18-membered macrocycle appended on resin for selective rare earth element extraction and separation

Abstract

The rare earth elements are critically important for a wide range of modern technologies. However, obtaining them selectively and efficiently from natural sources and recycled materials is challenging and often requires harsh or wasteful conditions. Here we show that a macrocyclic chelator appended to a solid resin can overcome this challenge by acting as a robust platform for both the extraction and separation of these elements. This resin preferably captures the large rare earth elements in mixtures of these ions, giving rise to higher extraction efficiencies for them over the smaller ions. We further demonstrate that this resin can be used to separate rare earth elements. As a proof-of-principle validation, this resin was demonstrated to selectively extract rare earth elements in the presence of many different types of competing metal ions in a bioleachate solution obtained from autoslag waste, leading to their enrichment.

Scheme 1

The chelators discussed in this study.

Fig. 1. Comparison of the Ln³⁺ complex stability constants (log K_ML values).

The stability constants of BZmacropa (red triangle), macropa (orange square), and CHX-macropa (green circle).

Fig. 2. Crystal structure of [La(NH₂-BZmacropa)(H₂O)]⁺.

Thermal ellipsoids are depicted at the 50% probability level. For clarity, outer-sphere solvent molecules, counter-anions, and hydrogen atoms attached to carbon centers have been omitted. The color scheme used is as follows: La (navy blue), O (red), N (blue), and C (gray). Only one of the two molecules present in the asymmetric unit is shown.

Scheme 2

NH₂-BZmacropa immobilization on NCS-XAD resin.

Fig. 3. The adsorption profiles of BZmacropa-XAD resin for La³⁺, Dy³⁺, and Lu³⁺.

a Ln³⁺ adsorption at different pH values; Adsorption condition: 30 mg resin, [Ln³⁺] = 600 µM in 100 mM buffer (glycine at pH 3 and ammonium acetate at pH 4 and 5), V = 10 mL, and t = 24 h. b Adsorption capacity comparison between BZmacropa-XAD resin and unmodified NCS-XAD resin at pH 5.

Fig. 4. Adsorption kinetics and resin reusability test.

a Plot of Ln³⁺ adsorption on the resin (Q_t in µmol g^-1) versus incubation time, fit using the integrated pseudo-second-order adsorption rate law for La³⁺, Dy³⁺, and Lu³⁺ at pH 5. Adsorption conditions: 30 mg resin, [Ln³⁺] = 600 µM, pH 5 (100 mM ammonium acetate buffer), V = 10 mL, T = 22 ± 1 °C. b La³⁺ over 6 adsorption/desorption cycles with 0.1 M HCl as eluent. Adsorption condition: 30 mg resin, [Ln³⁺] = 600 µM, pH 5 (100 mM ammonium acetate buffer), V = 10 mL, T = 22 ± 1 °C, and t = 24 h; Desorption condition: V = 10 mL, T = 22 ± 1 °C, and t = 4 h.

Fig. 5. The extraction performance of BZmacropa-XAD and NCS-XAD resin for mixed solution of 16 REEs.

A solution of the REE ions at equal concentration (50 µM) was treated with the resins, and the concentrations of Ln³⁺ ions remaining in solution after contact with the resin are reported. Adsorption condition: 30 mg resin, [Ln³⁺]_total = 800 µM, pH 5 (100 mM ammonium acetate), V = 7 mL, T = 22 ± 1 °C, and t = 24 h.

Fig. 6. The separation of equal mixtures of La³⁺/Dy³⁺ and La³⁺/Lu³⁺ with BZmacropa-XAD resin.

a La³⁺/Dy³⁺ and c La³⁺/Lu³⁺ mixture adsorption profiles. Adsorption condition: 1.5 g resin loaded in column, [La³⁺] = [Dy³⁺] = [Lu³⁺] = 1000 µM, V = 20 mL, pH 5 (100 mM ammonium acetate buffer), flow rate = 0.2 mL/min, T = 22 ± 1 °C. b La³⁺/Dy³⁺ and d La³⁺/Lu³⁺ mixture two-step pH-based desorption profiles. Desorption condition: pH 1.8 HCl was used for fractions 1–20 and 0.1 M HCl was used for fractions 21–40, flow rate = 0.2 mL/min, T = 22 ± 1 °C.

Fig. 7. Batch adsorption profiles of BZmacropa-XAD resin applied to slag bioleachate feedstock.

a Concentrations within the initial slag bioleachate feedstock, and the residual solution after adsorption. m_resin = 30 mg. 1 mL leachate, pH = 5.2, t = 24 h, and T = 22 ± 1 °C. b The ratios between C_initial and C_residual.

Fig. 8. REE extraction results from bioleachate using BZmacropa-XAD resin.

a Extraction efficiency of each adsorption fraction (A1–A10); b Overall adsorption efficiency of all metal ions; c The desorption efficiency using 0.01 M HCl of each metal relative to the metal amount adsorbed on the BZmacropa-XAD column. d The molar ratios between the total quantity REEs and total quantity of non-REEs in each desorption fraction (Ʃ is the total REE or non-REE amount in each fraction of eluent and initial feed). Condition: m_resin = 1.5 g, 10 mL leachate, pH 5.2, 1 fraction = 1 mL, Flow rate: 0.2 mL min⁻¹. 0.1 M HCl was used as eluent, T = 22 ± 1 °C.

Fig. 9. Molar percentage of the metals following different processing steps of the bioleachate feedstock.

The initial bioleachate feedstock (left), in the solution after passing through the column and subsequent elution with 0.10 M HCl (middle), and in the solid-state after precipitation of the eluate with oxalate (right).