Deciphering the curved profile of uranyl ions at the aqueous-organic interface by atomistic simulations

Abstract

The acceptance of nuclear energy is due to the safe and clean operation of the reactor along with the proper reprocessing of the aqueous nuclear waste. In that context, extraction of uranyl ion form aqueous solution has an immense importance in the field of fuel reprocessing. The extraction of uranyl ion is the key step on which the efficiency of the reprocessing depends. Tri-isoamyl phosphate (TiAP) is proven to be a potential alternative of Tri-butyl phosphate (TBP) in the Plutonium Uranium Extraction (PUREX) process. Therefore, understanding of PUREX process at the atomistic level using molecular dynamics simulations is of immense practical significance. Therefore, extensive molecular dynamics simulations have been conducted using calibrated all-atom optimized potentials. The simulation studies have been performed for a wide range of nitric acid concentration from 1 M to 8 M. MD studies reveal the importance of interfacial properties on the extraction of uranyl ions. The predicted interface thickness is seen to be increased with acidity whereas the calculated interfacial tension shows the opposite behavior. From the calculated results, it is observed that the capillary part of the total interface thickness is suppressed whereas the intrinsic part is expanded due to the packing of uranyl ions. The uranyl complexes remain adsorbed at the close proximity of the aqueous-organic interface. The predicted distribution constant of uranyl ion with increasing acidity exhibits a “curved profile“ as observed in the experiment. The computed interaction energy using DFT displays that the TiAP-UO22+ interaction energy is much higher compared to the TiAP-HNO3 and TiAP-H2O interaction energies thus support the higher distribution constant of uranyl ion. The high value of negative free energy difference, ΔΔG (ΔGSolvation - ΔGHydration) of uranyl ion in the organic and aqueous phase confirms that the partitioning of uranyl ion to the organic phase through complexation reaction is thermodynamically feasible process.