Equilibrium and kinetics modeling of fluoride adsorption onto activated alumina, alum and brick powder

For the present study three adsorbents namely, activated alumina, alum and brick powder has been studied and tested for its fluoride removal efficiency by kinetics and equilibrium models at varying dose and initial fluoride conc. The experimental values were modeled for different adsorption isotherm and kinetic models. Equilibrium studies suggests that even though a high coefficients of correlation obtained using Langmuir-1 and Langmuir-5 expressions, the model does not describe perfectly the equilibrium data because of the higher calculated error values. Elovich isotherm exhibited very poor coefficients of correlation, lower than those obtained for Langmuir and Freundlich equation, which nullifies the assumption of the exponential covering of adsorption sites that implies multilayer adsorption in the studied concentration range. Freundlich model suggests that, activated alumina and alum are good adsorbent for fluoride adsorption (1.380 ≤ n ≤ 2.992), but brick powder can proved to be a good adsorbent only at lower fluoride conc. as suggested by n < 1 for F− conc. of 5 mg/L. Error analysis suggests good agreement between the experimental and predicted values, suggesting Freundlich model for the experimental equilibrium data. Comparing pseudo-first, -second and Elovich models, it was observed that fluoride adsorption was best represented by pseudo-second order model. Liquid film diffusion model analysis suggests that, as the straight lines did not pass through the origin, resistance or film diffusion might not be the sole rate-limiting step. The adsorption kinetics might be controlled by both film diffusion as well as intra-particle diffusion simultaneously. © 2018 Elsevier B.V.