Understanding the solution behavior of minor actinides in the presence of EDTA^4-, carbonate, and hydroxide ligands

The aqueous solution behavior of An^III (An = Am or Cm) in the presence of EDTA^4- (ethylenediamine tetraacetate), CO ₃^2- (carbonate), and OH^- (hydroxide) ligands has been probed in aqueous nitrate solution (various concentrations) at room temperature by UV-vis absorption and luminescence spectroscopies (Cm systems analyzed using UV-vis only). Ternary complexes have been shown to exist, including [An(EDTA)(CO₃)]^3-_(aq), (where An = Am^III or Cm^III), which form over the pH range 8 to 11. It is likely that carbonate anions and water molecules are in dynamic exchange for complexation to the [An(EDTA)]^-_(aq) species. The carbonate ion is expected to bind as a bidentate ligand and replaces two coordinated water molecules in the [An(EDTA)]^-_(aq) complex. In a 1:1 Am^III/EDTA^4- binary system, luminescence spectroscopy shows that the number of coordinated water molecules (N_H2O) decreases from ∼8 to ∼3 as pH is increased from approximately 1 to 10. This is likely to represent the formation of the [Am(EDTA)(H₂O) ₃]^- species as pH is raised. For a 1:1:1 Am ^III/EDTA^4-/CO₃^2- ternary system, the N_H2O to the [Am(EDTA)]^-_(aq) species over the pH range 8 to 11 falls between 2 and 3 (cf. ∼3 to ∼4 in the binary system) indicating formation of the [An(EDTA)(CO₃)]^3- _(aq) species. As pH is further increased from approximately 10 to 12 in both systems, there is a sharp decrease in N_H2O from ∼3 to ∼2 in the binary system and from ∼2 to ∼1 in the ternary system. This is likely to correlate to the formation of hydrolyzed species (e.g., [Am(EDTA)(OH)]^2-_(aq) and/or Am(OH)_3(s)).