Thermodynamic referencedatenbasis for the hexary system of oceanic salts, extended by its acids & bases and CO₂/carbonate: Na⁺, K⁺, Mg²⁺, Ca²⁺, H⁺, Cl^-, SO₄^2-, OH^-, CO₂/CO₃^2- - H₂O

Evaporitic sediments (salt deposits) are widespread worldwide and often form geological formations several hundred to thousand metres thick. Over millions of years, evaporation processes have precipitated all dissolved salts from the seawater, with NaCl (halite) being the main component and all others, such as KCl, MgSO₄, MgCl₂ etc., form a variety of other salt minerals.

For the understanding or development of processes of crystallisation, redissolution and recrystallisation

• during the formation of a particular geological formation,
• for extraction processes (e.g. potassium salts etc.),
• for the formation of a particular geological formation
• .),
• for the safety analysis of a repository for radioactive waste in rock salt, among other things,

a model of the solubility equilibria in the hexagonal system of the oceanic salts Na-K-Mg-Ca-Cl-SO₄-H₂O is required that can represent the experimental findings with interpolation of data gaps. Pre-calculations for the development of the composition of possible access solutions, e.g. for the engineering design of a repository with regard to the location and selection of shaft and drift sealing elements for long-term storage, form a building block in the safety analysis of a repository for radioactive waste in the host rock salt.

We started developing a consistent thermodynamic data set for the oceanic salt system based on the Pitzer ion interaction model in 2006 as part of THEREDA (www.thereda.de) (funded by the BMBF, BMWi and currently by BGE). At present, this data set is the only one in the world that provides a comprehensive polythermal calculation (at least in the range from 0 to 110°C) of the hexary system, extended by its acids & bases as well as CO₂/carbonate (Na⁺, K⁺, Mg²⁺, Ca²⁺, H⁺, Cl^-, SO₄^2-, OH^-, CO₂/CO₃^2- - H₂O).

Dataset development & model quality

With the establishment of the Pitzer ion interaction model for solutions with high ionic strengths and the proof that this approach is suitable for describing the oceanic system at 25°C (HMW model, Harvie et al. 1984), the polythermal THEREDA dataset was developed using the HMW model as a "25°C anchor". The temperature-dependent Pitzer activity models (Papalan & Pitzer 1987) were implemented for the binary systems (NaCl-H₂O, KCl- ...). By reviewing all solubility data available in the literature, their critical evaluation and storage in ASCII format (approx. 16000 data points), the polythermal model was developed in a step-by-step evaluation up to the hexary system.

Some improvements were made to the underlying 25°C HMW model in line with the state of research and development. FOR EXAMPLE

• The chemical formula for kainite was corrected (4KCl∙4MgSO₄∙11H₂O instead of KCl-MgSO₄∙3H₂O);
• the missing mineral goergeyite (K₂SO₄∙5CaSO₄∙H₂O) was implemented, which has a crystallisation field at 25°C;
• the solubility of KCl is described more correctly;
• the water activity of the gypsum/anhydrite transition at NaCl saturation is shifted to the more acceptable value of 0. 84 - 0.85 (instead of 0.775 in the HMW model);
• two additional aqueous solution species (KMgSO₄⁺, KCaSO₄^-) have been implemented, which improve the description of solubilities at elevated temperatures.

The existing polythermal data set in THEREDA is a solubility data-checked data set. Visual evidence of the model quality achieved will soon be available here based on binary systems:

Contact person

Selected publications

• THEREDA: German Thermodynamic Reference Database for Geochemical Modelling of Nuclear Waste Disposal under Saline Conditions - Application, Overview, and new development.
  H. Moog, M. Altmaier, F. Bok, V. Brendler, D. Freyer, X. Gaona, S. Hagemann, C. Joseph, G.D. Miron, M. Pannach, J. Sohr, W. Voigt, M. Voss and L. Wissmeier.
  xxx xx, xx-xx (2025) in progress
• Implementation of Carbonates and CO₂ into the T-dependent Pitzer Model of Oceanic Systems. I. System NaOH - Mg(OH)₂ - Ca(OH)₂ - CO₂ - H₂O.
  W. Voigt.
  THEREDA-Journal 03(02) 1-77 (2023) read online (open access)

Scientific papers, national - international

-Selection-

• W. Voigt: T-dependent Pitzer model for carbonates and CO₂ in the THEREDA-database for aqueous geochemistry up to high ionic strengths.
  ISSP 21, Novi Sad, Serbia, 09-13 September 2024.
• J. Sohr: Representation of phase equilibria in the oceanic salt system by the THEREDA model.
  ABC-SALT VII, Santa Fe, New Mexico, USA, 15-16 June 2023.