Solubility of mimetite Pb5(AsO4)3Cl – Vanadinite Pb5(VO4)3Cl solid solution series at 5–65 °C

Abstract

Ternary isomorphic lead apatite system comprises mimetite (Pb5(AsO4)3Cl, Mim), vanadinite (Pb5(VO4)3Cl, Vna), and pyromorphite (Pb5(PO4)3Cl, Pym). These phases are frequently studied for environmental applications, such as immobilization of trace metals and metalloids. Despite extensive knowledge of the endmembers, thermodynamic data for the intermediate phases in the Mim–Vna series and their temperature-dependent stability are lacking. This gap was addressed by studying the effect of vanadate substitution on the solubility of the Mim–Vna series, the effect of temperature on dissolution mechanisms and secondary phase formation, and by quantifying the solubility constants (Ksp) at different temperatures (5–65 °C). The Ksp for Mim–Vna phases decreases linearly with V content, showing progressive dissolution with temperature. Dissolution in the Mim–Vna series transitions from congruent to non-congruent as the V content increases, leading to the precipitation of chervetite (Pb2V2O7), particularly at higher temperatures (45–65 °C). Therefore, the concentration of Pb2+ in solution is controlled by chervetite precipitation, unlike the Mim–Pym system, in which Pym controls lead solubility. These findings enhance our understanding of the behavior of lead apatites in contaminated environments. The new solubility constants can be directly implemented into existing thermodynamic databases for geochemical modeling, aiding in the prediction of lead and metalloid behavior in aqueous systems for optimization of remediation strategies.