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

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 OOH 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 ap-parent rate law of Rate kaH n (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] and1120 directions, respectively. Previous study, however, indicated a smaller n value (n 0.55–0.70) for fluorapatite powders at higher pHs. A non-linear 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 prin-cipal directions have the following relationship: [0001] 11201010 for pH0–1, but the order was reverse