Adsorption of Fluoride from Aqueous Solution by a Synthetic Iron(III)−Aluminum(III) Mixed Oxide

This paper presents the synthesis of iron(III)−aluminum(III) mixed oxide with some of its physicochemical characteristics and fluoride adsorption behavior thereon. Results showed that the optimum initial pH for fluoride adsorption is 4.0−10.0, and the equilibrium time required is 1.5 h. The isotherm data follow the order Redlich−Peterson ≥ Langmuir > Freundlich > Temkin. The Langmuir monolayer adsorption capacity of the adsorbent is determined to be 17.73 mg/g, which is higher than that of either of the pure oxides. The enthalpy change (ΔH°) and entropy change (ΔS°) for the adsorption reaction are +29.31 kJ/mol and +116.75 J mol-1 K-1, respectively. The adsorption is endothermic in nature. The kinetics follows a pseudo-second-order rate equation, and the reaction rate is a multistage-controlled diffusion process. The activation energy for this adsorption reaction is 6.35 kJ/mol.