Preparation and Characterization of Magadiite–Magnetite Nanocomposite with Its Sorption Performance Analyses on Removal of Methylene Blue from Aqueous Solutions

Abstract

The magadiite–magnetite (MAG–Fe₃O₄) nanocomposite has great potential applications in the field of biomaterials research. It has been used as a novel magnetic sorbent, prepared by co-precipitation method. It has the dual advantage of having the magnetism of Fe₃O₄ and the high adsorption capacity of pure magadiite (MAG). MAG–Fe₃O₄ was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The results showed that Fe₃O₄ nanoparticles were deposited on the interlayer and surface of magadiite. MAG–Fe₃O₄ was treated as an adsorbent for methylene blue (MB) removal from aqueous solutions. The adsorption properties of MAG–Fe₃O₄ were investigated on methylene blue; however, the results showed that the adsorption performance of MAG–Fe₃O₄ improved remarkably compared with MA and Fe₃O₄. The adsorption capacity of MAG–Fe₃O₄ and the removal ratio of methylene blue were 93.7 mg/g and 96.2%, respectively (at 25 °C for 60 min, pH = 7, methylene blue solution of 100 mg/L, and the adsorbent dosage 1 g/L). In this research, the adsorption experimental data were fitted and well described using a pseudo-second-order kinetic model and a Langmuir adsorption isotherm model. The research results further showed that the adsorption performance of MAG–Fe₃O₄ was better than that of MAG and Fe₃O₄. Moreover, the adsorption behavior of MB on MAG–Fe₃O₄ was investigated to fit well in the pseudo-second-order kinetic model with the adsorption kinetics. The authors also concluded that the isothermal adsorption was followed by the Langmuir adsorption isotherm model; however, it was found that the adsorption of the MAG–Fe₃O₄ nanocomposite was a monolayer adsorption.

1. Introduction

Water pollution is seriously damaging the environment as well as threatening human existence and development. According to reports, a large number of poisonous and harmful contaminants (e.g., heavy metal ions, nitrite, and organic dyes) were directly poured into natural water bodies [1]. Wastewater with organic dyes is especially toxic and non-biodegradable; dyes are difficultly treated, and, even at low concentration, are harmful to human beings and microorganisms [2,3]. Accordingly, the technologies to treat dye wastewater has attracted more attention, and multiple technologies were developed for remove organic dyes pollutant from various aqueous solutions [4]. Until now, the conventional methods of treating wastewater has included chemical precipitation, adsorption, filtration, and ion exchange [5,6,7,8]. Moreover, the adsorption method has been proven to be one of the favored techniques, owing to its convenience and effectiveness [9,10,11,12]. The common adsorbents, including activated carbon, cellulosic biomass [13], as well as—according to the latest reports—chitosan [14,15] and silicate materials [16,17,18] has a low cost but also excellent adsorption properties.

Magadiite (MAG) is a kind of layered silicate material [19,20] that possesses a large amount of potentially exchangeable hydrated sodium ions between its layers [21]; for this reason, MAG has unique properties, such as high surface area, good ion exchange, and expansion [22,23]. MAG showed excellent adsorption performance on heavy metal ions and organic pollutants [24,25,26], although it was difficult to separate from the water after adsorption. Fe₃O₄ is a typical superparamagnetic nanoparticle with chemical stability, large specific surface area, easy separation, and good biocompatibility [27,28,29,30], but it is also easy to agglomerate in the aqueous system to influence on its adsorption properties because of its small particle size [31]. Therefore, it has a conceivable capability as a magnetic composite, which loaded the magnetite on MAG to solve the problems of separation of MAG and the aggregation of magnetite (Fe₃O₄) in the aqueous solutions.

Until now, the Fe₃O₄ nano-composite has been prepared using the co-precipitation method, and the composite exhibits high adsorption ability for dyes, while the magnetic separation effect is remarkable [32]; it was reported that Fe₃O₄–montmorillonite nanocomposite, showing good stability and reusability, was synthesized using the co-precipitation method [33] and exhibited excellent adsorption properties for Pb²⁺, Cu²⁺, and Ni²⁺ [34]. In this research, the MAG–Fe₃O₄ nanocomposite was prepared using a co-precipitation method, with Fe₃O₄ nanoparticles deposited on the interlayer and surface of MAG. Methylene blue (MB) was chosen as a model organic dye to investigate the adsorption properties of the MAG–Fe₃O₄ nanocomposite.

2. Experimental

2.1. Materials Collection

Anhydrous ferric chloride (FeCl₃) (chemical pure) was purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Ferrous chloride (FeCl₂·4H₂O) (analytical pure), Hydrochloric acid (HCl) (analytical pure), and Methylene blue (MB) (analytical pure) were obtained from Guangzhou Chemical Reagent Factory, Guangzhou, China. Sodium hydroxide (NaOH) (analytical pure) was provided by Tianjin Chemical Reagent Factory, Tianjin, China, and South China University of Technology, Guangzhou, China.

2.2. Preparation of MAG-Fe₃O₄

MAG was prepared in the laboratory according to reference [35]. The MAG–Fe₃O₄ nanocomposite was prepared by co-precipitation method. A dose of 2.32 g MAG was dispersed in 50 mL deionized water, sonicated for 10 min, then stirred for 12 h by mechanical agitator to obtain the MAG dispersion solution. The iron salts, 0.04 mol FeCl₂·4H₂O and 0.03 mol FeCl₃, were added to a three-mouth flask containing 50 mL deionized water under the presence of N₂ gas, stirred by a mechanical agitator to obtain the solution of Fe²⁺ and Fe³⁺ (n(Fe²⁺):n(Fe³⁺) = 4:3). Then, the MAG dispersion solution was added to the solution of Fe²⁺ and Fe³⁺ under vigorous stirring and stable N₂ flow, the Fe²⁺ and Fe³⁺ enter the interlayer of MAG by ion exchange with the Na⁺ between the layers of MAG. Afterward, the 20 mL of 0.4 mol/L NaOH solution was added dropwise to the mixture solution, the Fe²⁺ and Fe³⁺ reacted with OH⁻ to form Fe₃O₄, and attached to the inside and outside of the layer of MAG; the chemical reaction equation is ${\mathrm{Fe}}^{2+}+2{\mathrm{Fe}}^{3+}+8{\mathrm{OH}}^{-}={\mathrm{Fe}}_3{\mathrm{O}}_4+4{\mathrm{H}}_2\mathrm{O}$. The reaction solution was kept at 80 °C for 2 h in the presence of an N₂ gas. The precipitate was separated by a permanent magnet, washed with absolute ethanol for three times, and dried at 80 °C for 24 h in a vacuum to obtain MAG–Fe₃O₄ nanocomposites.

2.3. Instruments

In order to evaluate the adsorbents and adsorption products, X-ray diffraction (XRD) analyses patterns on the samples were performed using an AXS D8 ADVANCE X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu-Kα radiation operated at 40 kV and 40 mA. Fourier transform infrared spectroscopy (FTIR) was used to record and investigate the structure of constituents as well as chemical changes in materials in the range of 400~4000 cm⁻¹ by NEXUS 670 type FTIR (Nicolet, Waltham, MA, USA). A scanning electron microscope (SEM) was used to observe the surface morphology of the adsorbent samples by LEO 1530VP type SEM (Zeiss, Oberkochen, Germany). A vibrating sample magnetometer (VSM) was used to measure the magnetic characterization of the material by the VSM 7404 type VSM (Lakeshore, Columbus, OH, USA) at 298 K with magnetic fields up to 8000 Oe.

2.4. Characterization Study

2.4.1. Batch Adsorption Experiments

In order to analyze the adsorption performance of MAG–Fe₃O₄, batch adsorption experiments were performed to investigate the effects of influential parameters, such as solution pH, adsorbent dose, contact time, and concentration. To perform the adsorption experiments, 50 mL MB solutions with different concentrations (30, 50, 80, 100, 120, and 150 mg/L) were carried out individually in 150 mL glass bottles. The adsorption experiments were conducted at room temperature (25 °C) and the pH values were kept at 7. Different amounts of MAG–Fe₃O₄ nanocomposites (12.5, 25, 37.5, 50, 62.5 and 75 mg) were added into the MB solution, the mixture solutions were stirred at 25 °C with the contact time between 5 and 120 min, and the adsorbents (MAG-Fe₃O₄ nanocomposite) were separated from the solution by the magnetic separation technique (MST). The concentrations of MB in the supernatants were measured at 664 nm with 53WUV/VIS type ultraviolet-visible spectrophotometer (UVS) (produced by Shanghai Analytical Instrument Factory, China). The results of the removal rate were found to express as the removal efficiency (%) of the adsorbent toward MB, which was defined as:

$$\mathrm{Removal}\mathrm{efficiency},\mathsf{\eta }(\%)=\left(\frac{C_0-C_t}{C_0}\right)\times 100\%$$

(1)

where C₀ is the initial concentrations (mg/L) of adsorbates (MB), and C_t is the concentration of adsorbates (MB) at time $t$ in the adsorbents (MAG-Fe₃O₄ nanocomposite) solution (mg/L).

The adsorption capacity of MB is the concentration of MB on the adsorbent mass, and it was calculated based on the mass balance principle:

$$\mathrm{Adsorption}\mathrm{capacity},q_t\left(\mathrm{mg}/\mathrm{g}\right)=\frac{\left(C_0-C_t\right)\times V}{m}$$

(2)

where q_t is the adsorbing capacity of adsorbents (MAG–Fe₃O₄ nanocomposite) (mg/g), C_t is the adsorbates (MB) concentration at time $t$ in the adsorbents (MAG–Fe₃O₄ nanocomposite) solution (mg/L), V is the volume of the adsorption adsorbates (MB) solution (mL), and m is the mass of the adsorbents (MAG–Fe₃O₄ nanocomposite) in the solution (g).

2.4.2. Adsorption Kinetics Study

The adsorption kinetics is the study of the influence of various factors on the adsorption rate, careful monitoring of the experimental conditions which influence the speed of a chemical reaction and helps to obtain the equilibrium in a reasonable length of time. The adsorption kinetics of solutes in a solution by solid adsorbents is described by the pseudo-first-order and pseudo-second-order kinetic models [36,37].

Pseudo-First-Order Kinetic Model

The pseudo-first-order kinetic model is the adsorption kinetic equation which was applied to the liquid phase. The pseudo-first-order kinetic equation can be expressed as the following:

$$\ln \left(q_{eq}-q_t\right)=\ln q_{eq}-K_{1}t$$

(3)

where $q_{eq}$ is the amount of adsorption (mg/g) at equilibrium, $q_t$ is the amount of adsorption (mg/g) at time t, and $K_1$ is the rate constant (min⁻¹) of the pseudo-first-order kinetic equation (g/mg min).

Pseudo-Second-Order Kinetic Model

The pseudo-second-order kinetic model was assumed that the adsorption process was controlled by the chemisorption mechanism. The pseudo-second-order kinetic equation can be expressed as in the following:

$$\frac{t}{q_t}=\frac{1}{K_2{q}_{eq}^2}+\frac{t}{q_{eq}}$$

(4)

where $q_{eq}$ is the amount of adsorption (mg/g) at equilibrium, $q_t$ is the amount of adsorption (mg/g) at time t, and $K_2$ (g/mg min) is the rate constant of the pseudo-second-order kinetic equation.

2.4.3. Adsorption Isotherms

The adsorption isotherm is a curve which shows the relationship between the adsorption capacity and the initial concentration of the solution under certain temperature conditions. The effects of the adsorbent and the adsorbate can be judged by the variation of the adsorption isotherm. The commonly used adsorption isotherm models were introduced as Langmuir and Freundlich models [38]. In this study, the adsorption isotherm data were analyzed with the classical Langmuir isotherm model and Freundlich isotherm models.

Langmuir Isotherm Model

The Langmuir model, based on the adsorption kinetics and conforming to the monolayer adsorption mechanism, is deduced by a series of hypotheses and can be used to calculate the maximum adsorption capacity q_m (the theoretical saturated adsorption capacity). To ensure the equilibrium conditions, the linear form of the Langmuir isotherm model was applied to the experimental data. The Langmuir isotherm equation is given as in the following:

$$\frac{c_{eq}}{q_{eq}}=\frac{1}{K_L{q}_m}+\frac{c_{eq}}{q_m}$$

(5)

$$R_L=\frac{1}{K_L{q}_m+c_0}$$

(6)

where $c_0$ (mg/L) is the initial concentration of MB solution, $c_{eq}$ (mg/L) is the concentration of MB solution at equilibrium, $q_{eq}$ (mg/g) is the adsorption capacity at equilibrium, $q_m$ (mg/g) is the maximum adsorption capacity of Langmuir adsorption model, $R_L$ is the adsorption strength, and $K_L$ is the adsorption constants.

Freundlich Isotherm Model

The Freundlich model is a rough estimate of the affinity between adsorbent (MAG–Fe₃O₄ nanocomposite) and adsorbates (MB). The Freundlich isotherm equation can be expressed as in the following:

$$q_{eq}=K_F{C}_{eq}^{\frac{1}{n}}$$

(7)

$$\ln q_{eq}=\frac{1}{n}\ln c_{eq}+\ln K_F$$

(8)

where $c_{eq}$ (mg/L) is the concentration of the MB solution at equilibrium, $q_{eq}$ (mg/g) is the adsorption amount unit at equilibrium, 1/n and K_F are Freundlich constants that are related to adsorption intensity. The value of n is an indication of the favorability of adsorption ($n$ is called as the characteristic constant).

3. Results and Discussion

3.1. Characterization and Structure Analysis of MAG–Fe₃O₄

3.1.1. XRD Analyses

The XRD patterns of MAG, Fe₃O₄, and MAG–Fe₃O₄ are shown in Figure 1. According to Figure 1a, it was clearly that the characteristic diffraction reflections of MAG appeared at 2θ = 5.58°, 11.32°, 25.64°, 26.76°, 28.18°, and 49.57° [39]. From Figure 1b, the characteristic diffraction reflections of Fe₃O₄ appeared at 2θ = 30.04°, 35.49°, 43.09°, and 57.16° [40,41,42]. However, according to Figure 1c, the characteristic diffraction reflections of MAG–Fe₃O₄ appeared at 2θ = 5.58°, 11.32°, 25.64°, 26.76°, 28.18°, 49.57°; and at 2θ = 30.04°, 35.49°, 43.09°, 57.16°. The XRD patterns of MAG–Fe₃O₄ has contained both characteristic reflections of MAG and Fe₃O₄, indicating that the MAG–Fe₃O₄ nanocomposite was successfully synthesized by co-precipitation method.

3.1.2. FTIR Analyses

The FTIR spectrum of MAG, Fe₃O₄, and MAG–Fe₃O₄ are shown in Figure 2. The FTIR spectroscopy was used to identify the chemical groups of MAG, Fe₃O₄, and MAG–Fe₃O₄. From the FTIR spectrum of MAG and Fe₃O₄, the sorption reflection around the wave number at 3430 cm⁻¹ and 1613 cm⁻¹ were attributed to the stretching and bending vibration of the O–H bond on the surfaces of MAG and Fe₃O₄. It was found that the reflection band of sorption around the wave number at 1079 cm⁻¹ was seen due to the symmetric stretching vibration of the [SiO₄] tetrahedron in MAG [43], whereas the wave number at 785 cm⁻¹ and 619 cm⁻¹ was assigned to the double rings vibrations in MAG [43]. However, the wave number at 576 cm⁻¹ had corresponded to the stretching vibration of Fe–O in Fe₃O₄ [44,45,46]. Therefore, the reflection band of sorption around the wave number at 462 cm⁻¹ was seen from the flexural vibration of Si–O–Si in MAG. It also showed that the basic skeleton of MAG did not change during the preparation of MAG–Fe₃O₄. The reason why the Fe₃O₄ particles could be loaded on the MAG by electrostatic attraction to reduce their surface energy was that there was a high surface energy of Fe₃O₄ nanoparticles and an intrinsic charge for MAG [47].

3.1.3. SEM Images Analyses

The SEM images of MAG and MAG–Fe₃O₄ are shown in Figure 3. Figure 3a shows that the MAG was rose-patterned and petal-shaped with a smooth surface. Figure 3b shows that many nanoscale spherical Fe₃O₄ particles adhered on the interlayer and surface of MAG, which indicated that the MAG–Fe₃O₄ nanocomposites were successfully synthesized.

3.1.4. Magnetism Analyses

In order to investigate the magnetic properties of Fe₃O₄ and MAG–Fe₃O₄, the hysteresis loops were tested by VSM. It can be seen from Figure 3c,d that the saturation magnetization of Fe₃O₄ and MAG–Fe₃O₄ were 64.25 emu/g and 3.09 emu/g, respectively. It was found that the MAG–Fe₃O₄ had a certain magnetization as well as the hysteresis loop of the MAG–Fe₃O₄ had passed through the origin, indicating that the MAG–Fe₃O₄ had no remanence and coercively, and it was a typical paramagnetic material [47]. The dispersion of MAG–Fe₃O₄ in deionized water before the action of an applied magnetic field are shown in Figure 4a,b respectively. Without applying the external magnetic field, the MAG–Fe₃O₄ was dispersed uniformly in deionized water, but the solution was muddy, as shown in Figure 4a. By applying the external magnetic field, the MAG–Fe₃O₄ moved to the side of the magnetic field in the solution, and the solution became clarified. As shown in Figure 4b, MAG–Fe₃O₄ was separated easily from the aqueous solution with the magnetic field. In fact, the magnetic separation technology obviously improved the solid–liquid separation and avoided secondary pollution.

3.2. Adsorption Properties of MB on MAG–Fe₃O₄

3.2.1. Adsorption Capacity Analyses of MAG, Fe₃O₄, and MAG–Fe₃O₄

The adsorption capacity of MAG, Fe₃O₄, and MAG–Fe₃O₄ (at 25 °C, pH = 7, 100 mg/L MB solution, 1.0 g/L MAG-Fe₃O₄) are shown in Figure 4c. The adsorption capacity of MAG, Fe₃O₄, and MAG–Fe₃O₄ were measured in order to compare the adsorption properties of MB, as shown in Figure 4c. The equilibrium adsorption capacity of MAG–Fe₃O₄ (94 mg/g) was much higher than the MAG (74.7 mg/g) and the Fe₃O₄ (25.3 mg/g). The adsorption capacity of MAG–Fe₃O₄ increased by about 26% compared with MAG alone, and therefore possessed excellent adsorption properties for MB. This phenomenon was attributed to the fact that Fe₃O₄ inserted itself into the interlays of MAG, and the agglomeration of Fe₃O₄ was inhibited, meaning that more and more active sites of MAG–Fe₃O₄ could be provided than that of MAG and Fe₃O₄. Consecutively, a large number of negative charges on the surfaces of MAG–Fe₃O₄ could contribute to the binding of cationic dye MB [49].

3.2.2. Effect of Adsorbent Dosage on the Adsorption

Adsorbent dosage plays an important role in the adsorption process, because it determines the capacity of an adsorbent for a given initial concentration of the adsorbate. The experiments were carried out in a 100 mg/L MB solution at a temperature of 25 °C, and the pH value of solutions for the adsorption was kept at 7 for 60 min; however, the experiments were carried out at different concentrations of adsorbent MAG–Fe₃O₄ (0.25, 0.5, 0.75, 1, 1.25, and 1.5 g/L).

The influences of the adsorbent dosage on the adsorption performance were the adsorption capacity and removal rate, shown in Figure 4d. The removal rate of MB increased with the increasing of adsorbent dosage; however, the adsorption capacity decreased with increasing adsorbent dosage. On the one hand, with the increasing of the adsorbent dose, the adsorption active sites of the adsorbent also increased and could be adsorbed more MB, and then the removal rate of MB became higher. On the other hand, with the increasing of the adsorbent dose, the adsorption capacity of MAG–Fe₃O₄ decreased, but when the adsorbent solution was 1 g/L, the adsorption capacity of MAG–Fe₃O₄ was 93.7 mg/g, and the removal rate of MB was 96.2%.

Not only was the MAG–Fe₃O₄ tested for its magnetic separation ability, it was also important that the adsorption capacity and removal rate were reached, at 93.7 mg/g and 96.2%, respectively; this was done at 25 °C for 60 min, pH = 7, methylene blue solution of 100 mg/L, and the adsorbent dosage of 1 g/L. Thus, MAG–Fe₃O₄ was proven to be an effective adsorbent.

The definition of adsorption capacity was stated in Equation (2); it decreased in the adsorption capacity though it did not decrease in the amount of adsorption, because the total amount of MB in the solution was constant with the increasing of the dosage of MAG–Fe₃O₄ but the adsorption capacity was gradually decreased. By increasing the dosage of MAG–Fe₃O₄, the removal rate increased but the removal rate of MAG–Fe₃O₄ was more than 85% when the dosage of MAG–Fe₃O₄ was 0.25 g/L; however, it was impossible to reach a removal rate of 100%, due to the dynamic equilibrium of adsorption and desorption. Thus, the solid phase increased by 5, but the removal rate increased by only 10%.

3.2.3. Effect of the Solution pH on Adsorption

The initial pH played an important role in the surface binding sites of the adsorbents and the whole adsorption process. Figure 4e showed the removal rate of MB and the adsorption capacity of MAG–Fe₃O₄ at pH ranges from 4 to 12 at room temperature for 60 min, 100 mg/L of MB solution, and 1 g/L of MAG–Fe₃O₄. It was proven that the adsorption performance of MAG–Fe₃O₄ was affected slightly by pH values; the removal rate was between 94.8% and 96.2%.

3.2.4. Effect of Initial Concentration of MB Solutions on the Adsorption

The effect of the initial concentration of MB was studied and the results were shown in Figure 4f. To investigate the effect of the initial concentration of MB solutions on the adsorption performance, several tests were carried out at 30, 50, 80, 100, 120, and 150 mg/L. As it can be seen from Figure 4f, the initial concentration of MB was significantly affected by the adsorption capacity and removal rate. The adsorption capacity increased quickly with the increasing initial concentration of MB as well as the decreasing removal rate. In addition, when the concentration of the MB solution was low, the active sites of MAG–Fe₃O₄ were sufficient in the solution; however, the MB was almost completely adsorbed by MAG–Fe₃O₄.

3.2.5. Effect of Contact Time on the Adsorption

Equilibrium time is one of the most important parameters in the design of economical adsorption treatment system. Figure 4g shows the effect of contact time on the adsorption of MB on MAG–Fe₃O₄ at temperature 25 °C, the pH value of 7, the concentration of 100 mg/L MB solution, and the amount of 1 g/L MAG–Fe₃O₄. Figure 4g shows that the adsorption capacity and removal rate increased rapidly during the first 20 min, then the adsorption capacity increased slowly as prolonged contact time, after 60 min, the adsorption equilibrium was reached finally. This fact had also reported by other researchers [50,51,52,53,54,55].

3.2.6. Adsorption Kinetics Analyses

In order to evaluate the adsorption, the kinetic of MB on MAG–Fe₃O₄, the pseudo-first-order and pseudo-second-order adsorption kinetics model equations were adopted to fit the experimental data. The simulated curves of the kinetic equation for the MB adsorption on MAG–Fe₃O₄ are shown in Figure 5a,b and, the related fitting parameters of the pseudo-first-order model and the pseudo-second-order model are listed in

Table 1. Fitting parameters of adsorption kinetic equations.

Model	qeq (mg/g)	qeqC (mg/g)	K	R2
Pseudo-first-order	94.1	16.4976	0.04915	0.94821
Pseudo-second-order	94.1	95.4198	0.00689	0.99996

(q_eq—Adsorption capacity of adsorption equilibrium by experiment; q_eqc—Adsorption capacity of adsorption equilibrium by calculation; K—Adsorption rate constants; and R²—Correlation coefficient). It was clearly shown that the correlation coefficient (R² = 0.99996) of the pseudo-second-order kinetic model was much closer to 1 than the correlation coefficient (R² = 0.94821) of the pseudo-first-order kinetic model. In addition, the equilibrium adsorption capacity $q_{eqc}$ was calculated from the pseudo-second-order, which was closer to the experimental value. Therefore, the pseudo-second-order model was more suitable to describe the adsorption process which was indicated that the reaction rate was linear of the concentration of the two reactants (MAG–Fe₃O₄ and MB). The most important fact is that it was used to calculate the reaction rate constant (K₂ = 0.00689) of the pseudo-second-order model, which intuitively showed the speed of adsorption rate in adsorption mechanism of the MB removal. The faster adsorption rate was found while the larger value of reaction rate constant (K₂) was performed, whereas the required time for the adsorption calculation was accounted according to the reaction rate constant (K₂).

3.2.7. Adsorption Isotherm Analyses

In order to study the interactive behavior between MAG–Fe₃O₄ and MB, the Langmuir and Freundlich adsorption isothermal models were employed to simulate the process of adsorption. The linear fitting result of the adsorption isotherms of MAG–Fe₃O₄ was presented in Figure 5c,d, whereas the fitting parameters and regression coefficients (R²) of isothermal models were tabulated in

Table 2. Fitting parameters of adsorption isotherm models.

Langmuir Model			Freundlich Model
${\mathit{q}}_{\mathit{m}}(\mathbf{mg}/\mathbf{g})$	${\mathit{K}}_{\mathit{L}} (\mathbf{L}/\mathbf{mg})$	R2	n		R2
128.5347	1.591	0.971	4.6	72.057	0.95725

. As seen from Table 2, the correlation coefficients of the Langmuir model (R² = 0.971) was better than the Freundlich model (R² = 0.95725). Furthermore, the maximum adsorption capacity of the Langmuir model was 128.5 mg/g, which was closed to the experimental value of 132.7 mg/g. The concluding remarks can be referenced from another research report that the adsorption process of MB on MAG–Fe₃O₄ was consistent with monolayer adsorption [33]. It was also reported that the adsorption intensity of the Langmuir model could be expressed by $R_L$, when 0 < $R{ }_L$ < 1, indicating favorable adsorption [56,57]. Therefore, in this research experiment, the initial concentration of $c_0$ was fitted between 30 mg/L and 150 mg/L as well as the regression coefficients were considered as $R_L$ = 0.0028~0.0043, which embodied a favorable adsorption.

MAG–Fe₃O₄ was generated by treating the adsorbed MAG–Fe₃O₄ with ethanol, then the removal rate of the regenerated MAG–Fe₃O₄ was measured, and the whole process was repeated five times to acquire excellent results. Figure 6 shows that the removal rate of MB by MAG–Fe₃O₄ was over 82.84% after five times, indicating that MAG–Fe₃O₄ has perfect reusability.

The concentration of 100 mg/L MB solution and 1 g/L MAG-Fe₃O₄ were stirred at 25 °C for 60 min at a pH value of 7; the adsorption capacity of MAG–Fe₃O₄ was 93.7 mg/g while the adsorption capacity of MAG was 74.7 mg/g, the rice biomass was 8.13 mg/g [58], synthetic nano-clay magadiite (SNCM) was 20.00 mg/g [59], Zeolite was 41.26 mg/g [60], TiO₂ was 57.14 mg/g [61], and montmorillonite (MMT) was 64.43 mg/g, as shown in

Table 3. Comparison of MB adsorption capacity with other reported systems.

Adsorbents	Adsorption Capacity (mg/g)	References
Rice biomass	8.13 mg/g	[58]
SNCM	20.00 mg/g	[59]
Zeolite	41.26 mg/g	[60]
TiO2 MMT	57.14 mg/g 64.43 mg/g	[61] [33]
MAG MAG-Fe3O4	74.7 mg/g 93.7 mg/g	This work This work

. The results indicate that MAG–Fe₃O₄ is an efficient low-cost adsorbent.

4. Conclusions

It can be concluded that the MAG–Fe₃O₄ nanocomposite was successfully prepared using the co-precipitation method, and further applied to adsorb cationic organic dye MB from aqueous solutions. The magnetic nanoparticles Fe₃O₄ were deposited on the interlayer and surface of MAG. The morphology, as well as structural properties of MAG–Fe₃O₄, was characterized by XRD, FTIR, SEM, and VSM. The concentration of 100 mg/L MB solution and 1 g/L MAG–Fe₃O₄ were stirred at a temperature of 25 °C for 60 min with a pH value of 7; the adsorption capacity of MAG–Fe₃O₄ and the removal ratio of methylene blue were found as 93.7 mg/g and 96.2%, respectively. The research results showed that the adsorption performance of MAG–Fe₃O₄ was better than that of MAG and Fe₃O₄. The adsorptions behaviors of MB on MAG–Fe₃O₄ were investigated to fit well in the pseudo-second-order kinetic model with the adsorption kinetics. However, it can be concluded that the isothermal adsorption was followed by the Langmuir adsorption isotherm model, which found and illustrated that the adsorption of MAG–Fe₃O₄ nanocomposite was a monolayer adsorption.