Development of Fluorophilic Magnetic Adsorbents with Ultrahigh Adsorption Capacity from Red Mud: Defluorination Behavior and Mechanisms

To address the low fluoride (F^-) adsorption capacity of red mud (RM)-based adsorbents, a novel Mn-coated RM adsorbent (Mn-RM) was prepared by Mn loading from red mud. Mn-RM features a 3D heteropolyhedral structure, which provides abundant functional groups for F^- adsorption. The maximum equilibrium adsorption capacity (q_max) of Mn-RM for F^- is 141.83 mg/g at 298 K and pH 3.0, among the highest reported for RM-based adsorbents. The adsorption kinetics and isotherms follow the pseudo-second-order and Langmuir models, respectively, suggesting monolayer adsorption involving electrostatic attraction, pore filling, ion exchange-disproportion reaction, and metal complexation. Mn-RM demonstrates excellent recyclability, maintaining an 86.69% removal rate after five reuse cycles. Competitive ion adsorption experiments reveal that PO₄^3- significantly inhibits F^- adsorption, while SO₄^2-, NO₃^-, and Cl^- have minimal effects. The adsorption efficiency and equilibrium capacity of Mn-RM for fluorinated wastewater from an AlF₃ industrial facility and a coal-fired power plant are 99.54% (56.26 mg/g) and 99.16% (44.38 mg/g), respectively. Moreover, F^- concentrations in both wastewaters after adsorption are below 0.2 mg/L, meeting the drinking water quality guidelines of both China and the World Health Organization (WHO).