically enhances mass transfer and exposes abundant active sites. The optimized Co-N/C catalyst demonstrates ultrafast catalytic activity, achieving over 98% degradation of methylene blue (MB) within 5 min via efficient peroxymonosulfate (PMS) activation. Mechanistic studies, supported by quenching experiments and density functional theory (DFT) calculations, reveal that the degradation proceeds via a nonradical-dominated pathway, with singlet oxygen (¹O₂) generated through interfacial electron transfer playing the primary role, while sulfate (SO₄•^-) and hydroxyl (•OH) radicals from Co²⁺/Co³⁺ redox cycling serve a secondary function. The catalyst exhibits excellent operational robustness, maintaining high efficiency (>92%) across a broad pH range (3-9) and showing remarkable reusability with minimal activity loss over five consecutive cycles. Furthermore, ecological structure-activity relationship (ECOSAR) modeling indicates a significant reduction in the environmental toxicity of the degradation intermediates. This work provides not only an effective and eco-friendly candidate for dye wastewater remediation but also fundamental insights into the rational design of MOF-derived functional materials for superior, nonradical-predominant AOPs.