Fluoride Adsorption by Apatite Systems: Equilibrium, Kinetic and Thermodynamic Studies

Adsorption is one important technique used in fluoride ion removal from aqueous solutions. The viability of adsorption techniques is greatly dependent on the solid adsorptive materials. In this study fluoride adsorption by both synthetic hydroxyapatite (HAP) and biogenic hydroxyapatite such as bone meal (BM) was investigated. The effects of the following factors: contact (residence) time, fluoride initial concentration, adsorbent dosage, solution pH and operating temperature on the adsorption process were considered. The defluoridation capacity increased with increase in the initial fluoride concentration and contact time but decreased with increase in sorbent dose for both systems. Most adsorption was achieved within the first hour of contact but equilibrium was attained by the fifth hour. For both systems the optimal adsorbent dosage was about 20g/L where almost 90% of fluoride was removed. The optimal pH range for removal of fluoride was between 5.2 and 6.0 for both adsorbents. The adsorption process was found to be pseudo-second order kinetics with correlation coefficients of 0.9898 and 0.9944 for HAP and BM, respectively. The adsorption process can be modeled by Freundlich isotherms. Thermodynamically the adsorption process was spontaneous and endothermic. From the enthalpy calculated, the mechanism can be described as physio-chemical adsorption process. The bone meal showed a higher adsorption capacity with a maximum value of 2.25 mg/g and maximum percent fluoride removal of 90% compared to 2.10mg/g and 85%.of HAP, respectively. This showed bone meal to be more effective defluoridating agent.