THEREDA: Polythermal extension of the Pitzer database for strontium

INTRODUCTION
Radioactive strontium isotopes are products of the fission of uranium in the context of nuclear energy production. Furthermore, strontium is often used in geochemical calculations as an analog for radium, both being alkaline earth metals.
In the THEREDA database [1,2], a thermodynamic Pitzer dataset for Sr in the system of oceanic salts (Na⁺, K⁺, Mg²⁺, Ca²⁺ | Cl⁻, SO₄²⁻ – H₂O) is available [3]. Yet, this dataset is valid for the temperature T = 25 °C only. However, several strontium solid phases show a large variation in solubility with temperature, e.g. strontium chloride hydrates (SrCl₂∙xH₂O with x = 6, 2 and 1) [4] or strontium hydroxide (Sr(OH)₂∙8H₂O) [5].
In this work, a polythermal extension of the existing Sr dataset for the chloride system is presented, which is valid in the range T = 0–100 °C.

DESCRIPTION OF THE WORK
Experimental data (osmotic coefficients) from literature were used to generate a temperature function for the binary Pitzer interaction coefficients (β⁰, β¹, and CΦ). Solubility data of SrCl₂ in water [4] were then used to parameterize temperature functions for the solubility products of the different strontium chloride hydrates. Consistency with the existing 25 °C data set was ensured.
Solubility data of SrCl₂ in water [4] were then used to parameterize temperature functions for the solubility products of the different strontium chloride hydrates. Polythermal expansions of the ternary Pitzer coefficients also were required for a few subsystems only.
Furthermore, the data set was extended to quaternary acidic systems (Sr²⁺, H⁺ | Cl⁻ – H₂O).

RESULTS
With the obtained dataset, it is possible to model the ternary system (Sr²⁺, Na⁺ | Cl⁻ – H₂O) in the temperature range T = 0–100 °C. The solubility of all known solid phases is correctly reproduced, see Fig. 1. The dataset now also allows polythermal calculation of higher systems such as the quaternary system (Sr²⁺, Na⁺, K⁺ | Cl⁻ – H2O) in the temperature range for which experimental solubility data are available (T = 15–50 °C), see Fig. 2. For those two systems, no adjustment of the existing ternary Pitzer interaction coefficients was necessary.
The presented polythermal extension of the strontium Pitzer model allows robust calculation of the geochemical behavior of strontium in the chloride system over a wide temperature range. This extension of the THEREDA data set will become part of the next official data release and will be complemented by polythermal data for strontium sulfate (SrSO₄) and hydroxide (Sr(OH)₂∙8H₂O) in future work.

ACKNOWLEDGMENTS
THEREDA is funded by the German “Bundesgesellschaft für Endlagerung (BGE)”, contract number 45181017.

REFERENCES
1. THEREDA – Thermodynamic Reference Database. Release 2021, https://www.thereda.de/, (2022).
2. H. C. Moog et al., “Disposal of Nuclear Waste in Host Rock formations featuring high-saline solutions - Implementation of a Thermodynamic Reference Database (THEREDA)” Appl. Geochem., 55, 72–84 (2015), DOI: 10.1016/j.apgeochem.2014.12.016.
3. T. Scharge “Thermodynamic model for the systems Sr – Na, K, Mg, Ca – Cl, SO₄ – H2O at 298.15 K”, THEREDA Report (2016).
4. B. S. Krumgalz “Temperature Dependence of Mineral Solubility in Water. Part I. Alkaline and Alkaline Earth Chlorides” J. Phys. Chem. Ref. Data, 46, 043101, DOI: 10.1063/1.5006028.
5. I. Lambert et al. “Alkaline Earth Hydroxides in Water and Aqueous Solutions” IUPAC Solubility Data Series Vol. 52, Pergamon Press, Oxford, 388 p.