Calculation on salinity solubility based on thermodynamic models at different temperatures

Saline soils are significantly affected by water-salt phase changes, evaporation, and groundwater during seasonal freezing and thawing. For the study of physical and mechanical properties of saline soils, solubility is an important indicator that varies with temperature. However, there have been very limited computational studies on solubility at low temperatures. The model for calculating the solubility of Na₂SO₄-NaCl-H₂O ternary system under low temperature conditions was constructed in this paper, based on the Pitzer and BET models. Improvements were made to the parameters $\varnothing$ and $\gamma$ in the Pitzer model, while improvements were made to the parameters $c$, $r$, and $aw$ in the BET model. The solubility changes within the range of 273.15 K–373.15 K were calculated and validated by combining them with indoor experiments. It was found that both the improved Pitzer model and BET model accurately predicted relative equilibrium solubility data of the Na₂SO₄-NaCl-H₂O ternary system at temperatures ranging from 273.15 K to 373.15 K. Additionally, compared with the Pitzer model, the BET model had advantages such as easy parameter acquisition and wider application range. The findings from this research hold great significance for understanding the process and patterns of salt analysis during soil freeze-thaw cycles as well as providing a scientific foundation for further comprehension of phase change laws and physical properties related to saline soils.