Heterogeneous iron-containing fluoridated apatites

The iron-containing fluoridated hydroxyapatites Fe-FeFAp and FeF-FeAp were synthesized heterogeneously in the presence of ferrous ions at 80 °C and pH 7.4 in two different types of solution, i.e. fluoride-free solution during the initial half of the experimental period and fluoride-containing solution during the second half, in the latter of which the fluoride concentration was equivalent to that of fluorapatite. Another experiment was carried out under the opposite conditions. The total fluoride contents of both heterogeneous iron-containing fluoridated apatites were almost the same (0.83 ± 0.01 and 0.89 ± 0.02 mmol g-1, respectively). Their X-ray diffraction patterns were not significantly different. However, scanning electron microscopy showed that Fe-FeFAp was composed of clusters of rod-like crystals and FeF-FeAp of bundles of many thin needle-like crystals. Energy dispersive spectroscopy coupled with scanning electron microscopy estimated that the fluoride intensity of Fe-FeFAp was higher than that of FeF-FeAp. The apparent solubility of the former (Ca = 8.83 ± 0.20, P = 4.27 ± 0.13 mmol l−1) was less than that of the latter (Ca = 10.8 ± 0.4, P = 5.56 ± 0.09 mmol l-1) at 37 °C and pH 4.0. These results suggest that two different types of heterogeneous iron-containing fluoridated apatites may be formed.