Abstract
Uranyl ion complexation with water and nitrate is a key aspect of the
uranium/plutonium extraction process. We have 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 by 5 water molecules in the equatorial plane. When associating with
nitrate ions, a water molecule is replaced by a nitrate ion, preserving the five-fold
coordination and planar symmetry. Analysis of the pair correlation function
between uranyl and nitrate suggests that nitrates bind to uranyl in aqueous
solution mainly in a monodentate mode, although a small portion of bidentates
occur. 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 mononitrate
complexes shows a dependence on acid concentration consistent with
equilibrium-constant analysis, namely, the concentration of [UO2NO3]+ increases
with nitric acid concentration.