On solubility of radium sulfate and carbonate
Abstract: In this work details of 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 study 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 radium stock solution. The radium stock solution was used to determine 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 NaCl ionic strength. The concentration of radium was measured by gamma spectrometry after separation of the aqueous phase from the solid phase using ultracentrifugation. Hybrid extended specific ion interaction theory was used to extrapolate solubility product constants to zero ionic strength (log10K0sp=10.16±0.06 for RaSO4 and log10K0sp=7.32±0.06 for RaCO3) and to calculate ion interaction coefficients. It is shown that electrostatic model fails to predict RaCO3 and RaSO4 solubilities by extrapolation from other alkali-earth carbonates and sulfates solubilities using effective ionic radii in 8-fold coordination. The comparison of the shapes of radium solubility curves for both anions with the shapes of corresponding barium solubility curves demonstrates the similarity of radium and barium salts behaviour in saline solutions. According to the present theoretical model change of the solubility at particular ionic strength from the solubility at hypothetical zero ionic strength is attributed to the change of activity coefficients or to the long-range electrostatic interactions. Taking this into account it can be argued that due to similarity of radium and barium effective ionic radii and the same charge these ions undergo the same long-range interactions.
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