Removal of Malachite Green Dye from Water Using MXene (Ti₃C₂) Nanosheets

Abstract

In the present study, new emerging 2D Mxene nanosheets (MXNSs) were synthesized from MAX phase powders of Ti₃AlC₂ and then characterized using a scanning electron microscope (SEM) and X-ray diffraction (XRD) to explore the chemical and physical properties of the prepared MXNS. The characterization of the synthesized MXNS indicated the formation of exfoliated 2D MXene nanosheets (Ti₃C₂) as a result of the HF treatment of the MAX phase, which was confirmed by XRD measurements, as the characteristic peaks of 2D MXene nanosheets were only observed. The synthesized MXNS was then used as a solid adsorbent for removing malachite green dye (MG) from water. The effects of different operational factors such as MXNS dose, solution temperature, time, MG concentration, solution pH, and ionic strength have also been evaluated. The adsorption results showed that the temperature of the solution, as well as its pH, significantly influenced MG removal when using MXNS. The optimum removal was obtained within 150 min, with 20 mg of MXNS at ambient temperature and a pH value of 6.0. The maximum removal capacity obtained was 4.6 mg MG per g of MXNS using 5 mg of MXNS with a removal efficacy of 46.0%, and the minimum removal capacity obtained was 2.5 mg MG per g of MXNS using 20 mg of MXNS with a removal efficacy of 99.1%. Finally, the results displayed that the MXNS solid adsorbent was able to absorb a high percentage of MG and maintained reasonable efficiency for four consecutive cycles, indicating that MXNS could be a promising adsorbent in wastewater remediation and environmental sustainability.

1. Introduction

Water is an essential element of life and is a very vital resource for all living things, as it supports the physiological activities of any biological cell [1]. Although more than 70% of the Earth’s crust is covered by this precious resource, unfortunately only a limited portion is portable [2]. This is due to growing industrialization leading to the discharge of different pollutants into water [3]. In addition, natural phenomena such as storms, volcanoes, earthquakes, and algae blooms also result in major changes in the ecological status and quality of water [1]. Water pollution is a serious problem that results in the deaths of over 14,000 people every day [1,4]. Moreover, the United Nations (UN) has projected that between 1/2 and 2/3 of the world’s population will experience scarcity of fresh water in 2025 [5]. The problem of pollution by hazardous organic pollutants such as dyes has become a matter of serious concern worldwide. Malachite green (MG) dye is one of the commonly detected hazardous organic dyes and is one of the highly toxic synthetic organic dyes that are used widely in industry, agriculture, food, and medical disinfectants, but its adverse effects such as carcinogenesis, mutagenesis, chromosomal fractures, respiratory toxicity, and teratogenicity have been observed [6,7].

Over the past few years, various technologies such as photocatalytic degradation [8,9], sonocatalytic degradation [10], biodegradation [11], reductive decolorization [12], ultrafiltration membranes [13], and adsorption [14] have been used for water treatment. The remediation of MG dye-polluted water by the adsorption process has received significant attention from environmentalists and researchers because the process is promising, efficient, technically feasible, and economically viable for wastewater treatment [14,15,16,17,18].

Exploring new solid adsorbents that could be used for the removal of different organic pollutants such as MG dye from polluted water is a prodigious task. MXene nanosheets (MXNSs) are a new class of two-dimensional (2D) nanomaterials based on transition-metal nitrides, carbides, and carbonitrides; similarly to graphene, they have attracted considerable attention from the scientific community and various industries due to their outstanding physicochemical properties. They have been used in different applications such as biomedical [19], energy storage [20,21], electromagnetic interference shielding [22], desalination [23], catalysis [24], and analytical applications [25], and they have been found to be effective in removing hazardous materials from the environment such as heavy metals [26], pharmaceutical compounds [27], and dyes [28,29].

In this research work, Ti-based MXene (Ti₃C₂T_x) nanosheets were synthesized, characterized, and then used for the removal of MG dye from aqueous solutions and real water samples as an environmental application. MXene was characterized by different chemical and physical techniques, and then different experimental conditions affecting the efficiency of removal of MG dye from synthetic water samples were explored. The adsorption process was studied kinetically and thermodynamically to understand the mechanism of removal and its spontaneity in order to enhance the efficiency of the removal process.

2. Experimental Section

2.1. Chemicals

Analytical-grade chemicals were used for all experiments. The experiments used the MAX phase powder of Ti₃AlC₂ (NANO SHEL, Dera Bassi, India) and 48.051% hydrofluoric acid (HF, Parmcac Applichem IT Reagents).

2.2. Synthesis of MXene Nanosheets

MXene nanosheets were prepared according to the procedure of Lim et al. [30] as follows: 5.0 g of Ti₃AlC₂ MAX phase powder was added slowly to 100.0 mL of hydrofluoric acid (HF, Merck, Germany, 49%) and stirred for 24.0 h at ambient temperature, as HF was used to etch aluminum layers from Ti₃AlC₂ and produced Ti₃C₂T_x. The Ti₃C₂T_x solid was washed regularly using deionized water and then centrifuged for 5 min at 3500 rpm for separation. The washing process was repeated until the pH of the supernatant reached a value of 6, and then the exfoliated Ti₃C₂ (MXene nanosheets) was dried for 12 h at 80 °C.

2.3. Characterizations

The identification of the crystalline phases of the MXNS was recorded by a Bruker D2 Phaser X-ray diffractometer (Billerica, MA, USA). XRD measurements were carried out by CuKα radiation (1.5418 Å). The external surface morphology of MXNS was studied using a scanning electron microscope (Gemini, Zeiss-Ultra 55, Oberkochen, Germany).

2.4. Adsorption Experiment

A certain amount of the MXNS adsorbent was used to treat 10.0 mL of 5.0 ppm MG dye for a certain period of time. After the experiment’s onset, the amount of MG dye adsorbed was monitored using SHIMADZU CPS-240A UV-VIS at 617 nm wavelength. The influences of experimental conditions such as MXNS mass, solution pH, initial MG dye concentration, solution temperature, and ionic strength were studied in order to determine the performance of the MXNS adsorbent in the adsorption of MG dye under different conditions in model and real water samples. The efficiency of MXNS in the removal/adsorption of MG dye was calculated using Equation (1), and the removal capacity (q_t, mg/g) was estimated using Equation (2).

$$\% adsorption=\frac{C_o-C_t }{C_o}$$

(1)

$$q_t=\frac{V\times \left(C_o-C_t\right) }{m}$$

(2)

C_o and C_t are the concentration of the MG dye at time zero and any time, respectively, in the solution (ppm, mg/L), V is the volume of the solution (L), and m is the MXene dose (g).

3. Results and Discussions

3.1. Characterization of MXNS

Figure 1 shows a typical SEM image of the MAX phase (Ti₃AlC₂) before and after HF etching treatment and the formation of the 2D MXene nanosheets (Ti₃C₂). It is clear that HF exfoliated the layered Ti₃AlC₂ due to the removal of Al atoms. Ti–Al bonds within the MAX phase (Ti₃AlC₂) are relatively weak compared to Ti–C bonds, and accordingly, Al atoms can be readily removed from Ti₃AlC₂ structure due to their utmost reactivity, forming 2D MXene nanosheets (Ti₃C₂). Figure 2 shows the XRD patterns of the MAX phase (Ti₃AlC₂) and 2D MXene nanosheets (Ti₃C₂). The classic Ti₃AlC₂ peaks at 9.5°, 19.1°, 34°, 38.8°, 41.7°, and 60.2° are consistent with the previously reported values [31]. After etching treatments, the diffraction peak around 39° corresponding to the (104) characteristic peak of MAX phase Ti₃AlC₂ nearly disappeared, which indicates that the Al element was successfully removed. The peaks of MXene appearing at 9.16° (002) and 36.01° (104) matched well with previously published reports on MXene (Ti₃C₂) [32].

3.2. Adsorption Studies

The effects of various operational parameters were studied in order to determine the performance of MXNS solid adsorbents in the adsorption of MG dye under different conditions, and they are discussed in the subsequent sections. The effect of the MXNS solid adsorbent dose on MG removal via adsorption was explored, and the results are explained in Figure 3a, which shows the change in the UV-vis absorbance spectrum, and Figure 3b, which shows changes in removal efficiency with MXNS doses. The maximum removal capacity obtained was 4.6 mg MG per g of MXNS using 5 mg of MXNS, with a removal efficacy of 46.0%; the minimum removal capacity obtained was 2.5 mg MG per g of MXNS using 20 mg of MXNS, with a removal efficacy of 99.1%. The MG removal using MXNS increased with increasing MXNS doses, and perhaps this is because of the accessibility to more adsorption active sites due to the increase in adsorbent dose [33]. Furthermore, 12 mg of MXNS was used for further experiments.

The influence of solution temperatures on MG removal using MXNS was explored, and the results are presented in Figure 4. The percentage of MG adsorbed on MXNS increases at high temperatures, indicating that the process is endothermic. This is attributed to the increase in mobility and diffusion of MG molecules, as more MG molecules could acquire sufficient energy to interact with MXNS’s surface [34].

Interaction time between the adsorbate, such as MG dye molecules, and solid adsorbents, such as MXNS, is one of the main operating parameters affecting the removal process. Therefore, the influence of the interaction time was explored, and the results are presented in Figure 5a, which depicts the variation of removal efficiency with the interaction time, and Figure 5b, which depicts the variation of removal capacity with the interaction time. The percentage of MG adsorbed on MXNS increases with increasing interaction time and reached almost complete removal within 120 min, with a removal capacity (q_t) of 3.18. Further increases in interaction time are associated with slight increases in removal efficiency and removal capacity. Accordingly, 120 min of interaction time between MG dye and MXNS was used for further experiments.

The solution pH value is very crucial for the control of any adsorption/removal process, especially for organic molecules such as MG dyes, which are charged based on the pH value. The solution’s pH also controls the surface charges of adsorbents such as MXNS. According to Figure 6, the removal efficiency of MG dye by MXNS was not affected significantly by increasing the solution’s pH from 2.0 to 6.0 as removal efficiency was around 95.0%. This may be attributed to the electrostatic attraction between the molecules of the MG cationic dye and the negatively charged MXNS surface. Meanwhile, at a pH value of 7.0, there was a dramatic decrease in the removal efficiency of MG dye by MXNS to 75.5%, which in turn decreased further to 68.6% by increasing the pH value to 10.0. This behavior could be attributed to the electrostatic repulsion between the negatively charged MG molecules and the negatively charged MXNS surface. This finding shows that the removal mechanism of the MG molecules by the MXNS solid adsorbent is electrostatic in nature. This electrostatic-in-nature mechanism was evident and confirmed by studying the electrostatic interaction mechanism for the adsorption/removal of MG dye by MXNS by conducting the removal experiment at different ionic strengths and by changing [KNO₃] in the studied solution. Figure 7 shows that the removal percentage of MG dye decreased dramatically from 92.8% to 53.9% after increasing the solution’s ionic strength by increasing the KNO₃ concentration of the solution from 5.0 mM to 0.1 M. This could be due to the formation of a double layer arising from positive (K⁺) and negative ions (NO₃⁻) of KNO₃ surrounding the MXNS surface, leading to electrostatic repulsion of the MG dye molecules. This finding validates the electrostatic interaction mechanism for MG dye removal by MXNS, which agreed well with previous studies that confirmed the electrostatic mechanism of the removal of organic dyes by solid clay-based adsorbents [35,36,37].

The interaction of the MG dye molecules with MXNS was explored based on the FT-IR measurements, and the results are presented in Figure 8. The spectra reflect two obvious FTIR bands of MXene [38], and the existence of hydroxyl groups was confirmed by the absorption peaks at 3470 cm⁻¹ and 1636 cm⁻¹, which were assigned to the absorbed external water and strongly hydrogen-bonded OH or extremely strong coordinated H₂O in MXNS before and after the adsorption of MG. Moreover, the peak at 620 cm⁻¹ was probably due to the deformation vibration of the Ti–O bond, and the peak at 1097 cm⁻¹ was due to the vibration of the C–F bond [38]. The change in the FTIR spectra of MXNS after the adsorption of the MG dye is a clear indication that the adsorption of MG molecules at the MXNS surface had taken place and that new bonds had been formed between MG molecules and MXNs.

To understand the interaction between the MG dye and MXNS, it is important to study the removal process kinetically based on the pseudo-first-order (PFO) [39] and pseudo-second-order (PSO) [40] kinetic models.

The following equation represents the linearized form of the PFO kinetic model.

$$\ln (q_e-q_t )=\ln q_e-k_{1}t$$

(3)

The following equation represents the linearized form of the PSO kinetic model:

$$\frac{t}{q_t}=\frac{1}{k_2{q}_t^2{}_{}}+\frac{t}{q_e}$$

(4)

where k₁ (min⁻¹) is the PFO rate coefficient, k₂ (g/(mg·min)) is the PSO rate coefficient, and q_e and q_t represent the amount of MG dye removed per unit mass of MXNS at equilibrium and at any time t, respectively. The application of the PFO and PSO kinetic models to experimental data in Figure 5 showed better convergence with the PFO kinetic model (R² value of 0.890) compared to the PSO kinetic model (R² value of 0.660), as presented in Figure 9 indicating the suitability of the PFO kinetic model for describing the adsorption of MG dye by MXNS with k₁ of 0.037 min⁻¹, and k₂ of 0.002 g/(mg·min).

3.3. Environmental Samples

To confirm the efficacy of the tested adsorbent, MXNS was used as a solid adsorbent for the removal of MG dye from three different real water samples (Figure 10). The compositions of the wastewater and sea water are reported elsewhere [41]. The removal efficacies were almost 100%. Such a result is an indication that MXNS could function and be used for the remediation of different water matrices.

3.4. Reusability Studies

The reusability of any adsorbent is vital for its practical application. It is a very essential aspect of adsorption from both environmental and economic points of view. Accordingly, the recycling efficiency of MXNS for the adsorption of MG dye was studied. After each adsorption cycle, MG dye molecules were desorbed from the MXNS solid adsorbent using a methanol/acetic acid mixture (95:5%, v/v) [42], and then MXNS was washed with deionized water and dried at room temperature before being used for another adsorption cycle. As presented in Figure 11, MG removal using MXNS remained significant even after the fourth cycle, which indicates that MXNS is a reusable adsorbent, and there were no significant changes in the morphology of MXNS as confirmed by the SEM image presented in Figure 12.

4. Conclusions

MXene nanosheets were synthesized and characterized using SEM and XRD, which shows a typical SEM image of 2D exfoliated MXene nanosheets and the XRD patterns of the 2D MXene nanosheets. The removal of MG dye onto MXNS adsorbent was explored, and it was found that increases in MXNS dose, removal time, and temperature were all associated with increased MG dye removal, and the removal is electrostatic in nature based on the effects of solution pH and ionic strength. Finally, the results showed that MXNS adsorbents were able to adsorb a high percentage of MG and maintained reasonable efficiency even after four cycles. Thus, MXNS could be a promising adsorbent in wastewater remediation.