Magnetite/MXene (Fe₃O₄/Ti₃C₂) Nanocomposite as a Novel Adsorbent for Environmental Remediation of Malachite Green Dye

Abstract

In this work, a novel adsorbent called magnetite/MXene (Fe₃O₄/Ti₃C₂) nanocomposite was prepared, characterized, and applied for the removal of organic dye, malachite green dye (MG), from both real water and model solutions. Numerous techniques were used to characterize the prepared Fe₃O₄/Ti₃C₂ nanocomposite: XRD, SEM, TEM, FTIR, and surface area analysis. The outcomes showed that the Al layer had been selectively etched, that the MAX phase (Ti₃AlC₂) had been transformed into layered Ti₃C₂ MXene, that the cubic Fe₃O₄ phase had been prepared, and that the prepared Fe₃O₄ NPs had been evenly distributed on the MXene surface. Also, SEM pictures showed the successful etching of the MAX phase and the formation of the ultrathin multi-layered MXene, which the Fe₃O₄ NPs covered upon forming the Fe₃O₄/Ti₃C₂ nanocomposite at the surface and inside the ultrathin multi-layered MXene. The effect of different operational parameters affecting the removal process was explored and optimized. The MG dye was removed mostly within 60 min, with a 4.68 mg/g removal capacity using 5 mg of the Fe₃O₄/Ti₃C₂ nanocomposite. The removal was examined from both kinetic and thermodynamic perspectives, and the findings demonstrated the spontaneity of the removal process as well as the applicability of fractal-like pseudo-first-order and fractal-like pseudo-second-order kinetics when compared to other kinetics models. The Fe₃O₄/Ti₃C₂ nanocomposite was used to remove MG dye from real spiked environmental water samples, and the results revealed the successful remediation of the real samples from the organic dye by the Fe₃O₄/Ti₃C₂ nanocomposite. Accordingly, Fe₃O₄/Ti₃C₂ nanocomposite could be considered a potential adsorbent for the environmental remediation of polluted water.

1. Introduction

Most consumer items, including textiles, paper, leather, food, and cosmetics, are colored using synthetic organic dyes that, when combined with other auxiliary chemicals, form large discharges that threaten aquatic life and humans with poisonous, carcinogenic, and mutagenic effects, in addition to their great susceptibility to chelate metal ions, increasing their toxicity to fish and other creatures [1]. Additionally, synthetic organic dyes in water bodies absorb light and prevent light penetration, decreasing photosynthetic activity and preventing the growth of aquatic vegetation [2].

The removal of organic dyes can be accomplished by utilizing a variety of treatment methods, including ion exchange, precipitation, coagulation–flocculation, filtration, liquid–liquid extraction, electrochemical methods, and adsorption [3], especially with carbon-based materials for adsorption and separation [4,5]. Adsorption is considered one of the most efficient removal techniques as it has numerous advantages, including a simple design, reusability of adsorbents/adsorbate, low cost, ease of operation, fewer chemical requirements, high efficiency, and a short time of removal [6].

One of the great challenges facing the application of the adsorption process is the search for new adsorbents with high efficacy. Therefore, exploring novel adsorbents is still a prodigious task for the scientific community. Nothing excites the community of material scientists more than the discovery of a new family of two-dimensional (2D) nanomaterials, such as transition metal dichalcogenides (e.g., MoS₂ and WS₂), black phosphorous layered double hydroxides (LDHs), g-C₃N₄, boron nitride, graphene, graphene oxide, metal–organic frameworks (MOFs), and MXenes, which are characterized by a wide range of potential compositions and tailorable features for varied uses [7], especially in applied environmental cleaning [8]. Moreover, the layered 2D materials can easily attach to the nano-sized encapsulated particles through strict layer spaces for various applications [9].

MXene is the most recent member of the 2D nanomaterials family; due to their strong negative surface charge, high chemical stability, adjustable chemistry, and high hydrophilicity, MXenes can be employed in a wide range of applications, such as absorbents, energy storage, supercapacitors, catalysis, gas sensors, polymer-based composites, electromagnetic interference (EMI) shielding antibiosis, and lubrication [10]. Many surface modification techniques have been cleverly developed recently to increase the potential of MXenes in various sectors, including surface-initiated polymerization and single-heteroatom doping [11]. MXenes were recently applied in dye removal [12,13]; for instance, they were used for eliminating methylene blue dye from water [14] and were also used for the removal of malachite green dyes [15]. MXene nanosheets were directly deposited on commercially available filters for fast and efficient dye removal, and the results indicated outstanding performance [16]. This demonstrates the possibility of using MXenes in the adsorption and environmental remediation of dyes from water. Recently, MXene modification was explored to enhance extraordinary properties [17,18,19]. Most recently, MXene/carbon foam hybrid aerogel (MCF) was synthesized and used for methylene blue (MB) and Congo red (CR) [20], MXene/NiFeMn-layered double hydroxide decorated gelatin for the removal of Congo red [21], and porous heterostructured MXene/biomass-activated carbon composites were synthesized and used for the removal of three anionic azo dyes: allure red, Congo red, and sunset yellow [22]. In the recent past, the field of environmental remediation has paid considerable attention to magnetite nanoparticles (Fe₃O₄ NPs) due to their tendency to aggregate and form clusters [23]. Magnetite nanoparticles (Fe₃O₄) are among the most promising transition metal oxides that are used for the modification of various adsorbents [24,25,26] because of their advantages of having excellent theoretical unique capacity, affordability, and nontoxicity.

The novelty of the current research is to develop a unique, efficient, and reliable solid hybrid adsorbent based on magnetite and MXene for the environmental remediation of polluted water with organic dyes such as malachite green dye.

Accordingly, in this study, magnetite nanoparticles were used for the modification of MXene to form the magnetite/MXene (Fe₃O₄/Ti₃C₂) nanocomposite and used to remove malachite green (MG) dye, as an example of synthetic organic dye, from a simulated dye aqueous solution. The adsorption process was investigated based on the operational factors adsorbent mass, interaction time, temperature, solution pH, and ionic strength, and the mechanism was explored using several adsorption kinetics models, in addition to thermodynamics.

2. Results and Discussion

2.1. Characterization

The XRD patterns of the Ti₃AlC₂ MAX, exfoliated Ti₃C₂ MXene, Fe₃O₄ nanoparticles, and Fe₃O_4/Ti₃C₂ nanocomposite are shown in Figure 1. At 2θ = 39°, the peak attributed to Ti₃AlC₂′s (1 0 4) peak disappeared after the procedure of HF exfoliation. Additionally, the peak at (0 0 2) for 2D Ti₃C₂ changed from 2θ = 9.7° to 2θ = 9.1°, indicating that the Ti₃AlC₂ phase transformed into layered Ti₃C₂ MXene and that the Al layer was carefully etched. There are four diffraction peaks for the generated Fe₃O₄, at 30.2°, 35.5°, 57.2°, and 62.8°, which belong to the cubic Fe₃O₄ phase’s (2 2 0), (3 1 1), (5 1 1), and (4 4 0) planes, respectively (JCPDS card no. 19-0629). By applying the Scherrer Formula and utilizing the (3 1 1) peak’s half-peak width, it is expected that the produced Fe₃O₄ crystallite size is about 27.5 nm. There are no discernible XRD peaks in response to new crystalline phases in the Fe₃O₄/Ti₃C₂ nanocomposite; instead, the XRD patterns are composed solely of cubic Fe₃O₄ and pure Ti₃C₂. This means that the nanocomposite is a true combination of Ti₃C₂ MXene and Fe₃O₄ nanoparticles, which is in line with previously published work [27].

The MAX phase, prepared Ti₃C₂ MXene, prepared Fe₃O₄ NPs, and Fe₃O₄/Ti₃C₂ nanocomposite microstructures were examined using scanning electron microscopy, and the micrographs are presented in Figure 2. The images of the MAX phase (Ti₃AlC₂) before (Figure 2a) and following an HF etching procedure that eliminated the Al atoms to produce the exfoliated 2D MXene nanosheets (Ti₃C₂) (Figure 2b) possessing a few nanometers’ thickness are shown. The MAX phase was characterized by an uneven surface and a closely packed structure, whereas the resulting exfoliated 2D MXene nanosheets were ultrathin and multi-layered, characterized by clear sharp edges, confirming the success of the etching process because of the weak Ti–Al bonds compared to the Ti–C bonds within the MAX phase. Also, due to the extreme reactivity of the Al atoms, they could be easily etched and removed from the Ti₃AlC₂ structure and form the 2D MXene nanosheets (Ti₃C₂). Moreover, the ultrathin sheets are parallel to one another, which suggests that the basal planes and Ti₃AlC₂ etching directions are parallel, which is similar to previous studies [28,29,30]. Figure 2c shows the Fe₃O₄ NPs, which are characterized by a large quantity of agglomerated nanoparticles with a very small size. Figure 2d displays the uniform dispersion of Fe₃O₄ nanoparticles both on the exterior and within the incredibly thin multi-layered MXene. The Fe₃O₄ nanoparticles could be easily inserted between the layers of Ti₃C₂ MXene or adsorb on their surface.

Further examination of the MAX phase, Ti₃C₂ MXene, Fe₃O₄ NPs, and Fe₃O₄/MXene nanocomposite was performed using TEM. Figure 3a,b show the MAX phase and Ti₃C₂ MXene nanosheets, which demonstrated overlapped and nearly transparent sheets with a few nanometers’ diameter. It is evident that the diameter of the MXene is smaller than the MAX phase due to the etching process, which decreased the size of MXene compared to the MAX phase: 78 nm and 122 nm, respectively. Additionally, the aggregation of Fe₃O₄ NPs with an average size of particles of 11 nm was visible in the TEM image, and the homogeneous distribution of the Fe₃O₄ NPs over the MXene nanosheets is displayed in Figure 3d. FT-IR analyses of the MAX phase, Ti₃C₂ MXene, Fe₃O₄ NPs, and Fe₃O₄/MXene nanocomposite were explored to provide details on the produced nanomaterials’ composition and surface functionality. The FTIR spectrum of the Ti₃AlC₂ MAX phase is displayed in Figure 4, with pronounced bands at 474, 617, 1087, and 1541 cm⁻¹ corresponding to the TiO₂, Ti–O, Al–O, and CH groups, respectively, in addition to the strong vibration band of the –OH group at 3500 cm⁻¹ due to the adsorbed external water. In the case of MXene, the vibrational stretching of the highly hydrogen-bonded –OH group and the bending vibration of the C–OH bond, respectively, could be the cause of the absorption peaks at 1638 and 3686 cm⁻¹ because MXene’s hydrophilic nature causes it to absorb external water molecules or strongly coordinated H₂O. Such distinct –OH groups’ stretching vibrational patterns may also be associated with Ti⁴⁺ cations. Moreover, Ti–O–Ti linkage and Ti–O bonding deformation vibrations account for the maxima at 877 and 617 cm⁻¹, respectively. Furthermore, the band at 617 cm⁻¹ might be a representation of rocking or twisting Ti–C out-of-plane vibrations [31,32]. The FT-IR spectrum of Fe₃O₄ nanoparticles showed absorption bands at 550–1650 cm⁻¹ due to Fe–O group flexural vibrations and strong vibration bands of the –OH group at 1638 and 3500 cm⁻¹ due to adsorbed external water. The FTIR spectrum of the Fe₃O₄/MXene nanocomposite showed most of the characteristic absorption bands of both MXene and the Fe₃O₄ NPs. The specific surface area values of the samples were calculated from the nitrogen gas adsorption/desorption isotherms at 77 K, and the values were 10.40, 40.25, 59.89, and 32.86 m²/g for the MAX phase, Ti₃C₂ MXene, Fe₃O₄ NPs, and Fe₃O₄/MXene nanocomposite, respectively. There was a significant increase in the MAX phase’s specific surface area following HF etching and the formation of exfoliated 2D nanosheets of MXene, from 10.40 to 40.25 m²/g. Upon the modification of MXene with the Fe₃O₄ NPs and the formation of the Fe₃O₄/MXene nanocomposite, this BET surface area was slightly reduced from 40.25 to 32.86 m²/g due to the distribution of the Fe₃O₄ NPs over the MXene nanosheets, as was presented earlier by the SEM (Figure 2) and TEM (Figure 3) analyses.

2.2. Adsorption Studies

The effects of several operating factors were investigated to ascertain the efficacy of Fe₃O₄/Ti₃C₂ nanocomposite solid adsorbent concerning the MG dye’s adsorption under diverse conditions, and they are detailed in the following parts. We investigated the amount of absorption of prepared nanomaterials for MG dye at a concentration of 10 mg/l in deionized water. The highest removal ratio (47.72%) of this dye was achieved by the Fe₃O₄/Ti₃C₂ nanocomposite. Figure 5a shows how the dosage results for the Fe₃O₄/Ti₃C₂ nanocomposite affect the removal efficiency and its UV–Vis. absorbance spectra (inset). Using 100 mg of Fe₃O₄/Ti₃C₂ nanocomposite, the maximum removal capacity was found to be 0.993 mg/g of MG, with a removal efficacy of 99.28%. While utilizing 5 mg of Fe₃O₄/Ti₃C₂ nanocomposite, the optimum removal capacity was found to be 4.68 mg/g, with a removal efficacy of 23.38%. MG clearance using Fe₃O₄/Ti₃C₂ nanocomposite increased with increasing Fe₃O₄/Ti₃C₂ nanocomposite dose, possibly due to the improved accessibility to extra adsorption active sites as a result of the increased adsorbent dose. Furthermore, 30 mg of Fe₃O₄/Ti₃C₂ nanocomposite was used for further experiments.

Temperature’s effect on the adsorption of MG utilizing Fe₃O₄/Ti₃C₂ nanocomposite was examined, and Figure 5b displays the findings. The figure makes it evident that the endothermic nature of the adsorption process is present because the percentage of MG adsorbed on the Fe₃O₄/Ti₃C₂ nanocomposite increases as temperature rises, which will be evaluated thoroughly regarding the thermodynamic parts. This is explained by the fact that more MG molecules were able to obtain enough energy to engage with the Fe₃O₄/Ti₃C₂ nanocomposite surface, which increased MG molecule mobility and diffusion. Furthermore, because of active diffusion, a temperature increase would cause pore sizes to grow, adding additional surface area for adsorption [33].

The duration of interaction between the solid adsorbents, Fe₃O₄/Ti₃C₂ nanocomposite, the adsorbate, and MG dye molecules is one of the critical operating parameters influencing the removal process. The results of this investigation on the impact of interaction time are shown in Figure 5c. The Fe₃O₄/Ti₃C₂ nanocomposite showed extremely rapid adsorption; full saturation happened in less than 1 h, and it had a removal capacity (q_t) of (3.14 mg MG each g of Fe₃O₄/Ti₃C₂ nanocomposite) because of the increase in active sites associated with increasing the mass of the Fe₃O₄/Ti₃C₂ nanocomposite and consequent electrostatic interaction between the negative Fe₃O₄/Ti₃C₂ nanocomposite surface and the MG dye increased. Therefore, the amount of MG adsorbed on the Fe₃O₄/Ti₃C₂ nanocomposite rises with increasing contact time. An increase in interaction duration is correlated with a small increase in removal efficiency and capacity. Furthermore, a 60 min duration of the interaction between MG dye and Fe₃O₄/Ti₃C₂ nanocomposite was used for further experiments.

Any adsorption/removal process that involves controlling surface charges of adsorbents, which changed according to pH value, depends heavily on the solution pH value. Surface charge is affected principally by the solution pH, which modifies the electrostatic conditions between adsorbent and adsorbate. The current results show that the adsorption of MG dye molecules onto Fe₃O₄/Ti₃C₂ nanocomposite increases when pH increases, indicating the degree of the surface charge that is negative. The information is consistent with the theory that adsorption is mostly caused by electrostatic attraction. The Fe₃O₄/Ti₃C₂ nanocomposite exhibited a noteworthy improvement in MG dye removal efficiency (87.63–99.7%) in Figure 5d when the pH of the solution was raised gradually from 2.0 to 12.0. This could be explained by electrostatic attraction between the MG neutral and cationic dye molecules (pK_a value 10.3) and the negatively charged Fe₃O₄/Ti₃C₂ nanocomposite surface (zeta potential value of −21 mV). In the meantime, the Fe₃O₄/Ti₃C₂ nanocomposite dramatically increased its ability to remove MG dye at a pH of 6.0, 80.88%, which in turn increased more through raising the pH to 12.0 (99.28%). The electrostatic attraction between the positively charged MG molecules and the negatively charged Fe₃O₄/Ti₃C₂ nanocomposite surface may be the cause of this phenomenon. This result demonstrates that the Fe₃O₄/Ti₃C₂ nanocomposite solid adsorbent’s mechanism for removing the MG molecules is electrostatic in nature. The existence of this electrostatic mechanism was demonstrated and confirmed by investigating the Fe₃O₄/Ti₃C₂ nanocomposite’s electrostatic interaction mechanism for the adsorption and removal of MG dye through varying the KNO₃ concentration in the investigated solution and performing the removal experiment at different ionic strength values. Figure 5e illustrates how, when the (KNO₃) concentration in the solution was increased to 5.0 mM, the percentage of MG dye removed dropped sharply to 76.89%, and further addition of KNO₃ slightly retarded the adsorption mechanism; 0.1 M KNO₃ was associated with 75.09% removal. The reason for this could be that the Fe₃O₄/Ti₃C₂ nanocomposite surface is surrounded by a double layer of positive ions (K⁺) and negative ions (NO₃⁻) of (KNO₃), which creates electrostatic repulsion with the MG dye molecules. This discovery supports the mechanism of electrostatic contact for the MG dye that had been removed by Fe₃O₄/Ti₃C₂ nanocomposite, which strongly corroborated earlier research that established the organic dyes’ electrostatic mechanism through solid clay-based adsorbents [34].

2.3. Kinetics Study

Information regarding the kinetics interaction between the adsorbent and adsorbate in the solid/solution system provides a comprehensive understanding of the possible interaction mechanism of adsorption in addition to the reaction pathways [35]. Several kinetics models assuming that the kinetic constants are time-invariant (a time-dependent system function that is not a direct function of time), such as pseudo-first-order (PFO) and pseudo-second-order (PSO), have been used for adsorption kinetics prediction at solid/solution interfaces. This assumption has been unacceptable when reactants (adsorbent and adsorbate) were spatially constrained by walls or phase boundaries [36]. Based on experimental and theoretical examinations, it was presented that the reaction rate coefficient is time-dependent (fractal-like kinetics), leading to the fractal-like pseudo-first-order (FL-PFO) and fractal-like pseudo-second-order (FL-PSO) kinetics models.

Therefore, the kinetic adsorption data were evaluated by using four nonlinear models: pseudo-first-order (PFO) [37], pseudo-second-order (PSO) [37], fractal-like pseudo-first-order (FL-PFO) [38], and fractal-like pseudo-second-order (FL-PSO) [38]. The mathematical equations of these respective models are shown in Equations (1)–(4).

$$q_t=q_e\times [1-{\mathrm{e}}^{-k_{1}t}]$$

(1)

$$q_t=\frac{k_2{\times q}_e^2\times t}{1+q_e{k}_2\times t}$$

(2)

$$q_t=q_e\times [1-{\mathrm{e}}^{{(-k_{1,0}\times t)}^n}]$$

(3)

$$q_t=\frac{k_{2,0}{\times q}_e^2\times t^n}{1+q_e{k}_{2,0}\times t^n}$$

(4)

where t is the interaction time (min); q_t, and q_e are the amount of MG dye adsorbed at time t and at equilibrium, respectively (mg g⁻¹); k₁ is the pseudo-first-order rate constant (min⁻¹); k₂ is the pseudo-second-order rate constant (g mg⁻¹ min⁻¹); k_1,0 is the fractal-like pseudo-first-order constant rate (min⁻¹), k_2,0 is the fractal-like pseudo-second-order rate constant (g mg⁻¹ min⁻¹), and n is the fractional-like exponent (n > 0). The kinetic and equilibrium models’ appropriateness was then examined and evaluated using different statistical error functions: the residual sum of squares (RSS), Equation (5); the determination coefficient (R²), Equation (6); the adjusted determination coefficient (R²_adj), Equation (7); the standard deviation of residues (SD), Equation (8); and the Bayesian Information Criterion (BIC), Equation (9) [37,39].

$$RSS={{\sum }_i^n(q_{i,exp}-q_{i,model})}^2$$

(5)

$$R^2=(\frac{{\sum _i^n(q_{i,exp}-{\overline{q}}_{i,exp})}^2-{\sum _i^n(q_{i,exp}-{\overline{q}}_{i,emodel})}^2}{{\sum _i^n(q_{i,exp}-{\overline{q}}_{i,exp})}^2})$$

(6)

$$R_{adj}^2=1-(1-R^2)(\frac{n-1}{n-p-1})$$

(7)

$$SD=\sqrt{(\frac{1}{n-p}){{\sum }_i^n(q_{i,exp}-{\overline{q}}_{i,emodel})}^2}$$

(8)

$$BIC=nLn\left(\frac{RSS}{n}\right)+pLn(n)$$

(9)

In the above equations, q_i,model is the individual theoretical q value predicted by the model; q_i,exp is individual experimental q value; ${\overline{q}}_{i,exp}$ is the average of all experimental q values measured; n is the number of experiments; and p is the number of parameters in the fitting model. In this work, the $R_{adj}^2$, SD, and BIC values will be used to examine the appropriateness of different kinetics models, taking into consideration that the best-fitted model would have $R_{adj}^2$ value closer to unity, in addition to lower values of SD and BIC. Figure 6 shows the fitting of the four kinetics models with the experimental adsorption capacities (q_t, mg/g) variation with interaction time (min), and

Table 1. Parameters of the kinetics models for the removal of MG dye from model solutions using Fe₃O₄/Ti₃C₂ nanocomposite.

Pseudo-First-Order Kinetics Model (PFO)
qe,exp (mg g−1)	qe,cal (mg g−1)	k1 (min−1)	R2	${\mathit{R}}_{\mathit{a}\mathit{d}\mathit{j}}^2$	SD	RSS	BIC
3.140	2.776	1.080	0.9573	0.9488	0.2392	0.2861	−16.544
Pseudo-second-order kinetics model (PSO)
qe,exp (mg g−1)	qe,cal (mg g−1)	k2 (g mg−1 min−1)	R2	${\mathit{R}}_{\mathit{a}\mathit{d}\mathit{j}}^2$	SD	RSS	BIC
3.140	2.917	0.5965	0.9813	0.9775	0.1584	0.1254	−22.315
Fractal-like Pseudo-first-order Kinetics model (FL-PFO)
qe,exp (mg g−1)	qe,cal (mg g−1)	k1,0 (min−1)	R2	${\mathit{R}}_{\mathit{a}\mathit{d}\mathit{j}}^2$	SD	RSS	BIC
3.140	4.446	0.0488	0.9963	0.9944	0.0790	0.0249	−31.668
Fractal-like Pseudo-second-order Kinetics model (FL-PSO)
qe,exp (mg g−1)	qe,cal (mg g−1)	k2,0 (g mg−1 min−1)	R2	${\mathit{R}}_{\mathit{a}\mathit{d}\mathit{j}}^2$	SD	RSS	BIC
3.140	5.358	0.1098	0.9963	0.9945	0.0784	0.0246	−31.783

shows the kinetic parameters for each model. The four kinetics models were evaluated statistically based on the $R_{adj}^2$, SD, and BIC values to evaluate the most suitable model. Surprisingly, both the FL-PFO and FL-PSO kinetics models showed the best $R_{adj}^2$ values of 0.9944 and 0.9945, and the lowest standard deviation (SD) of residue values of 0.0790 and 0.0784, and the lowest BIC values of −31.67 and −31.78 for MG dye using Fe₃O₄/Ti₃C₂ nanocomposite as adsorbent (Table 1), respectively.

Furthermore, the process of adsorption/removal process is controlled/governed by diffusion kinetics too as the adsorption process occurs typically in four separate stages: mass action, bulk diffusion, liquid film diffusion, and intra-particle diffusion, which is sometimes referred to as pore and surface diffusion. The identification of the adsorption/diffusion process’s rate-determining step is the main focus of this research segment; based on the liquid film diffusion model, it predicts the rate processes’ kinetics by assuming that the adsorbate molecules’ slowest-moving stage in the adsorption process is their flow through a liquid film that surrounds the solid adsorbent, and intra-particle diffusion model assumes that the adsorbate molecules’ slowest-moving stage in the adsorption process is their flow through the solid adsorbent pores.

The following equation [40] describes the liquid film diffusion model:

$$Ln\left(1-F\right)=k_{fd}\ast t$$

(10)

where the film diffusion rate coefficient is denoted by k_fd (min⁻¹), and F denotes the fractional attainment of equilibrium (F = q_t/q_e). Figure 7a demonstrates how the liquid film diffusion model plot of [$-$Ln (1 $-$ F)] against t converged when applied to the MG dye adsorption by Fe₃O₄/Ti₃C₂ nanocomposite, and a correlation coefficient of 0.856 was obtained, resulting in a straight line through the origin for the first 3 min. This suggests that the rate-determining step could be for the liquid film diffusion model within the first 3 min.

The following is the equation of the intra-particle diffusion model that Weber and Morris presented in 1963 [41]:

$$q_t=k_{id}{t}^{1/2}+C$$

(11)

C is a constant inversely proportional to the boundary layer thickness (mg/g), and k_id is the intra-particle diffusion rate constant (mg/g·min^1/2). Upon utilizing the intra-particle diffusion model on the experimental data, the adsorption of the MG dye by the Fe₃O₄/Ti₃C₂ nanocomposite was plotted as q_t versus t; the plot did not converge satisfactorily considering all the points, and, therefore, it was possible to draw a straight line through the origin, as observed in Figure 7b, obtained following the 3 initial minutes, with a correlation coefficient value of 0.973, indicating that the process that influences the rate may be the intra-particle diffusion model. Accordingly, both liquid film and intra-particle diffusion may have an impact on the removal diffusion kinetics in this work at the same time, as presented in Figure 7c.

Figure 7. (a) Liquid film diffusion kinetics model, (b) intra-particle diffusion kinetics model, and (c) change over time in the quantity of MG dye adsorbed per unit mass of the Fe₃O₄/Ti₃C₂ nanocomposite.

Figure 7. (a) Liquid film diffusion kinetics model, (b) intra-particle diffusion kinetics model, and (c) change over time in the quantity of MG dye adsorbed per unit mass of the Fe₃O₄/Ti₃C₂ nanocomposite.

2.4. Environmental Applications

Five separate real water samples were utilized to further confirm the effectiveness of the Fe₃O₄/Ti₃C₂ nanocomposite as an adsorbent for the removal of MG dye, and Figure 8 shows the results. After the real water sample collection, analysis was conducted on the MG dye concentrations, and it was discovered that they were below the detection limit; they were regarded as zero. Therefore, to simulate pollution and explore the possibility of using Fe₃O₄/Ti₃C₂ nanocomposite as a solid material for water pollution treatment, concentrated MG dye was added to real water samples until a final concentration of 10.0 mg/L was obtained. Afterward, the spiked real water samples were mixed with the Fe₃O₄/Ti₃C₂ nanocomposite. Regarding the MG dye, the samples of distilled water, mineral water, well water, wastewater, and seawater had removal percentages of 98.40%, 97.49%, 96.13%, 94.27%, and 93.14%, in that order. It is noteworthy that the minimal percentage of MG dye removal observed in the well water, wastewater, and seawater through the Fe₃O₄/Ti₃C₂ solid nanocomposite was mainly caused by the elevated concentrations of Na⁺, K⁺, Mg²⁺, and Ca²⁺ found in the selected real water samples as compared to the other real water samples. Because of these ions’ screening and shielding properties at such high concentrations, the amount of MG dye removed by the solid Fe₃O₄/Ti₃C₂ nanocomposite was significantly reduced. According to these findings, the Fe₃O₄/Ti₃C₂ nanocomposite was able to remove the majority of the MG dye from real environmental water samples.

2.5. Reusability Studies

For practical application, a solid adsorbent’s reusability is essential. From environmental and economic points of view, it is a highly crucial component of adsorption. Thus, the Fe₃O₄/Ti₃C₂ nanocomposite’s recycling effectiveness for MG dye adsorption was examined during four consecutive cycles. After each adsorption cycle, the MG dye molecules were adsorbed from the Fe₃O_4/Ti₃C₂ nanocomposite using a concentrated solution of NaCl; after that, the Fe₃O_4/Ti₃C₂ adsorbent was washed with acetone several times, and in the last step it was washed with deionized water and a centrifuge was used to separate the Fe₃O_4/Ti₃C₂ nanocomposite and then dried at room temperature before being used in another absorption cycle.

We discovered that, even after the four cycles with a high removal efficiency, the proportion of MG that was removed using the Fe₃O₄/Ti₃C₂ nanocomposite had somewhat decreased (Figure 9). It is clear from this that the Fe₃O₄/Ti₃C₂ nanocomposite is a reusable adsorbent for environmental cleanup.

2.6. Studies on Thermodynamics of the Removal

The three essential thermodynamic functions are entropy change (ΔS), enthalpy change (ΔH), and Gibbs free energy change (ΔG), which must be explored and measured to estimate thermodynamic viability and spontaneity using any physical or chemical procedure, such as using a solid adsorbent Fe₃O₄/Ti₃C₂ nanocomposite to remove MG dye from an aqueous solution. To determine the thermodynamic parameters, the following formulas were employed [42]:

$${\mathrm{K}}_{\mathrm{d}}=\frac{{\mathrm{q}}_{\mathrm{t}}}{{\mathrm{C}}_{\mathrm{t}}}\times 1000$$

(12)

$${\mathrm{l}\mathrm{n}\mathrm{K}}_{\mathrm{d}}=\frac{\Delta \mathrm{S}}{\mathrm{R}}-\frac{\Delta \mathrm{H}}{\mathrm{R}\mathrm{T}}$$

(13)

ΔG = ΔH – TΔS

(14)

where K_d, the dimensionless thermodynamic distribution coefficient, is calculated at equilibrium concerning the amount of MG dye removed (mg of MG dye for each gram of Fe₃O₄/Ti₃C₂ nanocomposite), and the equilibrium concentration of MG dye in the solution is represented by C_t (mg/L). Figure 10 shows that the plot of ln K_d vs. 1/T revealed a straight line, and the ΔH and ΔS were calculated using the straight-line slope and intercept, in that order. Fe₃O₄/Ti₃C₂ nanocomposite’s thermodynamic characteristics for MG dye elimination were calculated, where the corresponding values for ∆H, ∆S, and ∆G were +49.07 kJ/mole, +234.29 J/mole.K, and −20.75 kJ/mole (at 298 K). There was a positive value obtained for the entropy change, demonstrating an increase in the degrees of freedom at the boundary between liquid and solid due to the MG dye attaching itself to the Fe₃O₄/Ti₃C₂ nanocomposite surface, promoting the removal process’ spontaneity, whereas the enthalpy change had a positive value, indicating the endothermic nature of the removal process, which unfavored the removal process. As a result, a negative change in the free energy would be anticipated for a process of spontaneous elimination. Conversely, the removal of MG dye from aqueous solution by Fe₃O₄/Ti₃C₂ nanocomposite is an entropy-driven process, as indicated by the positive values of ∆H, ∆S, and the negative value of ∆G [43].

Based on the above results, the removal/adsorption mechanism of the MG dye from aqueous solution by Fe₃O₄/Ti₃C₂ nanocomposite was found to be electrostatic in nature due to the attraction forces arising between the MG neutral and cationic dye molecules to the negatively charged Fe₃O₄/Ti₃C₂ nanocomposite surface, which were confirmed by studying the effect of the solution pH and ionic strength. Also, the removal process was well-described by both the FL-PFO and FL-PSO kinetics models and controlled by both liquid film and intra-particle diffusion kinetics. In addition, the removal process was entropy-driven, as indicated by the positive values of ∆H, ∆S, and the negative value of ∆G.

Based on the above-mentioned results, the maximum removal capacity of MG dye (10.0 mg/L), obtained using 0.5 g/L loading of Fe₃O₄/Ti₃C₂ nanocomposite (5 mg/10 mL) within 60.0 min, was found to be 4.68 mg/g, which is very competitive with other removal methods based on removal capacity and treatment time, as presented in

Table 2. Removal capacity of MG dye by various removal techniques.

Material	Removal Capacity (mg/g)	Removal Time	Reference
Bottom ash	0.71	83 min	[44]
Polyethylene glycol micelles	1.00	10 min	[45]
Unsaturated Polyester Ce(IV) phosphate	1.01	35 min	[46]
Fum rosin alcoholpoly(acrylamide)	1.40	300 min	[47]
Hybrid nanocomposite	3.21	28 h	[48]
Neem sawdust	4.35	14 min	[49]
Fe3O4/Ti3C2 nanocomposite	4.68	60 min	[Current study]
TiO2 nanoparticles	6.25	40 min	[50]
Deinococcus radiodurans	7.63	30 min	[51]

.

3. Experimental

3.1. Substances

All studies were conducted using chemicals of analytical grade: MAX phase powder of Ti₃AlC₂ (NANOSHEL, Dera Bassi, India, purity: 99%); 48.051% hydrofluoric acid (HF, Parmcac Applichem IT Reagents, Darmstadt, Germany); polyethylene glycol (PEG 4000, BOC Sciences, New York, NY, USA); and 99.5% acetone (Abuljadayel & Sons Co., Jeddah, Saudi Arabia).

3.2. Fe₃O₄ Nanoparticles and MXene Nanosheet Synthesis

Powdered Ti₃AlC₂ (>98 weight percent pure) was offered commercially by Forsman Com., Ltd. (Shanghai, China) to produce Fe₃O₄ and Ti₃C₂ MXene nanoparticles. In order to remove the Al layers completely, 2 g of the Ti₃AlC₂ powder was stirred for 24 h at 60 °C while submerged in HF solutions (20 mL; >40 wt%). After removing the HF, the solution was centrifuged and repeatedly washed with deionized water until the pH reached 5–6. The generated Ti₃C₂ powdered form was dried under a vacuum at 80 °C for a whole day. The process of hydrothermal synthesis was used to create Fe₃O₄ nanoparticles. First, under vigorous stirring conditions for 30 min, 0.43 g of NaOH, 1.35 g of FeCl₃.6H₂O, and 10 g of polyethylene glycol (PEG 4000, BOC Sciences, New York, NY, USA) were dissolved in 40 mL of ethylene glycol. The solution was then instantly put into a 100 mL Teflon-lined autoclave and heated at 200 °C for 8 h. In order to get rid of the contaminant, the sediment was repeatedly cleaned with ethanol and distilled water. The Fe₃O₄ nanoparticles that were produced were then dried in a vacuum for 6 h at 80 °C.

3.3. Synthesis of Fe₃O₄/Ti₃C₂ Nanocomposite

For 30 min, deionized water was used to ultrasonically disperse 100 mg of Ti₃C₂ and 40 mg of Fe₃O₄ nanoparticles in 80 mL and 20 mL, respectively. Both were then combined, and they underwent 6 h ultra-sonification therapy after that. The Ti₃C₂/Fe₃O₄ nanocomposite was then obtained by filtering the suspension. The finished nanocomposite was dried in a vacuum for 24 h at 80 °C.

3.4. Characterizations

The crystalline structures of all the samples were analyzed using powder X-ray diffraction (XRD, Cu K radiation, Bruker D2 Phaser X-ray diffractometer) with a step scan of 0.02° per step. Scanning electron microscopy (SEM, Gemini, Zeiss-Ultra 55, Chiyoda, Tokyo) and transmission electron microscopy (TEM, Sirion 200FEI, Eindhoven, The Netherlands) were used to examine the microstructure and morphology of produced samples. The FT-IR spectra of the samples were recorded using an FT-IR spectrophotometer (Spectrum 100, Perkin Elmer, Waltham, MA, USA). NOVA3200e (Quantachrome, Boynton Beach, FL, USA) was utilized to determine the BET specific surface area of the materials by utilizing the nitrogen adsorption/desorption isotherm at 77 K.

3.5. Adsorption Experiment

First, the prepared nanomaterials studied the adsorption of 10 mL of 10 mg/L of malachite green dye for a certain period. After the experiment began, at a wavelength of 617 nm, the SHIMADZU CPS-240A UV–Vis (Kyoto, Japan) was employed to measure the quantity of adsorbed MG dye. The effects of experimental parameters such as Fe₃O₄/Ti₃C₂ nanocomposite mass, solution pH, initial MG dye concentration, solution temperature, and ionic strength were examined to estimate the efficacy of the nanocomposite adsorbent toward the adsorption of MG dye under various conditions in model and real water samples. The efficiency of the Fe₃O₄/Ti₃C₂ nanocomposite towards the removal/adsorption of MG dye was calculated using Equation (15), and the removal capacity (q_t, mg/g) was estimated using Equation (16).

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

(15)

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

(16)

where V is the volume of the solution (L), m is the dose (g) of the Fe₃O₄/Ti₃C₂ nanocomposite, and C₀ and C_t are the concentrations of the MG dye in the solution at time zero and any time thereafter.

3.6. Collecting Real Water Samples

Several real water samples were gathered to confirm the Fe₃O₄/Ti₃C₂ nanocomposite’s suitability as a solid adsorbent for the environmental cleanup of contaminated real water using MG dye. A wastewater sample was obtained from King Abdelaziz University (KAUWW), Jeddah City’s Membrane Bio-Reactor Technology Wastewater Treatment Plant (MBR 6000 STP) (Latitude deg. North 21.487954, Longitude deg. East 39.236748). At Abdraboh AL-Beladi Farm in the Khulais Province of Saudi Arabia, a deep well (latitude deg. North 22.124376, longitude deg. East 39.500618) was used to gather the well water sample. A sample of distilled water from a distillation unit from King Abdelaziz University’s laboratories also included a Row mineral water sample (commercial water in bottles) and a sample from the Red Sea. A 45.0 µm Millipore filter paper was used to filter all real water samples, which were then stored in Teflon^® bottles at 5 °C in the dark.

4. Conclusions

This study explored the preparation of the modified magnetite/MXene nanocomposite and its application for the removal/adsorption of malachite green dye from model solutions and environmental real water samples. The prepared Fe₃O₄/Ti₃C₂ nanocomposite was physically and chemically characterized using XRD, SEM, TEM, FT-IR, and surface area analysis, and the results demonstrated the successful preparation of MXene and magnetite, in addition to the homogenous distribution of Fe₃O₄ over the MXene surface. The removal/adsorption was explored, and the results revealed maximum removal achieved using 100 mg of the Fe₃O₄/Ti₃C₂ nanocomposite within 60 min under ambient conditions, with an optimum removal capacity of 4.676 mg/g using 5 mg only. Also, removal was studied kinetically, and the results revealed that both the fractal-like pseudo-first-order and fractal-like pseudo-second-order kinetics models could better explain the removal of MG dye by the Fe₃O₄/Ti₃C₂ nanocomposite. Also, the Fe₃O₄/Ti₃C₂nanocomposite was used for the MG dye removal/adsorption from spiked real environmental water samples collected from different places, and the results demonstrated extraordinary efficacy of the Fe₃O₄/Ti₃C₂ nanocomposite and the prospective application of the Fe₃O₄/Ti₃C₂ nanocomposite for the environmental remediation of polluted water. The recycling and reuse of the Fe₃O₄/Ti₃C₂ nanocomposite for the successive removal of the MG dye was studied, and the results showed the possibility of reusing the Fe₃O₄/Ti₃C₂ nanocomposite for four consecutive times with high efficiency. The thermodynamics of the removal/adsorption process were also explored and the results demonstrated the spontaneity of the process: a negative Gibbs free energy change value of −20.75 kJ/mole, a positive entropy change value of +234.29 J/mole.K, and a positive enthalpy change value of +49.07 kJ/mole, indicating that the removal/adsorption of the MG dye by the Fe₃O₄/Ti₃C₂nanocomposite is an entropy-driven process.