Sulfur-doped cyanobacterial biochar-supported Co-Fe spinel oxide with enriched oxygen vacancies: overcoming redox and electron transfer limitations in Fenton-like sulfadiazine removal.

Spinel-type CoFe2O4 is limited by poor electron transfer and insufficient metal redox kinetics in Fenton-like PMS activation for sulfadiazine (SDZ) removal. To address these bottlenecks, we designed a sulfur-doped cyanobacterial biochar (SBC)-supported Co-Fe spinel oxide (CFSBC0.5) with enriched oxygen vacancies (OV) via a synergistic regulation strategy. The BC matrix served as a "highway" for electron transfer while dispersing and stabilizing Co-Fe spinel particles. Crucially, sulfur doping executed a synergistic dual role: directly acting as a sacrificial electron donor to overcome the kinetic bottleneck of metal redox cycling, and indirectly inducing topological carbon defects to thermodynamically facilitate OV formation. This dual modulation fundamentally reshaped the electronic configuration of the active sites. Under the optimal conditions of 0.05 g/L catalyst dosage, 0.3 mM PMS, and 20 mg/L initial SDZ concentration, the CFSBC0.5/PMS system achieved 98.3% SDZ degradation within 5 min, with an apparent rate constant (kobs) 2.6-fold higher than that of pure CoFe2O4. The boosted catalytic performance was attributed to the synergistic effect of OV, bimetallic active sites, and the SBC matrix, which enhanced non-radical pathways (1O2 and electron transfer process) as the dominant degradation mechanisms. The catalyst exhibited excellent pH adaptability, low metal leaching, strong anti-interference, and reusability. This work developed a high-performance catalyst for SDZ removal, realized "treating waste with waste" via cyanobacterial valorization, and provided insights into "biochar loading-sulfur doping-OV engineering" for spinel-based catalysts in real wastewater treatment.