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

Background: The interaction between Ca-HAP and Pb2+ solution can result in the formation of a hydroxyapatite-hydroxypyromorphite solid solution [(PbxCa1-x)(5)(PO4)(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 (PbxCa1-x)(5)(PO4)(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 [(PbxCa1-x) 5(PO4) 3(OH)] in aqueous solution at 25 degrees 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 [(Pb0.89Ca0.11)(5)(PO4)(3)OH], the aqueous Pb2+ 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 Pb2+ 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(PO4)(3)OH] and 10(-58.38)+/- 0.07 (10(-58.31)-10(-58.46)) for calcium hydroxyapatite [Ca-5(PO4)(3)OH]. The Gibbs free energies of formation (Delta 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 (PbxCa1-x)(5)(PO4)(3)(OH). For the dissolution at 25 degrees C with an initial pH of 2.00, the experimental data plotted on the Lippmann diagram showed that the solid solution (PbxCa1-x)(5)(PO4)(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(PO4)(3)OH, and then the data points moved along the Lippmann solutus curve from right to left. The Pb-rich (PbxCa1-x)(5)(PO4)(3)(OH) was in equilibrium with the Ca-rich aqueous solution.