Emerging Nanomaterials toward Uranium Extraction from Seawater: Recent Advances and Perspectives

Nuclear energy holds great potential to facilitate the global energy transition and alleviate the increasing environmental issues due to its high energy density, stable energy output, and carbon-free emission merits. Despite being limited by the insufficient terrestrial uranium reserves, uranium extraction from seawater (UES) can offset the gap. However, the low uranium concentration, the complicated uranium speciation, the competitive metal ions, and the inevitable marine interference remarkably affect the kinetics, capacity, selectivity, and sustainability of UES materials. To date, massive efforts have been made with varying degrees of success to pursue a desirable UES performance on various nanomaterials. Nevertheless, comprehensive and systematic coverage and discussion on the emerging UES materials presenting the fast-growing progress of this field is still lacking. This review thus challenges this position and emphatically focuses on this topic covering the current mainstream UES technologies with the emerging UES materials. Specifically, this review elucidates the causality between the physiochemical properties of UES materials induced by the intellectual design strategies and the UES performances and further dissects the relationships of materials-properties-activities and the corresponding mechanisms in depth. This review is envisaged to inspire innovative ideas and bring technical solutions for developing technically and economically viable UES materials.

Efficient removal of uranium (VI) with a phytic acid-doped polypyrrole/ carbon felt electrode using double potential step technique

In order to efficiently extract uranium from uranium-containing wastewater, a novel acid doped polypyrrole/carbon felt (PA-PPy/CF) electrode was prepared via a facile electrodeposition method. For this material, PA and PPy combined to form a stable chemical structure by a charge compensation mechanism. The electrochemical characterization results showed that PA-PPy can significantly accelerate the electrochemical reduction rate of uranium ions. Moreover, a double potential step technique (DPST) was applied to prevent water splitting and maintained the electrocatalytic reduction activity of the surface groups during the electrochemical adsorption process. The removal efficiency obtained by the DPST method was six times higher than that obtained by the conventional chemical adsorption. When the concentrations of uranyl nitrate were 10, 20, 50, and 100 mg/L, the removal efficiencies of uranium were 98.8%, 98.1%, 94.6%, and 93.7%, and the adsorption capacities of uranium were 164.7, 326.9, 788.5, and 1562.0 mg/g, respectively. This material also showed an excellent recycling performance and remarkable selectivity for uranium ions. This work may shed light on the development of removal system for uranium (VI).

Iron phosphonate for highly efficient capture of U(VI) from acidic solution

In this study, a novel, high surface area iron phosphonate (IP) for highly efficient adsorption of uranyl ion in acidic medium was described. The as-prepared IP was amorphous with its specific surface area and total pore volume as high as 268 m²/g and 1.04 cm³/g, respectively. Particularly, the as-prepared IP with ferrous ions and oxygen, nitrogen-bearing functional groups prove excellent U(VI) adsorption capacity (154.6 mg/g) as compared to that of amorphous FePO₄ (67.3 mg/g) and Fe₃(PO₄)₂(H₂O)₈ (33.8 mg/g). Surprising, the saturation adsorption capacity could achieve up to 353.9 mg/g. Besides, the IP also had a fast adsorption rate for attaining adsorption equilibrium within 20 min, and followed pseudo-second-order kinetic and Freundlich models. Moreover, both the Dubinin-Radushkevich isotherm adsorption model and the value of enthalpy indicated a chemisorption process. Otherwise, the Na⁺-independent U(VI) adsorption on IP and the adsorption-desorption isotherm studies revealed that inner-layer surface complexation is the control step for U(VI) adsorption process, and the adsorbent featured an irreversible adsorption process. The structure and functional groups of the adsorbent remained unchanged after capture of U(VI). Further, X-ray photoelectron spectra (XPS) analysis demonstrated that the capture mechanism of U(VI) on IP from acidic aqueous solution was due to not only redox reaction, but also ascribed to the coordinated chemical adsorption.

Nano zero valent iron encapsulated in graphene oxide for reducing uranium

Nano zero-valent iron (Fe⁰) has been widely used to remove Uranium (U(VI)). In order to enhance the performance of Fe⁰ toward U(VI) removal, the Fe⁰ was assembled into graphene oxide (GO) sheets via 3-aminopropyl triethoxysilane (APTES) as Fe⁰/APTES-GO composites. The Fe⁰/APTES-GO composites were triumphantly prepared, characterized and analyzed by means of Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) together with Energy Dispersive Spectrometer (EDS), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). SEM and TEM-EDS results manifested that Fe⁰ particles were encapsulated into rolled-up GO, which greatly improved the stability of Fe⁰. Batch experiment showed that only a small amount of Fe²⁺ was leached in the first two leaching cycles of Fe⁰/APTES-GO composites. The removal capacity of Fe⁰/APTES-GO composites was up to 1357.99 mg/g at pH = 4.1 and T = 50 °C, which was mainly attributed to the reducing activity of Fe⁰ and an abundance of functional groups (i.e., -COOH, C-OH and -OH) on the Fe⁰/APTES-GO composites. The electrostatic potential (ESP) from the calculation also supported that U(VI) tended to be reduced at the back side of the GO-Fe⁰ cluster.

Facile construction of Fe, N and P co-doped carbon spheres by carbothermal strategy for the adsorption and reduction of U(vi)

In this work, nitrogen and phosphorus co-doped magnetic carbon spheres encapsulating well-dispersed active Fe nanocrystals (Fe/P-CN) were fabricated via a simple copolymer pyrolysis strategy. Benefiting from heteroatoms doping, Fe/P-CN could primarily adsorb soluble U(vi) ions through abundant functional groups, and subsequently, the adsorbed U(vi) could be reduced to insoluble U(iv) by Fe nanocrystals. Fe/P-CN pyrolyzed at 800 °C (Fe/P-CN-800) exhibited excellent U(vi) removal capacity of 306.76 mg g⁻¹, surpassing nitrogen and phosphorus co-doped carbon spheres and nano zero-valent iron. In addition, the magnetic separation and thermal reactivation properties endow Fe/P-CN-800 with excellent reusability. This research, especially, provides a promising synergistic adsorption and reduction strategy to effectively remove U(vi) using heteroatom-doped composites.

The constructed novel magnetic carbon sphere co-doped by N, P, Fe (Fe/P-CN) exhibits high U(vi) removal efficiency, excellent magnetic separation and reusability, evidencing the potential practical applications in environmental remediation.

Design Strategies to Enhance Amidoxime Chelators for Uranium Recovery

To move nuclear as a primary energy source, uranium resources must be secured beyond what terrestrial reserves can provide. Given the vast quantity of uranium naturally found in the ocean, adsorbent materials have been investigated to recover this vital fuel source. Amidoxime (AO) has been found to be the state-of-the-art functional group for this purpose, however, improvements must still be made to overcome the issues with selectively capturing uranium at such a low concentration found in the ocean. Herein, we report PAF-1 as a platform to study the effects of two amidoxime ligands. The synthesized adsorbents, PAF-1-CH₂NHAO and PAF-1-NH(CH₂)₂AO, with varying chain lengths and grafting degrees, were investigated for their uranium uptakes and kinetic efficiency. PAF-1-NH(CH₂)₂AO was found to outperform PAF-1-CH₂NHAO, with a maximum uptake capacity of 385 mg/g and able to reduce a uranium-spiked solution to ppb level within 10 min. Further studies with PAF-1-NH(CH₂)₂AO demonstrated effective elution for multiple adsorption cycles and showed promising results for uranium recovery in the diverse composition of a spiked seawater solution. The work presented here moves forward design principles for amidoxime-functionalized ligands and provides scope for strategies to enhance the capture of uranium as a sustainable nuclear fuel source.

Synthesis of nanoscale zero-valent iron loaded chitosan for synergistically enhanced removal of U(VI) based on adsorption and reduction

In this study, chitosan (CS) loading well-dispersed nanoscale zero-valent iron (NZVI/CS) was successfully prepared via the liquid-phase reduction method. Characterizations of the NZVI/CS with high-resolution transmission electron microscopy and X-ray diffraction suggested that the as-prepared NZVI/CS comprised numerous dispersed Fe⁰ nanoparticles. Synergistic adsorption and reduction occurred during the removal process based on X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Influence of pH, contract time and temperature on U(VI) removal were investigated. The high removal capacity and rapid removal kinetics were predominately ascribed to the existence of well-distributed NZVI, which could rapidly reduce U(VI) into U(IV). The removal process could be better depicted by the Langmuir isotherm model and the pseudo-second-order kinetic model. The thermodynamic parameters showed that the removal process was exothermic. These findings indicate that the synthesized NZVI/CS composites have potential application for the removal of U(VI) from the sewage water.