MOFs have emerged as effective materials for uranium removal due to their unique structural and functional properties. The removal of uranium by MOFs involves a combination of physical and chemical adsorption mechanisms that leverage the material’s high surface area, porosity, and functional versatility (Adil et al., 2022). The adsorption mechanism of uranium by MOFs materials involves coordination, ion exchange, and electrostatic interactions. Figure 3 illustrates several mechanisms of uranium adsorption by MOFs through various case studies.

Coordination is the dominant mechanism, where functional groups within MOFs, such as carboxyl and hydroxyl groups, form stable complexes with uranyl ions (UO₂ ²⁺). For instance, UiO-66-AO is functionalized with amidoxime groups to chelate uranium effectively (Chen et al., 2017). Similarly, Liu et al. (2019) used adenine and BPDC ligands to modify a zinc-MOF (JXNU-4) to capture U(VI). JXNU-4 uses adenine and BPDC ligands to provide binding sites through exposed carboxylate oxygen atoms (Figure 3A). MOFs also exhibit ion-exchange capabilities, where uranyl ions can displace exchangeable cations within the framework. This ion-exchange mechanism enhances the retention and immobilization of uranium, providing an additional layer of adsorption strength. Ion exchange involves the replacement of ions in the MOF with uranium ions. This is evident in Co-SLUG-35 (Li et al., 2017), where uranium ions exchange with carbonate ions, and in GO-COOH/UiO-66 composites (Vaddanam et al., 2023), where hydrogen ions in carboxyl groups are replaced by uranyl ions (Figure 3B).

In the process of uranium adsorption by MOF-based materials, electrostatic interactions play a crucial role. These interactions occur when the charged surfaces of MOFs attract oppositely charged uranium ions, a process that is significantly influenced by the pH of the solution. At specific pH levels, the surface charge of MOFs can either enhance or inhibit the adsorption of uranyl ions (UO₂ ²⁺), thereby affecting the overall efficiency of uranium removal. This mechanism is integral to optimizing MOF performance in various environmental conditions. In rGO/ZIF-67 aerogels, an increase in pH enhances the electrostatic attraction between negatively charged surfaces and uranyl ions, increasing removal efficiency (Zhao et al., 2020). Similarly, MOF-5’s adsorption of uranium is strongly dependent on pH, highlighting electrostatic interactions (Wu et al., 2019).

These mechanisms often work together, with the specific conditions and MOF types determining which interactions dominate. The combination of these mechanisms allows MOFs to efficiently capture uranium from aqueous solutions, making them highly effective for environmental remediation. This multifaceted approach is crucial for optimizing MOF design and improving uranium adsorption performance.

U(VI) is a typical Lewis acid that tends to bind with hard bases such as nitrogen, oxygen, and phosphorus-containing groups (e.g., amines, hydroxyl groups, and phosphorylurea groups) (Table 1). Studies have shown that the pore size of MOFs significantly affects their adsorption performance for U(VI), with larger pores facilitating the diffusion of U(VI) into the framework. Carboni et al. (2013) modified UiO-68 with amino and dihydroxy phosphorylurea groups to create MOF-2 and MOF-3, which were used to adsorb U(VI) from radioactive wastewater and simulated seawater. It was found that MOF-2 coordinated with UO₂ ²⁺ through two phosphorylurea ligands, resulting in a higher adsorption capacity (217 mg/g), whereas MOF-3, which coordinated with UO₂ ²⁺ through a single ligand, showed a lower capacity (109 mg/g). Additionally, ligand protonation under acidic conditions facilitated the formation of more stable coordination structures with UO₂ ²⁺, thereby improving the adsorption performance.

Studies have shown that post-synthetic modification can further enhance the adsorption capacity of MOFs for U(VI). For example Bai et al. (2015), introduced three different nitrogen-containing amine ligands into MIL-101(Cr) to create MIL-101-NH₂, MIL-101-ED, and MIL-101-DETA, with MIL-101-DETA exhibiting the highest uranium adsorption capacity (350 mg/g). Zhang Y. J et al. (2016) developed MOF-74-Zn by grafting coumarin onto unsaturated Zn (II) centers, which enhanced U(VI) adsorption (360 mg/g) due to larger pore sizes and additional adsorption sites. Furthermore, defect engineering strategies (such as adding modulators during synthesis) can introduce more active sites and larger pores by adjusting the pore size, thereby enhancing adsorption capacity (Liu et al., 2017).

Combining MOFs with other functional materials to create composites can significantly enhance adsorption performance through synergistic effects. For example Yang A. L. et al. (2018), designed a GO-COOH/UiO-66 composite for U(VI) extraction from simulated seawater, which maintained efficient adsorption in solutions with pH 6-9 and achieved an adsorption capacity of up to 188.3 mg/g. Magnetic composites such as Fe₃O₄@ZIF-8 also exhibited ultra-high adsorption capacity (523.5 mg/g) and excellent selectivity, mainly due to the coordination between Zn atoms and U(VI) ions and hydrogen bonding (Min et al., 2017).

In uranium removal, COFs rely on a synergistic interaction of physical and chemical adsorption. In nature, uranium primarily exists as UO₂ ²⁺, which have a high positive charge density and are the main target of COF adsorption. Physically, COFs with their highly ordered porous structures effectively capture uranyl ions through van der Waals forces and electrostatic interactions. These structures provide abundant adsorption sites and facilitate the chemical bonding of uranium ions with functional groups on the COF surface. The high surface area of COFs enables rapid concentration of uranium ions during the initial adsorption phase, enhancing overall adsorption efficiency.

Chemically, the functional groups in COFs, such as carboxyl, amide, amino, and hydroxyl groups, form stable coordination bonds with uranyl ions, effectively immobilizing them. These groups engage in strong coordination interactions with uranyl ions, forming stable complexes and enhancing selective adsorption. Additionally, negatively charged functional groups on the COF surface can electrostatically attract the positively charged uranyl ions, further improving selectivity and efficiency. In some cases, uranyl ions may also displace cations such as H⁺ or Na⁺ in the COF material through ion exchange, achieving uranium fixation. Research indicates that the presence of hydroxypropoxy groups strengthens the affinity and adsorptive capacity for UO₂ ²⁺ at the hydrazino-carbonyl sites of COFs by means of hydrogen bonding and charge stabilization (Zhang et al., 2022). Mei and Yan (2023) employed a post-synthetic modification approach to synthesize two COFs with rich phosphine group modifications, connected by β-ketoenamine and benzoxazole, overcoming the disadvantage that the affinity of hydrazine groups for vanadium is stronger than that for uranium.

The synergy between physical and chemical adsorption significantly enhances the overall adsorption capacity and efficiency of COF materials. Physical adsorption provides favorable conditions for the concentration and rapid kinetics of uranium ions, while chemical adsorption ensures stable binding and thermodynamic stability. This synergistic effect allows COFs to perform exceptionally well in uranium removal, demonstrating effectiveness in both simulated laboratory solutions and complex real-world environments.

Recent studies have demonstrated the potential of COFs for the selective adsorption and extraction of UO₂ ²⁺ from various environments (Table 2). By modifying COF materials with specific functional groups or structures, researchers have achieved significant improvements in their adsorption capacity and selectivity for UO₂ ²⁺. The following cases highlight different approaches to enhance the performance of COFs in uranium removal.

The adsorption of uranium on the surface of heterogeneous adsorbents, such as GO and rGO, is significantly influenced by the pH of the solution due to the varying forms of uranium species under different pH conditions. At pH levels below 4, uranium primarily exists as the cationic species UO₂ ²⁺, with a small fraction as UO₂ (OH)⁺ and (UO₂)₂(OH)₂ ²⁺. Between pH 4 and 8, uranium species such as (UO₂)₃(OH)₅ ⁺ and (UO₂)₄(OH) ₇ ⁺ become dominant, while at pH levels above 8, anionic species like (UO₂)₃(OH)₇ ⁻ and UO₂(OH)₃ ⁻ prevail.

GO, which bears oxygen-containing groups such as hydroxyl, carboxyl, and epoxy, carries a negative surface charge across a broad pH range (2–9) (Verma and Dutta, 2015). This negative charge makes GO and its composites effective adsorbents for cationic uranium species through mechanisms such as coordination, electrostatic interactions, hydrogen bonding, and π-π interactions. As the pH increases, the negative surface charge of GO becomes more pronounced, enhancing the adsorption of positively charged uranium species. At low pH levels, the adsorption efficiency is reduced due to competition between H⁺ ions and uranium species for active sites on the GO surface. For example, pure GO showed improved uranium removal efficiencies from 50% to 66% as the pH increased from 2 to 11 (Wang et al., 2020). Similarly, Cetyltrimethylammonium bromide-modified GO (MGO) demonstrated a marked increase in uranium removal efficiency from 50% at pH 2 to nearly 99% at neutral pH 7, due to favorable electrostatic interactions between the cationic uranium species and the negatively charged MGO surface (Wang et al., 2020). However, at pH levels above 7, the efficiency slightly decreases as anionic uranium species form, yet overall effectiveness remains above 90%. In the case of graphene oxide-activated carbon felt (GO-ACF) composites, maximum uranium adsorption was observed at pH 5.3, with reduced efficiency at lower pH levels due to competitive adsorption of H⁺ ions (Chen et al., 2013).

GO and rGO are effective in uranium removal because of their unique structures and functional groups (Figure 7B). GO, produced by oxidizing graphite, has abundant oxygen-containing groups that facilitate uranium adsorption. Meanwhile, rGO, formed by reducing GO, partially restores the electrical properties of graphene while retaining sufficient functional groups to bind uranium. The π-conjugated structure of rGO further aids in uranium capture, making both GO and rGO highly efficient for environmental remediation due to their strong interactions with uranium and structural versatility.

Graphene-based adsorbents, particularly those with abundant oxygen functional groups, play a key role in uranium adsorption. The effectiveness of these adsorbents depends heavily on synthesis conditions, influencing both the amount and type of functional groups present (Table 3). Few-layered GO prepared using a modified Hummer’s method shows high performance due to its large surface area and high oxygen content, achieving a high partition coefficient (PC) and removing nearly 99% of uranium at pH 5.0. In contrast, expanded graphite-derived GOs exhibit higher adsorption capacity but lower PC, indicating reduced efficiency due to the presence of more unadsorbed U species. The PC reflects the effectiveness of actual removal, while adsorption capacity more reflects the maximum adsorption ability under ideal conditions. Thus, both adsorption capacity and PC are critical for evaluating the performance of graphene-based adsorbents.

The adsorption capacity of GO for uranium can be enhanced by chemically modifying its oxygen-containing functional groups, particularly by incorporating nitrogen-containing groups such as amine, amide, imine, and oximes. For example, GO-polyethyleneimine (GO-PEI) macrostructures achieved a high PC of 513.40 mg·g⁻¹·µM⁻¹ and an adsorption capacity of 215.6 mg/g at pH 5 due to the strong coordination between uranium ions and amino and oxygen groups (Huang et al., 2018). In contrast, the capacity decreased at lower pH levels due to repulsion between cationic uranium ions and protonated sites. Additionally, oxime-functionalized adsorbents (RGO-PDA/oxime) showed excellent uranium adsorption with a capacity of 1,049 mg/g, attributed to the high affinity of oxime molecules for uranium ions and the stable molecular bridging provided by polydopamine (PDA) (Qian et al., 2018). Other factors, such as the porous 3D network and large surface area of RGO-PDA/oxime, further enhance U uptake (Gunathilake et al., 2015).

GO exhibits extreme hydrophilicity in aqueous solutions due to the presence of many oxygen-containing polar functional groups, making post-separation in wastewater treatment challenging. A solution is to load magnetic nanomaterials onto GO sheets, enabling easy separation with an external magnetic field and enhancing adsorption capacity. Magnetic GO-based adsorbents, such as mGO-PP, demonstrated over 90% uranium removal from simulated wastewater within 30 min, with an adsorption capacity of 200.4 mg/g and a high PC of 4,747.6 mg·g⁻¹·µM⁻¹ (Dai et al., 2019). In real mine wastewater, mGO-PP selectively adsorbed uranium with a removal rate of 93.68%, and the residual uranium concentration was well below the WHO limit of 30 μg/L (Verma and Dutta, 2015). This suggests that GO adsorbents with nitrogen functional groups are highly effective for the selective removal of uranium from both artificial and real wastewater.

Currently, there is no universally accepted mechanism for the adsorption of uranium on MXene materials. This is due to the diverse adsorption and reduction mechanisms exhibited by MXenes, which are influenced by various factors such as material type, surface modification, environmental conditions (e.g., pH), and specific application methods. Existing studies have proposed different mechanisms (Figure 10), but their dominant roles may vary under different conditions.

a. Chemisorption and Hydrogen Bonding: Studies have shown that uranyl ions (UO₂ ²⁺) can form chemical bonds and hydrogen bonds with the surface of MXene materials [such as Ti₃C₂(OH)₂]. Depending on the pH conditions, the hydroxyl groups (OH) on the MXene surface can be deprotonated or protonated, allowing uranyl ions to be adsorbed on the surface through bidentate or tridentate coordination. Experiments have demonstrated that forming a bidentate coordination structure with two U-O bonds is more energetically favorable, enhancing the adsorption capacity of uranium (Zhang C. R. et al., 2016; Zhang et al., 2017).

Recent advancements have demonstrated the potential of MXene materials in the field of uranium remediation, leveraging their unique structural and chemical properties to achieve efficient removal of uranium from aqueous solutions (Table 4). The following examples highlight the different types of MXene materials, such as V₂CT _x , Ti₂CT _x , and Ti₃C₂T _x , and their mechanisms of action in effectively capturing and reducing uranium under various conditions (Yu et al., 2022).

In 2016, Wang et al. (2016) first reported the use of multilayer V₂CT _x MXene materials for capturing U(VI) from aqueous solutions, achieving a maximum adsorption capacity of 174 mg/g. The primary mechanisms for U(VI) removal by V₂CT _x MXene were identified as coordination and ion exchange. Density Functional Theory calculations and Extended X-ray Absorption Fine Structure analysis revealed that U(VI) forms bidentate inner-sphere complexes by binding to heterogeneous adsorption sites on the MXene nanosheets. Additionally, the deprotonation of hydroxyl groups on the surface of V₂CT _x upon binding with U(VI) suggests that the adsorption process follows an ion exchange mechanism (Wang et al., 2016). Building on this initial success, researchers explored the potential of other MXenes, such as Ti₂CT _x and Ti₃C₂T _x , for uranium removal. In 2018, Wang et al. (2020) demonstrated that highly reactive Ti₂CT _x MXene could effectively reduce U(VI) to U(IV) and remove uranium through an adsorption-reduction strategy. This study showed that Ti₂CT _x MXene exhibits good adsorption of U(IV) over a wide pH range: at low pH, U(VI) is reduced to mononuclear U(IV) and binds to the MXene substrate, while at near-neutral pH, UO₂ ^+x nanoparticles adsorb onto Ti₂CT _x MXene, with some Ti₂CT _x converting to amorphous TiO₂. These findings highlighted the reduction-inducing effect of Ti₂CT _x MXene in the removal of U(VI), which is partially dependent on the solution’s pH.

Further advancing this field, Wang et al. (2017) utilized redox and adsorption processes to effectively remove U(VI) using Ti2CTx MXene, achieving a high adsorption capacity of 470 mg/g. Lin Wang et al. also conducted experiments using Ti₃C₂T _x MXene intercalated with dimethyl sulfoxide (DMSO), revealing that the DMSO-intercalated Ti₃C₂T _x had five times the adsorption capacity of non-intercalated Ti₃C₂T _x , indicating the critical role of interlayer spacing in U(VI) adsorption. In another advancement, Zhang C. R. et al. (2020) enhanced the application of MXenes by modifying them with carboxyl-terminated aryl diazonium salts. This modification improved the water stability and chelating capacity of MXenes for UO₂ ²⁺, achieving an adsorption capacity of up to 334.8 mg/g. The study aligned with the Langmuir and pseudo-second-order adsorption models and attributed the adsorption mechanisms to inner-sphere surface complexation and electrostatic interactions. Lee et al. (2018) proposed an innovative approach to extend the interlayer spacing of Ti₃C₂T _x MXene by preparing and storing pristine and intercalated MXenes under hydrated conditions. This strategy enhanced the sorption efficacy for radionuclide removal through ultrafast mild temperature post-treatment of adsorption calcination, confirming the potential for successful retention of U(VI) beneath the multi-pattern Ti₃C₂T _x structure.

Further research by Attia et al. (2021) using DFT and extensive X-ray absorption techniques identified heterogeneous sorption sites, such as -O, -F, and -OH, in MXene components as key factors for effective U(VI) capture. A bidentate adsorption mechanism for U(VI) in V₂C(OH)₂ was reported, achieving an uptake efficacy of approximately 535 mg/g. Meanwhile, another study found that uranyl ions tend to coordinate with hydroxyl groups on the V sites, forming a bidentate complex with an inner-sphere structure (Jun et al., 2020). The deprotonation of hydroxyl groups after binding with U(VI) facilitated an ion exchange mechanism. Sun Q. et al. (2018) further demonstrated that strong U-O linkages with hydroxyl groups and intense interactions between uranium and oxygen atoms at active adsorption sites, as well as strong hydrogen bonds, contributed to the maximum adsorptive energy.

These findings illustrate that MXene materials, particularly when modified or functionalized, offer significant promise for the removal of uranium from aqueous environments. Their high adsorption capacities, diverse adsorption mechanisms (such as coordination, ion exchange, and reduction), and tunable chemical properties make them highly effective for environmental remediation. Ongoing research to optimize MXene materials and refine their adsorption strategies could further expand their application potential in nuclear waste management and the removal of other toxic contaminants.