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Background  The interaction between Ca-HAP and Pb 2+  solution can result in the formation of a hydroxyapatite–hydroxypyromorphite solid solution [(Pb x Ca 1−x ) 5 (PO 4 ) 3 (OH)], which can greatly affect the transport and distribution of toxic Pb in water, rock and soil. Therefore, it’s necessary to know the physicochemical properties of (Pb x Ca 1−x ) 5 (PO 4 ) 3 (OH), predominantly its thermodynamic solubility and stability in aqueous solution. Nevertheless, no experiment on the dissolution and related thermodynamic data has been reported.    Results  Dissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb x Ca 1−x ) 5 (PO 4 ) 3 (OH)] in aqueous solution at 25 °C was experimentally studied. The aqueous concentrations were greatly affected by the Pb/(Pb + Ca) molar ratios (X Pb ) of the solids. For the solids with high X Pb  [(Pb 0.89 Ca 0.11 ) 5 (PO 4 ) 3 OH], the aqueous Pb 2+  concentrations increased rapidly with time and reached a peak value after 240–720 h dissolution, and then decreased gradually and reached a stable state after 5040 h dissolution. For the solids with low X Pb  (0.00–0.80), the aqueous Pb 2+  concentrations increased quickly with time and reached a peak value after 1–12 h dissolution, and then decreased gradually and attained a stable state after 720–2160 h dissolution.    Conclusions  The dissolution process of the solids with high X Pb  (0.89–1.00) was different from that of the solids with low X Pb  (0.00–0.80). The average  K  sp  values were estimated to be 10 −80.77±0.20  (10 −80.57 –10 −80.96 ) for hydroxypyromorphite [Pb 5 (PO 4 ) 3 OH] and 10 −58.38±0.07  (10 −58.31 –10 −58.46 ) for calcium hydroxyapatite [Ca 5 (PO 4 ) 3 OH]. The Gibbs free energies of formation (Δ G   f    o  ) were determined to be −3796.71 and −6314.63 kJ/mol, respectively. The solubility decreased with the increasing Pb/(Pb + Ca) molar ratios (X Pb ) of (Pb x Ca 1‒x ) 5 (PO 4 ) 3 (OH). For the dissolution at 25 °C with an initial pH of 2.00, the experimental data plotted on the Lippmann diagram showed that the solid solution (Pb x Ca 1−x ) 5 (PO 4 ) 3 (OH) dissolved stoichiometrically at the early stage of dissolution and moved gradually up to the Lippmann  solutus  curve and the saturation curve for Pb 5 (PO 4 ) 3 OH, and then the data points moved along the Lippmann  solutus  curve from right to left. The Pb-rich (Pb x Ca 1−x ) 5 (PO 4 ) 3 (OH) was in equilibrium with the Ca-rich aqueous solution.   Graphical abstract   Lippmann diagrams for dissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb x Ca 1−x ) 5 (PO 4 ) 3 OH] at 25 ˚C and an initial pH of 2.00.Electronic supplementary material  The online version of this article (doi:10.1186/s12932-016-0034-8) contains supplementary material, which is available to authorized users.

Characterization, dissolution and solubility of the hydroxypyromorphite–hydroxyapatite solid solution [(PbxCa1−x)5(PO4)3OH] at 25 °C and pH 2–9