The efficiency of nano metakaolin modified with zirconium oxide for fluoride adsorption from aqueous solution

Abstract

Groundwater pollution by fluoride is a global issue that has health implications for humans. Thus, it is important to have an understanding of how to efficiently remove it using the most effective and sustainable technique. In this research, a novel and economical adsorbent, zirconium-modified nano metakaolin (Zr@NMK), was synthesized through a chemical precipitation process and was explored in the adsorption technique for fluoride from water. Analytical techniques including FTIR, XRD, SEM, EDX, TGA, as well as BET were employed to analyze Zr@NMK. The response surface methodology was utilized to obtain the optimal crucial factors that affect fluoride adsorption. The adsorption process reaches equilibrium and follows the pseudo-second-order whereas fluoride isotherm fits the Langmuir and Freundlich model. The optimum fluoride adsorption capacity of Zr@NMK is 6.42 mg/g, corresponding to 96.4% fluoride removal efficiency, significantly higher than that of the unmodified nano metakaolin (NMK), under the optimum condition according to Response Surface Methodology (RSM); (pH$:$ 6, dose of adsorbent$:$ 3 g/L, fluoride level$:$ 20 mg/L, at 38 °C in 80 min). Additionally, the influence of concurrent anions in water on the elimination of fluoride was assessed, and it was determined that the existence of Cl⁻ and NO₃⁻ had a minor influence on removal efficiency, whereas the existence of SO₄²⁻ and CO₃²⁻ in water led to its substantial drop. However, the positive sign of ΔH° and ΔS°, and the negative sign of ΔG° indicate that the fluoride capture was spontaneous and endothermic on Zr@NMK. Moreover, desorption of Zr@NMK is possible for subsequent utilization and can be utilized for up to eight cycles. Zr@NMK offers a promising solution for fluoride removal and has the potential to promote a more sustainable future.