Preparation of biochar-supported nano-zero-valent iron materials via sodium salt-assisted carbon thermal reduction for efficient phosphorus removal from water: Performance and mechanism

Excess phosphorus in water is a major driver of eutrophication. In this study, peanut shells and corn stover were used as biomass feedstocks to prepare biochar-supported nanoscale zero-valent iron through sodium salt-assisted carbothermal reduction. The addition of sodium salts facilitated the formation of smaller, well-dispersed, and more reactive zero-valent iron particles. X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analyses confirmed the successful loading and uniform distribution of zero-valent iron on the biochar surface. Among the prepared materials, corn stover-derived nZVI@7YBC-Na synthesized at 700 °C showed the best phosphate removal performance, achieving 95.9% removal at a dosage of 1.2 g/L under acidic conditions. Its maximum adsorption capacity reached 53.85 mg/g, 1.35times that of nZVI@7YBC prepared by conventional carbothermal reduction. Phosphate removal involves multiple pathways, including ligand exchange, chemical precipitation, electrostatic attraction, and hydrogen bonding. After four adsorption–desorption cycles, nZVI@7YBC-Na retained 85.6% phosphate removal efficiency and showed low secondary pollution risk. Its production cost was only 0.0088 USD/g, indicating promising potential for low-cost phosphorus remediation.