Ion displacement following the adsorption of anionic macromolecules on hydroxyapatite

Certain small anionic molecules displace phosphate ions from the hydroxyapatite crystal hydration layer on adsorption. This follows the interaction of negatively charged groups with calcium ions in the crystal surface. The present study examines ion-exchange processes occurring during the adsorption of anionic macromolecules and attempts to determine the role of coulombic forces in the process. Various natural and synthetic polyanionic substances were added to a buffered hydroxyapatite slurry and adsorption measured by solution depletion. Calcium and phosphate ion displacement or uptake by apatite was determined after accounting for polymer-bound ions. Carboxymethylcellulose and a polyacrylamide sample in which 54% of amide groups had been hydrolyzed to carboxyl were adsorbed in the lowest amounts, but both caused consistent rises in the bulk solution phosphate level. Crude bovine submaxillary mucin and a polyacrylamide sample in which 28% of amide groups had been hydrolyzed were adsorbed in greater amounts but caused little or no phosphate displacement. In no case was there a stoichiometric relation between carboxyl adsorbed and phosphate displaced. The polyacrylamide samples also displaced Ca ions into the bulk solution, whereas the other macromolecules caused a decrease in solution Ca concentration. These results provide some evidence for the operation of an ion-exchange mechanism in the adsorption of polyanions on hydroxyapatite but do not exclude the involvement of other mechanisms, especially in the case of salivary mucin.