Plasma-Modified Carbon Materials for Radionuclide Absorption

Abstract

Carbon-based materials, characterized by their high specific surface area and exceptional chemical stability, have become integral to adsorption-based remediation methods. Carbon materials demonstrate exceptional efficiency, selectivity, and environmental compatibility in radionuclide adsorption. However, the practical application of conventional carbon materials is limited by their insufficient adsorption capacity and selectivity. Plasma modification has emerged as a highly effective strategy for enhancing the surface chemistry of carbon materials, thereby significantly improving their adsorption performance. This process increases the specific surface area of carbon materials and introduces a variety of functional groups, which in turn boost their capacity to adsorb radionuclides. This review systematically explores the progress made in modifying carbon-based adsorbents for the remediation of radioactive nuclides, with a particular emphasis on the mechanisms and effectiveness of plasma modification, covering studies on plasma-modified carbon materials for radionuclide adsorption published between 2009 and 2024. Furthermore, the review discusses the future prospects and practical applications of plasma-modified carbon materials in nuclear wastewater treatment, providing a scientific foundation for the development of efficient and sustainable remediation technologies.

1. Introduction

Energy systems’ environmental ramifications are inherently dictated by their intrinsic properties. The proliferation of fossil fuel utilization has catalyzed a pronounced escalation in global carbon dioxide emissions, intensifying the greenhouse effect and destabilizing ecological homeostasis [1]. Conversely, non-fossil energy sources impose significantly lower ecological burdens. Nuclear energy, exemplified by its low-carbon footprint, exceptional efficiency, and environmental compatibility, constitutes a pivotal instrument in addressing fossil fuel depletion and mitigating anthropogenic climate change. Empirical analyses have revealed an inverse correlation between nuclear power expansion and atmospheric carbon dioxide levels [2], underscoring its capacity to satisfy energy demands while curtailing the reliance on conventional hydrocarbons. Nuclear energy’s stature within low-carbon energy paradigms continues to rise. By 2020, its contribution reached 10.3% of global electricity generation, yielding 2553 TWh annually and accounting for 25% of all low-carbon electricity output, with upward trends persisting [3]. Nevertheless, extensive adoption of nuclear energy engenders the inevitable production of radioactive nuclides. These entities, characterized by acute toxicity, environmental mobility, and solubility, represent substantial hazards due to their proclivity for dispersion, jeopardizing both ecological stability and public health [4]. Within the current research framework, uranium (U), iodine (I), europium (Eu), thorium (Th), strontium (Sr), and cesium (Cs) constitute the predominant radionuclides of prioritized scientific interest. The adverse effects of radioactive nuclides encompass carcinogenesis, multi-organ impairment, and genetic inheritance anomalies [5,6], necessitating rigorous containment to mitigate systemic risks. Heightened public scrutiny has emerged following catastrophic events such as the Yonggwang Nuclear Power Plant Leak (South Korea), the Fukushima Daiichi Nuclear Disaster (Japan), and the Sellafield Facility Incident (United Kingdom) [7]. Such occurrences underscore the imperative for innovative, high-efficiency strategies to address radioactive contamination and safeguard both environmental integrity and human well-being.

The current strategies for treating radioactive nuclides in nuclear wastewater include chemical precipitation, ion exchange, adsorption, evaporation, biotechnological methods, membrane separation, and photocatalysis, among others [8]. Among these, adsorption has gained significant attention due to its cost-effectiveness, operational simplicity, and environmental friendliness. Adsorption has proven capable of achieving nuclide removal through selective interactions or a high adsorption capacity, driven by the intrinsic physical properties of the adsorbent material [9]. The effectiveness of adsorption largely depends on the choice of adsorbent material. Key parameters such as porosity, specific surface area, ion framework, and post-synthetic modifications play a critical role in determining the adsorption performance [10]. Carbon-based materials, known for their high specific surface area, excellent chemical stability, tunable pore structures, and adjustable surface functionalities, are widely utilized [11]. However, their application potential is often limited by intrinsic drawbacks, including a low adsorption capacity, poor selectivity, and limited reusability [12]. To address these challenges and improve the adsorption efficiency of carbon-based adsorbents, surface modification and functionalization have emerged as promising strategies [13]. These modifications enhance interactions with target nuclides, thereby expanding the applicability and performance of carbon-based adsorption systems.

Among the various material modification techniques, plasma technology has emerged as a prominent approach due to its exceptional efficiency, preservation of the internal structural integrity of materials, and minimal environmental influence [14]. This technique leverages high-energy ion bombardment to induce surface modification, enabling precise alterations to the material’s physicochemical properties without compromising its structural framework. It is worth noting that early research—motivated by the need to overcome the low adsorption capacity and selectivity of pristine carbon materials—relied on conventional chemical modification techniques. In contrast, more recent efforts have developed advanced plasma functionalization protocols to achieve precise control over the surface chemistry and grafting of targeted functional groups. For example, the controlled introduction of specific gaseous atmospheres during plasma treatment allows for the grafting of targeted functional groups onto the material surface, thereby enhancing its adsorption capacity and selectivity toward radioactive nuclides [15]. Elucidating the impact of plasma-induced modifications on carbon materials is of critical importance for advancing remediation strategies for radioactive nuclides. This review summarizes the recent advancements in the application of carbon-based materials and their composites in nuclide remediation, with a focus on the use of plasma technology for surface functionalization. We discuss the typical adsorption mechanisms of original and plasma-modified carbon materials for radioactive nuclides (as shown in Figure 1 and Figure 2), and explore the future development directions for plasma-modified carbon materials, offering valuable insights into their potential to address pressing challenges in radioactive waste management and environmental sustainability.

2. Carbon Materials for Radioactive Nuclide Removal

Carbon materials, distinguished by their extensive specific surface area and well-developed porous structures, provide a high density of adsorption sites for radionuclides. Their inherent physicochemical stability further establishes them as superior candidates for adsorption applications. Oxidation processes introduce an abundance of oxygen-containing functional groups onto carbon material surfaces, enhancing their hydrophilicity and substantially improving their surface ion-exchange capacities, thereby reinforcing adsorption efficiency [23]. In recent years, functionalization through surface modification of carbon-based adsorbents has become a prominent area of investigation. Notable examples include activated carbon (AC), graphene oxide (GO), carbon nanotubes (CNTs), fullerenes, and carbon-based composites, which have been extensively employed due to their tunable surface properties, structural versatility, and superior adsorption performance [24]. These advantages underscore the transformative potential of carbon materials in developing high-performance systems for radionuclide remediation.

2.1. Graphene and GO

Graphene, a two-dimensional nanostructure of carbon composed of sp²-hybridized atoms arranged in a honeycomb lattice, has garnered significant attention due to its well-developed porous architecture, exceptional thermal and electrical conductivity, extensive specific surface area, and abundance of oxygen-containing functional groups. These characteristics make it highly suitable for applications in environmental remediation and related domains [25]. As a promising material, graphene not only exhibits potential for U removal from nuclear waste but also demonstrates broad applicability in U extraction from seawater and U detection [26]. A notable derivative of graphene, GO, has emerged as an advanced material of interest. Its large specific surface area, rich oxygen-containing groups, and excellent chemical stability endow it with a high adsorption capacity and superior hydrophilicity [27]. Furthermore, the abundant active sites distributed across its surface significantly enhance its adsorption capabilities, positioning it as an ideal candidate for adsorbent applications. These unique properties underscore the potential of graphene and its derivatives in advancing environmental remediation technologies, particularly in the context of radioactive nuclide removal.

The exceptional tunability of GO facilitates functionalization through various modification strategies, positioning it as a premier carbon-based adsorbent material, which has been extensively investigated for radionuclide remediation. The adsorption properties of GO are highly dependent on the synthesis methodologies employed. Comparative analyses have indicated that GO synthesized via the Hummers and Tour methods (HGO and TGO) demonstrates significantly enhanced adsorption capacities and affinities relative to GO produced using the Brodie method (BGO), a divergence primarily ascribed to variances in the structural configuration and oxidation states. The high adsorption capacity of U by HGO, TGO, and BGO is clearly demonstrated in Figure 3a [28]. Advanced experimental techniques have elucidated the underlying interaction mechanisms between GO and radionuclides, paving the way for the rational design of novel GO-based adsorbents with superior performance. In this context, Boulanger refined the Hummers oxidation protocol to fabricate defect-enriched GO (dGO) using reduced GO (rGO) as a precursor. This modified material exhibited substantially enhanced adsorption efficiencies for U, americium, and Eu, with its U adsorption capacity surpassing that of conventionally synthesized GO by a factor of 15. Figure 3b further presents a comparative analysis of the adsorption capacity of dGO and HGO for U [16]. The augmented adsorption performance of dGO is attributed to its abundant carboxyl functional groups, which are preferentially localized at atomic edges surrounding the nanopores of GO sheets. A schematic representation of the synthesis process and adsorption mechanisms underscores this correlation. The specific adsorption mechanism is illustrated in Figure 1a, providing a clear depiction of the underlying interactions [16]. These findings underscore the transformative potential of introducing structural defects into GO, offering a robust framework for engineering the next generation of high-efficiency adsorbent materials tailored for radionuclide sequestration.

2.2. Biochar

Biochar, a stable carbonaceous solid residue, is produced via the pyrolysis of diverse agricultural and industrial biomass under oxygen-limited conditions across a range of temperatures [29,30]. Its large specific surface area, high permeability, well-developed porosity, and environmental benefits, such as soil enhancement, have garnered significant attention, establishing its widespread applicability in environmental remediation. Its specific adsorption mechanism is illustrated in the adsorption schematic shown in Figure 1b [17,29]. Despite the inherently favorable physicochemical properties of biochar, its practical efficacy is often undermined by limitations in its structural stability and adsorption efficiency, which are highly dependent on the feedstock characteristics [31]. Consequently, the activation and functional modification of biochar have emerged as pivotal strategies for enhancing its physicochemical attributes, paving the way for more robust and efficient applications in environmental management [32].

Studies have revealed two principal strategies for enhancing biochar as an adsorbent. The first involves doping biochar with additional materials to synthesize composites that integrate diverse functional properties. Guo et al. successfully fabricated a novel nitrogen-doped biochar (AL-N/BC-700) and investigated its adsorption mechanisms for U [33]. The study demonstrated that AL-N/BC-700 exhibits an extraordinary U(VI) adsorption capacity of 25,000 mg/g, significantly surpassing other adsorbents. Furthermore, it achieved an exceptional desorption efficiency of 94.5% in the presence of Na₂CO₃, addressing critical challenges such as adsorption instability, high costs, a low desorption efficiency, and interference from coexisting substances [33]. The second approach involves modifying or activating biochar surfaces through methods such as gas treatment, steam, microwave irradiation, acids, bases, or oxidants. Chen et al. developed phosphate-modified hydrothermal biochar (PHBB) derived from waste bamboo. This material exhibited a threefold increase in adsorption capacity compared to unmodified biochar due to its enhanced specific surface area, additional active sites, and higher adsorption efficiency. PHBB offers a cost-effective and environmentally sustainable solution for radioactive nuclide removal, underscoring its potential in environmental remediation [34]. Beyond acid activation, Sumalatha et al. synthesized potassium hydroxide-activated biochar from peanut shells (PSABC) through pyrolysis at 400 °C for 30 min. PSABC demonstrated remarkable performance, achieving a maximum radionuclide removal rate of 99.02%. Even in the presence of high concentrations of competing Ca²⁺ ions, it maintained a removal efficiency of 53.55%, providing innovative pathways for radionuclide adsorption [35]. In summary, biochar, as an eco-friendly and versatile adsorbent, offers transformative opportunities for environmental remediation, highlighting its broad application potential.

2.3. CNTs

CNTs, tubular structures formed by rolling single or multiple layers of graphene, have garnered significant attention as promising adsorbent materials. Their exceptional properties, including a high specific surface area, substantial adsorption capacity, tunable designability, outstanding chemical stability, and superior mechanical strength, render them highly effective for adsorption applications [36]. CNTs are classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) based on the number of graphene layers. While SWCNTs exhibit higher stability and larger specific surface areas [37], their limited surface functional groups and inherent chemical inertness have directed research efforts toward enhancing the performance and surface functionalization of MWCNTs, particularly for radionuclide adsorption, The specific functionalization reaction and adsorption mechanisms are illustrated in the schematic shown in Figure 1c [18]. Hassan et al. investigated the application of MWCNTs for the removal of radionuclides such as U-238, Th-232, and potassium-40 from wastewater. The study demonstrated that, under optimal conditions, MWCNTs achieved removal efficiencies exceeding 98% for all three radionuclides. These findings not only establish MWCNTs as potent adsorbents for radionuclide removal from aqueous solutions but also provide valuable insights for selecting and optimizing adsorbents in practical applications [38]. In summary, the functionalization and optimization of MWCNTs significantly enhance their adsorption efficiency, affirming their role as versatile and effective materials for radionuclide remediation.

To enhance the applicability of CNTs in radionuclide remediation, researchers have employed various functionalization and modification strategies to augment the surface functional groups and specific surface area, thereby improving adsorption capacity. Yılmaz et al. oxidized MWCNTs using potassium permanganate (KMnO₄) and citric acid (C₆H₈O₇) as oxidizing agents, synthesizing two adsorbents: PPM-MWCNTs and CA-MWCNTs. The study evaluated the capacity of these modified MWCNTs for Th removal from aqueous solutions. The experimental results revealed adsorption capacities of 105.28 mg/g and 50.67 mg/g for PPM-MWCNTs and CA-MWCNTs, respectively. These findings underscore the potential of functionalized MWCNTs as efficient adsorbents, offering novel strategies for the remediation of radionuclides in nuclear wastewater [39].

2.4. AC

AC, an amorphous carbon material, is characterized by its exceptional adsorption capacity, low cost, environmental compatibility, and abundant raw material availability [40,41,42]. Its highly porous structure, significant porosity, and extensive specific surface area make it a widely utilized solid adsorbent for organic pollutants, heavy metal ions, and radionuclides [43]. The abundant availability of raw materials ensures cost-effectiveness in AC production [44,45], while the structural diversity of AC can be tailored by employing different biomass precursors [41,44]. The selection of raw materials for AC production is primarily dictated by their inorganic content, the intended application of the produced AC, and the type of adsorbates being targeted [46]. Due to its widespread availability, low production cost, relatively simple manufacturing processes, and minimal environmental impact, AC is recognized as a highly efficient and cost-effective adsorbent. Its versatility and adaptability further underscore its significance in addressing a diverse array of adsorption challenges across environmental remediation and other application domains.

The abundant availability of raw materials ensures the scalable production of AC and provides a foundation for enhanced selectivity, enabling targeted surface modifications to achieve functionalization, such as improved selectivity and an improved adsorption capacity. Alahabadi et al. reported the preparation of a highly efficient magnetic acid-treated AC (M-AcidAC) derived from maple wood waste (SWAC) through NH₄Cl chemical activation, post-treatment with various mineral acids, and microwave-assisted magnetization. The study revealed that nitric acid-treated AC (M-HNAC) exhibited removal efficiencies of 98.29% and 92.27% for U and Th, respectively, under optimal conditions, highlighting its significant potential in radionuclide remediation [47]. Beyond acid treatment, other functionalization strategies, such as aminohydroxylation, have demonstrated remarkable enhancements in U(VI) adsorption performance. Amidoxime-based AC achieved a maximum adsorption capacity of 14.16 mg/g, with an adsorption efficiency exceeding 95% under optimal conditions as the adsorbent dosage increased. This material represents a promising candidate for the treatment of U-containing wastewater [48]. Figure 4 demonstrate that the modified carbon materials exhibit significantly enhanced removal efficiency for radionuclides [47,48]. In summary, AC and its modified derivatives can achieve adsorption efficiencies exceeding 90% under optimal conditions, underscoring their critical role in radionuclide remediation and environmental management. These findings emphasize the versatility and potential of AC-based materials in addressing diverse challenges in waste treatment and environmental sustainability.

2.5. Fullerenes

Fullerenes (C₆₀) and their derivatives (e.g., C₆₀O and C₆₀(OH)₂₄) demonstrate remarkable potential in radionuclide adsorption owing to their distinctive spherical configuration, high specific surface area, and tunable surface chemistry [49,50,51,52]. However, experimental investigations involving radioactive elements are substantially constrained by their inherent toxicity and radiological hazards. Consequently, researchers have increasingly adopted quantum chemical computations to elucidate the interaction mechanisms between actinide ions and solid surfaces, thereby facilitating the development of more efficient actinide sequestration materials.

Computational analyses have revealed that the nanoscale cavity and π-electron system of pristine C₆₀ provide multiple adsorption sites for radionuclides. Structural functionalization through hydroxylation, epoxidation, or carboxyl group grafting can significantly enhance both the chemical selectivity and adsorption capacity of fullerene derivatives [49,50,51]. Notably, the hydroxyl groups in C₆₀(OH)₂₄ adopt a “Saturn-ring” spatial distribution, forming a high-density hydrogen-bonding network that establishes multiple strong interactions with actinide ions via outer-sphere (OS) adsorption mechanisms. These hydrogen bonds exhibit dual characteristics, combining electrostatic interactions with partial covalent nature [51]. Malonic acid-functionalized C₆₀ demonstrates enhanced U chelation through deprotonated carboxyl groups under alkaline conditions, with binding energies substantially surpassing the weak physisorption observed in non-functionalized counterparts [50]. Furthermore, oxygen atoms on C₆₀O surfaces mediate synergistic hydrogen bonding and An-O covalent interactions, achieving adsorption energies comparable to GO, thereby illustrating the multi-mechanistic synergy in functionalized fullerenes for efficient radionuclide capture [49].

Notwithstanding these theoretical insights, the current research remains predominantly computational, and critical engineering parameters such as adsorption kinetics and column breakthrough curves have yet to be systematically investigated. Therefore, interdisciplinary collaborative efforts remain imperative to realize the practical implementation of fullerene-based adsorbent materials in nuclear waste management applications.

In summary, carbon-based adsorbents have garnered widespread application in radionuclide remediation owing to their exceptional physicochemical properties, playing a pivotal role in the treatment of radioactive wastewater. Materials such as AC, graphene, and CNTs exhibit significant potential due to their high specific surface areas and well-developed porous structures, which facilitate efficient radionuclide adsorption. However, these materials are not without limitations—AC demonstrates relatively low selectivity [53], graphene involves high production costs and complex synthesis procedures [54], and CNTs are constrained by high manufacturing costs and challenges in large-scale production. Including those of the previously discussed materials, Figure 1d also demonstrates the adsorption mechanism governing zeolite-functionalized carbon adsorbents, thereby providing a mechanistic foundation for advancing carbon-based strategies in radionuclide management [19]. Therefore, the selection of carbon-based adsorbents must involve a balanced evaluation of their respective advantages and drawbacks.

Table 1. Advantages and disadvantages of major carbon-based adsorbents in radionuclide adsorption.

Adsorbent	Advantages	Disadvantages
AC	Widely applicable, strong adsorption capacity, low cost, and wide array of sources [40,41,42].	Low selectivity, and adsorption capacity is influenced by environmental factors [53].
Biochar	High permeability, good porosity, large surface area, and environmentally friendly [30].	Poor environmental stability [42], and adsorption efficiency depends on raw materials [55].
Graphene	Excellent thermal/electrical conductivity, large specific surface area, and multiple oxygen-containing functional groups [25].	Limited oxygen-containing functional groups, high cost, and complex preparation process [54].
CNTs	High elastic modulus and tensile strength, and excellent electrical and thermal conductivity [36].	High cost, challenges in large-scale production, and insolubility issues [37].
Zeolites	High ion-exchange capacity, excellent selectivity, low cost, and compatibility with natural environments [56].	Small pore size and long diffusion paths reduce transport efficiency [57].

provides a comprehensive summary of the merits and limitations of commonly used carbon-based adsorbents.

2.6. Carbon Based Composite Materials

In recent years, in addition to conventional materials, various advanced carbon-based substances, such as mesoporous carbon and graphene, have gained significant attention as scaffolds for composite adsorbents due to their intrinsic tunability and versatility in radionuclide removal from aqueous environments. Mesoporous carbon, characterized by adjustable pore sizes, a high specific surface area, a three-dimensional porous network, low cost, and excellent design flexibility, has emerged as a novel adsorbent. While inherently lacking selectivity for radionuclides, functionalization through the incorporation of specific functional groups can substantially enhance its adsorption selectivity and capacity [58]. For instance, Zhang et al. utilized ordered mesoporous carbon derived from sucrose to synthesize an amidoxime (AO)-functionalized ordered mesoporous polymer-carbon composite (AO-OMC). The study demonstrated that the AO-OMC exhibited high affinity and selectivity toward U(VI), with its adsorption capacity increasing dramatically from 43.4 mg/g for mesoporous carbon to 322.6 mg/g—an approximately sevenfold enhancement. Furthermore, this adsorbent showcased remarkable reusability, retaining its adsorption capacity and structural stability after ten cycles, significantly reducing the environmental remediation cost and bolstering its practical applicability [59].

Given that adsorption technologies demand adsorbents with robust stability, a high adsorption capacity, excellent selectivity, and reusability potential, traditional materials often fall short of meeting these multifaceted requirements. As a result, the development of composite materials with tailored properties has become a focal point in recent research. Graphene, with its extensive specific surface area and superior tunability, is frequently selected as a foundational scaffold for composites, enabling synergy between the advantages of its constituents to achieve efficient radionuclide adsorption. For U-specific removal, batch adsorption experiments revealed that a zero-valent iron–polyaniline–graphene aerogel composite (Fe-PANI-GA) demonstrated exceptional removal efficiency for U(VI) in acidic solutions, with a maximum adsorption capacity of 350.47 mg/g. This material represents a cost-effective adsorbent suitable for practical nuclear waste treatment [60]. Similarly, a functionalized AC–graphite composite (Graphite/AC), prepared through oxidation and amination processes, exhibited superior U adsorption performance, achieving a maximum capacity of 100 mg/g. The composite successfully reduced U and heavy metal concentrations in real groundwater samples, underscoring its efficacy and applicability [46]. Beyond graphene, GO has also been utilized as a composite matrix. For example, a phosphorus-doped GO–chitosan composite (GO-CS-P) was synthesized via crosslinking GO, prepared using a modified Hummers method, with chitosan (CS), followed by phosphorization. Batch experiments demonstrated that this material exhibited exceptional selectivity, effectively capturing U(VI) from solutions containing multiple competing metal ions. At pH 5.0, GO–CS–P achieved a remarkable maximum adsorption capacity of 779.44 mg/g, reaching equilibrium within just 15 min. These findings highlight its potential as an innovative adsorbent for U-contaminated water purification, providing novel methodologies for nuclear wastewater treatment [61].

This section focused on the inherent advantages of GO, biochar, CNTs, AC, and carbon composite materials in the adsorption of radionuclides. Functional group modification of carbon materials as ligands has been proven to be an effective strategy for enhancing their adsorption performance [62]. However, most modification techniques for carbon materials still focus primarily on traditional chemical methods. For example, immobilizing amine groups through wet chemical methods is a common modification approach [63,64]. Nevertheless, such methods have several shortcomings, including complex operational processes, strong substrate dependency [65], and high waste generation [66], which limit their wide application in the field of adsorbent materials. Chemical grafting is another commonly used modification method [67]. However, this process typically requires the use of expensive and corrosive reagents, posing safety and environmental concerns. The challenges of handling large volumes of polluted wastewater further constrain its large-scale application [68,69]. Additionally, the chemical grafting process may damage or even completely destroy the deeper surface structures of the materials, negatively affecting the modification results [68]. To overcome the limitations of traditional chemical modification methods, it is necessary to develop a rapid, simple, substrate-independent, and environmentally friendly approach.

3. Plasma-Modified Carbon Materials for Nuclides Absorption

Plasma is considered the fourth state of matter and consists of a partially or fully ionized gas that includes electrons, ions, neutral atoms, and molecules, while maintaining overall electrical neutrality. The chain chemical reactions induced by plasma discharge excitation of gas molecules can generate various reactive oxygen and reactive nitrogen species [70]. These reactive species can interact with the material’s surface, altering its chemical properties and structure, thereby achieving surface functionalization and performance enhancement [71]. Plasma modification technology is acknowledged as a promising environmentally sustainable technology, as it does not involve the use of solvents and generates minimal waste, making it a viable alternative to traditional modification methods. Furthermore, plasma modification has attracted considerable attention for its high modification efficiency, operational simplicity, and lower operational temperature compared to other available techniques [68,72]. Among these, low-temperature plasma (LTP) technology has attracted wide attention due to its high efficiency, low cost, broad applicability, and ability to preserve the structural integrity of materials. LTP can enhance the chemical functionality and adsorption capacity of adsorbents by inducing the formation of chemically active species or grafting other materials to regulate the functional group characteristics on the surface of the adsorbent [15,73,74,75,76]. This chapter focuses on the application of LTP in the modification of carbon materials. It discusses the performance and research progress of different modified carbon materials in adsorbing radioactive nuclides (e.g., U and other nuclides).

3.1. Plasma-Modified Carbon Materials for U Absorption

This section systematically reviews the application of LTP technology in the modification of various carbon materials, including GO, biochar, carbon nanotubes, graphite, carbon-based composites, and other types of carbon materials. The section focuses on the surface modification of carbon materials using LTP, evaluates their adsorption capacity for radioactive nuclides, and discusses the adsorption mechanisms and development trends in radioactive nuclide treatment.

3.1.1. Plasma-Modified GO

Researchers have widely applied plasma-induced graft polymerization technology to modify GO, a technique in which the monomers required for the process polymerize on the surface of plasma-activated substrate materials, forming grafted brush layers on the surface, which provide abundant U adsorption active sites [15,77]. In addition, the numerous energy and material exchange processes generated by the interaction between highly reactive plasma species and materials endow surface modification with great flexibility [78]. The grafted surface can provide abundant U adsorption active sites [79], thereby significantly enhancing its adsorption capacity, efficiency, and selectivity. For example, this technique can graft polyacrylamide (PAM) onto GO (PAM/GO), which significantly increases the specific surface area of PAM/GO, providing more adsorption sites and functional groups for the capture of heavy metal ions such as U(VI). More importantly, compared with unmodified GO and other adsorbents, PAM/GO exhibited significantly enhanced adsorption capacity for U(VI), with a maximum adsorption capacity of 0.698 mmol/g. The improved adsorption performance is mainly attributed to the strong interaction between the PAM-grafted functional groups and U(VI) [80]. This technique can also graft AO groups onto the surface of magnetic GO (AO/mGO). The material achieved a maximum adsorption capacity of 435 mg/g for U(VI) at pH = 4.0 and T = 293 K, reaching adsorption equilibrium within 30 min. Figure 5a presents a comparison of the maximum adsorption capacities of various radionuclides, including U(VI), Th(IV), Eu(III), Sr(II), and Cs(I), on AO/mGO at pH = 4.0. The results indicate that the maximum adsorption capacity of U(VI) on AO/mGO (435 mg/g) is significantly higher than that of Th(IV) (55 mg/g), Eu(III) (68 mg/g), Sr(II) (43 mg/g), Pb(II) (108 mg/g), and Cs(I) (32 mg/g) [81]. These findings suggest that AO/mGO exhibits a highly effective adsorption of U(VI) compared to other radionuclides. Moreover, the material’s maximum enrichment capacity for U(VI) in South China Sea water was 2.85 mg/g. The high enrichment efficiency of U(VI) is attributed to inner-sphere surface complexation. The adsorption of U(VI) in simulated seawater can be effectively modeled by two types of inner-sphere surface complexes (e.g., SOUO₂⁺ and SOUO₂(CO₃)₂³⁻). As illustrated in Figure 5b, the pH-dependent adsorption of U(VI) on AO/mGO can be described by the diffused layer model within the acceptable error range. It was observed that SOUO₂⁺ was the predominant adsorption species at pH < 5.0, while SOUO₂(CO₃)₂³⁻ became the dominant species at pH > 6.0 [81]. The grafted functional groups are often adsorption-enhancing materials that are commonly used in wastewater treatment. For instance, PAM relies on its amide groups as active sites, forming stable metal ion–amide coordination bonds with metal ions, thereby effectively improving metal ion adsorption performance [82]. AO, on the other hand, exhibits highly selective enrichment performance for U(VI) due to its excellent chelating ability [83]. Adsorbent materials constructed based on these functional properties demonstrate excellent performance in removing environmental pollutants and show broad application prospects.

3.1.2. Plasma-Modified Biochar

Over the past five years, researchers have developed various modified biochars derived from everyday waste materials using LTP-induced graft polymerization technology for the adsorption of radioactive nuclide U. For instance, pumpkin vines [84] or coffee grounds [85] can be used as raw materials and modified using this technology. Compared to pumpkin vine-derived biochar (PVB), the acrylic acid-modified pumpkin vine-based biochar (p-PVB-PAA) prepared from pumpkin vines had a surface with more oxygen-containing functional groups, with its specific surface area increasing from 3.8 m²/g to 275.3 m²/g. At pH = 5 and 298 K, the maximum adsorption capacity of uranyl in aqueous solution increased from 67.58 mg/g to 207.02 mg/g. Its adsorption behavior followed the pseudo-second-order kinetic model and the Langmuir adsorption model. Additionally, the significant improvement in adsorption performance can be attributed to the synergistic effects of surface complexation and electrostatic interactions [84]. Coffee-derived biochar can be prepared from coffee grounds, with phosphate groups grafted onto its surface. At T = 298 K and pH = 6.0, this material achieved a maximum adsorption capacity of 648.54 mg/g, far exceeding that of most carbon-based composites. Its excellent adsorption performance is primarily related to electron transfer reactions and the complexation with -NH₂, P-OH/P=O, and C-OH groups [85]. In addition to LTP-induced graft polymerization technology, LTP etching technology has also shown significant applicability in biochar modification. For example, biochar with mesoporous structures was prepared from waste cucumber stems and treated with ammonia LTP. High-energy particles etched the surface of the carbon material, introduced nitrogen atoms, and partially reduced carbon–oxygen and nitrogen–oxygen groups [86]. The modified biochar exhibited excellent U-selective electroadsorption performance, with the U(VI) electroadsorption efficiency increasing from 49.75% to 94.45%. Moreover, its U selectivity coefficient (SU/M) exceeded 120. This study also demonstrated for the first time the influence of the previously overlooked Faradaic side reactions in U electroadsorption [87]. Thus, LTP technology is an effective method for modifying biochar, significantly enhancing the U adsorption capacity and selectivity of the modified materials. The raw biochar materials used for modification are primarily derived from common and abundant waste in daily life. In the future, by introducing materials with superior nuclide adsorption properties into these resource-rich biochar carriers, it will be possible to develop economically viable radioactive nuclide adsorbents with excellent adsorption performance.

3.1.3. Plasma-Modified CNTs

LTP-induced grafting technology has gradually become the primary method for the functionalization of CNTs, particularly MWCNTs. Researchers have systematically optimized their performance in U adsorption by grafting various functional groups. To enhance their application in the treatment of radioactive element contamination, Shao et al., in 2009, first used LTP-induced grafting technology on MWCNTs, and the adsorption capacity of the prepared material for UO₂²⁺ was significantly higher than that of raw MWCNTs [88]. In 2010, the same team used this technology to modify the surface of MWCNTs by grafting CS (MWCNT-g-CS) onto the surface. At pH = 5 and T = 20 °C, the adsorption capacity for UO₂²⁺ was 39.2 mg/g. The experimental data for UO₂²⁺ adsorption shown in Figure 6a were fitted using the Langmuir model, where C_s represents the concentration of metal ions on the solid phase, and C_e denotes the concentration of metal ions in the supernatant after centrifugation [89]. The CS used in this study was a natural amino polysaccharide composed of glucosamine and N-acetylglucosamine. Due to its high content of hydroxyl and amino groups in the polymer chains, it serves as an excellent adsorbent for removing organic and inorganic pollutants [90]. More importantly, the adsorption of U(VI) on both MWCNTs and MWCNT-CS is mainly dominated by inner-sphere surface complexation. After surface grafting, the adsorption of U(VI) significantly improved, as shown in Figure 6b, with the adsorption capacity at T = 20 °C and pH = 5 being approximately 2.4 times higher than that of the ungrafted material [91].

Although plasma-treated CNTs exhibit improved adsorption performance, their selectivity for target nuclides remains insufficient, and there is still room for further improvement in their adsorption capacity. Therefore, further research into enhancing the selectivity and loading capacity of CNTs for U is crucial. AO has been demonstrated to be an excellent functional group for extracting U from seawater due to its selectivity for U(VI) in highly saline brine solutions [92], making it a promising candidate for U(VI) adsorption. The grafting of AO can be achieved by first oxidizing MWCNTs with nitric acid and sulfuric acid, followed by grafting acrylonitrile (AN) onto the MWCNTs using LTP technology, and finally converting the nitrile groups into AO through a reaction with neutralized hydroxylamine hydrochloride, as shown in Figure 2b [20]. Compared to oxidized MWCNTs, the treated oxidized MWCNTs exhibited a significantly enhanced U adsorption capacity, with an optimal adsorption capacity of 145 mg/g (0.61 mmol/g) for U(VI) under conditions of pH = 4.5 and room temperature. As shown in Figure 6c, the strong adsorption and coordination ability of amidoxime functional groups for U enhanced the selective adsorption capacity of the CNT composites for U. In aqueous solutions containing coexisting ions (Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, Sr²⁺, Ba²⁺, and Cs⁺), the material could selectively adsorb U(VI) [20]. Recent studies have shown that the introduction of AO groups onto CNT surfaces through LTP-induced grafting technology not only effectively grafts target functional groups but also maintains the structural integrity of the CNTs without causing damage. As demonstrated in Figure 6d, all samples achieved adsorption equilibrium within a short period (under 4 h), which is essential for practical U extraction applications [93]. Furthermore, after grafting AO groups, the adsorption performance of the material significantly improved, primarily due to the highly efficient chelation between AO groups and U. Under conditions of pH = 6.0 and 303 K, the maximum adsorption capacity for U(VI) reached 275.98 mg/g, a significant improvement compared to the adsorption capacity of raw CNTs (120.12 mg/g). Additionally, the synergistic effect between the oxygen- and nitrogen-containing functional groups within the AO groups on the CNT surface further enhanced their adsorption performance for U(VI) [93]. Future research should focus on further improving the selectivity and loading capacity of CNTs for U while exploring better structure–performance relationships between functional group design and adsorption performance. This would provide more comprehensive theoretical support for developing efficient adsorbent materials and achieving superior adsorption effects in practical applications.

Figure 6. Comparison of the radionuclide adsorption capacity of LTP-modified CNTs. (a) Adsorption isotherms of UO₂²⁺ from solution on MWCNTs, plasma-treated MWCNTs, and MWCNT-g-CS (T = 20 ± 1 °C, equilibrium time = 24 h, m/v  =  0.40 g/L, C[NaClO₄] = 0.01 mol/L, pH = 5.0 ± 0.1) [89]. (b) Sorption isotherms of UO₂²⁺ from solution on MWCNTs and MWCNT-CS (T = 20 ± 1 °C, contact time = 48 h, m/v = 0.4 g/L, C[NaClO₄] = 0.01 mol/L, pH = 5.0 ± 0.1) [91]. (c) Competitive sorption capacities on oxidized MWCNTs and AO-g-MWCNTs (C₀ = 1.0 mmol/L for all cations, pH = 4.5, t = 240 min, V = 20 mL, T = 298.15 K, and w = 20 mg) [20]. (d) Kinetic study for U(VI) adsorption onto adsorbent samples (C[U(VI)] = 50.0 mg/L, pH = 6.0, T = 303 K, dosage = 0.2 g/L, C(Na₂CO₃) = 1.0 mmol/L, contact time = 24 h) [93].

Figure 6. Comparison of the radionuclide adsorption capacity of LTP-modified CNTs. (a) Adsorption isotherms of UO₂²⁺ from solution on MWCNTs, plasma-treated MWCNTs, and MWCNT-g-CS (T = 20 ± 1 °C, equilibrium time = 24 h, m/v  =  0.40 g/L, C[NaClO₄] = 0.01 mol/L, pH = 5.0 ± 0.1) [89]. (b) Sorption isotherms of UO₂²⁺ from solution on MWCNTs and MWCNT-CS (T = 20 ± 1 °C, contact time = 48 h, m/v = 0.4 g/L, C[NaClO₄] = 0.01 mol/L, pH = 5.0 ± 0.1) [91]. (c) Competitive sorption capacities on oxidized MWCNTs and AO-g-MWCNTs (C₀ = 1.0 mmol/L for all cations, pH = 4.5, t = 240 min, V = 20 mL, T = 298.15 K, and w = 20 mg) [20]. (d) Kinetic study for U(VI) adsorption onto adsorbent samples (C[U(VI)] = 50.0 mg/L, pH = 6.0, T = 303 K, dosage = 0.2 g/L, C(Na₂CO₃) = 1.0 mmol/L, contact time = 24 h) [93].

3.1.4. Plasma-Modified Graphite

Compared to other carbon materials, graphite less commonly applied in the field of adsorption and is usually not directly used as an adsorbent. For instance, graphite carbon substrates are often used as coating materials due to their high stability and extensive compatibility with various components (such as functional groups, drugs, and catalytic substances) [94]. Xiao et al. synthesized nitrogen-doped graphite-embedded magnetic nanoparticles in one step using an arc discharge method. During this process, the H free radicals generated by the decomposition of NH₃ in the arc atmosphere etched the graphite shell and simultaneously incorporated four types of nitrogen-containing components—quaternary nitrogen, pyrrolic nitrogen, amino nitrogen, and pyridinic nitrogen—into the graphite surface network. At pH = 4 and T = 298 K, the adsorption capacity was 47.28 mg/g, and its adsorption ability showed an excellent positive linear relationship with the amount of nitrogen-containing substances on the surface [95]. Flake graphite (FG), typically prepared from bulk graphite through physical or chemical methods, is considered one of the most stable carbon allotropes. Due to its unique sandwich structure and long interlayer spacing, FG exhibits a strong atomic or molecular intercalation capability [96]. Additionally, FG has advantages such as high intrinsic thermal conductivity, a large specific surface area, low cost, and good commercial availability. Duan et al. modified FG with -NH₂ functional groups under vacuum conditions using LTP. The surface active sites of the modified FG were significantly increased, which greatly enhanced the adsorption and embedding efficiency of U(VI) and facilitated subsequent -NH₂ group grafting. The schematic diagram of its modification and adsorption is shown in Figure 2a [15]. The adsorption mechanism of U(VI) embedding was achieved through the complexation of U(VI) with the -NH₂ and phenolic hydroxyl groups on the modified FG surface. Furthermore, the adsorption of U(VI) was studied on FG samples treated for different durations. At pH = 6.0, T = 333.15 K, and a treatment time of 2 h with NH₂, the maximum adsorption capacity reached 140.68 mg/g [15]. Graphite, after further processing, exhibits potential for application in the field of radioactive nuclide adsorption due to its unique structural characteristics and surface chemical properties. LTP technology provides an efficient means of functionalizing graphite materials by introducing various nitrogen-containing functional groups and enhancing surface active sites.

3.1.5. Plasma-Modified Carbon Composites

By combining carbon-based materials with functional materials that exhibit excellent adsorption performance for U ions, the overall adsorption capacity of the materials can be significantly improved. Plasma-induced polymerization technology, as an efficient method for synthesizing composite materials, has been widely applied in this field. Functional materials, such as polypyrrole (PPy), with its excellent thermal stability, easy synthesis, non-toxicity, and unique electronic and redox properties [97], have demonstrated strong affinity for various pollutants [98,99]. Aniline molecules contain -NH₂ functional groups that exhibit high affinity for U(VI) [22], while carboxymethyl cellulose (CMC) has attracted attention for its renewability, low cost, and environmental friendliness. The abundant oxygen donor sites (such as hydroxyl and carboxyl groups) in its molecular structure endow it with unique advantages in limiting U(VI) as a hard metal ion. Composite materials such as graphene oxide/polypyrrole (GO/PPy) [100], aniline/graphene oxide (AGO) [22], and carboxymethyl cellulose-supported magnetic GO (CMC/MGO) [101] have been prepared using plasma-induced polymerization technology. These composites exhibit significantly better U(VI) adsorption capacity than their individual components. Among them, Liao et al. evaluated the key role of -NH₂ functional groups in U(VI) enrichment by comparing the enrichment performance of AGO composites with pure GO. Under conditions of pH 3–7 and an aniline-to-GO mass ratio of 2.5–7.5, the -NH₂ functional groups significantly enhanced the enrichment capacity and selectivity for U(VI). This performance improvement was attributed to the strong coordination interaction between nitrogen donor atoms and U(VI) [22]. Meanwhile, CMC/MGO composites effectively overcame the limitations of GO in achieving efficient separation under centrifugal conditions. By introducing magnetic functionality, the materials could achieve rapid liquid-phase separation under the action of an external permanent magnetic field, significantly improving their separation efficiency and practical application potential [102,103]. Furthermore, Li et al. grafted arsenazo III (ASA) onto the Fe₃O₄@C core–shell surface during plasma discharge. The synthesized composite materials exhibited excellent adsorption performance in aqueous solutions, especially for trace U(VI) in the low pH range. In practical applications, these composites could recover U from acidic brine solutions with high concentrations of coexisting ions. Their magnetic responsiveness enabled a simple and fast separation process, further highlighting their application potential in treating complex water systems [104]. These functional composite materials, utilizing plasma-induced grafting technology, show broad application prospects in the treatment of U-ion-containing wastewater. The characteristics of single materials such as PPy, aniline, and carboxymethyl cellulose provide important references for the design and optimization of composite materials. The significant synergistic effects among the components of the composites effectively integrate and complement the unique properties of each component, pointing the way toward developing highly efficient and selective adsorbent systems.

3.1.6. Other Plasma-Modified Carbon Materials (Carbon Dots and Carbon Black)

In addition to traditional carbon materials, some novel carbon materials have also demonstrated significant research value in the field of radioactive nuclide adsorption. Among these, carbon dots (CDs), with their excellent photoluminescence properties, good water solubility, ease of functionalization, and tunable surface functional groups, have been innovatively applied to the detection and adsorption of radioactive nuclides [105]. Wang et al. first reported a microplasma-based method for the preparation of CDs [106], which were subsequently loaded onto mesoporous silica (SBA) for U detection and adsorption. This material not only exhibited high adsorption performance for U but also enabled real-time in situ monitoring of the adsorption process. Additionally, the CD/SBA-NH₂ nanocomposite prepared by microplasma-assisted technology retained both the high surface area of mesoporous silica and the fluorescence properties of CDs [107]. Compared with unmodified SBA-NH₂, the adsorption capacity of the CD/SBA-NH₂ composite material was significantly enhanced, showing excellent selectivity for uranyl ions. More importantly, the fluorescence intensity of this composite material decreased significantly as U adsorption increased, thus achieving real-time monitoring of the adsorption process. Furthermore, the adsorption selectivity of this composite material for metal ions was highly consistent with the fluorescence selectivity of the original CDs. This result indicates that by evaluating the fluorescence response of CDs to metal ions, it is possible to effectively predict the adsorption selectivity of CD-based composites. Figure 7a illustrates the adsorption and monitoring of U using CD/SBA-NH₂ nanocomposites [108]. This type of composite material, which combines efficient adsorption and fluorescence monitoring, provides a new research approach and application prospect for the efficient removal and real-time detection of radioactive nuclides.

In addition, carbon black (CB) may be a potential substrate due to its hexagonal mesh structure formed by numerous microcrystals stacked around the center [109]. The activated carbon/graphene-like structure of CB endows it with a high specific surface area and large porosity, which enhance its affinity for adsorbates [109,110]. However, the low adsorption selectivity of CB limits its ability to remove specific metal ions [111]. Therefore, grafting functional groups with adsorption selectivity (such as the AO group mentioned earlier) onto the surface of CB is an effective method to significantly improve its selective adsorption capacity for target metal ions. Moreover, the non-polar and hydrophobic nature of CB results in poor dispersion and solubility in aqueous solvents, as well as weak adhesion to heavy metal ions [112]. To address this, optimizing the surface composition and properties of CB through appropriate physical and chemical modification methods has become a key strategy to enhance its adsorption performance and dispersion in aqueous environments. In 2024, He et al. successfully grafted AO groups onto the CB surface using plasma-induced grafting technology. Figure 7b illustrates the mechanistic flowchart of CB-AO [113]. The modified adsorbent achieved U adsorption capacities of 220.95 mg/g in aqueous solutions and 3.2 mg/g in dynamic simulated seawater, demonstrating excellent adsorption performance [113]. Currently, an increasing number of novel carbon materials are being utilized as substrates for radioactive nuclide adsorbents. The successful application of plasma technology to these materials also indicates that plasma technology is expected to open up broader research opportunities in the development of novel materials and the treatment of radioactive wastewater in the future.

Figure 7. The application of other LTP-modified carbon materials. (a) Adsorption and monitoring of U using CDs/SBA-NH₂ nanocomposites [108]. (b) The mechanistic flowchart of CB-AO synthesis [113].

Figure 7. The application of other LTP-modified carbon materials. (a) Adsorption and monitoring of U using CDs/SBA-NH₂ nanocomposites [108]. (b) The mechanistic flowchart of CB-AO synthesis [113].

3.2. Plasma-Modified Carbon Materials for Absorption of Other Nuclides

In the study of plasma-modified carbon materials for the adsorption of radioactive nuclides, U has been the most extensively researched target, while studies on other nuclides such as I [114,115], Eu [81,95,116], Th [81,95], Sr, and Cs [81,117] are relatively limited. Carbon-based nanomaterials have gradually become the preferred substrates for the adsorption of other nuclides due to their unique advantages in mitigating radioactive contamination. For example, CNT-functionalized membranes prepared via Ar/O₂ plasma treatment exhibit high selectivity for Sr²⁺. As shown in Figure 8a, the distribution coefficient (K_d) values of CNT membranes for the target metal ions generally increased with rising solution pH within the studied pH range. The K_d value of Sr²⁺ on the P-MWCNT membrane ranged from 1.90 mmol/g to 4.36 mmol/g, while the K_d values for Ca²⁺ and Mg²⁺ exhibited minimal variation. These results indicate that, within the investigated pH range, the P-MWCNT membrane exhibited a higher adsorption capacity for all three divalent metal ions compared to the MWCNT membrane [117]. Additionally, MWCNTs are particularly favored for their remarkable surface properties. When combined with LTP modification techniques, the resulting adsorbents display excellent adsorption performance. For instance, a ZnO/MWCNT nanocomposite prepared using MWCNTs and the electric arc discharge method achieved a removal efficiency of 94.76% for radioactive ¹³¹I⁻. The filtrate from the first cycle was used to repeat the removal process, which was considered the second cycle, and this procedure continued until the fifth cycle. As depicted in Figure 8b, the removal efficiency for ¹³¹I⁻ reached an exceptionally high value after the fifth cycle [115]. Sodium carboxymethyl cellulose/iron oxide/MWCNT composites prepared by combining MWCNTs with LTP demonstrated an adsorption capacity for Eu(III) of up to 3.36 × 10⁻⁴ mol/g. This study also explained that the adsorption of Eu(III) ions by this material is primarily governed by outer-sphere surface complexation at low pH or inner-sphere surface complexation at high pH [116]. In comparison, studies on other carbon materials are relatively scarce. For example, under plasma discharge conditions, an I recovery of 89% was achieved by using waste granular activated carbon treated with 20% NaOH and 50% ethanol [114]. LTP-modified carbon materials have also demonstrated significant advantages in removing various radioactive nuclides, increasingly becoming a technical approach for developing efficient adsorbents. It is evident that LTP-induced grafting technology has become a key method that is widely applied in this field.

This section focused on the research progress on carbon materials modified by LTP for radioactive nuclides, as shown in

Table 2. Plasma modification of different carbon materials for adsorption of U and other nuclides.

No.	Adsorption Target	Carbon Material	Structure of Plasma Source and Its Discharge Modes	Excitation Source	Working Gas and Gas Pressure	Adsorption Mechanism	Functional Groups and Modification Methods	Adsorption Capacity	Ref.
1	U(VI) Eu(III) Co(II)	GO	Dielectric barrier discharge (DBD) plasma	Power: 240 W; voltage: 120 V; time: 30 min; room temperature	Atmospheric pressure	Complexation between nitrogen- and oxygen-containing functional groups and radionuclides	A large number of nitrogenous and oxygen-containing functional groups; grafting	At pH = 5.0 ± 0.1 and T = 295 K, the adsorption capacity of PAM/GO for U(VI), Eu(III), and Co(II) was 0.698, 1.245, and 1.621 mmol/g, respectively	Song et al., 2015 [80]
2	U(VI)	Magnetic GO	-	Power: 120 W; voltage: 600 V; current: 20 mA	N2, 10 Pa	Inner-sphere surface complexation	Oxygen- and nitrogen-containing functional groups; grafting	At pH = 4 and T = 293 K, the adsorption capacity of AO/mGO was 435 mg/g and 2.85 mg/g in the South China Sea	Hu et al., 2018 [81]
3	Uranyl	Biochar	-	A high-voltage pulsed DC voltage device; power: 100 W	-	Surface complexation and electrostatic interactions	C-O, C=O, and -COO; grafting	At pH = 5 and T = 298 K, the adsorption capacity was 207.02 mg/g	Yi et al., 2019 [84]
4	U(VI)	Biochar	-	Power: 100 W; time: 2 hr	N2, 1.8 Pa	1. Electron transfer reaction 2. Complexation of -NH2, P-OH/P=O and C-OH groups	-NH2, phosphate group, -OH group; grafting	At pH = 6, T = 298 K, and time = 1 h, the adsorption capacity was 648.54 mg/g	Chen et al., 2022 [85]
5	U(VI)	Biochar	Radio frequency (RF) plasma	Power: 200 W	NH3, 4.0 Pa	The Faraday side reaction was mainly introduced	Nitrogen-containing and oxygen-containing groups; etching	At pH = 4 and T = 298 K, the adsorption capacity was 466.72 mg/g and the electroadsorption efficiency of biocarbon for U(VI) was 94.45%; the electroadsorption capacity in seawater was 78.34 mg/g	Wang et al., 2023 [87]
6	UO22+	MWCNTs	Customized grafting reactors	Power: 70 W; voltage: 650 V; current: 60 mA	N2, 10 Pa	Strong complexation ability of CMC with metal ions	-NH2 and CMC; grafting	At pH = 5, T = 298 K, and m/v = 0.4 g/L, the ionic strength was 0.01 mol NaClO4 and the adsorption capacity was 111.86 mg/g	Shao et al., 2009 [88]
7	UO22+ Cu2+ Pb2+	MWCNTs	Customized grafting reactors	Power: 70 W; voltage: 650 V; current: 60 mA	N2, 10 Pa	The functional groups of the material formed strong complexes with metal ions	UO22+: -OH and other functional groups; grafting	At pH = 5.0 ± 0.1, T = 20 ± 1 °C, time = 24 h, m/v = 0.4 g/L, and C[NaClO4] = 0.01 mol/L, the adsorption capacity of UO22+ was 39.2 mg/g	Shao et al., 2010 [89]
8	U(VI)	MWCNTs	Graft reactor	Power: 70 W; voltage: 650 V; current: 60 mA	N2, 10 Pa	Inner-sphere surface complexation dominated	The functional groups of CS; grafting	At pH = 5.0 ± 0.1, T = 20 ± 1 °C, time = 48 h, m/v = 0.4 g/L, and C[NaClO4] = 0.01 mol/L, the adsorption capacity was 41 mg/g	Chen et al., 2012 [91]
9	U(VI)	MWCNTs	In a custom-made grafting reactor	Power: 100 W; voltage: 800 V; current: 15 mA	N2, 10 Pa	Surface complexation	AO; grafting	At pH = 4.5, the adsorption capacity for U(VI) was 145 mg/g (0.61 mmol/g)	Wang et al., 2014 [20]
10	U(VI)	CNTs	RF plasma	Power: 100 W; time: 20 min	O2, vacuum environment	The synergistic effect of abundant oxygen- and nitrogen-containing functional groups within AO groups on CNTs facilitated the process, and when U(VI) reached the surface of CNTs-AO, complex formation or ion exchange reactions took place	Oxygen-containing functional groups and nitrogen-containing functional groups; grafting	At pH = 6 and T = 303 K, the adsorption capacity was 275.98 mg/g	He et al., 2024 [93]
11	UO22+	MWCNTs	RF plasma	Power: 80 W	O2, 20 Pa	Ion exchange and outer-sphere surface complexation	-COOH, carbonyl (C=O), and -OH groups; modification	At pH = 5.6 ± 0.1, T = 343.15 K, m/v = 0.3 g/L, C(NaClO4) = 0.01 M, the adsorption capacity was 4.06 × 10−4 mol/g	Song et al., 2012 [118]
12	U(VI) Th(IV) Eu(III)	Graphite-embedded magnetic nanoparticles	One-step arc discharge	Voltage: 650 V; current: 120 A; the discharge was produced by gradually decreasing the distance between the two rods	Gas mixture of He/CH4/NH (NH3: 0–5.0%; He/CH4 = 2:1), 80 Torr	Inner-sphere surface complexation	Quaternary, pyrrolic, amino, and pyridinic N	At pH = 4.0 ± 0.1 and T = 298.15 K, the adsorption capacities for U(VI), Th(IV), and Eu(III) were 47.28 mg/g, 45.48 mg/g, and 32.21 mg/g, respectively	Xiao et al., 2018 [95]
13	U(VI)	FG	-	The HV pulsed DC voltage; power: 100 W	3.9 Pa	Complexation of U(VI) with -NH2 and phenolic hydroxyl groups on the surface of modified FG	-NH2 and -OH; grafting	At pH 6.0 ± 0.1 and T = 333.15 K, the adsorbent concentration = 0.25 g/L, the adsorption capacity was 140.68 mg/g	Duan et al., 2017 [15]
14	U(VI)	GO/PPy	DBD plasma	Power: 200 W; voltage: 100–110 V; time: 30 min; room temperature	N2	Mainly attributed to surface complexation due to the coordination of U(VI) ions with oxygen- and nitrogen-containing functional groups	Nitrogen- and oxygen-containing functional groups; grafting	At pH = 5.0 ± 0.1 and T = 298 ± 2 K, the adsorption capacity was 147.1 mg/g	Hu et al., 2014 [100]
15	U(VI)	AGO	RF plasma	Power: 100 W	Ar, 10 Pa	Coordination of -NH2 functional groups	Graphite’s original functional group and -NH2 group	At pH = 5 and T = 298 K, the adsorption capacity was 341.5 mg/g	Liao et al., 2021 [22]
16	U(VI)	CMC/MGOs	Customized reactors	Power: 120 W; voltage: 950 V	N2, 10 Pa	Inner-sphere surface complexation	Hydroxyl group, carboxymethyl group, epoxy group, etc.	At pH = 5.5 ± 0.1, T = 301 K, and m/v = 0.25 g/L, the adsorption capacity was 7.94 × 10−4 mol/g	Zong et al., 2019 [101]
17	U(VI)	Biochar/MoS2 composites	RF plasma	Power: 180 W	H2, 30 Pa	The S vacancies, S, C-O and P-O of the BDC/MoS2-PO4 were bonded to [O = U = O]2+ in the solution	Modification	At pH = 6, the adsorption capacity was 204.08 mg/g	Sun et al., 2022 [21]
18	238U (VI)241 Am(III)	AO/carbon nanofiber hybrids	Customized grafting reactors	Power: 70 W; voltage: 650 V; current: 60 mA	N2, 10 Pa	At pH = 5.0–7.0: inner-sphere surface complexation/surface precipitated; at pH = 3.5: inner-sphere surface complexation was formed on AO/CNF	AO; grafting	At pH = 3.5 and T = 293 K, the adsorption capacities for 238U(VI) and 241Am(III) were 588.24 mg/g and 40.79 mg/g, respectively	Sun et al., 2017 [119]
19	U(VI)	CDs	Atmospheric-pressure microplasma	Current: 10 mA	60 sccm Ar	-	−COOH, -OH, etc.	At pH = 5, T = 298.15 K, and m/v = 0.5 mg/mL, the adsorption capacity was 173.60 mg/g	Wang et al., 2017 [108]
20	U(VI)	CB	RF plasma	Power: 60 W; time: 30 min	Carrier gas O2/Ar (5:25 ratio), <30 Pa	Adsorption was closely related to the single-site or double-site chelation of U(VI) with -NH2 and -C=N-OH, respectively	AO, oxygen-containing functional groups (mainly -COO); grafting	At pH = 6, T = 303 K, dosage = 0.4 g/L, and time = 24 h, the adsorption capacity was 220.95 mg/g in aqueous solution; at pH = 8.3, T = 293 K, dosage = 0.1 mg/L, and C[U (VI)] = 4.0 μg/L, the adsorption capacity was 3.2 mg/g in dynamic simulated seawater	He et al., 2024 [113]
21	U(VI)	C core–shell	RF plasma	Voltage: 5000 V; current: 1.0 mA	Ar, 200 sccm	The uranyl and -AsO2(OH) groups produced a strong affinity through chelation	Arsenazo III; grafting	At pH = 4, T = 298 K, C[U(VI)] = 2 × 10−5 mol/L, m/v = 0.6 g/L, and ionic strength = 0.01 mol/L NaCl, the adsorption capacity was 46.2 mg/g	Li et al., 2018 [104]
22	I	AC	Underwater plasma discharge	Power: 600 W; voltage: 2100 V	-	-	-OH, etc.	At 20% NaOH and 50% ethanol and time = 48 h, the adsorption capacity was 849 mg/g in water and the recovery of I adsorption capacity was 89%	Park et al., 2018 [114]
23	127I− 131I−	ZnO/ MWCNTs nanocomposite	Arc discharge	Constant voltage: 2100 V alternating current: 15 A; each discharge time: 2 min; total discharge time: 30 min	-	Multi-layer physical adsorption	-	At pH = 5, T = 25 °C, and time = 60 min, the adsorption capacity was 15.25 mg/g	El- Khatib et al., 2024 [115]
24	Eu(III)	CMC/iron oxides/ MWCNTs	Customized grafting reactors	Power: 70 W; voltage: 650 V; current: 60 mA	N2, 10 Pa	At low pH, the main interaction mechanism was outer-sphere surface complexation, and at high pH, it was inner-sphere surface complexation	Multiple hydroxyl and carboxyl functional groups; grafting	At pH = 6.0 ± 0.1, T = 298 K, m/v = 0.6 g/L, and ionic strength = 0.01 mol/L NaNO3, the adsorption capacity was 3.36 × 10−4 mol/g	Zong et al., 2018 [116]
25	Sr(II) Cs(I)	CNT membrane	RF plasma	Power: 80 W	70 Sccm3 min−1 for Ar and 40 Sccm3 min−1 for O2, 2 Pa	The removal mechanisms of divalent cations by adsorbents usually involved inner-sphere complexation reactions between the metal ions and the electron-pair donor atoms available on the surface of the adsorbents and the monovalent cations was primarily induced by electrostatic or Coulombic attraction between negatively charged CNTs	Functionalization	The partition coefficient was 4.14 for Sr and 0.81 for Cs	Ali et al., 2020 [117]

. More importantly, Table 2 also compares the adsorption performance exhibited by different materials after employing LTP-induced grafting or etching modification techniques, thus providing valuable reference data for subsequent research. In the future, the development of adsorbents with higher selectivity and adsorption performance using LTP technology for radioactive nuclide adsorption will be an important goal. To this end, it is necessary to explore the adsorption mechanisms of materials in depth to optimize the selection of substrate materials. At the same time, adjusting the critical parameters of plasma treatment to further enhance the modification effect and strengthening research on adsorption applications under real environmental conditions will provide strong support for advancing this field.

4. Conclusions and Future Perspectives

This paper reviewed the research progress of carbon materials in the adsorption of radionuclides and focused on the recent advancements for plasma-modified carbon materials in this field. The research has mainly focused on the adsorption performance of carbon-based materials such as GO, biochar, CNTs, AC, fullerenes, and carbon composites. After plasma modification, the adsorption capacity of these materials for radionuclides is significantly enhanced. This approach could aid in nuclear waste management and environmental protection by reducing treatment costs and environmental impacts.

Despite the remarkable adsorption performance imparted to carbon-based materials through plasma modification, their practical application faces several significant limitations and challenges including (1) the high cost of modification equipment, coupled with insufficient investigation into the uniformity of modified products; (2) the unclear underlying modification mechanisms, hindering the industrial-scale production of high-performance adsorbents; (3) the interference from competing ions and environmental factors in complex systems, which hampers the realization of high selectivity and adsorption capacity for low-concentration radionuclides, necessitating further optimization; (4) the intricate and expensive synthesis and functionalization processes for some materials, restricting their large-scale adoption; (5) the limited lifespan, insufficient stability, and poor recyclability of certain materials substantially undermine their practical utility. Addressing these challenges requires advancements in modification techniques, a deeper understanding of adsorption mechanisms, and further efforts to reduce production and operational costs, thereby accelerating the practical deployment of plasma-modified carbon-based materials for radionuclide remediation.

The advancements in plasma-modified carbon materials hold transformative potential for addressing radioactive contamination. By enhancing adsorption capacities and enabling resource utilization of agricultural waste, this technology offers scalable, eco-friendly solutions for nuclear wastewater treatment. Such innovations reduce ecological risks in scenarios like the Fukushima disaster and align with circular economy principles by valorizing biomass waste. For academia, the integration of plasma physics with materials science has redefined carbon material functionalization, establishing quantitative links between plasma parameters and surface chemistry. This cross-disciplinary synergy, exemplified by carbon dots enabling simultaneous adsorption and fluorescence monitoring, could inspire novel research into smart remediation technologies. This research could also provide critical benchmarks that accelerate the development of next-generation adsorbents.

In summary, the development of carbon-based adsorbents with high adsorption capacities and selectivities, along with the establishment of efficient and cost-effective plasma-modification systems for their fabrication, represents a critical focus for future research. As plasma modification technology continues to integrate with advancements in materials science, its application potential in nuclear wastewater treatment, soil remediation, and emergency responses to nuclear incidents is expected to expand further. This progress will provide pivotal technological support for the efficient remediation of radioactive contamination.