(212c) Antagonistic Role of Aqueous Complexation in the Solvent Extraction and Separation of Rare Earth Ions

During solvent extraction of rare earth ions, extractants in an organic solvent complex with ions in water to facilitate their transport into the organic phase. A common extractant used in the separation of rare earth ions, bis(2-ethylhexyl) phosphate (HDEHP), is partially soluble in water and hypotheses for the site of initial ion-extractant complexation include both the liquid-liquid interface and the bulk aqueous phase. To explore the role of the bulk aqueous phase, we show that Nd and Er ions that complex with HDEHP within an isolated aqueous phase exhibit behavior that appears to contradict trends observed under normal conditions for liquid-liquid solvent extraction. Interfacial tension measurements suggest that complexes of HDEHP with lighter lanthanides are enhanced at the liquid-vapor interface over those containing heavier lanthanides. Measurements of the interface with x-rays support these results in the specific cases of Nd (a light lanthanide) and Er (a heavy lanthanide) by direct detection of these elements at the interface. The greater availability of Nd-HDEHP complexes at the liquid-vapor interface suggests that contacting the aqueous solution with an organic phase would lead to more efficient extraction of Nd over Er with HDEHP. In contrast, it is known that liquid-liquid extraction into the organic phase under similar conditions is greatly favored for Er over Nd. These results reveal the antagonistic role played by complexation within the bulk aqueous phase and clarify the advantages of extractants that interact with ions only at the interface during the process of liquid-liquid extraction.