Solubility of mimetite Pb5(AsO4)3Cl at 5–55°C

Environmetal context. The mobility of toxic arsenic compounds in the environment can be controlled by the solubility of certain minerals. To predict and model the fate and behaviour of these contaminants, the solubility and related thermodynamic properties of the lead and arsenic mineral mimetite were determined. The data obtained in this study will be used to optimise and increase the effectiveness of remediation procedures that are already applied to contaminated sites. Abstract. The solubility of the synthesised mimetite was measured in a series of dissolution experiments at 5–55°C and at pH values between 2.00 and 2.75. The solubility product logKSP for the reaction Pb5(AsO4)3Cl ? 5Pb2+ + 3AsO43– + Cl– at 25°C is –76.35 ± 1.01. The free energy of formation ?Gf,2980 calculated from this measured solubility product equals –2634.3 ± 5.9 kJ mol–1. The temperature dependence of the logKSP is non-linear, indicating that the enthalpy of the reaction depends on the temperature. The enthalpy of the formation of mimetite ?Hf0, is –2965.9 ± 4.7 kJ mol–1, the entropy, ?S0, is 39.5 J mol–1 K–1, and the heat capacity, ?Cp,f0 is –6172 ± 105 J mol–1 K–1. Hydrochemical modelling indicates that regardless of the composition of the background solution, Pb5(AsO4)3Cl is most stable at neutral to weakly alkaline pH.