Aryl Diazonium-Assisted Amidoximation of MXene for Boosting Water Stability and Uranyl Sequestration via Electrochemical Sorption

Advanced MXene-based materials for efficient extraction of uranium from seawater and wastewater

In order to realize the low-carbon development policy, the large-scale development and utilization of nuclear energy is very essential. Uranium is the key resource for nuclear industry. The extracting and recycling uranium from seawater and nuclear wastewater is necessary for secure uranium reserves, ensure energy security, control pollution and protect the environment. The novel nanomaterial MXene possesses the layered structure, high specific surface area, and modifiable surface terminal groups, which allowed it to enrich uranium. In addition, good photovoltaic and photothermal properties improves the ability to adsorb uranium. The excellent radiation resistance of the MAX phase strongly indicates the potential use of MXene as an effective uranium adsorbent. However, there are relatively few reviews on its application in uranium extraction and recovery. This review focuses on the recent advances in the use of MXene-based materials as highly efficient adsorbents for the recovery of uranium from seawater and nuclear wastewater. First, the structural, synthetic and characterization aspects of MXene materials are introduced. Subsequently, the adsorptive properties of MXene-based materials are evaluated in terms of uranium extraction recovery capability, selectivity, and reproducibility. Furthermore, the interaction mechanisms between uranium and MXene absorbers are discussed. Finally, the challenges for MXene materials in uranium adsorption applications are proposed for better design of new types of MXene-based adsorbents.

Unveiling the power of MXenes: Solid lubrication perspectives and future directions

The interaction between two surfaces leads to the generation of friction and wear of material. Friction and wear are some of the major challenges that may readily be overcome by the third part of tribology called lubrication. Utilizing solid lubricants including polymers, carbon-based materials, soft metals, transition metal dichalcogenides, along with their potential benefits and drawbacks in dry environments can reduce friction. Recently, an emerging class of two-dimensional (2D) transition metal nitrides, carbides or carbonitrides commonly known as MXenes have emerged as an attractive alternative for solid lubrication because of their ability to establish wear-resistant tribo layers and well as low friction and shear strength. Furthermore, the inherent hydrophilic nature of these substances has led to limited dispersion stability and phase compatibility when combined with pure base oils. As a result, their potential use as solid lubricants and lubricant additives has been impeded. To address this issue and enhance the applicability of MXenes as solid lubricants, their surface modification can be an attractive tool. Therefore, this review provides a succinct summary of the current state-of-the-art in surface functionalization of MXenes, a subject that has not yet been thoroughly addressed. Further, the mechanical behavior of MXenes and composites has been discussed, followed by the potential of MXenes as a solid lubricant at micro- and macro-scale. Finally, the existing opportunities and challenges of the research area have been discussed with possible future research directions. We believe, this article will be a valuable resource for MXenes and opens the door to improve the chemical, physical and mechanical properties of MXenes in various applications, such as solid lubrication.

Molten salt synthesis of MXene-derived hierarchical titanate for effective strontium removal

The removal and recovery of radioactive Sr(II) from wastewater and seawater has been of great concern due to the negative environmental impacts of nuclear energy development and the potential risk of nuclear accidents. Herein, a facile molten salt synthesis strategy was developed to systematically investigated the reaction of different types of MXenes with nitrates. Among the products, K+ intercalated hierarchical titanate nanostructures (K-HTNs) obtained from the direct chemical transformation of multilayered Ti3C2Tx exhibited unique layered structures, good physicochemical properties, and outstanding adsorption performance for Sr(II). The maximum adsorption capacity of Sr(II) by K-HTNs reached 204 mg·g-1 at ambient temperature, and the good regeneration and reusability of the titanate was also demonstrated. K-HTNs showed preferential selectivity for Sr(II) in different environmental media containing competing ions, and the removal efficiency of Sr(II) in real seawater was as high as 93.3 %. The removal mechanism was elaborated to be the exchange of Sr2+ with K+/H+ in the interlayers of K-HTNs, and the adsorbed Sr(II) had a strong interaction with Ti-O- termination on the titanate surface. Benefiting from the merits of rapid and scalable synthesis and excellent adsorption performance, MXene-derived K-HTNs have broad application prospects for the purification of 90Sr-contaminated wastewater and seawater.

Cinnamic Acid: A Low-Toxicity Natural Bidentate Ligand for Uranium Decorporation

Uranium accumulation in the kidneys and bones following internal contamination results in severe damage, emphasizing the pressing need for the discovery of actinide decorporation agents with efficient removal of uranium and low toxicity. In this work, cinnamic acid (3-phenyl-2-propenoic acid, CD), a natural aromatic carboxylic acid, is investigated as a potential uranium decorporation ligand. CD demonstrates markedly lower cytotoxicity than that of diethylenetriaminepentaacetic acid (DTPA), an actinide decorporation agent approved by the FDA, and effectively removes approximately 44.5% of uranyl from NRK-52E cells. More importantly, the results of the prompt administration of the CD solution remove 48.2 and 27.3% of uranyl from the kidneys and femurs of mice, respectively. Assessments of serum renal function reveal the potential of CD to ameliorate uranyl-induced renal injury. Furthermore, the single crystal of CD and uranyl compound (C9H7O2)2·UO2 (denoted as UO2-CD) reveals the formation of uranyl dimers as secondary building units. Thermodynamic analysis of the solution shows that CD coordinates with uranyl to form a 2:1 molar ratio complex at a physiological pH of 7.4. Density functional theory (DFT) calculations further show that CD exhibits a significant 7-fold heightened affinity for uranyl binding in comparison to DTPA.

Insights into enhanced immobilization of uranyl carbonate from seawater by Fe-doped MXene

Extracting uranium from seawater not only reduces radioactive contamination in seawater but also provides a source of uranium energy. However, due to the low concentration of uranium in seawater and the high salinity of seawater, extraction of uranium from seawater is challenging. In this work, we demonstrated a simple strategy to synthesize Fe-doped MXene (Fe@Ti3C2Tx) via a hydrothermal method and applied for uranium enrichment in seawater. The Fe@Ti3C2Tx exhibited excellent adsorption performance in high salinity environments. The removal capacity of Fe@Ti3C2Tx was determined to be 526.6 mg/g for UO2(CO3)22- at 328 K with quick reaction equilibrium (∼ 30 min). Kinetic and thermodynamic analyses of UO2(CO3)22- elimination process on Fe@Ti3C2Tx surface revealed it to be a spontaneous and endothermic single-phase elimination process. FT-IR and XPS analyses further indicated that the removal mechanism of UO2(CO3)22- by Fe@Ti3C2Tx was surface complexation. Our study suggests that Fe@Ti3C2Tx can provide a feasible solution for uranium enrichment in seawater.

Recent strategies, progress, and prospects of two-dimensional metal carbides (MXenes) materials in wastewater purification: A review

With the rapid development of industrialization, water pollution directly leads to the serious shortage of fresh water. As reported by the World Water Council, nearly 3.8 billion people will face water scarcity by 2030. Therefore, developing advanced nanomaterials to realize wastewater purification is a major challenge. Two-dimensional (2D) transition metal carbides (MXenes), as the emerging 2D layered nanomaterials, have been investigated for the applications of water purification treatment since first reported in 2011. Over 40 different MXenes have been developed for environmental remediation, and dozens more structures and properties are theoretically predicted. Here, we review the advances from the aspects of synthesis strategies for MXenes, purification mechanism, and their applications in wastewater treatment processes. The major points are 1) the synthesis and modification approaches for MXenes such as multi-layered stacked MXenes and delaminated MXenes 2) a discussion of current water remediation over MXene-based materials, 3) a brief introduction for removal behaviors and deep interaction mechanisms, 4) optimization strategies and key points for boosting the remediation performance of MXenes.

Flexible self-supporting Na3MnTi(PO4)3@C fibers for uranium extraction from seawater by electro sorption

As an eco-friendly technique with the superior adsorption performance, electroadsorption has shown great potential for application in uranium (U(VI)) recovery in recent years. However, the electrodes used in the electrosorption generally suffer the adsorbent to be loaded on the conductors, which greatly limited the adsorption performance of the electrodes for uranyl ions. In present study, a flexible self-supporting Na3MnTi(PO4)3@C fibers (NMTP@C fibers) electrode material was rationally designed and prepared by electrostatic spinning method and annealing technique, and its ability to capture U(VI) efficiently was preliminarily demonstrated by batch adsorption and electro sorption. The plentiful phosphate groups provide sufficient active sites for adsorption, while the axially continuous electron conduction and radially short-range ion transport give NMTP@C fibers fast charge/ion transport capability. The NMTP@C fiber can remove 99% of 5 ppm U(VI) in seawater by electro absorption within 1 h. After several cycles of adsorption under seawater conditions, the adsorbent can still maintain a stable adsorption capacity. The adsorption mechanism of NMTP@C nanofibers for U(VI) was investigated by XPS, FT-IR, Raman, SEM-EDS, and XRD, which was electrostatic interactions and surface complexation. These results suggest that NMTP@C fibers are promising high-capacity adsorbents for efficient and selective capture of U(VI) from seawater.

Synergistically adsorbing and reducing Uranium from water by a novel nano zero-valent copper/MXene 0D/2D nanocomposite

Proper disposal of uranium-containing waste is of utmost importance for safeguarding the environment and human health. In this study, we proposed a novel zero-dimensional (0D)/two-dimensional (2D) nanocomposite material, nZVC/Ti3C2, composed of nano zero-valent copper (nZVC) nanoparticles loaded onto Ti3C2 MXene nanoflakes, which was prepared using a simple in situ chemical reduction method. The uniform dispersion of 0D nZVC nanoparticles, with a size of approximately 5 nm, onto the 2D ultrathin Ti3C2 MXene effectively prevented agglomeration and corrosion of nZVC. This unique configuration provided numerous adsorption sites for UO22+and facilitated a fascinating charge channel for reducing adsorbed UO22+ into low-mobilized UO2 by nZVC. Under the synergistic effect of Ti3C2 MXene and nZVC, remarkable efficiency and selectivity of nZVC/Ti3C2 for U (VI) removal were demonstrated, which exhibited an exceptional adsorption capacity of up to 360 mg/g, coupled with a high removal efficiency of 97.5 % and rapid kinetics. Importantly, the presence of humic acid did not significantly affect the U (VI) removal efficiency of the composite because of the reduction effect of nZVC. The underlying mechanism of U (VI) removal was elucidated, revealing the involvement of reductive immobilization in the form of UO2 (as high as 73.6 %), inner-sphere surface complexation, and hydrolytic precipitation. This mechanism was dependent on the availability of active nZVC and the solution's pH. These findings highlight the potential of nZVC/Ti3C2 composites as efficient decontaminants for radioactive wastewater, thus contributing to advancements in environmental remediation endeavors.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Recent developments and challenges in uranium extraction from seawater through amidoxime-functionalized adsorbents

As per statistical estimations, we have only around 100 years of uranium life in terrestrial ores. In contrast, seawater has viable uranium resources that can secure the future of energy. However, to achieve this, environmental challenges need to be overcome, such as low uranium concentration (3.3 ppb), fouling of adsorbents, uranium speciation, oceanic temperature, and competition between elements for the active site of adsorbent (such as vanadium which has a significant influence on uranium adsorption). Furthermore, the deployability of adsorbent under seawater conditions is a gigantic challenge; hence, leaching-resistant stable adsorbents with good reusability and high elution rates are extremely needed. Powdered (nanostructured) adsorbents available today have limitations in fulfilling these requirements. An increase in the grafting density of functional ligands keeping in view economic sustainability is also a major obstacle but a necessity for high uranium uptake. To cope with these challenges, researchers reported hundreds of adsorbents of different kinds, but amidoxime-based polymeric adsorbents have shown some remarkable advantages and are considered the benchmark in uranium extraction history; they have a high affinity for uranium because of electron donors in their structure, and their amphoteric nature is responsible for effective uranium chelation under a wide range of pH. In this review, we have mainly focused on recent developments in uranium extraction from seawater through amidoxime-based adsorbents, their comparative analysis, and problematic factors that are needed to be considered for future research.

Can MXene be the Effective Nanomaterial Family for the Membrane and Adsorption Technologies to Reach a Sustainable Green World?

Environmental pollution has intensified and accelerated due to a steady increase in the number of industries, and exploring methods to remove hazardous contaminants, which can be typically divided into inorganic and organic compounds, have become inevitable. Therefore, the development of efficacious technology for the separation processes is of paramount importance to ensure the environmental remediation. Membrane and adsorption technologies garnered attention, especially with the use of novel and high performing nanomaterials, which provide a target-specific solution. Specifically, widespread use of MXene nanomaterials in membrane and adsorption technologies has emerged due to their intriguing characteristics, combined with outstanding separation performance. In this review, we demonstrated the intrinsic properties of the MXene family for several separation applications, namely, gas separation, solvent dehydration, dye removal, separation of oil-in-water emulsions, heavy metal ion removal, removal of radionuclides, desalination, and other prominent separation applications. We highlighted the recent advancements used to tune separation potential of the MXene family such as the manipulation of surface chemistry, delamination or intercalation methods, and fabrication of composite or nanocomposite materials. Moreover, we focused on the aspects of stability, fouling, regenerability, and swelling, which deserve special attention when the MXene family is implemented in membrane and adsorption-based separation applications.

Synthesis and applications of MXene-based composites: a review

Recently, there has been considerable interest in a new family of transition metal carbides, carbonitrides, and nitrides referred to as MXenes (Ti₃C₂T_x) due to the variety of their elemental compositions and surface terminations that exhibit many fascinating physical and chemical properties. As a result of their easy formability, MXenes may be combined with other materials, such as polymers, oxides, and carbon nanotubes, which can be used to tune their properties for various applications. As is widely known, MXenes and MXene-based composites have gained considerable prominence as electrode materials in the energy storage field. In addition to their high conductivity, reducibility, and biocompatibility, they have also demonstrated outstanding potential for applications related to the environment, including electro/photocatalytic water splitting, photocatalytic carbon dioxide reduction, water purification, and sensors. This review discusses MXene-based composite used in anode materials, while the electrochemical performance of MXene-based anodes for Li-based batteries (LiBs) is discussed in addition to key findings, operating processes, and factors influencing electrochemical performance.

Grafting Behavior of Amine Ligands for Surface Modification of MXene

Surface modification to improve the oxidation stability and dispersibility of MXene in diverse organic media is a facile strategy for broadening its application. Among the various ligands that can be grafted on the MXene surface, oleylamine (OAm), with amine functionalities, is an advantageous candidate owing to its strong interactions and commercial viability. OAms are grafted onto MXene through covalent bonds induced by nucleophilic reactions and H bonds in liquid interface reactions at room temperature. In addition, this grafting behavior of the ligand was characterized by a reduction in the slope with an increase in the ligand concentration (C_l), confirming that the OAms were grafted via Langmuir-like behavior, and the monolayer of OAms was developed via two distinct steps (I: lying-down phase; II: ordered monolayer). MXene nanosheets modified by OAm (OAm-MX) are highly dispersible in a wide range of organic solvents owing to the alkyl chain of the OAms, which induces hydrophobic properties on the surface of MXene. The OAm-MX dispersion exhibits outstanding oxidation and dispersion stability and remarkable coating performance on a wide range of substrates owing to their excellent solution processability. Therefore, this study provides fundamental insights into the adsorption behavior and interaction between amine ligands and MXene nanosheets for the surface chemistry of MXene.

Hierarchical Self-Assembled Polyimide Microspheres Functionalized with Amidoxime Groups for Uranium-Containing Wastewater Remediation

Through molecule self-assembly and subsequent surface functionalization, novel uranium adsorbent AO-OB hierarchical self-assembled polyimide microspheres (AO-OBHSPIMs) were obtained by introducing the amidoxime groups into hierarchical self-assembled polyimide microspheres for the efficient and selective recovery of uranium from wastewater. The results of Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD), and nitrogen adsorption-desorption isotherm showed that AO-OBHSPIMs were a semicrystalline polymer material with self-supporting hierarchical structure and low pore volume, and they were equipped with abundant amidoxime groups. Given the recognized selectivity of amidoxime groups and their hierarchical structure, AO-OBHSPIMs exhibited excellent selectivity to uranyl ions. Moreover, AO-OBHSPIMs exhibited good stability and recyclability and remarkable removal percentage within low-concentration solution (99.4%) and simulated uranium-containing wastewater (97.3%). AO-OBHSPIMs could be applied to fixed-bed column adsorption due to their large particle size and self-supporting hierarchical structure that can facilitate water flow. The in-depth discussion of the adsorption mechanism showed that the adsorption mainly depended on the combined action of electrostatic interactions and complexation, and the adsorption process was a spontaneous endothermic monolayer adsorption. In summary, AO-OBHSPIMs exhibited good application prospects in uranium-containing wastewater remediation.

Uranium capture by a layered 2D/2D niobium phosphate/holey graphene architecture via an electro-adsorption and electrocatalytic reduction coupling process

As an energy-efficient and eco-friendly technique, capacitive deionization (CDI) has shown great potential for uranium (U(VI)) capture recently. However, extracting U(VI) with high kinetics, capacity and selectivity remains a major challenge due to the current surface active sites-based material and co-existing ions in aqueous solution. Here we rationally designed a layered 2D/2D niobium phosphate/holey graphene (HGNbP) electrode material, and originally demonstrated its efficient U(VI) capture ability via an electro-adsorption and electrocatalytic reduction coupling process. The less-accumulative loose layered architecture, open polycrystalline construction of niobium phosphate with active phosphate sites, and rich in-plane nano-pores on conductive graphene nanosheets endowed HGNbP with fast charge/ion transport, high electroconductivity and superior pseudocapacitance, which enabled U(VI) ions first to be electro-adsorbed, then physico-chemical adsorbed, and finally electrocatalysis reduced/deposited onto electrode surface without the limitation of active sites under a low potential of 1.2 V. Based on these virtues, the HGNbP exhibited a fast adsorption kinetics, with a high removal rate of 99.9% within 30 min in 50 mg L^-1 U(VI) solution, and a high adsorption capacity up to 1340 mg g^-1 in 1000 mg L^-1 U(VI) solution. Furthermore, the good recyclability and selectivity towards U(VI) were also realized.

Rapid growth of MXene-based membranes for sustainable environmental pollution remediation

Water consumption has grown in recent years due to rising urbanization and industry. As a result, global water stocks are steadily depleting. As a result, it is critical to seek strategies for removing harmful elements from wastewater once it has been cleaned. In recent years, many studies have been conducted to develop new materials and innovative pathways for water purification and environmental remediation. Due to low energy consumption, low operating cost, and integrated facilities, membrane separation has gained significant attention as a potential technique for water treatment. In these directions, MXene which is the advanced 2D material has been explored and many applications were reported. However, research on MXene-based membranes is still in its early stages and reported applications are scatter. This review provides a broad overview of MXenes and their perspectives, including their synthesis, surface chemistry, interlayer tuning, membrane construction, and uses for water purification. Application of MXene based membrane for extracting pollutants such as heavy metals, organic contaminants, and radionuclides from the aqueous water bodies were briefly discussed. Furthermore, the performance of MXene-based separation membranes is compared to that of other nano-based membranes, and outcomes are very promising. In order to shed more light on the advancement of MXene-based membranes and their operational separation applications, significant advances in the fabrication of MXene-based membranes is also encapsulated. Finally, future prospects of MXene-based materials for diverse applications were discussed.

Bifunctional Amine- and Thiol-Modified Ti3C2Tx MXene for Trace Detection of Heavy Metals

Surface functionalization of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, also known as MXenes, is a powerful approach for modification of their physical and chemical properties for new applications. In this study, we demonstrate the synthesis of a bifunctional Ti₃C₂T_x MXene modified with amine and thiol groups through a facile condensation reaction. We successfully employed the resulting NH₂/SH-Ti₃C₂T_x MXene as a solid phase in the ultrasonic-assisted dispersive micro solid-phase extraction (d-μ-SPE) method for the analytical determination of heavy metals at trace levels in food and soil samples. The prepared NH₂/SH-Ti₃C₂T_x MXene showed remarkable performance in the ultrasonic-assisted d-μ-SPE method with limits of detection of 0.12 and 2.30 ng mL^-1, with linear dynamic ranges of 0.50-90 μg L^-1 and 10-120 μg L^-1 for cadmium (Cd²⁺) and lead (Pb²⁺) ions, respectively. Furthermore, the extraction efficiencies were greater than 97%, with a relative standard deviation of less than 3% for five separate batch experiments in the determination of 5.0 μg L^-1 of Cd²⁺ and Pb²⁺. This study shows that NH₂/SH-Ti₃C₂T_x can be used as a simple, rapid, reliable, selective, and sensitive material in the d-μ-SPE method for the trace determination of Cd²⁺ and Pb²⁺ in soil and agricultural products. This study demonstrates the utility of MXenes for analytical chemistry and suggests that further advances in methods for the functionalization of MXenes can open new applications for these already exciting materials.

An overview of MXene-Based nanomaterials and their potential applications towards hazardous pollutant adsorption

With the massive development of industrialization, multiple ecological contaminants in gaseous, liquid, and solid forms are vented into habitats, which is currently at the forefront of worldwide attention. Because of the possible damage to public health and eco-diversity, high-efficiency clearance of these environmental contaminants is a serious concern. Improved nanomaterials (NMs) could perform a significant part in the exclusion of contaminants from the atmosphere. MXenes, a class of two-dimensional (2D) compounds that have got tremendous consideration from researchers for a broad array of applications in a variety of industries and are viewed as a potential route for innovative solutions to identify and prevent a variety of obstreperous hazardous pollutants from environmental compartments due to their exceptional innate physicochemical and mechanical features, including high specific surface area, physiological interoperability, sturdy electrodynamics, and elevated wettability. This paper discusses the recent progress in MXene-based nanomaterials' applications such as environmental remediation, with a focus on their adsorption-reduction characteristics. The removal of heavy metals, dyes, and radionuclides by MXenes and MXene-based nanomaterials is depicted in detail, with the adsorption mechanism and regeneration potential highlighted. Finally, suggestions for future research are provided to ensure that MXenes and MXene-based nanomaterials are synthesized and applied more effectively.

Surface Functionalized MXenes for Wastewater Treatment-A Comprehensive Review

Over 80% of wastewater worldwide is released into the environment without proper treatment. Whilst environmental pollution continues to intensify due to the increase in the number of polluting industries, conventional techniques employed to clean the environment are poorly effective and are expensive. MXenes are a new class of 2D materials that have received a lot of attention for an extensive range of applications due to their tuneable interlayer spacing and tailorable surface chemistry. Several MXene-based nanomaterials with remarkable properties have been proposed, synthesized, and used in environmental remediation applications. In this work, a comprehensive review of the state-of-the-art research progress on the promising potential of surface functionalized MXenes as photocatalysts, adsorbents, and membranes for wastewater treatment is presented. The sources, composition, and effects of wastewater on human health and the environment are displayed. Furthermore, the synthesis, surface functionalization, and characterization techniques of merit used in the study of MXenes are discussed, detailing the effects of a range of factors (e.g., PH, temperature, precursor, etc.) on the synthesis, surface functionalization, and performance of the resulting MXenes. Finally, the limits of MXenes and MXene-based materials as well as their potential future research directions, especially for wastewater treatment applications are highlighted.

Covalent organic frameworks functionalized electrodes for simultaneous removal of UO22+ and ReO4- with fast kinetics and high capacities by electro-adsorption

The recovery of radioactive ions from high salinity low-level radioactive wastewater (LLRW) is important for the sustainable utilization of nuclear energy. Previous work primarily focuses on developing adsorbents that remove individual types of ions via physicochemical adsorption. Here, we report a new strategy for the simultaneous recovery of uranium (UO₂²⁺) and rhenium (ReO₄^-) as a non-radioactive surrogate of technetium from LLRW via electro-adsorption. Carboxyl functionalized covalent organic frameworks (COF-1) and cationic covalent organic frameworks (COF-2) were prepared as cathode and anode materials, respectively. The adsorption capacities were 411 mg U/g for COF-1 and 984 mg Re/g for COF-2 under 1.2 direct-current (DC) volts, 2.5 and 2.1 times higher than the capacities of the same adsorbents obtained by physicochemical adsorption. We also found that the electro-adsorption of uranium and rhenium follows pseudo-second-order kinetics with the adsorption rates of 0.45 and 1.05 g/mg/h at pH 7.0 and 298.15 K, again two times faster than those measured in physicochemical adsorption. Therefore, electro-adsorption improves both adsorption capacity and kinetics by maximizing the utility of available active sites in adsorbents and facilitating ion migration towards the adsorbents. The adsorption efficiencies for uranium and rhenium reached 65.9% and 89.2%, respectively, after electro-adsorption for 2 h. The high efficiencies can be maintained after five adsorption-desorption cycles. Furthermore, the electrodes showed high selectivity for uranium(VI) and rhenium(VII) and excellent salt resistance even in 1 mol/L NaCl solution. XPS studies revealed that covalent bonds were formed between uranium(VI) and carboxyl groups on COF-1, and rhenium(VII) was bound to cationic COF-2 through electrostatic interaction. Our asymmetric electrodes design can be extended to simultaneously and efficiently remove other types of radioactive or heavy metal ions from wastewater.

Efficient enrichment of U(VI) by two-dimensional layered transition metal carbide composite

With the rapid development of nuclear energy, how to safely and efficiently dispose of radioactive waste solution has become an urgent environmental problem of public concern. It is of great significance to construct a new type of high-efficiency adsorbent material to recover uranium from nuclear waste solution. In this work, the Ti3C2Tx material (an emerging two-dimensional inorganic layered material) with a stable layered structure was used as the matrix, and the amidoxime functionalized MXene composite material (PAO/Ti3C2Tx) was synthesized by in-situ polymerization. The amidoxime-functionalized Ti3C2Tx showed excellent capacity to capture U(VI), with a maximum adsorption capacity of 98.04 mg/g at 25 °C, which was significantly better than that of Ti3C2Tx, and the adsorption selectivity for U(VI) was greatly improved. The adsorption was conformed to Langmuir isotherm model and pseudo-second-order kinetic model. In addition, the adsorbed UO22+ could be effectively desorbed by 0.1 M HNO3, and the adsorption performance of PAO/Ti3C2Tx did not decrease significantly after 5 adsorption/desorption cycles. The results of ionic strength experiment, FT-IR, SEM, and XPS jointly indicated that adsorption mechanism of U(VI) on PAO/Ti3C2Tx was the combined effect of the amidoxime group and -O and -OH active groups on the surface of Ti3C2Tx, mainly inner complexation. These advantages make PAO/Ti3C2Tx composite a highly potential U(VI) adsorbent with great application prospects.

Efficient and selective adsorption of U(VI) by succinic acid modified iron oxide adsorbent

The iron oxide surface was modified with succinic acid moiety and the adsorbent obtained, Fe-SUC, was evaluated for the adsorption of U(VI) (Uranium (VI)) from aqueous solution. The Fe-SUC was characterized by FT-IR (Fourier Transform Infrared Spectroscopy), Raman spectroscopy, thermogravimetry, X-ray diffraction, SEM-EDX (Scanning Electron Microscope - Energy-dispersive X-ray Spectroscopy), and particle size analysis. The adsorption behavior of U(VI) on Fe-SUC was studied as a function of pH, contact time, and concentration of U(VI) in the aqueous phase. The adsorption of U(VI) increased with increase in the pH of aqueous phase, and the adsorption saturation occurred at pH = 6. The kinetic data obtained for the adsorption of U(VI) on Fe-SUC were modeled with the pseudo-first-order and pseudo-second-order rate models. Similarly, the U(VI) adsorption isotherm was fitted with Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich adsorption isotherm models. The Langmuir adsorption capacity of U(VI) on Fe-SUC was about ∼176 mg g−1. The selectivity of the adsorbent toward U(VI) was evaluated in the presence of several possible interfering ions. The adsorbed U(VI) was recovered by 0.5 M sodium carbonate solution and the spent adsorbent was tested for its reusability.

Roles of Metal Ions in MXene Synthesis, Processing and Applications: A Perspective

With a decade of effort, significant progress has been achieved in the synthesis, processing, and applications of MXenes. Metal ions play many crucial roles, such as in MXene delamination, structure regulation, surface modification, MXene composite construction, and even some unique applications. The different roles of metal ions are attributed to their many interactions with MXenes and the unique nature of MXenes, including their layered structure, surface chemistry, and the existence of multi-valent transition metals. Interactions with metal ions are crucial for the energy storage of MXene electrodes, especially in metal ion batteries and supercapacitors with neutral electrolytes. This review aims to provide a good understanding of the interactions between metal ions and MXenes, including the classification and fundamental chemistry of their interactions, in order to achieve their more effective utilization and rational design. It also provides new perspectives on MXene evolution and exfoliation, which may suggest optimized synthesis strategies. In this respect, the different effects of metal ions on MXene synthesis and processing are clarified, and the corresponding mechanisms are elaborated. Research progress on the roles metal ions have in MXene applications is also introduced.

MXenes as emerging nanomaterials in water purification and environmental remediation

Environmental pollution has accelerated and intensified because of the acceleration of industrialization, therefore fabricating excellent materials to remove hazardous pollutants has become inevitable. MXenes as emerging transition metal nitrides, carbides or carbonitrides with high conductivity, hydrophilicity, excellent structural stability, and versatile surface chemistry, become ideal candidates for water purification and environmental remediation. Particularly, MXenes reveal excellent sorption capability and efficient reduction performance for various contaminants of wastewater. In this regard, a comprehensive understanding of the removal behaviors of MXene-based nanomaterials is necessary to explain how they remove various pollutants in water. The eliminate process of MXene-based nanomaterials is collectively influenced by the physicochemical properties of the materials themselves and the chemical properties of different contaminants. Therefore, in this review paper, the synthesis strategies and properties of MXene-based nanomaterials are briefly introduced. Then, the chemical properties, removal behaviors and interaction mechanisms of heavy metal ions, radionuclides, and organic pollutants by MXene-based nanomaterials are highlighted. The overview also emphasizes associated toxicity, secondary contamination, the challenges, and prospects of the MXene-based nanomaterials in the applications of water treatment. This review can supply valuable ideas for fabricating versatile MXene nanomaterials in eliminating water pollution.

Methods of synthesis, characteristics, and environmental applications of MXene: A comprehensive review

MXene, comprised of two-dimensional transition metal carbides/nitride, has emerged as a novel material suitable for environmental remediation of toxic compounds. Due to their inherent and superior physical and chemical properties, MXene is employed in separation techniques like photocatalysis, adsorption, and membrane separation. MXene is equipped with a highly hydrophilic surface, ion exchange property, and robust surface functional groups. In this review paper, a comprehensive discussion on the structural patterns, preparation, properties of MXene and its application for the removal of toxic pollutants like Radionuclide, Uranium, Thorium, and dyes is presented. The mechanism of removal of the pollutants by MXene is extensively reviewed. Synthesis of MXene based membranes, their properties, and application for water purification and properties were also discussed. This review will be highly helpful to understand critically the methods of synthesis and use of MXene material for priority environmental pollutants removal. In addition, the challenges behind the synthesis and use of MXene for decontamination of pollutants were reviewed and reported.

Ultra-high mechanical property and multi-layer porous structure of amidoximation ethylene-acrylic acid copolymer balls for efficient and selective uranium adsorption from radioactive wastewater

Adsorption uranium [U(VI)] from U-containing radioactive wastewater (URW) is a critical strategy for solving the resource shortage and environmental pollution in pace with the sustainable development of nuclear energy. However, the URW universally exhibits acidity and contains co-existing metal ions with high concentration. Herein, the amidoximation ethylene-acrylic acid copolymer balls (EAA-AO) with aciduric and super-high mechanical property were successfully synthesized through grafting diaminomaleonitrile and further treatment of amidoximation. Significantly, the mechanical properties of EAA-AO were not affected by the grafting process and maintained super-high mechanical properties. Furthermore, the -NH₂ and unreacted -CN groups in diaminomaleonitrile adjusted the pK_a to make the optimal pH be 4. In addition, the microstructure of EAA-AO was transformed from the original dense to multi-layer porous structure, which promoted the mass transfer process and the contact between uranyl ions (UO₂²⁺) and internal adsorption active sites. The adsorption capacity of EAA-AO was about 1.78 times that of EAA at pH = 4, and the adsorption capacity for U(VI) was about 8.17 times that of Ba²⁺ with the second highest adsorption capacity. Therefore, the EAA-AO exhibited ultra-high adsorption performance (q_e = 3.196 mg g^-1) in the artificial radioactive wastewater, laying a good foundation for subsequent large-scale industrial adsorption of U(VI) in nuclear industrial wastewater.

Amino-functionalized POSS nanocage-intercalated titanium carbide (Ti3C2Tx) MXene stacks for efficient cesium and strontium radionuclide sequestration

In this work, we prepared two-dimensional (2D) stack-structured aminopropylIsobutyl polyhedral oligomeric silsesquioxane (POSS-NH₂) intercalated titanium carbide (Ti₃C₂T_x) MXene material (Ti₃C₂T_x/POSS-NH₂) using a post-intercalation strategy as a potential adsorbent for the removal of cesium (Cs⁺) and strontium (Sr²⁺) ions from aqueous solutions. Ti₃C₂T_x/POSS-NH₂ exhibited unprecedented adsorption capacities of 148 and 172 mg g^-1 for Cs⁺ and Sr²⁺ ions, respectively. Batch adsorption experimental data well fitted the Freundlich isotherm model, which revealed multilayer adsorption of Cs⁺ and Sr²⁺ ions onto heterogeneous -OH, -F, -O, and -NH₂ adsorption sites of Ti₃C₂T_x/POSS-NH₂ with different energies. Ti₃C₂T_x/POSS-NH₂ exhibited rapid Cs⁺/Sr²⁺ ions adsorption kinetics and attained equilibrium within 30 min. Also, Ti₃C₂T_x/POSS-NH₂ exhibited recyclable capability over three cycles and remarkable selectivities of 89% and 93% for Cs⁺ and Sr²⁺ ions, respectively, in the presence of co-existing mono- and divalent cations. We suggest the high adsorption capacity of Ti₃C₂T_x/POSS-NH₂ might be due to the synergistic effects of (i) increased inter-lamellar distance between Ti₃C₂T_x galleries due to POSS-NH₂ intercalation, enabling diffusion and encapsulation of large numbers of Cs⁺/Sr²⁺ ions, (ii) strong complexation of amine (-NH₂) groups of POSS-NH₂ with Cs⁺/Sr²⁺ ions, and (iii) the presence of large numbers of heterogeneous surface functional groups (e.g., -OH, -F, and -O), which resulted in the adsorptions of Cs⁺/Sr²⁺ ions through electrostatic, ion exchange, and surface complexation mechanisms. Given the extraordinary adsorption capacities observed, intercalation appears to be a promising strategy for the effective removal of radioactive Cs⁺ and Sr²⁺ ions from aqueous media.

Potential environmental applications of MXenes: A critical review

Various environmental pollutants (e.g., air, water and solid pollutants) are discharged into environments with the rapid development of industrializations, which is presently at the forefront of global attention. The high efficient removal of these environmental pollutants is of important concern due to their potential threat to human health and eco-diversity. Advanced nanomaterials may play an important role in the elimination of pollutants from environmental media. MXenes as the new intriguing class of graphene-like 2D transition metal carbides and/or carbonitrides have been widely used in energy storage, environmental remediation benefitting from exceptional structural properties such as highly active sites, high chemical stability, hydrophilicity, large interlayer spacing, huge specific surface area, superior sorption-reduction capacity. However, the comprehensive investigation concerning the removal of various environmental pollutants on MXenes is yet not available up to date. In this review, we summarized the synthesis and properties of MXenes to demonstrate the key roles in ameliorating their adsorption performance; then the recent advances and achievements in environmental application of MXenes on the removal of gases, organics, heavy metals and radionuclides were comprehensively reviewed in details; Finally, the formidable challenges and further perspectives regarding utilizing MXene in environmental remediation were proposed. Hopefully, this review can provide the useful information for environmental scientists and material engineers on designing versatile MXenes in actual environmental applications.