Abstract
The efficient and environmentally sustainable separation process for rare earth elements (REE), especially for adjacent lanthanoids, remains a challenge due to the chemical similarity of REEs. Tetravalent actinoids, thorium, and traces of uranium are also present in concentrates of REEs, making their separation relevant. This study reports six simple water-soluble aminobis(phosphonate) ligands, RN[CH2P(O)(OH)2]2 (1 R = CH2CH3, 2 R = (CH2)2CH3, 3 R = (CH2)3CH3, 4 R = (CH2)4CH3, 5 R = (CH2)5CH3, 6 R = CH2CH(C2H5)(CH2)3CH3) as precipitating agents for REEs, Th, and U, as well as gives insight into the coordination modes of the utilized ligands with REEs at the molecular level. Aminobis(phosphonates) 4–6 with longer carbon chains were found to separate selectively thorium, uranium, and scandium from REEs with short precipitation time (15 min) and excellent separation factors that generally range from 100 to 2000 in acidic aqueous solution. Ligands 1–6 also improved separation factors for adjacent lanthanoids in comparison to traditional oxalate precipitation agents. Importantly, precipitated metals can be recovered from the ligands with 3 molar HNO3 with no observed ligand decomposition enabling the possibility of recycling the ligands in the separation process. NMR-monitored pH titrations for 1 showed deprotonation steps at pKa 1.3, 5.55, and >10.5, which indicate that the ligands remain in a deprotonated [L]−1 form in the pH range of 0–4 used in the precipitation studies. 31P NMR titration studies between 1 and M(NO3)3 (M = Y, La, Lu) gave satisfactory fits for 1:3, 1:2, and 1:1 metal–ligand stoichiometries for Y, La, and Lu, respectively, according to an F-test. Therefore, aminobis(phosphonate) precipitation agents 1–6 are likely to form metal complexes with fewer ligands than traditional separation agents like DEHPA, which coordinates to REEs in 1:6 metal–ligand ratio.
Highlights
Rare earth elements (REE) consisting of lanthanoids, scandium, and yttrium are widely used in crucial technological applications such as computers, catalysts, batteries of electric cars, and renewable energy production; latter two play an important role in a shift toward greener technologies.[1]
To the best of our knowledge, this study demonstrates for the first time that simple aminobis(phosphonates), which can be synthesized by straightforward addition reactions, can be used as efficient precipitation agents with short precipitation times for REEs, Th, and U in aqueous solutions
NMR and large-scale complexation and precipitation studies were performed for six different simple aminobis(phosphonates) ligands 1−6 with REEs, Th, and U
Summary
Rare earth elements (REE) consisting of lanthanoids, scandium, and yttrium are widely used in crucial technological applications such as computers, catalysts, batteries of electric cars, and renewable energy production; latter two play an important role in a shift toward greener technologies.[1]. A liquid−liquid extraction process is the most suitable for industrial scale, one of the challenges has been to reduce the amount of used organic solvents to make the process more sustainable.[4] The liquid−liquid extraction utilizing commercial extraction agents, such as DEHPA and EHEHPA, can be improved by replacing organic solvents with ionic liquids.[9−13] This replacement has shown improvements in the separation of heavy and light adjacent lanthanoids,[9,10] selectivity to Nd over transition metals,[11] and high La/Ce separation.[12] Ionic liquids have been used in Th/U separation with a success as very good separation factor is obtained (SFTh/U 793).[13] the high viscosity of ionic liquids still remains a challenge in the extraction processes. Traditional precipitation agents, oxalates, have been reported to separate light lanthanoids from heavy ones by selective dissolution of rare earth oxalates.[14] Fractional crystallization from water solutions with borates[15] or coordination polymers[16,17] have yielded good separation factors, especially for Nd/Dy separation (SFNd/Dy > 300). To the best of our knowledge, this study demonstrates for the first time that simple aminobis(phosphonates), which can be synthesized by straightforward addition reactions, can be used as efficient precipitation agents with short precipitation times for REEs, Th, and U in aqueous solutions
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