Abstract
The extractant-assisted transport of metal ions from aqueous to organic environments by liquid–liquid extraction has been widely used to separate and recover critical elements on an industrial scale. While current efforts focus on designing better extractants and optimizing process conditions, the mechanism that underlies ionic transport remains poorly understood. Here, we report a nonequilibrium process in the bulk aqueous phase that influences interfacial ion transport: the formation of metastable ion–extractant precipitates away from the liquid–liquid interface, separated from it by a depletion region without precipitates. Although the precipitate is soluble in the organic phase, the depletion region separates the two and ions are sequestered in a long-lived metastable state. Since precipitation removes extractants from the aqueous phase, even extractants that are sparingly soluble in water will continue to be withdrawn from the organic phase to feed the aqueous precipitation process. Solute concentrations in both phases and the aqueous pH influence the temporal evolution of the process and ionic partitioning between the precipitate and organic phase. Aqueous ion–extractant precipitation during liquid–liquid extraction provides a reaction path that can influence the extraction kinetics, which plays an important role in designing advanced processes to separate rare earths and other minerals.
| Original language | English |
|---|---|
| Article number | e2315584121 |
| Journal | Proceedings of the National Academy of Sciences of the United States of America |
| Volume | 121 |
| Issue number | 13 |
| DOIs | |
| State | Published - Mar 26 2024 |
| Externally published | Yes |
Funding
ACKNOWLEDGMENTS. This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Separations Program under Award Number DE-SC0018200 to M.L.S. and from the NSF CHE-1834750 to NSF’s ChemMatCARS.We thank Artem Gelis (University of Nevada at Las Vegas) for purification of HDEHP, Benjamin J. Reinhart for assistance at Sector 12 of the APS,M.Alex Brown (Argonne National Laboratory) for advice on fitting EXAFS data, and Snezhana Abarzhi for a discussion on fluid instabilities. ICP-OES measurements were performed at the Center for Nanoscale Materials, Argonne National Laboratory. Use of the APS and the Center for Nanoscale Materials, both Office of Science User Facilities operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.
Keywords
- lanthanides
- liquid interfaces
- precipitate depletion
- rare earth elements
- solvent extraction