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), CO₃^2– (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 (<i>N</i>_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 <i>N</i>_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 <i>N</i>_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))