Surface complexation of calcium minerals in aqueous solution: 1. Surface protonation at fluorapatite—water interfaces

Abstract

The amphoteric properties of hydrous fluorapatite surfaces have been investigated. The measurements were performed as potentiometric titrations in 0.1 M NaCl medium at 25°C using a glass electrode. The experimental data could be explained by a model that assumes the surface equilibria PO −+H +=POH logβ s 101( int.)=6.6±0.1(3σ) CAOH 2 +−H +=CAOH logβ s -101( int.)=-9.7±0.1(3σ) The equilibrium constants were evaluated using the computer program FITEQL by Westall (Report 82-01, Department of Chemistry, Oregon State University, Corvallis, 1982), assuming a constant capacitance model for the electric double layer. The acidity constants result in a value of log[H +] pzc = −8.15, which shifts to log[H +] pzc = −7.13 upon exposure to atmospheric carbon dioxide.