Fluorapatite surface composition in aqueous solution deduced from potentiometric, electrokinetic, and solubility measurements, and spectroscopic observations

Abstract

International audienceThe surface chemistry of fluorapatite in aqueous solution was investigated using electrokinetic techniques, potentiometric titrations, solubility measurements, and attenuated total reflection infrared spectroscopy. All methods indicate the formation of Ca/F depleted, P enriched altered layer via exchange reactions between H+ and Ca2+, and OH? and F? at the fluorapatite (FAP) surface. Observations suggest that this leached layer has a di-calcium phosphate (CaHPO4) composition and that it controls the apparent solubility of FAP. Electrokinetic measurements yield an iso-electric point value of 1 ± 0.5 consistent with a negatively charged FAP surface at pH > 1. In contrast, surface titrations give an apparent pH of point of zero charge of ?7.7, consistent with a positively charged surface at pH < 7.7. These differences are shown to stem from proton consumption by both proton exchange and dissolution reactions at the FAP surface. After taking account for these effects, FAP surface charge is shown to be negative to at least pH 4 by surface titration analysis

Biotite surface chemistry as a function of aqueous fluid composition

“…Several recent studies reported that due to metal exchange reactions, the composition of the near-surface of multi-oxide minerals depends strongly on the composition of its surrounding aqueous phase (e.g. Pokrovsky and Schott, 2000;Chaïrat et al, 2007;Oelkers et al, 2009). In an attempt to extend these concepts to phyllosilicates, we have performed a series of batch potentiometric titration experiments and electrokinetic measurements on biotite surfaces as a function of aqueous solution pH from 1 to 12.…”

Transition from Endothermic to Exothermic Dissolution of Hydroxyapatite Ca5(PO4)3OH–Johnbaumite Ca5(AsO4)3OH Solid Solution Series at Temperatures Ranging from 5 to 65 °C

“…As presented in Figure 13, two nearly straight lines approximate the Roozeboom curve. This implies that in the case of the JBM-HAP solid solution series, even a negligible amount of aqueous phosphates in the system can alter the thermodynamic equilibrium to promote the formation of HAP-rich solids [32,37,38]. The increase in the phosphate content in the structure of JBM improves its stability.…”

Characterization, Dissolution, and Solubility of Zn-Substituted Hydroxylapatites [(ZnxCa1−x)5(PO4)3OH] at 25°C

“…Commonly, the solution [29]. Additionally, the experimental conditions could significantly affect the apatite dissolution [30].…”