Dissolution of mimetite Pb₅(AsO₄)₃Cl in malic acid solutions

Mimetite Pb₅(AsO₄)₃Cl is the most insoluble lead arsenate mineral and could be used in remediation techniques to reduce As(V) mobility in soils. However, low-molecular-weight organic acids such as malic acid increase its solubility. The effect of malic acid on the dissolution of mimetite strongly depends on the pH of the equilibrium solution. At low pH, mimetite is decomposed mostly by the mechanism of protonation. With increasing pH, the solubility of mimetite decreases and a greater influence in its dissolution is ascribed to complexation of Pb(II) by organic ligands. During dissolution experiments, the amount of arsenic released to solution was > 26% higher in organic solutions than in inorganic solutions, and the amount of lead was > 8% greater. The solubility product of mimetite (K_SP) was calculated in order to quantify the thermodynamic stability of the investigated mineral. The value obtained, i.e., -24.52 ± 0.49 fluctuates between the values of -21.69 ± 1.05 and -27.87 ± 0.42 reported in the literature.