Antagonistic Role of Aqueous Complexation in the Solvent Extraction and Separation of Rare Earth Ions

Abstract

During solvent extraction of rare earth ions, an aqueous electrolyte solution is placed in contact with an immiscible organic solution of extractants to enable extractant-facilitated transport of ions into the organic solvent. Although experimental methodologies such as x-ray and neutron scattering have been applied to characterize ion-extractant complexes, identifying the site of ion-extractant complexation has proven challenging. Here, we use tensiometry and surface-sensitive xray scattering to study the surface of aqueous solutions of lanthanide chlorides and the water-soluble extractant bis(2ethylhexyl) phosphate (HDEHP), in the absence of a coexisting organic solvent. These studies restrict interactions of HDEHP with trivalent lanthanide ions to the aqueous phase and the liquid-vapor interface, allowing us to explore the consequences that one or the other is the site of ion-extractant complexation. Unexpectedly, we find that light lanthanides preferentially occupy the liquid-vapor interface, with an overwhelming preference for a light lanthanide, Nd, when present in a mixture with a heavy lanthanide, Er. This contradicts our expectation that heavy lanthanides should have a higher interfacial density since they are preferentially extracted by HDEHP in the presence of an organic phase. These results reveal the antagonistic role played by ion-extractant complexation within the aqueous phase and clarify the potential advantages of water-insoluble extractants that interact with ions primarily at the interface during the process of solvent extraction.vapor interface, and found an enhanced presence of heavy over light Ln ions, as expected. Comparison of these results reveals the antagonistic role played by ion-extractant complexation within the aqueous phase. EXPERIMENTAL SECTIONMaterials. Ultrapure water from a Millipore system with resistivity of 18.2 MΩ•cm was used for all aqueous solutions. Bis(2-ethylhexyl)-phosphoric acid (HDEHP, Chart 1) was purchased from Alfa-Aesar (97%) and purified to >99.9% via a third-phase formation procedure. NdCl3•6H2O (99.9%) and ErCl3•6H2O (99.995%) LuCl3•6H2O (99.99%), GdCl3•6H2O (99.99%), DyCl3•6H2O (99.9%), were purchased from Sigma Aldrich and used without further purification. LaCl3•7H2O (99.99%) was purchased from Alfa-Aesar and used without further purification. Dihexadecyl phosphate (DHDP, >98% purity from Sigma-Aldrich, Chart 1) was purified by recrystallizing it twice from chloroform. Chart 1. Molecular Structure of (A) HDEHP and (B) DHDPSolutions. Aqueous solutions of bis(2-ethylhexyl) phosphate (HDEHP, Chart 1) and lanthanide chlorides (LnCl3) were prepared as described in SI1.2. Aqueous solutions were prepared at three different values of pH: 2.0, 3.0, and 4.5. The percentages of protonated HDEHP and deprotonated DEHPin a bulk aqueous solution without lanthanides were calculated for these pH values by using a pKa of 3.24: 40 5% DEHPand 95% HDEHP at pH 2.0, 37% DEHPand 63% HDEHP at pH 3.0, and 95% DEHPand 5% HDEHP at pH 4.5 (Fig. S1). Note that the presence of lanthani...

Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface

“…Similarly, X-ray reflectivity of the HDEHP at the air-water interface is not qualitatively different for light and heavy lanthanides. 23 With a shorter chain analogue of HDEHP, namely dibutyl phosphate, a structure similar to inverted-bilayer is observed. 63 Organic phase speciation of Er is also different when extracted with HDEHP (hexamer) or DHDP (trimer).…”

“…22 Interestingly, interfacial X-ray scattering and fluorescence results suggest a preference for lighter lanthanides at the Gibbs monolayers of HDEHP at the air/aqueous interface, due to the higher solubility of HDEHP complexes of heavier lanthanides. 23 Water-insoluble molecules, with longer alkyl tails, are limited to interface and can plausibly provide more information on the ion-extractant interactions. Dihexadecylphosphate (DHDP) forms an insoluble monolayer on water, and it has been used as an analogue of HDEHP for this purpose.…”