Selective Liquid–Liquid Extraction of Thorium(IV) from Rare-Earth Element Mixtures

• Published in: Inorganic chemistry 2024-07, Vol.63 (30), p.13815-13819
• Main Authors: Larrinaga, Wyatt B., Lake, Bailey J., Pacanowski, Victoria D., Patterson, Michael G., Hudson, Michael, Carlson, Faith M., Heselschwerdt, Gabriel, Balboa, Steven, Biros, Shannon M., Werner, Eric J.
• Format: Article
• Language: English
• Subjects: Communication
• ISSN: 0020-1669; 1520-510X; 1520-510X
• DOI: 10.1021/acs.inorgchem.4c01240

One of the major challenges in processing rare-earth element (REE) materials arises from 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 by using elemental analysis, NMR, Fourier transform infrared spectroscopy, 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), UO2 2+, and all rare-earth metal cations (except promethium) dissolved in nitric acid into an organic solvent. Th­(IV) extraction selectivity is maintained upon extraction 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 a larger TRPN capping scaffold in achieving Th­(IV) extraction selectivity.