Kinetic and Equilibrium Study for the Fluoride Adsorption using Pyrophyllite

Batch adsorption study was carried out to remove excess fluoride from water using pyrophyllite. Result showed that adsorption of fluoride was rapid in first 20min and thereafter increased slowly to reach the equilibrium in about 2 hrs. About 85% removal efficiency was obtained within 2 hrs at an adsorbent dose of 4 g/L for initial fluoride concentration of 10mg/L. Maximum fluoride adsorption takes place at pH 4.9. Thermodynamic parameters such as Gibb's free energy (Delta G degrees), enthalpy (Delta H degrees), and entropy (Delta S degrees) changes were determined for the adsorption process. Negative Delta H degrees value signified that the adsorption process was exothermic in nature. From the kinetic study it was found that fluoride adsorption by pyrophyllite followed pseudo- second- order kinetics with an average rate constant of 0.92 g/mg.min. Intraparticle diffusion model was studied to determine the rate limiting step of the adsorption process. The system followed the Langmuir isotherm with maximum adsorption capacity of 2.2 mg/g of fluoride.