On the solubility of radium sulfate and carbonate
Radium is one of the most toxic elements and its concentration in different human activities and migration from man-made wastes provokes a strong interest in environmental science. To be able to model the migration process, reliable experimental thermodynamic data of radium compounds are needed.
In this work details of the safe radium source disassembly which were previously used in brachytherapy are described and different methods for conversion of RaSO4 into aqueous solution are reviewed. The method of choice included three cycles of RaSO4 heating in 1.5 M Na2CO3 up to 85 ºC, cooling and subsequent removal of supernatant. X-ray diffraction studies showed that the method allows the synthesis of amorphous RaCO3, which can be dissolved in mineral acid. Gamma spectrometric measurements showed that most of the initial RaSO4 was converted into solution and that 7 ± 1 % of the initial 210Pb was co-precipitated with RaCO3. Synthesized RaCO3 was dissolved in HCl to prepare a radium stock solution.
The radium stock solution was used to determine the solubility of pure RaSO4 and RaCO3 from oversaturation using Na2SO4 and Na2CO3 as a source of sulfate and carbonate ions. The solubility was determined at 25.1 ºC as a function of ionic strength using NaCl media. The concentration of radium was measured by gamma spectrometry after separation of the aqueous phase from the solid phase using ultracentrifugation. The extended specific ion interaction theory was used to extrapolate solubility product constants to zero ionic strength (log10 Ksp° = -10.16 ± 0.05 for RaSO4 and log10 Ksp° = -7.73 ± 0.56 for RaCO3) and to calculate ion interaction coefficients.
A comparison of the shapes of the radium solubility curves for both anions with the shapes of corresponding barium solubility curves demonstrates the similarity of the behaviour of radium and barium salts in saline solutions. It can be supposed that due to the similarity of the radium and barium effective ionic radii, and the same charge, these ions undergo similar specific ion interactions in NaCl media.