Alkyl-Substituted Aminobis(phosphonates)—Efficient Precipitating Agents for Rare Earth Elements, Thorium, and Uranium in Aqueous Solutions

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[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 HNO 3 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 pK a 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. 31 P NMR titration studies between 1 and M(NO 3 ) 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.

α-Aminophosphonates, -Phosphinates, and -Phosphine Oxides as Extraction and Precipitation Agents for Rare Earth Metals, Thorium, and Uranium: A Review

“…α-Aminophosphonates have also been used as precipitation agents for REs and actinoids [ 9 ]. In 2021, Moilanen et al published a study focusing on the double-armed α-aminophosphonates ( 15 – 20 ) with short alkyl chains to increase their water solubility.…”

“…α-Aminobisphosphonates 15 – 20 were synthesized using water as a solvent and HCl as a catalyst instead of organic solvents and p -toluenesulfonic acid [ 9 ]. To obtain compounds with two phosphonic acid groups, two equivalents of phosphorous acid with respect to amine, as well as excess formaldehyde, were used in the reaction.…”

“…A study on α-aminobis(phosphonates) determined the compositions of the complexes via 31 P NMR titrations in D 2 O [ 9 ]. This method was successfully employed for 15 with Y( iii ), La( iii ), and Lu( iii ), and the results revealed the complex compositions of YL 3 , LaL 2 (NO 3 ), and LuL(NO 3 ) 2 , respectively.…”

“…While the majority of previous studies carried out for α-aminophosphonates have focused on solvent extraction, these compounds also work as precipitation agents. A series of α-aminobis(phosphonates) 15 – 20 , with variable hydrocarbon chain lengths, was studied for the recovery of REs, Th( iv ), and U( vi ) by direct precipitation of the formed complexes from the nitric acid solution [ 9 ]. α-Aminobis(phosphonates) 18 – 20 , with a longer hydrocarbon chain, separated Sc( iii ) and both actinoids from the rest of the investigated REs well in the pH range of 1–2.…”

“…Apart from solvent extraction, organophosphorus compounds have also been utilized in other separation methods to recover and separate REs. Illustrative examples of such methods are fractional precipitation and solid-phase extraction [ 6 , 7 , 8 , 9 ].…”