Multi-active sites of Co/N-biochar created by ball-milling for peroxymonosulfate-activated degradation of diclofenac

A cobalt-nitrogen co-doped biochar catalyst (Co/N-BC-300) was synthesized from spent coffee grounds via urea-assisted ball milling and low-temperature pyrolysis for peroxymonosulfate (PMS) activation in diclofenac (DCF) degradation. Ball milling successfully disperses cobalt precursors, increases surface defects, and enhances nitrogen doping. These structural improvements expose active sites, thereby accelerating PMS activation and electron transfer. The optimized catalyst achieves complete DCF removal within 7 min under mild conditions. Quenching and electron paramagnetic resonance analyses confirm singlet oxygen (¹O₂) was the dominant species in a non-radical degradation pathway with superoxide radicals (O₂^•-) serving as secondary contributors. Furthermore, density functional theory calculations are used to identify key electrophilic and radical attack sites, supporting the proposed mechanisms. The catalyst demonstrates excellent stability, maintaining >93 % activity after five regeneration cycles with negligible cobalt leaching. Also, the catalytic performance can be maintained in natural water matrices containing organic matter and coexisting anions. Toxicity assessments confirm that the degradation intermediates pose a low environmental risk. This study highlights the dual benefits of waste valorization and ball milling-assisted catalyst engineering in advancing PMS-based water treatment technologies.