Preparation of porous alumina adsorbent from kaolin using acid leach method: studies on removal of fluoride toxic ions from an aqueous system

Abstract

In the present study, feasibility of the porous alpha alumina beads for the removal of fluoride has been investigated. The porous alumina beads were fabricated by choosing the low-cost method i.e., oil drop method through extraction of alumina from kaolin. Extraction of alumina from kaolinite is a well-defined process from long decade. After preparation, alumina beads were characterized using powder X-ray diffractometer (XRD) which shows major phase as alpha alumina (α-Al2O3) with minor phase of theta alumina (θ-Al2O3) at 1050 °C. Further these beads were examined through scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) using N2 adsorption–desorption isotherm. The specific surface area of 54.54 m2/g was estimated through BET experiment. Alumina beads were then used to eliminate the fluoride toxic ions from the aqueous system using adsorption process. Further, the adsorbent dose and kinetics parameters were thoroughly studied to achieve the maximum removal in the aqueous system. The microstructural analysis was carried out before and after the adsorption process using SEM with EDX to compare the surface morphology of the alumina beads. Batch adsorption study show the mechanism and adsorption capacity of the porous alpha alumina beads which suggests the Freundlich adsorption i.e., heterogeneous behavior of fluoride ions towards alumina surface. Through batch adsorption study it was clearly observed that adsorption belongs to pseudo second order kinetic with inter particle diffusion. At initial fluoride concentration of 10 mg/L (adsorbent dose = 3 g/L; pH 6.7 ± 0.3; time = 48 h) the defluoridation efficiency was 26% with Freundlich adsorption capacity kf = 0.91795 (mg/g) (L/mg)1/n.