(339c) Dynamics of Actinyl Ions in Water

Actinides, mainly found in solution as actinyl ions
(AnO₂ⁿ⁺), are a key species in the nuclear fuel
cycle. The dynamics of actinyl ions in aqueous solutions is important
for the design of advanced separation processes used in recycling and
for understanding the fate of actinides in the environment. The
hazardous nature of actinides makes them difficult to study
experimentally, so predictive simulations are an attractive method
for probing the properties of these systems.

We report the results of extensive atomistic-level
molecular dynamics simulations of actinyl ions (of U, Np, Pu, and Am)
in their mono- and di-cation states in aqueous solution. Accurate
force field parameters developed from the first principle
calculations were used to compute self-diffusion coefficients of the
actinyl ions, coordination numbers, water exchange mechanisms and
residence times of water molecules surrounding the actinyl ions.
Mono-cation actinyl ions diffuse faster than their di-cation
counterparts. All the actinyl ions show a water coordination number
close to five in their equatorial region with di-cations having a
tighter first coordination shell compared to that of mono-cations. An
associative type water exchange mechanism was found for di-cation
actinyls, while a dissociative type exchange mechanism was found for
mono-cation actinyls. The differences in the exchange mechanism have
profound impact on the residence time of water molecules in the first
solvation shell. All the results except that of residence times agree
well with the available literature data. Residence times for
di-cation actinyls available in literature (~10^-6 s) show
a much slower rate of water exchange compared to our results (~10^-9
s), for which a suitable explanation will be given.