Does uranyl-TBP complex formation happen at the aqueous-organic interface? Revelation by molecular dynamics simulations

P Sahu and AK Deb and SM Ali and KT Shenoy, JOURNAL OF MOLECULAR LIQUIDS, 330, 115621 (2021).

DOI: 10.1016/j.molliq.2021.115621

In addition to solvent extraction experiments, the extensive molecular dynamics simulations have been performed for aqueous (water + uranyl ion + nitrate ion + nitric acid) organic (tributyl phosphate (TBP) + dodecane) biphasic systems in order to gain molecular insights of uranium extraction process. The simulation results well corroborate with the experimental observations. An attempt has been made to uncover the location of uranyl-TBP complexation by systematically studying the distribution of the complexes at the interface, mixed phase and the bulk phases. From the distribution constant profile, the extraction of UO2+2 ions to the organic phase was predicted to increase with increase in UO2+2 ion concentration. The experimentally observed bell shaped profile of distribution constant for uranyl extraction vs. acid concentration was first time captured by the MD simulations. The interface thickness was seen to be increased with increase in uranyl concentration. On the other hand, the inverse bell-shaped curve of interfacial thickness was noticed with increasing acid concentration. The extraction of uranyl ionswas predicted to be proportional to the interface thickness. Finally, the stoichiometry distribution of UO(2)(+2)ions and UO2+2-TBP complex, their hydration characteristics, and structural orientation was explored in the different zones of biphasic system. The MD results show that the UO2+2-TBP complexation happens at the interface not in the organic phase as recently reported by the vibrational sum frequency generation experiment. However, reorganization of UO2+2-TBP complex was observed while migration from interface to the bulk organic phase. More, simulations and experimental studies are needed further to resolve the issue of uranyl ion complexation at the interface. (C) 2021 Elsevier B.V. All rights reserved.

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