Highly efficient extraction of uranium from seawater by polyamide and amidoxime co-functionalized MXene

Uranium mainly exists in the form of uranyl carbonate in seawater. [UO₂(CO₃)₃]^4- has strong stability, which increases the difficulty of uranium extraction from seawater. Meanwhile, the complex marine environment, a large number of coexisting competing ions and biological pollution are all non-negligible disturbing factors. Herein, we introduced amidoxime (AO) groups into the surface of Ti₃C₂ and grafted polyamides (PA) by a simple one-step hydrothermal method to produce an efficient seawater uranium extraction adsorbent Ti₃C₂-AO-PA. Owing to the amidoxime groups, the material was highly selective for uranium. And the large number of amino groups in the polyamides gave it ideal resistance to biofouling. The possibility of Ti₃C₂-AO-PA as an adsorbent for uranium extraction from seawater was confirmed by various characterization techniques, numerous adsorption batch experiments, simulated seawater experiments and antibacterial performance tests. It was demonstrated that the uptake of [UO₂(CO₃)₃]^4- by Ti₃C₂-AO-PA showed fast reaction kinetics (about 120 min), brilliant absorption capacity (81.1 mg·g^-1 at pH 8.3), significant high selectivity (32.8 mg-U/g-Ads) and outstanding anti-biological contamination performance (92.9% antibacterial rate). XPS and DFT further indicated that the high extraction ability of Ti₃C₂-AO-PA for uranium was mainly attributed to the strong complexation of AO and -NH₂ with [UO₂(CO₃)₃]^4-. These conclusions showed that Ti₃C₂-AO-PA not only had an ideal application prospect for uranium extraction from seawater, but also provided an available strategy for rapid and selective uranium adsorption from real seawater.

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 possible applications of amidoxime-based adsorbents in neptunium and plutonium separation

Efficient separation of neptunium and plutonium from spent nuclear fuel is essential for advanced nuclear fuel cycles. At present, the development of effective actinide separation ligands has become a top priority. As common adsorbents for extracting uranium from seawater, amidoxime-based adsorbents may also be able to separate actinides from high-level liquid waste (HLLW). In this work, the complexation of Np(IV,V,VI) and Pu(IV) and alkyl chains (R = C₁₃H₂₆) modified with amidoximate (AO^-) and carboxyl (Ac^-) functional groups was systematically studied by quantum chemical calculations. For all the studied complexing species, the RAc^- and RAO^- ligands act as monodentate or bidentate ligands. Complexes with AO^- groups show higher covalency of the metal-ligand bonding than the analogues with Ac^- groups, in line with the binding energy analysis. Bonding analysis verifies that these amidoxime/carboxyl-based adsorbents possess higher coordination affinity toward Pu(IV) than toward Np(IV), and the Np(VI) complexes have stronger covalent interactions than Np(V). According to thermodynamic analysis, these adsorbents have the ability to separate Np(IV,V,VI) and Pu(IV), and also exhibit potential performance for partitioning Pu(IV) from Np(IV) under acidic conditions. This work can help to deeply understand the interaction between transuranium elements and amidoxime-based adsorbents, and provide a theoretical basis for the separation of actinides with amidoxime-based adsorbents.

Structural and Computational Characterization of a Bridging Zwitterionic-Amidoxime Uranyl Complex

A bridging (μ₂) neutral zwitterionic amidoxime binding mode previously unobserved between amidoximes and uranyl is reported and compared to other uranyl amidoxime complexes. Density functional theory computations show the dinuclear complex exhibits a shallow potential energy surface allowing for facile inclusion of a nonbonding water molecule in the solid-state.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.

Binding of oxime group to uranyl ion

Currently, the leading approach for extraction of uranium from seawater is selective sorption of UO2(2+) ions onto a poly(acrylamidoxime) fiber. Amidoxime functional groups are the most studied extractant moieties for this application, but are not perfectly selective, and understanding how these groups (and more generally the oxime groups) interact with UO2(2+) and competing ions in seawater is an important step in designing better extractants. We have started a new research programme aiming at in-depth studies of the uranyl-oxime/amidoxime interactions and we report here our first results which cover aspects of the coordination chemistry of 2-pyridyl ketoximes towards UO2(2+). Detailed synthetic investigations of various UO2(2+)/mepaoH and UO2(2+)/phpaoH reaction systems (mepaoH is methyl 2-pyridyl ketoxime and phpaoH is phenyl 2-pyridyl ketoxime) have provided access to the complexes [UO2(mepao)2(MeOH)2]{[UO2(NO3)(mepao)(MeOH)2]}2 (), [UO2(mepao)2(MeOH)2] (), [(UO2)2(O2)(O2CMe)2(mepaoH)2] () and [UO2(phpao)2(MeOH)2] (). The peroxido group in , which was isolated without the addition of external peroxide sources, probably arises from a bis(aquo)- and/or bis(hydroxido)-bridged diuranyl precursor in solution followed by photochemical oxidation of the bridging groups. The U(VI) atom in the [UO2(NO3)(mepao)(MeOH)2] molecules of () is surrounded by one nitrogen and seven oxygen atoms in a very distorted hexagonal bipyramidal geometry; two oxygen atoms from the terminal MeOH ligands, two oxygen atoms from the bidentate chelating nitrato group, and the oxygen and nitrogen atoms from the η(2) oximate group of the 1.110 (Harris notation) mepao(-) ligand define the equatorial plane. This plane consists of two terminal MeOH ligands and two η(2) oximate groups in the [UO2(mepao)2(MeOH)2] molecule () of . The structure of the [UO2(mepao)2(MeOH)2] molecule that is present in is very similar to the structure of the corresponding molecule in . The structure of the dinuclear molecule that is present in consists of two {UO2(O2CMe)(mepaoH)}(+) units bridged by a η(2):η(2):μ O2(2-) group. The equatorial plane of each uranyl site is composed of the pyridyl and oxime nitrogen atoms of a 1.011 mepaoH ligand, the oxygen atoms of an almost symmetrically coordinated bidentate chelating MeCO2(-) group and the two oxygen atoms of the peroxido groups. The core molecular structure of is similar to that of , the only difference being the presence of 1.110 phpao(-) ligands in the former instead of mepao(-) groups in the latter. The free pyridyl nitrogen atoms of mepao(-) and phpao(-) ligands of , and are acceptors of intramolecular H bonds from the ligated MeOH oxygen atoms. H-bonding and π-π stacking interactions build interesting supramolecular networks in the crystal structures of the four complexes. Compounds are the first structurally characterized uranyl complexes with 2-pyridyl aldoximes or ketoximes as ligands. IR data are discussed in terms of the coordination modes of the ligands in the complexes. (1)H NMR data in DMSO-d6 suggest that the complexes decompose in solution. The ESI(-) MS spectrum of dissolved in the NH4(O2CMe) buffer is indicative of the presence of [UO2(O2CMe)3](-), [UO2(O2CMe)2(phpao)](-), [UO2(O2CMe)(phpao)2](-) and [UO2(phpao)3](-) species. A common structural motif of the complexes containing the anionic mepao(-) (, ) and phpao(-) () ligands is that the deprotonated oximate group prefers to bind in the η(2) fashion forming a 3-membered chelating ring in spite of the presence of a pyridyl nitrogen atom, whose coordination would be expected to lead to 5- or 6-membered chelating rings.

Displacement of carbonates in Ca2UO2(CO3)3 by amidoxime-based ligands from free-energy simulations

Classical molecular dynamics simulations coupled with umbrella sampling reveal the atomistic processes and free-energy profiles of the displacement of carbonate groups in the Ca₂UO₂(CO₃)₃ complex by amidoxime-based ligands in a 0.5 M NaCl solution.

The enhanced uranyl–amidoxime binding by the electron-donating substituents

This work describes the enhanced binding between uranyl and amidoxime ligands due to the electron-donating effect of the substituents.

The coordination of amidoxime ligands with uranyl in the gas phase: a mass spectrometry and DFT study

The coordination of three amidoxime ligands (NAO, GIO, and GDO) with uranyl was compared by MS studies and DFT calculations in the gas phase to reveal the structural information.