Electrochemical preparation of hydroxyapatite and its removal of fluorine ions.

In this paper, hydroxyapatite (HAP) was synthesized by clean and simple multi-scan cycle voltammetry. The crystal structure and functional group of intermediate products calcium hydrogen phosphate dihydrate (DCPD) and hydroxyapatite(HAP) were characterized by X-ray diffraction and infrared spectroscopy. Scanning electron microscope showed that the surface of HAP was a rod-like structure with good dispersion. Taking fluorine ions in water as adsorption objects, the effects of the parameters such as pH, possible coexistence anions and temperature on the adsorption capacity of synthesized HAP were studied. The results showed that in the range of pH of 3-6, the adsorption capacity of HAP to fluorine ions gradually increased with the increase of pH and reached the maximum when the pH was 6; in the range of pH of 6-10, and the adsorption of HAP to fluorine ions gradually decreased with the increase of pH. Anions that may coexist in water, such as Cl^-, NO₃^-, SO_2-⁴, CO_2-⁴, did not interfere with HAP adsorption of fluorine ions. In the range of temperature of 15-55 °C, the adsorption capacity of HAP increased with the temperature increase, and reached the maxum of 8.36 mg/g at 55 °C, which was higher than the similar materials reported . After 4 times of adsorptiondesorption experiments, the removal effect of HAP on fluoride ions in simulated wastewater was still in line with the national standard. In addition, the adsorption of fluorine ions by HAP was a spontaneous process of entropy increase and heat absorption, which conformed to the adsorption isothermal models of Langmuir-Freundlich and Dubinin-Radushkevich, and followed the kinetic reaction mechanism of intra particle diffusion. [ABSTRACT FROM AUTHOR]