The efficient separation of heavy elements, including lanthanides and actinides, is an important task for many technological applications. Solvent extraction (SX) is the most common technology used in refining and reprocessing of heavy elements. Understanding, and controlling the mass transport through interfaces is a key challenge in SX. Extractant molecules used in SX are typically amphiphilic, so they alter the interface between the aqueous and organic phases. The precise nature of these interfacial effects on SX, in terms of extraction capacity and selectivity, are not well understood. Generally, heavier lanthanides are extracted better than the lighter lanthanides, which has been attributed to increasing ligand binding as the cation’s charge density increases due to the so called lanthanide contraction. However, in some extraction systems middle and lighter lanthanides are extracted better. A particularly interesting case is that of Aliquat 336, a quaternary ammonium extractant. While thiocyanate salt of Aliquat 336 extracts heavier lanthanides, the nitrate salt shows the reverse trend. The molecular-scale origins of this peculiar behavior is not understood. We present results from small angle X-ray scattering experiments on the organic phase obtained from extraction with trioctylmethylammonium (TOMA, a major component of Aliquat 336) nitrate and thiocyanate to understand the mesoscale origins of these trends. Our results show differences in clustering behavior of lanthanide-extractant complexes in nitrate and thiocyanate media. In the case of thiocyanate medium, number of lanthanide-extractant complexes per cluster appears to be directly correlated with extraction efficiency while in the nitrate medium it varies non-monotonically across the lanthanide series, suggesting different mechanisms dominate in the presence of these two anions. We discuss the implications of these results to our understanding of extractant assisted ion and mass transport at the oil/aqueous interface with an emphasis on transient species that cannot be observed in static experiments.

Ion-specific clustering of metal-amphiphile complexes in solvent extraction

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.