Theoretical insights on the interaction of uranium with amidoxime and carboxyl groups

Recovery of uranium from seawater is extremely challenging but important for the persistent development of nuclear energy, and thus exploring the coordination structures and bonding nature of uranyl complexes becomes essential for designing highly efficient uranium adsorbents. In this work, the interactions of uranium and a series of adsorbents with various well-known functional groups including amidoximate (AO^-), carboxyl (Ac^-), glutarimidedioximate (HA^-), and bifunctional AO^-/Ac^-, HA^-/Ac^- on different alkyl chains (R′=CH₃, R″=C₁₃H₂₆) were systematically studied by quantum chemical calculations. For all the uranyl complexes, the monodentate and η² coordination are the main binding modes for the AO^- groups, while Ac^- groups act as monodentate and bidentate ligands. Amidoximes can also form cyclic imide dioximes (H₂A), which coordinate to UO₂²⁺ as tridentate ligands. Kinetic analysis of the model displacement reaction confirms the rate-determining step in the extraction process, that is, the complexing of uranyl by amidoxime group coupled with the dissociation of the carbonate group from the uranyl tricarbonate complex [UO₂(CO₃)₃]^4-. Complexing species with AO^- groups show higher binding energies than the analogues with Ac^- groups. However, the obtained uranyl complexes with Ac^- seem to be more favorable according to reactions with [UO₂(CO₃)₃]^4- as reactant, which may be due to the higher stability of HAO compared to HAc^- This is also the reason that species with mixed functional group AO^-/Ac^- are more stable than those with monoligand. Thus, as reported in the literature, the adsorbability of uranium can be improved by the synergistic effects of amidoxime and carboxyl groups.