Uptake fluoride from water by caclined Mg-Al-CO3 hydrotalcite: Mg/Al ratio effect on its structure, electrical affinity and adsorptive property

• Higher Mg/Al ratio exhibited larger interlayer spacing and lower electrical affinity which had reverse effect on adsorption.
• Pseudo-second order and Langmuir model fitted kinetics and isotherm satisfactorily.
• From both kinetics and equilibrium study, the maximum adsorption rate and capacity was achieved when Mg/Al = 4.
• The adsorption process was spontaneous and endothermic.
• The detrimental effect of co-existing anion increased as: Cl− < SO42− < PO43−.

Previous studies have found that calcined Mg-Al-CO3 hydrotalcites exhibit high fluoride removal capacity. The effect of Mg/Al ratio on its structure, electrical affinity and adsorptive property, however, remained elusive, and their elucidation could enhance the functional optimization of the material. This study presents significant effect of Mg/Al ratio in calcined hydrotalcite (CHTx, x was the ratio: 2:1, 3:1, 4:1 and 5:1) on fluoride adsorption. The materials were characterized by X-ray diffraction (XRD), N2 adsorption/desorption analysis using BET method, scanning electron microscopy (SEM), Zeta-potential and Fourier transform infrared spectroscopy (FTIR) to confirm the adsorption mechanism. In this unique study, we found that different Mg/Al ratios resulted in different interlayer spacing and electrical properties, influencing the substance's adsorptive properties. As the Mg/Al ratio increased, the interlayer spacing increased and Zeta potential decreased, which had a reverse effect on adsorption. The CHT4 exhibits the highest adsorption rate and capacity (Qmax = 119.04 mg/g at 298 K). The adsorption kinetics data best fit a Pseudo-second-order kinetic model. The Langmuir isotherm model fit experiment data better than Freundlich model. The results of thermodynamic study highlighted the spontaneous and endothermic nature of the adsorption process. The detrimental effect of co-existing anion increased as: Cl− < SO42− < PO43−.

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