Study On Synthesis Of Different Types Of Hydroxyapatite And Their Adsorption Of Fluoride Ions

Fluorine is one of the essential micronutrients which sustain human physiological processes. A moderate intake of fluorine is important in preventing caries. The concentration of fluorine in drinking water would be in the abnormal range because of the natural and human activities. The large number of fluorine will cause water pollution, harm the human body, affect the plant growth and even cause esteofluorosis. Today, fluoride wastewater treatment has aroused widely concern. The solid adsorption has been an effective method to treat the fluorine wastewater because of its effluent stability, process train simplicity and cost-effectiveness. The key to the success of renovating of fluorine by adsorption is to search for a high-efficient and cost-effective sorbent.Hydroxyapatite widely exists in nature. It has been aroused widely concern due to its excellent biological properties and ion exchange, and excellent adsorption capacity. The three kinds of hydroxyapatites (normal hydroxyapatite, carbonate hydroxyapatite, rod hydroxyapatite) were synthesized by the method of sol-gel based on the similar studies at home and abroad. The raw materials and the conditions of the preparation of three types of hydroxyapatites varied with each other. X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscope (SEM), N2 adsorption-desorption measurement (BET) are employed to characterize the powders. The results show: the three obtained powders are pure-phase and nano-sized hydroxyapatite, they have good distribution. The rod hydroxyapatite crystalline form is better than the other two hydroxyapatites. The surface area of three kinds of hydroxyapatite is 53.50,56.42,24.63 m2/g, the larger of the surface area, the stronger capacity of adsorption. The Zeta potential values showed that the charge of the hydroxyapatite surface is always negative; the titration experiment showed that the hydroxyapatite had adsorbed the fluoride ions.In this study, the wastewater was simulated by sodium fluoride solution. The adsorption of fluoride ions with hydroxyapatites was divided into two parts, including the fluoride ion adsorbed on the normal hydroxyapatite and the compare of three different hydroxyapatites capacity of fluoride's adsorption. The influence of various parameters on the adsorption, including initial concentration of fluoride ion, adsorption time, pH value and the temperature were investigated. The equilibrium adsorption time was determined, the sorption isotherm, the dynamic and thermodynamic of process were studied. The thermodynamic and kinetics parameters were obtained. The mechanism of fluoride adsorbed on normal hydroxyapatite was also discussed. The compare of three different hydroxyapatites capacity of fluoride's adsorption was studied under the same conditions.On the basis of the experimental data, the following conclusions have been drawn: the equilibrium adsorption data showed that the adsorption process follows the Langmuir equation models. The values of RL indicate that fluoride can be easily adsorbed on hydroxyapatite. The adsorption was rapid during the initial 30min but equilibrium was attained within 180min. kinetic adsorption data showed that the adsorption process follows the pseudo-second order kinetic. The pH has a significant effect on the adsorption and the optimum equilibrium pH value is about 6.0, the adsorption amount increased with the pH increasing when the pH is 4-6 while pH at 6-12 was opposite. The adsorption amount increased with the temperature increasing and the activation energy was 17.66kJ/mol. The adsorption mechanism could be explained as the specific adsorption and ion exchange with hydroxyl on the hydroxyapatite.The comparison of three kinds of hydroxyapatites'adsorption capacity was discussed as above. The results show: normal hydroxyapatite, carbonate hydroxyapatite, rod hydroxyapatite's kinetic and equilibrium adsorption data showed that the adsorption process follows the pseudo-second order kinetic and Langmuir equation models. The influence of various parameters on the adsorption, including initial concentration of fluoride ions, adsorption time and pH value of the suspensions were investigated, then we can get their adsorption capacity is different in this order: carbonate hydroxyapatite> normal hydroxyapatite> rod hydroxyapatite.