A Low-Temperature Thermodynamic Model for the Na-K-Ca-Mg-Cl System Incorporating New Experimental Heat Capacities in KCl, MgCl2, and CaCl2 Solutions

Chloride salts and their solutions are common on Earth and perhaps on other solar system bodies. To better understand the effects of freezing and evaporation on mineral formation and the composition of residual brines in chloride salt systems, we developed a comprehensive Pitzer model in the Na-K-Ca-Mg-Cl system valid from 298.15 to 2, and MgCl2 solutions that have been measured using differential scanning calorimetry, as well as heat capacities of crystalline phases and enthalpies of solution from literature sources. These thermal properties constrain the temperature dependence of model activity coefficients. We also use water activity data and solubility data covering the full temperature range of equilibrium salt solubilities in binary, ternary, quaternary, and quinary systems. Importantly, our model accurately represents the thermodynamic properties of CaCl2 salts and their mixtures to high concentrations. This model is applicable to chloride-rich brines in a variety of settings and provides a thermodynamically consistent framework for the future addition of other brine chemistries (e.g., sulfates and perchlorates).