Ligand-exchange mechanism: new insight into solid-phase extraction of uranium based on a combined experimental and theoretical study

Abstract

In numerous reports on selective solid-phase extraction (SPE) of uranium, the extraction of uranium is generally accepted as a direct coordination of the ligands on the solid matrix with the uranyl, in which the critical effect of the hydration shell on the uranyl is neglected. The related mechanism in the extraction process remains unclear. Herein, the detailed calculation of activation energy and the geometry of the identified transition states reveal that the uranium extraction by a newly-synthesized urea-functionalized graphite oxide (Urea-GO) is in essence an exchange process between the ligands on Urea-GO and the coordinated water molecules in the first hydration shell of the uranyl. Moreover, we demonstrate that it is the ketone oxygen in the urea ligand to displace the coordinated water molecule of uranyl due to its stronger bonding ability and lower steric-hindrance, whereas the nitrogen atom in the same ligand is proved to be an electron donor that enables the oxygen atom to have stronger affinity for uranium through electron delocalization effects evaluated on the basis of calculations of the second-order interaction energy between donor and acceptor orbitals. We therefore propose a new ligand-exchange mechanism for the SPE process. This study advances the fundamental understanding of uranium extraction, and provides theoretical and practical guidance on ligand design for selective complexation of uranium(VI) and other metal ions in aqueous solution. Finally, the effect of nitrate ions on the extraction of uranyl was successfully explained based on the experimental and theoretical study.

Complexation of Manganese with Glutarimidedioxime: Implication for Extraction Uranium from Seawater

“…5a)2829. The complex formation is an exchange process in essence between the glutarimidedioxime and the coordinated water molecules in the first hydration shell of the Mn(II)30. The most practical solvation model should comprise the explicit inclusion of waters in the first hydration shell combined with continuum solvation model for the remainder of the hydration shell31.…”

Redox‐Active Two‐Dimensional Covalent Organic Frameworks (COFs) for Selective Reductive Separation of Valence‐Variable, Redox‐Sensitive and Long‐Lived Radionuclides

“…In principle, solid‐phase extraction as an auxiliary method for enhanced efficiency of the PUREX process should be in line with the goal pursued by the aqueous reprocessing process of spent nuclear fuel. Unfortunately, the interactions between solid‐phase extractants (such as, hydrothermal carbon, porous organic polymers, metal‐organic frameworks (MOFs), porous silica, graphene) and metal ions reported so far are mostly chemical adsorption (coordination/complexation by functional groups) and physical adsorption (trapping via pores/channels, or van der Waals adsorption) or both . Chemical adsorption is mainly based on coordination interaction between electron‐rich (donor) atoms, such as O/N/S, and electron‐deficient metal cations, but at higher acidity (usually pH<2), the separation effect will be lost due to the protonation of ligands .…”

Preparation of Polyamidoxime/Magnetic Graphene Oxide Composite and Its Application for Efficient Extraction of Uranium(VI) from Aqueous Solutions in an Ultrasonic Field

“…On the whole, the peaks of mGO-PAO became wider and shifted to higher binding energy after adsorption of U(VI), which is consistent with the result reported in the literature. 55,56 On the basis of the XPS analysis, it can be concluded that the U(VI) was adsorbed onto mGO-PAO through the complexation of U(VI) with the imine and the hydroxyl, which probably is due to the η 2 chelation, 57,58 as shown in Figure 14. The results of XPS analysis also indicated the successful preparation of mGO-PAO.…”