2.1 Heavy metal ions

The functionalities available on the external surface of MXene have a beneficial impact on the adsorption of heavy metals. For the adsorption of heavy metal ions such as copper, lead, chromium, nickel, cesium, and uranium, Ti₃C₂Tx represents the most studied MXene (Sheth et al., 2022). The heavy metal ion adsorption mechanism using MXene is presented in Fig. 5 (Pouramini et al., 2023). The performance of various MXene-based adsorbents for removing heavy metals is shown in Table 2. Tang et al. (Tang et al., 2018) synthesized the Ti₃C₂Tx MXene-based adsorbent for the adsorption of Cr⁺⁶ by utilizing the HF etching techniques by eliminating the Al form Ti₃AlC₂. After synthesizing MXene, various characterization techniques like XRD, SEM, TEM, and DRS have been used for their characterization. The synthesized MXene possessed higher adsorption capacity than its parent material, Ti₃AlC_2, because of the successful exfoliation. The synthesized adsorbent has an adsorption capability of up to 80 mg g⁻¹. They used intercalation and sonication techniques to enhance their adsorption capacity during the experiment. The experimental results show that both modification techniques have a meaningful impact on the adsorption capability of the adsorbent.

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Fig. 5

Mechanism of the adsorption of the metal ions on MXene-based adsorbent (Pouramini et al., 2023).

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Zhan et al. (Zhan et al., 2020) utilized the environmentally friendly alkalization-grafting modification technique to synthesize the alkMXene-NH₂ nanocomposites for the adsorption of lead (Pb²⁺). The synthesized MXene has an adsorption capacity of Pb²⁺ heavy metal 187.5 mg g⁻¹. The remarkable adsorption capacity of the synthesized MXene is because of their excellent SSA area of 129.21 m².g⁻¹ and the availability of the active functionalities on their surface. Some other factors are also discussed during the investigation and have a tremendous impact on the adsorbent's performance. The adsorption temperature, reaction time, adsorbent dose, pH of the system, and pressure also have little effect on the adsorption of Pb²⁺. These characteristics suggest that MXene-based adsorbents that have been amino-functionalized will make good options for use in the industry.

Mercury emissions substantially harm the natural balance and biological security of flue gas from coal-fired power stations. Acid gases (SO₂, NO) frequently present in flue gas will significantly impact (negatively) the mercury adsorbent's effectiveness. Xu et al. (Xu et al., 2022) utilized a cost-effective and environmentally friendly technique to develop CuS/Ti₃C₂ for the environment's safety and human health. The experimental findings show that the pristine MXene has less adsorption capacity and reusability than the CuS/Ti₃C₂-based composites which is approximately 20 mg g⁻¹. According to the findings, the reactivity of the pure Ti₃C₂ interface is increased by CuS dosing. This research thus confirms that CuS/Ti₃C₂ is an attractive source for eliminating mercury from coal-fired power stations and broadens the MXene's practical applicability.

Liu et al. (Liu et al., 2023) utilized the environmentally friendly electrostatic self-assembly technique to synthesize the Ti₃C₂@FeS-PDA/PEI nanocomposite. Various analytical methods were used to properly assess all as-prepared substances and evaluate the development of species with the specified structure and characteristics. Findings showed that the novel chemical composition of MXenes enhanced the FeS nanomaterials distribution and prevented their aggregation. Various cations and anions in the synthesized composite have no significant impact on the rejection capacity of the adsorbent. They also specified the endothermic process type. DFT analysis shows the strong chemical bond between FeS and active functionalities present on the layer of Ti₃C₂@FeS-PDA/PE; that is responsible for effective adsorption. The Ti₃C₂@FeS-PDA/PEI nanocomposite's adsorption capacity for removal of U⁶⁺ is 88.5 mg g⁻¹. The schematic illustration of U⁺⁶/Cr⁺⁶ removal pathways on Ti₃C₂@FeS-PDA-PEI is presented in Fig. 6.

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Fig. 6

Schematic illustration of U⁺⁶/Cr⁺⁶ removal pathways on Ti₃C₂@FeS-PDA-PEI (Liu et al., 2023).

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According to Zhang et al. (Zhang et al., 2022), when enzymatic hydrolysis lignin (EHL) reacts with 2D Ti₃C₂Tx MXene to synthesize the functionalized 2D Ti₃C₂Tx (TN-EHL) adsorbent for the removal of Cu²⁺ heavy metal. The introduction of EHL into Ti₃C₂T_x cannot disturb the availability of active functionalities on the surface of Ti₃C₂Tx, and their addition reduces the chance of oxidation. This role enhances the adsorption capacity of Ti₃C₂Tx. So, the adsorption capability of TN-EHL for Cu²⁺ is 293.7 mg g⁻¹. The reaction of Cu²⁺ adsorption is endothermic and uncontrolled. During the adsorption of Cu²⁺ on the surface of the developed composite, Cu²⁺ ions are converted to Cu₂O and CuO particles to reduce the concentration of Cu²⁺ ions.

2.2 Dyes

Because of the economic disadvantages of conventional adsorbent materials, MXene offers a superior alternative to clear effluent tainted with dye. Studies were reported on removing toxic dyes like methylene blue and acid blue using 2-D MXene by adsorption. These nanomaterials outperformed other 2D substances regarding adsorption. Concerning several conventional adsorbents, the adsorption rate of MXene-based adsorbents for removing various dyes-based contaminants, like Congo red, Methylene blue, and Methyl orange, was competitive (Liu et al., 2023). A comparison of adsorption conditions and performance of removal of various dyes by different MXene-based adsorbents is shown in Table 3.

Ibrahim et al. (Ibrahim et al., 2022) have used the ability of raw and modified MXenes to have high surface area and surface terminations for the adsorption of methylene blue (MB) and acid blue 80 (AB80). They have shown that they create many active sites for direct ion exchange and reductive-adsorptive removal of dyes and cations. For example, various salt-based Ti₃C₂Tx adsorbent was used to remove MB and AB80 dye from the aqueous solution. The adsorption capacity of MB reached about 39 mg g⁻¹. The adsorption capacity of NaOH-Ti₃C₂Tx (189 mg g⁻¹) was higher than LiOH- Ti₃C₂Tx (121 mg g⁻¹), Ti₃C₂Tx (100 mg g⁻¹), and KOH-Ti₃C₂Tx (75 mg g⁻¹). They have shown that Ti₃C₂Tx presents a higher adsorption capacity for MB than AB80, which is due to strong electrostatic interaction with MB. Based on their outcomes, it can be concluded that MXenes present an attractive option for the removal, adsorption, and degradation of numerous dyes like AB80, MB, and others in a variety of composites, membranes, sorbents, photo-catalysts, and electrodes.

To develop an MXene-based nanocomposite (PA-MXene) to extract MB from untreated wastewater, Cai et al. (Cai et al., 2020) employed environmentally friendly and inexpensive hydrothermal procedures. Fig. 7 provides a graphic representation of the PA-MXene nanocomposites created using hydrothermal and dye adsorption processes. The PA was not chemically bonded with MXene, as it was a physical coating of PA on the surface of MXene. The adsorption performance of the MXene-based adsorbent was excellent, up to 85 % after 12 cycles. The variety of characterization approaches is consistent with its excellent efficiency and chemical and thermal stability. The adsorption power of MXene-based adsorbent for removing MB and RhB was 106.7 mg.g⁻¹ and 72.4 mg.g⁻¹, respectively. This research offered new information for creating and enhancing MXene-based nanomaterials for various adsorption processes.

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Fig. 7

Schematic representation of the PA-MXene composites fabricated using a dye adsorption procedure and a hydrothermal method (Cai et al., 2020).

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Anionic azo dye methyl orange (MO) has been widely utilized in the textile sector for a long time to give orange-like colors to clothing. Sun et al. (Sun et al., 2021) used etching to eliminate the Al layer from their Ti₂AlC to synthesize the Ti₂CTx MXene-based nano-adsorbent. The various characterization techniques have been utilized to provide an extensive understanding of Ti₂CTx MXene via XPS, SEM, XRD, and FTIR techniques. The synthesized MXene-based adsorbent performance was evaluated at different pH levels, contact times, adsorbent concentrations, temperatures, and surrounding ions. The synthetic Ti₂CTx layer is positively charged and exhibits good MO adsorption properties. The adsorption efficiency of the adsorbent was 122.6 mg.g⁻¹ at reaction conditions (T = 298 °C, t = 24 min, pH = 6.0). Ti₂CTx offers outstanding recyclability for eliminating MO, as evidenced by its excellent rejection performance after four cycles. The results of this study demonstrate that members of the MXene family can be employed as adsorbents to remove anionic azo dye methyl orange contaminants from sewage.

To minimize Malachite Green (MG), particularly from effluent discharge, and to protect aquatic life, Wang et al. (Wang et al., 2023) created the alk-MXene/CoFe₂O₄/CS nanomaterials using an eco-friendly hydrothermal and self-assembly method. Specifically, it is found that etching for 24 h improves the composite's capacity for adsorption throughout the synthesis process. Under ideal experimental conditions, the alk-MXene/CoFe₂O₄/CS nanostructure can adsorb 537.6 mg of MG per gram of surface area. The dye concentration significantly influences the adsorption rate and capacity of the adsorbent in the water; the adsorption capacity and rate were high at high concentrations of MG and vice versa. The functions on the adsorbent's interfaces can be considerably increased by adding CoFe₂O₄/CS to the alk-MXene interlayer and surface, providing different active sites for dye adsorption. The MG dye and the SO₃ polar group in MG are easily attracted to the outer layer of the produced adsorbent nanostructure due to electrostatic interactions, the availability of functionalities, and positive charges. The ease of access of the H-bond on the adsorbent and dyes is also helpful for the adsorption of MG dyes. After five working cycles, it had reached a very high removal efficiency of about 80 %.

Rhodamine B (RhB) dye is largely used in the paper and textile industries for coloring. A luminous cationic RhB dye is employed in distemper paintings, inks, leather, wood stains, cosmetics, and shoe polish. Rethinasabapathy et al. (Rethinasabapathy et al., 2022) used the magnetic stirring technique to develop the Ti₂CTx/Fe₃O₄ nanostructure by mixing MXene and Fe₃O₄ at 353 K under an argon atmosphere to eliminate RhB organic dyes, especially from textile waste. The designed MXene-based adsorbent had excellent discharge ability of RhB up to 86 mg g⁻¹ even when it was just 45. The proposed composite demonstrated great reusability after four cycles with good workability due to magnetic Fe₃O₄ particles. According to their hypothesis, the outstanding RhB rejection effectiveness of Ti₂CTx/Fe₃O₄ is due to the synergistic relationship of electrostatic forces and H bonds between the active functionalities of Ti₂CTx/Fe₃O₄ and lone-pair electrons of the N group of RhB. Moreover, the excellent selectivity of cationic dyes in coexistence was 88 %. The Ti₂CT_x/Fe₃O₄ nanostructure might have been a desirable option for removing harmful cationic dyes from aqueous solutions because of its remarkable sorption.

Wang et al. (Wang et al., 2023) utilized the environmentally friendly hydrothermal and self-assembly approach to synthesize the alk-MXene/CoFe₂O₄/CS nanostructure to reduce Congo Red (CR), especially from industrial effluent, to save aquatic life. It has been determined that 24 h is the best etching period to enhance the composite's adsorption capability throughout the synthesis method. The dye concentration in the water also plays a key role in the adsorption rate and capacity of the adsorbent; the adsorption capacity and rate were high at high concentrations of CR and vice versa. Finally, it was discovered that adding CoFe₂O₄/CS to the alk-MXene interlayer and surface can significantly increase the functionalities on the adsorbent's interface and offer additional active sites for dye adsorption. Furthermore, the extensive availability of the H-bond on the adsorbent and dyes is advantageous for the adsorption of CR dyes. Contemporary science research presents a novel idea for synthesizing MXene-based magnetic adsorbent materials with an enhanced ability to remove different colors from aqueous pollutants. The synthesis process of the alk-MXene/CoFe₂O₄/CS nanostructure is shown in Fig. 8.

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Fig. 8

The alk-MXene/CoFe₂O₄/CS synthesizing via hydrothermal techniques (Wang et al., 2023).

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2.3 Pharmaceuticals

MXenes have been investigated as potential drug molecule adsorbents in medicinal applications (Ahmaruzzaman, 2022). On the other hand, drug molecules are attracted to the surface of the MXene material during the adsorption process, which can result in more effective medication delivery and tailored therapy. In Table 4, the adsorption of pharmaceuticals by fabricated MXenes is presented. Khatami and Iravani (Khatami and Iravani, 2021) has demonstrated that MXenes can successfully adsorb various medicinal molecules, such as antibiotics, anticancer medications, and anti-inflammatory drugs. The surface chemistry of the MXene material, the kind of drug molecule, and the ambient circumstances all affect how MXenes adsorb these substances. Overall, MXenes are a promising family of materials for pharmaceutical adsorption applications, and additional study is required to maximize their efficacy and promise for therapeutic drug delivery (Khatami and Iravani, 2021; Miri-Jahromi et al., 2022).

Researchers reviewed 2D MXenes, which have a large surface area and are chemically stable enough to be used as adsorption components for CLX, AMP, and AMX (Khatami and Iravani, 2021). Among Mn₂C, Ti₂C, and V₂C, the fabricated Ti₂C had a 100 % adsorption efficiency for cloxacillin. The increased effectiveness was ascribed to the functionalization of MXene with hydroxyl and amine groups, which increased the affinity of antibiotic molecules to the adsorbent surface. The electrostatic attraction between cloxacillin and functionalized Ti₂C is boosted by including the functional group, as shown in Fig. 9(Miri-Jahromi et al., 2022). Analysis of how three different kinds of MXenes interact with pharmaceuticals was conducted using molecular dynamic simulations (MD). MD offers a potent approach for investigating atomic-level intermolecular interactions and characteristics. This part of the study revealed that compared to AMX and AMP, the solution with CLX had greater negative adsorption energy, making adsorption easier for the MXenes (Ti₂C, V₂C, and Mn₂C). It was discovered that Ti₂C absorbed 100 % of CLX while V₂C and Mn₂C adsorbed only 66 % and 55 % of CLX, respectively. Ti exhibited the least negativity compared to V and Mn, even though the pore size of all three MXenes was the same. As a result, the Ti₂C structure had the maximum charge density. Increased charge density enhances the interaction of MXene with the antibiotics, increasing Ti₂C's capacity for adsorption. Hydrogen bonding was chosen as a useful index to evaluate the characteristics of antibiotic adsorption; the stronger the interaction between MXene and the antibiotics, the weaker the H-bonding of MXene with the H₂O molecule. This investigation revealed that CLX had the highest interaction with the MXene surface due to its low H-bond in an aqueous solution.

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Fig. 9

Adsorption of Cloxacillin on hydroxyl and amine groups functionalized Ti₂C (Miri-Jahromi et al., 2022).

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Ghani et al. (Bhuyan and Ahmaruzzaman, 2023) removed ciprofloxacin (CPX) by producing 2D MXene by intercalating Na ions using a batch experimentation method. Intercalation has the advantage of accelerating the spacing and surface terminations between layers of SI-MXene, which improves adsorption capacity and reaction rate. The as-fabricated nanocomposite was regenerated Using an electrochemical method in about 5 min. The elimination rate of ciprofloxacin after successive reuse of the nanocomposite is 99.7 %, indicating that the produced materials are highly efficient. After intercalation, SI-Ti₃C₂Tx MXene's CPX removal rate was twice as high as pure Ti₃C₂T_x MXene.

Kim et al. (Kim et al., 2021) fabricated Ti₃C₂Tx-MXene to adsorb amitriptyline (AMT), verapamil (VRP), carbamazepine (CBM), 17 α-ethinyl estradiol, ibuprofen (IBP), and diclofenac (DCF) from aqueous solution, with AMT showing highest adsorption with a capacity of 58.7 mg.g⁻¹. The effect of pH was studied from 3 to 11 by the researchers. Because MXenes have terminal –OH, −O, and −F groups, their surface charge alters with rising pH and becomes negatively charged. The VRP and AMT showed the best performance at pH 7, and the low negative charge of MXene may be responsible for the restricted adsorption at pH 3.5. AMT and VRP are neutralized when the pH value is higher than 10. The impact of pH is minimal with CBM because it cannot be ionizable at any pH. The researchers concluded that at pH levels greater than their pK_a values, electrostatic attraction between negatively charged MXene and ionizable medicines decreased adsorption affinity. Also, to compare AMT adsorption performance with pure MXene, the authors produced two sonicated MXenes at 28 and 580 kHz. In sonicated MXene, smaller, evenly dispersed particles with a high oxygen-functionalized group and hydrodynamic diameter were found. Better performance and an increased elimination rate were obtained thanks to the sonicated MXene increased adsorption capacity of 214 mg/g for AMT adsorption. This is because sonication-induced cavitation bubbles can produce MXene that is well-dispersed and can even create oxygenated functional groups on MXene. In particular, the best performance was demonstrated at lower frequencies by producing larger cavitation bubbles. In addition, using sonicated MXene, it was determined how different ions affected the effectiveness of adsorption on pharmaceutical substances because there are several ions in real aquatic habitats.

Most inorganic contaminants and a small number of organic pollutants, such as pharmaceuticals, are the focus of MXene adsorbent application. Even fewer findings are based on their ability to remove pharmaceutical substances through adsorption. The use of MXenes to treat pharmaceutical compounds is still in its infancy and requires more research (Bhuyan and Ahmaruzzaman, 2023).

2.4 Radionuclides

Removing radionuclides by adsorption has gained worldwide interest due to its simple procedure, low cost, excellent reliability, and availability of a wide range of adsorbents. A summary of radionuclides adsorption by MXene-based adsorbents is given in Table 5. From nuclear waste management's inception, various inorganic, organic, and organic–inorganic adsorbents have been investigated for radioactive extraction (Ahmaruzzaman, 2022; Wang et al., 2017; Fard et al., 2017; Li et al., 2019; Lai et al., 2018; Yan et al., 2022; Jun et al., 2020; Yu et al., 2015). However, these adsorbents suffer from low adsorption capacity, slow kinetics, and low selectivity. Various 2D layered materials are reported in the literature for the adsorptive removal of radionuclides. Among them, MXene has been extensively explored due to its high selectivity, high thermal and chemical stability, larger surface area, tunable structure, and presence of surface functional groups (Zhang et al., 2018).

Fard et al. (Fard et al., 2017) used Ti₃C₂Tx MXene synthesized by the intercalation followed by exfoliation using HF of MAX phase Ti₃AlC₂ for the first time in the literature. The authors claimed 100 % removal of Ba ions with a maximum adsorption capacity of 9.3 mg/g. About 90 % removal of barium was achieved within 10 min with high selectivity compared to other co-existing ions in the solution. Fig. 10 shows the adsorption mechanism of barium ions by alkali-treated Ti₃C₂Tx MXene. The adsorption of Ba ions was found to be physisorption on the surface in addition to the chemosorption by ion exchange with the surface functional groups according to the reactions: Ba²⁺ + 2OH^– → Ba(OH)₂; Ba²⁺ + 2F^- → BaF₂. However, a low adsorption capacity limits its practical use.

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Fig. 10

Schematic illustration of the adsorption of barium ions (Mu et al., 2018).

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Furthermore, Mu et al. (Mu et al., 2018) tried to improve the barium adsorption capacity of Ti₃C₂Tx MXene by surface modification and basic metal intercalation using NaOH. The XRD and TEM studies showed that the Na⁺ ions are readily intercalated within the Ti₃C₂Tx; causing an increase in the c-lattice parameter, which increased the interaction affinity of barium ions with the surface functional groups of MXene, resulting in improved removal efficiency. A maximum adsorption capacity of 46.5 mg/g could be achieved by alkali-treated Ti₃C₂Tx MXene, which is about three times higher than untreated MXene and higher than previously reported studies. Therefore, alkali activation of MXene could be an effective way to increase the adsorption capacity of MXenes.

Recently, Jun et al. (Jun et al., 2020) investigated the adsorption ability of Ti₃C₂Tx MXene to remove Ba²⁺ and Sr²⁺ ions from model fracking wastewater. The negatively charged MXene surface caused effective bimetallic barium and strontium ion adsorption via electrostatic interactions, with a maximum adsorption capacity reaching 180 and 225 mg.g⁻¹ for barium and strontium within 60 min. Additionally, the Ti₃C₂Tx MXene maintained excellent stability and reusability for four consecutive cycles of operations. Recently, a post-intercalation strategy was developed to intercalate POSS-NH₂ into Ti₃C₂Tx MXene and was employed to sequester radioactive strontium from wastewater (Rethinasabapathy et al., 2021). The complete removal of Sr²⁺ was achieved within 30 min with an adsorption capacity of 172 mg.g⁻¹ following the Freundlich adsorption isotherm.

Additionally, the MXene showed outstanding reusability by maintaining 93 % of its initial adsorption capacity after three cycles. The increased adsorption capacity was attributed to the increased interplanar distance allowing the strontium ions to get adsorbed between the layers, complexing NH₂ groups having high affinity for Sr²⁺ ions and the presence of surface functional groups. Thus, POSS-NH₂ MXene could be a potential candidate for removing strontium ions from aqueous media. Radioactive cesium (¹³⁷Cs), having a half-life of more than 30 years, is also one of the most toxic and hazardous radionuclides constantly emitting β and γ rays. Furthermore, Cs⁺ ions are highly mobile and highly soluble in water. Since the similarity in the hydrated radius of Cs⁺ and K⁺ readily replaces K⁺ ions in the human body's cells. More significantly, it easily penetrates the food chains of humans and animals by ingesting tainted water, meat, fish, and plants, which in turn causes cellular damage and cancer (Shen et al., 2018).

Khan et al. (Khan et al., 2019) investigated the use of Ti₃C₂Tx MXene for the adsorption of Cs⁺ ions from water samples. The adsorption followed the Freundlich isotherm model. A maximum adsorption capacity of 25.4 mg/g could be achieved within 1 min at room temperature. Here, the surface functional groups played a significant role in the sequestering of cesium ions. The adsorbent accumulated Cs⁺ ions in its pores and interlayer gaps despite having many competing cations present because of its layered architecture. Recently, Jun et al. (Jun et al., 2020) compared the adsorption capacity of Ti₃C₂Tx and porous activated carbon for the adsorption of Cs⁺ ions from radioactive waste. Despite having a higher surface area (470 m²/g), activated carbon showed an adsorption capacity of only 80 mg.g⁻¹. The Ti₃C₂Tx MXene with a low surface area (10 m².g⁻¹) showed an outstanding adsorption capacity of 148 mg.g⁻¹. Compared to activated carbon, the enhanced adsorption capacity was attributed to the highly negatively charged MXene surface. According to the findings, the electrostatic interaction between the adsorbent and Cs⁺ ions was an important factor in determining adsorption capacity. The change in the cesium removal rate in the presence of other competing ions (K⁺, Na⁺, Mg²⁺, and Ca²⁺) and organic compounds (oxalic acid, citric acid, and sodium oleate) indicated that the Cs⁺ ions get adsorbed according to the ion exchange mechanism. Traditional adsorbents, including graphene oxide, activated carbon, and MOF-based materials, have been used to remove uranium (Xie et al., 2022; Yu et al., 2019). However, Zhang et al. (Zhang et al., 2016), for the first time, reported the use of hydroxylated titanium carbide (Ti₃C₃(OH)₂) for the adsorption of uranyl with an adsorption capacity of 595 mg g^−1, which is the most effective material for removing uranium from the aqueous stream. The results showed that uranium ions preferably get adsorbed over deprotonated oxygen-containing functional groups compared to protonated ones.

Moreover, chemical interactions and hydrogen bonding were reported to be the major interactions between the uranyl ions and the MXene surface. Wang et al. (Wang et al., 2016) investigated using V₂CTx MXene to adsorb uranium from water samples with a significantly higher adsorption capacity of 175 mg.g⁻¹, higher than most previously reported inorganic adsorbents. The DFT study revealed that uranyl ions get attracted to hydroxyl groups present on the surface of V₂CTx MXene via forming a bidentate complex. The authors reported that the presence of functional groups like –OH, −F, and –O and the interfacial spacing are essential factors for uranium adsorption over the MXene surface. In another research, Wang et al. (Wang et al., 2017) reported a method for increasing the interlayer spacing of Ti₃C₂Tx MXene by exposing it to hydrated environments. The MXene soaked in DMSO showed enhanced adsorption capacity for U⁶⁺ ions with an adsorption capacity of 214 mg/g compared to untreated MXene. Recently, Xie et al. (Xie et al., 2022) synthesized chloroacetic acid-modified Ti₃C₂T_x MXene and investigated its potential for uranium adsorption. The prepared MXene showed an adsorption capacity of 165.43 mg g⁻¹ following second-order kinetics following the Langmuir isotherm model, suggesting monolayer-chemisorption. Moreover, the adsorbent maintained an efficiency of 78 % till the fifth cycle, indicating good stability and reusability of the prepared material. Several modifications in the structure and the surface functional groups were also reported to enhance the adsorption capacity of the MXenes (Liu et al., 2023; Zhang et al., 2023; Xu et al., 2023).

Thorium (²³²Th) is the best alternative nuclear fuel to uranium (²³³U). Like other radioactive elements, thorium is also harmful to human health and the environment, causes irreversible damage to bone tissues, and is carcinogenic to humans. Therefore, removing thorium from water bodies is a great concern (Meng et al., 2022; Liu et al., 2022). Due to surface functional groups, layered structure, excellent dispersibility, and negatively charged surface, Li et al. (Li et al., 2019) investigated the adsorption of Th⁴⁺ over Ti₃C₂Tx MXene. The Ti₃C₂Tx MXene was prepared via the lithium salt method under dry and hydrated conditions. The highest adsorption capacity of 213.2 mg g⁻¹ was achieved by hydrated Ti₃C₂Tx MXene attributed to the larger interlayer spacing allowing easy incorporation of Th⁴⁺ ions and inner sphere complexation originated from the interaction of Th⁴⁺ ions with the surface hydroxyl groups. Recently, Liu et al. (Liu et al., 2022) fabricated magnetically retrievable amidoxime-functionalized Mxene (Fe₃O₄@Ti₃C₂-PDA/OA) to effectively remove Th⁴⁺ ions from the aqueous phase. Fig. 11 illustrates the synthesis mechanism of Fe₃O₄@Ti₃C₂-PDA/OA. Similar to other results, a maximum adsorption capacity of 203 mg g⁻¹ could be achieved by forming inner-sphere complexation with hydroxyl groups. The results suggested that the MXene could be a potential adsorbent for removing radioactive elements from wastewater.

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Fig. 11

Schematic illustration of the synthesis process of Fe₃O₄@Ti₃C₂-PDA/OA MXene (Liu et al., 2022).

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Trivalent europium (^152,154Eu) is the unavoidable by-product of the nuclear fission reaction in the nuclear power plant discharged into the environment, causing major health hazards even at trace levels. As a result, the risks of radioactive contamination sparked research on the adsorptive removal of Eu³⁺ from water samples (Attia et al., 2021). Continuing this vein, Zhang et al. (Zhang et al., 2023) prepared hierarchical titanate nanostructures (HTN) by treating Ti₃C₂Tx with 1 M NaOH/KOH via an in-situ chemical conversion strategy and investigated the adsorption of europium ions. An enhanced adsorption capacity of 222 mg g⁻¹ was shown by Na-HTN compared to K-HTN (203 mg g⁻¹) via an ion exchange mechanism through electrostatic interactions and hydrogen bonding. The improved adsorption capacity was attributed to the increased interlayer spacing and a high hydration energy of Na^+, resulting in high diffusion and exchange of europium. Recently, Yan et al. (Yan et al., 2022) synthesized alkali-modified Ti₃C₂Tx MXene via a green strategy using citric acid as a surfactant rich in carboxylic and hydroxylic groups for the adsorption of Eu³⁺ ions. The prepared MXene showed an adsorption capacity of 118 mg g⁻¹ following pseudo-second-order kinetics and Freundlich adsorption isotherm. Thus, the literature indicates that the MXene and its composites could be a state-of-the-art new-generation heterogeneous adsorbent for the remediation of radionuclides from nuclear waste.

2.5 Phenolic compounds

Organic phenolic pollutants in industrial wastewater cause environmental pollution and physiological damage. Researchers have developed a thermosensitive composite hydrogel made of MXene and PA to remove phenolic pollutants from industrial wastewater. The adsorptive removal of phenolic compounds by MXene-based materials at different conditions is presented in Table 6. MXenes-based composites were obtained via the in-situ polymerization of ionic liquids on their surfaces for superior mechanical strength and adsorption efficiency. A composite hydrogel with ILs is mechanically more efficient than one with MXene/PA composites, in addition to adsorptive properties. MXene flakes are attached to the surface by electrostatic interactions, making MXene more thermally stable and oxidation-resistant. A 45-day aging experiment using modified MXene (MXene-EMT) was insignificant after adding 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMT). P-nitrophenol (4-NP) was very effectively absorbed by the composite hydrogels as prepared, demonstrating exceptional structural characteristics and high uptake efficiency. The highest uptake ability for sorbate was around 200 mg.g⁻¹ at 298 K. After five sorption cycles, it continued at 90 % of its starting amount, often because of the presence of EMT. As per the experimental value, Freundlich isotherm and pseudo 2nd order kinetics were used (Lee et al., 2015).

The immobilization of laccase has been achieved using a proprietary ferroelectric 2D hybrid structure of MXene/NiFe-LDH/Fe₃O₄ (MNLF). MXene nanosheets were co-precipitated with layered double hydroxide (LDH) in situ to achieve surface functional groups. After that, magnetic nanoparticles Fe₃O₄ were introduced, which had excellent biocompatibility and the capacity to separate materials from substrates quickly. The surface of the MNLF was treated with a silane coupling agent. The immobilization of laccase from Trametes-Versicolor was carried out using functionalized MNLF. A 167.9 mg g⁻¹ of enzymes were loaded into the nanocomposite (NC) material. The immobilized laccase demonstrated good stability in a wider pH 2–8, temperature range (25–60 °C), and organic solvent concentration 1–4 M range compared to free enzymes. 2, 4-Dichlorophenol exhibited a 55.5 % efficiency after seven cycles of repeated catalysis. Pyrocatechol demonstrated a 70.9 % efficiency after seven cycles. Laccase immobilization carriers are prepared in this study using a radically different strategy. Free laccase at room temperature does not display the same degree of stability as immobilized laccase. MNLF-laccase shows a higher activity at pH > 4 than at pH of 4.0, which is optimal for the free enzyme. In contrast, pH of 5 optimizes the immobilized enzyme (Li et al., 2023).

MXene/poly (N-isopropyl acrylamide) (MP) hydrogel heat-responsive adsorbents were developed to minimize phenolic toxicity and exhibit strong binding performance, superior thermoactive features, and no residual pollution. MXene also exhibited enhanced antioxidative properties and reactive functional groups due to the cyclodextrin fabrication technique. Solvent-based polymerization was used to make a heat-constrained hydrogel, which showed no significant changes after incubation in water for 21 days. 4-NP was removed by the sorbent after five adsorption–desorption cycles at room temperature, removing 82 % of 4-NP in a deionized medium at 308 K following five sorption–desorption cycles. As a result of heterogeneous layer-by-layer uptake and intra-molecular diffusion, the hybrid hydrogel could take up the target material spontaneously selectively. As a consequence of the electrostatic bonding between MXene and the polymer, the breakdown temperature of pure polypropylene acrylamide was increased to 633–723 K, implying that instantaneous dipole interactions between MXene planes and Si-O/Ti-C bonds significantly boosted the thermal stability of the hydrogel (Wang et al., 2022).

Molecular dynamics simulations, coarse-grained simulations, and DFT simulations have been conducted to investigate the adsorption intensity of phenol and chlorophenol on six novel 2D structure nanocomposites. Due to strong instantaneous dipole interactions between the lone pair of amine groups and phenol's p orbital electron cloud, the designed sorbent provides excellent uptake properties. A molecule like chlorophenol has an irregular electron density distribution due to atoms such as chlorine with electronegative properties. Thus, chlorophenol was more strongly attached to the adsorbents than phenols, resulting in bipolar connections. This resulted in greater chlorophenol adsorption by the MXene layers in the sorbents. Increasing the system temperature increases the contact area of sorbate molecules with water, reducing % removal due to a surge in monolayer displacements of MXene and weakening of hydrogen bonds. At room temperature, the experimental isotherm values fit the Freundlich isotherm and possess a maximum sorption capacity of 714.29 mg g⁻¹ (Miri-Jahromi et al., 2022).

3.1 Heavy metal ions

MXene-based photocatalysts have remarkable efficacy in eliminating heavy metal ions from polluted water, attributable to their superior conductivity, extensive surface area, and abundant surface functional groups. These features facilitate the robust adsorption of heavy metals such as Pb2⁺, Hg2⁺, and Cd2⁺, promoting their photocatalytic reduction or oxidation upon light irradiation. MXenes improve charge separation and augment active sites when integrated with other materials, expediting metal ions' breakdown. Moreover, the stability and recyclability of MXene render it an exemplary choice for removing heavy metal ions in water treatment, providing an environmentally friendly and efficient method for mitigating environmental contamination. Li et al. (Li et al., 2022) utilized the cost-effective and environmentally friendly heat polymerization technique to develop the excellent g-C₃N₄/Ti₃C₂ MXene (CN/TC-2) based photocatalyst for the photo catalytical degradation of very intensive heavy metal U⁺⁶. The adsorption capacity of the MXene-based composite is 14.05 times greater than the pristine MXene and graphene. Because the pristine MXene and graphene have lower Specific Surface area (SSA) than the g-C₃N₄/Ti₃C₂. Ti₃C₂ addition to CN substantially increased the adsorption selectivity for U⁶⁺. Under both dim and bright lighting, they looked at how produced photocatalysts performed regarding photodegradation.

In contrast to dim light, the U⁶⁺ photodegradation activity was significantly altered by visible light. Researchers concluded that adding cocatalysts to substances like MXenes and TiO₂ would enhance the photocatalytic activity of catalysts to destroy U⁶⁺. Chen et al. (Chen et al., 2024) effectively synthesized a 2D Ti₃C₂Tx/g-C₃N₄ composite utilizing an assembly technique. The substance possesses a stratified architecture, elevated conductivity, and chemical resilience. The amalgamation of MXenes with g-C₃N₄ enhanced the dispersion of g-C₃N₄ and inhibited agglomeration. This resulted in exceptional efficacy in eliminating U⁶⁺ from water, attaining complete removal within 120 min at a high reaction rate. It continued to be effective after five cycles. The material performed effectively at an ideal pH of 6.0 and a U⁶⁺ concentration of 10 mg/L, exhibiting no chemical alterations post-reaction, thereby demonstrating its stability. The study determined that superoxide radicals (.O₂⁻) play a crucial role in reducing U⁶⁺ to U⁴⁺. XPS research verified that Ti₃C₂Tx facilitated the reduction of U⁶⁺ and functioned as an electron sink, enhancing charge transfer. This work presents innovative concepts for developing efficient photocatalysts for environmental remediation.

Zhao et al. (Zhao and Cai, 2020) combined Bismuth molybdate (Bi₂MoO₆) and Ti₃C₂ MXene by hydrothermal technique to synthesize the MXene-based Bi₂MoO₆/Ti₃C₂ nanostructure for the photocatalytic breaking of toxic environmental pollutant Cr⁶⁺ from wastewater that Cr⁶⁺ contaminated water. The enhanced photocatalytic performance of the Bi₂MoO₆/Ti₃C₂ nanostructure in pure Bi₂MoO₆ was explained using various characterization techniques, including HRTEM, BET, XRD, EDS, PL, EIS, EPR, TEM, SEM, and DRS. Under sunlight, the higher degradation efficiency of the synthetic MXene-based photocatalyst reached 100 % in 60 min. The photocatalytic degradation effectiveness of the Bi₂MoO₆/Ti₃C₂ photocatalyst is 11.2 times more than that of pure Bi₂MoO₆, based on a scientific study. This finding creates new opportunities for researching low-cost photocatalysts that work well.

3.2 Dyes

MXene-based photocatalysts exhibit remarkable efficacy in eliminating dye ions from wastewater, attributed to their superior conductivity, extensive surface area, and capacity to promote efficient charge separation. MXene-based composites, when integrated with other semiconductor materials, augment photocatalytic degradation by facilitating the decomposition of dye molecules under visible light. These compounds can adsorb and decompose many hazardous dyes, including methylene blue, rhodamine B, and crystal violet, frequently present in industrial effluent. MXenes have distinctive characteristics, including elevated electron mobility and diverse surface chemistry. These facilitate rapid degradation and enhanced stability throughout numerous cycles, rendering them advantageous for sustained dye removal in water treatment applications. Cui et al. (Cui et al., 2020) synthesized the extremely thin Bi₂WO₆/Nb₂CTx nanoparticles using an environmentally friendly hydrothermal procedure to treat RhB and RhB from wastewater. The architectural functionality of Nb₂CTx, Bi₂WO_6, and Bi₂WO₆/Nb₂CTx nanostructures has been assessed using a variety of characterization approaches, including lateral size, zeta potential, XRD spectra, and mechanical integrity.

Because of its outstanding thermal, chemical, and photostability, bismuth tungstate (Bi₂WO₆) is one of the most popular photocatalysts for dye degradation. Due to “their slow dissociation of photogenerated electron-hole pairs, its employment is severely constrained by their poor photodegradation ability. One of the strongest ingredients for accelerating the dissociation of photogenerated electron-hole pairs is MXene. Thus, relative to Bi₂WO₆, Bi₂WO₆/Nb₂CTx photocatalysts demonstrated remarkable photodegradation efficiency for MB, 92.7 %, when Bi₂WO₆ and Nb₂CTx created a composite to change their photocatalytic features. Compared to pristine Bi₂WO, the photocatalysis kinetics of Bi₂WO₆ mixed with two weight percent Nb₂CTx for MB appear to be the value of 0.0285 min⁻¹, which is twice as high. This research showed that 2D Nb₂CTx is a useful co-catalyst for enhancing photocatalyst photocatalytic degradation performance and producing an efficient photocatalyst with 2D/2D nanomaterials.

Tran et al. (Tran et al., 2021) synthesized the MXene-based TiO₂/Ti₃C₂ heterostructure for the photocatalytic degradation of RhB synthetic dye to treat wastewater and save humans and aquatic animals. TiO₂/Ti₃C₂ nanostructure nanorods were used in a novel way to create microscale safflowers. Beginning with 2D Ti₃C₂Tx MXene, a sequential in-body transition was used to create this one-of-a-kind architecture. Ion exchange, heat treatment, hydrothermal oxidation, and alkalization were intricately coupled stepwise for the conversion. After those procedures, TiO₂/Ti₃C₂ nanostructures developed laterally from the layer-structured MXene sheets broken into nanomaterials. It was discovered that the heat treatment conditions significantly impacted the heterostructures' geometry, structural elements, and characteristics. The catalytical photo activity of synthesized MXene-based TiO₂/Ti₃C₂ photocatalyst was 95 %. The TiO₂/Ti₃C₂ heterostructures produced under ideal conditions exhibited semiconductor-like behavior, in contrast to Ti₃C₂Tx MXene. The excellent heterostructure photocatalytic activity was also exhibited. Chen et al. (Chen et al., 2020) synthesized the TiO₂/MXene heterostructure by employing the low price and echo-loving in-situ solvothermal procedure for the photo-catalytical degradation of MO in textile-contaminated water. Ti₃C₂Tx is used as a cocatalyst to increase the photocatalytic degradation ability of TiO₂ by capturing the photoexcited. By employing C₃H₈O molecules, the findings from experiments show that TiO₂ NPs are uniformly distributed over the Ti₃C₂Tx surface. The maximal photocatalytic activity of the TiO₂/ Ti₃C₂Tx- C₃H₈O nanostructure was 90.5 % during 75 min of Hg illumination, which is 57.9 % higher than the capacity of the pure Tio₂ nanoparticles. Due to the synergistic effects of Ti₃C₂T_x and TiO₂, the maximum photocatalytic lowering capability was attained. First, they synthesized the MXene Ti₃C₂Tx from the original material Ti₃AlTx by removing Al via echo enemy chemical-based HF etching procedure. After synthesizing MXene, they employed a friendly solvothermal procedure to synthesize the TiO₂/MXene heterostructure for MO adsorption, as shown in Fig. 12.

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Fig. 12

Synthesizing of TiO2/MXene procedure and MO photodegradation procedure (Chen et al., 2020).

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This section concludes that MXene-based photocatalysts are successful in dye degradation, emphasizing improving photocatalytic performance by integrating MXenes with other semiconductor materials. Research indicated that incorporating MXenes, namely Ti₃C₂Tx, markedly enhanced electron-hole separation, thus increasing photocatalytic efficiency. This improvement was apparent in degrading dyes such as methylene blue and rhodamine. The production of new heterostructures, including Bi₂WO₆/Nb₂CTx and TiO₂/Ti₃C₂, demonstrated enhanced photodegradation performance relative to their pristine counterparts, achieving up to 95 % degradation efficiencies. The findings highlight the function of MXenes as co-catalysts that improve photocatalyst efficiency through their distinctive 2D architecture, elevated conductivity, and interactions with other substances. The findings suggest that MXenes possess significant promise for efficiently eliminating dye contaminants in water treatment applications.

3.3 Pharmaceuticals

For the photocatalytic removal of pharmaceutical compounds from the water, many 2D nanomaterials are available. However, after a thorough review of the literature, we can conclude that MXene-based nanomaterials have superior properties to all other 2D nanomaterials such as layered double hydroxides (LDH), transition metal oxides (TMOs), graphene and transition metal dichalcogenides (TMDs) (Kemp et al., 2013; Bhattacharjee et al., 2022). MXene-based photocatalysts for the photo-catalytic degradation of various pharmaceutical compounds are presented in Table 8.

To improve the photocatalytic properties of MXenes, they may be hybridized with other substances such as MOFs, polymers, metal oxide, graphene, etc. Because of their interesting properties, such as adjustable band gap (0.92–1.75 eV), high structural stability, non-toxicity, high interlayer spacing, and hydrophilicity, MXene-based photocatalysts are used for the removal of a variety of pharmaceutical compounds and antibiotics from the aqueous system. MXenes can potentially increase the photo-catalytic efficiency of their composites when used as co-catalysts due to their distinctive lamellar nanostructures and excellent conductivity (Feng et al., 2021; Javaid et al., 2022).

In recent work, a multi-functional adsorbent photo-catalyst MXene-TiO₂ composite was synthesized using a rapid microwave hydrothermal method to remove enrofloxacin from water (Sukidpaneenid et al., 2023). Moreover, the layered structure of MXene improves photocatalytic efficiency by increasing the charge carrier separation and acting as a robust support (Kuang et al., 2020). MXenes showed great potential in the photodegradation of several pharmaceutical compounds and antibiotics. MXenes may also be utilized as a host substance to improve the catalytic activity of certain co-catalysts. Pharmaceuticals have been successfully degraded using MXene-based photo-catalysts synthesized using various techniques, including chemical vapor deposition, sol-hydrothermal method, anodization, and in-situ reductive deposition method (Ihsanullah, 2022).

For the degradation of tetracycline hydrochloride (TC-HCl), NH₂-MIL-125(Ti)(TiO₂)/Ti₃C₂ dual heterojunction exhibited 11.5 times higher degradation efficiency than pristine MIL-125-NH₂ under the irradiation of visible light (Wu et al., 2020). The enhancement in the photocatalytic activity of the heterojunction is due to the incorporation of Ti₃C₂, which increased the charge transfer. When NH₂-MIL-125 (Ti) is used, the TC-HCl degradation process comprises the generation of ˚OH by water oxidation, and electrons are excited from the valence band to the conduction band. Similarly, electron migration occurs across the hetero-junction from the NH₂-MIL-125 (Ti) CB to the TiO₂ CB and subsequently from TiO₂ to Ti₃C₂. The Schottky junction captures the photoinduced electrons transported to the Ti₃C₂ surface for electron-oxygen reduction (Ihsanullah, 2022; Wu et al., 2020).

The advanced oxidation process is also used to remove pharmaceuticals using MXene-based nanocomposite. Ranitidine is a toxic pharmaceutical compound, the nitrosamine dimethylamine (NDMA) source. Novel Ti₃C₂-based MXene nanosheets adorned with zero-valent iron particles (nZVIPs@Ti₃C₂ nanosheets) were synthesized by Ma et al. (Ma et al., 2021) to increase the catalytic activity of peroxymonosufate. This synthesized Ti₃C₂-based nanosheet exhibited notable degradation efficiency for quickly removing ranitidine by an advanced oxidation process under mild reaction conditions. The high stability of the catalyst is mostly due to the fast transfer of charge between the electron-rich active centers (Fe and Ti), with ranitidine degradation being carried out by SO₄˚^- and OH˚^- derived from PMS activation, with SO₄˚^- being the predominant contributor. Another study reported the synthesis of a highly photoactive Z–scheme of graphene layers anchored TiO₂/g–C₃N₄ (GTOCN) photocatalyst by a step-in-situ calcination process. It is used for photocatalytic degradation of two pharmaceutical compounds, ciprofloxacin and tetracycline (Wu et al., 2020). This MXene-based photocatalyst exhibited a degradation efficiency of 83.5 % of tetracycline within 80 min of visible light irradiation and 61.7 % of ciprofloxacin within 60 min of visible light irradiation. The electron spin resonance and free radical experiments confirmed that the active radicals responsible for the degradation of pharmaceuticals are O₂˚^- and OH˚. Schematic of the photocatalytic process for organic pollutants degradation over Z-scheme heterojunction of GTOCN3 under visible light irradiation, as shown in Fig. 13. Dai et al. (Dai et al., 2022) synthesized 2D CuO nanosheets by drip method. They introduced the layered MXenes as the charge transfer bridges into CuO and O-doped g-C₃N₄ nanosheets to prepare a 2D/2D/2D CuO-MXene-OCN S-scheme heterojunction. The CuO-MXene-OCN heterojunction exhibited higher photocatalytic efficiency than the CuO-OCN heterojunction because of the close interaction between the interfaces. This MXene-based photocatalyst exhibited a degradation efficiency of 98.2 % for tetracycline hydrochloride, 75.6 % for metoprolol, and 92.6 % diclofenac within 60 min of solar light irradiation.

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Fig. 13

Schematic of the photocatalytic process for organic pollutants degradation over Z-scheme heterojunction of GTOCN₃ under visible light irradiation (Wu et al., 2020).

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Cao et al. (Cao et al., 2020) reported the use of novel CuFe₂O₄/MXene hierarchical heterostructure for the photocatalytic degradation of the antibiotic sulfamethazine (SMZ) up to 59.4 % under the irradiation of visible light, which is attributed to the increased lifespan of carriers and charge transfer in the composite. The visible light activates the CuFe₂O₄ (CFO) photocatalyst with a band gap of 1.43 eV. The electrons are excited from the valence band to the conduction band. The photo-generated electrons are shifted from the CuFe₂O₄ to the 2D Ti₃C₂. However, many electrons emerge at the Fermi level in the Ti₃C₂ flakes. Moreover, because of its substantial specific surface area, Ti₃C₂ may provide many active adsorption sites. Furthermore, during photo-catalysis, the active material may potentially adsorb the sulfamethazine, a toxic antibiotic.

3.4 Radionuclides

Several methods have been used to remove radioactive nuclides from wastewater and aqueous solutions, such as adsorption, chemical precipitation, electrochemical treatment, membrane filtration, reverse osmosis, and photodegradation (Zhang et al., 2019; Rana et al., 2013). In practical applications, conventional adsorption and ion exchange methods using nanomaterials have several drawbacks, such as poor interaction with radionuclides. However, most published reduction techniques involve high reagent usage and readily produce secondary waste. A novel solution to the problems mentioned above is semiconductor photocatalysis. This green method uses semiconductor photocatalysts under solar light irradiation to reduce toxic heavy metals (Lu et al., 2017). However, very little work has been reported on the photoreduction of toxic radionuclides using MXene-based nanomaterials. A unique and effective method of U⁺⁶ transformation is the photocatalytic reduction of U⁺⁶, in which the free U⁺⁶ will first attach to the surface of the photocatalysts. Then, the generated photoelectrons reduce the U⁺⁶ to U⁺⁴. Yu et al. (Yu et al., 2022) synthesized an MXene-based photocatalyst by the deposition of Ag nanoparticles on Ti₃C₂T_x with profuse binding sites for U(VI) (Ag/Ti₃C₂Tx-O). Many characterization techniques were used to comprehend the physicochemical characteristics of the synthesized MXene-based composite. The photocatalytic activity of Ag/Ti₃C₂Tx-O photocatalyst for the photoreduction of U⁺⁶ was found to be 1257.6 mg g⁻¹ in 2 h of the treatment process, which is 11 times higher compared to conditions with the absence of light. This research advances our knowledge of the mechanism behind U⁺⁶ reduction. It proposes a strategy for developing efficient catalysts for U⁺⁶ photocatalytic reduction.

Li et al. (Li et al., 2022) developed an MXene-based photocatalyst by modifying g-C₃N₄ with oxidized MXene (Ti₃C₂). Modification of graphitic carbon nitride (g-C₃N₄) with MXene (Ti₃C₂) enhances the adsorption capacity for U⁺⁶, separation efficacy of photo-generated e^-/h^+, and optical absorptivity which results in the enhancement of photoreduction of U⁺⁶. The reaction constant for reducing U⁺⁶ to U⁺⁴ using the g-C₃N₄/MXene (0.267 min⁻¹) is 14.05 times higher than the pristine g-C₃N₄ (0.019 min⁻¹). TiO₂ was formed on the surface of Ti₃C_2, and a Z-scheme heterojunction was developed. This heterojunction helps effectively separate charge carriers and improves the photocatalytic activity of the g-C₃N₄ composite. Reduction of U⁺⁶ to U⁺⁴ occurs during the photocatalytic reaction, generating deposits of UO_2+x (x < 0.25) with ˚O₂^– serving as the primary reduction species (Fig. 14(a)).

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Fig. 14

Mechanism of photocatalytic reduction of U⁺⁶ by (a) g-C₃N₄/Ti₃C₂ heterojunction, (b) Ti₃C₂/SrTiO₃ heterostructure (Li et al., 2022).

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A novel Ti₃C₂/SrTiO₃ heterostructure was synthesized by partial oxidation of multi-layered Ti₃C₂ precursor depending on the simple crystallography. It was used as a photocatalyst for the photocatalytic reduction of U⁺⁶ (Solangi et al., 2023). This MXene-based composite showed an excellent photocatalytic efficiency of 77 % for removing UO₂²⁺ ions, which is 38 times higher than pure SrTiO₃ (Fig. 14(b)). The multi-layered Ti₃C₂ improved charge transfer while preventing electron recombination in the conduction band. This study revealed the alluring possibility of generating doped perovskite oxide materials based on MXene (Ti₃C₂) and using sunlight.

3.5 Phenolic compounds

Phenol, which is frequently utilized in industries such as the chemical and pharmaceutical industries, poses significant threats to both the environment and human health when it is released into water because it is poisonous and resistant to breakdown via microbes. One of the most important things is to achieve its complete mineralization using procedures that are energy efficient. Co-catalyst modification, which involves using metals such as platinum, gold, cobalt, or manganese, improves charge transfer and redox kinetics, increasing the effectiveness of photocatalytic reactions. Conventional co-catalysts, on the other hand, are either expensive, difficult to manufacture, or unstable. Ti₂C₂ MXene, on the other hand, is a two-dimensional material with great conductivity and exposed active sites. It has garnered interest due to its capacity to increase photocatalysis. The combination of semiconductors with MXene improves charge separation and raises photocatalytic efficiency, making it a potentially useful option for phenol degradation.

Zhang et al. (Zhang et al., 2024:) synthesized the BT composite photocatalysts with different Ti₃C₂ mass loadings by a hydrothermal technique to improve the separation of photogenerated carriers and phenol degradation. Characterization experiments validated the successful synthesis of a BiVO₄/Ti₃C₂ composite exhibiting good shape and carrier separation, particularly in the BT-3 sample. Studies on the breakdown of phenol demonstrated that PMS activation and Ti₃C₂ doping markedly enhanced efficacy, with BT-3 attaining an 87.2 % degradation rate within 60 min (rate constant k = 0.02537 min⁻¹). Elevated PMS dosage improved decomposition; however, increased starting phenol concentrations diminished effectiveness, with neutral pH being best. The catalysts exhibited commendable stability over ten cycles. Experiments on free radical capture indicated SO₄⋅⁻ as the paramount species in the breakdown of phenol. This study presents novel approaches for developing effective photocatalysts to degrade persistent organic pollutants such as phenol.

Hussain et al. (Hussain et al., 2024) synthesized pure CdAl₂O₄ and Ag–CdAl₂O₄ employing coprecipitation and Ag–CdAl₂O₄/MXene-based composite material via ultrasonication. The photocatalytic efficacy of these substances was evaluated by degrading contaminants such as crystal violet and phenol. The composite demonstrated markedly improved performance, decomposing approximately 95 % of crystal violet and around 84.6 % of phenol. The enhancement results from the interaction between silver doping and the MXene-based composite, which improves visible light absorption and charge carrier separation. Scavenging tests found active species responsible for pollutant degradation, and the process adhered to first-order kinetics. The findings underscore the composite's significant potential for decomposing organic contaminants.

In-situ preparation of composite photocatalysts using MXene − Ti₃C₂T_x was carried out using decahedral anatase particles and titanium carbide. Magnetron sputtering was employed to prepare Fe-modified composites containing decahedral anatase particles coupled with Ti₃C₂, which were applied to the photocatalytic degradation of phenol. UV–VIS light was used to evaluate photocatalytic activity during phenol decomposition. 20 mg/dm³ of phenol was applied at an initial pH of 6.8. In an air-supplied quartz reactor, photodegradation reactions were conducted. After introducing the photocatalyst to the reactor, an aqueous phenol solution was introduced and stirred continuously for 30 min. The reaction was kept in the dark for 30 min to achieve equilibrium before exposure to UV light. The irradiation source was an Xe lamp of 300 W. Aliquots of samples were collected at 0, 20, 40, and 60 min after irradiation. This experiment used 0.2 m syringe filters to separate photocatalyst particles from the solution. Phenol photodegradation progress and intermediate concentrations were analyzed through reverse-phase high-performance liquid chromatography. Tert-butanol was added to investigate the sorbent's capability to remediate free radicals and detect surface structural changes (Grzegórska et al., 2021).

An electrostatically driven self-assembly strategy successfully synthesized an Ag₃PO₄/Ti₃C₂ MXene Schottky catalyst. In the etching solution used for fabricating Ti₃C₂ MXene, Ti₃AlC₂ was immersed in Ti₃C₂. HCl solution of 20 mL was mixed with 3.35 g NaF and kept at 60 °C for 12 h with continuous stirring to prepare the etching solution. In the following steps, the residue was washed repeatedly with diluted hydrochloric acid, ethanol, and deionized water before centrifugation was performed to separate them. A vacuum at 80 °C was used to dry the obtained MXene powder. 2,4 DNP was photodegraded with visible photons under a given interval, and 1 mL of the suspension was centrifuged to separate it from the suspension. Schottky contact results show electron transfer from Ag₃PO₄ to the Ti₃C₂ surface by XPS. The Mott-Schottky measurements indicate that Ag₃PO₄ has a higher Fermi level than Ti₃C₂. Therefore, energies flow from Ag₃PO₄ to Ti₃C₂ because of the Fermi level difference between Ti₃C₂ and Ag₃PO₄, which leads to band bending and Schottky junctions. Furthermore, the efficient photocatalytic reduction of Cr⁶⁺ was achieved in an AgI/Ti₃C₂ system, demonstrating Ti₃C₂′s versatility as an electron sink in various environments (Cai et al., 2018). The band theory was employed to elucidate the interaction between Ti₃C₂ and Ag₃PO₄ at their contact. The Fermi level, indicative of the energy state of electrons, is elevated in Ag₃PO₄ relative to Ti₃C₂. Due to this disparity, electrons transfer from Ag₃PO₄ to Ti₃C₂ until their Fermi levels equilibrate. This movement induces the bending of energy bands, forming a Schottky junction, a specific type of interface between a metal and a semiconductor. A Schottky junction is established at the interface of Ag₃PO₄ to Ti₃C₂, facilitating unidirectional electron flow. Ti₃C₂ functions as an “electron sink,” signifying its ability to capture electrons. The built-in electric field generated at the junction enables this phenomenon. Consequently, the electrons and holes (carriers) produced during the photocatalytic process are more efficiently segregated, inhibiting their recombination. This enhanced separation improves photocatalytic efficiency by facilitating greater electron involvement in processes such as pollutant breakdown. The detailed mechanism of the degradation of the phenol by the Ag₃PO₄/Ti₃C₂ MXene Schottky catalyst is presented in Fig. 15. Table 9 shows the degradation of phenolic compounds by MXene-based photocatalysts.

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Fig. 15

Photocatalytic degradation of phenolic compounds by the Ag₃PO₄/Ti₃C₂ MXene-based Schottky catalyst MXene (Cai et al., 2018).

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In a nutshell, MXene-based photocatalysts exhibit significant potential for eliminating phenolic compounds owing to their superior conductivity, extensive surface area, and capacity to improve charge separation. MXene-based composites demonstrate markedly enhanced photocatalytic activity under visible light when integrated with other materials, as discussed above. These composites effectively decompose phenolic chemicals, leveraging improved light absorption and the inhibition of charge recombination, rendering them extremely suitable for environmental remediation.

4.1 Heavy metal ions

MXene can efficiently collect heavy metal ions like mercury Hg²⁺, copper Cu²⁺, lead Pb^2+, etc. The specific pollutants are also absorbed by MXene when its surface groups interact with impurities (Bai and Guan, 2023). The functionalization of MXenes can increase stability while simultaneously promoting sustainability and the elimination of heavy metals from water and wastewater. The hydrothermal process effectively synthesizes the magnetic Ti₃C₂T_x MXene (MGMX) composite. Table 10 presents the synthesizing techniques for reacting parameters and the refection performance of MXene-based membranes.

Ti₃C₂T_x composite has a maximal Hg²⁺ adsorption capacity of 1128.41 mg/g, and lead can be successfully removed throughout a wide pH range (Fu et al., 2020). After five desorption/ adsorption operations, high extraction efficiency was maintained, and MGMX nanomaterials were successfully reused. Likewise, Hg²⁺ removal is accomplished using 2D Ti₃C₂T_x MXene core–shell sodium alginate (SA) aerogel spheres (MX-SA) (II). The primary methods are ion exchange, electrostatic interaction, and complex formation. MX-SA has a saturation adsorption capability of 932.84 mg/g for Hg²⁺ (Fu et al., 2020). Removing Hg²⁺ out of an aqueous phase requires the synthesis of a multidimensional oxygen-functionalized Ti₃C₂ (MTi₃C₂) nanostructure. Fig. 16 shows that M demonstrated strong selectivity- Ti₃C₂, particularly a highly effective removal rate to Hg²⁺of 4806 mg g⁻¹. In addition, it also displayed a pH range of 3–12. The MoS₂/MXene nanostructures had outstanding Hg²⁺ extraction capability, demonstrating that the elimination efficiency of MXene may be enhanced through manufacturing, functionalization with suitable materials, and structural modification. As a result, it can be used to rehabilitate the ecosystem successfully (Shahzad et al., 2020).

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Fig. 16

(a) Adsorption kinetics for Hg²⁺onto M−Ti₃C₂ (b) Adsorption isotherms for different temperatures (from 288 to 308 K) for Hg²⁺ onto M−Ti₃C₂ (c) Effect of pH on Hg²⁺ removal (d) Effect of ionic species on Hg²⁺removal (Fu et al., 2020).

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Unique architectures and a wealth of MXene-based compounds have stimulated adsorbents to eliminate Cu²⁺ from wastewater. Delaminated (DL)-Ti₃C₂T_x exhibits strong capability for removing Cu²⁺; the sorption rate has reached 80 % in 1 min. To create innovative functionalized Ti₃C₂T_x, a facile one-step inspired fabrication process is adopted. As demonstrated in Fig. 17, the carboxyl group of DOPA and catechol groups lead Ti₃C₂Tx to establish a greater removal capability of Cu²⁺- PDOPA than Ti₃C₂T_x. To eliminate Cu²⁺, MXene-based materials can provide a range of active ligands (Gan et al., 2020). For instance, large oxygen-containing groups were used to generate MXene/alginate composites that demonstrated an improvement in extraction efficiency (87.6 mg.g⁻¹) and a reduction in equilibrium time (15 min). MXene/alginate composites and Cu²⁺ were bound by two distinct mechanisms: ion exchange and chemical interaction (Dong et al., 2019). For Ti₃C₂Tx to interact with amino acid residues in water, delaminated MXene (d- Ti₃C₂Tx) flakes must first generate rutile phase titanium dioxide (TiO₂). A degradation efficiency rate of 75 % results in the elimination of 94.6 mg g⁻¹ of Cu²⁺ in 5 min. The effective elimination of heavy metals from functionalized Ti₃C₂Tx after successful production makes them suitable for solving environmental issues (Elumalai et al., 2020).

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Fig. 17

(a) The resemblance of the Cu²⁺ sorption, (b) Adsorption kinetics for Cu²⁺ onto Ti₃C₂Tx-PDOPA (Gan et al., 2020).

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4.2 Dyes

The dye industry contributes significantly to the annual emissions of organic pollutants and creates serious environmental risks. The textile sector mostly releases dye. MXene makes it possible to quickly and effectively eliminate dye from wastewater released (Fayyaz et al., 2021). MXene-based membranes were proven to be the best option for removing several harmful dyes from colored water. Table 11 gives an overview of the existing literature on the synthesizing techniques, reacting parameters, and removal performance for different dyes.

Liu et al. (Liu et al., 2020) utilized a modified Hummers approach to synthesize the graphene oxide (GO) and develop Ti₃C₂T_x MXene by eliminating Al from their parent material Ti₃AlC₂ via chemical synthesizing techniques. So, for synthesizing GO/Ti₃C₂T_x MXene-based membranes, the commercial mixed cellulose ester (MCE) membrane techniques were utilized; based on the proportional quantities of GO and MXene, the GO/MXene membrane's distinctive diverse structure displayed a beneficial effect regarding substrate breakdown and permeability. Under comparable experimental conditions, a composite membrane (550 nm) with a GO/MXene mass ratio of 1/4 demonstrated significantly higher water flux (71.9 L.m⁻².h⁻¹.bar⁻¹) than a benchmark GO membrane (6.5 L.m⁻².h⁻¹.bar⁻¹) thanks to its beneficial two-dimensional (2D) interparticle channels and hydrophilicity. The sudden change in the membrane's interlayer distance and the decline in oxygen-based functional groups were primarily responsible for the GO/MXene combination membrane's improved water flow compared to the standard GO membrane. Lin et al. (Lin et al., 2022) produced the raw materials Bi₂O₂CO₃ and Ti₃C₂Tx by employing chemical etching and a low-temperature chemical modification process. They created the Bi₂O₂CO₃/Ti₃C₂Tx composite membranes by synthesizing the constituent materials and ultrasonic stirring. The experiment outcomes showed that adding N-doped Bi₂O₂CO₃ nanoparticles resulted in an extremely high water flux for the composite membrane (815.3 L.m^-2h⁻¹). The rejection capacities of the MXene-based membranes RhB and CR are 99.9 % and 98.4 %, respectively. It is crucial to note that the composite membrane kept its constant permeability and selectivity even after five continuous cycles of visible light irradiation. The number of electrons stored close to MXene was decreased due to the efficient extraction of photoexcited electron-hole pairs, which further encouraged the fast mobility of free electrons inside the crystal and of photogenerated charge carriers. In general, the extraction impacts of dyes from water were enhanced by a synergistic interaction between photocatalysts and 2D membrane composites. In contrast to conventional pulverized photocatalysts that lack transport, the photocatalytic membrane may be easily detached from the water without causing additional pollution from photocatalyst powder. The practical goal of lowering the cost of wastewater treatment was achieved by combining photocatalytic technology with membrane separation technology in this work. Gong et al. (Gong et al., 2022) utilized the vacuum filtration technique to synthesize the Ti₃C₂Tx /COF composite membranes to reject MG synthetic dyes for wastewater treatment. These techniques are FTIR, BET, AFM, and XRD for the extensive understanding of morphology and the internal structure of the developed Ti₃C₂Tx/COF composite membranes. The MXene has high instability and low permeability, and the addition of COF improves its stability and permeability to a large extent. Ti₃C₂Tx /COF composite membranes were rejected at 97 %, and the water flux is still as great as 194.5 L m⁻¹. At experimental conditions C₀ = 250 mg.L^-1, P = 0.01 MPa. The molecular sieving function of the MXene/COF composite membrane is shown in Fig. 18.

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Fig. 18

The molecular sieving function of the MXene/COF composite membrane (Gong et al., 2022).

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4.3 Pharmaceutical chemicals

Membrane-based filtration has long been used in the environment and medical and material sciences. However, their uses have been limited and for only removing selected environmental pollutants (Sutariya et al., 2023). Recently, excessive growth has been observed in the design and application of these membranes in environmental remediation. Since synthesizing 2D materials like MXene and its derivatives of titanium oxide, these membranes have been prepared for the selective removal of dyes, drugs, and other pollutants. The MXene-based membrane provides stability selectivity and a large surface area suitable for this purpose (Sutariya et al., 2023). Table 12 shows the MXene-based nanofiltration membranes for the separation of pharmaceutical chemicals.

Yue et al. (Yue and Sun, 2021) experimented with a Co^2+/Mxene-based membrane to remove tetracycline in peroxy mono-sulfate. They observed that the drug was removed more than 97 % within 5 min. In the presence of peroxy monosulfate, a higher flux of 52 % was observed compared to 22 % with peroxy monosulfate. They also concluded that sulfate and hydroxyl radicals were actively involved in the degradation. Zhang et al. (Liang et al., 2023) presented the solution to the swelling problem in MXene-based membranes by incorporating carboxylated cellulose nanofibers into the Mxene titanium oxide membrane. They discovered that incorporating these nanofibers greatly reduced the swelling of the membrane. They selected azithromycin to check the membrane application. They found that more than 99 % of azithromycin was filtered, and a very high water permeance of 26 Lm^-2h^-1bar⁻¹ was obtained.

Moreover, Wang et al. (Wang et al., 2023) proposed a greener method for synthesizing fluorine-free titanium carbide MXene. In this study, the degradation impacts of Doxycycline and Oxytetracycline reached about 90 %. Sun et al. (Sun et al., 2022) prepared a hybrid organic–inorganic MXene-pillarene-based membrane. They achieved 88 % rejection for ampicillin and 95 % rejection for bacitracin. Li et al. (Li et al., 2020) prepared a laminated titanium carbide membrane with regular slit-shaped channels. They filtered not only water-based drugs but also organic solvent-based drugs. Devaraj et al. (Devaraj et al., 2022) prepared alginate/MXene/CoFe₂O₄-based Composite material to enhance the adsorbing properties of MXenes. They observed that this composite material had highly improved absorption rates of 359.76 percent and 371 % respectively, for ciprofloxacin and copper ions; they also observed that using a rotating magnetic field can greatly enhance the adsorbing rate and performance. Kim et al. (Kim et al., 2021) studied the effects of sonication on the adsorption of selected pharmaceuticals added different pH by MXene deselected verapamil carbamazepine, ibuprofen, and diclofenac and used sonication to enhance adsorption by MXene. They observed that increasing the sonication frequency can increase the absorption capacity of MXene material because sonication costs the even distribution dispersion of MXene. Kim et al. (Kim et al., 2022) studied the effects of Ultrasonication on single and multi-layered MXenes. They also analyzed how Ultrasonication affected the adsorption performance of single-layered and multi-layered MXenes off diclofenac and verapamil. They concluded that using ultrasonic treatment can greatly enhance the efficiency of multilayered and single-layer MXenes. Park et al. (Park et al., 2023) prepared cobalt doped/ZnTiO₃ (ZTO)/TiC₂ Mxene. They checked the adsorption of tetracycline, a persistent pharmaceutical pollutant, and analyzed the effects of pH, background ions, and temperature; they performed different types of isotherms to study the adsorption of tetracycline by the prepared material. They found that prepared material had greater adsorption capacity than activated carbon and simple MXene. Sukidpaneenid et al. (Sukidpaneenid et al., 2023) observed that incorporating sodium chloride in MXene titanium composite material can greatly enhance the adsorption capacity and photocatalysis activity of the membrane. They found that incorporating sodium chloride greatly increased the absorption of norfloxacin by MXene titanium oxide composites.

5.1 Heavy metal ions

Up to date, various 2-D nanomaterials such as graphene, metal oxides, MoS₂, layered double hydroxides, transition metal dichalcogenides (TMDs), boron nitride (BN), silicenes and germanene have been exploited as an electrochemical sensor for the detection of heavy metal ions from environmental samples (Su et al., 2018). However, these 2-D materials have some drawbacks, including low electrical conductivity, high hydrophilicity, and difficult surface functionalization. Conversely, MXene-based electrochemical sensors have excellent properties such as ease of functionalization, high electrical conductivity, high hydrophilicity, good ion intercalation behavior, and feasibility for large-scale fabrication (Rizwan et al., 2022). Therefore, these are perfect for developing high-performance electrochemical sensors to determine heavy metal ions from environmental samples. Table 13 summarizes MXene-based electrochemical sensors for detecting heavy metals and phenolic-based pollutant species contaminants.

Zhu et al. (Zhu et al., 2017) have prepared a 2-dimensional accordion-like alk-Ti₃C₂ composite material through acid etching and alkaline intercalation treatment. The prepared alk-Ti₃C₂ composite was employed on the surface of a glassy carbon electrode and used for the determination of different heavy metal ions such as Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ using square wave anodic stripping voltammetry (SWAV) mode of the electrochemical workstation. The alk-Ti₃C₂/GCE modified electrode showed outstanding electrochemical ability compared to the Ti₃C₂/GCE modified electrode for determining various heavy metal ions. The proposed Ti₃C₂/GCE sensor exhibited tremendous selectivity and sensitivity towards the target analytes with the linear dynamic range of 1–1.5 µM, and the limit of detection for Cd²⁺, Pb²⁺, Cu^2+, and Hg²⁺ was found to be as 0.098, 0.041, 0.032 and 0.130 µM respectively. Also, the proposed alk-Ti₃C₂/GCE sensor exhibits brilliant selectivity in the presence of different interferant species.

Chen et al. (Chen et al., 2023) have synthesized three-dimensional melamine-doped graphene oxide/MXene composite aerogel (3D MGMA) by using the self-assembled method and applied it as a modified sensor for the detection of multiple heavy metal ions like Zn²⁺, Cd^2+, and Pb²⁺ from the environmental samples. Moreover, combining melamine 3-D structure melamine with a 2-D network like MXene and graphene oxide enhanced the surface area. It improved the electrical conductivity and uptake of the excess heavy metal ions from the environmental samples. Also, the synthesized 3D MGMA/SPCE modified electrode exhibited an outstanding linear dynamic range from 3-900 µg/L with detection limits of 0.48, 0.45, and 0.29 µg/L simultaneously from the environmental samples.

Wen et al. (Wen et al., 2022) fabricated a composite based on the MXene aerogel-CuO/carbon cloth (MXA-CuO/CC) for the sensitive electrochemical determination of Cd²⁺ and Pb²⁺ ions. The fabricated MXA-CuO/CC Modified electrode showed brilliant sensing properties towards target analytes because the oxygen vacancies of CuO have a strong capability for target analytes that promoted the adsorption of heavy metal ions, i.e., Cd²⁺ and Pb²⁺ on the surface of modified electrode. Moreover, the differential pulse anodic stripping voltammetry (DPASV) was exploited for the detection of Cd²⁺ and Pb²⁺ ions with a wide linear range from 4-800 µg/L and 4–1200 µg/L simultaneously with a limit of detections of 0.3 µg/L and 0.2 µg/L respectively. Also, the fabricated MXA-CuO/CC has exceptional anti-interference, good stability, and reproductivity properties.

Xia et al. (Liu et al., 2022) have engineered a novel MXenes nanoribbons/carbon nanotube (Ti₃C₂TxR/CNT) heterostructure composite and employed on the glassy carbon electrode surface as a sensing platform for detecting Hg²⁺ ion. The modified Ti₃C₂TxR/CNT/GCE showed tremendous selectivity and sensitivity towards Hg²⁺ ion with a good wide linearity range from 0.01-7.0 µM with a limit of detection 5.2 nM from real water samples in the presence of different foreign interferants. However, this improvement in the sensitivity and limit of detection is due to the high surface area and electrical conductivity of the MXenes that allow more adsorption sites for the target analytes. Similarly, Zhang et al. (Zhao et al., 2022) prepared a novel 2-dimensions nitrogen-doped carbon-coated Ti₃C₂-MXene heterostructure (Ti₃C₂@N-C) based electrochemical platform for the detection of Cd²⁺ and Pb²⁺ ions from seawater and tap water by using the square wave anodic stripping voltammetry (SWASV) mode. The developed Ti₃C₂@N-C/GCE demonstrated excellent electrochemical sensing capacity for the Cd²⁺ and Pb²⁺ ions with a wide linear dynamic range of 0.1–4.00 and 0.05–2.00 µM. The limit of detection was found to be 2.55 and 1.10 nM, respectively. The developed sensor showed excellent selectivity for the Cd²⁺ and Pb²⁺ ions in the existence of various metal ions and molecules.

Cheng et al. (Cheng and Yang, 2020) synthesized PANI-Ti₃C₂ composite for the electrochemical monitoring of Hg²⁺ ions from the natural environment. The synthesized PANI-Ti₃C₂ composite was employed on the surface of a glass carbon electrode for modification purposes and improved its properties. The developed sensor PANI-Ti₃C₂/GCE showed admirable electrochemical sensing towards Hg²⁺ ions with a wide range linear range between 0.1–20 µg/L with a limit of detection estimated 0.017 µg/L and the proposed senor also employed for the determination Hg²⁺ ions from tap water.

Rasheed et al. (Rasheed et al., 2022) fabricated the novel Nb₄C₃Tx (MXene)/Au/DNA composite. They fabricated and used it as an electrochemical sensor for the detection of Pb²⁺ ions from the aqueous system. The Au/DNA decorated sensor showed enhanced sensing ability toward the Pb(II) ions. For the Pb²⁺ sample detection, the proposed sensor displayed a linear response from 10 nM to 5 µM with a limit of detection of 4 nM in the presence of different interferant species. This study provides new ideas for developing stable and reliable electrochemical sensors for monitoring Pb(II) ions in environmental samples. He et al. (He et al., 2020) have reported an electrochemical sensor for determining Cd²⁺ and Pb²⁺ ions. The Bismuth/MXene (BiNPs/Ti₃C₂Tx) Nanocomposite showed tremendous electrochemical performance using SWASV mode, with a limit of detection found at 10.8 and 12. 4 nM.

5.2 Phenolic compounds

Synthetic phenolic compounds contain one or more hydroxyl groups than benzene. They may include other groups, which are also created artificially through chemical synthesis. Therefore, detecting these pollutants from the wastewater samples is very important. Huang et al. (Huang et al., 2021) suggested a straightforward self-assembled technique to create a novel heterostructure (MXene/ZIF) made of bimetallic Zn, Co embedded N-doped carbon (Zn-Co-NC) nanocages and derived from Zn-Co-ZIFs and Nb₂CTx. When detecting 4-nitrophenols electrochemically, modified Nb₂CTx/Zn-Co-NC hybrid electrodes were used. (4-NP). According to SEM pictures, Zn-Co-NC can be successfully inserted to prevent Nb₂CTx from restacking. The effective synthesis of a bimetallic Zn, Co co-embedded N-doped carbon nanocage was shown by EDS and XPS. The wide linear range that could be obtained in ideal circumstances ranged from 1 M to 500 M. The high sensitivity of 4.65 A M1 cm⁻² and low detection limit of 0.070 M was achieved. The Nb₂CTx/Zn-Co-NC sensor also possesses exceptional selectivity and durability.

Rasheed et al. (Rasheed et al., 2021) created a nanocomposite of platinum nanoparticles and Ti₃C₂Tx (Pt@Ti₃C₂Tx). They did this by using delaminated Ti₃C₂Tx nanosheets, which served as a reducing agent and a conductive scaffold, to self-reduce Pt salt to Pt nanoparticles. It was discovered that 10 %Pt@Ti₃C₂Tx had the greatest electrochemical activity in the anodic potential window after they characterized the electrochemical activity of Pt@Ti₃C₂Tx nanocomposites with varying Pt loading. Using this nanocomposite, they later developed an electrochemical sensor for detecting Bisphenol A (BPA), a prevalent environmental pollutant. Under ideal circumstances, the oxidation peak of BPA had a detection limit of 32 nM. It was proportionate to the analyte concentration from 50 nM to 5 M. The sensor was effectively tested using samples of fresh milk and drinking water.

Wang et al. (Wang et al., 2022) developed a sensor for p-Nitro phenol (p-NP), a significant environmental pollutant, which was quickly detected using a highly sensitive electrochemical sensor. The Ti₃C₂T_x MXene/graphene composite that served as the sensor's foundation was made using a minimally intensive layer delamination (MILD) technique and a self-assembly procedure. SEM, XRD, AFM, and Raman spectroscopy were all used to characterize the final product. The D-Ti₃C₂Tx/GR/GCE sensor was made by modifying a glassy carbon electrode (GCE) with the alloy. Chronocoulometry, electrochemical impedance spectroscopy, and cyclic voltammetry were all used to examine the sensor's electrochemical behavior. Due to the abundance of active sites, quick electron transfer, and good electro-catalytic performance of the D-Ti₃C₂Tx/GR composite, the sensor demonstrated a strong electrochemical response to the p-NP reduction reaction.

Liao et al. (Liao et al., 2022) used in-situ nucleation and conversion of ZIF-8 on 2D hierarchical Ti₃C₂ MXene nanosheets to effectively synthesize a novel composite, N-Ti3C2/PC. Adding ZIF-8 greatly enhanced the composite's electrochemical properties and stopped the restacking of Ti₃C₂ nanosheets. The construction of an electrochemical sensor for the combined detection of 4-aminophenol (4-AP) and acetaminophen using the N-Ti₃C₂/PC composite followed. (ACOP). Because N-Ti₃C₂ and PC work together to speed up electron transfer on the electrode surface, this sensor showed outstanding electrocatalytic activity for 4-AP and ACOP.

Sanko et al. (Sanko et al., 2022) created an electrochemical sensor using a Mo₂Ti₂AlC₃/MWCNT (multi-walled carbon nanotube) nanocomposite. The Mo₂Ti₂AlC₃ was used to develop the MAX phase of the sensor, and MWCNT was used to increase conductivity and sensitivity. The sensor's electrochemical capabilities were assessed using differential pulse voltammetry (DPV) and cyclic voltammetry. The outcomes demonstrated that the Mo₂Ti₂AlC₃/MWCNT nanocomposite demonstrated a single, diffusion-controlled, irreversible oxidation process against BPA. The sensor also had a linear working range of 0.01–8.50 M determined from DPV, a LOD of 2.7 nM, and a LOQ of 8.91 nM.

Rajendran et al. (Rajendran et al., 2022) the objective of this study was to create an electrochemical sensor for the detection of BPA, used a top-down approach to create a 2D mixed graphene/Ti₃C₂Tx nanocomposite (Gr/MXene) to accomplish this. XRD and Raman spectroscopy analysis were used to corroborate the successful formation of Gr sheets with MXene, and HR-SEM was used to confirm the formation of MXene and Gr/MXene nanocomposite. The researchers then used the Gr/MXene nanocomposite-modified GCE for BPA oxidation in 100 mM phosphate buffer solution to create an electrochemical BPA monitor in (PBS). The Gr/MXene/GCE showed linear BPA detection limits of 4.08 nM and 0.35 mM by differential pulse voltammetry (DPV) and a linear range of detection from 10 to 180 nM and 1 to 10 M BPA. Additionally, the sensor demonstrated good repeatability, selectivity, stability, and consistency regarding BPA detection. The researchers effectively used the Gr/MXene modified sensor to detect BPA in contemporary plastic products, with a recovery rate ranging from 99.2 % to 104.5 %. Overall, this study offers an electrochemical sensor that has the potential to be sensitive and specific for the detection of BPA, which could have a big effect on food and environmental safety.

Chandran et al. (Chandran et al., 2021), in their study, a glassy carbon electrode was immobilized with laccase (Lac) on Au/MXene to create a sensing electrode. XRD, SEM, and energy-dispersive X-ray spectroscopy were used to characterize the synthesized Au/MXene substance. The adsorbed enzymes could still function as biological electrocatalysts because the Au nanoparticles enabled the transfer of electrons between the Au surface and the T1 copper site of the electro-active Laccase. With a comparatively high sensitivity of 0.05 mA/mM and a detection limit of 0.05 mM, the Lac/Au/MXene/GCE electrode was used to study the electrochemical oxidation of catechol. This electrode showed a linear amperometric response in the 0.05 to 0.15 mM concentration range. Moreover, the biosensor electrode demonstrated outstanding repeatability, reproducibility, and durability.

Huang et al. (Huang et al., 2020) modified functional groups on the surface of Ti₃C₂ by alkalization treatment to create new electrode materials. Unfortunately, practical applications face major difficulties when trying to stack Ti₃C₂ sheets again. To get around this restriction, an innovative self-assembling technique was created to build a heterostructure of MOF and Ti₃C₂. Alk- Ti₃C₂ was combined with nitrogen-doped porous carbon (N-PC) produced from MOF-5-NH₂ to generate the structure known as alk- Ti₃C₂/N-PC. The latter was created by treating Ti₃C₂ with acid etching and alkaline intercalation. This method successfully stopped the Ti₃C₂ sheets from stacking again, enabling the detection of benzenediol via hydrogen-bond contact. The abundance of OH functional groups on alk- Ti₃C₂ and the many CeN bonds on N-PC cause this interaction. Using the benzenediol/benzoquinone redox reactions to detect hydroquinone (HQ) and catechol (CT), the alk- Ti₃C₂/N-PC electrode had good conductivity and surface area. The electrode demonstrated excellent performance with low detection limits of 4.8 nM (S/N = 3) for HQ and CT, respectively, and a broad linear detection range of 0.5–150 M under ideal circumstances. The electrode showed outstanding reproducibility and durability as well. Moreover, the alk- Ti₃C₂/N-PC electrochemical sensor was used to detect HQ and CT in industrial wastewater, with acceptable recoveries, to evaluate the viability of the electrode for environmental analysis. In conclusion, decorating alk- Ti₃C₂ with N-PC is a novel technique for developing different alk- Ti₃C₂-based composites to identify phenolic isomers and examine ecological materials.

Ranjith et al. (Ranjith et al., 2022) created a composite electrochemical sensor with a few-layered exfoliated 2D MXene and 1D MnMoO₄ nanofibers. The 1D MnMoO₄ and 2D MXene nanoarchitectures were created to improve electrocatalytic activity and synergistic signal enhancement for the oxidation of HQ and CC. The synthesized MnMoO₄-MXene-GCE sensor displayed oxidation potentials of 0.102 V and 0.203 V for HQ and CC, respectively. It also had a distinct and simultaneous sensing range of 0.101 V and a powerful anodic peak current. With low detection limits of 0.26 nM and 0.30 nM, respectively, the 1D-2D hybridized MnMoO₄-MXene-GCE developed sensor shows a broad linear response from 5 nM to 65 nM for both HQ and CC.

Additionally, the sensor showed significant stability. The 1D-2D MnMoO₄-MXene nanocomposite-based biosensor successfully detected HQ and CC in hazardous water pollutants. Recovery values were obtained using the differential pulse voltammetric method. In conclusion, the hybrid electrochemical sensor created in this study demonstrates promise for real-world analysis and dual HQ and CC detection in wastewater.

Yu et al. (Yu et al., 2023) focused on creating electrode materials that can perform double duty as pollutant monitors and supercapacitors. This research uses the two-step method to successfully synthesize the novel electrode material V2CTx@NiCoMn-OH. Using ZIF-67 as a framework and basic anion exchange, a 3D hollow structure of NiCoMn-OH was first created. The NiCoMn-OH structure was combined with the 2D layered V2CTx MXene in the subsequent phase. The self-accumulation of the MXene nanosheets was successfully constrained by intercalating the NiCoMn-OH structure with the V₂CTx MXene, and a 3D cross-linked hollow structure was created. This structure increased the conductivity of NiCoMn-OH, revealed more active sites of V₂CTx@NiCoMn-OH, and widened the ion transport channel.

5.3 Pesticides

Due to its sensitivity, selectivity, simplicity, quick response, and cost-effectiveness, electrochemical sensing has emerged as a viable method for detecting pesticides. The foundation of this technique is the measurement of the electrical signals produced by the contact of the pesticide molecules with the electrode surface (Wang et al., 2020). It is possible to detect and measure pesticide residues in various matrices, including water, soil, and food products, using the type and magnitude of these signals. The choice of electrode material is a crucial factor in electrochemical sensing because it greatly impacts how well the sensor works. Metals, metal oxides, carbon-based materials, and conducting polymers are just a few of the materials investigated for constructing electrochemical instruments for pesticide detection. Each of these substances demonstrates unique electrochemical characteristics, which impact the sensor's sensitivity, selectivity, and durability (Wang et al., 2020). Because of their exceptional transmission and biocompatibility, metal electrodes made of gold, silver, and platinum have been used extensively. Their high price and fouling propensity might constrain their practical uses. Metal oxides, like titanium dioxide, zinc oxide, and indium tin oxide, have benefits like improved electron transfer, excellent chemical stability, and adjustable electrocatalytic properties (Umapathi et al., 2022).

The development of electrochemical sensors for pesticide detection using various electrode materials and sensing techniques is the subject of eight recent studies we address in this review. In research (Zhong et al., 2023), the authors created a heterostructure Ti₃C₂Tx-TiO₂ composite electrode to detect thiabendazole (TBZ). The sensor obtained a linear range from 0.3 to 100.0 nM and a limit of detection (LOD) of 0.1 nM by tracking the anodic stripping peak signal change of media Cu²⁺. The sensor demonstrated outstanding anti-interference, repeatability, stability, and applicability with fruit and water samples. Another study showed an electrochemical ratio metric sensor for detecting carbendazim (CBZ) made of a composite of MXene@Ag nanoclusters and amino-functionalized multi-walled carbon nanotubes. The sensor had a LOD of 0.1 nM and a linear range of 0.3 nM to 10 M. It showed high selectivity, excellent reproducibility, secular stability, and acceptable applicability in vegetable samples.

Tu et al. (Tu et al., 2020) used an electrochemical sensing platform for CBZ detection made of nanoarchitecture of MXene/carbon nanohorns/-cyclodextrins-Metal-organic frameworks (MXene/CNHs/-CD-MOFs). The sensor showed a low LOD of 1.0 nM and a broad linear range of 3.0 nM to 10.0 nM. A tomato sample analysis also revealed high selectivity, reproducibility, long-term stability, and satisfactory applicability (Tu et al., 2020).

Zhong et al. (Zhong et al., 2022) suggested a hierarchical nano-CuO decorated MWCNTs-COOH/MXene composite-based highly sensitive and selective electrochemical sensor for real samples' benomyl (BN) detection. The sensor had a LOD of 3.0 nM and a linear range of 10.0 nM to 10.0 M. It also demonstrated successful application in apple samples, excellent long-term stability, and repeatability. In another study, the researchers developed an acetylcholinesterase (AChE) biosensor based on a SnO₂/Nb₂CTx MXene nanocomposite for pesticide detection. With a LOD of 5.1 X10^-14 M, the sensor showed a linear detection range for chlorpyrifos of 5.1 X 10^-14 − 5.1 X 10^-7 M. Additionally, another study described the creation of an ultrasensitive AChE-based electrochemical biosensor and the synthesis of MXene/AuPt nanocomposite for chlorpyrifos detection (Ding et al., 2023). The sensor's linear range was 10⁸ and 103 mg mL1, and its LOD was 1.55 pg mL¹. In another research, methiocarb, diethofencarb, and carbamate pesticides were quickly electroanalytical screened. The authors used colloidal single/few-layer Ti₃C₂Tx MXene flakes prepared via the minimally intensive layer delamination (MILD) method. The sensor showed acceptable recoveries in actual samples and detection limits of 0.19 g mL-1 for methiocarb and 0.46 g mL-1 for diethofencarb, respectively (Sinha et al., 2021).

Song et al. (Song et al., 2019) used MnO₂/Mn₃O₄ and Ti₃C₂ MXene/Au NP composites derived from MOFs to create a novel electrochemical sensing platform for detecting organophosphorus pesticides (OPs). Combining MXene/Au NPs with the 3D MnO₂/Mn₃O₄ hierarchical microcuboids formed from Mn-MOF produced excellent electrochemical performance, good environmental biocompatibility, and a synergistic signal amplification effect. The AChE-Chit/MXene/Au NPs/MnO₂/Mn₃O₄/GCE sensing platform was able to identify methamidophos in a concentration range of 1012–106 M with a low limit of detection (1.34–––10¹³ M) and good linearity (R² = 0.995). With excellent recoveries (95.2 %–101.3 %), the biosensor effectively detected methamidophos in real samples.

6.1 CO₂ reduction

CO₂ is a gas analyte of interest, and their capture and reduction using MXene as an adsorbent/catalyst are explored. CO₂ reduction over MXene-based composite surfaces is extensively investigated due to their brilliant photocatalytic efficiencies (Zhang et al., 2017). Cao et al. (Cao et al., 2018) discovered that the reduction efficiency of CO₂ (0.069 mmol/g) into methane and methanol over Ti₃C₂/Bi₂WO₆ nanosheet was 4.6 times that of pristine Bi₂WO₆. Parui et al. (Parui et al., 2022) described a simulation study (DFT calculations and AIMD-ab initio molecular dynamics) to describe the single-step selective reduction of CO₂ to HCOO^– and HCOOH using Ti₂C(OH)₂ MXene material. To make the material reactive, it transfers electrons to carbon dioxide (CO₂), and then CO₂ removes protons off the surface of titanium dioxide (Ti₂C(OH)₂) to produce these products. The formation of formate or formic acid is determined by how carbon dioxide is connected to the surface. It is possible for moisture to readily repair any surface imperfections that may have been generated throughout the procedure, allowing the reaction to continue. Ti₂C(OH)₂ is advantageous because it can donate electrons and protons, enabling it to convert carbon dioxide into compounds of great value effectively. Li et al. (Li et al., 2021) synthesized a novel mesoporous g-C₃N₄/Ti₃C₂Tx MXene photocatalyst and reported a higher yield of methane of about 2.117 µmol $g^{-1}{h}^{-1}$ on CO₂ reduction due to the contributing factor of Ti₃C₂Tx that allowed greater separation of holes and electrons. Furthermore, the material possesses a structure characterized by numerous minuscule pores, facilitating rapid electron transport and charge separation (electrons and holes), hence enhancing efficiency. MCT generates methane at a rate 2.4 times greater than a comparable material without Ti₃C₂Tx. The extensive surface area and many flaws in MCT offer ample sites for CO₂ adsorption, whereas Ti₃C₂Tx functions as a “capacitor,” facilitating efficient electron transport. This combination diminishes electron recombination, enhancing the efficacy of the CO₂ reduction reaction (CO₂RR). The research indicates that materials such as Ti₃C₂Tx may enhance the efficacy of photocatalysts.

6.2 Reduction/removal of other gases

Ngoc et al. (Wang et al., 2019) investigated Sc₂CF₂ MXene monolayer adsorption characteristics for common gaseous analytes using first-principles calculations. They chose Sc₂CF₂ MXene monolayer as a catalyst due to its small bandgap of 1.023 eV, high tunability under electric fields, doping and strain, and higher thermoelectric conductivities. They discovered how these gasses adhere to the material, found the amount of energy that is involved, and discovered how the electrical properties of the material change. The majority of gases have a weak attachment to Sc₂CF₂. Still, when molecules such as oxygen, nitrogen, and nitrogen dioxide are adsorbed onto the material, they transform it from a non-magnetic to a magnetic state. When nitrogen oxide is linked to the material, it exhibits the behavior of a semimetal, yet its conductivity remains the same for the majority of other gases. According to this, Sc₂CF₂ has the potential to be an effective material for the detection of NO gas. Exploiting Sc₂CF₂ MXene monolayer catalyst, Cheng et al. (Cheng et al., 2022) explored its application as a NO₂ gas sensor. They performed simulation activity to determine the adsorption of 11 gas analytes, such as NH₃, NO₂, SO₂, CO₂, H₂O, H₂S, CO, NO, O₂, N_2, and H₂. They observed a larger charge transfer quantity between the NO₂ analyte and Sc₂CF₂ due to the placement of LUMO (NO₂) between the valence band gap of Sc₂CF₂. This observation substantiated that Sc₂CF₂ can selectively detect NO₂ and thus act as a potential gas sensor. Wang et al. evaluated the selectivity of monolayer- Hf₂CO₂ for ammonia gas using theoretical first-principle calculations. They observed that SO₂, water, and CO₂ enhance ammonia adsorption on the Hf₂CO₂ layer (Wang et al., 2019). Recently, Li et al. (Li et al., 2023) utilized the MXene-Hf₂CO₂ monolayer for SO₂ capture through the adsorption of water molecules. Both the research works are based on first-principles calculations and substantiate that the reabsorption of water molecules before major analytes on MXene layers is due to hydrogen bonding.

6.3 Separation of gases analytes

MXene-based molecular sieving membranes are a lucrative technology with far-reaching implications for separating gaseous pollutants. With facile operations, lower energy consumption, and cost efficiency, MXene-based gas membrane sieves are gaining popularity. They are favoured over polymeric or carbon-based nanomembranes. Ding et al. (Ding et al., 2018) developed exfoliated MXenes nanosheets to selectively obtain laminar membranes to separate gaseous analytes. They demonstrated molecular sieving through MXenes for hydrogen and carbon dioxide gases. Fig. 19 shows the gas separation performance of the MXene membranes.

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Fig. 19

Gas separation performance of the MXene membranes, MXene molecular sieving membranes for highly efficient gas separation (Ding et al., 2023).

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6.4 Gas sensing applications

Due to high electrical conductivity, tunable surface properties, and diverse stacking patterns, MXenes have superseded carbon-based nanomaterials in gas sensing applications. Seminal work on the MXene-gas sensor was performed by Yu et al. (Yu et al., 2015), who described the behavior of NH₃, H₂, O₂, CH₄, CO, CO₂, N₂, and NO₂ on Ti₂CO₂ − MXene. They observed that ammonia was strongly chemisorbed amongst all gases, and the adsorption site was at the nitrogen atom sitting above the titanium atom of Ti₂CO₂. Their studies revealed adsorption energies and charge transfer of – 0.37 eV and 0.174e on ammonia gas in their novel MXene material. Xiao et al. (Xiao et al., 2016) further investigated the reusability of MXenes for ammonia capture and sensing, particularly focused on interactions between gas molecules and O-functionalized M₂CO₂ MXene where M was either Ti, Zr, Sc, and Hf. These reports were based on computational study and needed a practical demonstration to substantiate the data on MXene as potential gas sensors.

Also, MXenes are largely hydrophilic; hence, their sensing capability for non-polar gaseous analytes presents a serious challenge. Of all the gases, hydrogen and methane are nonpolar gas analytes. Various investigations pivoted towards experimental evidence to demonstrate that MXenes are ideal gas separation and sensing materials. Lee et al. (Lee et al., 2019) demonstrated a 2D vanadium carbide MXene gas sensor, illustrating LoD for hydrogen and methane analytes at 2 ppm and 25 ppm at room temperature, respectively. LoD values for V₂CTx-MXene were significantly lower than other nanomaterials and functionalized metal nanoparticles. Hence, these materials are also tunable to sense nonpolar gases, overcoming a major limitation to MXene applications.