Functionalized Carbon-Based Materials for Uranium Extraction: A Review

Abstract

The development of effective materials for uranium extraction from seawater is vital for advancing sustainable energy solutions. However, the efficient recovery of uranium from seawater presents significant challenges due to its extremely low concentration, the presence of competing ions, and the complex marine environment. To address these issues, various materials such as inorganic and organic sorbents, chelating resins, nanostructured sorbents, and composite materials have been explored. More recently, the functionalization of carbon-based materials for enhanced adsorption properties has attracted much interest because of their high specific surface area, excellent chemical and thermal stability, and tunable porosity. These materials include activated carbon, graphene oxide, biochar, carbon cloths, carbon nanotubes, and carbon aerogels. The enhancement of carbonaceous materials is typically achieved through surface functionalization with chelating groups and the synthesis of composite materials that integrate other high-performance sorbents. This review aims to summarize the work of these functionalized carbon materials, focusing on their adsorption capacity, selectivity, and durability for uranium adsorption. This is followed by a discussion on the binding mechanisms of uranium with major chelating functional groups grafted on carbonaceous sorbents. Finally, an outlook for future research is suggested. We hope that this review will be helpful to researchers engaged in related studies.

1. Introduction

Uranium, a naturally occurring radioactive element, is a low-emission energy source with exceptional energy density, making it highly valuable for nuclear power generation. Conventionally, uranium fuel is derived from terrestrial uraninite; however, these reserves are scarce and are projected to be depleted within the next century, creating a significant challenge in meeting steadily rising global energy demands [1,2,3].

Consequently, scientific and industrial interest has shifted towards the oceans, which contain an estimated 4.5 billion tons of uranium, a quantity nearly 1000 times greater than terrestrial reserves [4,5,6,7,8]. Despite this vast abundance, its recovery is complicated by an extremely low concentration of approximately 3.3 ppb. Furthermore, the extraction process faces numerous other significant challenges, including the presence of high concentrations of competing ions [9,10], high salinity (0.6–0.7 mol/L of metal ions, primarily Na⁺, Mg²⁺, Ca²⁺, and K⁺), alkaline pH (8.0–8.3), variable temperatures, and the complex speciation of uranium itself [11,12,13,14].

In seawater, uranium predominantly exists in its hexavalent state U(VI), and its chemical form is highly pH-dependent. At a pH of 5.5 or lower, the dominant species are UO₂²⁺ and UO₂(OH)⁺. However, as the pH rises to typical seawater levels, these species transition into more stable anionic uranyl carbonate complexes, such as UO₂(CO₃)₂²⁻ and UO₂(CO₃)₃⁴⁻ [15,16]. As illustrated in the distribution diagram in Figure 1, the most stable and dominant form of uranium at the typical seawater pH of 8.2 (indicated by the dotted line) is the neutral calcium uranyl tricarbonate complex, Ca₂[(UO₂)(CO₃)₃]. While this species predominates, minor contributions are also made by other complexes, including Mg[(UO₂)(CO₃)₃], Ca[(UO₂)(CO₃)₃]²⁻, and [UO₂(CO₃)₃]⁴⁻ [17].

Figure 1. Uranium (VI) speciation in seawater; the dashed gray line corresponds to an approximate seawater pH of 8.2. (1) UO₂²⁺, (2) (UO₂)(CO₃) (aq), (3) UO₂OH⁺, (4) [(UO₂) (CO₃)₂]²⁻, (5) Ca₂[(UO₂)(CO₃)₃] (aq), (6) Mg[(UO₂)(CO₃)₃]²⁻, (7) Ca[(UO₂)(CO₃)₃]²⁻, (8) [UO₂(CO₃)₃]⁴⁻. Reprinted with permission from Ref. [17]. Copyright 2016 American Chemical Society.

Figure 1. Uranium (VI) speciation in seawater; the dashed gray line corresponds to an approximate seawater pH of 8.2. (1) UO₂²⁺, (2) (UO₂)(CO₃) (aq), (3) UO₂OH⁺, (4) [(UO₂) (CO₃)₂]²⁻, (5) Ca₂[(UO₂)(CO₃)₃] (aq), (6) Mg[(UO₂)(CO₃)₃]²⁻, (7) Ca[(UO₂)(CO₃)₃]²⁻, (8) [UO₂(CO₃)₃]⁴⁻. Reprinted with permission from Ref. [17]. Copyright 2016 American Chemical Society.

To address the challenges of extracting uranium from seawater, a variety of techniques have been extensively investigated, including adsorption [18,19], ion exchange [20], solvent extraction [21], photocatalytic degradation [22], and the electrochemical method [23]. Among these diverse approaches, adsorption has emerged as a particularly promising method, primarily owing to its operational simplicity and high capture efficiency [24].

The pursuit of effective uranium adsorbents dates back to the 1950s. Initial research efforts centered on inorganic materials such as metal oxides (e.g., titanium dioxide) and clay minerals. Although these materials showed early promise, they were hampered by poor selectivity and a lack of stability in seawater [25,26]. A significant breakthrough occurred in the late 1970s with the discovery that grafting certain functional groups to carrier materials could substantially improve performance. These groups enhance adsorption capacity and selectivity by facilitating inner-sphere complexation and chelation with uranium ions [27,28,29,30,31]. Among the various functional groups explored, the amidoxime group draws the most attention due to its exceptional ability to bind uranium [32,33]. To date, amidoxime-functionalized adsorbents still remain the state-of-the-art material for uranium extraction. Following this discovery, in the 1980s and 1990s, various polymers, including polyacrylonitrile [34,35] and polyethylene fibers [36,37,38,39], were employed as carriers for these functional groups. Building on this foundational work, from 1999 to 2001, Japanese researchers conducted extensive marine field trials utilizing functionalized fabric adsorbents [40].

More recently, advanced nanostructured materials, such as metal–organic frameworks (MOFs), covalent organic frameworks (COFs), porous organic polymers (POPs), and porous silica, constitute the newest avenue for uranium adsorbents. These materials frequently exhibit markedly enhanced selectivity and uranium adsorption capacity, which stemmed from key attributes like high surface area, tunable pore sizes, and the relative ease of modification [41,42,43]. As a result, nanostructured sorbents are now widely regarded as the most advanced and efficient platforms for uranium extraction, fueling a rapid expansion of research in the field. Recent reviews already provide comprehensive coverage of nanostructured materials for uranium extraction, serving as essential resources for in-depth domain knowledge [41,44,45]. While nanoscale materials such as MOFs and COFs have exhibited exceptional adsorption performance, their practical application in large-scale uranium extraction calls for further improvements in three key aspects: reducing synthesis costs, enhancing scalability, and boosting stability under a harsh marine environment.

In recent years, surface-functionalized carbonaceous materials have drawn growing attention as a compelling alternative. This interest is largely due to the advantageous properties of the carbonaceous materials, which include high surface area, exceptional chemical and thermal stability, cost-effectiveness, and tunable porosity [11,41,46,47,48]. Building upon this potential, considerable work has been published detailing the use of various carbon-based adsorbents for uranium recovery [49,50,51]. Among these studies, carbonaceous materials such as activated carbon (AC), graphene oxide (GO), biochar, carbon cloths (CCs), carbon nanotubes (CNTs), and carbon aerogels [52,53] have been extensively investigated. These studies have shown notable uranium sorption performance, offering promising prospects for the large-scale application of these materials [54,55,56,57,58,59,60]. The performance of these carbon materials is significantly enhanced, particularly in terms of capacity and selectivity when modified with specific functional groups, such as amidoxime, phosphoryl, and amine [61,62,63]. Ultimately, these modifications not only enhance uranium recovery efficiency but also contribute to making the overall process more economically viable and environmentally sustainable.

Focused on surface-functionalized carbonaceous materials, this review intends to summarize the published literature, centering on their adsorption capacity, selectivity, and durability for uranium extraction from water. Following this, the adsorption mechanisms for uranium extraction by these surface-modified carbon-based materials are elaborated, with an emphasis on amidoxime, phosphoryl, and amine functional groups commonly used for their surface functionalization. Finally, an outlook for future research priorities is proposed at the end of this review.

2. Uranium Extraction by Functionalized Carbon-Based Materials

2.1. Activated Carbon

Activated carbon (AC), a porous carbonaceous material derived from carbon-rich precursors via activation, is a well-established, cost-effective adsorbent for uranium, owing to its highly developed porosity, large specific surface area (typically 500–3000 m²/g), and accessibility from renewable sources like coconut shells, bamboo, or wood [64]. Its adsorption efficacy is governed by both physical properties (e.g., surface area, pore structure) and surface chemical characteristics [65,66]. However, pristine AC shows limited affinity and selectivity for uranyl ions. Post-activation chemical modification addresses this limitation: grafting functional groups (e.g., amidoxime, -OH, -NH₂) tailors the surface for uranyl-specific binding, converting AC into a specialized adsorbent for uranium extraction [67]. Such modifications have yielded remarkable improvements, with reported uranium adsorption capacities up to 18-fold higher than unmodified AC [68,69].

Among the various functional groups explored, the amidoxime group (-C(NH₂)NOH) has emerged as a particularly potent ligand for uranium extraction. Its notable efficacy stems from the synergistic action of its amine (-NH₂) and hydroxyl (-OH) functionalities, which together form highly stable five-membered chelate rings with the uranyl ion [70,71]. This strong binding mechanism significantly enhances adsorption capacity, selectivity, and uptake kinetics [72,73]. For instance, Lu et al. [31] synthesized amidoxime-modified activated carbon fibers (ACFs-AO) and achieved a higher adsorption capacity of 191.6 mg/g, a nearly three-fold increase over the pristine ACFs (70.52 mg/g) and AO-AC [73], an enhancement directly attributed to the strong chelation afforded by the grafted amidoxime groups. The study also highlighted the critical role of pH, with optimal performance observed between pH 5 and 6. At lower pH values (<4), protonation of the amidoxime groups inhibits complexation, leading to a sharp decrease in capacity [74]. Despite this high capacity, however, a critical trade-off was observed: the system required 48 h to reach equilibrium. This slow kinetics, likely due to diffusional resistance within the complex pore network of the carbon substrate [75], represents a barrier to practical, large-scale applications.

In addition to amidoximes, another prominent class of functional groups involves primary or secondary amines, which enhance uranium adsorption and kinetics by forming stable metal-ligand complexes through nitrogen coordination [76,77]. This approach offers versatility, as different amine structures can be effective across a range of pH values. For instance, Ahmed et al. [78] functionalized AC with trioctylamine (TOA) and achieved efficient uranium recovery from a synthetic acidic sulfate solution (pH 3.4). While the reported capacity was moderate (50 mg/g), the material demonstrated exceptional reusability, achieving a high elution efficiency of 94.88% with a simple acid wash, thereby underscoring the importance of regeneration in developing economically viable processes. Building upon these modification strategies, innovative composite materials have also been developed. By impregnating activated carbon with magnetic nanoparticles (Fe₃O₄) and coating it with a dense polyethyleneimine layer, a novel sorbent was created that combines rapid, magnetically assisted separation with high surface area [79]. This specialized surface, rich with nitrogen and oxygen binding sites, forms strong inner-sphere coordination bonds with uranium ions, enabling the sorbent to capture uranium under optimal conditions.

In summary, amidoxime-functionalized AC offers a high capacity but is often hindered by slow kinetics, whereas amine-based systems can provide faster elution and broader pH, though typically with lower capacities. A central challenge, therefore, lies in balancing a high surface area with dense functionalization, as excessive grafting can block pores and impede ion diffusion. Furthermore, uranium extraction methods depend strongly on concentration, in seawater (ppb), adsorption with high-surface-area sorbents is most effective due to their selectivity at ultra-dilute levels. Whereas in industrial or aqueous effluents (ppm), adsorption becomes less practical because of limited capacity, frequent regeneration, and interference from competing ions. Thus, ion-exchange resins are generally preferred for efficient large-scale recovery. There is a further need to focus on the development of unique treatment technologies to deal with the highly diverse range of concentrations, the design of hierarchical pore structures for faster mass transport, and the creation of bifunctional materials. A summary of the performance of key functionalized AC adsorbents is presented in Table 1.

Table 1. Summary of uranium adsorption performance for representative functionalized activated carbon adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
ACFs-AO (Amidoxime functionalized activated carbon fibers)	191.6	Batch experiment conducted in aqueous solutions; pH 5; 25 °C; 48 h equilibrium time; exhibited good selectivity for U(VI) against competing ions (Ni2+, La3+, Sr2+, Eu3+, Ba2+, Zn2+, Nd3+); >95% desorption was achieved using 1.0 M HNO3 with only minor capacity loss after four regeneration cycles.	[31]
AO-AC (Amidoxime functionalized activated carbon)	66.35	Batch experiment conducted in aqueous solutions; pH 6; 25 °C; 1.3 h equilibrium time; exhibited good selectivity for U(VI) against competing ions (K+, Na+, Ca2+, Mg2+, CO32−, HCO3−, SO42−); desorption was achieved using 0.1 M HCl; 88.32% efficiency after five adsorption.	[73]
TOA-AC (Trioctylamine- AC)	50	Batch experiment conducted at laboratory scale in synthetic sulfuric acid solutions; pH 3.4; 25 °C; 0.5 h equilibrium time; 94.88% elution was achieved using 0.1 M HCl.	[78]
PAF/AC (Polyethyleneimine modified activated carbon/Fe)	115.3	Batch experiment conducted in aqueous solutions; pH 5; 20 °C; 1 h equilibrium time; desorption with 0.25–1 M HCl achieved 90–95% uranium recovery, with only a 12–13% performance loss after five cycles.	[79]

Table 1. Summary of uranium adsorption performance for representative functionalized activated carbon adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
ACFs-AO (Amidoxime functionalized activated carbon fibers)	191.6	Batch experiment conducted in aqueous solutions; pH 5; 25 °C; 48 h equilibrium time; exhibited good selectivity for U(VI) against competing ions (Ni2+, La3+, Sr2+, Eu3+, Ba2+, Zn2+, Nd3+); >95% desorption was achieved using 1.0 M HNO3 with only minor capacity loss after four regeneration cycles.	[31]
AO-AC (Amidoxime functionalized activated carbon)	66.35	Batch experiment conducted in aqueous solutions; pH 6; 25 °C; 1.3 h equilibrium time; exhibited good selectivity for U(VI) against competing ions (K+, Na+, Ca2+, Mg2+, CO32−, HCO3−, SO42−); desorption was achieved using 0.1 M HCl; 88.32% efficiency after five adsorption.	[73]
TOA-AC (Trioctylamine- AC)	50	Batch experiment conducted at laboratory scale in synthetic sulfuric acid solutions; pH 3.4; 25 °C; 0.5 h equilibrium time; 94.88% elution was achieved using 0.1 M HCl.	[78]
PAF/AC (Polyethyleneimine modified activated carbon/Fe)	115.3	Batch experiment conducted in aqueous solutions; pH 5; 20 °C; 1 h equilibrium time; desorption with 0.25–1 M HCl achieved 90–95% uranium recovery, with only a 12–13% performance loss after five cycles.	[79]

2.2. Graphene Oxide

Graphene oxide (GO) is a two-dimensional nanomaterial derived from the chemical oxidation and exfoliation of graphite. It has been widely studied for its large pore structure, high surface area, robust mechanical properties, and abundance of oxygen-containing functional groups [80,81,82,83]. These oxygen-containing groups enhance its baseline adsorption capacity, making it a promising candidate for removing uranium ions from aqueous solutions. However, to achieve good selectivity, efficiency, and reusability, GO is often modified with functional groups to create advanced composites [84,85,86]. For instance, Zhu et al. synthesized poly-amidoxime-hydroxamic acid functionalized graphene oxide (pAHA-GO), which achieved a considerable adsorption capacity of 178.7 mg/g [87]. This high performance is attributed to the amidoxime groups, which form strong coordination bonds with uranium ions through both nitrogen and oxygen atoms, thereby imparting a high affinity and selectivity for uranium [87,88].

As shown in current research, amidoxime functionalization has proven to be a premier strategy for enhancing the uranium affinity of GO, particularly in the presence of competing ions [89,90]. As illustrated in Figure 2a, amidoxime-based ligands typically interact with metal ions through three distinct coordination modes: direct coordination with the oxygen atom, simultaneous binding to both oxygen and nitrogen atoms to form a stable chelate ring, or η²-type coordination with the N–O bond [91]. This versatile binding capability translates directly into enhanced performance. For example, graphene oxide grafted with sodium carboxymethyl cellulose and amidoxime groups (GO-CMC-AO) exhibited impressive selectivity for uranium over coexisting ions like Nd³⁺, La³⁺, and Sr²⁺ [92]. This preference is primarily due to the strong and specific affinity between the grafted –NH₂ and –N–OH moieties and the target uranium species [92,93].

Beyond simple grafting, nanoscale strategies offer another avenue for developing high-performance GO-based materials. Among these, cycloaddition reactions serve as a key bi-functionalization method, enabling the covalent modification of GO with multiple active groups [94,95,96]. For example, diaminomaleonitrile (DM) is an ideal dienophile because its amino groups coordinate stably with U(VI), while its nitrile groups can be converted into amidoxime, further improving selectivity [97,98,99]. A post-functionalized graphene oxide composite developed through this method (GO-DM-AO) demonstrated exceptional uranium (VI) adsorption capacity. As shown in Figure 2b,c, concentration-dependent experiments revealed that GO-DM-AO achieved an extraordinary capacity of 935 mg/g, a 209% improvement over pristine GO (447.3 mg/g). This enhancement is attributed to both the introduction of additional binding sites and the activation of previously inert sites on the GO sheets via the cycloaddition reaction. However, the adsorption capacity was observed to decline with increasing salt concentration [100]. The proposed extraction mechanism, illustrated in Figure 2d, involves a combination of chelation by oxime groups, coordination by amino groups, and ion diffusion across the GO-DM-AO surface, where the GO sheet provides the structural support for efficient uranium capture [101].

Additionally, the incorporation of bifunctional groups has been shown to synergistically enhance the binding sites and improve the stability of GO-based adsorbents. This concept was demonstrated by Alexandratos, who developed amidoxime/amine-bifunctional fibers for uranium extraction from real seawater [102]. The synergy between amine and amidoxime functionalities is particularly effective, as it is known to enhance the stability of uranyl complexes [103,104]. This principle was leveraged by Yao et al. [105], who developed a composite (Cu-BDC-NH₂@GO-A) that achieved an exceptional uranium adsorption capacity of 1078.4 mg/g in simulated seawater, attributed to the synergistic interaction between amino and carboxylate groups. Similarly, biopolymers like chitosan, with their abundant –NH₂ and –OH groups, can be combined with GO to form highly effective composites [106,107]. For instance, a graphene-chitosan aerogel (GO-CS) demonstrated high efficiency in simulated seawater, with an adsorption capacity of 384.6 mg/g [108]. This performance was further enhanced to 561.09 mg/g by incorporating ZnO, where interactions between GO and zinc ions formed strong coordination bonds that improved the structural stability and adsorption properties of the aerogel [109]. Furthermore, regeneration tests using 0.1 mol/L solutions of citric acid, HCl, HNO₃, EDTA, water, and NaOH showed that acidic eluents achieved higher desorption efficiency than neutral or alkaline ones, with HCl providing the highest desorption rate (>98%). Therefore, 0.1 mol/L HCl was chosen as the eluent for subsequent experiments. Notably, both adsorption and desorption efficiencies remained nearly unchanged after five cycles, confirming that the CS-GO-DO/ZnO aerogel is stable, reusable, and effective [109,110,111].

Figure 2. (a) Three possible bonding motifs between amidoxime and metal ions: I, oxygen; II, chelate; III, η². Reprinted with permission from Ref. [91]. Copyright 2016, American Chemical Society, (b) Effects of various U(VI) concentrations on adsorption properties of GO—based materials, (c) removal rate of GO—based materials, (d) Schematic of the adsorption of U(VI) onto the surface of GO-DM-AO composites. Reprinted with permission from Ref. [101]. Copyright 2019, Elsevier B.V. All rights reserved.

Figure 2. (a) Three possible bonding motifs between amidoxime and metal ions: I, oxygen; II, chelate; III, η². Reprinted with permission from Ref. [91]. Copyright 2016, American Chemical Society, (b) Effects of various U(VI) concentrations on adsorption properties of GO—based materials, (c) removal rate of GO—based materials, (d) Schematic of the adsorption of U(VI) onto the surface of GO-DM-AO composites. Reprinted with permission from Ref. [101]. Copyright 2019, Elsevier B.V. All rights reserved.

Alongside amidoximes and amines, phosphoryl groups represent another potent functionalization strategy, owing to their strong chelating ability toward U(VI) ions [112]. While pristine GO contains hydroxyl and carboxyl groups that can coordinate with uranium, its practical application is often hindered by its tendency to aggregate in aqueous solutions [113,114]. Covalent functionalization helps overcome this issue by improving mechanical strength [115]. A recent study on a phosphoryl-functionalized graphene (PG) material highlighted this potential. The material provided highly accessible and selective binding sites, improving kinetics that removed approximately 77% of uranium within just 5 min. The PG adsorbent demonstrated capacities of 316 mg/g in distilled water and an impressive 117.8 mg/g in simulated seawater, showcasing its resilience against high concentrations of competing ions. Thermodynamic analysis confirmed that the adsorption process was endothermic and entropy-driven, indicating a favorable and spontaneous interaction between U(VI) and the PG surface [116].

In conclusion, the versatility of graphene oxide as a 2D scaffold has enabled the development of uranium adsorbents with capacities higher than those based on conventional activated carbon. Amidoxime-functionalized systems like GO-DM-AO have set a high benchmark for capacity and speed in laboratory settings and show good performance as compared to AC. Concurrently, synergistic composites incorporating MOFs or biopolymers (e.g., Cu-BDC-NH₂@GO-A, GO-CS/ZnO) have pushed these capacity limits even further. For practical application under harsh seawater conditions, future research could focus on optimizing material properties. Key strategies will likely include the design of hierarchical porous structures to improve mass transfer and the use of defect engineering to create additional binding sites. A summary of the adsorption capacities of various graphene oxide-based adsorbents is presented in Table 2.

Table 2. Summary of uranium adsorption performance for representative functionalized graphene oxide adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
PAHA-GO (Polyamidoxime-hydroxamic acid-GO)	178.7	Batch experiment conducted in aqueous solution containing Na+, K+, Ca2+, Mg2+, V5+, Cu2+, Pb2+, La3+, Ce4+, Ni2+, Co2+, Zn2+, Fe3+ ions; pH 3.6; 25 °C; 10 h equilibrium time; demonstrated efficient U(VI) extraction; efficiency retained >97% under 0.5 M HCl; good performance over four cycles.	[87]
GO-DM-AO (GO-diaminomaleonitrile-amidoxime)	935	Lab experiment conducted in artificial seawater containing Al3+, Ba2+, Na+, Zn2+, Cu2+, K+, Ni2+, Mg2+, Ca2+, Sr2+, Fe3+ ions; pH 8; 25 °C; 0.5 h equilibrium time; desorption was achieved using 0.2 M citric acid; 90% capacity was retained after five cycles; demonstrated efficient uranium selectivity.	[101]
Cu-BDC-NH2@GO-A (Copper-2-aminobenzene-1,4-dicarboxylate MOF/GO-Acid-modified)	1078.4	Batch experiment conducted in simulated seawater containing V+, Fe3+, Ni2+, Co2+, Zn2+, Na+, Mg2+, K+ ions; pH 8; 25 °C; demonstrated efficient uranium extraction against the competing ions.	[105]
GO-CS (Graphene oxide-chitosan Aerogel)	384.6	Batch experiment conducted in simulated seawater containing Sr2+, Ni2+, Co2+, Zn2+, La3+, Nd3+, Sm3+, Gd3+, Yb3+ ions; pH 8.3; 25 °C; 1 h equilibrium time; demonstrated efficient U recovery; 0.1 M HNO3 was used as the desorption agent; capacity retention was 93–87% after three cycles.	[108]
CS-GO-DO/ZnO (Chitosan-GO-Amidoximated Diaminomaleonitrile-Zinc Oxide)	561.09	Batch experiment conducted in a 96% pure uranium solution containing Na+, K+, Ca2+, Mg2+, Cu2+, Mn2+, Ce3+, La3+, Eu3+ ions; pH 6; 25 °C; 6.6 h equilibrium time; demonstrated efficient U recovery; 0.1 M HCl was used as the desorption agent; capacity retention was 98% after five cycles.	[109]
PG (Phosphoryl-functionalized graphene)	316	Batch experiment conducted in distilled water containing Na+, Mg2+, Ca2+, K+, Sr2+, Fe3+, VO43−, Cl−, SO42− ions; pH 7; 25 °C; 3 h equilibrium time; 91.76% efficiency after four cycles	[116]
rGO-PPy-Fe0 (reduced-GO-polypyrrole-zero-valent iron)	384.24	Lab experiment conducted in simulated seawater containing Na+, Ba2+, Mg2+, Zn2+, Cu2+, Cr3+, Ca2+, Al3+, V5+ competing ions; pH 8; 25 °C; 1.6 h equilibrium time; demonstrated efficient uranium recovery against coexisting ions.	[117]

Table 2. Summary of uranium adsorption performance for representative functionalized graphene oxide adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
PAHA-GO (Polyamidoxime-hydroxamic acid-GO)	178.7	Batch experiment conducted in aqueous solution containing Na+, K+, Ca2+, Mg2+, V5+, Cu2+, Pb2+, La3+, Ce4+, Ni2+, Co2+, Zn2+, Fe3+ ions; pH 3.6; 25 °C; 10 h equilibrium time; demonstrated efficient U(VI) extraction; efficiency retained >97% under 0.5 M HCl; good performance over four cycles.	[87]
GO-DM-AO (GO-diaminomaleonitrile-amidoxime)	935	Lab experiment conducted in artificial seawater containing Al3+, Ba2+, Na+, Zn2+, Cu2+, K+, Ni2+, Mg2+, Ca2+, Sr2+, Fe3+ ions; pH 8; 25 °C; 0.5 h equilibrium time; desorption was achieved using 0.2 M citric acid; 90% capacity was retained after five cycles; demonstrated efficient uranium selectivity.	[101]
Cu-BDC-NH2@GO-A (Copper-2-aminobenzene-1,4-dicarboxylate MOF/GO-Acid-modified)	1078.4	Batch experiment conducted in simulated seawater containing V+, Fe3+, Ni2+, Co2+, Zn2+, Na+, Mg2+, K+ ions; pH 8; 25 °C; demonstrated efficient uranium extraction against the competing ions.	[105]
GO-CS (Graphene oxide-chitosan Aerogel)	384.6	Batch experiment conducted in simulated seawater containing Sr2+, Ni2+, Co2+, Zn2+, La3+, Nd3+, Sm3+, Gd3+, Yb3+ ions; pH 8.3; 25 °C; 1 h equilibrium time; demonstrated efficient U recovery; 0.1 M HNO3 was used as the desorption agent; capacity retention was 93–87% after three cycles.	[108]
CS-GO-DO/ZnO (Chitosan-GO-Amidoximated Diaminomaleonitrile-Zinc Oxide)	561.09	Batch experiment conducted in a 96% pure uranium solution containing Na+, K+, Ca2+, Mg2+, Cu2+, Mn2+, Ce3+, La3+, Eu3+ ions; pH 6; 25 °C; 6.6 h equilibrium time; demonstrated efficient U recovery; 0.1 M HCl was used as the desorption agent; capacity retention was 98% after five cycles.	[109]
PG (Phosphoryl-functionalized graphene)	316	Batch experiment conducted in distilled water containing Na+, Mg2+, Ca2+, K+, Sr2+, Fe3+, VO43−, Cl−, SO42− ions; pH 7; 25 °C; 3 h equilibrium time; 91.76% efficiency after four cycles	[116]
rGO-PPy-Fe0 (reduced-GO-polypyrrole-zero-valent iron)	384.24	Lab experiment conducted in simulated seawater containing Na+, Ba2+, Mg2+, Zn2+, Cu2+, Cr3+, Ca2+, Al3+, V5+ competing ions; pH 8; 25 °C; 1.6 h equilibrium time; demonstrated efficient uranium recovery against coexisting ions.	[117]

2.3. Carbon Cloths

Carbon cloth (CC) is a versatile woven material predominantly composed of carbon fibers, which constitute at least 92–99% of its weight. This material is widely recognized for its exceptional properties, including high electrical conductivity, robust mechanical stability, excellent chemical resistance, and long-term durability, making it an ideal substrate for adsorption applications [118]. The production of carbon cloth begins with the manufacture of carbon fibers, typically derived from polyacrylonitrile (PAN) precursors. This process involves a stabilization step through oxidation at temperatures between 200 °C and 300 °C, which is subsequently followed by carbonization in an inert atmosphere at 1500 °C to 2000 °C. To further augment its performance, an optional graphitization step can be performed at temperatures ranging from 2000 °C to 3000 °C, which enhances electrical conductivity by improving the alignment of the carbon layers [119]. Moreover, the surface of carbon cloths can be modified with various functional groups, yielding lightweight, high-strength materials with a large surface area perfectly suited for extracting metals even under harsh environmental conditions [120].

Another application of this platform is electrosorption, a technique that utilizes an applied electric field to overcome the kinetic and capacity limitations often encountered in traditional adsorption processes [121,122]. For instance, by functionalizing a carbon cloth with amidoxime groups (CC-AO), researchers have successfully demonstrated a synergistic combination of strong surface chelation with electrochemical mechanisms. This dual-action approach resulted in an extraordinary uranium adsorption capacity of 1235 mg/g from a simulated acidic solution, accompanied by a remarkable selectivity of 94.1% against a suite of competing ions [123]. This enhanced performance stems from a process that involves not only physical adsorption but also electrochemical interactions, such as electron transfer, which significantly boost uranium uptake and improve separation efficiency [124].

The associated mechanism of amidoxime functionalized carbon cloth for uranium extraction is illustrated in Figure 3. Under specific conditions, such as an applied frequency of 800 Hz, the CC-AO material exhibited a substantially greater affinity for the uranyl cation (UO₂²⁺) over competing metal ions [123]. This pronounced preference is attributed to the strong chelation between the amidoxime groups and UO₂²⁺, a bond that provides both thermodynamic stability and kinetic favorability [123]. In a similar approach, amidoxime-functionalized carbon paper (CP-AO) achieved a high uranium adsorption of 1401 mg/g within 10 h under acidic conditions, far outperforming its unmodified counterpart. The applied electric field in this system aided the reduction in uranyl ions and their subsequent deposition as neutral oxides. The selectivity of CP-AO toward UO₂²⁺ was examined in the presence of competing ions. In the CP system, UO₂²⁺ removal was limited to 20%, reflecting its weaker affinity. By contrast, CP-AO achieved 90.4% removal, outperforming all other ions. This performance was attributed to the strong binding affinity of amidoxime groups for UO₂²⁺ and the optimized square wave periodic voltage (SWPV) conditions. Desorption experiments further confirmed the higher recovery of UO₂²⁺ compared to coexisting ions, underscoring the excellent selectivity of CP-AO. However, despite these impressive capacities, a clear limitation of the CP-AO electrode is the sharp decline in UO₂²⁺ removal once the pH falls below 3. This is a significant drawback, as many actual spent-fuel liquors operate at a pH well below 2. To address this gap, one proposed solution is to co-functionalize the amidoxime layer with acid-tolerant chelators, though direct studies on combined amidoxime and phosphonic systems under such highly acidic conditions remain limited [125,126].

Figure 3. The synthesis process of carbon cloths through electrosorption and the conditions for uranium extraction. Reprinted with permission from Ref. [123]. Copyright 2021, Elsevier B.V. All rights reserved.

Figure 3. The synthesis process of carbon cloths through electrosorption and the conditions for uranium extraction. Reprinted with permission from Ref. [123]. Copyright 2021, Elsevier B.V. All rights reserved.

Furthermore, the inherent conductivity of carbon cloths makes them ideal electrodes for capacitive deionization (CDI), an eco-friendly technology particularly suited for treating dilute waste streams [127]. In a notable application, Wang et al. [128] developed composite cathodes made of 2D transition metal sulfide-graphene oxide (TMDs-GO) for CDI systems, achieving a significant enhancement in uranium removal. Specifically, by fabricating a MoS₂-graphene oxide composite on a carbon cloth substrate (MoS₂-GO/CC), they created a specialized system for uranium remediation. Leveraging both electric double-layer capacitance and pseudocapacitive interactions, this cathode achieved a uranium capacity of 74.38 mg/g from a low-concentration (5 mg/L) feed. MoS₂-GO/CC showed strong uranium affinity despite interference from Fe²⁺, Na⁺, Mg²⁺, and Mn²⁺. Fe²⁺ minimally affected removal, likely forming Fe–U complexes, while Mn²⁺ slightly reduced adsorption due to its higher concentration occupying adsorption sites on MoS₂-GO/CC. This modest capacity, achieved via electrosorption, is highly effective for its intended application, demonstrating the platform’s adaptability for targeted, low-level decontamination tasks.

As shown above, functionalized carbon cloths exhibited a high uranium adsorption capacity and strong selectivity, making them highly efficient. However, they tended to show slow kinetics and only moderate reusability. Future research should focus on accelerating the kinetics and improving the pH stability of materials, while efforts for magnetic composites could benefit from structural optimization to enhance both capacity and stability. A comprehensive summary of the performance of these functionalized carbon cloth systems is provided in Table 3.

Table 3. Summary of uranium adsorption performance for representative functionalized carbon cloth adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
CC-AO (Amidoxime-modified carbon cloth)	1235	Lab scale experiment conducted in acidic uranium mine wastewater containing Zr4+, Fe3+, Pb2+, Ni2+, Ca2+, Cs+, Mn2+, Sr2+, Cu2+, Mg2+ competing ions; pH 2; 25 °C; 12.5 h equilibrium time; 0.1 M HNO3 was used as the desorption agent; 68.6% UO22+ uptake retained after seven cycles.	[123]
CP-AO (Amidoxime-modified carbon paper)	1401	Lab scale experiment conducted in acidic uranium-containing wastewater with Zr4+, Fe3+, Pb2+, Ca2+, Ni2+, Cs+, Mn2+, Sr2+, Cu2+, Mg2+ competing ions; pH 3; 10 h; 0.1 M HNO3 as desorption agent; showed 87.3% desorption after seven cycles.	[126]
MoS2-GO/CC (Sulfide-GO- composites-CC)	74.38	Lab-scale experiment conducted in simulated uranium-containing wastewater with Fe2+, Na+, Mg2+, Mn2+ competing ions (initial concentration: 5 ppm); pH 4; 25 °C; 12 h; 93.2% capacity retained after 10 cycles; demonstrated efficient uranium recovery against coexisting ions.	[128]

Table 3. Summary of uranium adsorption performance for representative functionalized carbon cloth adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
CC-AO (Amidoxime-modified carbon cloth)	1235	Lab scale experiment conducted in acidic uranium mine wastewater containing Zr4+, Fe3+, Pb2+, Ni2+, Ca2+, Cs+, Mn2+, Sr2+, Cu2+, Mg2+ competing ions; pH 2; 25 °C; 12.5 h equilibrium time; 0.1 M HNO3 was used as the desorption agent; 68.6% UO22+ uptake retained after seven cycles.	[123]
CP-AO (Amidoxime-modified carbon paper)	1401	Lab scale experiment conducted in acidic uranium-containing wastewater with Zr4+, Fe3+, Pb2+, Ca2+, Ni2+, Cs+, Mn2+, Sr2+, Cu2+, Mg2+ competing ions; pH 3; 10 h; 0.1 M HNO3 as desorption agent; showed 87.3% desorption after seven cycles.	[126]
MoS2-GO/CC (Sulfide-GO- composites-CC)	74.38	Lab-scale experiment conducted in simulated uranium-containing wastewater with Fe2+, Na+, Mg2+, Mn2+ competing ions (initial concentration: 5 ppm); pH 4; 25 °C; 12 h; 93.2% capacity retained after 10 cycles; demonstrated efficient uranium recovery against coexisting ions.	[128]

2.4. Biochar

Biochar is a carbon-rich material produced through the pyrolysis of organic biomass, such as wood chips, leaves, and husks, in an oxygen-limited environment. This thermochemical process involves heating the biomass to temperatures typically ranging from 200 °C to 700 °C in the absence of oxygen, which prevents combustion and results in the formation of a stable, carbonaceous solid [129]. Owing to its sustainable and cost-effective nature, biochar has garnered significant attention in environmental remediation, particularly for the adsorption of valuable metals like uranium [130]. As a porous material, it has emerged as a promising adsorbent for metal recovery due to its high surface area, tunable porosity, and diverse surface functional groups. To further augment its adsorption efficiency, biochar can be modified through various techniques, including mineral impregnation, nanoscale-metal incorporation, and surface modification [131].

Numerous advances in biochar modification have centered on developing materials that enhance metal binding by introducing specific functional groups. Consistent with trends observed across other carbon platforms, amidoxime functionalization provides a reliable route to improve the uranium affinity of biochar. For example, grafting polyamidoxime (PAO) onto a biochar substrate derived from fungal mycelium (PAO-BS) yielded a material with a respectable adsorption capacity of 211.35 mg/g and strong selectivity for uranium over most competing ions in simulated seawater [132]. The mechanism, illustrated in Figure 4a, involves the chemical crosslinking of PAO onto the carbonized hyphae. This composite architecture leverages the hydroxyl (-OH) and amine (-NH₂) groups of PAO, which coordinate with uranyl ions (UO₂²⁺) by donating lone electron pairs to form stable chelation bonds. Consequently, the carbonized mycelium enhances water permeability while the PAO component facilitates efficient and selective uranium adsorption.

To further improve both adsorption capacity and selectivity, researchers have explored the development of bifunctional composite adsorbents. Recently, Zhou et al. [133] synthesized a highly efficient bifunctional composite, designated PEA-CTS@WBC, by chemically crosslinking polyethanolamine amidoxime (rich in –NH₂ and –C(NH₂)=NOH groups) and chitosan onto winter melon-derived biochar. This was achieved via glutaraldehyde-mediated condensation, resulting in a composite that exhibited high porosity, excellent thermal stability, and a strong affinity for U(VI). Ultimately, this material achieved a maximum adsorption capacity of 552.75 mg/g and demonstrated both good selectivity and reusability.

Figure 4. (a) Mechanism for the chemical crosslinking of PAO onto the carbonized hyphae. Reprinted with permission from Ref. [132]. Copyright 2023, Elsevier Ltd. All rights reserved. (b) synthesis procedure and adsorption mechanism of U(VI) by PEA-CTS@WBC. Reprinted with permission from Ref. [134]. Copyright 2024, Elsevier Ltd. All rights reserved.

Figure 4. (a) Mechanism for the chemical crosslinking of PAO onto the carbonized hyphae. Reprinted with permission from Ref. [132]. Copyright 2023, Elsevier Ltd. All rights reserved. (b) synthesis procedure and adsorption mechanism of U(VI) by PEA-CTS@WBC. Reprinted with permission from Ref. [134]. Copyright 2024, Elsevier Ltd. All rights reserved.

While amine-based composites show promise, phosphate-functionalized biomass serves as a green and effective material for uranium extraction [135]. Wang et al. [134] developed a microbial-etched cotton straw biochar (CCS-P) through hydrothermal carbonization and subsequent calcium phosphate modification. As shown in Figure 4b, this advanced material achieved a remarkable adsorption capacity of 590.8 mg/g and maintained 96% efficiency over six cycles, even in the presence of most coexisting ions. Similarly, Hu et al. [136] developed phosphate-functionalized biochar (PBs) from bamboo sawdust, which exhibited a uranium (VI) adsorption capacity of 229.2 mg/g at pH 4.0. The slight effect of ionic strength confirmed that inner-sphere complexation governed the binding, reflecting high affinity. In terms of selectivity, U(VI) uptake was barely affected by Cs⁺, Sr²⁺, Co²⁺, Cu²⁺, and Ni²⁺, whereas Fe³⁺ and Eu³⁺ significantly reduced sorption owing to stronger competition. Taken together, these results demonstrate that PBs exhibit strong affinity and favorable selectivity toward U(VI), mainly due to stable U–O–P bonding and the effective Lewis acid–base interaction between UO₂²⁺ and phosphate groups. The adsorption mechanism for these phosphate-modified materials is multifaceted, involving coordination with P=O oxygen atoms, electrostatic attraction between negatively charged biochar surfaces and cationic U(VI) species, and cation-π interactions with the biochar’s π-electron-rich structure [134,137]. Indeed, molecular dynamics simulations have confirmed that phosphoryl groups demonstrate a particularly strong and preferential affinity for uranyl ions over other oxygen-containing functionalities [116].

In summary, the strategic modification of biochar has yielded some efficient adsorbents for uranium extraction. While amidoxime functionalization offers a proven pathway to high-capacity materials, the novel strategy of combining microbial pretreatment with phosphate mineralization has produced adsorbents with feasible capacities and fast kinetics. However, for these advanced materials to become industrially viable, future research would focus on improving their efficiency at reasonable pH and temperatures. Furthermore, a thorough evaluation of the life-cycle cost and scalability of these complex, multi-step synthesis routes will be crucial for determining their practical applicability. A summary of the performance of representative functionalized biochar adsorbents is presented in Table 4.

Table 4. Summary of uranium adsorption performance for representative functionalized biochar adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
CHMBC-PAO (Polyamidoxime-coated coconut haustorium-derived magnetic biochar)	810.22	Lab experiment conducted in nuclear wastewater with Al3+, Ba2+, Co2+, Cr3+, Fe3+, Ni2+, Mg2+, Sr2+, Zn2+, Mn2+, Cd2+ competing ions using coconut haustorium as carbon source; pH 5; 25 °C; 12 h; 1 M Na2CO3 used as desorption agent; after five cycles the adsorption and adsorption capacity decreased, around 3.54% per cycle.	[62]
PAO-BS (Polyamidoxime-Loaded Biochar Sphere)	211.35	Batch experiment conducted in simulated seawater with V5+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cr3+, Mn2+, Al3+ ions using fungal mycelium as carbon source; pH 6; 25 °C; 10 h; 1M Na2CO3 used as desorption agent; 22.5% adsorption loss after the fifth cycle.	[132]
PEA-CTS@WBC (Polyethanolamine amidoxime modified winter melon and chitosan-derived biochar)	552.75	Lab experiment conducted in nuclear wastewater with K+, Cs+, Mg2+, Zn2+, Sr2+, Ca2+, La3+, Gd3+, Sm3+, Eu3+, and Ce3+ ions using winter melon as a carbon source; pH 5; 25 °C; 3 h; 1 M HCl used as a desorption agent; desorption rate decreased by 5.3% after five consecutive cycles.	[133]
CCS-P (Microbial etch cotton straw with phosphate doping)	590.8	Lab experiment conducted in radioactive wastewater with K+, Cu2+, Mn2+, Cs+, Sr2+, and Fe3+ competing ions using cotton straw as a carbon source; pH 3.5; 1.6 h; 1 M Na2CO3 and 0.1 M H2O2 used as desorption agents; desorption rate decreased by 11% after six consecutive cycles.	[134]
PBs (Phosphate-functionalized biochars)	229.2	Lab experiment conducted in simulated aqueous solution with Cs+, Sr2+, Co2+, Cu2+, Fe3+, Ni2+, Eu3+ ions using bamboo sawdust as carbon source; pH 4; 25 °C; 8 h; 0.1 M Na2CO3 used as desorption agent; sorption capacity decreased by only 7.47% after six cycles.	[136]

Table 4. Summary of uranium adsorption performance for representative functionalized biochar adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
CHMBC-PAO (Polyamidoxime-coated coconut haustorium-derived magnetic biochar)	810.22	Lab experiment conducted in nuclear wastewater with Al3+, Ba2+, Co2+, Cr3+, Fe3+, Ni2+, Mg2+, Sr2+, Zn2+, Mn2+, Cd2+ competing ions using coconut haustorium as carbon source; pH 5; 25 °C; 12 h; 1 M Na2CO3 used as desorption agent; after five cycles the adsorption and adsorption capacity decreased, around 3.54% per cycle.	[62]
PAO-BS (Polyamidoxime-Loaded Biochar Sphere)	211.35	Batch experiment conducted in simulated seawater with V5+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cr3+, Mn2+, Al3+ ions using fungal mycelium as carbon source; pH 6; 25 °C; 10 h; 1M Na2CO3 used as desorption agent; 22.5% adsorption loss after the fifth cycle.	[132]
PEA-CTS@WBC (Polyethanolamine amidoxime modified winter melon and chitosan-derived biochar)	552.75	Lab experiment conducted in nuclear wastewater with K+, Cs+, Mg2+, Zn2+, Sr2+, Ca2+, La3+, Gd3+, Sm3+, Eu3+, and Ce3+ ions using winter melon as a carbon source; pH 5; 25 °C; 3 h; 1 M HCl used as a desorption agent; desorption rate decreased by 5.3% after five consecutive cycles.	[133]
CCS-P (Microbial etch cotton straw with phosphate doping)	590.8	Lab experiment conducted in radioactive wastewater with K+, Cu2+, Mn2+, Cs+, Sr2+, and Fe3+ competing ions using cotton straw as a carbon source; pH 3.5; 1.6 h; 1 M Na2CO3 and 0.1 M H2O2 used as desorption agents; desorption rate decreased by 11% after six consecutive cycles.	[134]
PBs (Phosphate-functionalized biochars)	229.2	Lab experiment conducted in simulated aqueous solution with Cs+, Sr2+, Co2+, Cu2+, Fe3+, Ni2+, Eu3+ ions using bamboo sawdust as carbon source; pH 4; 25 °C; 8 h; 0.1 M Na2CO3 used as desorption agent; sorption capacity decreased by only 7.47% after six cycles.	[136]

2.5. Carbon Nanotubes

Carbon nanotubes (CNTs) are cylindrical nanostructures composed of carbon atoms arranged in a hexagonal lattice. The primary method for their production is chemical vapor deposition (CVD), a process where a carbon-containing gas decomposes at high temperatures (700–1000 °C) in the presence of a metal catalyst, allowing carbon atoms to deposit onto catalyst particles and self-assemble into nanotubes [138]. Owing to their high surface area, exceptional chemical stability, and inherent adsorption efficiency, CNTs have emerged as highly effective materials for metal extraction applications, including uranium recovery [139]. However, to fully explore their potential, strategic surface modifications with specific functional groups are often necessary to enhance their adsorption capabilities and kinetics [140,141,142,143]. For instance, amide-functionalized multi-walled carbon nanotubes (MWCNTs) demonstrate improved metal ion adsorption because the amide groups (-CONH₂) on their surface readily coordinate with metal ions like UO₂²⁺ through lone electron pairs on their oxygen and nitrogen atoms, forming stable inner-sphere complexes [144,145,146]. Among various ligands, tridentate diglycolamides (DGAs) [147] are particularly effective due to their strong metal-binding affinity and eco-friendly properties, leading to the development of adsorbents like DGA-MWCNTs, which exhibit a maximum uranium adsorption capacity of 133.74 mg/g [147,148,149].

Among the various modification techniques, graft polymerization is a particularly effective approach for introducing functional groups onto material surfaces. For the modification of MWCNTs, two common strategies are chemical grafting and plasma-assisted modification [150,151]. The chemical grafting route, however, often involves high temperatures, initiators, and catalysts, which can complicate the process and introduce impurities. In contrast, plasma-based techniques rely on electron beam activation to generate surface free radicals, enabling the formation of high-purity grafted polymers under milder conditions [152]. Offering a simpler and more efficient alternative, radiation-induced graft polymerization (RIGP) employs γ-rays to generate free radicals, providing deeper material penetration and more effective surface etching. This technique can modify not only the surface but also the bulk of the material, making it highly suitable for large-scale applications [153]. For example, Wu et al. synthesized amidoxime-functionalized MWCNTs (AO-MWCNTs) via RIGP, achieving a uranium adsorption capacity of 67.9 mg/g within 60 min [154]. Similarly, Zhuang and Wang [63] demonstrated that polyamidoxime-functionalized CNTs (PAO@CNT) reached a significantly higher capacity of 247 mg/g. The optimal adsorption pH was found to be 4, as efficiency sharply declines at lower pH values due to competition from excess H⁺ ions for active sites, and at higher pH values due to the formation of soluble uranyl-hydroxy-carbonate complexes in solution [155,156].

Another prominent strategy involves the use of amine-functionalized materials, which leverage the strong coordination affinity of amine groups covalently bonded to the CNT surface to significantly improve chemical reactivity and interaction with target ions [157,158,159]. In one such study, Alijani et al. [160] explored the adsorption of UO₂²⁺ ions using MWCNTs functionalized with both ethylene diamine and triazine. Through batch sorption experiments at an optimized pH of 2.0, they achieved maximum adsorption capacities of 69.44 mg/g and 74.62 mg/g, respectively. The adsorption process was found to follow the pseudo-second-order kinetic model, which suggests a chemisorption-controlled process. To further clarify the mechanisms, uranium extraction in these systems typically involves the formation of inner-sphere complexes between UO₂²⁺ and surface functional groups. Donor atoms from ligands like phosphonate or amidoxime coordinate in monodentate or bidentate modes, ensuring the selective and stable binding of uranyl ions [161,162]

In a compelling demonstration of tailored functionalization, phosphoryl groups have emerged as a superior alternative for achieving both high performance and exceptional stability. The strong electron-donating character of the P=O bond leads to robust coordination with uranyl ions, enabling efficient extraction even from solutions with trace concentrations [163,164,165]. For instance, phosphoryl-functionalized MWCNTs (PS-MWCNTs), featuring P=O and P–O–C groups, demonstrated strong coordination with U(VI) ions [166]. Although performance can diminish under highly acidic (pH 1–3) or higher pH (pH > 5) conditions due to competition from H₃O⁺ ions or the formation of stable hydrolyzed uranium species, respectively [167,168]. Notably, the desorption efficiency of PS-MWCNTs increased with rising HCl concentration, reaching 96.9% at 1 mol/L, after which it remained stable. The U(VI) removal efficiency was 98.5% in the first cycle and 90.2% after eight cycles, demonstrating strong regeneration performance. Compared to other modified MWCNTs, PS-MWCNTs exhibited good uranium uptake across a broader pH range and attained adsorption equilibrium within 10 min, highlighting favorable reusability and kinetics for uranium recovery [166].

In conclusion, functionalized CNTs represent a promising platform for uranium extraction. To advance their practical application, future efforts should focus on optimizing material structures by increasing surface area or porosity and on further enhancing selectivity and capacity through co-functionalization with high-affinity ligands. Ultimately, testing under realistic conditions, such as in actual seawater or multi-ion industrial streams, will be essential to validate and boost their practical applicability. A summary of the performance of representative functionalized CNT adsorbents is presented in Table 5.

Table 5. Summary of uranium adsorption performance for representative functionalized carbon nanotube adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
PAO/CNT (Polyamidoxime-carbon nanotube)	247	Lab-scale experiment conducted in uranium-containing solution; pH 4; 20 °C; 2 h equilibrium time; demonstrated efficient uranium recovery.	[63]
DGA-MWCNTs (Diglycolamide-multi-walled CNTs)	133.74	Lab-scale experiment conducted in uranyl nitrate stock solution; 25 °C; 3 h equilibrium time; demonstrated efficient uranium recovery.	[147]
AO-MWCNTs (Amidoxime-MWCNTs)	67.9	Lab-scale experiment conducted in aqueous solution; pH 5; 25 °C; 1 h equilibrium time; demonstrated efficient recovery.	[154]
Ta-CNTs (Triazine- CNTs)	74.62	Lab experiment conducted in aqueous solution; pH 2; 1.5 h equilibrium time; demonstrated efficient uranium recovery.	[160]
PS-MWCNTs (Phosphoryl-multiwalled CNTs)	806.45	Lab-scale experiment conducted in aqueous solution; pH 5; 25 °C; 0.5 h equilibrium time; 1 M HCl used as desorption agent; adsorbent maintained 90.2% efficiency after eight cycles.	[166]
CS-CNTs (Chitosan-based composite -CNTs)	126.7	Lab-scale experiment conducted in wastewater with Th4+, Eu3+, La3+, Cs+, Yb3+ ions; pH 4; 25 °C; 1 h equilibrium time; 0.2 M acidified EDTA used as desorption agent; maintained 87.9% efficiency after five cycles.	[169]

Table 5. Summary of uranium adsorption performance for representative functionalized carbon nanotube adsorbents.

Adsorbent	Qmax (mg/g)	Adsorption Conditions and Key Findings	Ref.
PAO/CNT (Polyamidoxime-carbon nanotube)	247	Lab-scale experiment conducted in uranium-containing solution; pH 4; 20 °C; 2 h equilibrium time; demonstrated efficient uranium recovery.	[63]
DGA-MWCNTs (Diglycolamide-multi-walled CNTs)	133.74	Lab-scale experiment conducted in uranyl nitrate stock solution; 25 °C; 3 h equilibrium time; demonstrated efficient uranium recovery.	[147]
AO-MWCNTs (Amidoxime-MWCNTs)	67.9	Lab-scale experiment conducted in aqueous solution; pH 5; 25 °C; 1 h equilibrium time; demonstrated efficient recovery.	[154]
Ta-CNTs (Triazine- CNTs)	74.62	Lab experiment conducted in aqueous solution; pH 2; 1.5 h equilibrium time; demonstrated efficient uranium recovery.	[160]
PS-MWCNTs (Phosphoryl-multiwalled CNTs)	806.45	Lab-scale experiment conducted in aqueous solution; pH 5; 25 °C; 0.5 h equilibrium time; 1 M HCl used as desorption agent; adsorbent maintained 90.2% efficiency after eight cycles.	[166]
CS-CNTs (Chitosan-based composite -CNTs)	126.7	Lab-scale experiment conducted in wastewater with Th4+, Eu3+, La3+, Cs+, Yb3+ ions; pH 4; 25 °C; 1 h equilibrium time; 0.2 M acidified EDTA used as desorption agent; maintained 87.9% efficiency after five cycles.	[169]

2.6. Carbon Aerogels

Carbon aerogels (CAs) are a class of advanced materials characterized by their lightweight, highly porous structure, exceptionally high surface area (typically 400–1000 m²/g), and excellent electrical conductivity, making them highly valuable for both energy storage and environmental applications [170,171]. Their preparation typically involves a multi-step process beginning with the sol—gel polymerization of organic precursors. This is followed by an aging step to strengthen the gel structure, supercritical drying to preserve the delicate porosity, and finally, pyrolysis in an inert atmosphere to carbonize the gel into its final form [172]. These unique structural attributes, including ultra-low density and adjustable surface properties, create interconnected porous networks that are ideal for the capture and separation of target metal ions. Consequently, CAs have been proven to be highly effective for uranium extraction, where surface modifications with specific functional groups are often employed to further enhance their affinity and efficiency [120].

A key advantage of carbon aerogel systems is their ability to achieve high adsorption capacity and selectivity for uranium, even in the presence of competing ions within complex matrices [173]. In a recent breakthrough, a biomass-derived carbon aerogel functionalized with amidoxime groups achieved an exceptionally high adsorption capacity of 801.2 mg/g, with equilibrium reached rapidly within just one hour. This innovative synthesis employs Zn²⁺ ions as a catalytic “glue” to break the hydrogen bonds in cellulose and promote cross-linking, forming a 3D interconnected porous structure. The resulting material possesses bifunctional groups, both amidoxime and native oxygen-containing moieties (hydroxyl and carbonyl), which synergistically enhance uranyl ion binding.

The unique architecture of CAs also makes them ideal scaffolds for multifunctional composites that integrate other advanced properties. For instance, Wang et al. developed a konjac-derived carbon aerogel encapsulating carbon-doped ZnO (KC@C-ZnO), which achieved a uranium capacity of 738.4 mg/g [174]. Similarly, a composite incorporating nanoscale zero-valent iron (nZVI@KGMC) reached a capacity of 720.8 mg/g by leveraging both adsorptive and reductive capabilities [175]. In contrast, other more conventional composites, such as a graphene and ZIF-67-based aerogel (GCZCA), exhibited a more modest capacity of 278.64 mg/g, underscoring that sophisticated material design is paramount to achieving good performance [176].

In conclusion, carbon aerogel is uniquely capable of combining high capacity and kinetics, owing to its hierarchical porous structure. While functionalized CAs have set a new benchmark for balanced performance, advanced composites incorporating photocatalytic or reductive nanoparticles showcase the potential for creating multifunctional remediation systems. The primary challenge in this field will be to validate these laboratory-scale results under more realistic, low-concentration conditions. Furthermore, developing scalable and cost-effective synthesis methods will be essential to translate these material advantages into practical, industrial-scale applications. A summary of the performance of these functionalized carbon aerogel systems is provided in Table 6.

Table 6. Summary of uranium adsorption performance for representative functionalized carbon aerogel adsorbents.

Adsorbent	Qm (mg/g)	Adsorption Conditions and Key Findings	Ref.
KC@C-ZnO-CAs (Konjac glucomannan-derived carbon-encapsulated carbon-doped ZnO carbon aerogels)	738.4	Lab-scale experiment conducted in simulated aqueous solution with K+, Mg2+, Sr2+, Na+, Ca2+, Cu2+ competing ions; pH 5; 3 h equilibrium time; 0.1M Na2CO3 used as eluent; adsorbent maintained 92.9% U(VI) removal after five cycles.	[174]
nZVI@KGMC (Carbon-doped nano zero-valent iron particles with konjac glucomannan-derived CAs)	720.8	Lab-scale experiment conducted in simulated radioactive wastewater with Zn2+, Ca2+, Sr2+, K+, Co2+, Cu2+, Mg2+ competing ions; pH 5; 25 °C; 1 h equilibrium time; demonstrated efficient U recovery.	[175]
rGO/CNQDs/ZIF-67(GCZCA) (Reduced graphene oxide-carbon nitride quantum dots-zeolitic imidazolate framework-67-carbon aerogel)	278.64	Lab-scale experiment conducted in aqueous solution with Zn2+, Fe3+, Ni2+, Sr2+, Ca2+, K+, Na+ competing ions; pH 3; 35 °C; 0.001 M NaNO3 used as desorption agent; adsorbent maintained >80% removal rate after five cycles.	[176]
AO/BB-CAs (Amidoxime/ Biomass Bit-Derived Carbon Aerogel)	801.2	Lab-scale experiment conducted in nuclear wastewater with Cu2+, Zn2+, Sr2+, Ni2+, Co2+, Mg2+ ions; pH 5; 25 °C; 0.5 M HNO3 used as desorption agent; maintained 90% adsorbent efficiency after five cycles.	[177]
CS-CCN aerogel (Chitosan/carboxylated carbon nanotube composite Aerogels)	307.5	Lab-scale experiment conducted in radioactive wastewater with Gd3+, Yb3+, La3+, Nd3+, Sm3+, Co2+, Sr2+, Zn2+, Ni2+ competing ions (initial concentration: 120 ppm); pH 5; 25 °C; 1 h equilibrium time; demonstrated efficient uranium recovery.	[178]

Table 6. Summary of uranium adsorption performance for representative functionalized carbon aerogel adsorbents.

Adsorbent	Qm (mg/g)	Adsorption Conditions and Key Findings	Ref.
KC@C-ZnO-CAs (Konjac glucomannan-derived carbon-encapsulated carbon-doped ZnO carbon aerogels)	738.4	Lab-scale experiment conducted in simulated aqueous solution with K+, Mg2+, Sr2+, Na+, Ca2+, Cu2+ competing ions; pH 5; 3 h equilibrium time; 0.1M Na2CO3 used as eluent; adsorbent maintained 92.9% U(VI) removal after five cycles.	[174]
nZVI@KGMC (Carbon-doped nano zero-valent iron particles with konjac glucomannan-derived CAs)	720.8	Lab-scale experiment conducted in simulated radioactive wastewater with Zn2+, Ca2+, Sr2+, K+, Co2+, Cu2+, Mg2+ competing ions; pH 5; 25 °C; 1 h equilibrium time; demonstrated efficient U recovery.	[175]
rGO/CNQDs/ZIF-67(GCZCA) (Reduced graphene oxide-carbon nitride quantum dots-zeolitic imidazolate framework-67-carbon aerogel)	278.64	Lab-scale experiment conducted in aqueous solution with Zn2+, Fe3+, Ni2+, Sr2+, Ca2+, K+, Na+ competing ions; pH 3; 35 °C; 0.001 M NaNO3 used as desorption agent; adsorbent maintained >80% removal rate after five cycles.	[176]
AO/BB-CAs (Amidoxime/ Biomass Bit-Derived Carbon Aerogel)	801.2	Lab-scale experiment conducted in nuclear wastewater with Cu2+, Zn2+, Sr2+, Ni2+, Co2+, Mg2+ ions; pH 5; 25 °C; 0.5 M HNO3 used as desorption agent; maintained 90% adsorbent efficiency after five cycles.	[177]
CS-CCN aerogel (Chitosan/carboxylated carbon nanotube composite Aerogels)	307.5	Lab-scale experiment conducted in radioactive wastewater with Gd3+, Yb3+, La3+, Nd3+, Sm3+, Co2+, Sr2+, Zn2+, Ni2+ competing ions (initial concentration: 120 ppm); pH 5; 25 °C; 1 h equilibrium time; demonstrated efficient uranium recovery.	[178]

2.7. Other Carbon-Based Materials

In addition to the above-mentioned materials, numerous other carbon-containing materials are also employed for the adsorption of metals and environmental pollutants. Nanostructured materials such as metal—organic frameworks (MOFs) and covalent organic frameworks (COFs) have emerged as significant adsorbents for effectively removing uranium. These materials have garnered interest due to their exceptional structure and properties, including good porosity and the straightforward adjustability of their pore dimensions across the microporous to mesoporous range [105,179,180].

3. Binding Mechanisms of Uranium with Major Chelating Functional Groups Grafted on Carbon Sorbents

A lot of functional groups have been investigated for uranium extraction. Among them, amidoxime, phosphoryl, and amine functionalities are particularly prominent due to their unique properties. Specifically, the amidoxime group is selected for its selectivity in complex media, while the phosphoryl group provides powerful Lewis acid-base interactions that are highly effective in acidic conditions. Furthermore, the fundamental amine group serves as a crucial baseline for N-donor chelation. This section illustrates the binding mechanisms of uranium with representative chelating functional groups on carbon-based materials, in the hope of providing foundational insights for future work.

3.1. Amidoxime Functional Groups

Amidoxime functional groups (C(NH₂)=NOH) demonstrate remarkable efficiency in adsorbing uranium, primarily due to their capacity for robust chelation with uranyl ions (UO₂²⁺). Specifically, these groups, characterized by their nitrogen-oxygen bond, preferentially bind to uranium, thereby enhancing its extraction from seawater [74]. At the molecular level, this chelation process involves the formation of stable five-membered rings, which are created when lone pair electrons from both nitrogen and oxygen atoms are donated to the uranyl ion, forming a strong coordinate covalent bond [161,181]. Furthermore, amidoxime can bind to UO₂²⁺ through several motifs, as illustrated in Figure 5a. For instance, in a monodentate mode, the oxygen and nitrogen coordinate with UO₂²⁺ to form a stable five-membered chelate ring. This dual electron donation improves binding strength and selectivity over competing cations. However, a significantly stronger bond is formed in the η²-type mode, wherein uranium interacts with both the oxygen and the adjacent –NH₂ group [63]. Consequently, this multidentate chelation strongly stabilizes the UO₂²⁺ ion and minimizes desorption. This is particularly crucial in the complex ionic composition of seawater, as it allows the amidoxime group to effectively outcompete other dissolved species like carbonate and vanadate ions, while also demonstrating high selectivity over abundant competing cations such as Na⁺, Mg²⁺, and Ca²⁺. Moreover, the amidoxime group’s ability to disrupt the hydration shell of uranium ions further elevates adsorption efficiency by increasing ion accessibility. To leverage these properties, amidoxime groups are often integrated into advanced materials like poly(amidoxime)-reduced graphene oxide (PA-GO) and amidoxime-modified carbon nanotubes [182,183]. For a more detailed example, the mechanism depicted in Figure 5b for amidoxime-functionalized multi-walled carbon nanotubes (AO-g-MWCNTs) shows that the amine (-NH₂) and hydroxyl (-OH) groups interact with the uranyl ion through both coordination and electrostatic forces. Simultaneously, the oxime group (C=N-OH) forms strong coordination bonds, while the inherent porous structure of the MWCNTs enhances the available surface area, facilitating more efficient ion capture [151]. Ultimately, the resulting adsorbent complex firmly stabilizes the uranyl ion through this multidentate binding, enabling selective uranium recovery from aqueous solutions. Beyond their primary chelation function, amidoxime groups offer further practical advantages. For example, their ability to participate in hydrogen bonding with water can reduce solution viscosity, thereby enhancing the diffusion of uranium ions to the adsorbent surface. Finally, their stability across a wide pH range, coupled with their regenerability using mild acids, makes them highly effective and reusable for sustainable uranium extraction from seawater. The chemical structure of the amidoxime group can be represented as

R-C(NH₂)=NOH + UO₂²⁺ ⇌ R-C(NH₂)=N-O-UO₂⁺ + H⁺

(1)

Figure 5. (a) Possible binding motifs of the amidoxime functional group with the uranium ion. Reprinted with permission from Ref. [63]. Copyright 2020, Elsevier B.V. All rights reserved. (b) A selective uranium adsorption mechanism of AO-MWCNTs. Reprinted with permission from Ref. [151]. Copyright 2014, Elsevier B.V. All rights reserved.

Figure 5. (a) Possible binding motifs of the amidoxime functional group with the uranium ion. Reprinted with permission from Ref. [63]. Copyright 2020, Elsevier B.V. All rights reserved. (b) A selective uranium adsorption mechanism of AO-MWCNTs. Reprinted with permission from Ref. [151]. Copyright 2014, Elsevier B.V. All rights reserved.

3.2. Phosphoryl Functional Groups

Phosphoryl-functionalized carbon-based materials represent a highly efficient and selective approach for uranium extraction from seawater, operating through a coordination mechanism that leverages the unique chemical affinity between phosphoryl groups and uranyl ions. Essentially, the mechanism involves the strategic incorporation of phosphoryl groups (P═O) onto carbon substrates, particularly graphene and mesoporous carbon frameworks, where these functional groups serve as homogeneously distributed active binding sites to maximize uranium capture efficiency [116,184]. Under seawater conditions, phosphoryl groups exist in their deprotonated state (P–O⁻), which significantly enhances their ability to coordinate with dominant uranium species like uranyl ions (UO₂²⁺) through direct metal-ligand interactions [116,185]. Specifically, the interaction between the uranyl ions and the material is governed by two primary mechanisms: first, electrostatic attraction with the negatively charged oxygen atoms, and second, the formation of a coordinate bond with the lone-pair electrons of the phosphoryl (P=O) oxygen, as illustrated in Figure 6a [186]. This binding process involves the displacement of water molecules from the uranyl hydration sphere by phosphoryl oxygen atoms, forming stable coordination complexes in which the phosphoryl groups occupy equatorial positions around the linear uranyl moiety without altering its oxidation state [184]. Consequently, this strong and specific interaction leads to high selectivity for U(VI), even in the presence of competing ions. For a more detailed example, Figure 6b illustrates that on mesoporous carbon, uranyl ions are captured through inner-sphere complexation with surface phosphate groups. In this case, the –P–OH moieties interact directly with U(VI), forming stable P–O–U bonds that immobilize the uranyl species on the adsorbent surface [187]. Notably, because each phosphate unit attaches to the carbon framework at only one point, the resulting structure is a two-dimensional surface chelate rather than a three-dimensional coordination polymer. As a result, the surface becomes densely coated with isolated, chelated UO₂²⁺ units, which prevents extensive cross-linking. The labeled “L” denotes oxygen-containing ligands from the metal complex, which are reorganized upon binding to the phosphate sites. Ultimately, the formation of these strong coordination bonds enables the rapid and selective extraction of uranium from aqueous solutions, highlighting the high affinity and binding capacity of phosphate-functionalized adsorbents for uranium recovery [166,187,188,189].

Figure 6. (a) Schematic interactions of the uranyl ion and phosphate groups through electrostatic interaction and coordination method. Reprinted with permission from Ref. [186]. (b) Possible complexation mechanism between U(VI) and the phosphate-containing carbon materials (PC). Reprinted with permission from Ref. [187]. Copyright 2023 Elsevier B.V. All rights reserved.

Figure 6. (a) Schematic interactions of the uranyl ion and phosphate groups through electrostatic interaction and coordination method. Reprinted with permission from Ref. [186]. (b) Possible complexation mechanism between U(VI) and the phosphate-containing carbon materials (PC). Reprinted with permission from Ref. [187]. Copyright 2023 Elsevier B.V. All rights reserved.

3.3. Amine Functional Groups

Amine groups (-NH₂) effectively enhance uranium adsorption, primarily through coordination with the uranyl ion (UO₂²⁺) by utilizing the lone pair electrons on the nitrogen atom [190,191]. Essentially, the amine group acts as a Lewis base that coordinates with the Lewis acidic uranyl ion to form a stable bond. In addition to this direct bond, the interaction is further stabilized by hydrogen bonding with surrounding water molecules. This secondary effect is crucial, as it helps destabilize the uranium ion’s hydration shell, making it more accessible for adsorption. However, the efficiency of this process is highly pH-dependent. Specifically, at lower pH, protonated amines (-NH₃⁺) may electrostatically repel uranyl ions, whereas at higher pH, the deprotonated amines (-NH₂) enhance coordination [191]. To harness this capability, amine groups are often incorporated into carbon-based materials like activated carbon or biochar, which significantly improves their uranium adsorption capacity. This is because the high surface area and porous structure of these substrates provide an abundance of active sites for interaction [192,193]. Furthermore, amine groups can work synergistically with other functional groups to create even more powerful adsorbents. For instance, when combined with amidoxime, they form composites with exceptional adsorption capacity and selectivity, leveraging amidoxime’s strong chelating ability while benefiting from the amine’s additional coordination and hydrogen bonding capabilities [194]. A clear illustration of this is shown in Figure 7, where a Cu-BDC-NH₂ metal—organic framework serves as the primary adsorbent. In this system, both its amino (-NH₂) and carboxylate (-COO⁻) groups coordinate with UO₂²⁺ ions, while graphene oxide (GO) provides structural support and supplementary binding sites [105]. Similarly, amine-modified activated carbons have also proven effective. Herein, chemical modification increases the number of active sites for uranium binding and enhances the material’s hydrophilicity, thereby facilitating the diffusion of uranium ions to the adsorbent surface [28]. Finally, a key practical advantage of amine-based adsorbents is their robustness. They maintain their uranium-binding ability across a wide pH range and can be easily regenerated using mild acids, which enables multiple adsorption—desorption cycles without significant performance loss. The amine group’s structure can be represented as

R-NH₂ + UO₂²⁺ ⇌ R-NH-UO₂⁺ + H⁺

(2)

Figure 7. Schematic diagram of the mechanism of uranium adsorption by Cu-BDC-NH₂@ Graphene Oxide Membrane. Reprinted with permission from Ref. [105]. Copyright 2024 American Chemical Society.

Figure 7. Schematic diagram of the mechanism of uranium adsorption by Cu-BDC-NH₂@ Graphene Oxide Membrane. Reprinted with permission from Ref. [105]. Copyright 2024 American Chemical Society.

4. Challenges and Future Outlook

The extraction of uranium from seawater using functionalized carbon-based materials represents a promising frontier in sustainable energy, but its practical implementation faces significant hurdles. While substantial progress has been made, research remains largely confined to the laboratory scale. The critical next step is therefore bridging the gap between laboratory success and real-world application. This transition requires a dual focus: first, on systematically validating the materials’ adsorption performance and long-term durability in complex marine environments, and second, on ensuring their overall process is economically viable.

Leveraging Density Functional Theory (DFT) modeling and artificial intelligence (AI) can greatly accelerate the rational design of functionalized carbonaceous sorbents with superior adsorption capacity, selectivity, and anti-fouling properties for uranium extraction. For instance, DFT modeling can elucidate the binding mechanisms between uranium ions and adsorbent active sites at the atomic level, providing critical insights for material design. Furthermore, designing carbon-based sorbents with novel architectures, such as hierarchical porous structures and branched fibrous networks, can help to enhance mass transport of uranium ions, improve the materials’ long-term durability, and cost-effectiveness for practical marine applications.

Despite significant progress, research on functionalized carbonaceous materials remains at the laboratory stage, necessitating systematic testing to validate their performance and durability in real-world marine environments. Furthermore, prioritizing economic feasibility and process optimization is essential, requiring dedicated efforts to bridge the gap between innovation and commercial viability. Ultimately, the continued advancement of these materials pivots on a multi-pronged research effort focused on elucidating structureperformance relationships, developing novel and green functionalization techniques, promoting the integration of multifunctional capabilities, and ensuring economic viability.