This title appears in the Scientific Report : 2013 

Solid solution formation and uptake of Radium in the presence of barite
Brandt, Felix (Corresponding author)
Klinkenberg, Martina / Vinograd, Victor / Rozov, Konstantin / Bosbach, Dirk
Nukleare Entsorgung und Reaktorsicherheit; IEK-6
Goldschmidt Conference 2013, Firenze (Italy), 2013-08-25 - 2013-08-30
Conference Presentation
Slow processes in close-to-equilibrium conditions for radionuclides in water/solid systems of relevance to nuclear waste management
Safety Research for Nuclear Waste Disposal
The phase relations in the BaSO4-RaSO4-H2O system may determine the solubility of radium in natural waters due to the formation of a solid solution. In the near-field of nuclear waste repositories for spent fuel, radium may enter a system in which barite is in equilibrium with the aqueous solution. Thermodynamically, a RaxBa1-xSO4 solid solution is expected to form as solubility controlling phase rather than RaSO4. However, due to a lack of reliable data, the solid solution system RaSO4-BaSO4-H2O is currently not considered in long term safety assessments for nuclear waste repositories. The solubility product of the pure RaSO4 endmember is poorly constrained between pKRaSO4 = 10.26 to 10.41 by only very few experimental data [1,2]. Published interaction parameters WBaRa of the RaSO4-BaSO4-H2O system varies varies in different studies [3, 4] between 0.9 and 3.9 - 6.5 kJ/mol. In this study we have combined experimental data, atomistic calculations and thermodynamic modeling to study in detail how a radium containing solution will equilibrate with solid BaSO4 under repository relevant conditions. Batch sorption experiments at close to equilibrium conditions indicate the formation of a RaxBa1-xSO4 solid. Our first principles calculations based on the single defect method [5] indicate a value of WBaRa = 2.5 ± 1.0 kJ/mol, implying a non-ideal solid solution. Thermodynamic assessment calculations indicate that the final experimental Ra(aq) concentration at room temperature and 90 °C can be matched with WBaRa ≈ 1.5 kJ/mol and pKRaSO4 ≈ 10.41. [1] Lind, S. C., et al (1918). J Am Chem Soc 40, 465-472. [2] Paige, C. R. et al.(1998). Geochim. Cosmochim. Acta 62, 15-23. [3] Zhu, C., 2004. Geochim. Cosmochim. Acta 68, 3327-3337. [4] Curti, E., et al. (2010). Geochim. Cosmochim. Acta 74, 3553-3570. [5] Sluiter & Kawazoe (2002) Europhys Lett. 57, 526-532.