ANALYSIS OF CALCIUM HYDROXYAPATITE DISSOLUTION IN NONSTOICHIOMETRIC SOLUTIONS

The dissolution kinetics of calcium hydroxyapatite powder (HAP) are in vestigated under constant pH values of 5.0, 5.5 and 6.0, in solutions containing initially non-stoichiometric amounts of calcium and phospha te ions. Although a true steady state is never attained, it is possibl e to define a pseudo-steady state within a limited reaction domain. It is demonstrated that the kinetics follow an effective order of dissol ution relative to the calcium activity gradient near to 1. This proces s is quantitatively evaluated by a rate reduction factor which charact erizes the state of the interface. It is shown that calcium adsorption takes place at the interface during dissolution:on, and that the evol ution of the HAP surface state is responsible for the non-attainment o f a true steady state. In particular, for initial strong calcium under saturations, the attainment of the pseudo-steady state is very long. A t the pseudo-steady state, the rate reduction factor is only pH-depend ent, but not on the composition of the initial solution. (C) 1997 Else vier Science B.V.