A thermodynamic model for solution behavior and solid-liquid equilibrium in Na-K-Mg-Ca-Al(III)-Fe(III)-Cr(III)-Cl-H₂O system from low to very high concentration at 25°C

In this study we evaluated new mixing (θ and ψ) Pitzer parameters, and developed models for solution behavior and solid liquid equilibria for the following mixed systems: 1) KCl-AlCl₃-H₂O, 2) KCl-FeCl₃-H₂O, 3) KCl-CrCl₃-H₂O, 4) MgCl₂-AlCl₃-H₂O, 5) MgCl₂-FeCl₃-H₂O, 6) MgCl₂-CrCl₃-H₂O, 7) CaCl_2-AlCl₃-H₂O, 8) CaCl₂-FeCl₃-H₂O, and 9) CaCl₂-CrCl₃-H₂O at 25°C. The solubility modeling approach, implemented to the Pitzer specific interaction equations is employed. The values of the binary parameters for the binary sub-systems needed here to parameterize models for mixed systems are taken from our previous studies. Mixing solution parameters are evaluated in this study using activity (when available) and solubility data. Following an approach in our previous modeling studies on M(III) chloride and sulfate systems, in this work we accept that complex Al(III), Cr(III), and Fe(III) aqueous species do not exist in solutions. We test the new models by comparing model predictions with experimental data (activity data for unsaturated solutions and solubility data in ternary systems). The agreement between model predictions and experimental data is very good. Combining present parameterization, with our M(III) models developed previously we fully complete our at 25°C model for the 8^th component system Na-K-Mg-Ca-Al(III)-Cr(III)-Fe(III)-Cl-H₂O. The resulting model calculates solubilities and solution activities to high solution concentration within experimental uncertainty. Limitations of the model due to data insufficiencies are discussed. The resulting parameterization was developed for the Pitzer formalism based PHREEQC database.