Facile synthesis of Al/Fe bimetallic (oxyhydr)oxide-coated magnetite for efficient removal of fluoride from water

In this work, we developed a novel magnetic bimetallic Al/Fe (oxyhydr)oxide adsorbent through a facile and cost-effective method and explored its potential to adsorb fluoride in water. Its synthesis involved corrosion of natural magnetite in aluminium chloride solution, followed by titration with NaOH solution for in-situ synthesis of Al/Fe (oxyhydr)oxide-coated magnetite (Mag@Al₂Fe). Characterization data indicated a uniform coating of Al/Fe (oxyhydr)oxide on magnetite, and the resulting composite possessed large specific surface area (∼90 m²/g) and good magnetic property. In batch adsorption experiments, the isotherm and kinetic data fitted well to the Langmuir model and pseudo-second-order model, respectively. The maximum adsorption capacity of Mag@Al₂Fe is 26.5 mg/g, which was much higher than natural magnetite (0.44 mg/g). Moreover, this material retained high adsorption capacity toward fluoride within a wide pH range (3.0-8.0) and offered facile magnetic separation from water. Influence of competing ions was also evaluated which showed that the presence of Cl^- and NO₃ ^- posed negligible interference, while HCO₃ ^- and SO₄ ^2- had negative effects on fluoride adsorption. Thermodynamic investigations revealed that fluoride adsorption was exothermic and spontaneous. The observed increase in solution pH and formation of Al-F and Fe-F bonds (as indicated by XPS analysis) after fluoride adsorption suggested the major adsorption mechanism of ligand exchange. Besides, the adsorption/desorption cycle studies demonstrated the well-retained performance of Mag@Al₂Fe for repeated application after regeneration by 0.5 mol/L NaOH solution. Facile synthesis, high defluoridation, lower cost, and quick separation of Mag@Al₂Fe indicates its promising potential for drinking water defluoridation.