Extended X-ray Absorption Fine Structure and Density Functional Theory Studies on the Complexation Mechanism of Amidoximate Ligand to Uranyl Carbonate

Abstract

To shed some light on the uranium extraction mechanism of amidoximate (AO) ligands from uranyl carbonate solution, we present experimental data taken using extended X-ray absorption fine structure at the U L3 edge and theoretical calculation results. The EXAFS data were well simulated and confessedly shows that AO ligands directly substitute the CO32– group in the equatorial plane to form a stable [UO2(CO3)3-x(AO)x](4-x)– complex. Density functional theory calculation indicates that although they have a slightly weaker electrostatic attraction than CO32– ligands, AO ligands display stronger binding capability to uranyl because of the remarkable orbital hybridization between U 5f/6d and (N,O)2p in the uranyl–AO complex. This finding provides strong evidence supporting the substitutional mechanism, which implies that cationic adsorbents are preferred in the further design of higher efficient adsorbents, and furthermore highlights the crucial role of the covalent effect in the extraction process.