Solubility of pyromorphite Pb 5(PO4)3Cl–mimetite Pb5(AsO4)3Cl solid solution series

Pyromorphite Pb 5(PO4)3Cl and mimetite Pb 5(AsO4)3Cl are isostructural minerals with apatite. Due to their high environmental stability, they have gained considerable attention as metals sequestration agents in water treatment and contaminated soil remediation. Pyromorphite and mimetite can form a continuous solid solution series in near-Earth surface environments. Precipitation of the end members and intermediate members of the series is likely to occur in the areas where the cost-effective in situ immobilization reclamation method, based on phosphate amendments, is applied. In contrast to the widely studied thermodynamic parameters of pyromorphite and mimetite, knowledge of the thermodynamics of their solid solutions is sparse. To supplement the data, a number of compounds from the pyromorphite–mimetite series were synthesized at room temperature using a method to simulate the conditions in the near-Earth surface environments. Afterward, batch dissolution and dissolution–recrystallization experiments of seven synthesized precipitates were conducted at 25 /C to 176C, pH = 2, and in a 0.05 M KNO3 background electrolyte for periods of 6 and 14 months. The experiments indicated all seven dissolutions were congruent, and the IAPs for pyromorphite and mimetite exhibited a significant difference in values over three orders of magnitude, highlighting significant partitioning tendencies in the series.