Kinetics of hydroxyapatite precipitation at pH 10 to 11

The mechanism and kinetics of hydroxyapatite (HAP) precipitation from aqueo us solution at the conditions of pH 10-11 and at calcium ion concentrations of more than 0.5 mol/I were investigated. The results show that the reacti on undergoes the following process: transferring from octacalcium phosphate (OCP) to amorphous calcium phosphate (ACP) rapidly, and then from ACP to c alcium-deficient hydroxyapatite (DAP) and HAP. DAP was the non-stoichiometr ic material that ACP converts to HAP. Reaction temperature greatly affects the reaction rate of the conversion from ACP to HAP. It takes about 24 h to form pure-phase HAP at 25 degreesC while it takes only 5 min at 60 degrees C. The temperature also has a great deal of influence on the particle size and morphology of precipitated hydroxyapatite. The reaction of transformati on from ACP to DAP was second order and the activation energy was 95 kJ/mol (22.7 kCal/mol) and therefore it was inferred that the conversion reaction was a surface control process. (C) 2001 Elsevier Science Ltd. Ail rights r eserved.