# Kinetics, Isotherm, Thermodynamics, and Recyclability of Exfoliated Graphene-Decorated MnFe2O4 Nanocomposite Towards Congo Red Dye

Authors: Van Thinh Pham; Hong-Tham T. Nguyen; Thuan Van Tran; Duyen Thi Cam Nguyen; Hanh T. N. Le; Thuong Thi Nguyen; Dai-Viet N. Vo; Thi Hong Nhan Le; Duy Chinh Nguyen
Journal: Journal of Chemistry
 (2019)

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## Abstract

Herein, we described the use of exfoliated graphene- (EG-) decorated magnetic MnFe2O4 nanocomposite (EG@MnFe2O4) for the removal and adsorption of Congo red (CR) dye from wastewater. Firstly, the precursors (EG, MnFe2O4) and EG@MnFe2O4 were fabricated, characterized using several physical analytical techniques such as X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), and N2adsorption/desorption isotherm measurement. For the adsorption experiments, the effect of contact time (0–240 min), concentration (10–60 mg/L), solution pH (2–10), adsorbent dosage (0.03–0.07 g), and temperature (283–313 K) was rigorously studied. To elucidate the adsorption mechanism and behaviour of CR over EG@MnFe2O4 and MnFe2O4 adsorbents, the kinetic models (pseudo-first-order, pseudo-second-order, Elovich, and Bangham) and isotherm models (Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich) have been adopted. The kinetic results indicated that models adhered to the pseudo-second-order equation, exhibiting the chemisorption mechanism in heterogeneous phrase. Meanwhile, the isotherm results revealed the adsorption of CR over EG@MnFe2O4 obeyed the monolayer behaviour (Langmuir model) rather than multilayer behaviour (Freundlich equation) over MnFe2O4. The thermodynamic study also suggested that such adsorption was an endothermic and spontaneous process. With high maximum adsorption capacity (71.79 mg/g) and good recyclability (at least 4 times), EG@MnFe2O4 can be a potential alternative for the adsorptive removal of CR dye from water.


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## Body

## 1. Introduction

Research interest in nanomaterials (metalorganic frameworks, nanoparticles, porous nanomaterials, etc.) has become an integral part in the development of future technologies [1–4]. Among these, multiferroic nanomaterials (e.g., MFe2O4, M stands for transition metals) have afforded an abundance of widely practical applications, hence, giving rise in research interests, especially in environmental remediation [5, 6]. With their outstanding properties in inherent structure, such as excellent magnetism for easy separation, high chemical stability, and tunable production in both laboratory scale and industrial scale, many studies focused on ferrites and their modified compounds on the removal and degradation of contaminants [7–9]. Among these emergent pollutants [10–12], however, synthetic dyes (e.g., Congo red, CR) have been considered as potential carcinogenic chemicals because they can contain several toxic functional groups and nondegradable skeletons including amine, imine, and benzene rings (Scheme 1), and hence, this topic have been paid much attention over the past decades [9, 13–16].Scheme 1
Schematic process for the formation of EG@MnFe2O4 for the removal of Congo red.In terms of eliminating dye compounds, some works reported the outstanding removal efficiency using ferrite nanoparticles [17–19]. For example, Wojciech et al. investigated the adsorption of acid dye Acid Red 88 using magnetic ZnFe2O4 spinel ferrite nanoparticles. With relatively high surface area (139 m2/g), this ferroic material has given a desirable maximum adsorption capacity, at 111.1 mg/g [20]. Interestingly, Mahmoodi et al. also reported the use of sodium dodecyl sulfate (SDS) as a strongly modified agent for nickel ferrite nanoparticle (NFN) to remove a wide range of dyes including basic blue 41 (BB41), basic green 4 (BG4), and basic red 18 (BR18) with a considerable improvement in maximum adsorption capacity compared with nonmodified counterparts (control samples) [21]. These reports inspired many breakthroughs to chemically modify the ferrite structures to enhance the absorbability towards dye molecules.Generally, the ferrites can be easily modified by coatings containing diverse functional groups, which facilitate the capture of dyes. Exfoliated graphene (EG), a typically modified material synthesizing from natural graphene, can be a brilliant candidate [22]. Although EG presents as a primary adsorbent, one of the biggest drawbacks of EG material is the difficulty to separate it from the mixture after the adsorption process due to its low density towards water [23]. Moreover, EG itself can be hardly regenerated by common methods, thus, restraining its practical applications [24]. Therefore, combination between EG and ferrites may be an optimum solution aiming at taking advantage of both attractive properties.Herein, EG-decorated MnFe2O4 (namely, EG@MnFe2O4) as a promising adsorbent for the adsorption of CR as an emerging and typical dye was addressed. This material was firstly characterized using several analytical techniques such as X-ray powder diffraction (XRD), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and N2 adsorption/desorption isotherm measurement and used for kinetic and isotherms studies. Moreover, a series of parameters including contact time, dosage, solution pH, and temperature were employed to compare the adsorption between MnFe2O4 with and without EG decoration. To confirm the recyclability, EG@MnFe2O4 could be recycled for many times. To our best knowledge, its characterization and application for CR remediation was not previously addressed; hence, more investigations need to be conducted.


## 2. Materials and Methods

### 2.1. Chemicals and Instruments

Natural graphite flake (GF) was obtained from Yen Bai province, Vietnam. The material was selected to have the particle size of 60 mesh. Chemicals including Congo red, H2SO4 (98%), and H2O2 (30%) were purchased from Merck. The XRD profile was obtained using the D8 Advance Bruker powder diffractometer with Cu-Kα beams used as excitation sources. The SEM images with the magnification of 7000x were captured with the S4800 instrument (Japan) with an accelerating voltage source (15 kV). The infrared FT-IR spectra obtained by the Nicolet 6700 spectrophotometer were used to explore the characteristics of chemical bonds and functional groups. CR concentration was determined with UV-vis spectrophotometer at wavelength of 500 nm.


### 2.2. Synthesis of EG

The EG porous material was produced from the natural flaky graphite source by the microwave-irradiated method [22]. Initially, flaky graphite was carefully immersed in a mixture of H2SO4 (98%) and H2O2 (30%) (100 : 7 by volume) at room temperature during 2 hours. Next, the chemically treated solid was repeatedly washed with H2O and neutralized by diluted NaOH solution. The exfoliation of bulky powder was performed by the microwave-irradiated oven (750 W, 10 sec). The as-received EG sample can be collected and stored for the characterization and experiments.


### 2.3. Synthesis of MnFe2O4

Manganese-based magnetic nanoparticle MnFe2O4 was produced using the conventional polymerized-complex method according to our recent work [25]. Citric acid (93 g) was mixed with 140 mL of ethylene glycol and distilled water (2 : 5 by volume) and heated up 80°C under air atmosphere. Then, an amount of 0.303 g·MnCl2·6H2O solid was poured into the above mixture and heated up at 130°C. After 2 hours, the polymeric resin precursor was transferred into heat-resistant furnace and heated up at 1000°C for 2 h and allowed to cool down at room temperature. The black as-received sample can be collected and stored for the characterization and experiments.


### 2.4. Synthesis of EG@MnFe2O4

The synthesis procedure was followed as reported previously [26]. A mixture of 0.7 g Fe(NO3)3·9H2O and 0.25 g·Mn(NO3)2·6H2O was dissolved in 50 mL H2O, and heated up at 90°C under stirring continuously. After that, 50 mL citric acid solution (0.02 M) was added dropwise slowly and stirred for 60 min. 0.8 g·EG was carefully poured into such solution, and then NH3 solution was added to reach the weakly basic solution (pH 8-9). After 30 min, a slow addition of NH3 solution for the second time into the beaker (pH 10) was employed. The mixture was dried at 80 C and calcined at 700 C during 120 min to obtain as-received sample.


### 2.5. Experimental Batch

To determine the absorbability of EG towards CR, batch experiments could be conducted by an addition of adsorbent (0.5 g/L) into 100 mL of dye solutions (20–60 mg/L). The samples were employed to agitate on the shaker table. Preliminary runs indicated that the adsorption process reached an equilibrium state during 210 min. After the adsorption completion, the adsorbent was extracted from the aqueous solution using a filter syringe, while remaining concentration of dye was measured by the UV-vis spectrophotometer at 500 nm. The removal efficiency (H%) and adsorption capacity (Q) was calculated on the basis of the concentrations by the following equations:(1)H%=Co−CeCo⋅100,Qt=Co−Ctm⋅V,Qe=CoCem⋅V,where Co and Ce are, respectively, the initial and equilibrium dye concentrations, V is the volume of solution, and m represents the weight of adsorbent.


## 2.1. Chemicals and Instruments

Natural graphite flake (GF) was obtained from Yen Bai province, Vietnam. The material was selected to have the particle size of 60 mesh. Chemicals including Congo red, H2SO4 (98%), and H2O2 (30%) were purchased from Merck. The XRD profile was obtained using the D8 Advance Bruker powder diffractometer with Cu-Kα beams used as excitation sources. The SEM images with the magnification of 7000x were captured with the S4800 instrument (Japan) with an accelerating voltage source (15 kV). The infrared FT-IR spectra obtained by the Nicolet 6700 spectrophotometer were used to explore the characteristics of chemical bonds and functional groups. CR concentration was determined with UV-vis spectrophotometer at wavelength of 500 nm.


## 2.2. Synthesis of EG

The EG porous material was produced from the natural flaky graphite source by the microwave-irradiated method [22]. Initially, flaky graphite was carefully immersed in a mixture of H2SO4 (98%) and H2O2 (30%) (100 : 7 by volume) at room temperature during 2 hours. Next, the chemically treated solid was repeatedly washed with H2O and neutralized by diluted NaOH solution. The exfoliation of bulky powder was performed by the microwave-irradiated oven (750 W, 10 sec). The as-received EG sample can be collected and stored for the characterization and experiments.


## 2.3. Synthesis of MnFe2O4

Manganese-based magnetic nanoparticle MnFe2O4 was produced using the conventional polymerized-complex method according to our recent work [25]. Citric acid (93 g) was mixed with 140 mL of ethylene glycol and distilled water (2 : 5 by volume) and heated up 80°C under air atmosphere. Then, an amount of 0.303 g·MnCl2·6H2O solid was poured into the above mixture and heated up at 130°C. After 2 hours, the polymeric resin precursor was transferred into heat-resistant furnace and heated up at 1000°C for 2 h and allowed to cool down at room temperature. The black as-received sample can be collected and stored for the characterization and experiments.


## 2.4. Synthesis of EG@MnFe2O4

The synthesis procedure was followed as reported previously [26]. A mixture of 0.7 g Fe(NO3)3·9H2O and 0.25 g·Mn(NO3)2·6H2O was dissolved in 50 mL H2O, and heated up at 90°C under stirring continuously. After that, 50 mL citric acid solution (0.02 M) was added dropwise slowly and stirred for 60 min. 0.8 g·EG was carefully poured into such solution, and then NH3 solution was added to reach the weakly basic solution (pH 8-9). After 30 min, a slow addition of NH3 solution for the second time into the beaker (pH 10) was employed. The mixture was dried at 80 C and calcined at 700 C during 120 min to obtain as-received sample.


## 2.5. Experimental Batch

To determine the absorbability of EG towards CR, batch experiments could be conducted by an addition of adsorbent (0.5 g/L) into 100 mL of dye solutions (20–60 mg/L). The samples were employed to agitate on the shaker table. Preliminary runs indicated that the adsorption process reached an equilibrium state during 210 min. After the adsorption completion, the adsorbent was extracted from the aqueous solution using a filter syringe, while remaining concentration of dye was measured by the UV-vis spectrophotometer at 500 nm. The removal efficiency (H%) and adsorption capacity (Q) was calculated on the basis of the concentrations by the following equations:(1)H%=Co−CeCo⋅100,Qt=Co−Ctm⋅V,Qe=CoCem⋅V,where Co and Ce are, respectively, the initial and equilibrium dye concentrations, V is the volume of solution, and m represents the weight of adsorbent.


## 3. Results and Discussion

### 3.1. Structural Characterization

#### 3.1.1. PXRD Spectra of EG, MnFe2O4, and EG@MnFe2O4 Materials

To compare the crystallinity of EG@MnFe2O4 with their precursors including EG and MnFe2O4, the PRXD was used as a means of analysis. According to the observation from Figure 1, the profile of EG material witnessed a sharp peak at 26.6°, which was highly commensurate with previous publications, proving that EG has been successfully synthesized [27]. At a glance, the figure for MnFe2O4 had an apparent difference with that for EG mentioned. Indeed, there was an abundance of main peaks emerging at 24.4°, 34.0°, 36.7°, 50.0°, 54.5°, 62.5°, and 64.8°. Many works reported the same PXRD profiles of MnFe2O4 by various synthesis pathways (microwave-assisted ball-milling, wet-milling, solvothermal, etc.) [28–33]. The third diffraction spectrum belongs to EG@MnFe2O4, which had the mutual patterns of EG and MnFe2O4. Accordingly, a sharp peak at 26.6° again repeated at the constant position in the spectrum of EG@MnFe2O4 confirmed that the EG was successfully decorated in MnFe2O4. More interestingly, several peak traces of MnFe2O4 can be observed in the spectrum of EG@MnFe2O4. However, their signal intention seems very low, mainly because the EG may coat the peripheral shell of the MnFe2O4 nanoparticles. These results were totally in line with recent studies on the same structure [34–36].Figure 1
XRD spectra of EG, MnFe2O4, and EG@MnFe2O4.


#### 3.1.2. SEM Images of EG@MnFe2O4 Materials

To gain more understanding about the morphological properties of EG@MnFe2O4 material, the SEM technique can be used. Based on the SEM images in Figure 2, which showed at two various magnification levels (50 and 100 μm), it is evident that the EG@MnFe2O4 structure exposed a heterogeneous, highly defective, amorphous morphology. This phenomenon may be due to the effect of a full decoration by EG nanosheets, which MnFe2O4 is dispersed on the flexible graphene sheet, resulting in the typical kind of rough surface of EG@MnFe2O4. Kalimuthu also reported the same morphology of MnFe2O4/graphene produced by eco-friendly hydrothermal and in situ polymerization method, offering a deep degree of wrinkled and unsmooth surface [37].Figure 2
SEM images of EG@MnFe2O4.


#### 3.1.3. TEM Images of EG and EG@MnFe2O4 Materials

TEM technique is necessary to gain insight into inherent structure of exfoliated graphite and exfoliated graphite decorated MnFe2O4 materials. Figure 3 illustrates the SEM images of above materials at various scales 1 μm and 200 nm. Figures 3(a) and 3(b) show the highly opaque towards electron beams, and hence, implying that the EG obtained a thick structure [38]. In contrast, the structure of EG@MnFe2O4 in Figures 3(c) and 3(d) indicates a considerable difference from the EG structure. As illustrated, the existence of black spots in the opaque EG region can be due to the presence of MnFe2O4 particles, demonstrating the fact that MnFe2O4 particles were decorated by the EG sheets [39].Figure 3
TEM images of EG (a, b) and EG@MnFe2O4 (c, d) materials.


#### 3.1.4. N2 Adsorption/Desorption Isotherm Measurement of EG and EG@MnFe2O4

To characterize more properties of the inherent structure, the nitrogen adsorption/desorption isotherm of EG and EG@MnFe2O4 can be measured at 77 K and is illustrated in Figure 4(a). Generally, these isotherms mostly exhibit no hysteresis loops, representing a type II isotherm, means that both they were likely to offer a low degree of porosity. Indeed, the surface area values calculated by BET theory and pore volume of EG and EG@MnFe2O4 were relatively low, but those of EG were slightly higher than those of EG@MnFe2O4 composite, at 33.0 m2/g, 0.1299 cm3/g compared with 40.95 m2/g, and 0.1559 cm3/g, respectively. These results can be due to the effect of aggregation under magnetism of MnFe2O4, resulting in the depletion in porosity in EG@MnFe2O4 [40, 41]. Meanwhile, pore size distribution plots of both materials in Figure 4(b) also show the existence of both micropore (<2 nm) and mesopore (2–50 nm) in their structures.Figure 4
N2 adsorption desorption isotherm measurement (a) and pore size distribution (b) plots of EG and EG@MnFe2O4 materials.
(a)
(b)


#### 3.1.5. EDS Mapping Spectrum of EG@MnFe2O4

EDS mapping technique plays an important role in identifying how the components of EG@MnFe2O4 are included. Herein, Figure 5 shows the composition of elements existed in EG@MnFe2O4, which mainly consisted of carbon, iron, oxygen, and manganese. Especially, the mean content of iron in EG@MnFe2O4 can be measured, at 6.4%. In addition, the saturation magnetization value of EG@MnFe2O4 was found to be 1.5 emu/g, suggesting that EG@MnFe2O4 is possibly eligible to separate from an aqueous solution using a simple magnet [42, 43]. Consequently, the EG@MnFe2O4 structure obtained a combination of EG and MnFe2O4 components [34–36].Figure 5
EDS mapping spectrum of EG@MnFe2O4.
(a)
(b)


### 3.2. Adsorption Studies

#### 3.2.1. Effect of pH

Theoretically, the pH is one of the most influential parameters in any adsorption process, because the acidic, neutral, or basic solutions affect the charge natures (e.g., anionic, cationic, and zwitterionic) of adsorbate molecules and surface of adsorbent [44–48]. To compare the difference between MnFe2O4 and EG@MnFe2O4 materials in terms of CR adsorption efficiency, a range of pH from 2 to 12, which can be tuned by alkaline and acidic solutions, was investigated (Figure 6).Figure 6
Effect of pH on the adsorption efficiency of CR over the EG@MnFe2O4 (a) and MnFe2O4 (b) materials. Experimental conditions included adsorbent dose of 0.5 g/L, solution volume of 100 mL, and dye concentration of 60 mg/L at various pH (2–12). Experiments were run at room temperature (25°C) during 210 min.
(a)
(b)At a glance, it is evident that the adsorption uptake by EG@MnFe2O4 was remarkably higher than that by MnFe2O4 at any pH values. In detail, the highest adsorption capacity towards CR onto EG@MnFe2O4 could attain at nearly 66 mg/g under the pH condition of 6.0, while the optimal pH figure for MnFe2O4 was determined at 4.0, giving a capacity of only 35.5 mg/g. Enhancing the CR amount absorbed on EG@MnFe2O4 may be contributed by the component of EG coating, which contains functional groups essential for the adsorption. In our previous reports, we demonstrated the role of surface functional groups in improving the adsorption capacity of adsorbate [49, 50]. On the other hand, the CR adsorption of EG@MnFe2O4 by pH parameter seems to slightly drop, about 50 mg/g at the relatively weak acidic or basic media. By contrast, the adsorption of CR by MnFe2O4 at neutral or strongly basic solution was highly likely to be unconducive. These results suggested that the decoration of EG may be a considerable advantage because EG@MnFe2O4 material can obtain higher uptake at a harsher adsorption condition (e.g., at very strong basic/acidic solutions) in comparison to its precursor MnFe2O4. Based on the above results and analysis, we decided to conduct the next experiments under the pH condition at 4 and 6 for EG@MnFe2O4 and MnFe2O4 as adsorbents, respectively.


#### 3.2.2. Effect of Dosage

Optimizing the dosage of materials is of significance to boost the cost-effectiveness in any treatment process [51]. Herein, we investigated a series of dosage by adding the amount (0.03–0.07 g) of EG@MnFe2O4 (a) and MnFe2O4 into 100 mL CR solution at the initial concentration of 60 mg/L under room temperature. After that, the concentration residuals were determined by the spectroscopy method. The effect of dosage on CR adsorption capacity was plotted and is shown in Figure 7. It is evident that the adsorption uptake by EG@MnFe2O4 was remarkably higher than that by MnFe2O4 at any dosage values. Moreover, larger amount of adsorbents (0.03–0.05 g for both EG@MnFe2O4 and MnFe2O4) leaded to an enhancement in CR adsorption capacity, reached the peaks of capacity at 57 and 10 mg/g, respectively. However, the adsorption efficiency rapidly dropped down until pouring higher dosage of 0.05 g. This phenomenon may be mainly due to larger amount of adsorbents resulting in hampering the mass transfer of CR molecules into the pores of materials and changing the physical properties of solution (e.g., viscosity) [52, 53]. Consequently, the optimal dosage, which compromises all factors affecting the adsorption uptake, was found at 0.05 g.Figure 7
Effect of dosage the adsorption efficiency of CR over the EG@MnFe2O4 (a) and MnFe2O4 (b) materials. Experimental conditions included solution volume of 100 mL and dye concentration of 60 mg/L at pH 4 for MnFe2O4 and pH 6 for EG@MnFe2O4. Experiments were run at room temperature (25°C) during 210 min.
(a)
(b)


#### 3.2.3. Effect of Contact Time and Adsorption Kinetics

According to the optimized conditions obtained from Figures6 and 7, we carried out the kinetic experiments to investigate the influence of contact time on absorbability towards CR of EG@MnFe2O4 and MnFe2O4 at various concentrations (20–60 mg/L). Figure 8(a) shows the plots of the adsorption capacity (Qt, mg/g) against contact time (t, min). It is obvious that CR dye over EG@MnFe2O4 was rapidly absorbed during the first 60 minutes and steadily proceeded until the process became equilibrium. At the opposite trend, the plot in Figure 8(b) for MnFe2O4 showed a relatively gradual increase in adsorption capacity, in which adsorption at 50 mg/L gave the better adsorption results than others.Figure 8
Effect of material dosage on CR adsorption capacity onto EG@MnFe2O4 (a) and MnFe2O4 (b). Experimental conditions: solution volume of 100 mL, and dye concentrations of 20–60 mg/L at pH 4 for MnFe2O4 and pH 6 for EG@MnFe2O4. Experiments were run at room temperature (25°C) during the period of 0–240 min.
(a)
(b)To gain more insight into the profound effect of contact time, an array of commonplace kinetic equations (e.g., pseudo-first-order, pseudo-second-order, Elovich, and Bangham) were adopted and are shown in Figures9 and 10 [52, 53]. After evaluating these models based on the coefficients of determination (R2), adsorption mechanism in CR/EG@MnFe2O4 and CR/MnFe2O4 systems can be elucidated. Experimental data were transformed onto a mathematically linear form, which can be fitted by using the Origin Lab® version 9.0 software. Among the most prevalent kinetic models, the pseudo-first-order and pseudo-second-order models were applied herein. While equation (2) tends to explain the rate of adsorption relating to the number of unabsorbed sites from EG@MnFe2O4 and MnFe2O4, equation (3) describes the adsorption of CR over these magnetic nanocomposites through a chemisorption mechanism controlled by functional groups available on the surface of adsorbents [54].(2)logqe−qt=logqe−k1⋅t2.303,where k1 (1/min) is defined as the pseudo-first-order adsorption rate constant, qt (mg/g) is defined as the adsorption capacity at the period time t (min), and qe (mg/g) is defined as equilibrium adsorption capacity at the equilibrium period (min).(3)tqt=1k2·qe2+tqe,(4)H=k2⋅qe2,where k2 (g/mg min) is defined as the pseudo-second-order adsorption constant rate and H (mg/g min) is defined as initial adsorption rate (equation (4)).Figure 9
Linear plots of kinetic models: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich, and (d) Bangham for the adsorption of CR over EG@MnFe2O4.
(a)
(b)
(c)
(d)Figure 10
Kinetic plots of kinetic models: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich, and (d) Bangham for the adsorption of CR over MnFe2O4.
(a)
(b)
(c)
(d)Tables1 and 2 show the parameters of these models and their respective values at five CR concentrations (20, 30, 40, 50, and 60 mg/L) by over EG@MnFe2O4 and MnFe2O4, respectively. According to Table 1, which listed kinetic parameters of the CR adsorption models over EG@MnFe2O4, the coefficients of determination R2 for pseudo-second-order model (0.9987–0.9997) at all CR concentrations were far higher than those for pseudo-first-order model (0.8396–0.9749), indicating that the predicted data were well fitted with experimental data. This was also supported by Figures 9(a) and 9(b), which experimental data were depicted by the models. It is evident that the data points distributed well on the linear lines of pseudo-second-order model rather than the pseudo-first-order model. At the same trend for the CR adsorption models over MnFe2O4, Table 2 and Figures 10(a) and 10(b) show excellent fitness with R2 (0.8234–0.9706) better than the others (0.6957–0.9672). Therefore, the adsorption of CR over both adsorbents obeyed the pseudo-second-order model with the dominance of chemisorption process via electrostatic attraction between adsorbent and adsorbate, while the other tends to be ineligible to explain the adsorption mechanisms. Ali et al. also reported the lower fitness of pseudo-first-order model in describing the adsorption mechanism [55]. Liu et al. proved the role of the surface functional groups in enhancing the adsorbability on modified activated carbon [56]. More interestingly, based on the values of Q2, the adsorption of CR over EG@MnFe2O4 (29.61–57.54 mg/g) was observed to be so far higher than that over MnFe2O4 (6.34–18.19 mg/g).Table 1
Kinetic parameters of the CR adsorption models over EG@MnFe2O4.
Kinetic models
Parameters
Concentration (mg/L)
20
30
40
50
60
Pseudo-first-order
k
1 (min−1/(mg/L)1/n)
0.0152
0.01695
0.01519
0.01920
0.01365
Q
1 (mg/g)
6.92
10.82
15.43
24.71
18.98
R
2
0.8396
0.8380
0.9635
0.8622
0.9749
Pseudo-second-order
k
2 104 (g/(mg·min))
1.838
1.181
0.738
0.581
0.404
Q
2 (mg/g)
29.61
36.10
46.51
53.82
57.54
H = k2Q22
6.2065
6.4960
6.2657
5.914
7.471
R
2
0.9997
0.9992
0.9991
0.9974
0.9987
Elovich
β (g/mg)
0.32
0.29
0.20
0.18
0.17
α (mg/(g·min))
214.27
597.31
210.61
346.76
477.96
R
2
0.8489
0.8870
0.9128
0.9683
0.9705
Bangham
k
B (mL/(g/L))
0.0440
0.0373
0.0297
0.0289
0.0263
α
B
0.2280
0.1717
0.1974
0.1742
0.1631
R
2
0.8453
0.8578
0.8707
0.9571
0.9512Table 2
Kinetic parameters of the CR adsorption models over MnFe2O4.
Kinetic models
Parameters
Concentration (mg/L)
20
30
40
50
60
Pseudo-first-order
k
1 (min−1/(mg/L)1/n)
0.013
0.010
0.014
0.016
0.009
Q
1 (mg/g)
5.12
8.67
7.13
17.31
7.42
R
2
0.9187
0.6957
0.9672
0.7726
0.8762
Pseudo-second-order
k
2 (g/(mg·min))
0.238
0.07
0.03
0.019
0.035
Q
2 (mg/g)
6.34
9.63
10.00
18.19
10.57
H = k2Q22
9.57
6.52
2.94
6.14
3.92
R
2
0.9663
0.8234
0.9882
0.8312
0.9706
Elovich
β (g/mg)
0.82
0.58
0.57
0.32
0.48
α (mg/(g·min))
2.9
4.9
1.2
0.50
0.67
R
2
0.9592
0.8007
0.9616
0.8472
0.9558
Bangham
k
B 104 (mL/(g/L))
6.82
1.17
1.96
5.01
6.39
α
B
0.559
0.434
0.327
0.590
0.475
R
2
0.9439
0.9272
0.9901
0.9803
0.9333Otherwise, other two equations including (Elovich and Bangham) can be used to assess the adsorption kinetic of CR over EG@MnFe2O4 and MnFe2O4. In detail, the Elovich equation (equation (5)) assumes that the heterogeneous diffusion towards gases on heterogeneous surfaces or liquid/gas phase is related to the reaction rate and diffusion factor. Meanwhile, the Bangham equation (equation (6)) is typical for intraparticle diffusion mechanism of CR molecules over EG@MnFe2O4 and MnFe2O4 materials at room temperature. These equations can be described as follows:(5)Qt=1β⋅lnα⋅β+1β⋅lnt,where α (mg/g) and β (g/mg) are defined as adsorption and desorption rates of CR molecules over EG@MnFe2O4 and MnFe2O4.(6)loglogVCoVCo−Qt⋅m=logkB2.303V+αB⋅logt,where kB is defined as the Bangham equation constant and m (g) and V (mL) are defined as dosage and volume of adsorbent and solution, respectively.According to the results from Table1, all kinetic data by Elovich model fitted well with the experimental data due to their better goodness (R2 = 0.8427–0.9705) rather than those by Bangham model (R2 = 0.8453–0.9512), revealing the heterogeneous diffusion of CR over EG@MnFe2O4. Figures 9(c) and 9(d) were also eligible to support these results since the data points were distributed well on the linear lines of Elovich model. For analysing the adsorption data of CR onto MnFe2O4 via the Elovich and Bangham models, however, Figures 10(c) and 10(d) indicate the former model (0.9272–0.9901) was only better fitted with the adsorption of CR at concentrations 30–50 mg/L than the latter (0.8007–0.9616). For observation with more detail in Table 2, the CR adsorption rates (α, mg/g min) were extremely higher than CR desorption rates (β, g/mg) onto EG@MnFe2O4, while the figures for MnFe2O4 present the lower difference. This therefore follows that the adsorption of CR over EG@MnFe2O4 was more inclining to be favourable than over MnFe2O4.


#### 3.2.4. Effect of Concentration and Adsorption Isotherms

The isotherm models play a crucial role in better understanding the correlation between equilibrium concentration and adsorption capacity in liquid/solid phase at a constant temperature [57]. Several common isotherm equations including Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) can be used to investigate such relationship [58–61]. To conduct adsorption isotherm investigation, the initial concentration of CR was in the range from 20 to 60 mg/L. The plots of equilibrium adsorption capacity against equilibrium concentration are afforded to describe the mentioned models. In detail, the definition of four isotherms can be described as follows. Firstly, the Langmuir equation (equation (7)) assumes that the adsorption of CR molecules onto EG@MnFe2O4 and MnFe2O4 surface tends to reach the monolayer adsorption behaviour. This process may be caused by dynamically balancing the relative rates of adsorption/desorption without lateral interaction of CR molecules [62].(7)1Qe=1QmKL1Ce+1Qm,(8)RL=11+KLCo,where Qe (mg/g), Qm (mg/g), and Ce (mg/L) are defined as equilibrium adsorption capacity, maximum adsorption capacity, and equilibrium CR concentration, respectively. KL (L/mg) is defined as Langmuir’s constant [63]. RL is a separate parameter and can be defined by equation (8). Based on the magnitude of RL parameter, there are four kinds of adsorption processes: “RL is larger than 1.0” referring to unfavourable process, “RL is as equal as 1.0” referring to linear process, “RL is in range from 0 to 1.0” referring to favourable process, and “RL is as equal as 0” referring to irreversible process [64]. In addition, describing the multilayer adsorption behaviour is based on the Freundlich isotherm (equation (9)). This model proposes that the adsorption process occurs on heterogonous phase surfaces without any uniform distribution of heat of energies.(9)lnQe=lnKF+1nlnCe,where KF (mg/g) (L/mg) and 1/n are Freundlich’s constant and exponent of nonlinearity, respectively, which can determined from the intercept and slope of the Freundlich equation. Moreover, 1/n value shows the linearity of adsorption or the degree of curvature of the isotherms, and hence, its magnitude in the range from 0.1 to 0.5 indicates the good favourability of the adsorption of CR over EG@MnFe2O4 and MnFe2O4. One of the most useful isotherm models is the Temkin equation (equation (10)), which severs to describe the influence of indirect interactions between CR molecules and “adsorption sites” of EG@MnFe2O4 and MnFe2O4 adsorbents [65].(10)Qe=BTlnKT+BTlnCe,(11)BT=RTb,where BT, KT (L/g), and b (J/mol) are determined from slope and intercept from equations (10) and (11). R is the ideal gas constant (8.314 J/mol·K). Finally, the Dubinin–Radushkevich (D–R) isotherm (equations (12)) is used to explain the state of chemical/physical adsorption with D–R activity coefficient B (mol2/kJ2) and adsorption capacity Qm (mg/g), Polanyi potential ε (kJ/mol), and energy of adsorption E (kJ/mol), which can be calculated from equations (13) and (14):(12)lnqe=lnQm−Bε2,(13)ε=RTln1+1Ce,(14)E=1−2B.Table3 lists the parameters, values, and R2 of isotherm models for the adsorption of CR, and Figure 11 shows linear plots of isotherm models including Langmuir, Freundlich, Temkin, and D–R. It is clear that the adsorption of CR over EG@MnFe2O4 obeyed the Langmuir equation because of the highest R2 (0.9572) and experimental data well fitted on the linear, assuming that the monolayer adsorption behaviour is likely to be a dominant process [66]. Meanwhile, adsorption of CR over MnFe2O4 adhered to the Freundlich models (R2 = 0.8519), which is more inclining to occur upon multilayer adsorption behaviour. In addition, RL (0.4–0.9) and 1/n (0.39–0.63) constant values confirmed that the sorption of CR over EG@MnFe2O4 and MnFe2O4 was the favourable process. Based on the results from Table 3, the maximum adsorption capacity (Qm) calculated from Langmuir equation can be found at 71.79 and 19.57 mg/g for EG@MnFe2O4 and MnFe2O4, respectively. These results are compared with those of previous studies (Table 4), showing the higher Qm values than porous adsorbent mentioned. Therefore, EG@MnFe2O4 can be a promising candidate for the adsorption of CR in wastewater.Table 3
Isotherm parameters over EG@MnFe2O4 and MnFe2O4.
Model
Parameters
Value
EG@MnFe2O4
MnFe2O4
Langmuir
k
L (L/mg)
0.0018
0.0210
Q
m (mg/g)
71.79
19.57
R
L = 1/(1 + KL.Co)
0.90
0.44
R
2
0.9572
0.7310
Freundlich
k
F (mg/g)/(mg/L)1/n
14.956
0.885
1/n
0.39
0.63
R
2
0.9274
0.8519
Temkin
k
T (L/mg)
1.00
6.01
B
T
16.58
4.87
R
2
0.8687
0.8205
D–R
B (kJ2/mol2)
2.64
0.46
Q
m (mg/g)
50.69
11.302
E (J/mol)
435.19
103.75
R
2
0.5811
0.8483Figure 11
Linear plots of isotherm models: (a) Langmuir, (b) Freundlich, (c) Temkin, and (d) D-R for the adsorption of CR over EG@MnFe2O4 and MnFe2O4.
(a)
(b)
(c)
(d)Table 4
Compared BET surface area and maximum adsorption capacity of various materials.
No.
Adsorbents
BET surface area (m2/g)
Maximum adsorption capacity (mg/g)
Ref.
1
EG@MnFe2O4
33.0
71.79
This study
2
MnFe2O4
45.7
19.57
This study
3
Anilinepropylsilica xerogel
150
22.62
[67]
4
NaBentonite
25.7
35.84
[68]
5
Kaolin
20.28
5.44
[68]
6
Zeolite
8.31
3.77
[68]
7
Bentonite
32
40.4
[69]
8
Kaolin
168.8
11.88
[70]
9
Activated red mud
20.7
7.08
[71]
10
Activated coir pitch
—
6.72
[72]


### 3.3. Thermodynamic Study

In general, the thermodynamic equation, which is represented in equation (15), can be used to diagnose the adsorption occur spontaneously or not and to elucidate the influence of temperature on the adsorption of CR over EG@MnFe2O4 and MnFe2O4. Because the determination of KC is based on equation (16), the thermodynamic equation can be rewritten by a linear form (van’t Hoff isotherm equation) as shown in equation (17).(15)ΔG=−RTlnKC,(16)lnKC=CACe,(17)lnKC=−ΔHR⋅1T+ΔSR,where KC and T (K) are defined as the adsorption equilibrium constant and temperature, respectively; CA (mg/g) and Ce (mg/L) are the equilibrium CR concentrations in solid phase and solution phase, respectively; ΔH (kJ/mol), ΔS (kJ/mol K), and ΔG (kJ/mol) are defined as standard enthalpy and entropy and Gibb’s free energy.Figure12(a) plots the impact of temperature (283–313 K) on CR adsorption onto EG@MnFe2O4. Obviously, boosting the temperature led to a slight enhancement in the adsorption capacity. The correlation between temperature and equilibrium constant is described in Figure 11(b), which shows the plot of log (KC) against (1/T). As can be seen from Figure 12(b), experimental data were fitted well with the thermodynamic model. Moreover, high R2 in Table 5 confirms that the van’t Hoff equation obtained the excellent fitness; thus, it can be used to identify the standard thermodynamic constants (e.g., ∆H, ∆S, and ∆G). A positive ∆H reveals that the adsorption process of CR onto EG@MnFe2O4 tends to be endothermic. These results were highly in line with many previous works studying the adsorption of CR over various porous materials [73–77]. Meanwhile, positive value of ∆S shows an increase in disorder levels occurring in heterogeneous phase because of migration between aqueous solution and CR molecules during sorption [78]. Finally, the Gibbs free energy with minus values is eligible to assert that the adsorption of CR on EG@MnFe2O4 was a spontaneous process.Figure 12
Effect of temperature on adsorption of CR over EG@MnFe2O4 (a) and thermodynamic study (b). Experimental conditions included solution volume of 100 mL at pH 4 for MnFe2O4 and pH 6 for EG@MnFe2O4. Experiments were run at various temperatures (10–40°C).
(a)
(b)Table 5
Thermodynamic parameters for the adsorption of CR over EG@MnFe2O4.
van’t Hoff equation
ΔH° (kJ/mol)
ΔS° (J/mol K)
∆G283 (kJ/mol)
∆G293 (kJ/mol)
∆G303 (kJ/mol)
∆G313 (kJ/mol)
ln
K
C
=
−
1.097
1
/
T
+
4.081
R
2 = 0.9688
9.12
33.93
−0.652
−0.991
−1.331
−1.670


### 3.4. Recyclability Study

On the other hand, to assess the cost-effectiveness and practical applicability of any solid adsorbent, recyclability study needs to be investigated. Herein, we selected the best materials for recyclability performance. Therefore, EG@MnFe2O4 can be regenerated according to the following procedure. To begin with, CR-loaded EG@MnFe2O4 separated from the first run was washed with 10 mL ethanol for 3 times and then with 10 mL distilled water for another 3 times. The nanomaterial was reactivated at 105°C and then used for the next use. The number of recyclability experiments was repeated to be 5 runs. The first reuse was found to be 58.41%, which was the same percentage as the standard run (60%). However, the second and third runs witnessed the slight decrease in removal efficiency, at approximately 46 and 40%. This result was commensurate with a previous work reporting about the adsorption of Congo red from aqueous solution by zeolitic imidazolate framework-8 [79]. The percentage of CR removal for another runs was rapidly dropped down. As a result, the EG@MnFe2O4 can be totally recycled at least four times, revealing good stability and regeneration performance of EG@MnFe2O4 material in eliminating the CR dye.


### 3.5. Proposed Mechanism

It is known that the dissociation constant (pKa) of Congo red is 4.0 [80]. In addition, we measured the point of zero charge (pHpzc) of MnFe2O4 and EG@MnFe2O4 at 5.0, and 6.8, respectively. In adsorption factors, pH 6 is best condition to make the maximum removal efficiency. This can be explained based on the theory of electrostatic interaction.In fact, at pH < pKa (CR) = 4.0, the solute tends to contain more protons, and the surface of EG@MnFe2O4 also becomes more positively charged. This phenomenon appears an electrostatic repulsion between the surface of EG@MnFe2O4 and CR cations, thus resulting in a decrease in adsorption. In contrast, when the pH value is higher than pKa of CR but lower than pHpzc of EG@MnFe2O4, or 4.0 < pH < pHpzc = 6.8, CR molecules are deprotonated to transfer a form of anion while EG@MnFe2O4 surface is still positively charged due to pH < pHpzc. This results in an electrostatic attraction, leading to a considerable increase in adsorption. In this study, you can see the optimum pH at 6, which is appropriate to the above analysis. However, overcoming the pHpzc value tends to intercept the adsorption because both surfaces of EG@MnFe2O4 and CR molecules are negatively charged, causing a decline in the decontamination of CR dye. Consequently, the adsorption process is more likely to be favourable at pH varying from pKa to pHpzc.In addition, we measured the functional groups on the surface of EG@MnFe2O4 with the total acidic groups (carboxylic, lactonic, and phenolic groups) at 0.096 mmol/g and total basic groups at 0.156 mmol/g, while there was no detection of any groups on MnFe2O4 without EG decoration. In fact, the CR adsorption capacity of EG@MnFe2O4 (71.79 mg/g) was found to be higher than that of MnFe2O4 (19.57 mg/g); therefore, these groups can have an important role in improving the adsorption. In general, the surface functional groups can create a wide range of interactions such as the H-bond, π–π interaction, n–π interaction, and electrostatic force between CR molecules and adsorbate surface [81–83]. Meanwhile, the adsorption of MnFe2O4 was attributable to the weak forces such as “oxygen-metal” bridge and van der Waals [54].


## 3.1. Structural Characterization

### 3.1.1. PXRD Spectra of EG, MnFe2O4, and EG@MnFe2O4 Materials

To compare the crystallinity of EG@MnFe2O4 with their precursors including EG and MnFe2O4, the PRXD was used as a means of analysis. According to the observation from Figure 1, the profile of EG material witnessed a sharp peak at 26.6°, which was highly commensurate with previous publications, proving that EG has been successfully synthesized [27]. At a glance, the figure for MnFe2O4 had an apparent difference with that for EG mentioned. Indeed, there was an abundance of main peaks emerging at 24.4°, 34.0°, 36.7°, 50.0°, 54.5°, 62.5°, and 64.8°. Many works reported the same PXRD profiles of MnFe2O4 by various synthesis pathways (microwave-assisted ball-milling, wet-milling, solvothermal, etc.) [28–33]. The third diffraction spectrum belongs to EG@MnFe2O4, which had the mutual patterns of EG and MnFe2O4. Accordingly, a sharp peak at 26.6° again repeated at the constant position in the spectrum of EG@MnFe2O4 confirmed that the EG was successfully decorated in MnFe2O4. More interestingly, several peak traces of MnFe2O4 can be observed in the spectrum of EG@MnFe2O4. However, their signal intention seems very low, mainly because the EG may coat the peripheral shell of the MnFe2O4 nanoparticles. These results were totally in line with recent studies on the same structure [34–36].Figure 1
XRD spectra of EG, MnFe2O4, and EG@MnFe2O4.


### 3.1.2. SEM Images of EG@MnFe2O4 Materials

To gain more understanding about the morphological properties of EG@MnFe2O4 material, the SEM technique can be used. Based on the SEM images in Figure 2, which showed at two various magnification levels (50 and 100 μm), it is evident that the EG@MnFe2O4 structure exposed a heterogeneous, highly defective, amorphous morphology. This phenomenon may be due to the effect of a full decoration by EG nanosheets, which MnFe2O4 is dispersed on the flexible graphene sheet, resulting in the typical kind of rough surface of EG@MnFe2O4. Kalimuthu also reported the same morphology of MnFe2O4/graphene produced by eco-friendly hydrothermal and in situ polymerization method, offering a deep degree of wrinkled and unsmooth surface [37].Figure 2
SEM images of EG@MnFe2O4.


### 3.1.3. TEM Images of EG and EG@MnFe2O4 Materials

TEM technique is necessary to gain insight into inherent structure of exfoliated graphite and exfoliated graphite decorated MnFe2O4 materials. Figure 3 illustrates the SEM images of above materials at various scales 1 μm and 200 nm. Figures 3(a) and 3(b) show the highly opaque towards electron beams, and hence, implying that the EG obtained a thick structure [38]. In contrast, the structure of EG@MnFe2O4 in Figures 3(c) and 3(d) indicates a considerable difference from the EG structure. As illustrated, the existence of black spots in the opaque EG region can be due to the presence of MnFe2O4 particles, demonstrating the fact that MnFe2O4 particles were decorated by the EG sheets [39].Figure 3
TEM images of EG (a, b) and EG@MnFe2O4 (c, d) materials.


### 3.1.4. N2 Adsorption/Desorption Isotherm Measurement of EG and EG@MnFe2O4

To characterize more properties of the inherent structure, the nitrogen adsorption/desorption isotherm of EG and EG@MnFe2O4 can be measured at 77 K and is illustrated in Figure 4(a). Generally, these isotherms mostly exhibit no hysteresis loops, representing a type II isotherm, means that both they were likely to offer a low degree of porosity. Indeed, the surface area values calculated by BET theory and pore volume of EG and EG@MnFe2O4 were relatively low, but those of EG were slightly higher than those of EG@MnFe2O4 composite, at 33.0 m2/g, 0.1299 cm3/g compared with 40.95 m2/g, and 0.1559 cm3/g, respectively. These results can be due to the effect of aggregation under magnetism of MnFe2O4, resulting in the depletion in porosity in EG@MnFe2O4 [40, 41]. Meanwhile, pore size distribution plots of both materials in Figure 4(b) also show the existence of both micropore (<2 nm) and mesopore (2–50 nm) in their structures.Figure 4
N2 adsorption desorption isotherm measurement (a) and pore size distribution (b) plots of EG and EG@MnFe2O4 materials.
(a)
(b)


### 3.1.5. EDS Mapping Spectrum of EG@MnFe2O4

EDS mapping technique plays an important role in identifying how the components of EG@MnFe2O4 are included. Herein, Figure 5 shows the composition of elements existed in EG@MnFe2O4, which mainly consisted of carbon, iron, oxygen, and manganese. Especially, the mean content of iron in EG@MnFe2O4 can be measured, at 6.4%. In addition, the saturation magnetization value of EG@MnFe2O4 was found to be 1.5 emu/g, suggesting that EG@MnFe2O4 is possibly eligible to separate from an aqueous solution using a simple magnet [42, 43]. Consequently, the EG@MnFe2O4 structure obtained a combination of EG and MnFe2O4 components [34–36].Figure 5
EDS mapping spectrum of EG@MnFe2O4.
(a)
(b)


## 3.1.1. PXRD Spectra of EG, MnFe2O4, and EG@MnFe2O4 Materials

To compare the crystallinity of EG@MnFe2O4 with their precursors including EG and MnFe2O4, the PRXD was used as a means of analysis. According to the observation from Figure 1, the profile of EG material witnessed a sharp peak at 26.6°, which was highly commensurate with previous publications, proving that EG has been successfully synthesized [27]. At a glance, the figure for MnFe2O4 had an apparent difference with that for EG mentioned. Indeed, there was an abundance of main peaks emerging at 24.4°, 34.0°, 36.7°, 50.0°, 54.5°, 62.5°, and 64.8°. Many works reported the same PXRD profiles of MnFe2O4 by various synthesis pathways (microwave-assisted ball-milling, wet-milling, solvothermal, etc.) [28–33]. The third diffraction spectrum belongs to EG@MnFe2O4, which had the mutual patterns of EG and MnFe2O4. Accordingly, a sharp peak at 26.6° again repeated at the constant position in the spectrum of EG@MnFe2O4 confirmed that the EG was successfully decorated in MnFe2O4. More interestingly, several peak traces of MnFe2O4 can be observed in the spectrum of EG@MnFe2O4. However, their signal intention seems very low, mainly because the EG may coat the peripheral shell of the MnFe2O4 nanoparticles. These results were totally in line with recent studies on the same structure [34–36].Figure 1
XRD spectra of EG, MnFe2O4, and EG@MnFe2O4.


## 3.1.2. SEM Images of EG@MnFe2O4 Materials

To gain more understanding about the morphological properties of EG@MnFe2O4 material, the SEM technique can be used. Based on the SEM images in Figure 2, which showed at two various magnification levels (50 and 100 μm), it is evident that the EG@MnFe2O4 structure exposed a heterogeneous, highly defective, amorphous morphology. This phenomenon may be due to the effect of a full decoration by EG nanosheets, which MnFe2O4 is dispersed on the flexible graphene sheet, resulting in the typical kind of rough surface of EG@MnFe2O4. Kalimuthu also reported the same morphology of MnFe2O4/graphene produced by eco-friendly hydrothermal and in situ polymerization method, offering a deep degree of wrinkled and unsmooth surface [37].Figure 2
SEM images of EG@MnFe2O4.


## 3.1.3. TEM Images of EG and EG@MnFe2O4 Materials

TEM technique is necessary to gain insight into inherent structure of exfoliated graphite and exfoliated graphite decorated MnFe2O4 materials. Figure 3 illustrates the SEM images of above materials at various scales 1 μm and 200 nm. Figures 3(a) and 3(b) show the highly opaque towards electron beams, and hence, implying that the EG obtained a thick structure [38]. In contrast, the structure of EG@MnFe2O4 in Figures 3(c) and 3(d) indicates a considerable difference from the EG structure. As illustrated, the existence of black spots in the opaque EG region can be due to the presence of MnFe2O4 particles, demonstrating the fact that MnFe2O4 particles were decorated by the EG sheets [39].Figure 3
TEM images of EG (a, b) and EG@MnFe2O4 (c, d) materials.


## 3.1.4. N2 Adsorption/Desorption Isotherm Measurement of EG and EG@MnFe2O4

To characterize more properties of the inherent structure, the nitrogen adsorption/desorption isotherm of EG and EG@MnFe2O4 can be measured at 77 K and is illustrated in Figure 4(a). Generally, these isotherms mostly exhibit no hysteresis loops, representing a type II isotherm, means that both they were likely to offer a low degree of porosity. Indeed, the surface area values calculated by BET theory and pore volume of EG and EG@MnFe2O4 were relatively low, but those of EG were slightly higher than those of EG@MnFe2O4 composite, at 33.0 m2/g, 0.1299 cm3/g compared with 40.95 m2/g, and 0.1559 cm3/g, respectively. These results can be due to the effect of aggregation under magnetism of MnFe2O4, resulting in the depletion in porosity in EG@MnFe2O4 [40, 41]. Meanwhile, pore size distribution plots of both materials in Figure 4(b) also show the existence of both micropore (<2 nm) and mesopore (2–50 nm) in their structures.Figure 4
N2 adsorption desorption isotherm measurement (a) and pore size distribution (b) plots of EG and EG@MnFe2O4 materials.
(a)
(b)


## 3.1.5. EDS Mapping Spectrum of EG@MnFe2O4

EDS mapping technique plays an important role in identifying how the components of EG@MnFe2O4 are included. Herein, Figure 5 shows the composition of elements existed in EG@MnFe2O4, which mainly consisted of carbon, iron, oxygen, and manganese. Especially, the mean content of iron in EG@MnFe2O4 can be measured, at 6.4%. In addition, the saturation magnetization value of EG@MnFe2O4 was found to be 1.5 emu/g, suggesting that EG@MnFe2O4 is possibly eligible to separate from an aqueous solution using a simple magnet [42, 43]. Consequently, the EG@MnFe2O4 structure obtained a combination of EG and MnFe2O4 components [34–36].Figure 5
EDS mapping spectrum of EG@MnFe2O4.
(a)
(b)


## 3.2. Adsorption Studies

### 3.2.1. Effect of pH

Theoretically, the pH is one of the most influential parameters in any adsorption process, because the acidic, neutral, or basic solutions affect the charge natures (e.g., anionic, cationic, and zwitterionic) of adsorbate molecules and surface of adsorbent [44–48]. To compare the difference between MnFe2O4 and EG@MnFe2O4 materials in terms of CR adsorption efficiency, a range of pH from 2 to 12, which can be tuned by alkaline and acidic solutions, was investigated (Figure 6).Figure 6
Effect of pH on the adsorption efficiency of CR over the EG@MnFe2O4 (a) and MnFe2O4 (b) materials. Experimental conditions included adsorbent dose of 0.5 g/L, solution volume of 100 mL, and dye concentration of 60 mg/L at various pH (2–12). Experiments were run at room temperature (25°C) during 210 min.
(a)
(b)At a glance, it is evident that the adsorption uptake by EG@MnFe2O4 was remarkably higher than that by MnFe2O4 at any pH values. In detail, the highest adsorption capacity towards CR onto EG@MnFe2O4 could attain at nearly 66 mg/g under the pH condition of 6.0, while the optimal pH figure for MnFe2O4 was determined at 4.0, giving a capacity of only 35.5 mg/g. Enhancing the CR amount absorbed on EG@MnFe2O4 may be contributed by the component of EG coating, which contains functional groups essential for the adsorption. In our previous reports, we demonstrated the role of surface functional groups in improving the adsorption capacity of adsorbate [49, 50]. On the other hand, the CR adsorption of EG@MnFe2O4 by pH parameter seems to slightly drop, about 50 mg/g at the relatively weak acidic or basic media. By contrast, the adsorption of CR by MnFe2O4 at neutral or strongly basic solution was highly likely to be unconducive. These results suggested that the decoration of EG may be a considerable advantage because EG@MnFe2O4 material can obtain higher uptake at a harsher adsorption condition (e.g., at very strong basic/acidic solutions) in comparison to its precursor MnFe2O4. Based on the above results and analysis, we decided to conduct the next experiments under the pH condition at 4 and 6 for EG@MnFe2O4 and MnFe2O4 as adsorbents, respectively.


### 3.2.2. Effect of Dosage

Optimizing the dosage of materials is of significance to boost the cost-effectiveness in any treatment process [51]. Herein, we investigated a series of dosage by adding the amount (0.03–0.07 g) of EG@MnFe2O4 (a) and MnFe2O4 into 100 mL CR solution at the initial concentration of 60 mg/L under room temperature. After that, the concentration residuals were determined by the spectroscopy method. The effect of dosage on CR adsorption capacity was plotted and is shown in Figure 7. It is evident that the adsorption uptake by EG@MnFe2O4 was remarkably higher than that by MnFe2O4 at any dosage values. Moreover, larger amount of adsorbents (0.03–0.05 g for both EG@MnFe2O4 and MnFe2O4) leaded to an enhancement in CR adsorption capacity, reached the peaks of capacity at 57 and 10 mg/g, respectively. However, the adsorption efficiency rapidly dropped down until pouring higher dosage of 0.05 g. This phenomenon may be mainly due to larger amount of adsorbents resulting in hampering the mass transfer of CR molecules into the pores of materials and changing the physical properties of solution (e.g., viscosity) [52, 53]. Consequently, the optimal dosage, which compromises all factors affecting the adsorption uptake, was found at 0.05 g.Figure 7
Effect of dosage the adsorption efficiency of CR over the EG@MnFe2O4 (a) and MnFe2O4 (b) materials. Experimental conditions included solution volume of 100 mL and dye concentration of 60 mg/L at pH 4 for MnFe2O4 and pH 6 for EG@MnFe2O4. Experiments were run at room temperature (25°C) during 210 min.
(a)
(b)


### 3.2.3. Effect of Contact Time and Adsorption Kinetics

According to the optimized conditions obtained from Figures6 and 7, we carried out the kinetic experiments to investigate the influence of contact time on absorbability towards CR of EG@MnFe2O4 and MnFe2O4 at various concentrations (20–60 mg/L). Figure 8(a) shows the plots of the adsorption capacity (Qt, mg/g) against contact time (t, min). It is obvious that CR dye over EG@MnFe2O4 was rapidly absorbed during the first 60 minutes and steadily proceeded until the process became equilibrium. At the opposite trend, the plot in Figure 8(b) for MnFe2O4 showed a relatively gradual increase in adsorption capacity, in which adsorption at 50 mg/L gave the better adsorption results than others.Figure 8
Effect of material dosage on CR adsorption capacity onto EG@MnFe2O4 (a) and MnFe2O4 (b). Experimental conditions: solution volume of 100 mL, and dye concentrations of 20–60 mg/L at pH 4 for MnFe2O4 and pH 6 for EG@MnFe2O4. Experiments were run at room temperature (25°C) during the period of 0–240 min.
(a)
(b)To gain more insight into the profound effect of contact time, an array of commonplace kinetic equations (e.g., pseudo-first-order, pseudo-second-order, Elovich, and Bangham) were adopted and are shown in Figures9 and 10 [52, 53]. After evaluating these models based on the coefficients of determination (R2), adsorption mechanism in CR/EG@MnFe2O4 and CR/MnFe2O4 systems can be elucidated. Experimental data were transformed onto a mathematically linear form, which can be fitted by using the Origin Lab® version 9.0 software. Among the most prevalent kinetic models, the pseudo-first-order and pseudo-second-order models were applied herein. While equation (2) tends to explain the rate of adsorption relating to the number of unabsorbed sites from EG@MnFe2O4 and MnFe2O4, equation (3) describes the adsorption of CR over these magnetic nanocomposites through a chemisorption mechanism controlled by functional groups available on the surface of adsorbents [54].(2)logqe−qt=logqe−k1⋅t2.303,where k1 (1/min) is defined as the pseudo-first-order adsorption rate constant, qt (mg/g) is defined as the adsorption capacity at the period time t (min), and qe (mg/g) is defined as equilibrium adsorption capacity at the equilibrium period (min).(3)tqt=1k2·qe2+tqe,(4)H=k2⋅qe2,where k2 (g/mg min) is defined as the pseudo-second-order adsorption constant rate and H (mg/g min) is defined as initial adsorption rate (equation (4)).Figure 9
Linear plots of kinetic models: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich, and (d) Bangham for the adsorption of CR over EG@MnFe2O4.
(a)
(b)
(c)
(d)Figure 10
Kinetic plots of kinetic models: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich, and (d) Bangham for the adsorption of CR over MnFe2O4.
(a)
(b)
(c)
(d)Tables1 and 2 show the parameters of these models and their respective values at five CR concentrations (20, 30, 40, 50, and 60 mg/L) by over EG@MnFe2O4 and MnFe2O4, respectively. According to Table 1, which listed kinetic parameters of the CR adsorption models over EG@MnFe2O4, the coefficients of determination R2 for pseudo-second-order model (0.9987–0.9997) at all CR concentrations were far higher than those for pseudo-first-order model (0.8396–0.9749), indicating that the predicted data were well fitted with experimental data. This was also supported by Figures 9(a) and 9(b), which experimental data were depicted by the models. It is evident that the data points distributed well on the linear lines of pseudo-second-order model rather than the pseudo-first-order model. At the same trend for the CR adsorption models over MnFe2O4, Table 2 and Figures 10(a) and 10(b) show excellent fitness with R2 (0.8234–0.9706) better than the others (0.6957–0.9672). Therefore, the adsorption of CR over both adsorbents obeyed the pseudo-second-order model with the dominance of chemisorption process via electrostatic attraction between adsorbent and adsorbate, while the other tends to be ineligible to explain the adsorption mechanisms. Ali et al. also reported the lower fitness of pseudo-first-order model in describing the adsorption mechanism [55]. Liu et al. proved the role of the surface functional groups in enhancing the adsorbability on modified activated carbon [56]. More interestingly, based on the values of Q2, the adsorption of CR over EG@MnFe2O4 (29.61–57.54 mg/g) was observed to be so far higher than that over MnFe2O4 (6.34–18.19 mg/g).Table 1
Kinetic parameters of the CR adsorption models over EG@MnFe2O4.
Kinetic models
Parameters
Concentration (mg/L)
20
30
40
50
60
Pseudo-first-order
k
1 (min−1/(mg/L)1/n)
0.0152
0.01695
0.01519
0.01920
0.01365
Q
1 (mg/g)
6.92
10.82
15.43
24.71
18.98
R
2
0.8396
0.8380
0.9635
0.8622
0.9749
Pseudo-second-order
k
2 104 (g/(mg·min))
1.838
1.181
0.738
0.581
0.404
Q
2 (mg/g)
29.61
36.10
46.51
53.82
57.54
H = k2Q22
6.2065
6.4960
6.2657
5.914
7.471
R
2
0.9997
0.9992
0.9991
0.9974
0.9987
Elovich
β (g/mg)
0.32
0.29
0.20
0.18
0.17
α (mg/(g·min))
214.27
597.31
210.61
346.76
477.96
R
2
0.8489
0.8870
0.9128
0.9683
0.9705
Bangham
k
B (mL/(g/L))
0.0440
0.0373
0.0297
0.0289
0.0263
α
B
0.2280
0.1717
0.1974
0.1742
0.1631
R
2
0.8453
0.8578
0.8707
0.9571
0.9512Table 2
Kinetic parameters of the CR adsorption models over MnFe2O4.
Kinetic models
Parameters
Concentration (mg/L)
20
30
40
50
60
Pseudo-first-order
k
1 (min−1/(mg/L)1/n)
0.013
0.010
0.014
0.016
0.009
Q
1 (mg/g)
5.12
8.67
7.13
17.31
7.42
R
2
0.9187
0.6957
0.9672
0.7726
0.8762
Pseudo-second-order
k
2 (g/(mg·min))
0.238
0.07
0.03
0.019
0.035
Q
2 (mg/g)
6.34
9.63
10.00
18.19
10.57
H = k2Q22
9.57
6.52
2.94
6.14
3.92
R
2
0.9663
0.8234
0.9882
0.8312
0.9706
Elovich
β (g/mg)
0.82
0.58
0.57
0.32
0.48
α (mg/(g·min))
2.9
4.9
1.2
0.50
0.67
R
2
0.9592
0.8007
0.9616
0.8472
0.9558
Bangham
k
B 104 (mL/(g/L))
6.82
1.17
1.96
5.01
6.39
α
B
0.559
0.434
0.327
0.590
0.475
R
2
0.9439
0.9272
0.9901
0.9803
0.9333Otherwise, other two equations including (Elovich and Bangham) can be used to assess the adsorption kinetic of CR over EG@MnFe2O4 and MnFe2O4. In detail, the Elovich equation (equation (5)) assumes that the heterogeneous diffusion towards gases on heterogeneous surfaces or liquid/gas phase is related to the reaction rate and diffusion factor. Meanwhile, the Bangham equation (equation (6)) is typical for intraparticle diffusion mechanism of CR molecules over EG@MnFe2O4 and MnFe2O4 materials at room temperature. These equations can be described as follows:(5)Qt=1β⋅lnα⋅β+1β⋅lnt,where α (mg/g) and β (g/mg) are defined as adsorption and desorption rates of CR molecules over EG@MnFe2O4 and MnFe2O4.(6)loglogVCoVCo−Qt⋅m=logkB2.303V+αB⋅logt,where kB is defined as the Bangham equation constant and m (g) and V (mL) are defined as dosage and volume of adsorbent and solution, respectively.According to the results from Table1, all kinetic data by Elovich model fitted well with the experimental data due to their better goodness (R2 = 0.8427–0.9705) rather than those by Bangham model (R2 = 0.8453–0.9512), revealing the heterogeneous diffusion of CR over EG@MnFe2O4. Figures 9(c) and 9(d) were also eligible to support these results since the data points were distributed well on the linear lines of Elovich model. For analysing the adsorption data of CR onto MnFe2O4 via the Elovich and Bangham models, however, Figures 10(c) and 10(d) indicate the former model (0.9272–0.9901) was only better fitted with the adsorption of CR at concentrations 30–50 mg/L than the latter (0.8007–0.9616). For observation with more detail in Table 2, the CR adsorption rates (α, mg/g min) were extremely higher than CR desorption rates (β, g/mg) onto EG@MnFe2O4, while the figures for MnFe2O4 present the lower difference. This therefore follows that the adsorption of CR over EG@MnFe2O4 was more inclining to be favourable than over MnFe2O4.


### 3.2.4. Effect of Concentration and Adsorption Isotherms

The isotherm models play a crucial role in better understanding the correlation between equilibrium concentration and adsorption capacity in liquid/solid phase at a constant temperature [57]. Several common isotherm equations including Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) can be used to investigate such relationship [58–61]. To conduct adsorption isotherm investigation, the initial concentration of CR was in the range from 20 to 60 mg/L. The plots of equilibrium adsorption capacity against equilibrium concentration are afforded to describe the mentioned models. In detail, the definition of four isotherms can be described as follows. Firstly, the Langmuir equation (equation (7)) assumes that the adsorption of CR molecules onto EG@MnFe2O4 and MnFe2O4 surface tends to reach the monolayer adsorption behaviour. This process may be caused by dynamically balancing the relative rates of adsorption/desorption without lateral interaction of CR molecules [62].(7)1Qe=1QmKL1Ce+1Qm,(8)RL=11+KLCo,where Qe (mg/g), Qm (mg/g), and Ce (mg/L) are defined as equilibrium adsorption capacity, maximum adsorption capacity, and equilibrium CR concentration, respectively. KL (L/mg) is defined as Langmuir’s constant [63]. RL is a separate parameter and can be defined by equation (8). Based on the magnitude of RL parameter, there are four kinds of adsorption processes: “RL is larger than 1.0” referring to unfavourable process, “RL is as equal as 1.0” referring to linear process, “RL is in range from 0 to 1.0” referring to favourable process, and “RL is as equal as 0” referring to irreversible process [64]. In addition, describing the multilayer adsorption behaviour is based on the Freundlich isotherm (equation (9)). This model proposes that the adsorption process occurs on heterogonous phase surfaces without any uniform distribution of heat of energies.(9)lnQe=lnKF+1nlnCe,where KF (mg/g) (L/mg) and 1/n are Freundlich’s constant and exponent of nonlinearity, respectively, which can determined from the intercept and slope of the Freundlich equation. Moreover, 1/n value shows the linearity of adsorption or the degree of curvature of the isotherms, and hence, its magnitude in the range from 0.1 to 0.5 indicates the good favourability of the adsorption of CR over EG@MnFe2O4 and MnFe2O4. One of the most useful isotherm models is the Temkin equation (equation (10)), which severs to describe the influence of indirect interactions between CR molecules and “adsorption sites” of EG@MnFe2O4 and MnFe2O4 adsorbents [65].(10)Qe=BTlnKT+BTlnCe,(11)BT=RTb,where BT, KT (L/g), and b (J/mol) are determined from slope and intercept from equations (10) and (11). R is the ideal gas constant (8.314 J/mol·K). Finally, the Dubinin–Radushkevich (D–R) isotherm (equations (12)) is used to explain the state of chemical/physical adsorption with D–R activity coefficient B (mol2/kJ2) and adsorption capacity Qm (mg/g), Polanyi potential ε (kJ/mol), and energy of adsorption E (kJ/mol), which can be calculated from equations (13) and (14):(12)lnqe=lnQm−Bε2,(13)ε=RTln1+1Ce,(14)E=1−2B.Table3 lists the parameters, values, and R2 of isotherm models for the adsorption of CR, and Figure 11 shows linear plots of isotherm models including Langmuir, Freundlich, Temkin, and D–R. It is clear that the adsorption of CR over EG@MnFe2O4 obeyed the Langmuir equation because of the highest R2 (0.9572) and experimental data well fitted on the linear, assuming that the monolayer adsorption behaviour is likely to be a dominant process [66]. Meanwhile, adsorption of CR over MnFe2O4 adhered to the Freundlich models (R2 = 0.8519), which is more inclining to occur upon multilayer adsorption behaviour. In addition, RL (0.4–0.9) and 1/n (0.39–0.63) constant values confirmed that the sorption of CR over EG@MnFe2O4 and MnFe2O4 was the favourable process. Based on the results from Table 3, the maximum adsorption capacity (Qm) calculated from Langmuir equation can be found at 71.79 and 19.57 mg/g for EG@MnFe2O4 and MnFe2O4, respectively. These results are compared with those of previous studies (Table 4), showing the higher Qm values than porous adsorbent mentioned. Therefore, EG@MnFe2O4 can be a promising candidate for the adsorption of CR in wastewater.Table 3
Isotherm parameters over EG@MnFe2O4 and MnFe2O4.
Model
Parameters
Value
EG@MnFe2O4
MnFe2O4
Langmuir
k
L (L/mg)
0.0018
0.0210
Q
m (mg/g)
71.79
19.57
R
L = 1/(1 + KL.Co)
0.90
0.44
R
2
0.9572
0.7310
Freundlich
k
F (mg/g)/(mg/L)1/n
14.956
0.885
1/n
0.39
0.63
R
2
0.9274
0.8519
Temkin
k
T (L/mg)
1.00
6.01
B
T
16.58
4.87
R
2
0.8687
0.8205
D–R
B (kJ2/mol2)
2.64
0.46
Q
m (mg/g)
50.69
11.302
E (J/mol)
435.19
103.75
R
2
0.5811
0.8483Figure 11
Linear plots of isotherm models: (a) Langmuir, (b) Freundlich, (c) Temkin, and (d) D-R for the adsorption of CR over EG@MnFe2O4 and MnFe2O4.
(a)
(b)
(c)
(d)Table 4
Compared BET surface area and maximum adsorption capacity of various materials.
No.
Adsorbents
BET surface area (m2/g)
Maximum adsorption capacity (mg/g)
Ref.
1
EG@MnFe2O4
33.0
71.79
This study
2
MnFe2O4
45.7
19.57
This study
3
Anilinepropylsilica xerogel
150
22.62
[67]
4
NaBentonite
25.7
35.84
[68]
5
Kaolin
20.28
5.44
[68]
6
Zeolite
8.31
3.77
[68]
7
Bentonite
32
40.4
[69]
8
Kaolin
168.8
11.88
[70]
9
Activated red mud
20.7
7.08
[71]
10
Activated coir pitch
—
6.72
[72]


## 3.2.1. Effect of pH

Theoretically, the pH is one of the most influential parameters in any adsorption process, because the acidic, neutral, or basic solutions affect the charge natures (e.g., anionic, cationic, and zwitterionic) of adsorbate molecules and surface of adsorbent [44–48]. To compare the difference between MnFe2O4 and EG@MnFe2O4 materials in terms of CR adsorption efficiency, a range of pH from 2 to 12, which can be tuned by alkaline and acidic solutions, was investigated (Figure 6).Figure 6
Effect of pH on the adsorption efficiency of CR over the EG@MnFe2O4 (a) and MnFe2O4 (b) materials. Experimental conditions included adsorbent dose of 0.5 g/L, solution volume of 100 mL, and dye concentration of 60 mg/L at various pH (2–12). Experiments were run at room temperature (25°C) during 210 min.
(a)
(b)At a glance, it is evident that the adsorption uptake by EG@MnFe2O4 was remarkably higher than that by MnFe2O4 at any pH values. In detail, the highest adsorption capacity towards CR onto EG@MnFe2O4 could attain at nearly 66 mg/g under the pH condition of 6.0, while the optimal pH figure for MnFe2O4 was determined at 4.0, giving a capacity of only 35.5 mg/g. Enhancing the CR amount absorbed on EG@MnFe2O4 may be contributed by the component of EG coating, which contains functional groups essential for the adsorption. In our previous reports, we demonstrated the role of surface functional groups in improving the adsorption capacity of adsorbate [49, 50]. On the other hand, the CR adsorption of EG@MnFe2O4 by pH parameter seems to slightly drop, about 50 mg/g at the relatively weak acidic or basic media. By contrast, the adsorption of CR by MnFe2O4 at neutral or strongly basic solution was highly likely to be unconducive. These results suggested that the decoration of EG may be a considerable advantage because EG@MnFe2O4 material can obtain higher uptake at a harsher adsorption condition (e.g., at very strong basic/acidic solutions) in comparison to its precursor MnFe2O4. Based on the above results and analysis, we decided to conduct the next experiments under the pH condition at 4 and 6 for EG@MnFe2O4 and MnFe2O4 as adsorbents, respectively.


## 3.2.2. Effect of Dosage

Optimizing the dosage of materials is of significance to boost the cost-effectiveness in any treatment process [51]. Herein, we investigated a series of dosage by adding the amount (0.03–0.07 g) of EG@MnFe2O4 (a) and MnFe2O4 into 100 mL CR solution at the initial concentration of 60 mg/L under room temperature. After that, the concentration residuals were determined by the spectroscopy method. The effect of dosage on CR adsorption capacity was plotted and is shown in Figure 7. It is evident that the adsorption uptake by EG@MnFe2O4 was remarkably higher than that by MnFe2O4 at any dosage values. Moreover, larger amount of adsorbents (0.03–0.05 g for both EG@MnFe2O4 and MnFe2O4) leaded to an enhancement in CR adsorption capacity, reached the peaks of capacity at 57 and 10 mg/g, respectively. However, the adsorption efficiency rapidly dropped down until pouring higher dosage of 0.05 g. This phenomenon may be mainly due to larger amount of adsorbents resulting in hampering the mass transfer of CR molecules into the pores of materials and changing the physical properties of solution (e.g., viscosity) [52, 53]. Consequently, the optimal dosage, which compromises all factors affecting the adsorption uptake, was found at 0.05 g.Figure 7
Effect of dosage the adsorption efficiency of CR over the EG@MnFe2O4 (a) and MnFe2O4 (b) materials. Experimental conditions included solution volume of 100 mL and dye concentration of 60 mg/L at pH 4 for MnFe2O4 and pH 6 for EG@MnFe2O4. Experiments were run at room temperature (25°C) during 210 min.
(a)
(b)


## 3.2.3. Effect of Contact Time and Adsorption Kinetics

According to the optimized conditions obtained from Figures6 and 7, we carried out the kinetic experiments to investigate the influence of contact time on absorbability towards CR of EG@MnFe2O4 and MnFe2O4 at various concentrations (20–60 mg/L). Figure 8(a) shows the plots of the adsorption capacity (Qt, mg/g) against contact time (t, min). It is obvious that CR dye over EG@MnFe2O4 was rapidly absorbed during the first 60 minutes and steadily proceeded until the process became equilibrium. At the opposite trend, the plot in Figure 8(b) for MnFe2O4 showed a relatively gradual increase in adsorption capacity, in which adsorption at 50 mg/L gave the better adsorption results than others.Figure 8
Effect of material dosage on CR adsorption capacity onto EG@MnFe2O4 (a) and MnFe2O4 (b). Experimental conditions: solution volume of 100 mL, and dye concentrations of 20–60 mg/L at pH 4 for MnFe2O4 and pH 6 for EG@MnFe2O4. Experiments were run at room temperature (25°C) during the period of 0–240 min.
(a)
(b)To gain more insight into the profound effect of contact time, an array of commonplace kinetic equations (e.g., pseudo-first-order, pseudo-second-order, Elovich, and Bangham) were adopted and are shown in Figures9 and 10 [52, 53]. After evaluating these models based on the coefficients of determination (R2), adsorption mechanism in CR/EG@MnFe2O4 and CR/MnFe2O4 systems can be elucidated. Experimental data were transformed onto a mathematically linear form, which can be fitted by using the Origin Lab® version 9.0 software. Among the most prevalent kinetic models, the pseudo-first-order and pseudo-second-order models were applied herein. While equation (2) tends to explain the rate of adsorption relating to the number of unabsorbed sites from EG@MnFe2O4 and MnFe2O4, equation (3) describes the adsorption of CR over these magnetic nanocomposites through a chemisorption mechanism controlled by functional groups available on the surface of adsorbents [54].(2)logqe−qt=logqe−k1⋅t2.303,where k1 (1/min) is defined as the pseudo-first-order adsorption rate constant, qt (mg/g) is defined as the adsorption capacity at the period time t (min), and qe (mg/g) is defined as equilibrium adsorption capacity at the equilibrium period (min).(3)tqt=1k2·qe2+tqe,(4)H=k2⋅qe2,where k2 (g/mg min) is defined as the pseudo-second-order adsorption constant rate and H (mg/g min) is defined as initial adsorption rate (equation (4)).Figure 9
Linear plots of kinetic models: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich, and (d) Bangham for the adsorption of CR over EG@MnFe2O4.
(a)
(b)
(c)
(d)Figure 10
Kinetic plots of kinetic models: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich, and (d) Bangham for the adsorption of CR over MnFe2O4.
(a)
(b)
(c)
(d)Tables1 and 2 show the parameters of these models and their respective values at five CR concentrations (20, 30, 40, 50, and 60 mg/L) by over EG@MnFe2O4 and MnFe2O4, respectively. According to Table 1, which listed kinetic parameters of the CR adsorption models over EG@MnFe2O4, the coefficients of determination R2 for pseudo-second-order model (0.9987–0.9997) at all CR concentrations were far higher than those for pseudo-first-order model (0.8396–0.9749), indicating that the predicted data were well fitted with experimental data. This was also supported by Figures 9(a) and 9(b), which experimental data were depicted by the models. It is evident that the data points distributed well on the linear lines of pseudo-second-order model rather than the pseudo-first-order model. At the same trend for the CR adsorption models over MnFe2O4, Table 2 and Figures 10(a) and 10(b) show excellent fitness with R2 (0.8234–0.9706) better than the others (0.6957–0.9672). Therefore, the adsorption of CR over both adsorbents obeyed the pseudo-second-order model with the dominance of chemisorption process via electrostatic attraction between adsorbent and adsorbate, while the other tends to be ineligible to explain the adsorption mechanisms. Ali et al. also reported the lower fitness of pseudo-first-order model in describing the adsorption mechanism [55]. Liu et al. proved the role of the surface functional groups in enhancing the adsorbability on modified activated carbon [56]. More interestingly, based on the values of Q2, the adsorption of CR over EG@MnFe2O4 (29.61–57.54 mg/g) was observed to be so far higher than that over MnFe2O4 (6.34–18.19 mg/g).Table 1
Kinetic parameters of the CR adsorption models over EG@MnFe2O4.
Kinetic models
Parameters
Concentration (mg/L)
20
30
40
50
60
Pseudo-first-order
k
1 (min−1/(mg/L)1/n)
0.0152
0.01695
0.01519
0.01920
0.01365
Q
1 (mg/g)
6.92
10.82
15.43
24.71
18.98
R
2
0.8396
0.8380
0.9635
0.8622
0.9749
Pseudo-second-order
k
2 104 (g/(mg·min))
1.838
1.181
0.738
0.581
0.404
Q
2 (mg/g)
29.61
36.10
46.51
53.82
57.54
H = k2Q22
6.2065
6.4960
6.2657
5.914
7.471
R
2
0.9997
0.9992
0.9991
0.9974
0.9987
Elovich
β (g/mg)
0.32
0.29
0.20
0.18
0.17
α (mg/(g·min))
214.27
597.31
210.61
346.76
477.96
R
2
0.8489
0.8870
0.9128
0.9683
0.9705
Bangham
k
B (mL/(g/L))
0.0440
0.0373
0.0297
0.0289
0.0263
α
B
0.2280
0.1717
0.1974
0.1742
0.1631
R
2
0.8453
0.8578
0.8707
0.9571
0.9512Table 2
Kinetic parameters of the CR adsorption models over MnFe2O4.
Kinetic models
Parameters
Concentration (mg/L)
20
30
40
50
60
Pseudo-first-order
k
1 (min−1/(mg/L)1/n)
0.013
0.010
0.014
0.016
0.009
Q
1 (mg/g)
5.12
8.67
7.13
17.31
7.42
R
2
0.9187
0.6957
0.9672
0.7726
0.8762
Pseudo-second-order
k
2 (g/(mg·min))
0.238
0.07
0.03
0.019
0.035
Q
2 (mg/g)
6.34
9.63
10.00
18.19
10.57
H = k2Q22
9.57
6.52
2.94
6.14
3.92
R
2
0.9663
0.8234
0.9882
0.8312
0.9706
Elovich
β (g/mg)
0.82
0.58
0.57
0.32
0.48
α (mg/(g·min))
2.9
4.9
1.2
0.50
0.67
R
2
0.9592
0.8007
0.9616
0.8472
0.9558
Bangham
k
B 104 (mL/(g/L))
6.82
1.17
1.96
5.01
6.39
α
B
0.559
0.434
0.327
0.590
0.475
R
2
0.9439
0.9272
0.9901
0.9803
0.9333Otherwise, other two equations including (Elovich and Bangham) can be used to assess the adsorption kinetic of CR over EG@MnFe2O4 and MnFe2O4. In detail, the Elovich equation (equation (5)) assumes that the heterogeneous diffusion towards gases on heterogeneous surfaces or liquid/gas phase is related to the reaction rate and diffusion factor. Meanwhile, the Bangham equation (equation (6)) is typical for intraparticle diffusion mechanism of CR molecules over EG@MnFe2O4 and MnFe2O4 materials at room temperature. These equations can be described as follows:(5)Qt=1β⋅lnα⋅β+1β⋅lnt,where α (mg/g) and β (g/mg) are defined as adsorption and desorption rates of CR molecules over EG@MnFe2O4 and MnFe2O4.(6)loglogVCoVCo−Qt⋅m=logkB2.303V+αB⋅logt,where kB is defined as the Bangham equation constant and m (g) and V (mL) are defined as dosage and volume of adsorbent and solution, respectively.According to the results from Table1, all kinetic data by Elovich model fitted well with the experimental data due to their better goodness (R2 = 0.8427–0.9705) rather than those by Bangham model (R2 = 0.8453–0.9512), revealing the heterogeneous diffusion of CR over EG@MnFe2O4. Figures 9(c) and 9(d) were also eligible to support these results since the data points were distributed well on the linear lines of Elovich model. For analysing the adsorption data of CR onto MnFe2O4 via the Elovich and Bangham models, however, Figures 10(c) and 10(d) indicate the former model (0.9272–0.9901) was only better fitted with the adsorption of CR at concentrations 30–50 mg/L than the latter (0.8007–0.9616). For observation with more detail in Table 2, the CR adsorption rates (α, mg/g min) were extremely higher than CR desorption rates (β, g/mg) onto EG@MnFe2O4, while the figures for MnFe2O4 present the lower difference. This therefore follows that the adsorption of CR over EG@MnFe2O4 was more inclining to be favourable than over MnFe2O4.


## 3.2.4. Effect of Concentration and Adsorption Isotherms

The isotherm models play a crucial role in better understanding the correlation between equilibrium concentration and adsorption capacity in liquid/solid phase at a constant temperature [57]. Several common isotherm equations including Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) can be used to investigate such relationship [58–61]. To conduct adsorption isotherm investigation, the initial concentration of CR was in the range from 20 to 60 mg/L. The plots of equilibrium adsorption capacity against equilibrium concentration are afforded to describe the mentioned models. In detail, the definition of four isotherms can be described as follows. Firstly, the Langmuir equation (equation (7)) assumes that the adsorption of CR molecules onto EG@MnFe2O4 and MnFe2O4 surface tends to reach the monolayer adsorption behaviour. This process may be caused by dynamically balancing the relative rates of adsorption/desorption without lateral interaction of CR molecules [62].(7)1Qe=1QmKL1Ce+1Qm,(8)RL=11+KLCo,where Qe (mg/g), Qm (mg/g), and Ce (mg/L) are defined as equilibrium adsorption capacity, maximum adsorption capacity, and equilibrium CR concentration, respectively. KL (L/mg) is defined as Langmuir’s constant [63]. RL is a separate parameter and can be defined by equation (8). Based on the magnitude of RL parameter, there are four kinds of adsorption processes: “RL is larger than 1.0” referring to unfavourable process, “RL is as equal as 1.0” referring to linear process, “RL is in range from 0 to 1.0” referring to favourable process, and “RL is as equal as 0” referring to irreversible process [64]. In addition, describing the multilayer adsorption behaviour is based on the Freundlich isotherm (equation (9)). This model proposes that the adsorption process occurs on heterogonous phase surfaces without any uniform distribution of heat of energies.(9)lnQe=lnKF+1nlnCe,where KF (mg/g) (L/mg) and 1/n are Freundlich’s constant and exponent of nonlinearity, respectively, which can determined from the intercept and slope of the Freundlich equation. Moreover, 1/n value shows the linearity of adsorption or the degree of curvature of the isotherms, and hence, its magnitude in the range from 0.1 to 0.5 indicates the good favourability of the adsorption of CR over EG@MnFe2O4 and MnFe2O4. One of the most useful isotherm models is the Temkin equation (equation (10)), which severs to describe the influence of indirect interactions between CR molecules and “adsorption sites” of EG@MnFe2O4 and MnFe2O4 adsorbents [65].(10)Qe=BTlnKT+BTlnCe,(11)BT=RTb,where BT, KT (L/g), and b (J/mol) are determined from slope and intercept from equations (10) and (11). R is the ideal gas constant (8.314 J/mol·K). Finally, the Dubinin–Radushkevich (D–R) isotherm (equations (12)) is used to explain the state of chemical/physical adsorption with D–R activity coefficient B (mol2/kJ2) and adsorption capacity Qm (mg/g), Polanyi potential ε (kJ/mol), and energy of adsorption E (kJ/mol), which can be calculated from equations (13) and (14):(12)lnqe=lnQm−Bε2,(13)ε=RTln1+1Ce,(14)E=1−2B.Table3 lists the parameters, values, and R2 of isotherm models for the adsorption of CR, and Figure 11 shows linear plots of isotherm models including Langmuir, Freundlich, Temkin, and D–R. It is clear that the adsorption of CR over EG@MnFe2O4 obeyed the Langmuir equation because of the highest R2 (0.9572) and experimental data well fitted on the linear, assuming that the monolayer adsorption behaviour is likely to be a dominant process [66]. Meanwhile, adsorption of CR over MnFe2O4 adhered to the Freundlich models (R2 = 0.8519), which is more inclining to occur upon multilayer adsorption behaviour. In addition, RL (0.4–0.9) and 1/n (0.39–0.63) constant values confirmed that the sorption of CR over EG@MnFe2O4 and MnFe2O4 was the favourable process. Based on the results from Table 3, the maximum adsorption capacity (Qm) calculated from Langmuir equation can be found at 71.79 and 19.57 mg/g for EG@MnFe2O4 and MnFe2O4, respectively. These results are compared with those of previous studies (Table 4), showing the higher Qm values than porous adsorbent mentioned. Therefore, EG@MnFe2O4 can be a promising candidate for the adsorption of CR in wastewater.Table 3
Isotherm parameters over EG@MnFe2O4 and MnFe2O4.
Model
Parameters
Value
EG@MnFe2O4
MnFe2O4
Langmuir
k
L (L/mg)
0.0018
0.0210
Q
m (mg/g)
71.79
19.57
R
L = 1/(1 + KL.Co)
0.90
0.44
R
2
0.9572
0.7310
Freundlich
k
F (mg/g)/(mg/L)1/n
14.956
0.885
1/n
0.39
0.63
R
2
0.9274
0.8519
Temkin
k
T (L/mg)
1.00
6.01
B
T
16.58
4.87
R
2
0.8687
0.8205
D–R
B (kJ2/mol2)
2.64
0.46
Q
m (mg/g)
50.69
11.302
E (J/mol)
435.19
103.75
R
2
0.5811
0.8483Figure 11
Linear plots of isotherm models: (a) Langmuir, (b) Freundlich, (c) Temkin, and (d) D-R for the adsorption of CR over EG@MnFe2O4 and MnFe2O4.
(a)
(b)
(c)
(d)Table 4
Compared BET surface area and maximum adsorption capacity of various materials.
No.
Adsorbents
BET surface area (m2/g)
Maximum adsorption capacity (mg/g)
Ref.
1
EG@MnFe2O4
33.0
71.79
This study
2
MnFe2O4
45.7
19.57
This study
3
Anilinepropylsilica xerogel
150
22.62
[67]
4
NaBentonite
25.7
35.84
[68]
5
Kaolin
20.28
5.44
[68]
6
Zeolite
8.31
3.77
[68]
7
Bentonite
32
40.4
[69]
8
Kaolin
168.8
11.88
[70]
9
Activated red mud
20.7
7.08
[71]
10
Activated coir pitch
—
6.72
[72]


## 3.3. Thermodynamic Study

In general, the thermodynamic equation, which is represented in equation (15), can be used to diagnose the adsorption occur spontaneously or not and to elucidate the influence of temperature on the adsorption of CR over EG@MnFe2O4 and MnFe2O4. Because the determination of KC is based on equation (16), the thermodynamic equation can be rewritten by a linear form (van’t Hoff isotherm equation) as shown in equation (17).(15)ΔG=−RTlnKC,(16)lnKC=CACe,(17)lnKC=−ΔHR⋅1T+ΔSR,where KC and T (K) are defined as the adsorption equilibrium constant and temperature, respectively; CA (mg/g) and Ce (mg/L) are the equilibrium CR concentrations in solid phase and solution phase, respectively; ΔH (kJ/mol), ΔS (kJ/mol K), and ΔG (kJ/mol) are defined as standard enthalpy and entropy and Gibb’s free energy.Figure12(a) plots the impact of temperature (283–313 K) on CR adsorption onto EG@MnFe2O4. Obviously, boosting the temperature led to a slight enhancement in the adsorption capacity. The correlation between temperature and equilibrium constant is described in Figure 11(b), which shows the plot of log (KC) against (1/T). As can be seen from Figure 12(b), experimental data were fitted well with the thermodynamic model. Moreover, high R2 in Table 5 confirms that the van’t Hoff equation obtained the excellent fitness; thus, it can be used to identify the standard thermodynamic constants (e.g., ∆H, ∆S, and ∆G). A positive ∆H reveals that the adsorption process of CR onto EG@MnFe2O4 tends to be endothermic. These results were highly in line with many previous works studying the adsorption of CR over various porous materials [73–77]. Meanwhile, positive value of ∆S shows an increase in disorder levels occurring in heterogeneous phase because of migration between aqueous solution and CR molecules during sorption [78]. Finally, the Gibbs free energy with minus values is eligible to assert that the adsorption of CR on EG@MnFe2O4 was a spontaneous process.Figure 12
Effect of temperature on adsorption of CR over EG@MnFe2O4 (a) and thermodynamic study (b). Experimental conditions included solution volume of 100 mL at pH 4 for MnFe2O4 and pH 6 for EG@MnFe2O4. Experiments were run at various temperatures (10–40°C).
(a)
(b)Table 5
Thermodynamic parameters for the adsorption of CR over EG@MnFe2O4.
van’t Hoff equation
ΔH° (kJ/mol)
ΔS° (J/mol K)
∆G283 (kJ/mol)
∆G293 (kJ/mol)
∆G303 (kJ/mol)
∆G313 (kJ/mol)
ln
K
C
=
−
1.097
1
/
T
+
4.081
R
2 = 0.9688
9.12
33.93
−0.652
−0.991
−1.331
−1.670


## 3.4. Recyclability Study

On the other hand, to assess the cost-effectiveness and practical applicability of any solid adsorbent, recyclability study needs to be investigated. Herein, we selected the best materials for recyclability performance. Therefore, EG@MnFe2O4 can be regenerated according to the following procedure. To begin with, CR-loaded EG@MnFe2O4 separated from the first run was washed with 10 mL ethanol for 3 times and then with 10 mL distilled water for another 3 times. The nanomaterial was reactivated at 105°C and then used for the next use. The number of recyclability experiments was repeated to be 5 runs. The first reuse was found to be 58.41%, which was the same percentage as the standard run (60%). However, the second and third runs witnessed the slight decrease in removal efficiency, at approximately 46 and 40%. This result was commensurate with a previous work reporting about the adsorption of Congo red from aqueous solution by zeolitic imidazolate framework-8 [79]. The percentage of CR removal for another runs was rapidly dropped down. As a result, the EG@MnFe2O4 can be totally recycled at least four times, revealing good stability and regeneration performance of EG@MnFe2O4 material in eliminating the CR dye.


## 3.5. Proposed Mechanism

It is known that the dissociation constant (pKa) of Congo red is 4.0 [80]. In addition, we measured the point of zero charge (pHpzc) of MnFe2O4 and EG@MnFe2O4 at 5.0, and 6.8, respectively. In adsorption factors, pH 6 is best condition to make the maximum removal efficiency. This can be explained based on the theory of electrostatic interaction.In fact, at pH < pKa (CR) = 4.0, the solute tends to contain more protons, and the surface of EG@MnFe2O4 also becomes more positively charged. This phenomenon appears an electrostatic repulsion between the surface of EG@MnFe2O4 and CR cations, thus resulting in a decrease in adsorption. In contrast, when the pH value is higher than pKa of CR but lower than pHpzc of EG@MnFe2O4, or 4.0 < pH < pHpzc = 6.8, CR molecules are deprotonated to transfer a form of anion while EG@MnFe2O4 surface is still positively charged due to pH < pHpzc. This results in an electrostatic attraction, leading to a considerable increase in adsorption. In this study, you can see the optimum pH at 6, which is appropriate to the above analysis. However, overcoming the pHpzc value tends to intercept the adsorption because both surfaces of EG@MnFe2O4 and CR molecules are negatively charged, causing a decline in the decontamination of CR dye. Consequently, the adsorption process is more likely to be favourable at pH varying from pKa to pHpzc.In addition, we measured the functional groups on the surface of EG@MnFe2O4 with the total acidic groups (carboxylic, lactonic, and phenolic groups) at 0.096 mmol/g and total basic groups at 0.156 mmol/g, while there was no detection of any groups on MnFe2O4 without EG decoration. In fact, the CR adsorption capacity of EG@MnFe2O4 (71.79 mg/g) was found to be higher than that of MnFe2O4 (19.57 mg/g); therefore, these groups can have an important role in improving the adsorption. In general, the surface functional groups can create a wide range of interactions such as the H-bond, π–π interaction, n–π interaction, and electrostatic force between CR molecules and adsorbate surface [81–83]. Meanwhile, the adsorption of MnFe2O4 was attributable to the weak forces such as “oxygen-metal” bridge and van der Waals [54].


## 4. Conclusions

The present study successfully fabricated the EG, MnFe2O4, and EG@MnFe2O4 materials. The characterization results showed the EG@MnFe2O4 obtained a heterogeneous, highly defective, amorphous morphology with surface area of 33 m2/g. The adsorption results showed the equilibrium time at 240 min, optimal dosage of 0.05 g and solution pH 6 for EG@MnFe2O4 and pH 4 for MnFe2O4. Moreover, kinetic and isotherm models pointed out that the adsorption of CR over EG@MnFe2O4 at various concentrations adhered to chemisorption mechanism (pseudo-second-order) and monolayer adsorption behaviour (Langmuir equation). In addition, the thermodynamic study confirmed that the nature of adsorption is an endothermic and spontaneous process. The maximum adsorption capacity obtained from the Langmuir model for EG@MnFe2O4 was calculated to be 71.79 mg/g, which was so far higher than that of MnFe2O4 and several previous studies, indicating that EG@MnFe2O4 can be a potential adsorbent for the adsorption of CR dye in water.



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*Source: 5234585-2019-06-17.xml*