A critical overview of MXenes adsorption behavior toward heavy metals

Multisite synergistic interaction induced selective adsorption of CB5-Ti3C2T2 complex for strontium ion: A combined theoretical and experimental study

In this work, we use a well-defined water-soluble macrocyclic molecule cucurbit[5]uril (CB5) to modify 2D Ti3C2T2 MXene and assemble a novel high-performance adsorbent CB5-Ti3C2T2 for Sr ion by density functional theory and experimental methods. The structural stabilities of two distinct types of CB5-Ti3C2T2 (T = F, O and OH) complexes, i.e., CB5-Ti3C2T2(V) and CB5-Ti3C2T2(P) configurations are proved by binding energy and ab initio molecular dynamics (AIMD) simulations. Calculations of adsorption properties reveal that all the considered CB5-Ti3C2T2 complexes can act as efficient adsorbents for Sr ion, among which CB5-Ti3C2O2 complex possesses the best performance. The high affinity (the calculated adsorption energies < -7.6 eV) and selectivity of CB5-Ti3C2T2 complex for Sr ion are attributed to the synergistic effect between CB5 molecule and Ti3C2T2 MXene in the adsorption process, which arises from the multisite interactions of portal carbonyl groups of CB5 and surface functional groups of Ti3C2T2 towards Sr. Finally, CB5-Ti3C2T2 complex was successfully synthesized, and experimental results confirm its synergistic effect, good selectivity and high removal efficiency for Sr ions. In the treatment of strontium-containing wastewater with low concentrations, the distribution coefficient and decontamination factor of CB5-Ti3C2T2 for Sr were determined to be as high as 2.88 × 105 mL/g and 144.9, respectively, and the separation factor (SFSr/Ca) achieved a notable value of 67. 5. This work is expected to present a new strategy for the construction of high-performance MXene-based adsorbent for radionuclide elimination.

K2Cu3(Fe(CN)6)2 In Situ-Modified MXene Nanosheets for Selective Enrichment of Cs+ and the Mechanism

Selective enrichment of cesium ions (Cs+) at ultralow concentrations is essential for resource recovery and radioactive waste disposal, yet efficient adsorbents are lacking. Herein, we reported a Prussian blue analogue (K2Cu3(Fe(CN)6)2, Cu-PBA) decorated on MXene nanosheets by in situ fabrication, forming a composite material termed PMX, for enhanced adsorption of Cs+ in acidic solutions and seawater. The stable, negatively charged MXene effectively anchors Cu2+ precursors and promotes Cs+ adsorption. The synergistic interaction between MXene and the in situ-synthesized Cu-PBA significantly enhances the adsorption performance and water stability of PMX in both acidic solutions and seawater. PMX achieves rapid adsorption equilibrium within 5 min, with a high adsorption capacity of 408.2 mg/g at pH 1, surpassing conventional adsorbents. Moreover, PMX shows excellent Cs+ selectivity (Kd = 68,361.7 mL/g), cycle stability, and notable anti-irradiation ability, demonstrating superior efficiency in Cs+ enrichment from complex matrices. The adsorption mechanism involves electrostatic attraction and K+/Cs+ ion exchange, facilitated by MXene's functional groups and the Cu-PBA structure. These findings underscore the excellent potential of PMX as an efficient adsorbent for resource enrichment and the removal of radioactive elements such as Cs+.

High-Performance Ti3C2Tx-MXene/Mycelium Hybrid Membrane for Efficient Lead Remediation: Design and Mechanistic Insights

This study presents a hybrid microfiltration technology designed for high-performance lead (Pb(II)) remediation, especially from aqueous solutions with high Pb(II) concentrations, by utilizing two-dimensional (2D) Ti3C2Tx-MXene layers deposited on dry mycelium membranes. The hybrid Ti3C2Tx-MXene/mycelium (MyMX) membranes were fabricated via a single-step electrochemical deposition (ECD) technique, which enabled a uniform coating of 2D Ti3C2Tx-MXene onto individual hyphal fibers of a prefabricated mycelium membrane. Optimized ECD parameters for high Pb(II) uptake were identified using scanning electron microscopy and energy-dispersive X-ray spectroscopy. In immersion-based (no-flow) Pb(II) remediation experiments, MyMX membranes demonstrated significantly high Pb(II) removal efficiency (>87-99%) and rapid sorption kinetics across an initial Pb(II) concentration range of 60-1500 ppm in both single-ion and co-ion solutions. The enhanced Pb(II) sorption was attributed to electrostatic interactions and surface complexation assisted by hyphal surface proteins and Ti3C2Tx-MXene functional groups, as confirmed by infrared and X-ray photoelectron spectroscopies. In cross-flow studies, the MyMX membranes achieved a Pb(II) sorption capacity of ∼1347 mg/g while maintaining a high permeation rate of 51,800 L m-2 bar-1 h-1 at 1500 ppm Pb(II), surpassing the performance of various polymer-based and MXene-based microporous membranes for heavy metal remediation. The biomaterial-based hybrid MyMX membrane represents a significant advancement in water treatment technology, providing a cost-effective, sustainable solution for Pb(II) remediation in contaminated water sources.

Flexible self-standing amidoxime-functionalized MXene membrane for electrochemical uranium extraction

The presence of radioactive U(VI) ions in sewage poses a significant threat to both the ecological environment and human health. In recent years, an electricity-driven remediation strategy has emerged as aprominent technique for the elimination of radionuclides. Specifically, the square wave transformation method is an emerging technology for electrochemical separation and enrichment of uranium. It offers several merits, such as high extraction capacity, facile recovery of products, and effective suppression of water decomposition. However, conventional electrodes typically require the utilization of adhesives, which severely restricts their adsorption performance. In this study, we fabricated a self-supporting membrane electrode based on amidoxime-functionalized Ti3C2Tx MXene (TCP) for efficient extraction of uranium from radioactive solutions. The polyamidoxime (PAO) is incorporated into the interlayer of Ti3C2Tx through non-covalent bonds, leading to an increase in interlayer spacing and specific surface area while also providing an abundance of specific binding sites. Notably, by applying an alternating current (AC) voltage ranging from -5 to 0 V, uranyl ions can migrate and concentrate onto TCP membrane, achieving an exceptional extraction capacity of 2809 mg/g. Further characterization confirmed that the captured uranium(VI) was reduced to U(V), and then the unstable U(V) was re-oxidized to U(VI), eventually generating Na2O7U22 precipitates in the presence of Na+. Considering its remarkable electro-extraction performance and convenient preparation process, the TCP membrane electrode is regarded as a promising candidate for U(VI) extraction from wastewater.

Cr2TiC2Tx MXene as an adsorbent material in ultrasonic-assisted d-μ-solid phase extraction for trace detection of heavy metals

MXenes are a large family of two-dimensional transition metal carbides, nitrides, and carbonitrides. While MXenes have great potential for applications in analytical chemistry, most of the studies in this field are focused on Ti3C2Tx, the most popular MXene material. For example, several studies employed Ti3C2Tx as an adsorbent for the trace detection of toxic analytes, but there is limited knowledge on the utility of other MXene materials for this application. In this work, we investigated the potential of Cr2TiC2Tx, one of the least studied MXenes, for application as an adsorbent material in ultrasonic-assisted dispersive micro solid-phase extraction (d-μ-SPE) method for the detection of heavy metals at trace levels in food and soil samples. We synthesized large monolayer flakes of Cr2TiC2Tx and characterized it by a variety of microscopic and spectroscopic techniques. Cr2TiC2Tx MXene showed remarkable performance in the d-μ-SPE method with the detection limits of 0.09 and 1.9 ng mL-1, and dynamic ranges of 0.3-90 μg L-1 and 6-120 μg L-1 for cadmium (Cd2+) and lead (Pb2+) ions, respectively. The great performance of Cr2TiC2Tx MXene as an adsorbent for the trace detection of heavy metals highlights the importance of investigating other MXenes beyond Ti3C2Tx for analytical applications.

Enhancing Methylene Blue Adsorption Performance of Ti3C2Tx@Sodium Alginate Foam Through Pore Structure Regulation

Pore structural regulation is expected to be a facile way to enhance the adsorption performance of MXene. In this work, spherical foam composites consisting of Ti3C2Tx and sodium alginate (SA) were synthesized via a vacuum freeze-drying technique. By varying the solution volume of Ti3C2Tx, four distinct Ti3C2Tx@SA spherical foams with honeycomb-like and lamellar structures with a pore diameter in the range of 100-300 μm were fabricated. Their methylene blue (MB) adsorption performances were then systematically compared. The results revealed that the honeycomb-like porous-structured spherical foams have a significantly higher adsorption capacity than their lamellar counterparts. Notably, the Ti3C2Tx@SA honeycomb-like porous foam exhibited a remarkable maximum adsorption capacity (qm) of 969 mg/g, positioning it at the forefront of MB adsorbent materials. Respective analysis of the adsorption kinetics, thermodynamics, and isotherm model indicated that this MB adsorption of Ti3C2Tx@SA honeycomb-like porous foam is characterized to be a physical, endothermic, and monolayer adsorption. The Ti3C2Tx@SA honeycomb-like porous foam also demonstrated excellent resistance to ion interference and good reusability, further attesting to its substantial potential for practical applications. X-ray photoelectron spectroscopy (XPS) analysis was employed to elucidate the adsorption mechanism, which was found to involve the synergistic effect of electrostatic adsorption and amidation reaction. This work not only offers new avenues for the development of high-performance adsorption materials but also provides crucial insights into the structural design and performance optimization of porous materials.

Quasi-2D MIL-100 (Fe) synthesis via benzene-1,3,5-tricarboxylic acid self-assembly: organic dye adsorption at room temperature with dramatically enhanced kinetics

Amid increasing environmental pollution, two-dimensional materials have played pivotal roles in environmental remediation. However, two-dimensional metal-organic frameworks (2D-MOFs) have yet to be thoroughly explored. This study introduces a novel approach to synthesize 2D-MOFs, particularly focusing on MIL-100-(Fe), for the adsorption of emerging organic dyes. By harnessing the self-assembly of benzene-1,3,5-tricarboxylic acid (BTC), we formed thin solid interfaces of BTC as building blocks to control the growth of MIL-100-(Fe). This resulted in quasi-2D structures that showed over a 35% increase in adsorption capacity and a 5.5-fold increase in the adsorption kinetics of Rhodamine B removal compared to their 3D counterparts. This new method overcomes traditional synthesis limitations, offering a replicable and high-yield procedure for 2D-MOF synthesis. Compared to its three-dimensional counterpart (3D MIL-100 Fe), the prepared adsorbent exhibited remarkably higher efficacy in the adsorption of Rhodamine B, with high structural stability and recyclability. The prepared adsorbent shows over 99% adsorption within 90 minutes for initial dye concentrations of 1-40 mg L-1via the Langmuir adsorption mechanism and pseudo-second-order kinetics. Our research pioneers a method for the synthesis of quasi 2D-MIL-100-(Fe), laying the groundwork for fabricating other 2D-MOF structures, particularly those based on carboxylic acids.

A critical review of MXene-based composites in the adsorptive and photocatalysis of hexavalent chromium removal from industrial wastewater

The growing concern of water pollution is a critical issue stemming from industrialization and urbanization. One of the specific concerns within this broader problem is the toxicity associated with chromium (Cr), especially in its Cr (VI) form. Transition metal carbides/nitrides (MXenes) are attractive materials for the treatment of water due to their unique properties such as layered structure, high surface area, conductivity, flexibility, scalable manufacture, and surface functions. Adsorption and photocatalysis reactions are the two promising methods for the removal of Cr (VI) by using MXenes. Still, most of the previous reviews were limited to the single application area. Hence, this review covers recent developments in MXene-based composites, highlighting their dual role as both adsorbents and photocatalysts in the removal of Cr (VI). MXene-based composites are found to be effective in both adsorption and photodegradation of Cr (VI). Most MXene-based composites have demonstrated exceptional removal efficiency for Cr (VI), achieving impressive adsorption capacities ranging from 100 to 1500 mg g-1 and degradation percentages between 80% and 100% in a relatively short period. The active functional groups present on the surface of MXene have a viable impact on the adsorption and photodegradation performance. The mechanism of Cr (VI) removal is explained, with MXenes playing a key role in electrostatic attraction for adsorption and as co-catalysts in photocatalysis. However, MXene-based composites have limitations such as instability, competition with co-existing ions, and regeneration challenges. Further research is needed to address these limitations. Additionally, MXene-based composites hold promise for addressing water contamination, heavy metal removal, hydrogen production, energy storage, gas sensing, and biomedical applications.

Cost-Effective Synthesis of MXene Cadmium Sulfide (CdS) for Heavy Metal Removal

Background Environmental contamination resulting from the release of untreated industrial wastewater has emerged as a critical worldwide issue. These effluents frequently have high levels of heavy metals and antibiotics, which are bad for aquatic ecosystems and human health. Oftentimes, conventional wastewater treatment techniques fall short of effectively eliminating these pollutants. Innovative materials that may efficiently absorb or break down contaminants from contaminated water sources are, therefore, desperately needed. Hydrothermally produced MXene cadmium sulfide (CdS) composites have shown great promise as an adsorbent material because of their special qualities, which include high surface area, chemical stability, and customizable surface functions that improve their adsorption capacity for heavy metals and antibiotics alike.  Aim The aim of this study is to produce MXene-CdS nanoparticles in a cost-effective method for the simultaneous removal of heavy metals from aqueous contaminants for water pollution control. Methods and materials MXenes were synthesized by selectively etching Ti3AlC2 MAX-phase ceramics using aqueous HF. CdS nanoparticles were synthesized separately and integrated with MXenes via a hydrothermal process. The resulting MXene CdS nanocomposites were characterized using scanning electron microscopy (SEM) for morphology, energy dispersion spectrum (EDS) for elemental composition, X-ray diffraction (XRD) study for phase identification, and removal of heavy metals via MXene CdS.  Results Consistent distribution of CdS nanoparticles on the MXene surface and the creation of MXene CdS nanomembranes with a well-defined shape were observed by SEM analysis. Ti, C, Cd, and S elements, indiciaries of a successful composite formation, were confirmed to be present by EDS. The crystalline structure of both the MXene and CdS phases was confirmed by the distinctive peaks seen in the XRD patterns. MXene-CdS composites facilitate the effective removal of chromium ions from contaminated water. The excellent hydrophilicity of the produced nanomembrane allowed for effective interaction with watery contaminants. Conclusion This study showcases the successful synthesis and characterization of MXene-CdS nanocomposites for environmental remediation, particularly in removing toxic metals like chromium from industrial effluents. SEM analysis confirmed the uniform distribution of CdS nanoparticles on the MXene surface, while elemental composition validated their integration. XRD analysis confirmed the crystalline structures of both components. The nanocomposite exhibited excellent hydrophilicity, enhancing the efficient adsorption of heavy metals. Its large surface area and chemical stability contribute to high adsorption efficiency, making it ideal for wastewater treatment. The scalable synthesis process supports practical applications. This research highlights MXene-CdS nanocomposites as a cost-effective, sustainable solution for water pollution control.

Graphene oxide intercalated Alk-MXene adsorbents for efficient removal of Malachite green and Congo red from aqueous solutions

Currently, adsorbents with high adsorption performance for eliminating pollutants from discharged wastewater have received many researchers' attention. To this aim, a novel AMXGO absorbent was fabricated by intercalating graphene oxide (GO) into alkalized MXene (Alk-MXene) layer which exhibited high efficacy for the removal of cationic Malachite Green (MG) and anionic Congo Red (CR). Analysis of FTIR, XRD, SEM and TG presented that AMXGO absorbent have a typical three-dimensional layer by layer structure and abundant oxygen-containing groups and its thermal stability was remarkably improved. BET results elucidated that AMXGO1 adsorbent has larger specific surface area and pore volume (16.686 m2 g-1, 0.04733 cm3 g-1) as compared to Alk-MXene (4.729 m2 g-1, 0.02522 cm3 g-1). A dependence of adsorption performance on mass ratio between Alk-MXene and GO, initial dye concentration, contact time, temperature and pH was revealed. Maximum adsorption capacity of MG (1111.6 mg/g) and CR (1133.7 mg/g) were particularly found for AMXGO1 absorbent with a mass ratio of 3:1 and its removal for both dyes were higher than 92%. The adsorption process of AMXGO1 adsorbent for both MG and CR complies with pseudo-second-order kinetic model and Freundlich isotherm model. In addition, adsorption mechanism was explored that synergism effects as electrostatic attraction, π-π conjugates, intercalation adsorption and pore filling were the main driving force for the high adsorption performance of dye. Therefore, AMXGO adsorbent has a potential application prospect in the purification of dye wastewater.

Remediation of Safranin-O and Acid Fuchsin by Using Ti3C2 MXene /rGo-Cu2O Nanocomposite: Preparation, Characterization, Isotherm, Kinetics and Thermodynamic Studies

In recent years, MXene has become one of the most intriguing two-dimensional layered (2Dl) materials extensively explored for various applications. In this study, a Ti3C2 MXene/rGo-Cu2O Nanocomposite (TGCNCs) was developed to eliminate Safranin-O effectively (SO) and Acid Fuchsin (AF) as cationic dyes from the aquatic environment. Multistep was involved in the preparation of the adsorbent system, including the Preparation of Ti3C2, after that, GO synthesis by the Humer method, followed by rGO production, then added CuSO4 to obtain a final Nanocomposite (NCs) called "TGCNCs". The structure of TGCNCs can be varied in several ways, including FTIR, SEM, TGA, Zeta, EDX, XRD, and BET, to affirm the efficacious preparation of TGCNCs. A novel adsorbent system was developed to remove SO and AF, both cationic dyes. Various adsorption conditions have been optimized through batch adsorption tests, including the pH of the solution (4-12), the effect of dosage (0.003-0.03 g), the impact of the contact time (5-30 min), and the effect of beginning dye concentration (25-250 mg/L). Accordingly, the TGCNCs exhibited excellent fitting for Freundlich isotherm mode, resulting in maximum AF and SO adsorption capacities of 909.09 and 769.23 mg.g-1. This research on adsorption kinetics suggests that a pseudo-second-order (PSO) model would fit well with the experimental data ( = 0.998 and = 0.990). It is evident from the thermodynamic parameters that adsorption is an endothermic process that is spontaneous and favourable. During the adsorption of SO and AF onto NCs, it is hypothesized that these molecules interact intramolecularly through stacking interactions, H-bond interactions, electrostatic interactions, and entrapment within the polymeric Poros structure nanocomposite. Regeneration studies lasting up to five cycles were the most effective for both organic dyes under study.

Utilizing adsorption of wood and its derivatives as an emerging strategy for the treatment of heavy metal-contaminated wastewater

The rapid development of the industrial sector has resulted in tremendous economic growth. However, this growth has also presented environmental challenges, specifically due to the substantial sewage generated and its contribution to the early warning of global water resource depletion. Large concentrations of poisonous heavy metals, including cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), and nickel (Ni), are found in industrial effluent. Therefore, various studies are currently underway to provide effective solutions to alleviate heavy metal ion pollution in sewage. One emerging strategy for sewage pollution remediation is adsorption using wood and its derivatives. This approach is gaining popularity due to the porous structure, excellent mechanical properties, and easy chemical modification of wood. Recent studies have focused on removing heavy metal ions from sewage, summarising and analysing different technical principles, affecting factors, and mainstream chemical modification methods on wood. Furthermore, this work provides insight into potential future development direction for enhanced adsorption of heavy metal ions using wood and its derivatives in wastewater treatment. Overall, this review aims to raise awareness of environmental pollution caused by heavy metals in sewage and promote green environmental protection, low-carbon energy-saving, and sustainable solutions for sewage heavy metal treatment.

MXene/AgNW composite material for selective and efficient removal of radioactive cesium and iodine from water

Toxic fission products, such as cesium (137Cs) and iodine (129I) are of great concern because of their long half-lives and high solubility in water. The simultaneous removal of Cs and I using a single adsorbent is an area of increasing interest. In this study, MXene/silver nanowire (AgNW) composite was synthesized through physical mixing and employed for simultaneous removal of iodide (I-) and cesium (Cs+) ions from contaminated water. The MXene/AgNW composite demonstrated excellent adsorption capacities of 84.70 and 26.22 mg/g for I- and Cs+, respectively. The experimental data supported the hypothesis of multilayer adsorption of Cs+ owing to the inter-lamellar structures and the presence of heterogeneous adsorption sites in MXene. The interaction between I- and the AgNW involved chemisorption followed by monolayer adsorption. MXene/AgNW composite material exhibited promising results in the presence of competitive ions under extreme pH conditions. Thus, synthesized composite materials holds promising potential as an adsorbent for the remediation of radioactive liquid waste.

Synchronously Enhanced Removal Ability and Stability of MXene through Biomimetic Modification

Increasing environmental problems intensify the demand for high-performance environmental purification materials. MXene is a typical transition-metal carbide/nitride material with a two-dimensional geometric feature and a good deal of functional groups, and it is considered as an efficient adsorbent for removing pollutants from wastewater. However, the easy oxidation and relatively low adsorption capacity greatly restrict its application. In this study, the MXene/polydopamine (PDA) composite particles were fabricated through the biomimetic modification method of inducing the self-polymerization of dopamine in an MXene aqueous solution. Microstructure characterizations demonstrate that PDA facilitates the exfoliation of MXene. Adsorption measurements show that MXene and PDA exhibit an apparent synergistic effect in removing chromium hexavalent Cr(VI) from aqueous solution, and more PDA content leads to a larger synergistic effect. Consequently, the composite particles exhibit an ultrahigh adsorption capacity (862.3 mg/g). Specifically, even if the composite particles were stored in aqueous solution for 2 months, they still exhibit high adsorption ability with only a 3.3% loss in adsorption capacity, indirectly confirming the enhanced stability of MXene induced by PDA. Furthermore, the composite particles also show reduction ability to Cr(VI) and about 54.3% Cr(VI) can be reduced to harmless chromium trivalent Cr(III). This study provides a new method for the preparation of MXene-based adsorbents with excellent adsorption capacity and high stability, which has broad application prospects in the field of wastewater treatment.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

Applications of advanced MXene-based composite membranes for sustainable water desalination

MXenes are an innovative class of 2D nanostructured materials gaining popularity for various uses in medicine, chemistry, and the environment. A larger outer layer area, exceptional stability and conductivity of heat, high porosity, and environmental friendliness are all characteristics of MXenes and their composites. As a result, MXenes have been used to produce Li-ion batteries, semiconductors, water desalination membranes, and hydrogen storage. MXenes have recently been used in many environmental remediations, frequently surpassing conventional materials, to treat groundwater contamination, surface waters, industrial and municipal wastewaters, and desalination. Due to their outstanding structural characteristics and the enormous specific surface area, they are widely utilized as adsorbents or membrane materials for the desalination of seawater. When used for electrochemical applications, MXene-composites can deionize via Faradaic capacitive deionization (CDI) and adsorb various organic and inorganic pollutants to treat the water. In general, as compared to other 2D nanomaterials, MXene has superb characteristics; because of their magnificent characteristics and they exhibit strong desalination capability. The current review paper discusses the desalination capability of MXenes and their composites. Focusing on the desalination capacity of MXene-based nanomaterials, this study discusses the characteristics and synthesis techniques of MXenes their composites along with their ion-rejection capability and pervaporation desalination of water via MXene-based membranes, capacitive deionization capability, solar desalination capability. Furthermore, the challenges and prospects of MXenes and their composites are highlighted.

A review on recent advancements on removal of harmful metal/metal ions using graphene oxide: Experimental and theoretical approaches

Graphene oxide is a two-dimensional carbon nanomaterial and has gained huge popularity over the last decade. Because, the graphene oxide can be dispersed in water easily and it is one of the most researched two-dimensional materials in the current time. The extraordinary properties shown by graphene oxide (GO) are due to its unique chemical structure; includes various hydrophilic functional groups containing oxygen such as carboxyl, hydroxyl, carbonyl and tiny sp² carbon domains surrounded by sp³ domains. These groups are very peculiar for various applications as they allow covalent functionalisation with a plethora of compounds. Large surface area, intrinsic fluorescence, excellent surface functionality, amphiphilicity, improved conductivity, high adsorption capacity and superior biocompatibility are some of the chemical properties have drawn research from various fields. Graphene oxide has various interactions such as coordination, chelation, hydrogen bonding, electrostatic interaction, hydrophobic effects, π-π interaction, acid base interaction etc., with various metal ions. This review is focused on the removal of metals and metal ions due to their interactions mentioned above. Further, potential of composites of graphene oxide in the removal of metal and metal ions is also discussed. Further, the current challenges in this field at industrial-scale are also discussed.

Advances in 2D MXenes-based materials for water purification and disinfection: Synthesis approaches and photocatalytic mechanistic pathways

MXenes two-dimensional materials have recently excited researchers' curiosity for various industrial applications. MXenes are promising materials for environmental remediation technologies to sense and mitigate various intractable hazardous pollutants from the atmosphere due to their inherent mechanical and physicochemical properties, such as high surface area, increased hydrophilicity, high conductivity, changing band gaps, and robust electrochemistry. This review discusses the versatile applications of MXenes and MXene-based nanocomposites in various environmental remediation processes. A brief description of synthetic procedures of MXenes nanocomposites and their different properties are highlighted. Afterward, the photocatalytic abilities of MXene-based nanocomposites for degrading organic pollutants, removal of heavy metals, and inactivation of microorganisms are discussed. In addition, the role of MXenes anti-corrosion support in the lifetime of some semiconductors was addressed. Current challenges and future perspectives toward the application of MXene materials for environmental remediation and energy production are summarized for plausible real-world use.

Effects of synthesis temperature on ε-MnO2 microstructures and performance: Selective adsorption of heavy metals and the mechanism onto (100) facet compared with (001)

The heavy-metal adsorbent ε-MnO₂ was produced through a simple, one-step oxidation-reduction reaction at three different synthesis temperatures (25 °C, 50 °C and 75 °C) and their morphology and chemical-physical properties were compared. Of the three materials, MnO₂-25 had the largest specific surface area and the highest surface hydroxyl concentration. Its optimal performance was demonstrated by batch adsorption experiments with Pb²⁺, Cd²⁺ and Cu²⁺. Of the three metals, Pb²⁺ was adsorbed best (339.15 mg/g), followed by Cd²⁺ (107.50 mg/g) and Cu²⁺ (86.30 mg/g). When all three metals were present, Pb²⁺ was still absorbed best but now more Cu²⁺ was adsorbed than Cd²⁺. In order to explore the mechanism for the inconsistent adsorption order of Cd²⁺ and Cu²⁺ in single and competitive adsorption, we combined experimental data with density functional theory (DFT) calculations to elucidate the distinct adsorption nature of MnO₂-25 towards these three metals. This revealed that the adsorption affinity of the (100) facet was superior to (001), and since the surface complexes were also more stable on (100), this facet was most likely determining the adsorption order for the single metals. When the metals were present in combination, Pb²⁺ preferentially occupied the active adsorption sites of (100), forcing Cu²⁺ to be adsorbed on the (001) facet where Cd²⁺ was only poorly bound. Thus, the adsorption behavior was affected by MnO₂-25 surface chemistry at a molecular scale. This study provides an in-depth understanding of the adsorption mechanisms of the heavy metals on this adsorbent and offers theoretical guidance for production of adsorbent with improved removal efficiency.

Thermoresponsive MXene composite system with high adsorption capacity for quick and simple removal of toxic metal ions from aqueous environment

High-performance adsorption and easy-to-recycle property of adsorbents are desirable in wastewater treatment, and a suitably smart adsorbent with responsive phase separation capacity is promising in this regard. Herein, a thermoresponsive composite system is designed through the combination of transition metal carbides (MXene) and poly(N-isopropylacrylamide) (PNIPAM) for removal of toxic metal ions from water. As a thermoresponsive switch, the PNIPAM endows such composite system with superior thermoresponsiveness (i.e., gel-water phase separation) in water, which facilitates to the control of adsorption. The gel phase triggered by an elevated temperature (e.g., 40 °C) quickly adsorbs toxic metal ions, and then a solid-liquid extraction way is used to conveniently separated the gel phase from water phase for simple removal of toxic metal ions. A very high adsorption capacity (e.g., ~224 mg·g^-1 for Cu²⁺) can be achieved due to the synergistic effects of the composite system. Moreover, the separated gel can be back to a redispersed state at low temperature (e.g., 20 °C), enabling its effective regeneration and recovery. Notably, the PNIPAM as a protective agent prevents the oxidation of MXene so as to retain good stability during the multiple adsorption/desorption cycles. This simple and smart adsorption strategy is great promising for water purification application.

Efficient mercury removal from aqueous solutions using carboxylated Ti3C2Tx MXene

Water supplies contaminated with heavy metals are a worldwide concern. MXenes have properties that make them attractive for the removal of metal ions from water. This work presents a simple one-step method of Ti₃C₂T_x carboxylation that involves the use of a chelating agent with a linear structure, providing strong carboxylic acid groups with high mobility. The carboxylation decreases the zeta-potential of Ti₃C₂T_x by ~16 to ~18 mV over a pH range of 2.0-8.5 and improves Ti₃C₂T_x stability in the presence of molecular oxygen. pH in the range of 2-6 has a negligible effect on the adsorption capacity of Ti₃C₂T_x and COOH-Ti₃C₂T_x. Compared to Ti₃C₂T_x, COOH-Ti₃C₂T_x has a slightly higher and much faster mercury uptake, and the concentration of mercury ions leached out from COOH-Ti₃C₂T_x is lower. For both Ti₃C₂T_x and COOH-Ti₃C₂T_x, the leached mercury ion concentration is far below the U.S.-EPA maximum level. At an initial Hg²⁺ concentration of 50 ppm and pH of 6, COOH-Ti₃C₂T_x has the equilibrium adsorption capacity of 499.7 mg/g and removes 95% of Hg²⁺ in less than 1 min. Moreover, it has an equilibrium time of 5 min, which is significantly shorter than that of Ti₃C₂T_x (~ 60 min). Finally, its mercury-ion uptake capacity is higher than commercially available adsorbents reported in the literature. Its mercury removal is mainly via chemisorption and monolayer adsorption.

Synthesis, Toxicity Assessment, Environmental and Biomedical Applications of MXenes: A Review

MXenes are a family of two-dimensional (2D) composite materials based on transition metal carbides, nitrides and carbonitrides that have been attracting attention since 2011. Combination of electrical and mechanical properties with hydrophilicity makes them promising materials for biomedical applications. This review briefly discusses methods for the synthesis of MXenes, their potential applications in medicine, ranging from sensors and antibacterial agents to targeted drug delivery, cancer photo/chemotherapy, tissue engineering, bioimaging, and environmental applications such as sensors and adsorbents. We focus on in vitro and in vivo toxicity and possible mechanisms. We discuss the toxicity analogies of MXenes and other 2D materials such as graphene, mentioning the greater biocompatibility of MXenes. We identify existing barriers that hinder the formation of objective knowledge about the toxicity of MXenes. The most important of these barriers are the differences in the methods of synthesis of MXenes, their composition and structure, including the level of oxidation, the number of layers and flake size; functionalization, test concentrations, duration of exposure, and individual characteristics of biological test objects Finally, we discuss key areas for further research that need to involve new methods of nanotoxicology, including predictive computational methods. Such studies will bring closer the prospect of widespread industrial production and safe use of MXene-based products.

Computational-Based Approaches for Predicting Biochemical Oxygen Demand (BOD) Removal in Adsorption Process

Predicting the adsorption performance to remove organic pollutants from wastewater is an essential environmental-related topic, requiring knowledge of various statistical tools and artificial intelligence techniques. Hence, this study is the first to develop a quadratic regression model and artificial neural network (ANN) for predicting biochemical oxygen demand (BOD) removal under different adsorption conditions. Nanozero-valent iron encapsulated into cellulose acetate (CA/nZVI) was synthesized, characterized by XRD, SEM, and EDS, and used as an efficient adsorbent for BOD reduction. Results indicated that the medium pH and adsorption time should be adjusted around 7 and 30 min, respectively, to maintain the highest BOD removal efficiency of 96.4% at initial BOD $\text{concentration}\left(C_{\text{o}}\right)=100$  mg/L, mixing $\text{rate}=200$  rpm, and adsorbent dosage of 3 g/L. An optimized ANN structure of 5–10–1, with the “trainlm” back-propagation learning algorithm, achieved the highest predictive performance for BOD removal ( $R^2$ : 0.972, Adj- $R^2$ : 0.971, RMSE: 1.449, and SSE: 56.680). Based on the ANN sensitivity analysis, the relative importance of the adsorption factors could be arranged as $\text{pH}>\text{adsorbent}\text{dosage}>\text{time}\approx \text{stirring}\text{speed}>C_{\text{o}}$ . A quadratic regression model was developed to visualize the impacts of adsorption factors on the BOD removal efficiency, optimizing pH at 7.3 and time at 46.2 min. The accuracy of the quadratic regression and ANN models in predicting BOD removal was approximately comparable. Hence, these computational-based methods could further maximize the performance of CA/nZVI material for removing BOD from wastewater under different adsorption conditions. The applicability of these modeling techniques would guide the stakeholders and industrial sector to overcome the nonlinearity and complexity issues related to the adsorption process.

Preparations and Applications of MXene–Metal Composites: A Review

MXene, an advanced family of 2D ceramic material resembling graphene, has had a considerable impact on the field of research because of its unique physiochemical properties. MXene has been synthesized by the selective etching of MAX via different techniques. However, with the passage of time, due to the need for further progress and improvement in MXene materials, ideas have turned toward composite fabrication, which has aided boosting the MXene composites regarding their properties and applications in various areas. Many review papers are published on MXene and their composites with polymer, carbon nanotube, graphene, other carbon, metal oxides and sulfides, etc., except metal composite, and such papers discuss these composites thoroughly. In this review article, we illustrate and explain the development of MXene-based metal composites. Furthermore, we highlight the synthesis techniques utilized for the preparation of MXene composites with metal. We briefly discuss the enhancement of properties of the composites and a wide range of applications as an electrode substance for energy storage devices, electrochemical cells, supercapacitors, and catalytic and anti-corrosive performance. Major obstacles in MXene and metal composite are mentioned and provide future recommendations. Together, they can overcome problems and enable MXene and composites on commercial-scale production.