Interfacial Complex Formation in Uranyl Extraction by Tributyl Phosphate in Dodecane Diluent: A Molecular Dynamics Study

Abstract

Atomistic simulations have been carried out in a multicomponent two-phase system (aqueous and organic phases in direct contact) to investigate the interfacial molecular mechanisms leading to uranyl extraction from the aqueous to organic phase. The aqueous phase consists of the dissolved ions UO2(2+) and nitrate NO3-, with or without H3O+, in water to describe acidic or neutral condition; the organic phase consists of tributyl phosphate, the extractant, in dodecane as the diluent. We find that the interface facilitates the formation of various uranyl complexes, with a general formula UO2(2+)(NO3-)n *mTBP*kH2O, with n+m+k=5, suggesting a 5-fold coordination. The coordination for all three molecular entities has the common feature that they all bind to the uranyl at the uranium atom with an oxygen atom in the equatorial plane perpendicular to the molecular axis of the uranyl, forming a 5-fold symmetry plane. Nitric acid has a strong effect in enhancing the formation of extractable species, which is consistent with experimental findings.

Insights into Uranyl Chemistry from Molecular Dynamics Simulations

“…These features have been supported by more recent studies: firstly, by a potential of mean force (PMF) simulation (similar to the PTI technique used in conjunction with CPMD) of interface crossing by TBP, [UO 2 (NO 3 ) 2 ] and the [UO 2 (NO 3 ) 2 (TBP) 2 ] complex at the water/ TBP + hexane interface, [57] and secondly by an MD study on UO 2 2 + NO 3 À (plus added H 3 O + ions) at the water/dodecane + TBP interface. [58] Other MD simulations at liquid/liquid interfaces involve (UO 2 (NO 3 ) 2 (CMPO) n complexes with bidentate CMPO ligands (Scheme 1). [59] and the inclusion uranyl complex with calix [4]tetraamides.…”

Status of Safeguards and Separations Model Development at Plant and Molecular Levels

“…Recent work [10,11] on the simulation of interfacial molecular dynamics of uranyl nitrate led to the observation that extraction into the bulk of an organic phase containing tributylphosphate (TBP) is limited and does not reproduce the complex commonly used in empirical stoichiometry of solvent extraction for this system. The closest structure observed in the simulations was (Fig.…”

“…The results of this investigation [12] firmly demonstrate the need for future studies of coupled molecular dynamics and quantum mechanics before a realistic assessment of the performance of a solvent extraction agent in solution can be made. Another objective of this sub-task was to generate initial molecular dynamics configurations for a two-phase system similar to the acidic composition in [10] on the order of 1 million of atoms for future scale-up simulations. This entailed equilibrating an aqueous phase and an organic phase individually and then bringing both systems into close contact in order to form an interface.…”

“…First, it has shown that molecular solvent extraction phenomena can be captured qualitatively by modeling in the classical limit when properly calibrated. Second, there exists knowledge to consider fairly realistic systems including realistic diluents and acidic conditions; reference [10] was the first publication in the open literature to consider a realistic organic mixture and dissociated nitric acid for the TBP-dodecane, uranyl nitrate extraction problem. It remains to be explored whether the approach can be used to provide quantitave estimates of the rates of extractions, and corresponding speciation details.…”

A DFT study of uranyl hydroxyl complexes: structure and stability of trimers and tetramers

“…Their concentration in aqueous solutions in the pH range of 5–6 is comparable to that of trimeric complexes (Fig. S1) [23], which means that the knowledge of their structure would help specialists understand the numerous complex equilibria present in the aqueous phase during various industrial processes, such as ion-exchange, solvent extraction, adsorption, etc., used for the reprocessing of uranium minerals or for the recovery of U from nuclear wastes [24–27]. …”