Uranyl extraction by beta-diketonate ligands to SC-CO2: theoretical studies on the effect of ligand fluorination and on the synergistic effect of TBP

We report theoretical investigations on the effect of H --> F substitution in acetylacetonate ligands in order to understand why fluorination promotes the extraction of uranyl to supercritical CO(2) with a marked synergistic effect of tri-n-butyl phosphate "TBP". The neutral LH and deprotonated L(-) forms of the ligand, and the uranyl complexes UO(2)L(2) and UO(2)L(2)S (S = H(2)O versus trimethyl phosphate "TMP" which mimics TBP) are studied by quantum mechanics (QM) in the gas phase, whereas the ligands LH and their UO(2)L(2) and UO(2)L(2)S complexes are studied by molecular dynamics (MD) in SC-CO(2) solution as well as at a CO(2)-water interface. Several effects are found to favor F ligands over the H ligands. (i) First, intrinsically (in the gas phase), the complexation reaction 2 LH + UO(2)(2+) --> UO(2)L(2) is more exothermic for the F ligands, mainly due to their higher acidity, compared to the H ligands. (ii) The unsaturated UO(2)L(2) complexes with F ligands bind more strongly TMP than H(2)O, thus preferentially leading to the UO(2)L(2)(TMP) complex, more hydrophobic than UO(2)L(2)(H(2)O). (iii) Molecular dynamics simulations of SC-CO(2) solutions show that the F ligands and their UO(2)L(2) and UO(2)L(2)S complexes are better solvated than their H analogues, and that the UO(2)L(2)(TBP) complex with F ligands is the most CO(2)-philic. (iv) Concentrated solutions of UO(2)L(2)(TBP) complexes at the CO(2)-water interface display an equilibrium between adsorbed and extracted species, and the proportion of extracted species is larger with F- than with H- ligands, in agreement with experimental observations. Thus, TBP plays a dual synergistic role: its co-complexation by UO(2)L(2) yields a hydrophobic and CO(2)-philic complex suitable for extraction, whereas TBP in excess at the interface facilitates the migration of the complex to the supercritical phase.