Extraction of cesium nitrate from concentrated sodium nitrate solutions with 21-crown-7 ethers: Selectivity and equilibrium modeling

The extraction of cesium nitrate from a high concentration of sodium nitrate by a family of 21-crown-7 ethers in 1,2-dichloroethane has been investigated. Dibenzo-21-crown-7 (DB21C7) and bis[4(5),4′(5′)-tert-butylbenzo]-21-crown-7 (BtBB21C7) ethers have higher selectivity for cesium but lower extraction efficiency than dicyclohexano-21-crown-7 (DC21C7) ether. As measured by the distribution coefficient ratio DC_s/D_Na, the cesium selectivity averaged 78 and 93 for DB21C7 and BtBB21C7, respectively. However, in the case of DC21C7, the cesium selectivity was lower and decreased approximately three-fold from 10 to 3 as cesium loading increased. Alkyl substitution on the benzo group has only a small effect on the extraction behavior. It was shown by use of the equilibrium modeling program SXLSQI that the extraction of sodium and cesium could be adequately modeled by augmenting the previously determined cesium nitrate equilibrium model with a 1:1 Na⁺:crown organic-phase complex. No evidence for a mixed-metal species was found. In accord with our previous study, cesium nitrate extraction entails formation of a 1:1 Cs⁺:crown complex, but a minor 1:2 complex also forms in the case of DC21C7. Although the sodium and cesium organic-phase complexes are largely ion-paired with nitrate anion under the experimental conditions employed, observed slope behavior suggests slight dissociation of the nitrate anion, and in fact, improved fits are obtained assuming ion-pair dissociation in the modeling. The modeling further suggests that the three-fold loss of selectivity toward cesium in the case of DC21C7 is related in part to the diminishing contribution of the 1:2 Cs⁺:crown complex as the crown ether is loaded with cesium.