Apatite formation on alumina: The role of the initial adsorption of calcium and phosphate ions

Apatite formation on alumina was studied using two different pretreatments: the previous adsorption of calcium or phosphate ions before its immersion on two calcium-phosphate solutions. At pH 7.4, alumina that adsorbed initially phosphate and then calcium ions was used as substrate, but no apatite growth was detected after 16 days of immersion. However, apatite growth was observed at pH 8.6. In this case, alumina was first immersed in a calcium solution at pH 11.3, later suspended in a phosphate solution, and finally in a calcium-phosphate solution at pH 8.6. Results indicate that, when phosphate ions are firstly attached to the alumina surface at pH 7.4, although they attract calcium ions, apatite nucleation is inhibited. On the contrary, it seems that the initial calcium adsorption to the alumina surface forms appropriate sites for apatite nucleation and growth. Apparently, the alumina positive surface charge is not directly responsible for its inability towards apatite nucleation at pH 7.4, as has been previously suggested.