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

Abstract

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Background

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.

Results

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 ...... was determined to be -3,970.47 kJ/mol (-3,969.92 to -3,970.96 kJ/mol). Thermodynamic parameters, [Delta]G ⁰, [Delta]H ⁰, [Delta]S ⁰, and ...... 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 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).[Figure not available: see fulltext.]