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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    \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}    Δ   G   f   owas determined to be −3,970.47 kJ/mol (−3,969.92 to −3,970.96 kJ/mol). Thermodynamic parameters, Δ G  0 , Δ H  0 , Δ S  0 , and    \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}    Δ   C   p   0for 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).  Graphical abstract   Dissolution of cadmium hydroxylapatite [Cd 5 (PO 4 ) 3 OH].

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