Characterization, dissolution and solubility of synthetic cadmium hydroxylapatite [Cd5(PO4)3OH] at 25–45°C

The substitution of Ca²⁺ in Ca-hydroxylapatite by toxic Cd²⁺ can cause the forming of Cd-hydroxylapatite and is a significant issue in a great variety of research areas, which hence needs an understanding of the essential physicochemical characteristics. Unfortunately, the solubility product and thermodynamic data for Cd-hydroxylapatite in water under a variety of conditions now are lacking. Little information has been reported by previous researchers. Additionally, the dissolution mechanism of Cd-hydroxylapatite has never been studied.

Dissolution of the synthetic cadmium hydroxylapatite [Cd-HAP, Cd₅(PO₄)₃OH] in HNO₃ solution (pH?=?2), ultrapure water (pH?=?5.6) and NaOH solution (pH?=?9) was experimentally studied at 25, 35 and 45°C. Characterization by XRD, FT-IR and FE-SEM proved that Cd-HAP solids showed no recognizable change during dissolution. For the Cd-HAP dissolution in aqueous acidic media at initial pH 2 and 25°C, the solution cadmium and phosphate concentrations increased rapidly and reached the peak values after 20–30?days and 10?days reaction, respectively. Thereafter, the Cd-HAP dissolution rate decreased slowly, whereas the solution Cd/P molar ratio increased constantly from 1.65–1.69 to 6.61–6.76. The mean K _sp values for Cd₅(PO₄)₃OH were determined to be 10^?64.62 (10^?64.53–10^?64.71) at 25°C, 10^?65.58 (10^?65.31–10^?65.80) at 35°C and 10^?66.57 (10^?66.24–10^?66.90) at 45°C. Based on the obtained solubility data from the dissolution at initial pH 2 and 25°C, the Gibbs free energy of Cd₅(PO₄)₃OH forming \(\Delta G_{f}^{o}\) was determined to be ?3,970.47?kJ/mol (?3,969.92 to ?3,970.96?kJ/mol). Thermodynamic parameters, ΔG ⁰, ΔH ⁰, ΔS ⁰, and \(\Delta C_{\text{p}}^{ 0}\) for the dissolution process of Cd-HAP in aqueous acidic media at initial pH 2 and 25°C were calculated 368,710.12?J/K?mol, ?158,809.54?J/mol, ?1,770.20 and ?869.53?J/K?mol, respectively.

Based on the experimental results of the present work and some previous researches, the cadmium hydroxylapatite (Cd-HAP) dissolution in aqueous media is considered to have the following coincident processes: the stoichiometric dissolution coupled with protonation and complexation reactions, the non-stoichiometric dissolution with Cd²⁺ release and PO₄ ^3? sorption and the sorption of Cd²⁺ and PO₄ ^3? species from solution backwards onto Cd-HAP surface. The obtained solubility products (K _sp) 10^?64.62 (10^?64.53–10^?64.71) for Cd-HAP was approximately 7.62–5.62 log units lower than 10^?57–10^?59 for calcium hydroxylapatite (Ca-HAP).