Solvent extraction, also known as liquid–liquid extraction or partitioning, is a method to separate a compound based on the solubility of its parts using two liquids that don’t mix, for example water and an organic solvent. Solvent extraction has a wide range of important applications, from the everyday in the processing of perfumes, vegetable oil, or biodiesel to critical technologies such as recovering plutonium from irradiated nuclear fuel so that the recovered plutonium can be re-used as nuclear fuel and for environmental remediation.
Although the kinetics and efficiency of solvent extraction rely upon the formation of ion-extractant complexes, identifying the location of ion-extractant complexation (i.e., making an atom or compound form a complex with another) during the solvent extraction process and its impact on the separation has been challenging.
To fill in this information, the authors of this study employed a variety of research techniques including synchrotron x-ray liquid surface scattering at beamline 15-ID of the NSF’s ChemMatCARS facility at the Argonne Advanced Photon Source to characterize the surface of aqueous solutions of lanthanide chlorides and the water-soluble extractant bis(2- ethylhexyl) phosphoric acid (HDEHP), in the absence of a coexisting organic solvent.
A surprising result was the finding that light lanthanides preferentially occupy the liquid-vapor interface, which runs counter to the expectation that heavy lanthanides should have a higher interfacial density since they are preferentially extracted by HDEHP in the solvent extraction processes.
These results reveal the antagonistic role played by ion−-extractant complexation within the aqueous phase and clarify the advantages of complexation at the interface. Extractants in common use are often soluble in water, in addition to their organic phase solubility, and similar effects to those described here are expected to be relevant to a variety of separations processes.
Pan Sun, Erik A. Binter, Zhu Liang, M. Alex Brown, Artem V. Gelis, Ilan Benjamin, Mrinal K. Bera, Binhua Lin, Wei Bu, and Mark L. Schlossman, “Antagonistic Role of Aqueous Complexation in the Solvent Extraction and Separation of Rare Earth Ions” ACS Cent. Sci. 2021, 7, 1908−1918. https://doi.org/10.1021/acscentsci.1c00960