Precise adsorption behavior and mechanism of Ni(II) ions on nano-hydroxyapatite

The goal of this study was to synthesize use of hydroxyapatite as a high-efficiency adsorbent for Ni(II) ions, and to study its adsorption behavior. Three tests--Fourier-transform infrared spectroscopy, transmission electron microscopy, and Brunauer-Emmett-Teller were carried out to determine the chemical functionality of the hydroxyapatite powders, to observe its crystal morphology, and to measure the specific surface area. Results indicate that proves the n-HA synthesized by chemical precipitation is an effective adsorbent for the removal of Ni(II) ions from water solution. The synthesized, needle-like nano-hydroxyapatite (n-HA) have a uniform average size of 31.9 X 21.3nm, a large specific surface area (135 m2/g), and typically is a weak crystal with a broad pore distribution. The adsorption isotherm shows the Langmuir model is applicable only when the initial Ni2+ concentration is lower than 0.1 mol/L. Multilayer adsorption was attributed to uneven pore distribution that occurred at higher Ni2+ concentration. The adsorption of Ni2+ onto n-HA was attributed to electrostatic attraction, ion exchange, and dissolution-precipitation reaction. As the result, Ni2+ substitutes Ca2+ and binds with the oxygen atom on the surface, which resulted from the change in crystal-phase composition and in the binding energy of surface elements of n-HA before and after adsorption.