Efficient Phosphate Adsorption by Ball-Milled Fe₃O₄-Modified Biochar Derived from Agricultural Waste

Phosphorus is a critical factor contributing to eutrophication in aquatic environments, with agricultural nonpoint source pollution identified as its primary source. The development of efficient, eco-friendly, and renewable adsorbents is of significant importance for controlling phosphorus pollution in rural aquatic bodies. Biochar, a porous carbonaceous material, has demonstrated considerable adsorption potential; yet it exhibits limited affinity for phosphate, necessitating performance enhancements through modification. In this study, biochar was prepared from pig manure and wheat husk via high-temperature pyrolysis, and modified by loading nano-Fe₃O₄ using a ball milling process, which offers a greener alternative to chemical treatments. The material's structure was characterized using Brunauer-Emmett-Teller analysis and scanning electron microscopy. Adsorption experiments, including kinetic modeling, isotherm fitting, pH variation, ion interference, and regeneration assessments, were conducted to investigate phosphate removal performance. The wheat husk-derived biochar modified with iron oxide at 800 °C exhibited a specific surface area of 181.71 m²·g^-1, approximately 420 times greater than that of its unmodified counterpart. It followed a pseudo-second-order kinetic model and Langmuir isotherm, with a maximum phosphate adsorption capacity of 125.38 mg·g^-1 under an initial concentration of (50 mg·L^-1, pH 7.0, and 8 h equilibrium time), indicating that chemisorption may play a role in the adsorption process. The material demonstrated optimal performance at neutral pH, with calcium ions showing the greatest inhibitory effect on adsorption. After five adsorption-desorption cycles, the removal efficiency remained above 89%, confirming the material's robust regeneration capacity. Overall, the ball-milled iron oxide modification provides a sustainable and effective means to enhance biochar functionality. The resulting magnetic biochar exhibits high adsorption efficiency, rapid kinetics, pH sensitivity, and excellent reusability, making it a promising candidate for addressing nonpoint source phosphorus pollution in aquatic systems.