Adsorption of Mono-and Divalent Metal Cations on Hydroxyapatite in Water

Abstract

The order of affinity of various metal ions for synthetic hydroxyapatite (HAP) in water was determined by measuring the sedimentation volume, mean particle diameter, suspension pH, and the amounts of metal chloride adsorbed and of cetylpyridinium chloride adsorbed on HAP in competition with metal ions. Direct chemical analysis of divalent alkaline earth cations showed that the binding affinity to HAP is in the order Ca²⁺>Ba²⁺>Mg²⁺, in accordance with that deduced from the other experimental results ; Ca²⁺ presumably has the highest affinity because it is the parent ion of HAP. By comparing the effects of mono-and divalent cations, it was concluded that the order of binding affinity of monovalent alkali metal cations is Na⁺>K⁺>Li⁺ ; Na⁺ presumably has the highest affinity because its crystal ion radius is almost the same as that of Ca²⁺. It was also shown that these ions bind electrostatically by at least two type of mechanism, ionexchange with Ca²⁺ on HAP and specific adsorption on HAP. It was assumed that the binding sites are Ca²⁺-defects or dislocations on HAP, and that adsorbate ions are dehydrated when they are adsorbed. Some properties of biological mineralization can be explained on the basis of the results presented here.