Development of MXene for removal of lead (Pb⁺) ions from wastewater Open Access

Heavy metals have directly affected environmental safety and human health through water contamination, which has become a significant issue for people worldwide in recent years. Since lead (Pb), one of the dangerous heavy metals is poisonous and non-biodegradable, removal from drinking water and wastewater prevents food poisoning and kidney and nervous system ailments significantly look out. Research on the recovery of lead ions from an aqueous solution is required to lessen the likelihood and level of hazard that lead ions generate (Wang et al. 2020). Heavy metals other than lead have been removed from water and wastewater using a variety of processes, including membrane separation, adsorption and electrochemical deposition. Adsorption is a particularly appealing process for removing heavy metal ions because of how affordable and straightforward it is to use (Shahzad et al. 2017). The adsorbent materials are crucial when using adsorption to remove contaminants such as heavy metals released by businesses. Researchers have worked tirelessly to create lead ion adsorbent materials that are better and more widely accessible. Two-dimensional (2D) materials have been extensively investigated in recent years. Many 2D materials have characteristics like surface functional groups and specific surface areas, showing their significant potential for heavy metal ion adsorption and water treatment (Wang et al. 2020).

Due to their distinctive physical and chemical characteristics, 2D transition metal carbides (MXenes), a new class of two-dimensional materials, have attracted much interest. Selective etching of the MAX phase precursor (Ti₃AlC₂ and Ti₂AlC) was carried out to obtain a hexagonal layered structure MXenes (Ti₃C₂T_x and Ti₂CT_x). MXenes are more effective at removing organic dyes, heavy metal ions and other toxic pollutants from the aqueous solution because they are widely available and have several active sites. Despite the fact that MXenes are rapidly oxidized in water, contact with water is a necessary part of the adsorption process, which reduces their active sites for adsorption and constrains their adsorption capabilities (Yang et al. 2014). Additionally, restacking is an insurmountable difficulty in preparing 2D materials such as graphene and MXenes. Since the MXenes restacking wastes enormous active sites, steps must be taken to resolve this issue. In this work, a safe synthesis of the MXene using fluoride salt and strong acid was carried out to effectively remove the lead ions from the wastewater using adsorption (Peng et al. 2014). The parameters like the effect of time, dosage, temperature, initial concentration and pH studies were carried out to prove the effective removal. Reaction kinetics, thermodynamics and adsorption isotherms were studied to prove the adsorption mechanism of lead ions, and chemisorption and physisorption were discussed in detail (Mishra & Verma 2017).

The research limits using other fluoride salts and their etching capacity using safe synthesis routes. The etching of the A layer is more effective when pure hydrofluoric acid (HF) is used. When in situ HF is used, the etching capacity is reduced, but the method is much safer than directly using HF. The limitations can be solved by utilizing the alternative of the HCl and increasing the etching capacity to up to 98% using the safe synthesis approach. This research also limits the safe handling of the lead ions post-adsorption studies.

The research can be termed novel as the nanoadsorbent MXene used has not been effectively used in metal ion remediation from industrial wastewater. The material in its first adsorption cycle states an adsorption effectiveness of around 96%, which can still be improved to about 92% after regeneration. As per the results, one can say that the MXenes, when used as an adsorbent for treating the lead ions from industrial wastewater, can be an effective adsorbent for longer adsorption cycles, without affecting much of its surface activity. The material can be termed novel as it is easy to regenerate, reuse, stable and efficient for metal ion removal.

This research aims to study the replacement of traditional adsorbents with the new family of 2D materials, MXenes, which can potentially treat heavy metals that tend to contaminate wastewater. The study also states that the regenerability of the MXenes over longer periods and cycles also ensures the effective removal of selective metal ions (lead in this research is focused). The effect of time and dosage, which resulted in the best adsorption, has also been studied.

Titanium aluminum carbide powder (Ti₃AlC₂), having a purity of 99%, was purchased from Intelligent Materials Pvt. Ltd. Sodium fluoride (NaF) was procured from SD Fine-Chem Limited. Hydrochloric acid (HCl) (35%) was procured from Fine Chemicals. All the chemicals were used as they were procured.

The reaction (1) demonstrates the preparation of HF from fluoride salts and strong acids.

The reaction (2) demonstrates the preparation of MXene from its MAX phase by etching of the A layer which is confirmed by the X-ray diffraction (XRD) analysis. The MAX phase precursor titanium aluminum carbide (Ti₃AlC₂) was etched using the prepared in situ to eliminate the A layer (Al) from the MAX phase precursor resulting in the titanium carbide (Ti₃C₂) which was MXene. The A layer is etched using a strong etching agent, in this case, HF. For preparing the in situ HF solution, 3.35 g of sodium fluoride (NaF) was taken in the reaction vessel to which 20 mL of hydrochloric acid (HCl) was added dropwise. The solution was stirred vigorously at 40°C for 30 min for an effective reaction. A turbid white solution of HF is prepared, which is a safe method. The direct HF solution is toxic and difficult to handle. The in situ HF method is safe as compared to the direct HF method, the results of which will be displayed after the experimental work (Liu et al. 2017).

One gram of Ti₃AlC₂ MAX phase precursor was added slowly to the as-prepared in situ HF etchant to remove the A layer from the MAX phase. The solution was stirred for 24 h at 40°C. The MXene solution was centrifuged and washed with deonized (DI) water to maintain a neutral pH. Solvent washing was done to remove any impurities in the MXene powder. The sample was then vacuum dried in the vacuum drying oven at 70°C. The final MXene was confirmed after the characterization was done.

Scanning electron microscopy (SEM) was carried out to understand the surface morphology and the layered structure of the synthesized MXenes. Fourier transform infrared (FTIR) spectroscopy was used to understand the functional groups and the surface terminations on the MXenes. Wide-angle XRD analysis was carried out to understand the effective etching of A layers from the MAX phase and to define the crystallographic structure of the MXene crystals. Zeta-potential studies were carried out to understand the surface electric charge on the MXene for possible separation due to electrostatic separation.

Here, q is the removal rate (%), Q is the adsorption capacity (mg/g), C₀ is the initial concentration of the wastewater sample (mg/L), C is the final concentration of wastewater after the adsorption (mg/L) and V is the volume of the wastewater sample (Lü et al. 2007; Suzuki et al. 2007).

The surface structures and morphologies can be observed by SEM (Carl Zeiss EVO 18).

Wide-angle diffraction analysis was performed on Tabletop Rigaku MiniFlex 600-C X-Ray Diffraction equipment; scanning speed was maintained at 5° min⁻¹ having a 2θ range between 5° and 80°. XRD confirmed the crystallographic structure of the Ti₃C₂T_x MXene. The same analysis confirmed the effective etching of A layers from the MAX phase precursor (Li et al. 2019).

FTIR spectroscopy is an analytical method for determining the composition of organic, polymeric and occasionally inorganic materials. FTIR spectrum obtained from the Perkin-Elmer spectrum two model indicates the functional groups on the MXene surface responsible for the adsorption of the lead metal ions onto the surface of the MXene nanoadsorbent. Figure 3 represents the FTIR spectra of the MXene prepared using the in situ HF method.

After the adsorption studies, the FTIR spectra of the MXene did not show any specific peak shifts, which states that the MXenes can be highly stable and selective to the lead ions. It does not tend to react with the ions and form other products during the removal of the ions. FTIR shows that the MXenes are highly stable and unreactive to particles other than lead ions.

The graph shows that the MXene prepared using the in situ HF method and the sodium fluoride salts have a zeta of −22 mV, a near-stable compound. The surface of the nanoparticles has a negative charge, which can help in separating lead ions from the wastewater by electrostatic attraction. The lead ions tend to attract onto the negatively-charged surface of the MXene nanoadsorbent, which aids in the separation. The results of the adsorption isotherms confirmed the separation either by chemisorption or physisorption (Jastrzebska et al. 2017; Rozmysłowska et al. 2018). Based on the basics of the zeta-potential studies, the nanoparticles show the following stability behavior.

Table 1 explains that the MXenes are partially unstable because some particles are highly stable against the unstable particles. When mixed with the water sample, MXenes do not tend to coagulate and sediment at the bottom of the solution. The MXenes can easily interact with the water molecules and some of the partially stable atoms in the MXenes tend to attract and adhere to the lead metal ions on the active surface. As the MXenes are not excellently stable, their adsorption capacity is low. Even at moderately stable nanoparticles, if MXenes can have more than 80% adsorption, we can conclude that the MXenes are the perfect candidate for treating the lead ions in the wastewater.

Adsorption of the lead ions onto the surface of the MXene nanoadsorbent was carried out by studying the effect of time, dosage and temperature. The adsorption and removal of the metal ions were a result of these parameters. Adsorption is the surface phenomenon wherein the adsorbate (lead ions) adheres to the surface of the adsorbent (MXenes). The adsorbate adsorption occurs either by weak attraction forces, known as physical adsorption or physisorption, or by strong chemical bonds or interaction, known as chemical adsorption or chemisorption. The adsorption experiments here confirm the chemisorption of the lead ion adsorbate onto the highly surface-active MXene adsorbent. The confirmation can be done by studying the adsorption isotherms by varying the various parameters that can affect the adsorption rates and capacity (Jun et al. 2020a). Every sample's initial and final concentrations were carried out using an inductive coupled plasma–optical emission spectrophotometer (Perkin-Elmer 7300 DV).

Figure 6 shows that the removal rate increases with the increase in adsorption time. The pristine MXene removes around 87.6% of lead ions in the first 10 min. Here, the dosage of the MXene was kept constant. There may be significant amounts of unoccupied active sites on the adsorbent surface, which could account for this adsorption behavior. After that, the adsorption rate increases until an equilibrium concentration is achieved on the active surface of the MXene when the contact period is extended as the vacant sites get enough adsorbate, reducing the surface's effectiveness from adsorbing more ions. The removal rate of 97% and the adsorption capacity of 43.86 mg/g were achieved using the MXene as the nanoadsorbent. The adsorbent sites can be made effective after the regeneration of the adsorbents. Even after the regeneration, the sites might show effective adsorption performance as they have active surfaces. The MXene's regeneration until the second adsorption cycle showed effective but reduced adsorption performance (Mahar et al. 2019).

Excessive adsorbent, however, can reduce effective surface area, wasting the active sites and lowering adsorption capacity. The optimum dosage taken was 0.4 g/L. When the adsorbent dosage increases, nearly 100% of adsorption does not occur due to the possibility of aggregation of MXene adsorbent and unutilized adsorbent sites. However, when the amount of the adsorbent is increased, the active sites increase, leading to an increase in the adsorption rate, as seen in the results. But economically, it is not feasible. The surface increases as the dosage increases, resulting in more adsorption sites. Eventually, suppose we increase the dosage to 1 g. In that case, it can show an adsorption rate closer to 99%, which can hamper the economy of the process as nanomaterial synthesis can be economically not feasible. The optimum dosage of the MXene adsorbent must be noted here and chosen accordingly (Asandei et al. 2009).

From Figure 9, we can see that as the pH of the solution reaches 4–5, there is a rapid increase in the adsorption of lead ions, the reason being the acidic nature helps to separate the lead ions from the water molecules and aids in the adsorption on the sites of the MXenes. On the other hand, when alkali is added, the lead ions undergo hydrolysis to form insoluble precipitate components that are not easily adsorbed on the surface of the MXenes, reducing the performance at higher pH (Khurshid et al. 2022).

As shown in the above results, we can conclude that as the initial concentration increases, there needs to be an increase in the dosage of the MXene to maintain the removal rate. In the experiments, the MXene dosage was kept constant for every batch that needed to be studied. The MXene used was 0.1 g for both 5 ppm solution and 50 ppm solution. From the results, we can see that almost 97% removal is done with a 5-ppm initial lead concentration on the 0.1 g dose of MXene, whereas only 60% removal is possible with a 50-ppm solution with the same (0.1 g) dose of MXene. The MXene performs excellently at higher initial concentrations of lead ions in the water (Guo et al. 2015).

As industrial wastewater was used for the adsorption experiment, the effect of initial concentration studies was tough, but the concentration of the samples was varied.

The study of the optimized results is shown in Figure 11.

The adsorption of the lead metal ions was carried out from 5 to 120 min. From Figure 11, it was observed that the adsorption equilibrium was obtained at 95%, which was obtained at 90 min. The optimization for the effect of time is shown in Figure 11.

The dosage of the MXenes was varied from 0.05 to 0.5 g. The removal percentage of the metal ions increased rapidly from 89 to 96.81%, as the dosage was seen to increase. There was an increase in the removal as a result of increased dosage. As the dosage increased, the active surface responsible for the adsorption of metal ions increased. Therefore, we can see an increase in the removal percentage of the adsorbate materials (Bhat et al. 2015).

The MXene nanoadsorbent used for the adsorption of the metal ions from the wastewater was ensured to be stable for the studies by conducting the thermogravimetric analysis (TGA). The results obtained from the TGA are depicted in Figure 11 through graphical representation. The MXene behaves stable at higher temperatures and hence it is applicable for applications possessing higher temperatures. The MXene oxidizes at higher temperatures or becomes hygroscopic if not kept in the proper atmosphere. The MXene studied might have moisture content on it or hold weak bonds with the alcohols that tend to affect the surface of the MXene (Li et al. 2015).

Regaining MXene's ability to adsorb lead ions previously employed to remove impurities from a solution is known as the ‘regeneration of MXene’. It is essential to economical and environmentally friendly wastewater treatment and remediation techniques. In this experiment, the regeneration of the MXenes was carried out by desorption of the lead ions from the MXene surface using nitric acid as the agent to desorb the lead ions from the surface. The solution's pH was maintained by adding the alkali solution, in this case, NaOH.

Figure 18 shows that the regenerated MXenes using nitric acid and maintaining pH by adding NaOH show good reusability in the second adsorption cycle. In the regenerated MXene batch, the removal was seen to be around 87%.

From Figure 19, we can see that the regeneration of the MXene which was previously applied to study the effect of dose was regenerated in a similar way by using nitric acid. The results show promising adsorption efficiency for the regenerated MXene. The MXene after regeneration confirmed the removal of around 92%. As compared to the first adsorption cycle, the materials almost displayed safe adsorption rates, highest being at 0.15 g. The MXene when regenerated by aid of nitric acid and NaOH helps in effective desorption, formation of single-layered MXene and maintaining the pH of the MXene.

The number of atoms/ions/molecules that are adsorbed to the nanoadsorbent surface as a function of the heavy metal ion concentration (adsorbate) is known as the adsorption isotherm of any adsorption system. The adsorption mechanism for lead ions onto the adsorbent surface was studied using the Langmuir, Freundlich and Temkin isotherms. The adsorption process, governed by monolayers adhering to the adsorbent surface, is typically described by the Langmuir isotherm model. Freundlich isotherm states that the adsorbent surface is heterogeneous. The heterogeneous multi-site adsorption has a variable heat of adsorption depending upon the sites occupied. This isotherm also states that adsorbate forms a monomolecular layer on the surface of the adsorbent. The lead ions form an adsorptive layer on the MXene surface governing the adsorption rate and mechanism. The adsorption mechanism was concluded by studies of Langmuir, Freundlich and Temkin adsorption isotherms, out of which the results concluded the nature, rate and capacity of the adsorption (Table 2) (Chen 2015).

According to the Langmuir theory, molecules adhering to a solid surface depend on a kinetic concept wherein molecules constantly bombard the surface and match desorption or evaporation of molecules from the contact with the zero accumulating rate at the surface. The adsorption capacity of several novel nano-sorbents has been quantified and contrasted using the Langmuir isotherm model (Chen 2015). Due to the variety of surface chemistry and structural layout of solid materials, Langmuir could identify and categorize six distinct and straightforward adsorption mechanisms:

• i.

  Single-site: The simplest type of adsorption is Langmuir adsorption, in which its surface has identical primary adsorption sites that may accommodate a single adsorbed molecule.

• ii.

  Multiple elementary adsorption site types are present on the surface in multi-site Langmuir adsorption, and each site may accommodate a single adsorbed molecule. The adsorbate and adsorbent interactions are disregarded because their binding sites are separate.

• iii.

  Generalized Langmuir adsorption, where an amorphous material that is treated as a continuum might consist of an intractable number of various adsorption sites with various adsorbate affinities. Since adsorbate–adsorbent interactions are negligible, the adsorption isotherm follows the binding energy distribution of the adsorption sites.

• iv.

  Cooperative adsorption, in which numerous molecules can coexist at the same surface-bounding site. The presence of different adsorbates at the exact same adsorption site affects the energetics of future adsorption.

• v.

  Dissociative adsorption is assumed to be a process in which chemical bonding will lead to molecular dissociation and residence at the adsorption site, and it will then go through desorption, where two nearby atoms on the surface must reassociate into a diatomic molecule alongside leave the surface.

• vi.

  Because there is no cap on the number of molecules that can be adsorbed, it is assumed that each adsorption site in a multilayer adsorption process is independent and identical.

An empirical model called the Langmuir isotherm assumes that the layer of adsorbed molecules is one molecule thick (monolayer adsorption) and that the adsorption process takes place at equivalent and identical localized spots. The adsorbed molecules should not interact laterally or sterically, even at adjacent sites. According to the Langmuir isotherm model, adsorption is assumed to be uniform, with each molecule possessing a constant enthalpy and sorption activation energy. No adsorbate reincarnation should exist in the surface layer, and equivalent adsorbate affinities should be found at all sites. According to Langmuir's theoretical terms, there is a correlation between increasing distance and a swift reduction in the attraction forces between molecules. Moreover, adsorption by Langmuir isotherm indicates linear adsorption at low adsorption densities and maximum surface coverage (Rajahmundry et al. 2021).

The equilibrium between metal ions (adsorbate) and the MXene (nanoadsorbent) system, when the adsorbate is restricted to the monomolecular layer at or before a relative pressure of unity is reached, is described by the Langmuir adsorption isotherm. When ionic or covalent chemical bonds are formed between the MXene (nanoadsorbent) and the heavy metal ion (adsorbate), the Langmuir isotherm is typically appropriate for representing the chemisorption process.

The value of the R_L is the favorability or unfavorability of the isotherm. If:

• R_L > 1, unfavorable isotherm

• R_L = 1, linear isotherm

• 0 < R_L < 1, favorable isotherm

• R_L = 0, irreversible isotherm

In the interpretation of the Langmuir isotherm data, Q_max and K_L are the most important parameters that give the information on adsorption capacity and the rate of adsorption, respectively.

Because sediments are heterogeneous adsorbents with different adsorption energies at each site, and the Langmuir approach assumes a uniform adsorbent surface with similar adsorption energy at every site, there are some shortcomings in the description of contaminants adsorption onto the sediments. However, due to its simplicity of usage, adsorption researchers frequently utilize this isotherm approach and its coefficients to ascertain the distribution of pollutants between water and sediments. The complexity of adsorption behavior as well as the restrictions on the accuracy and completeness of the experimental data for adsorption serve as barriers to the development and deployment of adsorption technology (Gessner & Hasan 1987).

The Freundlich isotherm approach describes adsorption in multiple layers with interaction between adsorbed molecules and heterogeneous surface energy systems. The number of metal ions adsorbed (adsorbate) on an equilibrium concentration of MXene (adsorbent) has a power law dependence on the concentration of the solute.

The reversible yet non-ideal adsorption process is described by the Freundlich adsorption isotherm model. Unlike the model developed by Langmuir, the Freundlich model is applicable to multilayer adsorption and is not limited to monolayer formation. Adsorption heats and affinities do not have to be equally distributed across the heterogeneous surface according to this isotherm model. The formulation of the Freundlich isotherm model describes the surface's heterogeneity in addition to the exponentially varying distribution of the sites that are active and their energy. It is shown that the amount of solute to adsorbate on a given mass of a particular adsorbent was not constant at varying concentrations of the solution. The amount adsorbed in this instance is the total of the adsorption at each site. The more intense binding sites will first be occupied, and once the adsorption process is complete, the energy required for adsorption will begin to exponentially fall.

An empirical equation makes up the Freundlich isotherm model equation. It was initially achieved by presuming patch-wise topography and the heterogeneity of the layer on which the adsorption energy is distributed. The energy generated by the interaction between the adsorbent (MXene) and the adsorbate (lead metal ions) is known as adsorption energy (Skopp 2009).

The value of the 1/n defines the linearity of the equation. If:

The Temkin isotherm model excludes the extremely high and low concentrations while considering the relationship between the adsorbent (MXene) and the adsorbate (lead metal ions). This model implies that as surface coverage increases, the adsorption heat (ΔH_ads) of all molecules in the layer drops linearly instead of logarithmically as a result of temperature. The only range of intermediate concentrations for which this adsorption isotherm theory is applicable.

The Temkin isotherm approach presupposes that an equal distribution of interaction energies, to the point of maximum binding energy, characterizes adsorption and that the adsorption energy of all molecules reduces gradually with the rise in coverage of the surface of the adsorbent. The Temkin isotherm model presupposes that adsorption is characterized by a uniform distribution of binding energies, up to a maximum binding energy and that the adsorption heat of all molecules falls linearly with the increase in coverage of the adsorbent surface. According to the Temkin isotherm model, all molecules' adsorption heat will linearly decrease as their surface area is covered up (Musah et al. 2022).

In order to understand the monolayer adsorption process, the Langmuir isotherm model is employed to illustrate the adsorption of lead ions onto the MXene surface. According to the results obtained from the adsorption isotherm modeling, the Langmuir model can be best fit for understanding the adsorption mechanism of lead ions onto MXene active sites. The MXene might have a very homogeneous surface onto which the lead ions deposit on, assuming no chemical interactions between the adsorbed molecules, confirming that adsorbed lead ions do not play role in further adsorption process. Through the result, it can be clear that the lead ions form monolayers on the MXene surface. As there is an increase in the lead ion concentration, the active vacant sites tend to form monolayers, preventing the multilayers over the surface. K_L relates to the energy of adsorption. K_L in the range of 0–1 suggests stronger affinity between the lead ions and MXene, achieving higher rate of adsorption. Temperature variations with respect to time are not considered during adsorption.

The chemisorption mechanism is confirmed over the surface of the MXene. Chemical bonds are formed during the sorption of lead ions onto MXene sites, that entail the exchange or transfer of ions within the adsorbate and MXene. The surface of the MXenes is equipped by the lead ions till the equilibria are reached and fill the active sites for adsorption. Furthermore, the MXenes are washed with acid to remove the ions and regenerate the MXenes. Based on the isotherm values obtained, the Langmuir adsorption isotherm model can be the best fit to explain the adsorption mechanism of lead ions onto the MXene active surface. From the values of the Langmuir isotherm model, the equilibrium adsorption capacity of lead ions onto the MXene surface is 43.86 mg/g, the K_L value is favorable and ensure good adsorption binding between lead ions and MXene (Sheth et al. 2022). The R² is the regression data that help to understand the MXene nanoadsorbent's reproducibility. The R² values closer to 1 are named to be the best adsorption isotherm fit models. The R² value using the Temkin model was obtained around 0.98. Which suggested excellent reproducibility and regenerability of MXene after the adsorption experiment. Lead ions absorbed by the MXenes can be reversible adsorption–desorption process.

The research focuses on the treatment of the lead ions that are present in the wastewater and can be considered to be major threat to fauna and fauna. A new family of 2D material MXene, having more advanced properties than the graphene family, was applied for remediation by the adsorption method. The MXene was prepared by the in situ HF synthesis method, keeping safe synthesis as a major aspect. The synthesized material was characterized and proved to be fit as a nanoadsorbent for remediation of water. The parametric studies involving the effect of time, dosage, temperature, pH and initial concentration were carried out in systematic manner to understand the possible adsorption capacity, followed by adsorption isotherm modeling and thermodynamic studies in order to confirm the adsorption mechanism. The adsorption isotherm modeling confirms the Langmuir behavior fit, which was later confirmed by the thermodynamics stating that the adsorption was spontaneous and dependent on the entropy. The material was then regenerated in order to understand the regenerability and reusability for further adsorption cycles. The material stands to be stable at higher temperatures, which aids in adsorption at higher temperature. The material was easily regenerated by using the regenerating agent, nitric acid, which serves dual purpose of breaking down the inorganic lead ions from the adsorbent surface and delaminating the MXene and increasing its surface activity. Though in this work, after second regeneration cycles, the material is under the permissible limits assigned by World Health Organization, so further analysis was tough. The material proved to be around 90% reusable for further adsorption cycles. The safe utilization of the removed lead components is still a limitation. The second limitation which needs to be studied in further research is the disposal or usability of the A layer that was etched and washed away during the synthesis of MXene.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.