Group Hexavalent Actinide Separations: A New Approach to Used Nuclear Fuel Recycling

Hexavalent Np, Pu, and Am individually, and as a group, have all been cocrystallized with UO₂(NO₃)₂·6H₂O, constituting the first demonstration of an An­(VI) group cocrystallization. The hexavalent dioxo cations of Np, Pu, and Am cocrystallize with UO₂(NO₃)₂·6H₂O in near proportion with a simple reduction in temperature, while the lower valence states, An­(III) and An­(IV), are only slightly removed from solution. A separation of An­(VI) species from An­(III) ions by crystallization has been demonstrated, with an observed separation factor of 14. Separation of An­(VI) species from key fission products, ⁹⁵Zr, ⁹⁵Nb, ¹³⁷Cs, and ¹⁴⁴Ce, has also been demonstrated by crystallization, with separation factors ranging from 6.5 to 71 in the absence of Am­(VI), while in the presence of Am­(VI), the separation factors were reduced to 0.99–7.7. One interesting observation is that Am­(VI) shows increased stability in the cocrystallized form, with no reduction observed after 13 days, as opposed to in solution, in which >50% is reduced after only 10 days. The ability to cocrystallize and stabilize hexavalent actinides from solution, especially Am­(VI), introduces a new separations approach that can be applied to closing the nuclear fuel cycle

Selectivity Control in Synergistic Liquid–Liquid Anion Exchange of Univalent Anions via Structure-Specific Cooperativity between Quaternary Ammonium Cations and Anion Receptors

Two anion receptors enhance liquid–liquid anion exchange when added to quaternary alkylammonium chloride anion exchangers, but with a striking dependence on the structure of the alkylammonium cation that suggests a supramolecular cooperative effect. Two anion receptors were investigated, <i>meso</i>-octamethylcalix­[4]­pyrrole (C4P) and the bisthiourea tweezer 1,1′-(propane-1,3-diyl)­bis­(3-(4-<i>sec</i>-butylphenyl)­thiourea (BTU). Whereas synergism is comparatively weak when either methyltri­(C_8,10)­alkylammonium chloride (Aliquat 336) or tetraheptylammonium chloride is used with the BTU receptor, synergism between C4P and Aliquat 336 is so pronounced that anion exchange prefers chloride over more extractable nitrate and trifluoroacetate, effectively overcoming the ubiquitous Hofmeister bias. A thermochemical analysis of synergistic anion exchange has been provided for the first time, resulting in the estimation of binding constants for C4P with the ion pairs of A336⁺ with Cl^–, Br^–, OAc_F3^–, NO₃^–, and I^–

Simple guanidinium motif for the selective binding and extraction of sulfate

<p>It is shown that a simple guanidinium molecule binds sulfate selectively in methanol/water solution, and a synthesized lipophilic analog permits the selective extraction of sulfate from aqueous sodium chloride into 1,2-dichloroethane. This receptor, <i>N</i>,<i>N</i>’-bis(2-pyridyl)guanidinium, features a rigid pseudo-bicyclic conformation in binding anions in the solid state. It selectively binds sulfate in 10% water/90% MeOD-d₄ solutions with stepwise log <i>K</i>₁ and log <i>K</i>₂ values of 3.78 ± 0.12 and 2.10 ± 0.23, respectively. Density functional theory calculations were performed to predict the conformational preferences of guanidinium receptors upon anion coordination in solution.</p

Thermal stability study of a new guanidine suppressor for the next-generation caustic-side solvent extraction process

<p>Cesium stripping performance of thermally stressed solvent worsens slowly over time in batch tests of the Next-Generation Caustic-Side Solvent Extraction (NG-CSSX) process. NG-CSSX is currently used in full-scale equipment at the Savannah River Site for the selective removal of caesium from high-level salt waste. Recently, a new guanidine, <i>N,N</i>’,<i>N</i>”-tris(3,7-dimethyloctyl)guanidine (TiDG), was chosen for use as the suppressor, a lipophilic organic base needed for stripping, and the present study was undertaken to address the question of its stability. The NG-CSSX process solvent was evaluated for a period of three months under a variety of temperature and storage conditions. The performance of the solvent was tested at 30-day increments using an extraction, scrub, strip and extraction (ES₂S₃E) sequence. The results provide insights into the effects of storage and process conditions, the stripping behaviour of TiDG and the stability of the new solvent composition.</p

Sodium Sulfate Separation from Aqueous Alkaline Solutions via Crystalline Urea-Functionalized Capsules: Thermodynamics and Kinetics of Crystallization

The thermodynamics and kinetics of crystallization of sodium sulfate with a tripodal tris-urea receptor (L1) from aqueous alkaline solutions have been measured in the 15–55 °C temperature range for a fundamental understanding of the elementary steps involved in this sulfate separation method. The use of radiolabeled Na₂³⁵SO₄ provided a practical way to monitor the sulfate concentration in solution by β liquid scintillation counting. Our results are consistent with a two-step crystallization mechanism, involving relatively quick dissolution of crystalline L1 followed by the rate-limiting crystallization of the Na₂SO₄(L1)₂­(H₂O)₄ capsules. We found that temperature exerted relatively little influence over the equilibrium sulfate concentration, which ranged between 0.004 and 0.011 M. This corresponds to 77–91% removal of sulfate from a solution containing 0.0475 M initial sulfate concentration, as found in a typical Hanford waste tank. The apparent pseudo-first-order rate constant for sulfate removal increased 20-fold from 15 to 55 °C, corresponding to an activation energy of 14.1 kcal/mol. At the highest measured temperature of 55 °C, 63% and 75% of sulfate was removed from solution within 8 and 24 h, respectively. These results indicate the capsule crystallization method is a viable approach to sulfate separation from nuclear wastes

Trefoil-Shaped Outer-Sphere Ion Clusters Mediate Lanthanide(III) Ion Transport with Diglycolamide Ligands

Outer-sphere ion clusters are inferred in many important natural and technological processes, but their mechanisms of assembly and solution structures are difficult to define. Here, we characterize trefoil-shaped outer-sphere lanthanide chloride and nitrate ion clusters in hydrocarbon solutions formed during liquid–liquid extraction with diglycolamide ligands. These are assembled through steric and electrostatic forces, where the anions reside in equidistant “clefts” between coordinating diglycolamide ligands in positions that satisfy both repulsive and attractive ion–ion interactions. Our study shows how sterically directed electrostatic interactions may assemble stable outer-sphere ion clusters in organic solutions, elucidating new strategies for controlling ion cluster assembly and extraction

Bipyrrole-Strapped Calix[4]pyrroles: Strong Anion Receptors That Extract the Sulfate Anion

Cage-type calix­[4]­pyrroles <b>2</b> and <b>3</b> bearing two additional pyrrole groups on the strap have been synthesized. Compared with the parent calix[4]­pyrrole (<b>1</b>), they were found to exhibit remarkably enhanced affinities for anions, including the sulfate anion (TBA⁺ salts), in organic media (CD₂Cl₂). This increase is ascribed to participation of the bipyrrole units in anion binding. Receptors <b>2</b> and <b>3</b> extract the hydrophilic sulfate anion (as the methyltrialkyl­(C_8–10)­ammonium (A336⁺) salt) from aqueous media into a chloroform phase with significantly improved efficiency (>10-fold relative to calix[4]­pyrrole <b>1</b>). These two receptors also solubilize into chloroform the otherwise insoluble sulfate salt, (TMA)₂SO₄ (tetramethylammonium sulfate)

Bipyrrole-Strapped Calix[4]pyrroles: Strong Anion Receptors That Extract the Sulfate Anion

Cage-type calix­[4]­pyrroles <b>2</b> and <b>3</b> bearing two additional pyrrole groups on the strap have been synthesized. Compared with the parent calix[4]­pyrrole (<b>1</b>), they were found to exhibit remarkably enhanced affinities for anions, including the sulfate anion (TBA⁺ salts), in organic media (CD₂Cl₂). This increase is ascribed to participation of the bipyrrole units in anion binding. Receptors <b>2</b> and <b>3</b> extract the hydrophilic sulfate anion (as the methyltrialkyl­(C_8–10)­ammonium (A336⁺) salt) from aqueous media into a chloroform phase with significantly improved efficiency (>10-fold relative to calix[4]­pyrrole <b>1</b>). These two receptors also solubilize into chloroform the otherwise insoluble sulfate salt, (TMA)₂SO₄ (tetramethylammonium sulfate)

Outer-Sphere Water Clusters Tune the Lanthanide Selectivity of Diglycolamides

Fundamental understanding of the selective recognition and separation of <i>f</i>-block metal ions by chelating agents is of crucial importance for advancing sustainable energy systems. Current investigations in this area are mostly focused on the study of inner-sphere interactions between metal ions and donor groups of ligands, while the effects on the selectivity resulting from molecular interactions in the outer-sphere region have been largely overlooked. Herein, we explore the fundamental origins of the selectivity of the solvating extractant <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraoctyl diglycolamide (TODGA) for adjacent lanthanides in a liquid–liquid extraction system, which is of relevance to nuclear fuel reprocessing and rare-earth refining technologies. Complementary investigations integrating distribution studies, quantum mechanical calculations, and classical molecular dynamics simulations establish a relationship between coextracted water and lanthanide extraction by TODGA across the series, pointing to the importance of the hydrogen-bonding interactions between outer-sphere nitrate ions and water clusters in a nonpolar environment. Our findings have significant implications for the design of novel efficient separation systems and processes, emphasizing the importance of tuning both inner- and outer-sphere interactions to obtain total control over selectivity in the biphasic extraction of lanthanides

Radiolytic Treatment of the Next-Generation Caustic-Side Solvent Extraction (NGS) Solvent and its Effect on the NGS Process

<div><p>It is shown in this work that the solvent used in the Next Generation Caustic-Side Solvent Extraction (NGS) process can withstand a radiation dose well in excess of the dose it would receive in multiple years of treating legacy salt waste at the US Department of Energy Savannah River Site. The solvent was subjected to a maximum of 50 kGy of gamma radiation while in dynamic contact with each of the aqueous phases of the current NGS process, namely SRS−15 (a highly caustic waste simulant), sodium hydroxide scrub solution (0.025 M), and boric acid strip solution (0.01 M). Bench-top testing of irradiated solvent confirmed that irradiation has inconsequential impact on the extraction, scrubbing, and stripping performance of the solvent up to 13 times the estimated 0.73 kGy/y annual absorbed dose. Stripping performance is the most sensitive step to radiation, deteriorating more due to buildup of p-sec-butylphenol (SBP) and possibly other proton-ionizable products than to degradation of the guanidine suppressor, as shown by chemical analyses. </p></div