Calcareous Soil as a Promising Adsorbent to Remove Fluoride from Aqueous Solution: Equilibrium, Kinetic and Thermodynamic Study

Abstract

In this work, the feasibility of employing calcareous soil to remove fluoride ions from its aqueous solutions was investigated under batch mode. The influence of solution pH, sorbent dose, initial fluoride concentration, contact time, stirring rate and temperature on the removal process were investigated. The equilibrium adsorption data were analyzed using Langmuir, Freundlich and Temkin isotherm models. The kinetics of fluoride ion was discussed by pseudo-first-order, pseudo-second-order and intraparticle diffusion models. It was shown that the adsorption of fluoride ions could be described by the pseudo-second-order kinetic model. Thermodynamic parameters such as Gibbs free energy (rG o ), enthalpy (rH o ) and the entropy change of sorption (rS o ) have also been evaluated and it has been found that the adsorption process was spontaneous, feasible and exothermic in nature. A six-layered feed forward neural network with back propagation training algorithm was developed using thirty-one experimental data sets obtained from laboratory batch study. The ANN predicted results were compared with the experimental results of the laboratory test. Regeneration study indicates that about 81.4% fluorides can regenerate from the adsorbent. It was concluded that calcareous soil has potential for application as an effective adsorbent for removal of fluoride ions from aqueous solution