Characterization, dissolution and solubility of cadmium–calcium hydroxyapatite solid solutions at 25 °C

Abstract A series of cadmium–calcium hydroxyapatite solid solutions [Cd–Ca-HAP, (Ca 1 − x Cd x ) 5 (PO 4 ) 3 (OH)] was synthesized and characterized, and then the dissolution of the prepared solids in aqueous solutions was experimentally studied. The element concentrations in the aqueous solutions were considerably influenced by the Cd/(Cd + Ca) atomic ratio of the Cd–Ca-HAP solids (X Cd ). For solids with high X Cd (> 0.6), the aqueous Ca 2 + concentration increased gradually with time and reached a stable state after approximately 5040 h of dissolution; the aqueous Cd 2 + concentration increased rapidly with time and reached a peak value within 480–720 h, and then decreased gradually and attained a stable state after 5040 h of dissolution. For solids with low X Cd (≤ 0.6), the aqueous Ca 2 + concentration increased rapidly with time and reached a peak value after approximately 240–360 h of dissolution, and then decrease slightly and are relatively stable after 720 h; the aqueous Cd 2 + concentration increased quickly with time and attained a peak value within 1 h and then decreased gradually and attained a stable state after 480 h of dissolution. The dissolution process of the solid samples with high X Cd (> 0.6) was different from that of the solids with low X Cd (≤ 0.6), which can be related to a slight tendency of Cd 2 + in preferring the M(2) sites and Ca 2 + in occupying the M(1) sites in the apatite structure. The average solubility products ( K sp ) were estimated for Cd 5 (PO 4 ) 3 OH to be 10 –64.62 (10 –64.53 –10 –64.71 ) at 25 °C and for Ca 5 (PO 4 ) 3 OH to be 10 –57.65 (10 –57.56 –10 –57.79 ) at 25 °C. The corresponding free energies of formation (Δ G f o ) were − 3970.47 kJ/mol and − 6310.45 kJ/mol. The solubility of the solid solution (Cd x Ca 1 − x ) 5 (PO 4 ) 3 (OH) decreased with increasing X Cd . For the dissolution at an initial pH of 2 and 25 °C, plotting of the experimental data on the Lippmann diagram indicated that (Cd 0.71 Ca 0.29 ) 5 (PO 4 ) 3 (OH) dissolved stoichiometrically during the initial period and approached gradually up to the Lippmann solutus before overshooting it. Finally, the data points moved along the Lippmann solutus curve from right to left. The Cd 5 (PO 4 ) 3 (OH)-rich solid phase was in equilibrium with the Cd-poor aqueous solution. These findings provided a better understanding of the processes controlling the solubility, transport and distribution of cadmium in the environment.