Maximising carbonate content in sodium-carbonate Co-substituted hydroxyapatites prepared by aqueous precipitation reaction

We report the synthesis of a range of sodium-carbonate co-substituted hydroxyapatite compositions with sodium and measured carbonate contents ranging from approximately 0.4-0.8 wt% and 4.4-14.2 wt%, respectively, via aqueous precipitation reaction between calcium hydroxide, phosphoric acid and either sodium carbonate or sodium hydrogen-carbonate. A subsequent heat treatment in dry CO₂ at 600°C allowed for a Na-CO₃ co-substituted apatite containing approximately 17.7 wt% carbonate to be prepared, one of the largest carbonate contents reported to date for such a material. Deconvolution of FTIR data showed that the incorporated carbonate ions were situated on both hydroxyl and phosphate sites. Increasing the heat treatment in dry CO₂ from 300 up to 600°C, prior to the decomposition point of these compositions, showed a trend towards an increase in the distribution of carbonate on the main B-site at the expense of a decrease on the main A-site, although overall the total carbonate content increased with increasing temperature. Changes in the a lattice parameter with increasing carbonate content dominated, with a marked decrease in the a parameter with increasing addition of sodium carbonate, and an increase when samples were further heated in dry CO₂. These results demonstrate that highly carbonated sodium-carbonate co-substituted hydroxyapatites can be obtained using a simple, room temperature, aqueous precipitation reaction with starting reagents unlikely to pose significant environmental risks, adding a further degree of flexibility to the preparation of these materials and an increase in the scope of their application to fields beyond biomaterials.