Unraveling the synergistic cobalt-nitrogen cooperation in biochar for enhanced peroxymonosulfate activation: Mechanistic insights into nitrogen configuration-dependent radical pathways and direct electron transfer

The persulfate-based advanced oxidation processes employing biochar catalysts present a promising strategy for refractory organic pollutant removal, yet the interfacial reaction mechanisms governing pollutant degradation remain insufficiently elucidated. Herein, we innovatively developed a cobalt‑nitrogen co-embedded hierarchical porous biochar (Co-N@BC) through impregnation and pyrolysis of waste fir sawdust, demonstrating exceptional peroxymonosulfate (PMS) activation capability for chloroquine phosphate (CQP) degradation. Systematic investigations revealed that the Co-N@BC/PMS system achieved 97 % CQP removal within 45 min (k_obs = 0.061 min⁻¹) accompanied by 72 % total organic carbon reduction, outperforming conventional persulfate activation systems. The electrochemical analysis confirmed the formation of a metastable reactive complex between Co-N@BC and PMS, with its open-circuit potential change (ΔE = 0.27 V) being significantly higher than that of the nitrogen-doped system (ΔE = 0.17 V). Distinguished from the conventional radical-dominated mechanism, the study confirms the existence of a unique dual-pathway synergistic effect in the catalytic process: (1) Radical pathways dominated by surface-bound SO₄⁻• and •OH through Co²⁺/Co³⁺ redox cycling; (2) Non-radical pathways featuring ¹O₂ generation via surface group-mediated PMS activation and direct electron transfer through Co-N@BC-PMS* metastable complexes. Notably, we unveiled that the formed Co-N@BC-PMS* complexes function as interfacial electron-transfer mediators, simultaneously enhancing the efficiency of PMS activation and accelerating pollutant oxidation through interfacial-confined redox reactions. The engineered catalyst demonstrated remarkable environmental robustness, maintaining more than 90 % efficiency across four cycles, and exhibited broad pH adaptability (pH 3–11, less than 5 % variation). This study offers a new insight into the dynamic interactions between the multifunctional active sites of biochar and the mechanisms of persulfate activation. It also establishes a sustainable framework for converting waste biomass into highly efficient environmental catalysts.