Dynamics of actinyl ions in water: a molecular dynamics simulation study

Area of Science:

• Nuclear chemistry and radiochemistry
• Computational chemistry and molecular modeling
• Environmental science and geochemistry

Background:

• Actinyl ion (AnO2(n+)) dynamics in aqueous solutions are crucial for separation technologies and understanding environmental actinide behavior.
• Experimental measurement of actinide transport and thermodynamic properties is challenging due to their hazardous nature, necessitating predictive simulation methods.

Purpose of the Study:

• To investigate the atomistic-level dynamics of various actinyl ions (U, Np, Pu, Am) in mono- and dication states in aqueous solutions using molecular dynamics simulations.
• To elucidate water exchange mechanisms and calculate residence times of water molecules in the first solvation shell of these actinyl ions.

Main Methods:

• Atomistic-level molecular dynamics simulations were performed for actinyl ions (U, Np, Pu, Am) in aqueous solutions.
• Quantum mechanically derived force field parameters were employed to accurately model interatomic interactions.
• Key properties computed include self-diffusion coefficients, water exchange mechanisms, and water molecule residence times.

Main Results:

• Monocation actinyl ions exhibit slightly faster diffusion compared to their dication counterparts.
• Two distinct water exchange mechanisms were identified: associative interchange for dications and dissociative for monocations.
• Water molecule residence times in the first solvation shell are influenced by the exchange mechanism, actinyl bond stiffness (dications), and coordination distance (monocations).

Conclusions:

• The study provides detailed insights into the hydration dynamics and transport properties of key actinyl ions.
• Simulation results offer valuable data for refining models of actinide behavior in nuclear fuel cycles and environmental systems.
• While generally consistent with experimental data, simulations predict faster water exchange for uranyl ion (UO2(2+)) than observed via NMR, highlighting areas for further investigation.