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

Understanding the solution behavior of minor actinides in the presence of EDTA(4-), carbonate, and hydroxide ligand

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))

Lanthanide Speciation in Potential SANEX and GANEX Actinide/Lanthanide Separations Using Tetra-N-Donor Extractants

Lanthanide­(III) complexes with N-donor extractants, which exhibit the potential for the separation of minor actinides from lanthanides in the management of spent nuclear fuel, have been directly synthesized and characterized in both solution and solid states. Crystal structures of the Pr³⁺, Eu³⁺, Tb³⁺, and Yb³⁺ complexes of 2,9-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe₄-BTPhen) and the Pr³⁺, Eu³⁺, and Tb³⁺ complexes of 6,6′-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2′-bypyridine (CyMe₄-BTBP) were obtained. The majority of these structures displayed coordination of two of the tetra-N-donor ligands to each Ln³⁺ ion, even when in some cases the complexations were performed with equimolar amounts of lanthanide and N-donor ligand. The structures showed that generally the lighter lanthanides had their coordination spheres completed by a bidentate nitrate ion, giving a 2+ charged complex cation, whereas the structures of the heavier lanthanides displayed tricationic complex species with a single water molecule completing their coordination environments. Electronic absorption spectroscopic titrations showed formation of the 1:2 Ln³⁺/L_N₄‑donor species (Ln = Pr³⁺, Eu³⁺, Tb³⁺) in methanol when the N-donor ligand was in excess. When the Ln³⁺ ion was in excess, evidence for formation of a 1:1 Ln³⁺/L_N₄‑donor complex species was observed. Luminescent lifetime studies of mixtures of Eu³⁺ with excess CyMe₄-BTBP and CyMe₄-BTPhen in methanol indicated that the nitrate-coordinated species is dominant in solution. X-ray absorption spectra of Eu³⁺ and Tb³⁺ species, formed by extraction from an acidic aqueous phase into an organic solution consisting of excess N-donor extractant in pure cyclohexanone or 30% tri-<i>n</i>-butyl phosphate (TBP) in cyclohexanone, were obtained. The presence of TBP in the organic phase did not alter lanthanide speciation. Extended X-ray absorption fine structure data from these spectra were fitted using chemical models established by crystallography and solution spectroscopy and showed the dominant lanthanide species in the bulk organic phase was a 1:2 Ln³⁺/L_N‑donor species