Study on equilibrium and kinetics of hydroxyapatite fluoride removal filter

Key words: 
• defluoridation /
• HAP /
• adsorption thermodynamics /
• adsorption kinetics

Abstract: Hydroxyapatite (HAP) is a new kind of defluoridation material with high capacity and friendly to the environment. HAP was synthesized by chemical precipitation method using calcium hydroxide and phosphoric acid as raw materials. The products were characterized by FTIR and XRD. The effects of contact time， pH， temperature and other factors on fluoride removal performance were explored. And thermodynamics and kinetics parameters were calculated to understand the nature of the adsorption processes. The results show that the adsorption rate is much rapid within 60 mins， the adsorption process approaches to equilibrium at about 70 mins. The adsorption results in acidic environment are better than those in an alkaline environment； and high temperature is conducive to adsorption. The adsorption process is more accord with Langmuir adsorption model， which indicates that chemical adsorption is dominant. Adsorption process is a spontaneous endothermic process， in which ΔGo is less than zero， and ΔHo is 13.10 kJ/mol， and ΔSo is 31.42 J/(mol?K) and Ea is 15.24 kJ/mol. Kinetic experiments show that the adsorption process is in line with pseudo second dynamic model.