Elucidation of the Decomplexation and Reverse Migration Mechanism of Uranyl Ions from the Oil Phase to the Aqueous...

Interphase ion transfer is ubiquitous in chemistry, physics, biology, and various engineering sciences. Ion transfer from the aqueous phase to the oil phase or vice versa is a complex chemical phenomenon, and its fundamental understanding is crucial for efficient and economical mass transfer. This ion transfer is much more complex for radionuclide metal ions. Therefore, an attempt has been made to elucidate the decomplexation and mechanism of reverse migration of uranyl ions from the loaded oil phase to the aqueous phase using large time-scale molecular dynamics simulations (microseconds) and density functional theory (DFT). The strength of the metal-ligand complex is the key factor for stripping among other mass transfer parameters. Stronger the metal-ligand complex, the lower the tendency for reverse migration into the aqueous phase, which was demonstrated by three different ligands for reverse migration using water as the stripping agent. The interaction energy of the metal-ligand complex has been calculated using DFT. The reverse migration is validated by the distance between the centers of mass. Higher interface thickness and lower interfacial tension belong to N,N,N',N'-tetra-octyldiglycolamide (TODGA), which are favorable for mass transfer across the liquid-liquid interface. The computed distribution coefficient (K_D) is favorable for tributyl phosphate (TBP) and tri-iso-amyl phosphate (TiAP), whereas TODGA shows a higher K_D, indicating a less favorable value for reverse migration. The distance between the uranyl ion and the TODGA molecule confirms that the entrapment of uranyl ions in the TODGA phase might be attributed to aggregation. Further, the aggregation tendency of TODGA molecules reduces the back extraction and the recovery of uranyl ions into the aqueous phase. The present MD results might be important for predicting an efficient back-extracting agent for the recovery of the metal ions from the oil phase.