The Role of pH in the Adsorption of Citrate Ions on Hydroxyapatite

Abstract

The adsorption of citrate on hydroxyapatite (HA) has been studied at 25°C, pH 6 and 8, and at 37°C, pH 8. The experimental results agree with the Langmuir adsorption model at low citrate concentrations, with similar values of the affinity coefficientK. The amount adsorbedQdecreases with the increase of pH and the rise in temperature. However, the fraction of sites occupied on the HA surface is the same at both values of pH. On the other hand, the desorption is higher at pH 8 and zeta potential values of suspensions prepared from HA preadsorbed with citrate are negative and decrease with the rise of pH. The adsorption takes place by ionic exchange of phosphate by citrate ions at the solid-solution interface, caused by a higher affinity of citrate than phosphate species for the Ca-sites on the HA surface. To explain experimental data we propose a model where citrate species interact in different ways: cit3−interacts weakly in a bidentate manner (1 citrate per 2 Ca sites), whereas the Hcit2−interaction is stronger (1 citrate per 1 Ca site) than the previous one, possibly due to certain resonance between a monodentate (using 1 -COO−group) and a surface–chelate interaction (using 2 -COO−groups).