(720f) Atomistic Simulation of Uranyl Ion Extraction by 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⁺ + 2NO₃^- + 2TBP ↔ UO₂(NO₃)₂•2TBP (1)

Basically, it is envisioned 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. Specifically, the aqueous phase consists of: water, uranyl ions, nitrate ions, and in some cases hydronium ions. The organic phase consists of TBP in dodecane diluent. Our atomistic simulations demonstrate the formation of the UO₂(NO₃)₂•2TBP•H₂O complex under acidic condition. In particular, most uranyl complexes are in the form of UO₂(NO₃)_n•mTBP•kH₂O (n+m+k=5) with a coordination number 5 under these conditions while under neutral conditions we predominantly observe UO₂²⁺•5TBP . Overall, our simulations clearly demonstrate that the extraction efficiency and even the types of complexes formed are a strong function of the acidity of solution or equivalently the nitrate concentration.