Recent and present activities in THEREDA

INTRODUCTION
THEREDA is the acronym for “Thermodynamic Reference Database”, a multi-institutional, national joint project to create a mutually accepted database for the geochemical modeling of solubilities in the near field of an underground repository for high-level waste in a rock salt formation (Moog et al. 2015).
As of April 2010 THEREDA became operative in the world wide web, and since 30th of June 2011 ready-to-use parameterfiles for EQ3/6, Geochemist’s Workbench, PHREEQC, and ChemApp are released. During the following years the team issued altogether eleven data releases. Each release covers data for particular subsystems only.
RECENT ACTIVITIES
Data releases
During the last two years three data releases were issued. Each release was qualified and documented with test calculations. Apart from ChemApp the releases are accompanied by code-specific scripts which enable the user to reproduce our calculations.
R-09 covered the systems

• U(IV) – (Na, K) – HCO3 – H2O
• U(IV) – Na – Cl – H2O
• U(VI) – (Na, K, Ca) – Cl – H2O

R-10 covered the systems

• Na - Cl - Tc(IV) / Tc(VII) - OH,
• Mg - Cl - Tc(IV) - OH,
• Ca - Cl - Tc(IV) - OH

R-11 covered the systems

• Sr – (Na, K, Mg, Ca) – Cl – H2O
• Sr - Na – SO4 – H2O

R-10 was the first release to cover a redox reaction. Our efforts to produce parameter files for different codes which deliver the same result posed unexpected difficulties and revealed differences on how the different codes tackle redox transitions.
Extension of internal calculation
THEREDA features a system of internal calculations of dependent data from independent data (the latter ones directly entered). It has been extended to provide for temperature extrapolations from standard reaction data and for the calculation of the latter from temperature functions of equilibrium constants of standard molar Gibbs free enthalpies of reaction.
PRESENT ACTIVITIES
Data releases
The following data releases are in preparation and have priority for 2017.

• Solubility of oxygen in pure water and high saline solutions
• Na, K – Cl,SO4 – (PO43-, HPO42-, H2PO4-) –H2O (binary, ternary, and some quaternary systems tested)
• This will lead to an upgrade of R-09 covering uranium solubility in phosphate bearing solutions
• Merge of R-01 (Na, K, Mg, Ca – Cl, SO4 – H2O polythermal) and R-03 (Na, K, Mg, Ca – Cl, SO4 – HCO3/CO2(g) – H2O 298.15K only) with an upgrade of the latter for elevated temperatures.

As a side product, the release on the oxygen solubilities will lead to a re-evaluation of logK-values for the reactions
2H2O(l) ⇌ O2(aq) + 2H2(g)
2H2O(l) ⇌ O2(g) + 2H2(g)
REFERENCES
1. 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). http://dx.doi.org/10.1016/j.apgeochem.2014.12.016
E. Yalçintaş et al., Thermodynamic description of Tc(IV) solubility and hydrolysis in dilute to concentrated NaCl, MgCl2 and CaCl2 solutions, Dalton Transactions (45), 8916-8936 (2016). -> [YAL/GAO2016]
2. J. B. Macaskill et al., Isopiestic measurements on aqueous Mixtures of Sodium Chloride and Strontium Chloride. J. Solution Chem. (7), 339-347 (1978). -> [MAC/WHI1978]
3. V. K. Filippov et al., Thermodynamics of phase equilibria in the potassium, strontium, sodium, chloride, water (K+,Sr2+|| Cl--H2O, Na+,Sr2+|| Cl--H2O and Na+,K+,Sr2+|| Cl--H2O) systems at 25°, Russ. J. Gen. Chem. (60), 427-431 (1990). -> [FIL/FED1990]
4. J. A. Rard and D. G. Miller, Isopiestic determination of the osmotic and activity coefficients of aqueous mixtures of sodium chloride and strontium chloride at 25° C, J. Chem. Eng. Data (27), 342-346 (1982). -> [RAR/MIL1982b]
5. S. L. Clegg et al., Isopiestic Determination of the Osmotic and Activity Coefficients of NaCl + SrCl2 + H2O at 298.15 K and Representation with an Extended Ion-Interaction Model, J. Chem. Eng. Data (50), 1162-1170 (2005). -> [CLE/RAR2005]
6. M. Kydynov et al., Solubility in the quaternary system consisting of lithium, sodium and strontium chlorides and water at 25 °C, J. Appl. Chem. (London) (4), 736-739 (1969). -> [KYD/LOM1969]