Removal and Recovery of Phosphate and Fluoride from Water with Reusable Mesoporous Fe3O4@mSiO2@mLDH Composites as Sorbents

Three core/shell/shell MgAl-LDH composites using Fe₃O₄ microspheres as the core, a SiO₂ matrix as the inner layer and a MgAl-LDH layer as the outer shell have been synthesized for the removal and recovery of phosphate and fluoride from water by a magnetic separation technique. The synthetic mesoporous MgAl-LDH composites show good magnetic separability, well-defined pore distributions, and have specific surface areas of 73 m² g⁻¹, 168 m² g⁻¹, and 137 m² g⁻¹ for Fe₃O₄@SiO₂@LDH350, Fe₃O₄@SiO₂@mLDH350, and Fe₃O₄@mSiO₂@mLDH350, respectively. The adsorption isotherms of both phosphate and fluoride on these MgAl-LDH composites can be well fitted with the Langmuir model. The maximum adsorption capacities of 57.07 mg g⁻¹ and 28.51 mg g⁻¹ were obtained on Fe₃O₄@mSiO₂@mLDH350 for phosphate and fluoride, respectively, much higher than those of other LDH-type materials. The adsorbed phosphate and fluoride could be successfully recovered by NaNO₃-NaOH solution, and the regenerated MgAl-LDH composites could be reused for phosphate and fluoride removal. Owing to their high adsorption capacities of both phosphate and fluoride, easy magnetic separation from solution, and great reusability, the mesoporous MgAl-LDH composites are expected to have potential applications in removal or recovery of fluoride or phosphate from water.