Analysis of calcium hydroxyapatite dissolution in non-stoichiometric solutions

The dissolution kinetics of calcium hydroxyapatite powder (HAP) are investigated under constant pH values of 5.0, 5.5 and 6.0, in solutions containing initially non-stoichiometric amounts of calcium and phosphate ions. Although a true steady state is never attained, it is possible to define a pseudo-steady state within a limited reaction domain. It is demonstrated that the kinetics follow an effective order of dissolution relative to the calcium activity gradient near to 1. This process is quantitatively evaluated by a rate reduction factor which characterizes the state of the interface. It is shown that calcium adsorption takes place at the interface during dissolution:on, and that the evolution of the HAP surface state is responsible for the non-attainment of a true steady state. In particular, for initial strong calcium undersaturations, the attainment of the pseudo-steady state is very long. At the pseudo-steady state, the rate reduction factor is only pH-dependent, but not on the composition of the initial solution. (C) 1997 Elsevier Science B.V.