Abstract
While the recovery of rare earth elements (REEs) from aqueous solution by ionic liquids (ILs) has been well documented, the metal compounds that are formed in the organic phase remain poorly characterized. Using spectroscopic, analytical, and computational techniques, we provide detailed chemical analysis of the compounds formed in the organic phase during the solvent extraction of REEs by [(n-octyl)3NMe][NO3] (IL). These experiments show that REE recovery using IL is a rapid process and that IL is highly durable. Karl-Fischer measurements signify that the mode of action is unlikely to be micellar, while ions of the general formula REE(NO3)4(IL)2− are seen by negative ion electrospray ionization mass spectrometry. Additionally, variable temperature 139La nuclear magnetic resonance spectroscopy suggests the presence of multiple, low symmetry nitrato species. Classical molecular dynamics simulations show aggregation of multiple ILs around a microhydrated La3+ cation with four nitrates completing the inner coordination sphere. This increased understanding is now being exploited to develop stronger and more selective, functionalized ILs for REE recovery.
Highlights
Ionic liquids (ILs) are desirable as reagents and solvents due to their ability to be tailored through the modification of their cation and anion components, and their perception as having “green credentials” due to their negligible vapor pressure, high thermal stability, and non-flammability [1]
They are often immiscible with aqueous phases, and this feature, combined with their other properties, has been exploited in the separation and recovery of metals such as the rare earth elements (REEs) by solvent extraction [2,3,4,5,6,7,8]
Extraction is fast and efficient, relatively selective for light REEs, and back-extraction may show additional selectivity if aqueous NaNO3 is used. It can be concluded from the mode of action studies that the interactions between ionic liquids (ILs) and REE nitrates are not micellar; instead, ElectroSpray Ionization Mass Spectrometry (ESI-MS), Nuclear Magnetic Resonance (NMR), and computational studies suggest that multiple NO3 − anions are coordinated in the inner-sphere of a partially hydrated
Summary
Ionic liquids (ILs) are desirable as reagents and solvents due to their ability to be tailored through the modification of their cation and anion components, and their perception as having “green credentials” due to their negligible vapor pressure, high thermal stability, and non-flammability [1]. They are often immiscible with aqueous phases, and this feature, combined with their other properties, has been exploited in the separation and recovery of metals such as the rare earth elements (REEs) by solvent extraction [2,3,4,5,6,7,8].
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