THE ROLE OF PH IN THE ADSORPTION OF CITRATE IONS ON HYDROXYAPATITE

The adsorption of citrate on hydroxyapatite (HA) has been studied at 2 5 degrees C, pH 6 and 8, and at 37 degrees C, pH 8, The experimental r esults agree with the Langmuir adsorption model at low citrate concent rations, with similar values of the affinity coefficient K, The amount adsorbed Q decreases with the increase of pH and the rise in temperat ure. 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 pre adsorbed with citrate are negative and decrease with the rise of pH. T he adsorption takes place by ionic exchange of phosphate by citrate io ns at the solid-solution interface, caused by a higher affinity of cit rate than phosphate species for the Ca-sites on the HA surface, To exp lain experimental data we propose a model where citrate species intera ct in different ways: cit(3-) interacts weakly in a bidentate manner ( 1 citrate per 2 Ca sites), whereas the Hcit(2-) interaction is stronge r(1 citrate per 1 Ca site) than the previous one, possibly due to cert ain resonance between a monodentate (using 1 -COO- group) and a surfac e-chelate interaction (using 2 -COO- groups). (C) 1998 Academic Press.