An integrated experimental and modeling approach to understand pyromorphite solubility

The lead apatite mineral, pyromorphite (Pb₅(PO₄)₃Cl), has been of great interest due to its stable nature under most chemical conditions found in the environment; yet there are lingering questions about pyromorphite solubility needing to be successfully addressed to utilize this mineral in Pb remediation efforts. To address these gaps in our knowledge, we conducted a series of solubility experiments over the pH range of 2-10. A K_sp of 10^-79.58 is calculated for pyromorphite which is slightly higher than recent estimates of Ksp that range from ∼10^-80 to 10^-81. FTIR analysis of the synthesized material in this study indicates that the higher K_sp is mainly caused by the lower degree of crystallinity due to the shorter aging period compared to previous studies (one week vs. 2 weeks to a month). Despite predicting Pb release accurately at acidic pH values, PHREEQC simulations of pyromorphite dissolution using the K_sp calculated at pH values between 6 and 8 predicts aqueous Pb concentrations approximately an order of magnitude less than observed in the batch experiments. When a second phase (PbCl₂) at 0.2-0.4 % percent weight is included in the dissolution simulations, PHREEQC accurately models the results of the experiments up to pH 8. These findings indicate that the presence of a more soluble Pb bearing phase at levels undetectable to techniques such as FTIR and XRD can have a notable effect on the effective solubility of pyromorphite. Furthermore, these findings suggest that the pH conditions during phosphate remediation need to be closely monitored to minimize more soluble Pb minerals from coprecipitating.