Adsorption of phosphate ion by hydroxyapatite in water.

The amount of phosphate ion, xp, adsorbed by synthetic hydroxyapatite (HAP) was measured under various conditions in water. A long time was required to reach adsorption equilibrium, and the adsorption process was largely irreversible with respect to concentration. The adsorption isotherm was not of Langmuir type, but was dominated by the ionic product of adsorbate, depending on the species of counter ion. The value of xp decreased with pH due to mutual electrostatic repulsion between adsorbed phosphate ions, which are deprotonated, and due to competitive adsorption between phosphate ion and OH-ion. The adsorption process was also found to be endothermic, because the adsorption increased with temperature. The adsorbate ion and adsorbent surface may be dehydrated when they are bound together. From all these results, it was concluded that the adsorbate phosphate ion is oriented and coordinated to calcium ion on the surface of HAP, and this is the reason why the adsorption of phosphate ion is irreversible.