Triple-phase Separation of Alkaline Earth Metal Ions

Abstract

Both Sr²⁺ and Cs⁺ have been acknowledged as serious components of high level radioactive waste. The separation of Cs⁺ from solutions containing other alkali metal ions has already been accomplished by ion-exchange method using dihydrogen tetratitanate hydrate fibers. The separation of Sr²⁺ from other alkaline earth metal ions, however, is complicated because Sr²⁺ is in the middle of the selectivity series of any known separation method involving either solvent-extraction and ion-exchange methods. Separation of alkaline earth metal ions was studied by a triple-phase separation method which combined ion-exchange and solvent-extraction methods. In this study, the complete separation of individual alkaline earth metal ion has been sought for an aqueous solution containing Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺. The selectivity series of the ion-exchange system using dihydrogen tetratitanate hydrate fibers was Ba²⁺ _??_Sr²⁺>Ca²⁺_??_Mg²⁺. Ba²⁺ and Mg²⁺ can be separated from other alkaline earth metal ions because of the large separation factors to neighboring ions, but Sr²⁺ and Ca²⁺ cannot be separated each other in this manner. The extractability of alkaline earth metal ions with the solvent extraction system using thenoyltrifluoroacetylacetone (TTA, extractant) is Mg²⁺>Ca²⁺>Sr²⁺>Ba²⁺. Although the separation between Sr²⁺ and Ca²⁺ is better than that by the ion-exchange method, it is not sufficient for complete separation. To accomplish complete separation of Sr²⁺ and Ca²⁺, the organic phase containing TTA was added to the ion-exchange system. Because of their reverse relationship of the selectivity series, the smaller metal ions in an aqueous phase were extracted into the organic phase, while the larger metal ions were sorbed onto the ion exchanger. The separation factors of metal ions are much larger than those obtained by solvent extraction and ion exchange methods used separately.