title: 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 degrees 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(H+) (n) (in mole/m(2)h), 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 <1,120> 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] > <1,120> > <1,010> for pH=0-1, but the order was reversed for pH > 3.
authors.affiliation: Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, Republic of China.
authors.forename: Wan-Ju
authors.initials: WJ
authors.lastname: Tseng
authors.forename: Chung-Cherng
authors.initials: CC
authors.lastname: Lin
authors.forename: Po-Wen
authors.initials: PW
authors.lastname: Shen
authors.forename: Pouyan
authors.initials: P
authors.lastname: Shen
mesh_terms: D000143:Acids / Q000737:chemistry*; D001031:Apatites / Q000737:chemistry*; D002713:Chlorine / Q000737:chemistry*; D005461:Fluorine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D017665:Hydroxyl Radical / Q000737:chemistry*; D007700:Kinetics; D012995:Solubility; D013059:Spectrum Analysis, Raman
chemical_list: D000143:Acids; D001031:Apatites; D005461:Fluorine; D017665:Hydroxyl Radical; D002713:Chlorine