Speciation of AnCl62- (An=U,Np and Pu) are investigated in aqueous-nitrate solution using density functional theory (DFT). All the possible structures within the formula of [An(Cl)x(H2O)y(NO3)z]4-x-z (x=0-6, y=0-8 and z=0-6) are considered to develop complete speciation profile for each actinide halides. Gibbs free energy of solvation of each of the free An4+ ion is suitably reproduced using thermodynamic cluster cycle approach within DFT frame. Among the three halides, UCl62- and NpCl62- are found to be more reactive in aqueous solution and readily form several U-chloro-aquo-nitrato and Np-chloro-aquo-nitrato complex while PuCl62- is comparatively stable in solution. Stability of many mixed-ligand complexes is shown to be improved by tuning the concentration of ligands present in the solution. Low concentration of chloride and high concentration of nitrate is suitable to drive the speciation forward and stabilizes more mixed-ligand complexes. Observed trend of speciation were rationalized through a few benchmark calculations. This study provides an overall picture of the fate of An(IV)-hexachloride in the aqueous-nitrate solution and give an idea to the experimentalist about the probable species evolved in such condition.

Speciation of AnCl62- (An=U, Np and Pu) in aqueous solution: A computational Investigation

ACS Spring 2022

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