Synthesis of Pure Hydroxyapatite by Ion Exchange Reaction

An ion-exchange reaction for the synthesis of hydroxyapatite (HAP) in a constant alkaline medium using preliminary dry homogenization is proposed in this work. Sodium hydrogen phosphate Na2HPO4, calcium chloride CaCl2 and calcium hydroxide Ca(OH)2 were chosen as precursors. When Na2HPO4 interacts with calcium salts in an alkaline medium, Na+ ions are replaced by Ca2+ ions. The CaCl2 salt was chosen as the source of Ca2+ ions, since in this case one of the reaction products is sodium chloride, which is subsequently easily washed out. The required pH of the medium was achieved by adding calcium hydroxide to the starting materials, which was also a source of Ca2+ ions. Heating in a water bath provided a constant influx of water molecules, which is necessary for the dissociation of the molecules of the substances involved in the reaction. In dry synthesis, it is rather difficult to obtain stoichiometric HAP due to the uneven distribution of ions in the reacting mixture. The chemical reaction in a humid environment can reduce the influence of this factor. It was assumed that since, in addition to HAP, the reaction products are water-soluble compounds, a slight change in the ratio of the starting components can be used to control the stoichiometry of HAP, as well as the ratio of the final reaction products. Additional products formed during the reaction were removed by washing the resulting powder with distilled water. The washed and dried HAP powder was annealed in an oven at T  700 °C. Studies carried out by methods of X-ray phase analysis, scanning electron and transmission microscopy showed that the material obtained is a single-phase hydroxyapatite powder with a Ca/P ratio close to 1.67. It was found that the particles of HAP powder obtained as a result of synthesis with subsequent washing have the shape of needles with a thickness of 10 to 40 nm and a length of 30 to 200 nm. Annealing of the powder at a temperature of 700 °C leads to agglomeration of particles, as well as to rounding of their shape.