(265a) Atomistic Simulation of Uranyl Ion Extraction by a TBP/Dodecane Solution

In the UREX-based (Uranium Extraction) process, tri-butyl-phosphate (TBP) in a diluent dodecane is used as the extraction agent. The aqueous phase containing the spent nuclear fuel, namely uranyl, is in contact with the organic phase containing TBP. Based on experimental findings, the uranium in the form of uranyl is extracted into the organic phase as UO2(NO₃)₂?2TBP. One of the main chemical processes giving rise to the extraction is believed to be the following interfacial reaction:

(UO₂²⁺)_aq + 2(NO₃^-)_aq + 2(TBP)_org ↔ (UO₂(NO₃)₂?2TBP)_org (1)

Basically, Eq. (1) envisions that the extractant molecules bind to the cation and form a complex at the aqueous-organic interface. The complex, UO₂(NO₃)₂?2TBP, as a neutral entity, then migrates to the organic phase. This simple picture, however, only gives the initial reactants and final products. Hence, many molecular processes including the migration of reactants to the interface, the formation of various intermediates, the binding kinetics of the extractant and the cation; and the migration of the extractant-cation complex from the interface to the organic phase need to be elucidated before a mechanistic understanding of the extraction process is attained. To this end, we have carried out a comprehensive molecular dynamics study in a multi-component two-phase system to investigate the interfacial molecular mechanisms leading to uranyl extraction from the aqueous to organic phase. Similar to experimental protocols, bulk aqueous and organic phases are in contact at an interface (aqueous and organic phases in direct contact). The aqueous phase consists of the dissolved ions UO₂²⁺, nitrate NO₃^-, and 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 UO₂²⁺(NO₃^-)_n ?mTBP?kH₂O, with n+m+k = 5, suggesting a 5-fold coordination. Such coordination is consistent with analysis of recent experimental results [1-4]. 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 five-fold symmetry plane. Nitric acid has a strong effect in enhancing the formation of extractable species, which is consistent with experimental findings [5, 6].

Also, Uranyl ion complexation with water and nitrate in bulk aqueous phase is a key aspect of the uranium extraction process [1-4, 7]. We have also carried out a molecular dynamics simulation study to investigate this complexation process, including: the molecular composition of the various complex species, the corresponding structure, and the equilibrium distribution of the complexes. The observed structures of the complexes suggest that in aqueous solution, uranyls are generally hydrated with 5 water molecules in the equatorial plane, which is consistent with the experimental finding [4]. When associating with nitrate ions, a water molecule is replaced by a nitrate ion, preserving the five-fold coordination and planar symmetry. Analysis of pair correlation function between uranyl and nitrate suggests that nitrates bind to uranyl in aqueous solution mainly as a monodentate mode, although a small portion of bidentates occur, consistent with conclusions from experiment [1-3]. Dynamic association and dissociation between uranyls and nitrates take place in aqueous solution with a substantial amount of fluctuation in the number of various uranyl nitrate species. The average number of the uranyl mono-nitrate complexes shows a dependence on acid concentration consistent with equilibrium constant analysis, i.e., the concentration of [UO2NO3]+ increases with nitric acid concentration [7].

Reference

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