Adsorption of nickel on synthetic hydroxyapatite from aqueous solutions

The sorption of nickel on synthetic hydroxyapatite was investigated using a batch method and radiotracer technique. The hydroxyapatite samples used in experiments were a commercial hydroxyapatite and hydroxyapatite of high crystallinity with Ca/P ratio of 1.563 and 1.688, respectively, prepared by a wet precipitation process. The sorption of nickel on hydroxyapatite was pH independent ranging from 4.5 to 6.5 as a result of buffering properties of hydroxyapatite. The adsorption of nickel was rapid and the percentage of Ni sorption on both samples of hydroxyapatite was >98 % during the first 15–30 min of the contact time for initial Ni²⁺ concentration of 1 × 10^?4 mol dm^?3. The experimental data for sorption of nickel have been interpreted in the term of Langmuir isotherm and the value of maximum sorption capacity of nickel on a commercial hydroxyapatite and hydroxyapatite prepared by wet precipitation process was calculated to be 0.184 and 0.247 mmol g^?1, respectively. The sorption of Ni²⁺ ions was performed by ion-exchange with Ca²⁺ cations on the crystal surface of hydroxyapatite under experimental conditions. The competition effect of Co²⁺ and Fe²⁺ towards Ni²⁺ sorption was stronger than that of Ca²⁺ ions. NH₄ ⁺ ions have no apparent effect on nickel sorption.