Preparation of Red Mud-Modified Sludge-Based Biochar and Its Phosphorus Adsorption Efficiency and Mechanism

• • RMSBC800, prepared at 800 °C with a red mud-to-sludge mass ratio of 3:1, achieved a maximum phosphorus adsorption capacity of 28.57 mg/g, a 350% improvement over unmodified biochar (SBC), enabling high-efficiency phosphate removal from wastewater. • • Red mud modification enlarged the average pore size from 7.91 nm to 20.33 nm and increased the pHpzc from 2.01 to 3.37, reducing electronegativity and enhancing electrostatic attraction for phosphate anions. • • Adsorption followed pseudo-second-order kinetics and the Langmuir isotherm (1/n = 0.34), indicating monolayer chemisorption with a theoretical maximum capacity of 31.7 mg/g at pH 5 and an adsorbent dose of 4 g/L, achieving 83% removal in real wastewater. • • The material retained 61% of its initial adsorption capacity after five regeneration cycles, demonstrating reusability and cost-effectiveness for industrial applications, though sulfate and carbonate ions inhibited adsorption by 42.5% and 39%, respectively.