Preparation of Red Mud-Modified Sludge-Based Biochar and Its Phosphorus Adsorption Efficiency and Mechanism
To address the dual challenges of phosphorus resource scarcity and eutrophication control, this study proposed a sustainable waste-treats-waste strategy by preparing red-mud-modified sludge-based biochar (RMSBC) via co-pyrolysis of red mud and sewage sludge. The optimal material (RMSBC800), produced 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% enhancement over unmodified biochar (SBC). Characterization (SEM, XRD, EDS, FT-IR, XPS) revealed that red mud modification increased the average pore size from 7.91 nm to 20.33 nm, reduced electronegativity, and raised the pH at point of zero charge (pHpzc) from 2.01 to 3.37. Adsorption kinetics followed the pseudo-second-order model, and isotherms fitted the Langmuir model, indicating monolayer chemisorption. The Freundlich parameter 1/n was 0.34, suggesting favorable adsorption. Mechanistic studies identified electrostatic attraction and surface precipitation as dominant, with molecular dynamics simulations confirming the critical role of Fe3O4 over Fe2O3 in adsorbing HPO4^2- due to stronger electrostatic interactions. The material retained 61% of its initial capacity after five regeneration cycles and achieved 83% phosphorus removal from real wastewater. This work demonstrates synergistic valorization of industrial wastes, offering an economically viable solution for phosphorus pollution control and resource recovery.