Directional/acidic dissolution kinetics of (OH,F,Cl)‐bearing apatite

Abstract Directional dissolution of a natural (OH,F,Cl)‐bearing apatite has been studied at various solution pH values (0–3) and 30°C. This apatite showed abnormally high OH stretching frequencies due to the substitution of Cl for OH. The advance of dissolution front indicated that steady‐state directional dissolution for pH = 0–2 followed an apparent rate law of Rate = ka (in mole/m²h), where the rate constants (k) are 2.15 and 1.61; and the rate orders (n) are 1.44 and 1.30 for [0001] and <1120> directions, respectively. Previous study, however, indicated a smaller n value (n = 0.55–0.70) for fluorapatite powders at higher pHs. A nonlinear pH dependence of logarithmic dissolution rate at a wide pH range implied that the surface active sites and/or rate‐determining steps have changed when the acidity of solution and/or the composition of the apatite were changed. The opening of etch pits on basal planes further indicated that the dissolution rates along the three principal directions have the following relationship: [0001] > <1120> > <1010> for pH=0–1, but the order was reversed for pH > 3. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res, 2006