Abstract
Solid solution formation between RaSO4 and other isostructural sulfates has been known for a long time as a process potentially controlling Ra concentrations in aquifers. Here, we measured the Ra uptake in the Sr-rich corner of the ternary (Sr,Ba,Ra)SO4 solid solution - aqueous solution (SS-AS) system by equilibrating SrSO4, BaSO4 and mixed (Sr,Ba)SO4 solids with Ra-bearing aqueous solutions for up to 1302 days at 90 °C at a solid/liquid ratio of 5 g/L. The recrystallization experiments were combined with electron microscopy studies of the solids. The evolution of the solid and aqueous phases was interpreted based on thermodynamic modelling applying a recently revised thermodynamic dataset for the (Sr,Ba,Ra)SO4 + H2O system. The recrystallization process involved several metastable phases, starting from the least soluble, Ba- and Ra-rich precipitates and ending with Sr-rich solids, whose compositions approached the predicted equilibrium states. The composition and the time evolution of metastable phases were correlated with changes in the computed supersaturation functions. Particularly, the formation of Ra- Ba- and Sr-rich rims on primary barite grains in the experiment with the mechanical mixture of 99% of celestine and 1% of barite has been explained by combining calculated supersaturation conditions with considerations of structural misfit. A key result was the observed final Raaq concentration which is about one order of magnitude lower compared to the initial concentration, implying an efficient uptake into the newly formed solid phases. These concentrations appeared to be just slightly lower than those predicted by the thermodynamic calculations, implying that the complete SS-AS equilibrium was close to be reached in Sr-rich systems in the recrystallization experiments lasting for about 3.5 years.
Original language | English |
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Pages (from-to) | 1-17 |
Number of pages | 17 |
Journal | Chemical Geology |
Volume | 497 |
DOIs | |
State | Published - Oct 10 2018 |
Funding
The research leading to these results has received partial funding from the German Federal Ministry of Education and Research (BMBF) ImmoRad project (project number 02NUK019C ) and ThermAc project (project number 02NUK039D ). We are grateful to F. Sadowski, C. Kalitz, K. Dahmen, R. Engelmann, G. Deissmann and G. Modolo for their support. We thank Dr. D. Kulik (PSI, Switzerland) for discussions and supporting the thermodynamic calculations (GEMS). Prof. Dr. Th. Nagel (former Bonn University, Germany now University Arhus, Denmark) is acknowledged for electron microprobe measurements of the synthesized solid solutions.
Keywords
- (Sr,Ba,Ra)SO solid solution
- Barite
- Celestine
- Radium uptake
- Recrystallization
- Ternary solid solution thermodynamics