Process Optimization by a Response Surface Methodology for Adsorption of Congo Red Dye onto Exfoliated Graphite-Decorated MnFe₂O₄ Nanocomposite: The Pivotal Role of Surface Chemistry

Abstract

Natural graphite, a locally available, eco-friendly, and low-cost carbonaceous source, can be easily transformed into exfoliated graphite (EG) with many surface functional groups via a chemical oxidation route. Combination between EG and magnetic MnFe₂O₄ is a promising strategy to create a hybrid kind of nanocomposite (EG@MnFe₂O₄) for the efficient adsorptive removal of Congo red (CR) dye from water. Here, we reported the facile synthesis and characterization of chemical bonds of EG@MnFe₂O₄ using several techniques such as Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). In particular, the quantity method by Boehm titration was employed to identify the content of functional groups: Carboxylic acid (0.044 mmol/g), phenol (0.032 mmol/g), lactone (0.020 mmol/g), and total base (0.0156 mmol/g) on the surface of EG@MnFe₂O₄. Through the response surface methodology-optimized models, we found a clear difference in the adsorption capacity between EG-decorated MnFe₂O₄ (62.0 mg/g) and MnFe₂O₄ without EG decoration (11.1 mg/g). This result was also interpreted via a proposed mechanism to elucidate the contribution of surface functional groups of EG@MnFe₂O₄ to adsorption efficiency towards CR dye.

1. Introduction

Over the past decades, numerous synthetic dyes have been discovered, developed, and applied in industrial fields, such as textile, paper, pharmaceuticals, and food [1]. It is understandable that the increasing consumption of dyes can result in a vast volume of pollutants. Without any pretreatment measures, disposals of such contaminants could be threatening for the aquatic systems due to, for example, depletion of the penetration of light or inhibition of photosynthetic processes that may be caused by such dyes [2]. Among the emergent dyes, Congo red (CR) in terms of molecular structure presents a kind of complex azo compound constituting of many carcinogenic aromatic rings, amines, and imines (Figure 1) [3]. Under appropriate aerobic reactions, these CR molecules are likely to react/combine with functional groups on the surface of other molecular systems in the body, raising potential risks of genetic mutation (GM) [4,5]. Therefore, treatment of persistent and non-degradable CR molecules should be a priority regardless of the huge challenges relating to cost and technologies.

It has been reported that exfoliated graphene (EG), a chemically modified compound from natural graphite under oxidative conditions, can possess a series of functional groups on the surface [6,7,8]. Zheng-Hong et al. found the surface chemistry of EG containing hydroxyl (–OH), carbonyl (–C=O), and carboxylic acid (–COOH) groups [9]. Wang et al. also asserted that these groups played the main role in tailoring the chemisorption towards toxic dyes [10]. However, the main drawback of EG material is their reliance on its difficult separation from the aqueous solution during post-treatment. It is therefore reasonable to introduce a magnetic component (e.g., iron-based particles) to the EG structure, making it integrated magnetically. For standards of high magnetism, eco-friendliness, chemical stability, and tunable synthesis, MnFe₂O₄ is a suitable additive [11,12,13]. Combining mentioned precursors to create a novel type of EG-decorated MnFe₂O₄ nanocomposite affords opportunities to utilize the materials in adsorption applications.

Here, a prevalent approach to investigate and optimize the effect of input parameters, namely, response surface methodology (RSM), is adopted. The main variables consist of solution pH, CR concentration, and contact time. We attempted to interpret the optimized adsorption results based on a proposed mechanism with the contribution of surface chemistry (i.e., surface functional groups), which was analyzed by physical techniques (e.g., Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and quantity of surface functional groups via Boehm titrations. Scheme 1 illustrates the total process for adsorption of CR dye onto exfoliated graphite-decorated MnFe₂O₄ nanocomposite.

2. Materials and Methods

2.1. Chemicals and Instruments

The chemicals were commercially purchased from Merck (Kenilworth, New Jersey, USA) without any purification methods prior to the utilization. Natural graphite samples were locally purchased from Yen Bai province, Vietnam.

The D8 Advance Bruker powder diffractometer (Bruker, Billerica, MA, USA) was used to record the X-ray powder diffraction (XRD) profiles using Cu-Kα beams as excitation sources. The X-ray photoelectron spectroscopy (XPS) was performed on the ESCALab MKII spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using Mg-Kα radiation. The characteristics of chemical bonds and functional groups were investigated using the FT-IR spectra on the Nicolet 6700 spectrophotometer (Thermo Fischer Scientific, Waltham, MA, USA). The UV–VIS spectrophotometer (Shimadzu, Kyoto, Japan) was used to determine the CR concentration at 500 nm.

2.2. Synthesis of EG

Natural graphite samples (5.0 g) was carefully added in 100 mL of mixture containing H₂SO₄ (96%) and H₂O₂ (33%) (100:7 by volume) and vigorously stirred. After the process had been finished (2 h), the solid was washed with water many times until the aqueous residual became a neutral solution. To dry the sample, the solid was placed in an oven at 110 °C overnight. The EG was formed by the microwave-assisted irradiation of the bulky powder (750 W, 10 s).

2.3. Synthesis of MnFe₂O₄

The production of magnetic MnFe₂O₄ nanoparticles was based on recent work [14]. A mixture prepared by dissolving chemicals including citric acid (93 g), ethylene glycol (140 mL), and H₂O (40 mL) was heated up 80 °C beneath open air. The MnCl₂·6H₂O crystals (0.303 g) were slowly added into the above mixture and their temperature was allowed to rise at 130 °C during 2 h. The as-received polymeric resin was embarked on the heat-resistant furnace before being heated up at 1000 °C during 2 h. After the sample was cooled, the MnFe₂O₄ was stored in a desiccator.

2.4. Synthesis of EG@MnFe₂O₄

To prepare the EG@MnFe₂O₄ nanocomposite, we used the synthesized EG as a precursor. To begin with, a solution (50 mL) of citric acid (0.02 M) was slowly added into another mixture consisting of Fe(NO₃)₃·9H₂O (0.7 g) and Mn(NO₃)₂·6H₂O (0.25 g) in 50 mL H₂O at 90 °C for 1 h under vigorous stirring. The homogeneous solution was very slowly loaded with 0.8 g EG during 30 min. Next, NH₃ solution was added dropwise until a solution with pH ranging between 8 and 9 was obtained. After 30 min, NH₃ was repeatedly used to adjust pH to 8, and then the sample was dehydrated at 80 °C. The solid calcination was employed at 700 °C, 2 h to obtain a black magnetic sample.

2.5. Experimental Batch

Herein, EG@MnFe₂O₄ and MnFe₂O₄ were used as adsorbents to compare their adsorption capacity towards CR dyes. The adsorption experiments were based on the adsorbent dose (0.05 g/L) and the volume of dye solutions (100 mL). Other parameters including solution pH, CR concentration and contact time were described by experimental design with RSM. The samples in beakers (250 mL) were agitated on a shaking table (200 rpm). After the adsorption experiments had been finished, the adsorbent was separated from aqueous solution using a simple magnet and remaining concentration was measured by the UV–VIS spectrophotometer at 500 nm. The removal efficiency (H%), adsorption capacity (Q) at equilibrium period was calculated on the basis of the concentrations before and after adsorption process by the following equations:

$$H(\%)=\frac{C_o-C_e}{C_o}·100$$

(1)

$$Q_t=\frac{C_o-C_t}{m}·V$$

(2)

where, C_o and C_t are the dye concentrations (mg/L) at the initial and final periods, respectively. V and m represent the volume of solution (mL), and weight of adsorbent (g), respectively.

2.6. Experimental Design with RSM

To optimize conditions for the highest CR adsorption capacity in water, the RSM would be used for both EG@MnFe₂O₄ and MnFe₂O₄. In details, three parameters including solution pH (4.3–7.7), CR concentration (43.2–76.8 mg/L) and contact time (163.2–196.8 min) were incorporated into the model to investigate the effect of experimental conditions on the CR adsorption capacity of EG@MnFe₂O₄ and MnFe₂O₄ (

Table 1. List of variables for optimization of Congo red (CR) removal.

No	Independent Factors	Unit	Code	Levels
				$-\mathit{\alpha }$	−1	0	+1	$+\mathit{\alpha }$
1	pH of solution (pH)	-	$x_1$	4.3	5	6	7	7.7
2	Concentration (Co)	g/L	$x_2$	43.2	50	60	70	76.8
3	Time	min	$x_3$	163.2	170	180	190	196.8

). A second-order polynomial equation in which y and x represent the response and independent variables respectively could be established to describe the relationship between the adsorption capacity and experiment conditions as follows (Equation (3)). In this study, two equations were established representing CR adsorption of two materials, EG@MnFe₂O₄ and MnFe₂O₄

$$\begin{gathered} y=f\left(x\right)={\beta }_o+{\displaystyle \sum }_{i=1}^k{\beta }_i{x}_i+{\displaystyle \sum }_{i=1}^k{\displaystyle \sum }_{j=1}^k{\beta }_{ij}{x}_i{x}_j+{\displaystyle \sum }_{i=1}^k{\beta }_{ii}{x}_i^2 \\ \mathrm{N}=2^k+2k+c \end{gathered}$$

(3)

where y is the predicted response; x_i and x_j are the independent variables (i, j = 1, 2, 3, 4…k). The parameter β_o is the model constant; β_i is the linear coefficient; β_ii is the second-order coefficient and β_ij is the interaction coefficient. The total number of experiments is defined by Equation (4). The Design-Expert^® Software Version 10 (DX10) from Stat-Ease, Inc. (Minneapolis, Minnesota, USA) was used as a means of data analysis [15]. Table 1 summarizes independent factors and associated data levels used in the real experimental attempts.

3. Results

3.1. Structural Characterization

The crystallinity profile of both MnFe₂O₄ and EG@MnFe₂O₄ was analyzed using the X-ray diffraction technique, which is presented in Figure 2. It is obvious that the MnFe₂O₄ particles in Figure 2a offered a high degree of crystallinity with the presence of typical peaks at 24.4°, 34.0°, 36.7°, 50.0°, 54.5°, 62.5°, 64.8°. This observation was in line with many previous works, pointing out the successful synthesis of MnFe₂O₄ [16,17,18,19,20,21]. On the other hand, the EG@MnFe₂O₄ pattern in Figure 2b disclosed a very emergent peak at around 26.6°, corresponding to the presence of EG [7]. Although the intensity of peaks from 30° to 60° was low, the existence of MnFe₂O₄ in the structure of EG@MnFe₂O₄ could still be observed with the same position of mentioned peaks of MnFe₂O₄.

Figure 3 also provides more information about the magnetization of EG@MnFe₂O₄, whose saturation magnetization value (1.5 emu/g) was found to be drastically lower than that of original MnFe₂O₄ [14]. Meanwhile, EDS mapping results also provided the average iron content at 6.4%. This phenomenon may be due to the decoration of non-magnetic EG, leading to a depletion in magnetization and crystallinity of original MnFe₂O₄. However, with an eligible magnetization, EG@MnFe₂O₄ can be separated from solution by inducing a magnetic field. Consequently, the EG@MnFe₂O₄ structure obtained a combination of EG and MnFe₂O₄ components [22,23,24].

To gain more insight into various types of chemical bonds on the surface of EG@MnFe₂O₄, the FT-IR spectra were explored. According to Figure 4 and

Table 2. The surface functional groups analysis of EG@MnFe₂O_4.

Functional Groups	Experimental Frequency (cm−1)	Class	Ref.
O–H and N–H stretching	3300–3500 (broad band)	Primary amines, secondary amines, hydroxyls of absorbed water.	[25]
C=O stretch	1730 (very strong band)	Carbonyls of aldehydes (–CHO) or ketones (–C=O), lactone, esters (–COO–) or acid carboxylic (–COOH)	[26]
	1639 (very strong band)	Conjugation lowers frequency amides (–NHCO), or N–H stretching	[27]
C=C bending	1520	Aromatic rings, alkenes	[28]
C–O stretch	1195 (strong band)	Phenolic compounds or tertiary alcohol	[29]
	1076	Primary alcohols	[30]

, the hydroxyl (–OH) and amine (–NH) groups can be ascribed to a broad band at 3400 cm⁻¹ [25]. The aldehyde/ketone/acid/ester groups (C=O) were confirmed at around 1730 cm⁻¹ and 1639 cm⁻¹ regions with a strong intensity [26,27]. In addition, regions at round 1520 cm⁻¹ and 1195 cm⁻¹ were attributable to the existence of C=C, and C–O bonds, respectively [28,29]. A peak at 1076 cm⁻¹ could be ascribed to possible existence of primary alcohol [30]. Moreover, apart from the main peaks in EG@MnFe₂O₄, the spectrum of CR-loaded EG@MnFe₂O₄ was present in Figure 2. It is clear that an emergent peak at 1346 cm⁻¹ was of importance for C–N bond [31]. Meanwhile, three peaks at 1210 cm⁻¹ (narrow), 1178 cm⁻¹ (narrow), 1029 cm⁻¹ (very strong) were devoted for the absorption of –SO₃⁻ groups, and at around 800 cm⁻¹ for the ring vibrations of p-di-substituted aromatic compounds [31]. Moreover, the weak peaks at around 1580 cm⁻¹ could be assigned to the N=N typical stretching [32]. To sum up, the EG@MnFe₂O₄ shows a variety of chemical bonds, which were essential for adsorption.

To illustrate more information about surface functional groups, the XPS spectrum is shown in Figure 5. At a glance, the XPS survey revealed the presence of four elements for EG@MnFe₂O₄: Carbon (C 1s), oxygen (O 1s), iron (Fe 2p), and manganese (Mn 2p). However, among those elements, the C 1s peak was measured with high intensity. This preliminary observation can be explained mainly because EG with carbonaceous components covers overall MnFe₂O₄ nanoparticles, leading a vague detection of the typical MnFe₂O₄ signal (note that XPS sensitivity works out within a certain nanoscale depth <10 nm) [33]. This kind of pattern is commensurate with the very weak signal of MnFe₂O₄ in the XRD spectrum as illustrated in Figure 2.

It was found that the C 1s XPS spectrum in Figure 5b indicated the presence of π–π interaction 289.6 (eV), C=O (286.0 eV), C–C (284.2 eV) [34]. Meanwhile, the O 1s XPS signals in Figure 5c can be broken down into three curves with peaks at binding energies 535.1, 532.5, 530.0 eV, corresponding to chemisorbed O, C–O/C=O, and iron oxides Fe–O [33]. Fe 2p spectrum in Figure 5d is divided into two sub levels including Fe 2p_3/2 and Fe 2p_1/2. It is obvious that a spin-orbit separation energy was found to be 13.5 eV, while the distance from Fe 2p1/2 to satellite position was only 8.1 eV, which represents Fe³⁺ cations [35]. Mn 2p spectrum in Figure 5e shows two sub-level of spin-orbit-splits between 2p_3/2 and 2p_1/2 with their binding energy gap of around 11.8 eV. This distance is in close proximity to spin-orbit separation energy (~11.62 eV) of manganese (II) oxide [35]. In particular, a satellite speak appeared at 647 eV, which is nearly 6.8 eV as far as the 2p½ state, suggesting the existence of Mn²⁺ in the structure of EG@MnFe₂O₄ [36].

To quantify the functional groups, the Boehm titration can be used. This experiment allows identification of the amount of phenolic, lactonic, carboxylic groups, and basic groups [37]. It is assumed that NaOH (a very strong base) can neutralize Brønsted acids including phenol, lactone, and carboxylic acid, Na₂CO₃ can neutralize lactone, and carboxylic acid, and finally, NaHCO₃ can neutralize carboxylic groups [38]. According to

Table 3. Surface groups obtained from Boehm titrations and textual properties of MnFe₂O₄ and EG@MnFe₂O_4.

No	Materials	MnFe2O4	EG@MnFe2O4
1	Carboxylic groups (mmol/g)	0	0.044
2	Lactonic groups (mmol/g)	0	0.032
3	Phenolic groups (mmol/g)	0	0.020
4	Total oxygenated groups (mmol/g)	0	0.096
5	Total basic groups (mmol/g)	0	0.156

, the EG@MnFe₂O₄ contains a wide range of functional groups (phenolic, lactonic, carboxylic groups, and basic groups) with the amount of 0.044, 0.032, 0.020, and 0.156 mmol/g, respectively. These functional group are obviously derived from the EG component since MnFe₂O₄ in absence of EG decoration failed to produce the same results. Therefore, compared with MnFe₂O₄ without EG, EG@MnFe₂O₄ can own many surface functional groups, which are responsible for enhanced adsorption of CR. The existence of functional groups on the surface of adsorbents can not only create the interaction between the adsorbent and adsorbate, but also, enhance the retention of CR molecules on the adsorbent surface; thus the more the dye molecules are captured, the better the removal efficiency is [39].

3.2. Optimization with RSM

To compare the adsorption capacity towards CR dye between EG@MnFe₂O₄ and MnFe₂O₄, we set up two models with the three variables including solution pH (4.3–7.7), CR concentration (43.2–76.8 mg/L) and contact time (163.2–196.8 min) as shown in Table 1.

Table 4. Matrix of observed and predicted values for CR adsorption capacity.

Run	Independent Factors			Onto EG@MnFe2O4		Onto MnFe2O4
	${\mathit{x}}_{\mathbf{1}}$	${\mathit{x}}_{\mathbf{2}}$	${\mathit{x}}_{\mathbf{3}}$	Actual (mg/g)	Predicted (mg/g)	Actual (mg/g)	Predicted (mg/g)
1	50	5	170	49.92	50.48	6.38	6.79
2	50	7	170	35.10	37.83	5.98	6.27
3	70	5	170	30.36	30.31	3.14	4.45
4	70	7	170	20.01	20.47	2.51	3.61
5	50	5	190	52.62	53.92	9.01	9.04
6	50	7	190	41.81	43.62	8.64	8.46
7	70	5	190	34.36	33.38	6.42	7.26
8	70	7	190	24.70	25.89	5.63	6.36
9	60	4.3	180	33.50	33.85	6.75	5.75
10	60	7.7	180	19.74	16.91	5.16	3.34
11	6	43.2	180	58.21	55.26	6.86	7.08
12	6	76.8	180	22.91	23.39	5.16	3.34
13	6	60	163.2	47.52	46.17	9.07	7.77
14	6	60	196.8	54.75	53.62	12.28	11.97
15	6	60	180	57.91	58.21	10.35	10.28
16	6	60	180	57.45	58.21	10.30	10.28
17	6	60	180	58.41	58.21	11.13	10.28
18	6	60	180	57.68	58.21	9.18	10.28
19	6	60	180	58.30	58.21	10.48	10.28
20	6	60	180	59.08	58.21	10.01	10.28

presents the observed and predicted values of 20 experiments (8 axial points, 6 cube points and 6 replicates) using the central composites design (CCD) for two CR adsorption models of EG@MnFe₂O₄ and MnFe₂O₄. The values of response as adsorption capacity were also displayed in Table 4.

For all experiments, it is evident that CR adsorption capacities by EG@MnFe₂O₄ were significantly higher than those by MnFe₂O₄ materials. For example, in the last five entries, the former material offered the CR adsorption capacity at 58.21 mg/g, compared with 10.28 mg/g of MnFe₂O₄ without EG decoration. These results could be due to the existence of EG decorated on the surface of EG@MnFe₂O₄, leading to an enhancement of functional groups, which greatly contribute to the adsorption. To further investigate impact of experimental conditions on adsorption, we analyzed the ANOVA results extracted from Design Expert program, as shown in

Table 5. ANOVA data for the model of CR adsorption.

Material	Source	Sum of Squares	Degree of Freedom	Mean Square	F-Value	Prob. > F	Comment
EG@ MnFe2O4	Model	4056.21	9	450.69	121.02	<0.0001	SD = 1.93
	$x_1$	1225.91	1	1225.91	329.18	<0.0001	Mean = 43.72
	$x_2$	346.28	1	346.28	92.98	<0.0001	CV(%) = 4.41
	$x_3$	67.03	1	67.03	18.00	0.0017	R2 = 0.9909
	$x_1{}^2$	642.60	1	642.60	172.55	<0.0001	AP = 30.2649
	$x_2{}^2$	1941.60	1	1941.60	521.36	<0.0001
	$x_3{}^2$	124.46	1	124.46	33.42	0.0002
	$x_1{x}_2$	3.94	1	3.94	1.06	0.3279
	$x_1{x}_3$	0.0655	1	0.0655	0.0176	0.8971
	$x_2{x}_3$	2.76	1	2.76	0.7406	0.4096
MnFe2O4	Mode	125.91	9	13.99	10.75	0.0005	SD = 1.14
	$x_1$	1.73	1	1.73	1.33	0.2763	Mean = 7.72
	$x_2$	16.84	1	16.84	12.94	0.0049	CV(%) = 14.78
	$x_3$	21.34	1	21.34	16.39	0.0023	R2 = 0.9063
	$x_1{}^2$	47.42	1	47.42	36.43	0.0001	AP = 10.6995
	$x_2{}^2$	46.37	1	46.37	35.63	0.0001
	$x_3{}^2$	0.3050	1	0.3050	0.2343	0.6388
	$x_1{x}_2$	0.0558	1	0.0588	0.0428	0.8402
	$x_1{x}_3$	0.0018	1	0.0018	0.0014	0.9713
	$x_2{x}_3$	0.1578	1	0.1578	0.1212	0.7349

Note that: Prob. is probability, SD is standard deviations, CV is coefficient of variation, R² is coefficient of determination, AP is adequate precision.

.

According to Table 5, ANOVA data is fully listed main statistical parameters of two models, including sum of squares, degree of freedom, mean square, F-value, p-value, coefficient of determination R², and adequate precision ratio (AP). To obtain the most “statistically significant level”, several conditions of model parameters should be met [40,41,42]. Whereas the larger the F-value and the smaller p-value are desirable for model significance, R² and AP should be greater than 0.9 and 4 respectively [28]. In that case, proposed models can be considered to be statistically significant at 95% significance level. Comparing these standards with statistical parameters of both models, it is obvious that Prob. > F values were lower than 0.0001, F-values (10.75–121.02) were reliable, along with very high R² (0.9026–0.9909) and AP ratios (10.6995–30.2649), suggesting that both models offered an excellent degree of compatibility between predicted data and experimental data [43,44]. The R² reported in this study well meets the criteria of Tugba et al. who suggested that R² ≥ 0.80 is sufficient for model fitting [45]. Therefore, models for the adsorption of CR onto EG@MnFe₂O₄ and MnFe₂O₄ were well-designed, and hence eligible to predict the optimal conditions.

The residual analysis (Figure 6, Figure 7 and Figure 8) is an integral part of evaluating the model suitability. In detail, normal plots of residuals in Figure 6a,b were inclined to be “S-shape” lines rather than linear, possibly resulting in potential errors in predicting the trends of experimental data. However, Grace et al. reported that mentioned residual patterns are still eligible to analyze the transformation of the response [46]. Meanwhile, residuals versus run plots in Figure 7a,b illustrate relatively random scatters, proposing that the variance values are the constants against the residuals variables. At the same trend, Figure 8a,b show predicted and actual points distributed on the 45-degree line, therefore, it is reliable to predict the trends of models [47,48].

Three-dimensional surfaces and contour plots as shown in Figure 9, Figure 10 and Figure 11 reflect the effect of parameters on the response [49]. Figure 9a,b demonstrated the effect of concentration and solution pH on the adsorption of CR onto EG@MnFe₂O₄ and MnFe₂O₄. It is obvious that CR adsorption capacity obtained by the former materials was significantly higher than that of the latter. In addition, both variables showed a profound impact on the adsorption capacity of CR onto EG@MnFe₂O₄ and MnFe₂O₄, leading to the respective contour plots in Figure 9c,d reaching the convergent regions in the range of investigated values. Therefore, these regions would present the optimal conditions for the adsorption of CR. Figure 10a,b shows the major effect of concentration and modest effect of contact time on the adsorption of CR onto EG@MnFe₂O₄ and MnFe₂O₄. Clearly, highest CR adsorption capacities could be achieved at a moderate level of concentration and elevating the concentration past the optimal point may reduce the adsorption efficiency. For contact time, prolonging the exposure time could slightly improve CR uptake, as shown in contour plots of Figure 10c,d, particularly in the contour plot of MnFe₂O₄, where regions of optimal adsorption tended to deviate from the investigated regions to the top of the plot.. Similarly, Figure 11a–d show the effect of solution pH and contact time on the adsorption of CR onto EG@MnFe₂O₄ and MnFe₂O₄. Generally, pH is the most influential factor, while contact time presents a minor role in the adsorption process of CR. Contour plots in Figure 11c,d also demonstrate that the optimum pH was around 6.0.

To maximize the adsorption capacity value, the optimal conditions was set up based on the RSM as summarized in

Table 6. Confirmation of experiment results.

Sample	pH (-)	Concentration (mg/L)	Time (min)	Adsorption Capacity (mg/g)			Desirability
				Predicted	Tested	Error
EG@MnFe2O4	5.7	57.7	181	60.6	62.0	1.4	1.0000
MnFe2O4	6.0	62.0	182	10.4	11.1	0.7	1.0000

. Under the optimized conditions, confirmation tests were conducted under optimized conditions to verify the suitability between proposed and actual data. Highest adsorption capacities for the EG@MnFe₂O₄ and MnFe₂O₄ were recorded as 62.0 and 11.1 mg/g respectively. Also, it is evident that the results obtained by tested experiments were in line with those by predicted experiments, suggesting the model design was successfully applied.

Table 7. Comparation of adsorption capacity using various adsorbents.

No.	Adsorbents	Adsorption Capacity (mg/g)	Reference
1	EG@MnFe2O4	62.0	This work
2	MnFe2O4	11.1	This work
3	Anilinepropylsilica xerogel	22.62	[50]
4	Kaolin	5.44	[51]
5	Waste orange peel	22.4	[52]
6	Bentonite	40.4	[53]
7	CTS powder	74.7	[54]
8	CTS-MMT	54.5	[54]
9	m-Cell/Fe3O4/ACCs	60.5	[55]

compared adsorption capacity using various adsorbents, for which this study showed the better results.

3.3. Proposed Mechanism

Based on optimized conditions via RSM models, it was revealed that CR adsorption capacity of EG@MnFe₂O₄ was approximately six-fold higher than that of MnFe₂O₄. This result can be explained due to the role of chemical functional groups on the surface of EG@MnFe₂O₄ [56]. As previously mentioned, EG@MnFe₂O₄ is proved to contain many kinds of functional groups including carboxylic acid, lactone, phenol, and base groups, which are non-existent in MnFe₂O₄ (Table 3). During the adsorption process, the presence of functional groups may contribute to the interaction with CR molecules [56]. As a result, the CR molecules were more easily captured on the surface of EG@MnFe₂O₄ than on the surface of MnFe₂O₄.

Herein, we propose several kinds of plausible mechanism including H-bonding, and π–π interaction (Figure 12). It is known that CR molecules are constituted of aromatic rings, amines (–NH₂) and imines (–N=N–) as shown in Figure 1, while the four mentioned functional groups contain both H-donors (hydrogen atoms belonging to groups such as –OH, –NH₂, –C₆H₄OH) and H-acceptors (electron-rich oxygen or nitrogen atoms such as –CHO, N=N, –COO⁻). Therefore, a H-bond type can be formed between these CR molecules and functional groups, enhancing the adsorption efficiency [57,58]. In addition, the EG@MnFe₂O₄ is decorated with the outer EG layer. Because the EG is the carbonaceous source that abundantly contains aromatic rings in the structure. As a result, π–π interaction can be formed between aromatic rings of CR molecules and EG layers of EG@MnFe₂O₄ material, leading to an improvement in adsorption capacity.

In MnFe₂O₄, the adsorption of CR may be attributable to the existence of weak forces including “oxygen–metal” bridge and van der Waals [59]. It was reported that the electron-rich atoms such as oxygen can interact with a metal/oxides site to form an intermediate bridge called “oxygen–metal” [59]. Because these kinds of force are weak, the adsorption of CR over MnFe₂O₄ was unconducive.

4. Conclusions

The EG@MnFe₂O₄ has been successfully synthesized and characterized. The XPS, FT-IR and Boehm titration results indicated that EG@MnFe₂O₄ contains various kind of functional groups with carboxylic acid (0.044 mmol/g), phenol (0.032 mmol/g), lactone (0.020 mmol/g), and total base (0.156 mmol/g) on the surface. In addition, EG@MnFe₂O₄ and MnFe₂O₄ were used to absorb the CR dye from water via the experimental design by RSM for three parameters: Solution pH (4.3–7.7), CR concentration (43.2–76.8 mg/L) and contact time. The quadratic regression models were proved to be statistically significant at 95% significance level. Verification of the optimized results revealed that the CR adsorption capacity onto EG@MnFe₂O₄ (62.0 mg/g) was significantly higher than that onto MnFe₂O₄ (11.1 mg/g). To explain these results, the plausible mechanisms including H-bonding, and π–π interaction were proposed based on the Boehm titration results, assuming that functional groups on the surface of EG@MnFe₂O₄ play a crucial role in enhancing the adsorption of CR dye.