Investigation of Kinetics and Thermodynamics of Fluoride Ions Removal from Aqueous Solutions by Nanostructured Hydroxyapatite

The most extensively used de-fluoridation process is the adsorption process. Hydroxyapatite is a high efficiency adsorbent and its adsorption properties attract attention recently. In this research، nanostructured calcium hydroxyapatite (HAp) were synthesized by co-precipitation method and characterized using TEM، XRD، FTIR methods. The analysis confirms the formation of pure nanorods hydroxyapatite with the diameter of less than 10 nm and length of about 20 nm. The adsorption capacity of the nano- HAp was investigated for the de-fluoridation of fluoride contaminated water using a batch equilibrium procedure. The various parameters such as pH، contact time، initial fluoride concentration and temperature were varied as part of the study to investigate their effects on the fluoride adsorption capability of the nano-HAp material. Residual fluoride level was measured using ion-chromatography analysis. The Freundlich model was obtained as describing isotherm model. Investigation of three kinetics model of pseudo first، pseudo two and two proved that the adsorption of fluoride ions on hydroxyapatite follows the second order model. The enthalpy، entropy and Gibbs free energy of the process were calculated by thermodynamic equations.