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.