Removal of Fluoride using Activated Alumina: Adsorption Isotherms, Kinetics and Thermodynamic Analysis

Abstract:

Removal of fluoride is a desalination technology in which fluoride ions from aqueous solution are adsorbed on suitable adsorbent surfaces. In this study, adsorption potential was performed on both activated alumina (AA) and grinded activated alumina (GAA) by batch experiments for different contact time, pH, fluoride concentration, and adsorbent dose. Results indicated that adsorption occurred rapidly in the beginning, and equilibrium adsorption capacity was achieved as 23.73 and 28 mg g^-1 in case of AA, whereas it was noted as 26.33 and 39 mg g^-1 for GAA for pH of 3.0 and fixed fluoride concentration of 75 and 100 mg L^-1 respectively in aqueous solution. Furthermore, adsorption isotherms were performed using three isotherm models in which the Freundlich model indicated a better fit (in the order of Freundlich>Langmuir>Dubinin-Radushkevich), showing heterogeneous nonlinear monolayer sorption. Consequently, pseudo kinetics for fluoride adsorption on alumina particles for second order (R² >0.98) had better fit over the pseudo first order kinetic adsorption of fluoride. The exothermic behaviour (∆H^o = -58.78 J mol^-1) at solid-liquid interface delivered decrease in fluoride adsorption with increase in temperature of GAA. It has been noted that the adsorption of fluoride reduced in the presence of co-existing ions on AA and GAA, as the sorption capacity on adsorbent found in the order of (CO₃)^2- > (PO₄)^2- > (SO₄)^2- > (NO₃)^- > Cl^-. Results also revealed that the GAA could be a prospective adsorbent which can be regenerated as well as reused for the removal of fluoride ions from contaminated water.