The SIT model parameters for interactions of sulfate ion with alkali metal ions accounting for complex formation

• Published in: Applied geochemistry 2023-09, Vol.156, p.105740, Article 105740
• Main Author: Plyasunov, Andrey V.
• Format: Article
• Language: English
• Subjects: Activity coefficients; Alkali metal cations; Interaction coefficients accounting for association; Stability constants; Sulfate ion; The SIT model
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2023.105740

The goal of this study is to determine the interaction coefficients between alkali metal ions M+ (M = Li, Na, K, Rb, Cs, NH4), the sulfate ion SO42−, and ion pairs MSO4− for the use in the SIT model for activity coefficients, which is popular among solution chemists and adopted by OECD Nuclear Energy Agency (NEA) to develop a thermodynamic database relevant to the safety of radioactive waste repositories. As alkali metal sulfates are partially associated electrolytes, relations from the thermodynamics of associated solutions (“equilibrium model”) are employed for the correct treatment of tabulated activity and osmotic coefficients. As a part of this work, a compilation of stability constants of the ion pairs MSO4− is made together with selection of the recommended values. In addition, solubility values in ternary M2SO4−N2SO4−H2O are used to expand the list of relevant interactions. In total, the numerical values of the SIT coefficients are determined for 38 ionic interactions among indicated species. All values refer to 298.15 K and 0.1 MPa. •Solutions of alkali metal sulfates are partially associated in water.•Stability constants of these ion pairs are evaluated.•The SIT model interaction coefficients for these sulfate species are determined.•Solubility diagrams in ternary water-salt systems are calculated.