Unveiling the Reduction Mechanism of Pu(IV) by Acetaldoxime

The separation of plutonium (Pu) from spent nuclear fuel was achieved by effectively adjusting the oxidation state of Pu from +IV to +III in the plutonium uranium reduction extraction (PUREX) process. Acetaldoxime (CH3CHNOH) as a free salt reductant can rapidly reduce Pu(IV), but the reduction mechanism remains indistinct. Herein, we explore the reduction mechanism of two Pu(IV) ions by one CH3CHNOH molecule, where the second Pu(IV) reduction is the rate-determining step with the energy barrier of 19.24 kcal mol-1, which is in line with the experimental activation energy (20.95 ± 2.34 kcal mol-1). Additionally, the results of structure and spin density analyses demonstrate that the first and second Pu(IV) reduction is attributed to hydrogen atom transfer and hydroxyl ligand transfer, respectively. Analysis of localized molecular orbitals unveils that the reduction process is accompanied by the breaking of the Pu-OOH bond and the formation of the OOH-H and C-OOH bonds. The reaction energies confirm that the reduction of Pu(IV) by acetaldoxime is both thermodynamically and kinetically accessible. In this work, we elucidate the reduction mechanism of Pu(IV) with CH3CHNOH, which provides a theoretical understanding of the rapid reduction of Pu(IV).

Reduction of Np(VI) with hydrazinopropionitrile via water-mediated proton transfer

Effectively adjusting and controlling the valence state of neptunium (Np) is essential in its separation during spent fuel reprocessing. Hydrazine and its derivatives as free-salts can selectively reduce Np(VI) to Np(V). Reduction mechanisms of Np(VI) with hydrazine and four derivatives have been explored using multiple theoretical methods in our previous works. Herein, we examine the reduction mechanism of Np(VI) with hydrazinopropionitrile (NCCH₂N₂H₃) which exhibits faster kinetics than most other hydrazine derivatives probably due to its σ-π hyperconjugation effect. Free radical ion pathways I, II and III involving the three types of hydrazine H atoms were found that correspond to the experimentally established mechanism of reduction of two Np(VI) via initial oxidation to [NCCH₂N₂H₃]⁺˙, followed by conversion to NCCH₂N₂H (+2H₃O⁺) and ultimately to CH₃CN + N₂. Potential energy profiles suggest that the second redox stage is rate-determining for all three pathways. Pathway I with water-mediated proton transfer is energetically preferred for hydrazinopropionitrile. Analyses using the approaches of localized molecular orbitals (LMOs), quantum theory of atoms in molecules (QTAIM), and intrinsic reaction coordinate (IRC) elucidate the bonding evolution for the structures on the reaction pathways. The results of the spin density reveal that the reduction of the first Np(VI) ion is the outer-sphere electron transfer, while that of the second Np(VI) ion is the hydrogen transfer. This work offers new insights into the nature of reduction of Np(VI) by hydrazinopropionitrile via water-mediated proton transfer, and provides a basis for designing free-salt reductants for Np separations.

Theoretical insights into selective extraction of uranium from seawater with tetradentate N,O-mixed donor ligands

The competition of uranium and vanadium ions is a major challenge in extracting uranium from seawater. In-depth exploration of the complexation of uranium and vanadium ions with promising ligands is essential to design highly efficient ligands for selective recovery of uranium. In this work, we systematically explored the uranyl and vanadium extraction complexes with three tetradentate N,O-mixed donor analogues including the rigid backbone ligands 1,10-phenanthroline-2,9-dicarboxylic acid (PDA, L¹) and 5H-cyclopenta[2,1-b:3,4-b']dipyridine-2,8-dicarboxylate acid (L³), as well as the flexible ligand [2,2'-bipyridine]-6,6'-dicarboxylate acid (L²) using density functional theory (DFT). These ligands coordinate to the uranyl cation in a tetradentate fashion, while L¹ and L³ act as tridentate ligands toward VO₂⁺ due to the smaller ionic radius of VO₂⁺ and larger cleft sizes of L¹ and L³. Bonding analyses show that the metal-ligand bonding orbitals of the uranyl complexes [UO₂L(CO₃)]^2-, [UO₂L(OH)]^-, and [UO₂L(H₂O)] mainly arise from the interactions of the U 5f, 6d orbitals and N, O 2p orbitals. Because of the rigid structure and more suitable chelate ring size, the L¹ ligand possesses a stronger complexing ability for uranyl ions than other ligands, while the L³ ligand has weaker binding affinity than L¹ and L². All these ligands prefer to coordinate with the uranyl cation rather than vanadium ion, indicating the selectivity of these ligands to [UO₂(CO₃)₃]^4- over H₂VO₄^- and HVO₄^2- in seawater. This is mainly attributed to the metal ion size-based selectivity and structural preorganization of the ligands. These results demonstrate that the backbone of these ligands affect their extraction behaviors. It is expected that this work might prove useful in designing efficient ligands for uranium extraction from seawater.

Theoretical insights into the reduction mechanism of neptunyl nitrate by hydrazine derivatives

In the advanced spent fuel cycle, the control and adjustment of neptunium valence state is greatly important for the highly efficient separation of neptunium. Hydrazine and its derivatives as salt-free reagents can selectively reduce Np(VI) to Np(V), but their reduction mechanisms are still unclear. We explored the reduction of [NpVIO2(H2O)2(NO3)2] by N2H4 and its two derivatives HOC2H4N2H3 and CHON2H3 using scalar relativistic density functional theory. The thermodynamic energy of the reactions [NpVIO2(H2O)2(NO3)2] with three reductants are sensitive to the substitution group, HOC2H4N2H3 enhances thermodynamic ability of the reaction and CHON2H3 shows contrary result. Both HOC2H4N2H3 and CHON2H3 have lower energy barrier compared to N2H4 based on the potential energy profiles (PEPs), which probably attributes to the intramolecular hydrogen bond of hydrazine derivatives. The nature of these redox reactions is that the hydrogen atom of reductants is gradually transferred to the axis oxygen atom of neptunyl, which accompanies the N–H bond dissociation and Oax–H bond formation. The reduction of Np(VI) with HOC2H4N2H3 is the most favorable reaction based on the thermodynamic and kinetic results. This work provides theoretical perspective into the reduction of Np(VI) to Np(V), which is beneficial to the development of more effective free-salt reductants for the separation of neptunium from uranium and plutonium in spent fuel reprocessing.

Theoretical Insights on Improving Amidoxime Selectivity for Potential Uranium Extraction from Seawater

Extraction of uranium from seawater is one of the important ways to solve the shortage of terrestrial uranium resources. Thereinto, the competition between uranyl and vanadium cations is a significant challenge in the commonly used amidoxime-based adsorbents for extracting uranium from seawater. An in-depth understanding of the extraction behaviors of modified amidoxime groups with uranyl and vanadium ions is one of the effective means to design and develop efficient adsorbents for selective uranium sequestration. In this work, we have designed and systematically investigated the alkyl and amino functionalized amidoxime, (Z)-2-amino-N'-hydroxy-N,N-dimethylbenzimidamide (L¹), and its phenyl and methoxy derivatives ((Z)-3-amino-N'-hydroxy-N,N-dimethyl-2-naphthimidamide (L²) and (Z)-2-amino-N'-hydroxy-4-methoxy-N,N-dimethylbenzimidamide (L³)) by quantum chemistry calculations. In the uranyl complexes, the amidoxime groups prefer to act as η²-coordinated ligands as the amidoximes increase, and there exist substantial hydrogen bond interactions, which are different from the vanadium complexes. Various bonding analyses show that the L¹ ligand possesses a stronger binding affinity to UO₂²⁺, and the -C₆H₅ and -CH₃O substituent groups seem to have no effect on the improvement of extraction ability. Thermodynamic analysis confirms that the L¹ ligand has a stronger extraction capability to uranyl ion compared to L² and L³. According to the calculations of the vanadium (V) (VO₂⁺ and VO³⁺) complexes with the L¹ ligand, L¹ is more likely to react with [H₂VO₄]^- and [HVO₄]^2- to form VO₂⁺ complexes. Expectantly, thermodynamic analysis displays a higher extraction capacity for uranyl ions than vanadium ions. Therefore, these alkyl and amino functionalized amidoxime ligands demonstrate high selectivity for uranyl over vanadium ions, which is mainly due to the coordination mode changes of these ligands toward vanadium in conjunction with the considerable hydrogen bonds in the uranyl complexes. These results are expected to afford useful clues for the design of efficient adsorbents for uranium extraction from seawater.

Theoretical Insights into the Reduction Mechanism of Np(VI) with Phenylhydrazine

Effectively adjusting and controlling the valence state of neptunium from the spent fuel reprocessing process is essential to separating neptunium. Hydrazine and its derivatives as free-salt reductants have been experimentally demonstrated to effectively reduce Np(VI) to Np(V). We have theoretically investigated the reduction mechanisms of Np(VI) with hydrazine and three derivatives (HOC₂H₄N₂H₃, CH₃N₂H₃, and CHON₂H₃) in previous works. Herein, we further explored the reduction reaction of Np(VI) with phenylhydrazine (C₆H₅N₂H₃) including the free radical ion mechanism and the free radical mechanism. Potential energy profiles (PEPs) indicate that the rate-determining step of both mechanisms is the first stage. Moreover, for the free radical ion mechanism, phenylhydrazine possesses better reduction ability to Np(VI) compared to HOC₂H₄N₂H₃, CH₃N₂H₃, and CHON₂H₃, which falls completely in line with the experimental results. Additionally, the analyses of the quantum theory of atoms in molecules (QTAIM), natural bond orbitals (NBOs), electron localization function (ELF), and localized molecular orbitals (LMOs) have been put forward to elucidate the bonding evolution for the structures of the reaction pathways. This work offers insights into the reduction mechanism of Np(VI) with phenylhydrazine from the theory point of view and contributes to design more high-efficiency reductants for the separation of U/Np and Np/Pu in spent fuel reprocessing.

Theoretical Study on the Reduction Mechanism of Np(VI) by Hydrazine Derivatives

The key to effective separation of neptunium from the spent fuel reprocessing process is to adjust and control its valence state. Hydrazine and its derivatives have been experimentally confirmed to be effective salt-free reductants for reducing Np(VI) to Np(V). We theoretically studied the reduction reactions of Np(VI) with three hydrazine derivatives (2-hydroxyethyl hydrazine (HOC₂H₄N₂H₃), methyl hydrazine (CH₃N₂H₃), and formyl hydrazide (CHON₂H₃)) and obtained the free radical ion mechanism and the free radical mechanism. Their potential energy profiles (PEPs) suggest that the free radical mechanism is the most probable reaction. Based on the energy barrier of the free radical ion mechanism, the trend of the reduction ability of the three hydrazine derivatives is HOC₂H₄N₂H₃ > CH₃N₂H₃ > CHON₂H₃, which is in excellent agreement with the experimental results. Lastly, the analyses of natural bond orbitals (NBOs), quantum theory of atoms-in-molecules (QTAIM), and electron localization function (ELF) have been carried out to explore the bonding evolution of the structures along the reaction pathways. This work provides an insight into the reduction mechanism of Np(VI) with hydrazine derivatives from the theoretical perspective and helps to design more effective reductants for the separation of U/Np and Np/Pu in spent fuel reprocessing.