Molecular dynamics simulation study of distribution and dynamics of aqueous solutions of uranyl ions: the effect of varying temperature and concentration

Abstract

Investigating the characteristics of actinyl ions has been of great interest due to their direct relevance in the nuclear fuel cycle. All-atom molecular dynamics simulations have been employed to study the orientational structure and dynamics of aqueous solutions of uranyl ions of various concentrations. The orientational structure of water around a uranyl ion has been thoroughly investigated by calculating different orientational probability distributions corresponding to different molecular axes of water. The orientational distribution of water molecules in the first coordination shell of a uranyl ion is found to be markedly different from that in bulk water. Analysis of counterion distribution around the uranyl ion reveals the presence of nitrate ions along with water molecules in the first solvation shell. From the comparison of the number of coordinated water and nitrate ions at various uranyl nitrate concentrations, it is evident that these two species compete for occupying the first solvation shell of the uranyl ion. Orientational dynamics of water molecules about different molecular axes of water in the vicinity of uranyl ions have also been investigated and decreasing orientational mobility of water with increasing uranyl concentration has been found. However, it is observed that the orientational dynamics remains more or less the same whether we consider all the water molecules in the aqueous solution or only the solvation shell water molecules. The effect of temperature on the translational and orientational characteristics of the aqueous uranyl solutions has also been studied in detail.

Molecular Dynamics Simulation Studies on Structure, Dynamics, and Thermodynamics of Uranyl Nitrate Solution at Various Acid Concentrations

“…Therefore, it can be concluded that both the effects may alter the water diffusivity but in different magnitudes. The reported self-diffusivity of water for the 5U system (2.26 × 10 −9 m 2 /s) 65 is in quantitative agreement with the present simulation results.…”

“…Similar observation in CN was also reported by Chopra and Choudhury. 65 The predominant monodentate binding mode was observed in an aqueous environment using Raman spectroscopy by Brooker et al and Nguyenttung et al 70,71 Again, Chiarizia and co-workers also established the monodentate structure in a water-rich medium using extended X-ray absorption fine structure measurements. 72 According to the work of Buḧl et al, the solvent effect facilitated the transition from bidentate to monodentate and the nitrate ions are rather involved in intermolecular HBs with the solvent instead of intramolecular HBs with uranyl water.…”

“…Similar reduction in CN with increasing concentration of the uranyl ion was also reported by Chopra and Choudhury. 65 However, the change in CN is not very significant and can be concluded that the CN of U−O W PCFs is independent of uranyl ion concentration and system composition. Thereafter, on further away from the uranium atom, the second peaks for all the systems appeared at a distance of ∼4.65 Å similar to the first peak.…”

“…Additionally, the self-diffusivities of water have been evaluated for all systems, which were found to change with uranyl nitrate concentrations rather than decrease as observed earlier. 65 Such reduction in self-diffusion is not only due to the influence of UO 2 2+ ions but also due to the presence of NO 3 − ions. When the hydrated ions form complexes, water shielding is decreased, resulting in reduction of water exchange rate because of structure breaking and forming, which is responsible for the development of shielding.…”

“…Again, in the case of 5U and 10U, the first peaks are associated with the second peaks and only one peak appeared, which is located near the second peaks of the rest of the systems and was absent in their studies. 65 The unusual characteristics appear because of the presence of 3 M acid in the medium. Therefore, it can be concluded that the acid medium may alter the orientation in the solvation shell.…”