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

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.


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. Fe 3 O 4 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 (Fe 3 O 4 ) in the aqueous solutions.
Until now, the Fe 3 O 4 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 Fe 3 O 4 -montmorillonite nanocomposite, showing good stability and reusability, was synthesized using the co-precipitation method [33] and exhibited excellent adsorption properties for Pb 2+ , Cu 2+ , and Ni 2+ [34]. In this research, the MAG-Fe 3 O 4 nanocomposite was prepared using a co-precipitation method, with Fe 3 O 4 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-Fe 3 O 4 nanocomposite.

Preparation of MAG-Fe 3 O 4
MAG was prepared in the laboratory according to reference [35]. The MAG-Fe 3 O 4 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 FeCl 2 ·4H 2 O and 0.03 mol FeCl 3 , were added to a three-mouth flask containing 50 mL deionized water under the presence of N 2 gas, stirred by a mechanical agitator to obtain the solution of Fe 2+ and Fe 3+ (n(Fe 2+ ):n(Fe 3+ ) = 4:3). Then, the MAG dispersion solution was added to the solution of Fe 2+ and Fe 3+ under vigorous stirring and stable N 2 flow, the Fe 2+ and Fe 3+ 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 Fe 2+ and Fe 3+ reacted with OH − to form Fe 3 O 4 , and attached to the inside and outside of the layer of MAG; the chemical reaction equation is Fe 2+ + 2Fe 3+ + 8OH − = Fe 3 O 4 + 4H 2 O. The reaction solution was kept at 80 • C for 2 h in the presence of an N 2 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-Fe 3 O 4 nanocomposites.

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.

Batch Adsorption Experiments
In order to analyze the adsorption performance of MAG-Fe 3 O 4 , 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-Fe 3 O 4 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-Fe 3 O 4 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: where C 0 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-Fe 3 O 4 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: where q t is the adsorbing capacity of adsorbents (MAG-

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: 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: 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.

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: 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-Fe 3 O 4 nanocomposite) and adsorbates (MB). The Freundlich isotherm equation can be expressed as in the following: 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). 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:

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

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],  4 ] 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 Fe 3 O 4 [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-Fe 3 O 4 . The reason why the Fe 3 O 4 particles could be loaded on the MAG by electrostatic attraction to reduce their surface energy was that there was a high surface energy of Fe 3 O 4 nanoparticles and an intrinsic charge for MAG [47]. 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].

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.

SEM Images Analyses
The SEM images of MAG and MAG-Fe 3 O 4 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 Fe 3 O 4 particles adhered on the interlayer and surface of MAG, which indicated that the MAG-Fe 3 O 4 nanocomposites were successfully synthesized. 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].

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.

Magnetism Analyses
In order to investigate the magnetic properties of Fe 3 O 4 and MAG-Fe 3 O 4 , the hysteresis loops were tested by VSM. It can be seen from Figure 3c,d that the saturation magnetization of Fe 3 O 4 and MAG-Fe 3 O 4 were 64.25 emu/g and 3.09 emu/g, respectively. It was found that the MAG-Fe 3 O 4 had a certain magnetization as well as the hysteresis loop of the MAG-Fe 3 O 4 had passed through the origin, indicating that the MAG-Fe 3 O 4 had no remanence and coercively, and it was a typical paramagnetic material [47]. The dispersion of MAG-Fe 3 O 4 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-Fe 3 O 4 was dispersed uniformly in deionized water, but the solution was muddy, as shown in Figure 4a. By applying the external magnetic field, the MAG-Fe 3 O 4 moved to the side of the magnetic field in the solution, and the solution became clarified. As shown in Figure 4b, MAG-Fe 3 O 4 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. 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.

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

Effect of Adsorbent Dosage on the Adsorption
Adsorbent dosage plays an important role in the adsorption process, because it determines capacity of an adsorbent for a given initial concentration of the adsorbate. The experiments w carried out in a 100 mg/L MB solution at a temperature of 25 °C, and the pH value of solutions the adsorption was kept at 7 for 60 min; however, the experiments were carried out at diffe 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 adsorp capacity and removal rate, shown in Figure 4d. The removal rate of MB increased with the increa of adsorbent dosage; however, the adsorption capacity decreased with increasing adsorbent dos On the one hand, with the increasing of the adsorbent dose, the adsorption active sites of adsorbent also increased and could be adsorbed more MB, and then the removal rate of MB bec higher. On the other hand, with the increasing of the adsorbent dose, the adsorption capacit MAG-Fe3O4 decreased, but when the adsorbent solution was 1 g/L, the adsorption capacity of MA 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 impor that the adsorption capacity and removal rate were reached, at 93.7 mg/g and 96.2%, respectiv this was done at 25 °C for 60 min, pH = 7, methylene blue solution of 100 mg/L, and the adsorb 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 adsorp capacity though it did not decrease in the amount of adsorption, because the total amount of M  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-Fe 3 O 4 decreased, but when the adsorbent solution was 1 g/L, the adsorption capacity of MAG-Fe 3 O 4 was 93.7 mg/g, and the removal rate of MB was 96.2%.
Not only was the MAG-Fe 3 O 4 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-Fe 3 O 4 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-Fe 3 O 4 but the adsorption capacity was gradually decreased. By increasing the dosage of MAG-Fe 3 O 4 , the removal rate increased but the removal rate of MAG-Fe 3 O 4 was more than 85% when the dosage of MAG-Fe 3 O 4 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%.

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-Fe 3 O 4 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-Fe 3 O 4 . It was proven that the adsorption performance of MAG-Fe 3 O 4 was affected slightly by pH values; the removal rate was between 94.8% and 96.2%.

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-Fe 3 O 4 were sufficient in the solution; however, the MB was almost completely adsorbed by MAG-Fe 3 O 4 .

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-Fe 3 O 4 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-Fe 3 O 4 . 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].

Adsorption Kinetics Analyses
In order to evaluate the adsorption, the kinetic of MB on MAG-Fe 3 O 4 , 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-Fe 3 O 4 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 (q eq -Adsorption capacity of adsorption equilibrium by experiment; q eqc -Adsorption capacity of adsorption equilibrium by calculation; K-Adsorption rate constants; and R 2 -Correlation coefficient). It was clearly shown that the correlation coefficient (R 2 = 0.99996) of the pseudo-second-order kinetic model was much closer to 1 than the correlation coefficient (R 2 = 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-Fe 3 O 4 and MB). The most important fact is that it was used to calculate the reaction rate constant (K 2 = 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 (K 2 ) was performed, whereas the required time for the adsorption calculation was accounted according to the reaction rate constant (K 2 ).

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 and 5d, whereas the fitting parameters and regression coefficients (R 2 ) of isothermal models were tabulated in Table 2. As seen from Table 2, the correlation coefficients of the Langmuir model (R 2 = 0.971) was better than the Freundlich model (R 2 = 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 , when 0 < < 1, indicating favorable adsorption [56,57]. Therefore, in this research experiment, the initial concentration of was fitted between 30 mg/L and 150 mg/L as well as the regression coefficients were considered as = 0.0028~0.0043, which embodied a favorable adsorption.

Adsorption Isotherm Analyses
In order to study the interactive behavior between MAG-Fe 3 O 4 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-Fe 3 O 4 was presented in Figure 5c,d, whereas the fitting parameters and regression coefficients (R 2 ) of isothermal models were tabulated in Table 2. As seen from Table 2, the correlation coefficients of the Langmuir model (R 2 = 0.971) was better than the Freundlich model (R 2 = 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-Fe 3 O 4 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. The results indicate that MAG-Fe3O4 is an efficient low-cost adsorbent.

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 pseudosecond-order kinetic model with the adsorption kinetics. However, it can be concluded that the The concentration of 100 mg/L MB solution and 1 g/L MAG-Fe 3 O 4 were stirred at 25 • C for 60 min at a pH value of 7; the adsorption capacity of MAG-Fe 3 O 4 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], TiO 2 was 57.14 mg/g [61], and montmorillonite (MMT) was 64.43 mg/g, as shown in Table 3. The results indicate that MAG-Fe 3 O 4 is an efficient low-cost adsorbent.

Conclusions
It can be concluded that the MAG-Fe 3 O 4 nanocomposite was successfully prepared using the co-precipitation method, and further applied to adsorb cationic organic dye MB from aqueous solutions. The magnetic nanoparticles Fe 3 O 4 were deposited on the interlayer and surface of MAG. The morphology, as well as structural properties of MAG-Fe 3 O 4 , was characterized by XRD, FTIR, SEM, and VSM. The concentration of 100 mg/L MB solution and 1 g/L MAG-Fe 3 O 4 were stirred at a temperature of 25 • C for 60 min with a pH value of 7; the adsorption capacity of MAG-Fe 3 O 4 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-Fe 3 O 4 was better than that of MAG and Fe 3 O 4 . The adsorptions behaviors of MB on MAG-Fe 3 O 4 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-Fe 3 O 4 nanocomposite was a monolayer adsorption.