Development of Granular Porous Alumina Adsorbents Using Kaolin for the Mitigation of Excess Fluoride Ions from Aqueous System

Abstract

Alumina adsorbent has wide acceptance for mitigation of excess fluoride ions from water. The challenging part is preparing this adsorbent in the granule shape with high porosity and PZC value, ensuring its applicability under normal pH of water, i.e., 6.5 to 7.5, and easy separation after use. This research has attempted to prepare highly porous alumina adsorbents in granule form using low-cost ceramic raw material kaolin. The study aims to find a suitable, environment-friendly, and highly effective alumina adsorbent for defluoridation. The adsorbents are prepared both in bead shape and granule shape through oil drop method using aluminium rich leachate extracted from kaolin. Al3+ leachate is obtained by thermal treatment followed by acid leaching of kaolin. Incorporating pore former in the bead-shaped adsorbent (prepared directly using concentrated acid leachate) is challenging, so two sols, namely sol-1 and sol-2, are prepared from Al3+ leachate with and without impurity removal. The sols are mixed with different pore-formers like PEG, PVP, spent tea leave waste (STLW), starch, sucrose individually, and PVA binder to convert into a semi-gel state. This semi gel is then injected into a column containing paraffin oil as an upper layer and ammonia as a bottom layer with the help of a syringe pump to form porous alumina adsorbents. All the prepared adsorbents, both bead-shaped and granule, are washed, dried, and calcined at different temperatures to get the desired alumina phase. Different characterization techniques like XRD, FTIR, FESEM, and BET surface area analysis are employed to get preliminary information about the applicability of the prepared adsorbent for the fluoride adsorption study. A batch adsorption study is then carried out systematically using each type of prepared alumina adsorbents. The adsorbent dose and kinetics parameters are thoroughly studied to achieve the maximum fluoride removal in the fluoride contaminated aqueous system. Bead-shaped adsorbents show lower surface area and porosity than their granular counterparts, resulting in low removal efficiency. The microstructural analysis is carried out before and after the adsorption process using FESEM with EDX to compare the alumina adsorbents' surface morphology and confirm fluoride ions' adsorption on the adsorbent surface. The batch adsorption study shows the mechanism and adsorption capacity of the alumina adsorbents using various kinetic models (i.e., Pseudo-first-order kinetic, Pseudo-second-order kinetic, Intraparticle diffusion and Elovich kinetic) and equilibrium isotherm models (i.e., Langmuir, Freundlich, Redlich-Peterson, and Sips isotherm models). The regeneration of the spent adsorbent and the effect of other co-existing ions on the adsorption process are also estimated. It is observed that at an initial fluoride concentration of 10 mg/L (adsorbent dose = 3 g/L; pH 6.7 ± 0.3; time = 12 h), the defluoridation efficiency is estimated as in the range of 26 to 96 %, in which Sol 1- PEG and Sol 2- Sucrose are the best adsorbent showing ~96 % of fluoride removal efficiency. Groundwater with the presence of fluoride ions always contains some other anions like nitrate (NO3-), chloride (Cl-), phosphate (PO4-3), and bicarbonate (HCO3-2) which competing for the adsorption sites during the defluoridation process. Hence, it's become necessary to investigate the effect of these co-existing ions. Further, a regeneration study of all the spent adsorbents are carried out.Alumina adsorbent has wide acceptance for mitigation of excess fluoride ions from water. The challenging part is preparing this adsorbent in the granule shape with high porosity and PZC value, ensuring its applicability under normal pH of water, i.e., 6.5 to 7.5, and easy separation after use. This research has attempted to prepare highly porous alumina adsorbents in granule form using low-cost ceramic raw material kaolin. The study aims to find a suitable, environment-friendly, and highly effective alumina adsorbent for defluoridation. The adsorbents are prepared both in bead shape and granule shape through oil drop method using aluminium rich leachate extracted from kaolin. Al3+ leachate is obtained by thermal treatment followed by acid leaching of kaolin. Incorporating pore former in the bead-shaped adsorbent (prepared directly using concentrated acid leachate) is challenging, so two sols, namely sol-1 and sol-2, are prepared from Al3+ leachate with and without impurity removal. The sols are mixed with different pore-formers like PEG, PVP, spent tea leave waste (STLW), starch, sucrose individually, and PVA binder to convert into a semi-gel state. This semi gel is then injected into a column containing paraffin oil as an upper layer and ammonia as a bottom layer with the help of a syringe pump to form porous alumina adsorbents. All the prepared adsorbents, both bead-shaped and granule, are washed, dried, and calcined at different temperatures to get the desired alumina phase. Different characterization techniques like XRD, FTIR, FESEM, and BET surface area analysis are employed to get preliminary information about the applicability of the prepared adsorbent for the fluoride adsorption study. A batch adsorption study is then carried out systematically using each type of prepared alumina adsorbents. The adsorbent dose and kinetics parameters are thoroughly studied to achieve the maximum fluoride removal in the fluoride contaminated aqueous system. Bead-shaped adsorbents show lower surface area and porosity than their granular counterparts, resulting in low removal efficiency. The microstructural analysis is carried out before and after the adsorption process using FESEM with EDX to compare the alumina adsorbents' surface morphology and confirm fluoride ions' adsorption on the adsorbent surface. The batch adsorption study shows the mechanism and adsorption capacity of the alumina adsorbents using various kinetic models (i.e., Pseudo-first-order kinetic, Pseudo-second-order kinetic, Intraparticle diffusion and Elovich kinetic) and equilibrium isotherm models (i.e., Langmuir, Freundlich, Redlich-Peterson, and Sips isotherm models). The regeneration of the spent adsorbent and the effect of other co-existing ions on the adsorption process are also estimated. It is observed that at an initial fluoride concentration of 10 mg/L (adsorbent dose = 3 g/L; pH 6.7 ± 0.3; time = 12 h), the defluoridation efficiency is estimated as in the range of 26 to 96 %, in which Sol 1- PEG and Sol 2- Sucrose are the best adsorbent showing ~96 % of fluoride removal efficiency. Groundwater with the presence of fluoride ions always contains some other anions like nitrate (NO3-), chloride (Cl-), phosphate (PO4-3), and bicarbonate (HCO3-2) which competing for the adsorption sites during the defluoridation process. Hence, it's become necessary to investigate the effect of these co-existing ions. Further, a regeneration study of all the spent adsorbents are carried out.