Magnetically separable Fe₃O₄-modified mangrove ash as a sustainable adsorbent for fluoride remediation in water

Fluoride contamination in drinking water poses significant environmental and public health challenges, necessitating the development of efficient and low-cost adsorbents. In this study, a magnetically recoverable Fe₃O₄-functionalized mangrove ash composite (MA/Fe₃O₄) was synthesized and evaluated for fluoride removal from aqueous solutions. Structural characterization using X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy with energy-dispersive X-ray analysis, Brunauer-Emmett-Teller, and elemental analysis confirmed the successful incorporation of Fe₃O₄ nanoparticles onto the ash surface, resulting in increased surface roughness, enhanced surface functionality, and improved mesoporosity compared with pristine mangrove ash. Batch adsorption experiments demonstrated that fluoride removal was influenced by adsorbent dosage, initial fluoride concentration, and contact time. The MA/Fe₃O₄ composite exhibited moderately improved adsorption performance with an experimental adsorption capacity of 6.83 mg g^-1 under near-neutral conditions. Adsorption kinetics were best described by the pseudo-second-order model (R² > 0.998), while equilibrium data were best described by the Langmuir isotherm (R² ≈ 0.996-0.997), indicating monolayer adsorption. The pHₚzc values (6.2-6.6) revealed favorable adsorption under near-neutral pH conditions. Furthermore, the composite retained approximately 85-90% of its adsorption capacity after five regeneration cycles, demonstrating good stability and reusability. These results highlight Fe₃O₄-modified mangrove ash as a promising, magnetically recoverable, and sustainable adsorbent for fluoride remediation in water treatment applications.