Aqueous Speciation of Tetravalent Actinides in the Presence of Chloride and Nitrate Ligands

Speciation of hexachloride tetravalent uranium, neptunium,
and
plutonium species in aqueous media has been investigated using density
functional theory in the presence of inner sphere ligands such as
chloride, nitrate, and solvent molecules. All possible structures
with the formula [AnIV(Cl)x(H2O)y(NO3)z]4–x−z (An = U, Np, and Pu; x = 0–6, y = 0–8, and z = 0–6) were
considered to explore the speciation chemical space of each actinide.
The nature of the mixed-ligand complexes present in solution is controlled
by the concentration of free ligands in solution. A low chloride concentration
is suitable to drive the speciation away from the highly thermodynamically
stable hexachloride species. Furthermore, the formation of dimeric
species can proceed through both olation and oxolation mechanisms.
Oxolation is preferred for monomers that contain fewer water ligands,
while olation becomes favorable for complexes with more water ligands.