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

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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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].

[figure omitted; refer to PDF]

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:$$\begin{array}{c}\text{(1)}& H\left(\%\right)=\frac{C_{\text{o}}-C_{\text{e}}}{C_{\text{o}}}\cdot 100,Q_{\text{t}}=\frac{C_{\text{o}}-C_{\text{t}}}{m}\cdot V,\\ Q_{\text{e}}=\frac{C_{\text{o}}{C}_{\text{e}}}{m}\cdot V,\end{array}$$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.

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].

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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 omitted; refer to PDF]

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 omitted; refer to PDF]

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.

[figures omitted; refer to PDF]

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].

[figures omitted; refer to PDF]

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).

[figures omitted; refer to PDF]

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.

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3.2.3. Effect of Contact Time and Adsorption Kinetics

According to the optimized conditions obtained from Figures 6 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 (Q_t, 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.

[figures omitted; refer to PDF]

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 Figures 9 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].$$\begin{array}{}\text{(2)}& \log \left(q_{\text{e}}-q_{\text{t}}\right)=\log q_{\text{e}}-\frac{k_1\cdot t}{2.303},\end{array}$$where k₁ (1/min) is defined as the pseudo-first-order adsorption rate constant, q_t (mg/g) is defined as the adsorption capacity at the period time t (min), and q_e (mg/g) is defined as equilibrium adsorption capacity at the equilibrium period (min).$$\begin{array}{}\text{(3)}& \frac{t}{q_{\text{t}}}=\frac{1}{k_2·q_{\text{e}}^2}+\frac{t}{q_{\text{e}}},\text{(4)}& H=k_2\cdot q_{\text{e}}^2,\end{array}$$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)).

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[figures omitted; refer to PDF]

Tables 1 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@MnFe₂O_4.

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	0.9512

Table 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	0.9333

Otherwise, 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:$$\begin{array}{}\text{(5)}& Q_{\text{t}}=\frac{1}{\beta }\cdot \ln \left(\alpha \cdot \beta \right)+\frac{1}{\beta }\cdot \ln \left(t\right),\end{array}$$where α (mg/g) and β (g/mg) are defined as adsorption and desorption rates of CR molecules over EG@MnFe₂O₄ and MnFe₂O₄.$$\begin{array}{}\text{(6)}& \log \hspace{0.17em}\hspace{0.17em}\log \left(\frac{V{C}_{\text{o}}}{V{C}_{\text{o}}-Q_{\text{t}}\cdot m}\right)=\log \left(\frac{k_{\text{B}}}{2.303V}\right)+{\alpha }_{\text{B}}\cdot \log \left(t\right),\end{array}$$where k_B 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 Table 1, 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].$$\begin{array}{}\text{(7)}& \frac{1}{Q_{\text{e}}}=\left(\frac{1}{Q_{\text{m}}{K}_{\text{L}}}\right)\frac{1}{C_{\text{e}}}+\frac{1}{Q_{\text{m}}},\text{(8)}& R_{\text{L}}=\frac{1}{1+K_{\text{L}}{C}_{\text{o}}},\end{array}$$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)). This model proposes that the adsorption process occurs on heterogonous phase surfaces without any uniform distribution of heat of energies.$$\begin{array}{}\text{(9)}& \ln Q_{\text{e}}=\ln K_{\text{F}}+\frac{1}{n}\ln C_{\text{e}},\end{array}$$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@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].$$\begin{array}{}\text{(10)}& Q_{\text{e}}=B_{\text{T}}\ln K_{\text{T}}+B_{\text{T}}\ln C_{\text{e}},\text{(11)}& B_{\text{T}}=\frac{\text{RT}}{b},\end{array}$$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 (mol²/kJ²) 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):$$\begin{array}{}\text{(12)}& \ln q_{\text{e}}=\ln Q_{\text{m}}-B{\epsilon }^2,\text{(13)}& \epsilon =\text{RT}\ln \left(1+\frac{1}{C_{\text{e}}}\right),\text{(14)}& E=\frac{1}{\sqrt{-2B}}.\end{array}$$

Table 3 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, R_L (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 (Q_m) 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 Q_m 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	0.8483

[figures omitted; refer to PDF]

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 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).$$\begin{array}{}\text{(15)}& \Delta G=-\text{RT}\ln K_{\text{C}},\text{(16)}& \ln K_{\text{C}}=\frac{C_{\text{A}}}{C_{\text{e}}},\text{(17)}& \ln K_{\text{C}}=\left(\frac{-\Delta H}{R}\right)\cdot \frac{1}{T}+\frac{\Delta S}{R},\end{array}$$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@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 (K_C) 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.

[figures omitted; refer to PDF]

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)
$\ln K_{\text{C}}=-1.097\left(1/T\right)+4.081$ 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 pK_a of CR but lower than pH_pzc of EG@MnFe₂O₄, or 4.0 < pH < pH_pzc = 6.8, CR molecules are deprotonated to transfer a form of anion while EG@MnFe₂O₄ surface is still positively charged due to pH < pH_pzc. 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 pH_pzc 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 pK_a to pH_pzc.

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.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgments

The Foundation for Science and Technology Development, Nguyen Tat Thanh University, Ho Chi Minh city, Vietnam, is acknowledged.

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