Development of a method for screening uranyl chelates: applications for selective extraction of U(VI) in sulfuric media

The search of molecules with a specific affinity for a metal is at the basis of many processes for selective recovery. This is the case for uranium that is extracted from mines, and isolated as a result of several steps among which a solvent extraction step from a concentrated juice of sulfuric acid. To overcome some drawbacks of the current extraction process, new chelating molecules of uranium must be determined. This is possible by using a screening method to quickly compare a large number of molecules. To date, there is no method to effectively test the complexing power and selectivity of molecules for uranium in highly acidic media. The screening method proposed in this work is based on ligand displacement reactions observed by UV-visible from the Arsenazo III-UO₂²⁺ reference complex where Arsenazo III is a chromogenic molecule displaying a strong affinity for uranium. Several ligands out of four molecular families (hydrazides, dihydrazones and (mono, bis)-phosphonates) were screened. These tests - carried out in monophasic MeOH/H₂O/SO₄ mixtures - provided an affinity order of di-hydrazones related to the nature of functionalisation of the 'di-hydrazone skeleton'. The effect of substituents on coordination with the UO₂²⁺ ion could be reproduced by DFT calculations (Density Functional Theory). Moreover, an independent approach by electro-spray ionization mass spectrometry permitted to corroborate the affinity order and to evaluate the UVI/FeIII selectivity, as iron(III) is often found quantitatively in juices. The screening method applied to bis-phosphonate ligands (in H₂SO₄ aqueous medium), already studied for the in vivo removal of uranium in biological conditions, showed very strong uranophiles while some of them were unexpected. These results highlight relationships between the bis-phosphonate structures and their complexation efficiency. With this new method of search, validated on some molecular families, new trails can be considered for the selection of molecular design or the use of innovative molecules in extraction processes. (author)