Thermodynamics of the solid solution - Aqueous solution system (Ba,Sr,Ra)SO4 + H2O: II. Radium retention in barite-type minerals at elevated temperatures

V. L. Vinograd, D. A. Kulik, F. Brandt, M. Klinkenberg, J. Weber, B. Winkler, D. Bosbach

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Abstract

The effect of temperature on the solid solution – aqueous solution (SS-AS) equilibria in the (Ba,Sr,Ra)SO4+H2O system is primarily determined by a change in the aqueous solubilities of the end members BaSO4, SrSO4 and RaSO4. The dependence of the solubility vs. the temperature for an MSO4 sulphate is, in turn, determined by the entropy and the heat capacity effects of the reaction MSO4 = M2+ + SO4 2−. The missing data for M = Ra are estimated here by a combination of atomistic simulations, a Debye-Einstein extrapolation of known thermodynamic properties of MSO4 sulphates, direct experimental measurements of solid solubility in the system (Ba,Ra)SO4+H2O at 70 and 90 °C, and thermodynamic modelling with the aid of the GEM-Selektor code. Finally, the GEM simulations together with the data assessed here and in Part I of this study (Vinograd et al. 2017) are applied to model the uptake of Ra in the (Ba,Sr,Ra)SO4+H2O system in the temperature range of 0–300 °C. Our results, consistent with earlier studies, show that the uptake of Ra by (Ba,Sr)SO4 barite solid solutions at about 25 °C is favoured by a lower solubility of RaSO4 relative to those of BaSO4 and SrSO4, however, with increasing temperature, the solubilities of MSO4 solids converge. Consequently, an increase in temperature makes the Ra uptake by Ba,Sr sulphates less efficient. In a radioactive waste repository relevant system, this effect would be partially compensated by the common anion and the dilution effects, which both enhance the Ra-uptake.

Original languageEnglish
Pages (from-to)190-208
Number of pages19
JournalApplied Geochemistry
Volume93
DOIs
StatePublished - Jun 2018

Funding

The research leading to these results has received partial funding from the German Federal Ministry of Education and Research (BMBF) joint projects ImmoRad (grant 02NUK019 ) and ThermAc3 (grants 02NUK039 ) and from the DFG project ( Wi 1232/44-1 ). DK is also grateful for the partial financial support from Nagra, Wettingen . VV, FB and MK acknowledge contributions of Konstantin Rozov at earlier stages of ImmoRad and ThermAc3 projects, which were valuable for defining the scope of this MS. We are also grateful to Fabian Sadowski, Guido Deissmann and Giuseppe Modolo for their support. The DFT computations were performed at the Jülich Supercomputer Centre.

Keywords

  • Barite
  • Celestite
  • Radium sulphate
  • Solid solution
  • Temperature dependence

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