Promising porous materials for uranium extraction from seawater

Abstract

Uranium extraction from seawater is a promising approach for ensuring continued uranium fuel supply to the nuclear power industry. However, extracting uranium by this route is challenging due to the low concentration of uranium, high ionic strength, and marine micro-organisms in seawater. Recently, a range of novel porous adsorbent materials have been developed for uranium extraction from ocean water. These adsorbents rely on specific pore characteristics and functional groups (hydroxyl, carboxyl, amidoxime, phosphate, etc.) to achieve a high affinity and selectivity for uranyl ions (UO₂²⁺) relative to other ions. Relying strongly on coordination principles, specific binding sites for uranium are assembled in these porous materials, with cooperative actions of several functional groups often used to achieve strong uranium capture and adsorption selectivity. In addition to traditional adsorbents, adsorption-photocatalytic and adsorption-electrocatalytic materials are also being pursued, which include both specific adsorption sites and photocatalytic or electrocatalytic moieties in their frameworks. These innovative strategies allow the conversion of uranyl ions into harvestable solid products (such as UO₂ or Na₂O(UO₃·H₂O)_x) and result in high extraction efficiencies together with good biofouling resistance. This perspective aims to capture some of the recent breakthroughs in the design of porous materials for selective uranium extraction from seawater.

Nuclear energy is expected to play an important transitional role in decarbonization of the global energy sector. The nuclear energy industry relies on ²³⁵U as a fuel, which is typically obtained by enrichment of uranium extracted from ore [1]. However, the limited reserves of uranium ore on land will not satisfy long-term production requirements, making it essential to find alternative sources of uranium. The earth's oceans contain approximately 4.5 billion tons of uranium (mostly in the form of UO₂²⁺ ions), thus motivating the search for effective technologies to extract uranium from seawater [2]. Compared to the pollution caused by uranium ore mining and processing, seawater extraction of uranium is expected to have minimal impact on the environment [3]. However, extraction of uranium from seawater is technically challenging due to the low uranium concentration (3.3 ppb), high concentrations of other ions, high ionic strength conditions, and risk of marine biofouling.

To date, a wide range of adsorbent materials have been investigated for uranium extraction from seawater, including functionalized carbons, zeolites, layered inorganic adsorbents, ion-exchange resins, fibers, and silica composites. The performance of these traditional materials is limited by low adsorption capacities, slow adsorption rates, and limited selectivity [4]. Emerging porous materials, including covalent organic frameworks (COFs), porous organic polymers (POPs), and metal-organic frameworks (MOFs), offer promise for overcoming these limitations [5]. The pore structures, inner surfaces, and functionalities of these materials can be tuned for specific applications, offering a rich toolbox for the design of porous adsorbents for uranium. Key features these materials need to possess include: (i) high stability in seawater, (ii) availability of suitable functional sites for binding uranyl ions, (iii) selectivity for uranyl ions over other ions in seawater, (iv) high resistance to marine biofouling caused by marine bacteria and algae, and (v) low-cost, non-toxicity and recyclability. In recent years, porous materials have been developed possessing all these characteristics with good performance demonstrated for uranium extraction from seawater at the laboratory level and in scaled-up marine field tests.

Functional groups such as hydroxyl, carboxyl, amino, amide, imidazole, amidoxime, and phosphate have been shown to be effective for uranyl ion binding when incorporated in porous materials (Fig. 1a). The exact chemical coordination of uranium determines the capacity and selectivity of uranium extraction. For example, microporous membranes supporting phenol functionalities allowed capture of 27.81 µg of uranium from 10 L of seawater. Imidazole groups with positive charges in a SII-PNF adsorbent displayed high selectivity towards uranium (K_d value of 2.01 ×  10⁵ mL/g) over vanadium [3]. Compared with these simple groups, amidoxime groups show a very high affinity for uranyl ions. Sun et al. [6] designed an adsorbent POP-oNH₂-AO rich in amidoxime groups, which utilized two amidoxime ligands and one carbonate ion to coordinate with uranyl. POP-oNH₂-AO, which offers robustness and high chemical stability (in both 1 M HCl and 1 M NaOH), was applied for uranium extraction from natural seawater, demonstrating a uranium uptake of 4.36 g/g and high affinity towards uranium (K_d value of 8.36  ×  10⁶ mL/g). To improve the affinity for uranium over vanadium and other metal ions, phosphate groups in a DNA-UEH adsorbent enabled a uranium uptake capacity of 6.60 mg/g (Fig. 2a), which was 17.95 times higher than that towards vanadium, surpassing the U/V selectivity of amidoxime groups (maximum 2.5 times) [3]. In addition to utilizing fixed functional groups, constructing specific sites and modulating local chemical environments offer further pathways for enhancing the binding affinity towards uranium. Different strategies are now being utilized to address the challenge of selective uranium extraction from seawater, such as hydrazine-carbonyl chelating sites, guanidinium‑hydroxyl sites, carboxyl-amino groups and salicylaldoxime-pyridine sites (Fig. 1b). The carboxyl and amino groups in MOF UiO-66-3C4N are spatially arranged in a manner that affords a high selectivity and strong affinity for uranium (Fig. 2a) [7]. UiO-66-3C4N exhibited a uranium capture capacity that was 17.03 times higher than that of vanadium (as vanadyl, VO₂⁺), with the uranium capture capacity reaching 6.85 mg/g. Density functional theory (DFT) calculations and Extended X-ray absorption fine structure (EXAFS) confirmed two oxygen atoms of a carboxyl group, a Zr atom, and the N atom of an amino group coordinated the uranyl ion. Furthermore, salicylaldoxime-pyridine sites show excellent selectivity towards uranyl ions [8], with uranyl uptake not being affected by competing ions, including vanadyl. It should be noted that the functional groups in these adsorbents, such as guanidine, quaternary ammonium, and peptides, contribute antifouling activity due to their positive charges which can effectively kill most microorganisms. Harnessing such multi-purpose functional groups, to boost adsorption capacity, selectivity, and anti-biofouling properties, holds great promise in the construction of adsorbents for uranium extraction from seawater.

Fig. 1.

Schematic illustration of the functional groups and constructed binding sites commonly used for selective uranium adsorption.

Fig. 2.

Schematic illustration of adsorbents, adsorption-photocatalysts, and adsorption-electrocatalysts for uranium extraction from seawater[3,7,9,12].

Compared to simple adsorption systems, adsorption-photocatalytic or adsorption-electrochemical approaches offer faster kinetics, higher uranium extraction efficiencies, and enhanced anti-biofouling abilities, representing a fast-growing research trend in the field of uranium extraction from seawater. Photocatalytic functional groups (e.g. keto-enamine, triazine, porphyrin, pyrene, thiazole, thiophene) can be incorporated as building blocks or linkers into porous adsorbents such as COFs, MOFs, or POPs to form adsorption-photocatalyst systems. Under visible light irradiation, the photocatalytic components generate electron-hole pairs, with electrons reducing adsorbed uranyl ions to solid U(IV) products for collection. COFs can readily be constructed with linkers containing electron donors, acceptors, and photosensitizers, whilst the introduction of uranyl adsorption nanotraps into the donor-acceptor structure of the COF enables captured uranyl ions to be converted directly into UO₂ (Fig. 2b) [9]. For instance, COF 2-Ru-AO possessed a high uranium uptake capacity, with the selectivity towards uranium over vanadium reaching 2.9 in seawater. Furthermore, hydrazide, as the key connecter, provides improved stability in acidic media. It was utilized as an adsorption site and electron transport platform to achieve simultaneous adsorption-photocatalytic uranium reduction [10]. The photocatalytic uranium extraction performance was ∼6.84 mg/g/day, which was 16.35 and 2.44 times higher than that of vanadium and copper, respectively. The chromenoquinoline rings and electron-attracting/donating groups of COFs enhanced stability, delivering an impressive uranium extraction performance, state-of-the-art for COF-based adsorbents or adsorption-photocatalyst systems in natural seawater [11]. Furthermore, reactive oxygen species produced by COFs under visible light irradiation can destroy marine microorganisms, thereby achieving outstanding anti-biofouling properties.

Furthermore, adsorption-electrochemical extraction systems show great promise for uranium extraction from seawater. Typically, an electrode surface rich in amidoxime groups is prepared for the selective adsorption of uranyl ions, with an electric current then applied to reduce adsorbed uranium [12]. Recently, Fe-N_x-C-R and In-N_x-C-R adsorption-electrocatalysts were prepared containing Fe and In single atoms, respectively, dispersed on porous N-doped carbon capsules (N_x-C) functionalized with chelating amidoxime groups (R) (Fig. 2c) [13,14]. Under square wave potential cycling, the single atom sites transformed adsorbed U(VI) to solid Na₂O(UO₃·H₂O)_x precipitates, allowing facile uranium collection. Notably, In-N_x-C-R offered a uranium capture capacity of 402.9 mg/g, greatly exceeding the performance of a In-N_x-C without amidoxime groups (118.9 mg/g) under similar conditions. Furthermore, the uranium extraction capacity was evaluated to be 6.35 mg/g/day for In-N_x-C-R, 8.75 times higher than that of vanadium in natural seawater. Such an adsorption-electrocatalyst system avoids the laborious elution process needed to recover uranium from traditional adsorbents, thus reducing extraction costs.

In addition to the aforementioned strategies, a number of promising proof-of-concept adsorbent systems have recently been reported in the literature. Polyethylene nonwoven fabrics grafted with amidoxime groups were used to recover more than 1 kg of yellowcake over 240 d in marine tests in Japan, affording an average capacity of 0.5 mg/g [15]. Further, kilogram-scale nanofiber-based membranes captured nearly 20 g of yellow cake after 30 d in the East China Sea [3]. These projects demonstrate the promise of using porous materials to extract uranium from seawater. However, for such systems to be commercially viable, the capture capacity and manufacturing costs of adsorbents still need to be improved.

Significant advances have been made relating to the development of porous adsorbent systems for uranium extraction from seawater. Uranium extraction capacities are constantly improving, whilst improving structure-function relationships are enabling the rational design of better adsorbent materials. The effectiveness, robustness, biofouling resistance, and economic viability of porous materials in scaled-up marine field tests remain to be further studied, to understand the viability and practicality of uranium harvesting from seawater. Active cooperation between scientists and engineers will be vital for achieving a reliable technology for uranium capture from seawater, with such collaboration essential to the future sustainable development of the nuclear energy industry (whilst nuclear fission remains the dominant technology).

Declaration of competing interest

The authors declare that they have no conflicts of interest in this work.

Biographies

Mengjie Hao received her B.S. degree from North China Electric Power University in 2020. Now, she is a Ph.D. candidate at the College of Environmental Science and Engineering, North China Electric Power University. Her research focuses on the design and synthesis of highly efficient covalent organic framework (COF) materials for environmental management.

Hui Yang (BRID: 07832.00.92592) earned his Ph.D. degree from Northwestern Polytechnical University in 2013. He is currently a professor at North China Electric Power University. His current research interest is the synthesis and application of porous materials for radioactive and environmental-related applications.