P. Schaad et al., ANALYSIS OF CALCIUM HYDROXYAPATITE DISSOLUTION IN NONSTOICHIOMETRIC SOLUTIONS, Colloids and surfaces. A, Physicochemical and engineering aspects, 121(2-3), 1997, pp. 217-228
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.