Kinetics , Isotherm , Thermodynamics , and Recyclability of Exfoliated Graphene-Decorated MnFe 2 O 4 Nanocomposite Towards Congo Red Dye

NTT Hi-Tech Institute, Nguyen Tat anh University, Ho Chi Minh City, Vietnam Center of Excellence for Green Energy and Environmental Nanomaterials, Nguyen Tat anh University, Ho Chi Minh City, Vietnam Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Ha Noi City, Vietnam Department of Pharmacy, Nguyen Tat anh University, Ho Chi Minh City, Vietnam Institute of Hygiene and Public Health, Ho Chi Minh City, Vietnam Ho Chi Minh City University of Technology, Vietnam National University-Ho Chi Minh City, Ho Chi Minh City 703500, Vietnam Center of Excellence for Functional Polymers and NanoEngineering, Nguyen Tat anh University, Ho Chi Minh City, Vietnam


Introduction
Research interest in nanomaterials (metalorganic frameworks, nanoparticles, porous nanomaterials, etc.) has become an integral part in the development of future technologies [1][2][3][4]. Among these, multiferroic nanomaterials (e.g., MFe 2 O 4 , 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][8][9]. Among these emergent pollutants [10][11][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][14][15][16].
In terms of eliminating dye compounds, some works reported the outstanding removal efficiency using ferrite nanoparticles [17][18][19]. For example, Wojciech et al. investigated the adsorption of acid dye Acid Red 88 using magnetic ZnFe 2 O 4 spinel ferrite nanoparticles. With relatively high surface area (139 m 2 /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]. ese 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]. erefore, combination between EG and ferrites may be an optimum solution aiming at taking advantage of both attractive properties.
Herein, EG-decorated MnFe 2 O 4 (namely, EG@MnFe 2 O 4 ) as a promising adsorbent for the adsorption of CR as an emerging and typical dye was addressed. is 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 N 2 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 MnFe 2 O 4 with and without EG decoration. To confirm the recyclability, EG@MnFe 2 O 4 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.

Chemicals and Instruments.
Natural graphite flake (GF) was obtained from Yen Bai province, Vietnam. e material was selected to have the particle size of 60 mesh. Chemicals including Congo red, H 2 SO 4 (98%), and H 2 O 2 (30%) were purchased from Merck. e XRD profile was obtained using the D8 Advance Bruker powder diffractometer with Cu-Kα beams used as excitation sources. e SEM images with the magnification of 7000x were captured with the S4800 instrument (Japan) with an accelerating voltage source (15 kV). e 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.

Synthesis of EG.
e EG porous material was produced from the natural flaky graphite source by the microwaveirradiated method [22]. Initially, flaky graphite was carefully immersed in a mixture of H 2 SO 4 (98%) and H 2 O 2 (30%) (100 : 7 by volume) at room temperature during 2 hours. Next, the chemically treated solid was repeatedly washed with H 2 O and neutralized by diluted NaOH solution. e exfoliation of bulky powder was performed by the microwave-irradiated oven (750 W, 10 sec). e as-received EG sample can be collected and stored for the characterization and experiments. 2 O 4 . Manganese-based magnetic nanoparticle MnFe 2 O 4 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.

Synthesis of MnFe
en, an amount of 0.303 g·MnCl 2 ·6H 2 O 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. e black as-received sample can be collected and stored for the characterization and experiments. 2 O 4 . e synthesis procedure was followed as reported previously [26]. A mixture of 0.7 g Fe(NO 3 ) 3 ·9H 2 O and 0.25 g·Mn(NO 3 ) 2 ·6H 2 O was dissolved in 50 mL H 2 O, 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 NH 3 solution was added to reach the weakly basic solution (pH 8-9). After 30 min, a slow addition of NH 3 solution for the second time into the beaker (pH 10) was employed. e mixture was dried at 80 C and calcined at 700 C during 120 min to obtain as-received sample.

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). e samples were employed to agitate on the shaker table. Preliminary runs indicated that the adsorption process reached an equilibrium state during 2 Journal of Chemistry 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. e removal efficiency (H%) and adsorption capacity (Q) was calculated on the basis of the concentrations by the following equations: where C o and C e are, respectively, the initial and equilibrium dye concentrations, V is the volume of solution, and m represents the weight of adsorbent.  O 4 Materials. To compare the crystallinity of EG@MnFe 2 O 4 with their precursors including EG and MnFe 2 O 4 , 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 MnFe 2 [28][29][30][31][32][33]. e third diffraction spectrum belongs to EG@MnFe 2 O 4 , which had the mutual patterns of EG and MnFe 2 O 4 . Accordingly, a sharp peak at 26.6°again repeated at the constant position in the spectrum of EG@MnFe 2 O 4 confirmed that the EG was successfully decorated in MnFe 2 O 4 . More interestingly, several peak traces of MnFe 2 O 4 can be observed in the spectrum of EG@ MnFe 2 O 4 . However, their signal intention seems very low, mainly because the EG may coat the peripheral shell of the MnFe 2 O 4 nanoparticles. ese results were totally in line with recent studies on the same structure [34][35][36]. 2 O 4 Materials. To gain more understanding about the morphological properties of EG@ MnFe 2 O 4 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@MnFe 2 O 4 structure exposed a heterogeneous, highly defective, amorphous morphology. is phenomenon may be due to the effect of a full decoration by EG nanosheets, which MnFe 2 O 4 is dispersed on the flexible graphene sheet, resulting in the typical kind of rough surface of EG@ MnFe 2 O 4 . Kalimuthu also reported the same morphology of MnFe 2 O 4 /graphene produced by eco-friendly hydrothermal and in situ polymerization method, offering a deep degree of wrinkled and unsmooth surface [37]. 2 O 4 Materials. TEM technique is necessary to gain insight into inherent structure of exfoliated graphite and exfoliated graphite decorated MnFe 2 O 4 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@ MnFe 2 O 4 2 O 4 . To characterize more properties of the inherent structure, the nitrogen adsorption/desorption isotherm of EG and EG@MnFe 2 O 4 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@MnFe 2 O 4 were relatively low, but those of EG were slightly higher than those of EG@MnFe 2 O 4 composite, at 33.0 m 2 /g, 0.1299 cm 3 /g compared with 40.95 m 2 /g, and 0.1559 cm 3 /g, respectively. ese results can be due to the effect of aggregation under magnetism of MnFe 2 O 4 , resulting in the depletion in porosity in EG@MnFe 2 O 4 [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. 2 O 4 . EDS mapping technique plays an important role in identifying how the components of EG@MnFe 2 O 4 are included. Herein, Figure 5 shows the composition of elements existed in EG@ MnFe 2 O 4 , which mainly consisted of carbon, iron, oxygen, and manganese. Especially, the mean content of iron in EG@ MnFe 2 O 4 can be measured, at 6.4%. In addition, the saturation magnetization value of EG@MnFe 2 O 4 was found to be 1.5 emu/g, suggesting that EG@MnFe 2 O 4 is possibly eligible to separate from an aqueous solution using a simple magnet [42,43]. Consequently, the EG@MnFe 2 O 4 structure obtained a combination of EG and MnFe 2 O 4 components [34][35][36].

Effect of pH.
eoretically, the pH is one of the most influential parameters in any adsorption process, because the acidic, neutral, or basic solutions a ect the charge natures (e.g., anionic, cationic, and zwitterionic) of adsorbate molecules and surface of adsorbent [44][45][46][47][48]. To compare the di erence between MnFe 2 O 4 and EG@MnFe 2 O 4 materials in terms of CR adsorption e ciency, a range of pH from 2 to 12, which can be tuned by alkaline and acidic solutions, was investigated ( Figure 6). At a glance, it is evident that the adsorption uptake by EG@MnFe 2 O 4 was remarkably higher than that by MnFe 2 O 4 at any pH values. In detail, the highest adsorption capacity towards CR onto EG@MnFe 2 O 4 could attain at nearly 66 mg/g under the pH condition of 6.0, while the optimal pH gure for MnFe 2 O 4 was determined at 4.0, giving a capacity of only 35.5 mg/g. Enhancing the CR amount absorbed on EG@MnFe 2 O 4 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@MnFe 2 O 4 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 MnFe 2 O 4 at neutral or strongly basic solution was highly likely to be unconducive. ese results suggested that the decoration of EG may be a considerable advantage because EG@MnFe 2 O 4 material can obtain higher uptake at a harsher adsorption condition (e.g., at very strong basic/ acidic solutions) in comparison to its precursor MnFe 2 O 4 . Based on the above results and analysis, we decided to conduct the next experiments under the pH condition at 4 and 6 for EG@MnFe 2 O 4 and MnFe 2 O 4 as adsorbents, respectively.

E ect of Dosage.
Optimizing the dosage of materials is of signi cance to boost the cost-e ectiveness in any treatment process [51]. Herein, we investigated a series of dosage by adding the amount (0.03-0.07 g) of EG@MnFe 2 O 4 (a) and MnFe 2 O 4 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. e e ect of dosage on CR adsorption capacity was plotted and is shown in Figure 7. It is evident that the adsorption uptake by EG@MnFe 2 O 4 was     O 4 ) leaded to an enhancement in CR adsorption capacity, reached the peaks of capacity at 57 and 10 mg/g, respectively. However, the adsorption e ciency rapidly dropped down until pouring higher dosage of 0.05 g. is 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 a ecting the adsorption uptake, was found at 0.05 g.

E ect of Contact Time and Adsorption Kinetics.
According to the optimized conditions obtained from  Figure 8(a) shows the plots of the adsorption capacity (Q t , mg/g) against contact time (t, min). It is obvious that CR dye over EG@MnFe 2 O 4 was rapidly absorbed during the rst 60 minutes and steadily proceeded until the process became equilibrium. At the opposite trend, the plot in Figure 8(b) for MnFe 2 O 4 showed a relatively gradual increase in adsorption capacity, in which adsorption at 50 mg/L gave the better adsorption results than others.   Journal of Chemistry To gain more insight into the profound e ect of contact time, an array of commonplace kinetic equations (e.g., pseudorst-order, pseudo-second-order, Elovich, and Bangham) were adopted and are shown in Figures 9 and 10 [52,53]. After evaluating these models based on the coe cients of determination (R 2 ), adsorption mechanism in CR/EG@ MnFe 2 O 4 and CR/MnFe 2 O 4 systems can be elucidated. Experimental data were transformed onto a mathematically linear form, which can be tted by using the Origin Lab ® version 9.0 software. Among the most prevalent kinetic models, the pseudo-rst-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@MnFe 2 O 4 and MnFe 2 O 4 , 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].
where k 1 (1/min) is de ned as the pseudo-rst-order adsorption rate constant, q t (mg/g) is de ned as the adsorption capacity at the period time t (min), and q e (mg/g) is de ned as equilibrium adsorption capacity at the equilibrium period (min).
where k 2 (g/mg min) is de ned as the pseudo-second-order adsorption constant rate and H (mg/g min) is de ned as initial adsorption rate (equation (4)).
Tables 1 and 2 show the parameters of these models and their respective values at ve CR concentrations (20,30,40,50, and 60 mg/L) by over EG@MnFe 2 O 4 and MnFe 2 O 4 , respectively. According to Table 1, which listed kinetic parameters of the CR adsorption models over EG@ MnFe 2 O 4 , the coe cients of determination R 2 for pseudosecond-order model (0.9987-0.9997) at all CR concentrations were far higher than those for pseudo-rst-order model (0.8396-0.9749), indicating that the predicted data were well tted with experimental data. is 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-rst-order model. At the same trend for the CR adsorption models over MnFe 2 O 4 , Table 2 and Figures 10(a) and 10(b) show excellent tness with R 2 (0.8234-0.9706) better than the others (0.6957-0.9672). erefore, 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 tness of pseudo-rst-order model in describing the adsorption mechanism [55]. Liu et al. proved the role of the surface functional groups in enhancing the adsorbability on modi ed activated carbon [56]. More interestingly, based on the values of Q 2 , the adsorption of CR over EG@MnFe 2 O 4 (29.61-57.54 mg/g) was observed to be so far higher than that over MnFe 2 O 4 (6.34-18.19 mg/g).
Otherwise, other two equations including (Elovich and Bangham) can be used to assess the adsorption kinetic of CR over EG@MnFe 2 O 4 and MnFe 2 O 4 . In detail, the Elovich equation (equation (5)) assumes that the heterogeneous di usion towards gases on heterogeneous surfaces or liquid/ gas phase is related to the reaction rate and di usion factor. Meanwhile, the Bangham equation (equation (6)) is typical for intraparticle di usion mechanism of CR molecules over EG@MnFe 2 O 4 and MnFe 2 O 4 materials at room temperature. ese equations can be described as follows: where α (mg/g) and β (g/mg) are de ned as adsorption and desorption rates of CR molecules over EG@MnFe 2 O 4 and MnFe 2 O 4 .
where k B is de ned as the Bangham equation constant and m (g) and V (mL) are de ned as dosage and volume of adsorbent and solution, respectively.
According to the results from Table 1, all kinetic data by Elovich model tted well with the experimental data due to their better goodness (R 2 0.8427-0.9705) rather than those by Bangham model (R 2 0.8453-0.9512), revealing the heterogeneous di usion of CR over EG@MnFe 2 O 4 .  Table 2, the CR adsorption rates (α, mg/g min) were extremely higher than CR desorption rates (β, g/mg) onto EG@MnFe 2 O 4 , while the gures for MnFe 2 O 4 present the lower di erence. is therefore follows that the adsorption of CR over EG@MnFe 2 O 4 was more inclining to be favourable than over MnFe 2 O 4 .   Journal of Chemistry

E ect of Concentration and Adsorption
Isotherms. e 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][59][60][61]. To conduct adsorption isotherm investigation, the initial concentration of CR was in the range from 20 to 60 mg/L. e plots of equilibrium   (7)) assumes that the adsorption of CR molecules onto EG@MnFe 2 O 4 and MnFe 2 O 4 surface tends to reach the monolayer adsorption behaviour. is process may be caused by dynamically balancing the relative rates of adsorption/desorption without lateral interaction of CR molecules [62].
where Q e (mg/g), Q m (mg/g), and C e (mg/L) are defined as equilibrium adsorption capacity, maximum adsorption capacity, and equilibrium CR concentration, respectively. K L (L/mg) is defined as Langmuir's constant [63]. R L is a separate parameter and can be defined by equation (8).
Based on the magnitude of R L parameter, there are four kinds of adsorption processes: "R L is larger than 1.0" referring to unfavourable process, "R L is as equal as 1.0" referring to linear process, "R L is in range from 0 to 1.0" referring to favourable process, and "R L 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)). is model proposes that the adsorption process occurs on heterogonous phase surfaces without any uniform distribution of heat of energies.
where K F (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@ MnFe 2 O 4 and MnFe 2 O 4 . 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@MnFe 2 O 4 and MnFe 2 O 4 adsorbents [65].
where B T , K T (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 (mol 2 /kJ 2 ) and adsorption capacity Q m (mg/g), Polanyi potential ε (kJ/mol), and energy of adsorption E (kJ/mol), which can be calculated from equations (13) and (14): Table 3 lists the parameters, values, and R 2 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@MnFe 2 O 4 obeyed the Langmuir equation because of the highest R 2 (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 MnFe 2 O 4 adhered to the Freundlich models (R 2 � 0.8519), which is more inclining to occur upon multilayer adsorption behaviour. In addition, R L (0.4-0.9) and 1/ n (0.39-0.63) constant values confirmed that the sorption of   the higher Q m values than porous adsorbent mentioned. erefore, EG@MnFe 2 O 4 can be a promising candidate for the adsorption of CR in wastewater.

ermodynamic 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@MnFe 2 O 4 and MnFe 2 O 4 . Because the determination of K C 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).
where K C and T (K) are defined as the adsorption equilibrium constant and temperature, respectively; C A (mg/g) and C e (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. Figure 12(a) plots the impact of temperature (283-313 K) on CR adsorption onto EG@MnFe 2 O 4 . Obviously, boosting the temperature led to a slight enhancement in the adsorption capacity. e correlation between temperature and equilibrium constant is described in Figure 11(b), which shows the plot of log (K C ) against (1/T). As can be seen from Figure 12(b), experimental data were fitted well with the thermodynamic model. Moreover, high R 2 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@ MnFe 2 O 4 tends to be endothermic. ese results were highly in line with many previous works studying the adsorption of CR over various porous materials [73][74][75][76][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@MnFe 2 O 4 was a spontaneous process.

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. erefore, EG@MnFe 2 O 4 can be regenerated according to the following procedure. To begin with, CRloaded EG@MnFe 2 O 4 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. e nanomaterial was reactivated at 105°C and then used for the next use. e number of recyclability experiments was repeated to be 5 runs. e 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%. is result was commensurate with a previous work reporting about the adsorption of Congo red from aqueous solution by zeolitic imidazolate framework-8 [79]. e percentage of CR removal for another runs was rapidly dropped down. As a result, the EG@MnFe 2 O 4 can be totally recycled at least four times, revealing good stability and regeneration performance of EG@MnFe 2 O 4 material in eliminating the CR dye.

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 MnFe 2 O 4 and EG@MnFe 2 O 4 at 5.0, and 6.8, respectively. In adsorption factors, pH 6 is best condition to make the maximum removal efficiency. is 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@MnFe 2 O 4 also becomes more positively charged.
is phenomenon appears an electrostatic repulsion between the surface of EG@ MnFe 2 O 4 and CR cations, thus resulting in a decrease in adsorption. In contrast, when the pH value is higher than pK a of CR but lower than pH pzc of EG@MnFe 2 O 4 , or 4.0 < pH < pH pzc � 6.8, CR molecules are deprotonated to transfer a form of anion while EG@MnFe 2 O 4 surface is still positively charged due to pH < pH pzc .
is results in an electrostatic attraction, leading to a considerable increase in Activated coir pitch -6.72 [72] adsorption. In this study, you can see the optimum pH at 6, which is appropriate to the above analysis. However, overcoming the pH pzc value tends to intercept the adsorption because both surfaces of EG@MnFe 2 O 4 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 pK a to pH pzc . In addition, we measured the functional groups on the surface of EG@MnFe 2 O 4 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 MnFe 2 O 4 without EG decoration. In fact, the CR adsorption capacity of EG@MnFe 2 O 4 (71.79 mg/g) was found to be higher than that of MnFe 2 O 4 (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][82][83]. Meanwhile, the adsorption of MnFe 2 O 4 was attributable to the weak forces such as "oxygen-metal" bridge and van der Waals [54].

Conclusions
e present study successfully fabricated the EG, MnFe 2 O 4 , and EG@MnFe 2 O 4 materials. e characterization results showed the EG@MnFe 2 O 4 obtained a heterogeneous, highly defective, amorphous morphology with surface area of 33 m 2 /g. e adsorption results showed the equilibrium time at 240 min, optimal dosage of 0.05 g and solution pH 6 for EG@ MnFe 2 O 4 and pH 4 for MnFe 2 O 4 . Moreover, kinetic and isotherm models pointed out that the adsorption of CR over EG@MnFe 2 O 4 at various concentrations adhered to chemisorption mechanism (pseudo-second-order) and monolayer adsorption behaviour (Langmuir equation). In addition, the thermodynamic study con rmed that the nature of adsorption is an endothermic and spontaneous process. e maximum adsorption capacity obtained from the Langmuir model for EG@MnFe 2 O 4 was calculated to be 71.79 mg/g, which was so far higher than that of MnFe 2 O 4 and several previous studies, indicating that EG@MnFe 2 O 4 can be a potential adsorbent for the adsorption of CR dye in water.
Data Availability e data used to support the ndings of this study are available from the corresponding author upon request.    Figure 12: E ect of temperature on adsorption of CR over EG@MnFe 2 O 4 (a) and thermodynamic study (b). Experimental conditions included solution volume of 100 mL at pH 4 for MnFe 2 O 4 and pH 6 for EG@MnFe 2 O 4 . Experiments were run at various temperatures (10-40°C).