Synthesis, structure, and solubility of carbonated barium chlor- and hydroxylapatites

Two series of carbonated barium apatites [BaApZ = Ba10(PO4)6Z2] were prepared in aqueous solution. Carbonated barium hydroxylapatites (CBaApOH) were prepared by mixing solutions of Ba(NO3)2 with (NH4)2HPO4 and (NH4)2CO3 at pH 12. Carbonated barium chlorapatites (CBaApCl) could only be prepared by stirring barium chlorapatite with (NH4)2CO3 at pH 10 for 1 week. Powder X-ray diffraction showed no evidence of the presence of BaCO3 or other possible impurities such as BaHPO4 in the carbonated products. Variations in carbonate content of the apatites had little effect on the width of peaks in their XRD patterns. The cell parameters determined by full pattern analysis were also relatively independent of the carbonate content of the apatites, although CBaApCl showed a slight decrease in a-axis length with increasing carbonate content. The slight decrease in the a-axis length, the amount of carbonate incorporated in the apatites, and the specific IR absorbances in the carbonate spectral regions support the mechanism of carbonate substitution for phosphate (B-type). In contrast, the barium:phosphate ratio determined for several apatites of different carbonate concentrations suggests carbonate substitution for the monovalent anion (A-type). Raman spectroscopy of CBaApOH confirmed that carbonate incorporation causes atomic disorder within the structure and that apparently carbonate-saturated CBaApOH still contains appreciable hydroxyl. Activity-based Ksp values were determined for both series of carbonated apatites and show that the solubilities of both CBaApCl and CBaApOH remain almost constant at low carbonate concentrations but increase at higher carbonate values (assuming B-type substitution). The Ksp values for uncarbonated BaApCl and BaApOH were determined by extrapolation to 0% carbonate (10−103 and 10−107 for BaApCl and BaApOH, respectively).

pKsp for CBaApOH and CBaApCl as a function of carbonate content, using the B-type substitution formula Ba10−x(PO4)6−x(CO3)x(Z)2−x.Figure optionsDownload as PowerPoint slideHighlights
► Structures of barium chlor- and hydroxylapatites little affected by carbonate.
► Unit cell a-axis for chlorapatites decreases as carbonate content increases.
► B-type substitution occurs for barium chlor- and hydroxylapatites.
► Solubilities of carbonated derivatives are slightly higher than noncarbonated.