Scaling high-speed counter-current chromatography for preparative neodymium purification: Insights and challenges.

Efficient rare earth element (REE) separations are becoming increasingly important to technologies ranging from renewable energy and high-performance magnets to applied radioisotope separations. These separations are made challenging by the extremely similar chemical and physical characteristics of the individual elements, which almost always occupy the 3+ oxidation state under ambient conditions. Herein, we discuss the development of a novel REE separation aimed at obtaining purified samples of neodymium (Nd) on a multi-milligram scale using high-speed counter-current chromatography (HSCCC). The method takes advantage of the subtle differences in ionic radii between neighboring REEs to tune elution rates in dilute acid through implementation of the di-(2-ethylhexyl)phosphoric acid (HDEHP)-infused stationary phase (SP) of the column. A La/Ce/Nd/Sm separation was demonstrated at a significantly higher metal loading than previously accomplished by HSCCC (15 mg, RNd/REE > 0.85), while the Pr/Nd separation was achieved at lower metal loadings (0.3 mg, RNd/Pr = 0.75 - 0.83). The challenges associated with scaling REE separations via HSCCC are presented and discussed within.

Separation of rare earth element radioisotopes by reverse-phase high-speed counter-current chromatography.

Analytical scale purification of rare earth element (REE) radioisotopes is typically accomplished using cation-exchange resins (e.g. AG 50W-X8) and high-performance liquid chromatography (HPLC). Despite the variety of improvements made since the development of this separation process in the 1950s, nearest neighbor separations remain a challenge, as does the issue of irreversible sample adsorption. Herein, we report a study that evaluates the potential of high-speed counter-current chromatography (HSCCC) as an alternative method for purifying REE elements, with specific reference to separations of fission product REE of interest to nuclear forensics. Complementary HSCCC REE separation experiments, one spiked with radiotracer and REE fission product activity, allowed for in depth analysis of resulting fractions from both an elemental (inductively coupled plasma atomic emission spectroscopy, ICP-AES) and radiological (gamma-ray spectrometry, beta counting) purity perspective. The highly reproducible nature of separation profiles generated from HSCCC instruments was leveraged to simplify work-up of samples containing radioisotopes. Subsequent radioanalytical evaluation revealed minimal carryover of Eu into neighboring Sm and Tb fractions (as indicated by presence of 150Eu), and trace contamination of the Tb fraction with Y (as indicated by presence of 91Y). Subtle differences in stationary phase retention across the two columns were reflected in significant variations in decontamination factors of duplicate parallel separations. These differences paired with obtained distribution of radioisotopes provided valuable insights into future improvements. Collectively, this study represents a significant step forward in development of HSCCC technology for task specific REE radioisotope purification.

Rare earth element separations by high-speed counter-current chromatography.

Following the initial development of High-Speed Counter-Current Chromatography (HSCCC) in the 1960s, several studies have explored its applicability in the separation of rare earth elements (REEs). More recently, however, HSCCC publications have transitioned towards the separation of natural products or pharmaceuticals, leaving the application for REEs largely unexplored from a practical standpoint. Herein, we expand upon prior work in this field by evaluating the suitability of HSCCC to separation of a subset of non-radioactive REEs (Nd, Sm, Eu, Tb, and Y) at 10-4 mol levels using di-(2-ethylhexyl)phosphoric acid (HDEHP) in n-heptane as the stationary phase and hydrochloric acid as the mobile phase. First, the effect of flow rate on the stationary phase volume retention ratio and resolution of Nd/Sm/Eu subgroup was evaluated followed by optimization of step-gradient elution profiles resulting in additional recovery of Tb and Y within a seven-hour window. The five REEs were separated at the baseline resolution level or above. Elution profiles obtained from multiple runs across two independently operated columns and across independent runs were cross analyzed. Reproducibility in elution profiles point to future applications in radioelement separation chemistry, where both chemical and radiochemical purity are of importance.

Anion exchange and extraction chromatography tandem column isolation of zirconium-89 (89Zr) from cyclotron bombarded targets using an automated fluidic platform.

The long-lived positron emitter 89Zr is a highly promising nuclide employed in diagnostic Positron Emission Tomography (PET) imaging. Methods of radiochemical processing to obtain 89Zr for clinical use are traditionally performed with a single hydroxamate resin column. Herein, we present a tandem column purification method for the preparation of high-purity 89Zr from cyclotron bombarded natural Y metal foils. The primary column is a macroporous, strongly basic anion exchange resin on styrene divinylbenzene co-polymer. The secondary microcolumn, with an internal volume of 33 µL, is packed with an extraction chromatography resin (ExCR) loaded with di-(2-ethylhexyl)phosphoric acid (HDEHP). A condition of "inverted selectivity" is presented, wherein the 89Zr elution from the primary column is synonymous with the load condition on the secondary column. The ability to transfer 89Zr from one column to the next allows two sequential purification steps to be performed prior to the final elution of the 89Zr product. This approach assures delivery of high purity 89Zr. The tandem column purification process has been implemented into a prototype automated fluidic system. Optimization of the method is presented, followed by evaluation of the process using seven cyclotron bombarded Y metal foil targets. Once optimized, we found that 93.7 ± 2.3% of the 89Zr present in the foils was recovered in the secondary column elution fraction (0.8 M oxalic acid). Radiochromatograms of the product elution peaks enabled determination of full width at half-maximum (FWHM) and 89Zr collection yields as a function of volume. Because of the small size of the secondary microcolumn, a 89Zr product volume of ∼0.28 mL is reported, which provides a substantially increased nuclide concentration over traditional methods. Finally, we evaluated the transchelation of the resulting 89Zr oxalate product to deferoxamine mesylate (DFOM) salt. We observed effective specific activities (ESA) and bindable metals concentrations ([MB]) that exceed those reported by the traditional single hydroxamate column method.

Development of a rapid technique for sequential separation of plutonium/americium in nasal swab using solvent extraction and liquid scintillation spectrometry.

A rapid radiochemical method has been developed for estimation of plutonium and americium in nasal swab using extractive liquid scintillation spectrometry. The method involves solvent extraction of plutonium and americium from pre-treated nasal swab using 0.2 M Di-(2-ethylhexyl)phosphoricacid prepared in toluene scintillator & back extraction of americium in aqueous phase using 0.35 M HNO3. Activity assessment was carried out using liquid scintillation spectrometry. Overall recovery obtained was 96% for plutonium and 76% for americium with a sample turnaround time of 3 h.

Radiochemical separation of 224Ra from 232U and 228Th sources for 224Ra/212Pb/212Bi generator.

The application of diagnostic and therapeutic radionuclides in nuclear medicine has grown significantly and has translated into the increased interest in radionuclide generators and their development. 224Ra and its shorter-lived daughters, 212Pb and 212Bi, are very interesting radionuclides from Targeted Alpha Therapy point of view for treatment of small cancers or metastatic forms. The purpose of the present work was to develop a simple generator for rapid elution of carrier-free 224Ra from 232U or 228Th sources by radiochemical separation based on extraction chromatography with the utilization of a home-made material. The bis(2-ethylhexyl) hydrogen phosphate (HDEHP) extractant was immobilized on polytetrafluroethylene (PTFE) grains and its ability to selectively adsorb 232U and 228Th, with simultaneous high elution recovery of 224Ra, was checked over few years. The 224Ra was quantitatively eluted with small volume (3-5 mL) of 0.1 M HNO3 with low breakthrough (<0.005%) and was used for further milking of 212Bi and 212Pb from DOWEX 50WX12 by 0.75 M and 2.0 M HCl, respectively. The elaborated here methods allowed high recovery of 224Ra, 212Pb and 212Bi radionuclides and their application in radiolabeling of various biomolecules.

TALSPEAK process on hollow fiber renewable liquid membrane apropos to the remedial maneuver of high level nuclear waste.

In concurrence with objectives of advanced high level nuclear waste(HLW) management, separation of chemically similar trivalent actinides and lanthanides is accomplished using TALSPEAK (Trivalent Actinide - Lanthanide Separation by Phosphorous reagent Extraction from Aqueous Komplexes) process on hollow fibre renewable liquid membrane (HFRLM). Permeability coefficient(Kf) of metal ions are determined under varying concentrations of diethylene triamine pentacaetic acid (DTPA) and H+ in the feed solution, containing 241Am with other metal impurities usually occurred in the HLW, and di(2-ethylhexyl) phosphoric acid (HDEHP) in liquid membrane and receiving emulsion phase. Optimized process conditions obtained are: 5 ± 0.25 L feed solution: containing 0.05 M DTPA, 1 M lactic acid and metal ions under the agitation of 400 ± 15 rpm, receiving phase: emulsion of 400 ± 15 mL 2 M HNO3 + 100 mL 0.2 M HDEHP/dodecane under stirring at 650 ± 25 rpm. The Kf of metal ions obtained under optimized process conditions are in the order: Am(III)<

Microcolumn lanthanide separation using bis-(2-ethylhexyl) phosphoric acid functionalized ordered mesoporous carbon materials.

Adjacent lanthanides are among the most challenging elements to separate, to the extent that current separations materials would benefit from transformative improvement. Ordered mesoporous carbon (OMC) materials are excellent candidates, owing to their small mesh size and uniform morphology. Herein, OMC materials were physisorbed with bis-(2-ethylhexyl) phosphoric acid (HDEHP) and sorption of Eu3+ was investigated under static and dynamic conditions. The HDEHP-OMC materials displayed higher distribution coefficients and loading capacities than current state-of-the-art materials. Using a small, unpressurized column, a separation between Eu3+ and Nd3+ was achieved. Based on these experimental results, HDEHP-OMC have shown potential as a solid phase sorbent for chromatographic, intragroup, lanthanide separations.

High-capacity extraction chromatographic materials based on polysulfone microcapsules for the separation and preconcentration of lanthanides from aqueous solution.

Extractant-loaded polysulfone (PS) capsules suitable for the separation and preconcentration of rare earth ions from acidic media have been prepared and characterized. Specifically, PS macro- and microcapsules have been impregnated with bis(2-ethylhexyl)phosphoric acid (HDEHP), and their performance in the extraction of europium (Eu3+) from nitric acid solution evaluated. The HDEHP-loaded microcapsules were found to exhibit sorption efficiency superior to those of analogous macrocapsules. Comparison to a conventional (i.e., commercial) extraction chromatographic (EXC) material (Ln•Resin), comprising HDEHP-loaded beads of poly(methyl methacrylate) (PMMA) of comparable size (50-100 µm), showed that the capacity of the HDEHP-loaded microcapsules for europium is ca. 2.5-fold greater than that of the conventional material. This larger capacity is the apparent result of both the higher extractant loading achievable with the microcapsules and an unexpected change in the complexation stoichiometry of europium by HDEHP upon extractant encapsulation. The microcapsule-based sorbent equaled or exceeded the performance of the commercial EXC material in other respects as well, most notably uptake kinetics and column efficiency, making it a promising alternative to established EXC resins.

A rapid impregnation method for loading desired amounts of extractant on prepacked reversed-phase columns for high performance liquid chromatographic separation of metal ions.

A time-efficient impregnation method for loading extractant onto reversed-phase columns was developed, using di-(2-ethylhexyl) phosphoric acid (HDEHP) as a model extractant. The optimal loading conditions for the impregnation process of a standard analytical scale column was achieved by dissolving an appropriate amount of HDEHP (per void volume) in n-pentane, flushing the column with two void volumes (5mL) of impregnation solution and heating the column for a short time to remove the solvent. The process takes about one hour, a significant time reduction compared to commonly used impregnation methods (17-23h). The chromatographic traits for separation of the lighter lanthanides (La-Gd) using columns impregnated under different conditions were evaluated; heating for short period of time gave improved column performance most likely due to the presence of n-pentane in the pores of the support material. A linear relation was found (R2=0.9934) for the amount of HDEHP loaded as a function of HDEHP concentration in the impregnation solution. The coated amounts of HDEHP were in the range of 0.29-2.25mmol per column by flushing with 5mL of impregnation solution containing 0.3-5.0mmol of HDEHP per void volume. This 'flush-evaporate' impregnation method allowed for loading a pre-determined amount of extractant and produces very small amounts of organic waste. An overview of the various impregnation approaches previously used for extractant coating on prepacked columns and bulk support materials is also presented.

Experimental productivity rate optimization of rare earth element separation through preparative solid phase extraction chromatography.

Separating individual rare earth elements from a complex mixture with several elements is difficult and this is emphasized for the middle elements: Samarium, Europium and Gadolinium. In this study we have accomplished an overloaded one-step separation of these rare earth elements through preparative ion-exchange high-performance liquid chromatography with an bis (2-ethylhexyl) phosphoric acid impregnated column and nitric acid as eluent. An inductively coupled plasma mass spectrometry unit was used for post column element detection. The main focus was to optimize the productivity rate, subject to a yield requirement of 80% and a purity requirement of 99% for each element, by varying the flow rate and batch load size. The optimal productivity rate in this study was 1.32kgSamarium/(hmcolumn(3)), 0.38kgEuropium/(hmcolumn(3)) and 0.81kgGadolinium/(hmcolumn(3)).

Central metal ion exchange in a coordination polymer based on lanthanide ions and di(2-ethylhexyl)phosphoric acid: exchange rate and tunable affinity.

In this paper the exchange of lanthanide(III) ions (Ln(3+)) between a solution and a coordination polymer (CP) of di(2-ethylhexyl)phosphoric acid (Hdehp), [Ln(dehp)3], is studied. Kinetic and selectivity studies suggest that a polymeric network of [Ln(dehp)3] has different characteristics than the corresponding monomeric complex. The reaction rate is remarkably slow and requires over 600 h to reach in nearly equilibrium, and this can be explained by the polymeric crystalline structure and high valency of Ln(3+). The affinity of the exchange reaction reaches a maximum with the Ln(3+) possessing an ionic radius 7% smaller than that of the central Ln(3+), therefore, the affinity of the [Ln(dehp)3] is tunable based on the choice of the central metal ion. Such unique affinity, which differs from the monomeric complex, can be explained by two factors: the coordination preference and steric strain caused by the polymeric structure. The latter likely becomes predominant for Ln(3+) exchange when the ionic radius of the ion in solution is smaller than the original Ln(3+) by more than 7%. Structural studies suggest that the incoming Ln(3+) forms a new phase though an exchange reaction, and this could plausibly cause the structural strain.