Study on the Synthesis of Hydroxyapatite under Highly Alkaline Conditions

The present research work studies the direct synthesis of hydroxyapatite (Ca10(PO4)6(OH)2), via a heat-assisted precipitation technique based on the reaction between Ca2+, phosphate (PO4)3-, and (OH)- ions under highly alkaline conditions. An assessment carried out on the thermodynamics involved in the formation of hydroxyapatite (HAP) via precipitation reactions highlighted the favorability of using a high pH to perform HAP synthesis in terms of ΔGReaction and element speciation. On the basis of these results, laboratory experiments performing the synthesis of hydroxyapatite at pH 11 using four different chemical reactions and different ripening times suggested that HAP nanocrystals are formed through the route of amorphous calcium phosphate (ACP) toward hydroxyapatite. Our results pointed out that the product obtained was a nonstoichiometric hydroxyapatite with a calcium to phosphorus molar ratio in the range Ca/P = 1.40 to Ca/P = 1.95, with carbonate ions (CO3)2- being present in the HAP crystals obtained under these experimental settings. In addition, XRD characterization suggested that the reagent used to achieve a high pH exerts an influence on the arrangement of HAP, as the reflection of the (300) plane was more prominent when alkali metal hydroxides were used as the source of (OH)- ions. Lastly, SEM imaging confirmed that carrying out the synthesis under highly alkaline conditions consistently yields agglomerated clusters of primary HAP nanoparticles, irrespective of the ripening time or the particular reaction used.