Characterization, dissolution and solubility of synthetic cadmium hydroxylapatite [Cd(5)(PO(4))(3)OH] at 25–45°C

BACKGROUND: The substitution of Ca(2+) in Ca-hydroxylapatite by toxic Cd(2+) 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. RESULTS: Dissolution of the synthetic cadmium hydroxylapatite [Cd-HAP, Cd(5)(PO(4))(3)OH] in HNO(3) 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 Cd(5)(PO(4))(3)OH 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 Cd(5)(PO(4))(3)OH forming [Formula: see text] 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 [Formula: see text] 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. CONCLUSIONS: 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 Cd(2+) release and PO(4)(3−) sorption and the sorption of Cd(2+) and PO(4)(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). [Figure: see text]