Enhanced adsorption-oxidation-coagulation synergistic removal of Sb(III) by magnetic biochar: Performance and mechanism

Antimony (Sb) pollution caused by the leaching from tailings and slag heaps has become an urgent water environmental problem. Traditional iron-based coagulation easily causes secondary pollution from excessive dosing, and high-efficiency adsorbents are hard to recycle. In this study, magnetic biochar (BC@x/yFe₃O₄) was prepared by loading Fe(II) and Fe(III) onto biochar to address problems. Magnetic biochar BC@1/2Fe₃O₄ with 1:2 Fe(II):Fe(III) ratio showed superior Sb(III) adsorption, reaching 98.58% removal at 0.1 g dosage for 5 mg/L Sb(III).The material maintained stable adsorption performance over a wide pH range of 5–10. The abundant surface functional groups (e.g., hydroxyl, carboxyl) and Fe O bonds on BC@1/2Fe₃O₄ acted as the core active sites for the adsorption. The Sb(III) adsorption onto BC@1/2Fe₃O₄ well followed the pseudo-second-order model (R 2 = 0.9514), revealing a chemisorption-dominated process with auxiliary physical adsorption. The removal mechanisms involved pore filling, surface complexation, electrostatic attraction, hydrogen bonding, and efficient Sb(III) oxidation to less toxic Sb(V), with over 90% conversion achieved within 30 min. Fe₃O₄ served as the key active phase for both adsorption and oxidation and Fe ion released to liquid phase along with the Sb(III) removal. The released Fe can further remove residual or high-concentration Sb(III) via coagulation at pH 6–11, reducing effluent total Sb to 14.57 μg/L. The BC@1/2Fe₃O₄, acting as a recyclable adsorbent and sustained-release Fe-based coagulant, combines excellent Sb(III) removal capacity with favorable magnetic separation properties, providing an alternative technical solution for the efficient treatment of Sb-containing wastewater in mining areas.