Interaction between low molecular weight organic acids and hydroxyapatite with different degrees of crystallinity

In this study, two types of hydroxyapatite (HAP) with different degrees of crystallinity were prepared by a sol–gel method and a chemical precipitation method. Influences of crystallinity on the adsorption and dissolution properties of HAP, and the release of phosphorus (total phosphorus) during the adsorption of organic acid were investigated. Results showed that crystallinity had a great effect on the adsorption capacity and dissolution properties of HAP, as well as the adsorption mechanisms of organic acids on HAP surfaces. The poorly crystallized (the degree of crystallinity X_c = 0.23) HAP adsorbed greater amounts of oxalic, citric, or malic acid than the well crystallized (X_c = 0.86) HAP, and the former could release more phosphorus in the presence of organic acids. The adsorption capacity of oxalic acid was much higher than citric and malic acids on both the well and the poorly crystallized HAP, which was due to the strong coordination of oxalic acid with calcium on HAP surface, and that physical adsorption was more inclined to dominate the adsorption of malic or citric acid on the well crystallized HAP. These findings might be of importance in understanding the effects of crystallinity and organic acid binding on the dissolution of calcium phosphates and the adsorption characteristics of HAP.