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Four tripodal carbamoylmethylphosphine oxide (CMPO)-based ligands are reported here and assessed with regard to lanthanide (Ln) coordination chemistry and selective extraction of lanthanide ions from aqueous solution. Inspired by previous liquid–liquid extraction studies that suggested a preference for terbium( iii ), the current work further probes the extraction behavior of a tris-(2-aminoethyl)amine (TREN) capped, ethoxy substituted CMPO ligand with respect to the entire series of lanthanides. Upon confirmation of Tb 3+ extraction selectivity versus the whole series, experiments were conducted to assess the effect of increasing the alkyl chain length within the ligand TREN cap, as well as changing the CMPO substituents by replacing the ethoxy groups with more hydrophobic phenyl groups to promote solubility in the organic extraction solvent. Extraction efficiencies remained low for most lanthanides upon increasing the cap size, with % E values consistently around 5%, and a complete loss of Tb 3+ preference was noted with a decrease in % E from 18% to 3.5%. For the agent employing the original, smaller TREN cap but with phenyl substituents on the CMPO units, an increase in extraction toward the middle of the row was again observed, albeit modest, with relatively high % E values for both Gd 3+ and Tb 3+ versus the other lanthanides (13 and 11%, respectively). A more dramatic extraction selectivity for the phenyl substituted ligand was achieved upon modification of the ligand to metal ratio, with a 100 : 1 ratio resulting in a near linear decrease in % E from 41% for La 3+ to 3.7% for Lu 3+ . Finally, modification of the TREN capping scaffold by adding an oxygen atom to the central nitrogen led to consistently low % E values, revealing the effect of TREN cap oxidation on Ln extraction for this tripodal CMPO ligand system.
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Selective Liquid–Liquid Extraction of Thorium(IV) from Rare-Earth Element Mixtures
One of the major challenges in processing rare earth element (REE) materials arises with the large amounts of radioactive thorium (Th) that are often found within REE minerals, encouraging enhanced metal separation procedures. We report here a study aimed at developing improved systems for REE processing with the goal of efficient extraction of Th(IV) from acidic solution. A tripodal ligand, TRPN-CMPO-Ph, was prepared that utilizes carbamoylmethylphosphine oxide (CMPO) chelators tethered to a tris(3-aminopropyl)amine (TRPN) capping scaffold. The ligand and its metal complexes were characterized using elemental analysis, NMR, FT-IR, mass spectrometry, and luminescence spectroscopy. Using a liquid-liquid metal extraction protocol, TRPN-CMPO-Ph selectively extracts Th(IV) at an efficiency of 79% from a mixture of Th(IV), UO22+, and all rare earth metal cations (except Pm) dissolved in nitric acid into an organic solvent. Thorium(IV) extraction selectivity is maintained upon extracting from a mixture that approximates a typical monazite leach solution containing several relevant lanthanide ions, including two ions at higher concentration relative to Th(IV). Comparative studies with a tris(2-aminoethyl)amine (TREN) capped derivative are presented and support the need for the larger TRPN capping scaffold in achieving Th(IV) extraction selectivity.
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- PAR ID:
- 10525519
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Inorganic Chemistry
- ISSN:
- 0020-1669
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acs.inorgchem.4c01240
