Thermodynamics of Fluoride Adsorption on Aluminum/Olivine Composite (AOC): Influence of Temperature on Isotherm, Kinetics, and Adsorption Mechanism

Abstract

The kinetic and thermodynamic study of the adsorptive removal of fluoride by a novel, low-cost, environmental friendly adsorbent, aluminum/olivine composite (AOC) has been conducted to demonstrate the mechanism and feasibility of the adsorption system. The isotherm and kinetic modeling were conducted at the different temperatures to assess its effect on the adsorption equilibrium and uptake rate. The Freundlich isotherm was found to be the best-fitted model and the Kf values arrived as 5.207–1.975 (mg/g) (L/mg)1/n at 10–50 °C. The negative value of ΔH0 suggested the reaction is exothermic and the negative values of ΔG0 in all the temperature confirmed the spontaneity of the adsorption. The thermodynamic parameter, such as Ead (3.279–1.069 kJ/mol), Ea (− 11.97 kJ/mol), ΔH0 (− 18.669 kJ/mol), ΔH# (− 13.799 kJ/mol), etc., established that the adsorption is dominated by physical forces and the ΔHx (− 0.899 to − 53.904 kJ/mol) suggested significant surface heterogeneity in AOC. The kinetic modeling showed the uptake is supported by pseudo-second-order rate kinetics and the film diffusion together with intraparticle diffusion controls the rate-limiting steps of adsorption. The absence of aluminum in the treated water near neural pH confirmed the significant applicability of AOC as an adsorbent without creating any post-adsorption secondary hazard.