An insight into thermodynamics of adsorptive removal of fluoride by calcined Ca–Al–(NO3) layered double hydroxide

Abstract

The conventional method for the estimation of ΔG⁰ through the van't Hoff equation was critically analysed. The development of an analytical framework for the determination of the thermodynamic parameters for the adsorption process had been attempted in this work. A calcined Ca–Al–(NO₃) layered double hydroxide was employed for the adsorptive removal of fluoride. The effect of the temperature on the adsorption process was assessed through thermodynamic and statistical methods. The adsorption equilibrium was described by Freundlich and linear isotherms. The isotherm constants were used in the van't Hoff equation by the proposed method. The thermodynamic assessment revealed that the adsorption reaction is endothermic and spontaneous. Furthermore, the ΔH⁰, E_a, E_ad, and ΔH_x values confirmed that the adsorption process is in between physisorption and chemisorption in nature. The interactive effect of the temperature with other important process parameters, i.e., pH, initial fluoride concentration and adsorbent dose, was evaluated through response surface methodology. The solution pH had very little effect on the adsorption. On the contrary, the adsorbent dose and initial concentration influenced the adsorption process significantly. The interactive effect of temperature was prominent at a lower adsorbent dose and a higher initial concentration. The highest adsorption capacity, obtained in the RSM study, was 59.60 mg g⁻¹. However, the highest K_f value from Freundlich isotherm was 8.48 (mg g⁻¹) (L mg⁻¹)^1/n at 50 °C.