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

Ion Interaction Models and Measurements of Eu³⁺ Complexation: DTPA in Aqueous Solutions at 25 °C Containing 1:1 Na⁺ Salts and Malonate pH Buffer

The separation of lanthanides from actinides in the TALSPEAK liquid–liquid distribution process is accomplished using an aminopolycarboxylate complexing agent, for example diethylenetriamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-pentaacetic acid (DTPA, CAS Reg. No. 67-43-6), in a low pH buffered aqueous phase in contact with an organic phase containing an extractant such as di­(2-ethylhexyl)­phosphoric acid (HDEHP, CAS Reg. No. 298-07-7). Literature measurements show that the partitioning of lanthanides to the organic phase falls with rising pH whereas thermodynamic equilibrium models suggest that, at pH above approximately 3.5, the partitioning should increase. In this study, the partitioning of Eu³⁺ between an aqueous phase (with NaNO₃ background electrolyte, malonate buffer, and DTPA complexing agent), and an organic phase (HDEHP in <i>n</i>-dodecane) is measured from pH 2 to 4.5 and for ionic strengths from 0.25 to 1.0 mol kg^–1. The measurements include systems with reduced (by 10×) concentrations of buffer, DTPA, and Eu³⁺. A Pitzer activity coefficient model of the aqueous mixture is developed based upon available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the DTPA and buffer speciation, and complexation of Eu³⁺ by both DTPA and malonate, to be calculated for different solution compositions and pH. The measured distribution coefficients are consistent with model predictions up to pH 3.5 and, below this pH, vary little with ionic strength. At higher pH, the distribution coefficients at different ionic strengths deviate both from the model and each other, consistent with other reactions occurring in the organic phase than the simple exchange of lanthanide and H⁺ embodied in the TALSPEAK phase transfer reaction

Coordination Chemistry and f‑Element Complexation by Diethylenetriamine‑<i>N</i>,<i>N</i>″‑bis(acetylglycine)‑<i>N</i>,<i>N</i>′,<i>N</i>″‑triacetic Acid

Potentiometric and spectroscopic techniques were used to evaluate the coordination behavior and thermodynamic features of trivalent f-element complexation by diethylenetriamine-<i>N</i>,<i>N</i>″-bis­(acetylglycine)-<i>N</i>,<i>N</i>′,<i>N</i>″-triacetic acid (DTTA-DAG) and its di­(acetylglycine ethyl ester) analogue [diethylenetriamine-<i>N</i>,<i>N</i>″-bis­(acetylglycine ethyl ester)-<i>N</i>,<i>N</i>′,<i>N</i>″-triacetic acid (DTTA-DAGEE)]. Protonation constants and stability constants of trivalent lanthanide complexes (except Pm³⁺) were determined by potentiometry. Six protonation sites and three metal–ligand complexes [ML^2–, MHL^–, and MH₂L­(aq)] were quantified for DTTA-DAG. Four protonation sites and one metal–ligand complex [ML­(aq)] were observed for DTTA-DAGEE, consistent with the presence of two ester groups. Absorption spectroscopy was utilized to measure the stability constants for complexation of trivalent neodymium and americium by DTTA-DAG and trivalent neodymium by DTTA-DAGEE. The coordination environment of trivalent europium in the presence of DTTA-DAG was investigated at various acidities by luminescence lifetime measurements. Decay constants indicate one water molecule in the inner coordination sphere across the 1.0 < pH < 5.5 range, presumably due to octadentate coordination by DTTA-DAG. A trans-lanthanide pattern of complex stabilities for DTTA-DAG was found to be analogous to that observed for DTPA, with a ∼10⁶ reduction of the complex stability. The lessened strength of complexation, relative to DTPA, was attributed to significant reduction of the total ligand basicity for DTTA-DAG due to the electronic influence of amide functionalization. When DTTA-DAG is used as an aqueous holdback complexant in liquid–liquid distribution experiments, the preferential coordination of Am³⁺ in the aqueous environment offers efficient An/Ln differentiation. The separation extends to pH 2 conditions, where the kinetics of phase transfer in such liquid–liquid systems are aided by the acid-catalyzed dissociation of a metal/aminopolycarboxylate complex

Ion Interaction Models and Measurements of Eu³⁺ Complexation: HEDTA in Aqueous Solutions at 25 °C Containing 1:1 Na⁺ Salts and Citrate pH Buffer

In the TALSPEAK liquid–liquid distribution process, dissolved lanthanides can be separated from actinides using a complexing agent such as <i>N</i>-(2-hydroxyethyl)­ethylenediamine-<i>N</i>,<i>N</i>′,<i>N</i>′-triacetic acid (HEDTA, CAS Reg. No. 150-39-0) in a low pH buffered aqueous phase in contact with an organic phase containing a suitable extractant. This study focuses on the chemical speciation of HEDTA, citrate pH buffer, and Eu³⁺ in aqueous solutions of 1:1 Na⁺ salts (mainly NaNO₃) as a function of ionic strength and pH. New measurements of stoichiometric protonation constants of HEDTA, and the HEDTA complex of Eu³⁺, in aqueous NaNO₃ are reported for ionic strengths from 0.5 to 4.0 M at 25 °C. A Pitzer activity coefficient model of the aqueous mixture has been developed based upon these measurements, available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the HEDTA and buffer speciation, and complexation of Eu³⁺ by both HEDTA and citrate, to be calculated for different solution compositions and pH values. The model of the citrate buffer, which is based on an extensive range of data for NaCl and NaNO₃ media, should also be useful in other practical applications

Thermodynamic and Spectroscopic Studies of Trivalent <i>f</i>‑element Complexation with Ethylenediamine-<i>N,N</i>′‑di(acetylglycine)-<i>N,N</i>′‑diacetic Acid

The coordination behavior and thermodynamic features of complexation of trivalent lanthanides and americium by ethylenediamine-<i>N,N</i>′-di­(acetylglycine)-<i>N,N</i>′-diacetic acid (EDDAG-DA) (bisamide-substituted-EDTA) were investigated by potentiometric and spectroscopic techniques. Acid dissociation constants (<i>K</i>_a) and complexation constants (β) of lanthanides (except Pm) were determined by potentiometric analysis. Absorption spectroscopy was used to determine stability constants for the binding of trivalent americium and neodymium by EDDAG-DA under similar conditions. The potentiometry revealed 5 discernible protonation constants and 3 distinct metal–ligand complexes (identified as ML^–, MHL, and MH₂L⁺). Time-resolved fluorescence studies of Eu-(EDDAG-DA) solutions (at varying pH) identified a constant inner-sphere hydration number of 3, suggesting that glycine functionalities contained in the amide pendant arms are not involved in metal complexation and are protonated under more acidic conditions. The thermodynamic studies identified that f-element coordination by EDDAG-DA is similar to that observed for ethylenediamine-<i>N,N,N</i>′<i>,N</i>′-tetraacetic acid (EDTA). However, coordination via two amidic oxygens of EDDAG-DA lowers its trivalent f-element complex stability by roughly 3 orders of magnitude relative to EDTA

Influence of a Heterocyclic Nitrogen-Donor Group on the Coordination of Trivalent Actinides and Lanthanides by Aminopolycarboxylate Complexants

The novel metal chelator <i>N</i>-2-(pyridylmethyl)­diethylenetriamine-<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-tetraacetic acid (DTTA-PyM) was designed to replace a single oxygen-donor acetate group of the well-known aminopolycarboxylate complexant diethylenetriamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-pentaacetic acid (DTPA) with a nitrogen-donor 2-pyridylmethyl. Potentiometric, spectroscopic, computational, and radioisotope distribution methods show distinct differences for the 4<i>f</i> and 5<i>f</i> coordination environments and enhanced actinide binding due to the nitrogen-bearing heterocyclic moiety. The Am³⁺, Cm³⁺, and Ln³⁺ complexation studies for DTTA-PyM reveal an enhanced preference, relative to DTPA, for trivalent actinide binding. Fluorescence studies indicate no changes to the octadentate coordination of trivalent curium, while evidence of heptadentate complexation of trivalent europium is found in mixtures containing EuHL_(aq) complexes at the same aqueous acidity. The denticity change observed for Eu³⁺ suggests that complex protonation occurs on the pyridyl nitrogen. Formation of the CmHL_(aq) complex is likely due to the protonation of an available carboxylate group because the carbonyl oxygen can maintain octadentate coordination through a rotation. The observed suppressed protonation of the pyridyl nitrogen in the curium complexes may be attributed to stronger trivalent actinide binding by DTTA-PyM. Density functional theory calculations indicate that added stabilization of the actinide complexes with DTTA-PyM may originate from π-back-bonding interactions between singly occupied 5<i>f</i> orbitals of Am³⁺ and the pyridyl nitrogen. The differences between the stabilities of trivalent actinide chelates (Am³⁺, Cm³⁺) and trivalent lanthanide chelates (La³⁺–Lu³⁺) are observed in liquid–liquid extraction systems, yielding unprecedented 4<i>f</i>/5<i>f</i> differentiation when using DTTA-PyM as an aqueous holdback reagent. In addition, the enhanced nitrogen-donor softness of the new DTTA-PyM chelator was perturbed by adding a fluorine onto the pyridine group. The comparative characterization of <i>N</i>-(3-fluoro-2-pyridylmethyl)­diethylenetriamine-<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-tetraacetic acid (DTTA-3-F-PyM) showed subdued 4<i>f</i>/5<i>f</i> differentiation due to the presence of this electron-withdrawing group

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))