Two-Phase Calorimetry. II. Studies on the Thermodynamics of Cesium and Strontium Extraction by Mixtures of H~+CCD~- and PEG-400 in FS-13

摘要

Thermochemical characterization of the partitioning of cesium and strontium from nitric acid solutions into mixtures of the acid form of chlorinated cobalt dicarbollide (H~+CCD~-) and polyethylene glycol (PEG-400) in FS-13 diluent has been completed using isothermal titration rnicrocalorimetry and radiotracer distribution methods. The phase transfer reaction for Cs~+ is a straightforward (H~+ for Cs~+) cation exchange reaction. In contrast, the extraction of Sr~(2+) does not proceed in the absence of the co-solvent molecule PEG-400. This molecule is believed to facilitate the dehydration of the Sr~(2+) aquo cation to overcome its resistance to partitioning. The phase transfer reactions for both Cs~+ and Sr~(2+) are enthalpy driven (exothermic), but partially compensated by an unfavorable entropy. The results of the calorimetry studies suggest that the PEG-400 functions as a stoichiometric phase transfer reagent rather than acting simply as a phase transfer catalyst or phase modifier. The calorimetry results also demonstrate that the extraction of Si~(2+) is complex, including evidence for both the partitioning of Sr(NO3)~+ and endothermic ion pairing interactions in the organic phase that contribute to the net enthalpic effect. The thermodynamics of the liquid-liquid distribution equilibria are discussed mainly considering the basic features of the ion solvation thermochemistry.