ABSTRACT The increasing use of extraction chromatography resins across fields such as hydrometallurgy, nuclear medicine, and environmental analysis has created a need for a deeper understanding of their interactions with transition metals. Despite extensive research on f-element separations, the behavior of transition metals in these systems remains relatively understudied. This review provides a comprehensive overview of the current state of knowledge on the extraction behavior of transition metals with neutral extractants, including TODGA, TEHDGA, TBP, and CMPO, and their corresponding resins, such as DGA, BDGA, UTEVA, TBP, and TRU. The review summarizes extraction data, extracted complex coordination environments, separation reaction stoichiometries, and associated thermodynamics, highlighting inconsistencies and knowledge gaps in the literature. The study emphasizes the need for further research using spectroscopy and computational methods to elucidate extraction mechanisms and to improve the efficiency and selectivity of transition metal separations. By identifying areas for future research and development, this review aims to stimulate advancements in the field and promote the development of innovative separation technologies. The implications of this research are far-reaching, with potential applications in nuclear waste management, nuclear forensics, metal recovery, and environmental remediation. Overall, this review provides a foundation for future studies on the extraction of transition metals using neutral extractants and resins.

ABSTRACT Efficient separation of Sr from multi-element mixtures is crucial in various industrial processes, including nuclear waste management. In this study, the solvent extraction behavior of Sr using a crown ether extractant, DtBuCH18C6, was systematically investigated with particular emphasis on controlling acid co-extraction using a 1-octanol/dodecane mixed solvent. The results revealed that increasing the 1-octanol volume ratio enhanced Sr extraction, which is attributed to improved solvation of the extractant and stabilization of extracted Sr species in the organic phase. However, this also led to the co-extraction of nitric acid, significantly hindering back-extraction of Sr due to acid accumulation in the organic phase. In contrast, reducing the 1-octanol content suppressed acid loading of the organic phase and facilitated efficient Sr back-extraction. Slope analysis indicated that the apparent dependence of Sr extraction on nitric acid concentration varied with solvent composition, reflecting changes in the extraction environment. Moreover, separation factors of Sr against various coexisting metals, including Ba, Mo, and Ag were evaluated. Back-extraction experiments under multi-element conditions demonstrated that a 1-octanol content of 10% provided high Sr stripping efficiency ( > 90%) in the recovered aqueous phase. The findings highlight solvent composition control as a complementary design parameter for achieving high Sr selectivity and recovery in separation systems treating complex multi-element solutions.

ABSTRACT The recovery of valuable vanadium (V) and molybdenum (Mo) from spent residue hydrogenation catalysts is crucial due to their high content. However, their efficient separation from the catalyst leaching solution remains challenging. This study investigates a single-stage extraction and stripping process for V and Mo recovery. Under the optimal conditions with 20 vol% of the extractant Aliquat 336, 10 vol% of the phase modifier n-octanol, and an initial aqueous phase pH of 1.0, V and Mo could be effectively extracted, with a distribution ratio of V being 7.02 and that of Mo greater than 1000. The separation factors of V relative to other impurity elements exceeded 360, while those of Mo with respect to impurity elements were greater than 10,000. Afterwards, selective stripping of V was achieved based on its valence state change using an ascorbic acid (VC)-H2SO4 solution. Mo was subsequently stripped with an NH4HCO3 solution. Applied to an actual leaching solution, this process achieved extraction efficiencies over 99.9% for both metals via a four-stage extraction process, with the co-extraction of impurities below 4.50%. A three-stage stripping with 0.3 mol/L VC–0.5 mol/L H2SO4 yielded V stripping efficiency over 99.9% andminimal Mo costripping efficiency of 1.22%. Subsequently, 99.9% of the retained Mo was recovered using NH4HCO3. Therefore, this study provides an efficient strategy for V and Mo recovery, which has significant practical implications for the sustainable management of spent catalysts.

ABSTRACT The CHALMEX (Grouped ActiNide Extraction) process, derived from the GANEX process, aims to improve upon existing nuclear fuel recycling methods by co-extracting all the actinides in one step. In this study, the use of dibutyl octanamide (DBOA) as an alternative extractant to Tributyl phosphate (TBP) in the CHALMEX process is demonstrated. A solvent extraction system composed of DBOA, FS-13, and CyMe4-BTBP was tested. Promising performance of DBOA in extracting tetravalent Pu(IV) and hexavalent U(VI) was observed. The distribution ratios for Pu(IV) were significantly higher than those achieved with TBP, indicating improved actinide recovery efficiency. Future investigation of this behavior under high loading conditions would provide valuable insights into the applicability of this system for Pu-rich raffinate in the GANEX second cycle. The radiolytic stability of DBOA was investigated up to 300 kGy, and distribution ratios showed dose-dependent changes. However, some challenges remain, notably the slight increase in co-extraction of lanthanides by CyMe4-BTBP compared with the TBP system, as well as the need for further optimization of hydrodynamic properties to ensure efficient phase separation. Despite these issues, the results build upon existing research by showing the strong potential of DBOA to address key limitations of conventional extractants, particularly in the CHALMEX process.

ABSTRACT A description of the EXTREQ-2 software package for the MS Windows XP/Windows 7 operating system is presented. This package is designed for the mathematical modeling of extraction isotherms for a single component using mixtures of two extractants. The EXTREQ-2 program can automatically evaluate up to 6 extracted complex compositions simultaneously. The software allows selection of the composition of extracted complexes in synergistic and binary extraction, calculation of thermodynamic extraction constants and hydration parameters for each extracted complex, as well as determination of the equilibrium concentrations of the extracted complexes and extractants in the organic phase. The organic phase compositions obtained from all examined systems were found to be entirely consistent with the independent physicochemical analyses of the extracted complexes documented in previous studies in the literature. The EXTREQ-2 software suite employs a hybrid optimization approach to improve both user-friendliness and computational accuracy. Several optimization methods, including a Genetic Algorithm, Simulated Annealing, and “Pattern Searching”, were independently evaluated and tested. The following results were obtained: Simulated Annealing: 3.67% (calculation speed comparable to Nelder-Mead); Pattern Search: 7.74% (the fastest method); Genetic Algorithm: 3.38% (the slowest); Our utilized Nelder-Mead method: 3.29%. The EXTREQ-2 software package is capable solely of determining the composition of extractable substances within a given extraction system and calculating the thermodynamic parameters associated with every extraction reaction taking place in that system.

ABSTRACT The increasing demand for light rare earth elements (LREEs), including lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd), motivates the development of solvent extraction predictive models to design a more efficient separation system. However, LREEs solvent extraction reactions with D2EHPA or PC88A are usually assumed to follow M 3 + + 3 HA 2 ‾ ⇌ M H A 2 3 ‾ + 3 H + , which is not correct for all cases. By implementing genetic algorithms, the present study concluded that species like MCl2+ and MCl2 + (M = La, Ce, Pr, or Nd) have more distinct roles compared to M3+ in a concentrated chloride system using D2EHPA or PC88A, proving the involvement of speciation in solvent extraction reactions. The current study evaluated and offered a genetic algorithm method to model reactions governing LREEs solvent extraction considering either a single-reaction system or a multi-reaction system involving speciation, resulting in the reactions as shown in Eqs. 6, 10, 11, and 9. (6) { LaC l 2 + + 3 HA 2 ‾ ⇌ LaCl A 2 H 2 AH 2 ‾ + 2 H + , HA = D 2 EHPA LaCl 2 + + 2 HA 2 ‾ ⇌ LaC l 2 A 2 H ‾ + H + , HA = PC 88 A (10) { CeC l 2 + + 3 HA 2 ‾ ⇌ CeCl A 2 H 2 AH 2 ‾ + 2 H + , HA = D 2 EHPA CeCl 2 + + 2 HA 2 ‾ ⇌ CeC l 2 A 2 H ‾ + H + , HA = PC 88 A (11) { PrC l 2 + + 3 HA 2 ‾ ⇌ PrCl A 2 H 2 AH 2 ‾ + 2 H + , HA = D 2 EHPA PrC l 2 + + 2.5 HA 2 ‾ ⇌ PrCl A 2 H 2 AH 2 ‾ + 2 H + , HA = PC 88 A (9) NdC l 2 + + 2.5 HA 2 ⇌ NdCl A 2 H 2 AH 2 + 2 H + , HA = PC 88 A

ABSTRACT Efficient separation of long-lived, trivalent actinides from the chemically similar lanthanide fission products remains a significant challenge in nuclear waste management. In solvent extraction processes, phase modifiers are often employed to enhance the solubility of extractants in process-relevant, aliphatic diluents while suppressing the formation of third phases. This work investigates the use of 2,6-bis(5,6-bis(3-butyoxyphenyl)-1,2,4-triazinyl-3-yl)pyridine (BOB-BTP) for the extraction of 241Am3+ over 154–152Eu3+. The impact of the phase modifier (1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol) (Cs-7SB), routinely employed for 137Cs and 90Sr extractions in an Isopar/HNO3(aq) biphasic system was systematically assessed for potential utility with BOB-BTP. A 10:1 Isopar:Cs-7SB ratio greatly enhanced complexant solubility and extraction efficiency, while simultaneously suppressing the formation of precipitates. Extraction studies conducted up to 2M HNO3(aq) confirmed high chemoselectivity for 241Am3+ and excellent resistance to hydrolytic degradation of the complexant. Evaluation of decomplexation was also studied. BOB-BTP was recycled through two successive extraction sequences, thereby suggesting a high potential for reusability without diminished performance. The findings demonstrate the potential of this complexant-diluent combination for advancing partitioning applications toward closure of the nuclear fuel cycle and mitigation of the long-term radiotoxicity of spent nuclear fuel. Evaluation of acid and complexant concentration variance, in addition to decomplexation and recyclability for multiple extractions are reported herein.

ABSTRACT Zirconium (Zr) and hafnium (Hf) are pivotal in the nuclear industry. This study introduces a novel extraction system utilizing Methyl isobutyl ketone (MIBK), thiocyanic acid (HSCN), and glycine to enhance the separation of Zr and Hf. Under optimized extraction conditions, the separation coefficient of Hf to Zr reaches 8.58, nearly double that of the traditional MIBK – HSCN system. Monitoring the flow direction of glycine reveals that it does not enter the organic phase; rather, glycine predominantly improves the separation coefficient by modulating the complexation of Zr and Hf in the aqueous phase, as evidenced by FTIR, Raman spectroscopy, and NMR analysis. The aqueous phase spectrum shows that glycine forms a complex with metal ions in the aqueous medium. The complexation form of Hf evolves from Hf(OH)(SCN)3 to Hf(OH)2(SCN)2 upon the addition of glycine, while the complexation of Zr remains unchanged, resulting in an increase in the ratio of K Hf to K Zr from 1.4 to 18.2. Following this, the scrubbing process is systematically analyzed, yielding a separation coefficient of 10.0. During the cleaning process, as the acidity of the washing solution increases, SCN− in the oil phase becomes more difficult to enter the aqueous phase. Finally, a process comprising six theoretical stages of extraction coupled with six theoretical stages of scrubbing is designed based on a split-flow extraction model, achieving a purity and product yield of Hf exceeding 99%.

ABSTRACT Conventional removal of oxalate from plutonium oxalate supernatant is typically carried out through oxidation using potassium permanganate, followed by neutralization with hydrogen peroxide. While effective, this approach results in the generation of large quantities of secondary solid waste and a significant increase in solution volume, posing challenges for downstream waste management. In the present study, an advanced and integrated process strategy is proposed, which combines Mn²+-catalysed oxalate destruction with formaldehyde-mediated denitration, followed by in-situ concentration through evaporation within a single pot-type reactor. This integrated methodology eliminates the need for additional reagents apart from trace levels of Mn²+ catalyst and leads to a substantial reduction (approximately 80–90%) in secondary solid waste generation compared to conventional practices. Laboratory-scale experiments were conducted to determine key process parameters such as residence time and steady-state oxalate concentration, and the results were systematically compared with predictions from a Continuous Stirred Tank Reactor (CSTR) model. The strong agreement between experimental observations and model predictions validates the robustness and applicability of the CSTR framework. Model analysis further indicates that, for a recovery (RA) cycle evaporator, operation with 0.005–0.010 M Mn²+ enables near-complete oxalate destruction (residual oxalate ≤ 0.008 M) along with effective volume reduction, offering directly scalable parameters for industrial deployment.

ABSTRACT The recovery of solutes from ionic liquids (ILs) remains a key challenge in the design of green separation processes. In this study, the distribution constants ( K D , SC − C O 2 / IL ) of six halogenated 8-quinolinol derivatives between supercritical carbon dioxide (SC-CO2) and the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]) were determined. The results showed that K D , S C − C O 2 / IL values for all derivatives increased with CO2 density, reflecting enhanced intermolecular interactions. Analysis of the K D , SC − C O 2 / IL dependence on CO2 density, based on the Chrastil model, indicated that the number of solvating CO2 molecules around the derivatives in the SC-CO2 phase remained constant, regardless of whether the co-existing phase was the IL or water. It was also confirmed that derivatives with bulkier substituents exhibited a greater dependence of K D , SC − C O 2 / IL on CO2 density. Regarding the K D , S C − C O 2 / IL sequence, no consistent trend was observed with halogen substitution. This is because increased hydrophobicity enhances solute affinity for both SC-CO2 and IL phases simultaneously, causing these effects to cancel each other out in the biphasic system. To verify this interpretation, values for HQ and 5-Cl-HQ were estimated using existing SC-CO2/water and IL/water data. The estimated values agreed well with experimental results. These findings suggest the feasibility of predictive modeling for solute distribution in SC-CO2/IL systems, contributing to the rational design of sustainable extraction processes.