Adsorption characteristics of phosphate in water by magnetically modified biochar

To address phosphorus pollution in water bodies, biochar （BC-700） is derived from highland barley straw as a carbon-based material. Magnetic biochar material （MBC-700） is developed by immobilizing iron-based compounds onto BC-700 surfaces through co-precipitation. Adsorption performance of MBC-700 is systematically investigated under varying conditions, including pH, dosage, initial phosphate concentration, and contact time. Characterization techniques （SEM-EDS, XRD, XPS, FTIR, and VSM） combined with kinetic and isothermal adsorption models are employed to elucidate the adsorption mechanism. Results demonstrate that MBC-700 exhibits a porous structure with iron species （FeOOH and Fe₃O₄） effectively immobilized on its surface, where Fe₃O₄ contributes to its strong magnetic properties. Post-adsorption analysis reveals phosphate retention primarily as FePO₄ precipitates and Fe—O—P complexes. The adsorption process follows spontaneous monolayer chemisorption kinetics, influenced by intraparticle diffusion, surface heterogeneous diffusion, and boundary layer diffusion, achieving a theoretical maximum adsorption capacity of 5.97 mg/g. MBC-700 maintains effective phosphate removal across a broad pH range （3–7）, with enhanced performance in acidic conditions and inhibition in alkaline environments. Notably, coexisting anions （\mathrmCO_3^2- and \mathrmHCO_3^- ） promote adsorption, while magnetic recovery achieves >98% efficiency. Mechanistic studies identify pore filling, electrostatic attraction, ligand exchange, precipitation, and complexation as dominant adsorption pathways. This work provides scientific insights and technical support for addressing phosphorus pollution and advancing biochar recovery technologies.