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

Geochemical Transactions

Research Article

Yinian Zhu¹  Zongqiang Zhu¹  Yanpeng Liang¹  Yanhua Huang¹  Liuqin Dai¹  Xin Zhao² 

[1] College of Environmental Science and Engineering, Guilin University of Technology, 541004, Guilin, People’s Republic of China;College of Light Industry and Food Engineering, Guangxi University, 530004, Nanning, People’s Republic of China;

关键词: Cadmium hydroxylapatite;    Dissolution mechanism;    Solubility product;    Gibbs free energy of forming;   

DOI  :  10.1186/s12932-015-0025-1

 received in 2014-12-16, accepted in 2015-07-01,  发布年份 2015

来源: Springer

【 摘 要 】

BackgroundThe 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.ResultsDissolution 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 Ksp 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 ΔGfo\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$\Delta G_{f}^{o}$$\end{document} was determined to be −3,970.47 kJ/mol (−3,969.92 to −3,970.96 kJ/mol). Thermodynamic parameters, ΔG0, ΔH0, ΔS0, and ΔCp0\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$\Delta C_{\text{p}}^{ 0}$$\end{document} 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.ConclusionsBased 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 (Ksp) 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).Graphical abstractDissolution of cadmium hydroxylapatite [Cd5(PO4)3OH].

【 授权许可】

CC BY   
© Zhu et al. 2015

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