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

Abstract

The dynamics of actinyl ions (AnO2(n+)) in aqueous solutions is important not only for the design of advanced separation processes but also for understanding the fate of actinides in the environment. The hazardous nature of actinides makes it difficult to measure transport and thermodynamic properties experimentally, so predictive simulations are an attractive method for studying these systems. Here, we report the results of atomistic-level molecular dynamics simulations of actinyl ions (of U, Np, Pu, and Am) in their mono- and dication states in aqueous solution. Quantum mechanically derived force field parameters are used to compute self-diffusion coefficients of the actinyl ions, water exchange mechanisms, and residence times of water molecules in the first solvation shell of the actinyl ions. We find that monocation actinyl ions diffuse slightly faster than their dication counterparts. Our simulations suggest that there are two distinct water exchange mechanisms for mono and dications. An associative interchange pathway is observed for water exchange involving dication actinyls, while in monocation actinyls the exchange occurs via a dissociative mechanism. The residence time of water molecules in the first solvation shell depends on the water exchange mechanism. In the case of dications, a stiffer actinyl bond angle results in a longer residence time, while for monocations, a shorter water coordination distance leads to a longer residence time. The simulations predict much faster water exchange for UO2(2+) than what is observed experimentally with NMR, but other properties are consistent with experiments.

Effect of Uranyl Ion Concentration on Structure and Dynamics of Aqueous Uranyl Solution: A Molecular Dynamics Simulation Study

“…The values of diffusion coefficients for bulk water, water in the U1 system, and uranium atom of uranyl ion in the U1 system are similar to those given by Tiwari et al for the TIP3P model of water. 24 The self-diffusion coefficients for uranyl ion were observed to be lower for SPC/E water as compared to those for TIP3P water. The diffusivity values obtained for water molecules and uranyl ions in aqueous uranyl solution with SPC/E water model are in good agreement with those obtained earlier using molecular dynamic simulations.…”

Ab Initio Molecular Dynamics Study on the Interactions between Carboxylate Ions and Metal Ions in Water

“…In that way, the electrostatic interactions may affect the behaviour of the complex and may, for example, reduce the probability for the magnesium ion to associate with a second carboxylate ion, since it is screened with the most negative charge by the surrounding water molecules. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 In Table 3, the number of exchanges of water molecules [34][35][36][37][38] in the first solvation shell has been reported for separation distances from 2 Å to 6 Å. For the divalent ions, most exchanges are found for the bigger ions, Ba 2+ and Sr 2+ , compared with the small ones (Mg 2+ and Ca 2+ ).…”

Ab initiomolecular dynamics studies of hydroxide coordination of alkaline earth metals and uranyl

“…Various DFT investigations of the aquo solvation structure of uranyl found a equatorial uranyl CN of 5, [91][92][93][94][95][96][97][98] with equatorial U-O bond lengths of 2.4 93,95 to 2.53 Å. 94,97 In general both AIMD [99][100][101] and MD 87,[102][103][104][105] simulations of the uranyl solvation environment indicated a coordination number of 5 and the U-O bond distance is identified as 2.36 102 to 2.48 Å. 104,105 The MD study by Rodríguez-Jeangros et al 106 identified an average CN of 4.39 as uranyl is equatorially coordinated by either 4 or 5 waters.…”