Oxygen-vacancy mediated CoFe bimetallic modified biochar for peroxymonosulfate activated degradation of organochlorine pesticides: Effect of the metal carbon interface.

Limited interfacial electron transfer and sluggish metal redox cycling constrain peroxymonosulfate (PMS) activation by iron-based biochar. Here, Co-, Cu-, Ni-, and Zn-doped iron biochars were synthesized via two-step pyrolysis and evaluated for the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D). CoFe/BC exhibited the highest activity, removing 92.56% of 2,4-D within 60 min with an observed rate constant of 0.0216 min-1, 6.0-12.7 times those of the other bimetallic catalysts. Spectroscopic characterization and density functional theory calculations showed that Co incorporation increased carbon graphitization, enriched oxygen-vacancy sites, and promoted charge redistribution across the CoFe oxide/carbon interface. These structural and electronic changes strengthened PMS adsorption and facilitated interfacial electron transfer. Selective site masking and postreaction X-ray photoelectron spectroscopy identified surface Co(II) as the primary PMS activation site, whereas the Fe(II)/Fe(III) couple acted mainly as an electron buffer that sustained Co redox cycling. Quenching experiments and electron paramagnetic resonance spectroscopy revealed coupled radical and nonradical oxidation pathways, with singlet oxygen playing the dominant role in 2,4-D degradation. The system retained 85.75% and 76.01% removal in river water and reclaimed water, respectively, although strongly alkaline conditions and increasing concentrations of chloride, bicarbonate, and humic acid inhibited degradation. After five cycles, 2,4-D removal decreased to 45.05% because of metal leaching and surface fouling but recovered to 86.37% after pyrolytic regeneration. In continuous flow, a fixed bed packed with CoFe/BC-loaded hydrogel beads maintained greater than 50% removal after treating 6.46 L of wastewater. These findings demonstrate that engineering oxygen-vacancy-rich metal‑carbon interfaces can enhance PMS activation and support continuous treatment of water contaminated with chlorinated herbicides.