Modeling Adsorption Isotherm for Defluoridation by Calcined Ca-Al-(NO3)-LDH: State-of-the-Art Technique

Abstract

The calcined Ca-Al-(NO3) layered double hydroxide (LDH) was synthesized by a coprecipitation method and was applied for the adsorptive removal of fluoride from the aqueous solution. The isotherm experiments were conducted in the various initial fluoride concentration ranges. The estimation of isotherm parameters from the various isotherm models as well as the adequacy of these models exhibited the significant variation in the different concentration ranges. A generalized isotherm model was employed based on the Langmuir and Freundlich isotherms. The proposed model segregates the adsorption from the other influencing factors. The model was applied in different concentration ranges and provides maximum adsorption capacity as 351.066, 364.868, and 365.425 mg/g in the concentration ranges of 0–10, 0–100, and 0–1,000 mg/L. However, the same was found as 8.964, 1.175×105, and 477 mg/g from the Langmuir isotherm. The significant variation of the isotherm parameters in the different concentration ranges clearly indicates an ambiguity in this field. The proposed model overcomes the drawback associated with the estimation of the isotherm parameters and can be applied as a generalized model to demonstrate the adsorption equilibrium.