Peter R. Zalupski

Complete Recovery of Actinides from UREX-like Raffinates using a Combination of Hard and Soft Donor Ligands

Studies have shown a much enhanced differentiation between trivalent actinides and trivalent lanthanides when substituted alkyl dithiophosphinic acids are utilized as the active phase transfer reagents. We present a study evaluating the use of a synergistic combination of bis(o-trifluoromethylphenyl)dithiophosphinic acid and trioctylphosphine oxide for the complete actinide recovery from UREX-like raffinates. A complete single-step separation of uranium, neptunium, plutonium, and americium from aqueous mixtures containing 0.5 M nitric acid and a total fission product content of 5.5 g/L (45 mM) has been demonstrated.

The Adsorption of Gold, Palladium, and Platinum from Acidic Chloride Solutions on Mesoporous Carbons

Studies on the adsorption characteristics of gold, palladium, and platinum on mesoporous carbon (CMK-3) and sulfur-impregnated mesoporous carbon (CMK-3/S) evaluated the benefits/drawbacks of the presence of a layer of elemental sulfur inside mesoporous carbon structures. Adsorption isotherms collected for Au(III), Pd(II), and Pt(IV) on those materials suggest that sulfur does enhance the adsorption of those metal ions in mildly acidic environment (pH 3). The isotherms collected in 1 M HCl show that the benefit of sulfur disappears due to the competing influence of large concentration of chloride ions on the ion-exchanging mechanism of metal ions sorption on mesoporous carbon surfaces. The collected acid dependencies illustrate similar adsorption characteristics for CMK-3 and CMK-3/S in 1-5 M HCl concentration range. Sorption of metal ions from diluted aqueous acidic mixtures of actual leached electronic waste demonstrated the feasibility of recovery of gold from such liquors.

Redox-Based Separation of Americium from Lanthanides in Sulfate Media

In the development of advanced nuclear fuel cycles, a primary motivation is the reduction of the long-term radiotoxicity of the wastes. From a few decades to several thousand years after removal of the fuel from the reactor, americium (Am) dominates the radiotoxicity of used fuel, thus its transmutation represents an attractive option for improved management of the residues. However, every viable scenario for transmutation of Am demands some degree of separation of Am from fission product lanthanides. Partitioning of Am from curium further simplifies the transmutation process. The mutual separation of these elements is very challenging due to their similar chemistry. The focus of this work is on the utilization of the upper oxidation states of americium (Am(V/VI)) to facilitate a more efficient separation of Am from fission product lanthanides and curium. A minimum 90% efficient separation of americium from the lanthanides and curium has been achieved in the laboratory by selectively oxidizing trivalent Am to the hexavalent state using Na2S2O8. At equilibrium, lanthanides are precipitated as sodium lanthanide sulfate double salts, while oxidized Am species remain predominantly in the supernatant phase. Trivalent curium is readily coprecipitated with the NaLn(SO4)2 solid. Parallel studies of oxidized uranium, neptunium, and plutonium solutions support the conclusion that Am(VI) has sufficient stability to allow the separation to be completed in a reasonable time frame. A particular advantage of this approach is the absence of readily oxidized species that can quickly reverse the oxidation of Am and thus negate the separation. A variety of chemical and physical characterization techniques have been applied to profile the performance of the system.

Understanding the Solution Behavior of Minor Actinides in the Presence of EDTA4–, 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 (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))).

Complete recovery of actinides from UREX-like raffinates using a combination of hard and soft donor ligands. II. soft donor structure variation

The feasibility of simultaneous separation of uranium, neptunium, plutonium, americium, and curium from a simulated dissolved used fuel simulant adjusted to 1.0 M nitric acid is investigated using a mixture of the soft donor bis(bis-3,5-trifluoromethyl)phenyl) dithiophosphinic acid (“0”) and the hard donor synergist trioctylphosphine oxide (TOPO) dissolved in toluene. The results reported in this work are compared to our recent demonstration of a complete actinide recovery from a simulated dissolved fuel solution using a synergistic combination of bis(o-trifluoromethylphenyl)dithiophosphinic acid (“1”) and TOPO dissolved in either toluene or trifluoromethylphenyl sulfone. While the extraction efficiency of americium was enhanced for the liquid-liquid system containing “0”, enabling to accomplish a trivalent An/Ln separation at 1.0 M HNO3, the extraction of neptunium was drastically diminished, relative to “1”. The partitioning behavior of curium was also negatively impacted, introducing an effective opportunity for americium/curium separation. Radiometric and spectrophotometric studies demonstrate that the complete actinide recovery using the solvent based upon “0” and TOPO is not feasible. In addition, the importance of radiolytic degradation processes is discussed through the comparisons of extraction properties of liquid-liquid systems based on both soft donor reagents.

Determination of Activity Coefficients of di-(2-ethylhexyl) Phosphoric Acid Dimer in Select Organic Solvents Using Vapor Phase Osmometry

Effective models for solvent extraction require accurate characterization of the nonideality effects for each component, including the extractants. In this study, the nonideal behavior of the industrial extractant di(2-ethylhexyl) phosphoric acid has been investigated using vapor pressure osmometry (VPO). From the osmometry data, activity coefficients for the HDEHP dimer were obtained based on a formulation of the regular solution theory of Scatchard and Hildebrand, and the Margules two- and three-suffix equations. The results show similarity with a slope-analysis based relation from previous literature, although important differences are highlighted. The work points towards VPO as a useful technique for this type of study, but care must be taken with the choice of standard and method of analysis.

Thermodynamics and Kinetics of Advanced Separations Systems – FY 2010 Summary Report

This report presents a summary of the work performed in the area of thermodynamics and kinetics of advanced separations systems under the Fuel Cycle Research and Development (FCR&D) program during FY 2010. Thermodynamic investigations into metal extraction dependencies on lactate and HDEHP have been performed. These metal distribution studies indicate a substantial deviation from the expected behavior at conditions that are typical of TALSPEAK process operational platform. These studies also identify that no thermodynamically stable mixed complexes exist in the aqueous solutions and increasing the complexity of the organic medium appears to influence the observed deviations. Following on from this, the first calorimetric measurement of the heat of extraction of americium across a liquid-liquid boundary was performed.