Adsorption of fluoride ions onto naturally occurring earth materials

Batch sorption system using two naturally occurring earth materials (EM) as adsorbents was investigated to remove fluoride ions from aqueous solution. The system variables studied include initial concentration of the sorbate, agitation time, adsorbent dose, pH, co-ions and temperature. The experimental data fitted well to the Freundlich isotherm. The amount of fluoride ions adsorbed per unit mass of the adsorbent was found to be 0.011 and 0.007 mg/g, at 30°C from 4 mg/L fluoride solution, for EM1 and EM2 respectively. Thermodynamic parameters such as ∆H°, ∆S° and ∆G° were calculated which indicates that the removal of fluoride ions is an endothermic process. Kinetic studies reveal that the adsorption follows reversible first order kinetics. X-ray diffraction patterns of the adsorbents before and after adsorption and Dubinin-Radushkevick (D-R) isotherm indicate that the adsorption of fluoride ions onto these materials is a physisorption process