Solubility and dissolution mechanisms of vanadinite Pb5(VO4)3Cl: Effects of temperature and PO4 substitutions

Abstract The effects of temperature and PO4 substitutions on the solubility of a series of vanadinite (Van) Pb5(VO4)3Cl–pyromorphite (Py) Pb5(PO4)3Cl solid solutions were experimentally investigated to determine differences in dissolution mechanisms and to estimate the Ksp and thermodynamic state functions ΔH°f, ΔG°f, S°, and ΔC°p. Dissolution of Van and the selected phases from the Van–Py series at pH 2.0 was congruent at 5 °C, but it became noncongruent with precipitation of a secondary phase - chervetite Pb2V2O7 in the temperature range of 25–65 °C. The isomorphic substitution of VO4 with PO4 reduced the level of Pb released during dissolution into the equilibrium solution from approximately 207.2 mg Pb L−1 for Van to 131 mg Pb L−1 for Py (at 5 °C). This substitution, however, simultaneously caused a very strong increase in the thermodynamic solubility constant Ksp, which showed a large difference (over 14 orders of magnitude) for the end members of the series. The experimentally determined Ksp at 5 °C varied from 10−96.89±0.53 for Van to 10−80.91±0.70 for Py. The variation in Ksp was linear with substitution of VO4 with PO4 and curvilinear with change in temperature. The more the amount of vanadium in the structure, the stronger was the temperature effect. Because of the noncongruent dissolution followed by far from equilibrium state for V-reach samples at higher temperatures, the experimental determination of thermodynamic state functions was possible only for pyromorphite and some P-reach members of Py–Van solid solution series. For Py, ΔH°f = −4098 ± 5 (kJ mol−1), ΔG°f = −3763 ± 4 (kJ mol−1), S° = 667 ± 313.4 (kJ mol−1 K−1), and ΔC°p = −930.9 ± 345.2 (kJ mol−1 K−1). For Van, ΔH°f = −3796 ± 10 (kJ mol−1), ΔG°f = −3486 ± 4 (kJ mol−1), S° = 719 ± 694 (kJ mol−1 K−1), and ΔC°p = −1037 ± 605 (kJ mol−1 K−1) were estimated.