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

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, Cd-5(PO4)(3)OH] in HNO3 solution (pH = 2), ultrapure water (pH = 5.6) and NaOH solution (pH = 9) was experimentally studied at 25, 35 and 45 degrees 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 degrees 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(PO4)(3)OH were determined to be 10(-64.62) (10(-64.53)-10(-64.71)) at 25 degrees C, 10(-65.58) (10(-65.31)-10(-65.80)) at 35 degrees C and 10(-66.57) (10(-66.24)-10(-66.90)) at 45 degrees C. Based on the obtained solubility data from the dissolution at initial pH 2 and 25 degrees C, the Gibbs free energy of Cd-5(PO4)(3)OH forming Delta G(f)(0) 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 Delta C-p(0) for the dissolution process of Cd-HAP in aqueous acidic media at initial pH 2 and 25 degrees 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 PO43- sorption and the sorption of Cd2+ and PO43- 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).