English

• 0.   Introduction

  With the rapid development of modern industry, dye pollution has become increasingly serious. Some technologies have been adopted to treat dye wastewater (e. g., physical adsorption, biological enzyme, and oxidation-reduction). To be specific, physical adsorption has aroused wide attention for its low equipment requirements, simple operation process, and high treatment capacity. Adsorbent takes on a critical significance in physical adsorption, and numerous adsorbents (e. g., porous carbon materials (PCMs) ^[1], molecular sieve^[2], alumina^[3], and clay^[4]) have been developed and employed as the adsorbents to remove dyes. To be specific, PCMs are the most extensively used materials due to their various sources and simple fabrication methods. However, most PCMs with powdery structures are difficult to separate from the water column after adsorption saturation^[5-7], thus significantly limiting their practical application and predisposing them to secondary pollution.

  Recent research has suggested that the combination of magnetic nanoparticles and PCMs is effective to enhance the separation performance of carbon materials. magnetic carbon spheres (MCSs) have received increasing research interest among the composite of magnetic nanoparticles and carbon materials due to their tunable properties and potential applications^[8-10]. A considerable number of methods (e. g., chemical vapor deposition^[11-12], the self-generated template approach^[13], pyrolysis of organometallic polymers^[14-17], resin sphere-based carbonization method^[18-20], and hydrothermal carbon of saccharide solution containing magnetic particle^[21-22]) have been developed to prepare MCSs over the past few decades. However, the complex procedure or expensive preparation cost significantly limits the practical application of the above methods. Accordingly, it is still challenging to develop new synthesis strategies for MCSs.

  In this study, a new two-step method was developed to prepare activated magnetic porous Fe₃O₄/carbon microspheres (A-Fe₃O₄/C). First, magnetic Fe₃O₄/chitosan microspheres (Fe₃O₄/CS) were prepared through electrostatic spraying with CS and FeCl₂/FeCl₃ as the precursor. Subsequently, a combination method of high-temperature carbonization and alkali activation was employed to convert Fe₃O₄/CS into A - Fe₃O₄/C. Methylene blue (MB) was selected as the model dye to adsorb A -Fe₃O₄/C, and batch studies were conducted to examine the effects of different experimental parameters (e. g., pH, adsorbent dosage, contact time, initial concentration, and alkali activator). The adsorption mechanism of A - Fe₃O₄ /C was investigated using the isotherm model and the kinetic model. Above all, this study provides a simple and easy method for the preparation of magnetic porous carbon spheres.

  1.   Experimental

  1.1   Materials

  Iron (Ⅱ) chloride tetrahydrate (FeCl₂ ·4H₂O, 98%) was purchased from Shanghai Macklin Biochemical Co., Ltd (China). MB (C₁₆H₁₈ClN₃S·3H₂O) was purchased from Tianjin Chemical Reagent Research Institute Co. CS (degree of deacetylation: 80.0% - 95.0%), sodium hydroxide (NaOH, AR), iron trichloride hexahydrate (FeCl₃·6H₂O, AR) were provided by Sinopharm Chemical Reagent Co., Ltd. Acetic acid (CH₃COOH, w > 99.5%) was purchased from Nanjing Chemical Reagent Co., Ltd (China). All chemicals were used as received, and the deionized water was applied throughout the experiments.

  1.2   Preparation of Fe₃O₄/CS

  Fe₃O₄/CS was prepared using the combination of electrostatic spray and the coprecipitation method. In brief, 0.20 g FeCl₂, 0.36 g FeCl₃, and 0.30 g CS were first dissolved in acetic acid aqueous solution (3%) to produce a mixed gel solution. Subsequently, the gel solution was loaded into a 10 mL syringe with a syringe needle of 0.86 mm inner diameter. The electrostatic spray process was performed in accordance with the previous literature^[23]. The prepared Fe₃O₄/CS gel was washed to neutral with distilled water and then chemically crosslinked with glutaraldehyde aqueous solution (2%) for 1 h. Afterward, the microspheres were washed with deionized water several times. Next, Fe₃O₄/CS was prepared through freeze-drying for 12 h.

  1.3   Preparation of A-Fe₃O₄/C

  A - Fe₃O₄/C was prepared using a combination method of high-temperature carbonization and alkali activation with NaOH as the alkali activator. Mainly, Fe₃O₄/CS was immersed in 1.0% NaOH solution for 0.5 h and freeze-dried directly, then, the dried microspheres were carbonized in N₂ atmosphere in a tube furnace at a set temperature of 700 ℃ for 2 h with the heating rate of 3 ℃ ·min^-1 from 25 ℃. After natural cooling to ambient temperature, the black magnetic carbon microspheres were obtained after washing to neutral with deionized water and freeze-drying and named A_NaOH-Fe₃O₄/C. The sample was labeled as A_KOH-Fe₃O₄/C in accordance with the alkali activator of KOH.

  1.4   Characterization

  The morphology and structure of prepared magnetic microspheres were observed by JSM - 7600F scanning electron microscope (SEM, Rigaku, Japan) operating at an accelerating voltage of 10-15 kV. The average diameter of prepared samples was examined from the SEM image by the National Institutes of Health (MD, USA), where the samples were adhered to an aluminum plate and coated with gold under high vacuum conditions (20 mA, 120 s). The elemental composition was characterized using an energy dispersive spectrometer (EDS) attached to JSM - 7600F SEM. The MB concentration was examined by a UV - 2450 (SHIMADZU, Japan). Fourier transform infrared (FT-IR) spectra were recorded by a Perkin-Elmer 100 spectrometer using a KBr pellet in a range of 4 000 - 400 cm^-1. Nitrogen sorption measurements were performed using the ASAP 2000 system (Micromeritics, USA) at 77 K and degassed at 175 ℃ for 500 min. Furthermore, the surface area and the pore size were obtained using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods.

  1.5   Adsorption experiment

  The adsorption performance of A - Fe₃O₄/C was examined through batch adsorption experiments with MB as the model pollutant. The effects of the adsorption experimental parameters (e. g., pH of the solution and alkali activator) were studied using a batch system. The adsorption kinetics was achieved with the initial concentration of 30, 50, and 80 mg·L^-1 at 298 K and lasted for 8 h. The adsorption isotherms of MB were set according to the initial concentrations ranging from 30 to 400 mg·L^-1 at 298, 308, and 318 K, respectively. The regeneration experiments were performed through thermal treatment for cyclic utilization. The sorption experiments were performed in 50 mL flasks containing 10 mg of the samples in 20 mL of MB solution on an air bath temperature-controllable shaker at 100 r· min^-1. In this study, the concentrations of MB were examined using an ultraviolet spectrophotometer (UV-2450). The adsorption capacity (Q_e) is expressed as:

  $ Q_{\mathrm{e}}=\frac{\left(\rho_0-\rho_{\mathrm{e}}\right) V}{m} $

  (1)

  where ρ₀ (mg·L^-1) and ρ_e (mg·L^-1) denote the initial and equilibrium concentrations of MB solution; V (mL) represents the volume of the solution; m (g) is the mass of the absorbent.

  2.   Results and discussion

  2.1   Materials characterization

  The morphology and structure of Fe₃O₄/CS and A - Fe₃O₄/C were observed by SEM, as presented in Fig. 1. Fe₃O₄/CS exhibited a spherical shape with a smooth surface (Fig. 1a). After carbonization and alkali activation, A_NaOH - Fe₃O₄/C and A_KOH - Fe₃O₄/C exhibited the spherical morphology, whereas the surface became rough (Fig. 1b and 1c). Moreover, Fig. 1d-1f showed that the average diameter of Fe₃O₄/CS, A_NaOH - Fe₃O₄/C, and A_KOH - Fe₃O ₄/C were 0.49, 0.40, and 0.34 mm, respectively. The reduction of the diameter of A_NaOH - Fe₃O₄/C and A_KOH - Fe₃O₄/C was due to the shrinking during the process of alkali etching^[24]. Fig. 2a-2c depict the honeycomb-like structure of interiors of Fe₃ O₄/CS, A_NaOH - Fe₃O₄/C, and A_KOH - Fe₃O₄/C, and some collapse and destruction were easy to find on the honeycomb walls and surfaces with the alkali activation process. The embedding of Fe₃O₄ nanoparticles in the prepared microspheres was confirmed using elemental mapping. As depicted in Fig. 2d-2f, the elemental mappings suggest that Fe elements were evenly distributed in Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C.

  Figure 1

  

  Figure 1.  SEM images (a-c) and particle size distribution (d-f) of Fe₃O₄/CS (a, d), A_NaOH-Fe₃O₄/C (b, e), and A_KOH-Fe₃O₄/C (c, f)

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  Figure 2

  

  Figure 2.  EDS representative SEM image (a-c) and corresponding elemental mappings (d-f) of Fe₃O₄/CS (a, d), A_NaOH-Fe₃O₄/C (b, e), and A_KOH-Fe₃O₄/C (c, f)

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  FT-IR analysis was conducted to identify the type of functional groups on the samples before and after activation. As depicted in Fig. 3, the typical absorption peak at 583 cm^-1 belonging to the stretching vibration of the Fe—O bond confirmed that the three samples all have Fe₃O₄ components. The strong peak observed at 3 437 cm^-1 should be the overlapping signals of O—H and N—H stretching vibrations, whereas the two tiny peaks at 2 924 and 2 853 cm^-1 belong to the C—H stretching vibration. Furthermore, the characteristic peaks at 1 632, 1 388, and 1 073 cm^-1 belonging to the amide Ⅰ band, C—O—C stretching vibration, and the C—N stretching vibration of the amide Ⅲ band were all presented in the above three samples, suggesting that the activation process slightly affects the functional groups of the samples^[25-26].

  Figure 3

  

  Figure 3.  FT-IR spectra of Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C

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  Fig. 4a and 4b show nitrogen adsorption-desorption isotherms and pore size distributions of the three samples. Fe₃O₄/CS, A_NaOH - Fe₃O₄/C, and A_NaOH - Fe₃O₄/C all exhibited typical type - Ⅳ hysteresis, suggesting the presence of mesopores^[27]. The specific surface areas (S_BET) of Fe₃O₄/CS were examined as 286 m²·g^-1. After being activated by NaOH and KOH, the surface areas of A_NaOH-Fe₃O₄/C and A_KOH-Fe₃O₄/C rapidly increased to 350 and 368 m²·g^-1, respectively. The activation process led to an increase in the specific surface area of A-Fe₃O₄/C. The pore volume and pore size of the samples were examined using the BJH conventional method, and the characteristics are listed in Table 1. As depicted in Fig. 4b, the pore size of the material was mainly distributed at 3.8 nm, and it also confirmed the presence of micropores and mesopores in the materials accompanied by the use of alkali activators. The above differences between the three samples confirm that alkali activation takes on a critical significance in porosity, and KOH has the best result. The above differences between the three composites confirm that activating agent plays an important role in porosity, and activators lead to larger S_BET and the production of numerous pores.

  Figure 4

  

  Figure 4.  Nitrogen adsorption-desorption isotherms at 77 K (a) and pore size distributions (b) of Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C

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  Table 1

  

  Table 1.  Texture properties of Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C

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  Sample	SBET/(m2·g-1)	VM/(cm3·g-1)
  Fe3O4/CS	286	0.38
  ANaOH-Fe3O4/C	350	0.48
  AKOH-Fe3O4/C	368	0.52

  2.2   Adsorption performance

  MB was used as a model dye to examine the adsorption performance of the prepared samples, and the effect of pH was investigated over a wide pH range from 3 to 11 at an initial concentration of 100 mg·L^-1. Fig. 5a depicts the effect of the pH of the solution on the adsorption of MB on the three materials. All three adsorbents showed the same trend of the removal rate of MB, i. e., the removal rate increased with the increase in pH. Specifically, the MB removal rate increased rapidly at the initial pH (3-5) and then increased slowly in the pH range of 5 - 10. The maximum removals were observed at pH 11, and the values for Fe₃O₄/CS, A_NaOH - Fe₃O₄/C, and A_KOH - Fe₃O₄/C were obtained as 71%, 84%, and 96%, respectively. MB is a cationic dye, and the adsorption of MB is dependent on the pH value since the main force of the adsorption is the electrostatic attraction between the dyes and the amino on the surface of the adsorbent^[28]. When the solution is rich in OH^-, the number of MB cations and the negative charge site on the surface of carbon microspheres will both increase, thus leading to an increase in the removal rate of MB^[29-30]. Under acidic conditions, MB exists mainly in the form of molecular molecules, and the electrostatic effect between carbon microspheres and MB is weakened. Accordingly, the adsorbent adsorption capacity of MB can be enhanced at high pH values.

  Figure 5

  

  Figure 5.  (a) Effect of pH on the adsorption of MB (ρ₀ of MB: 100 mg·L^-1, adsorbent dosage: 10 mg, sample volume: 20 mL, t: 8 h); (b) Effect of contact time on the adsorption of MB; (c) Pseudo‐first‐order and (d) pseudo‐second‐order kinetic models for the adsorption of MB by the adsorbents

  Inset: image showing the separation of the sample by a magnet

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  The effect of contact time on the adsorption of MB on the three samples is presented in Fig. 5. The samples can be quickly separated from the solution under the condition of an applied magnetic field, which is shown in the inset of Fig. 5c. The experiments were performed under the initial MB concentration of 30 mg· L^-1 (Fe₃O₄/CS), 50 mg·L^-1 (A_NaOH-Fe₃O₄/C) and 80 mg· L^-1 (A_KOH - Fe₃O₄/C). As depicted in Fig. 5b, Fe₃O₄/CS and A-Fe₃O₄/C exhibited a fast adsorption rate during the initial 40 min. The whole adsorption process reached equilibrium in about 200 min. The interactions between dye molecules and binding sites on the outer surface of Fe₃O₄/CS and A-Fe₃O₄/C accounted for the rapid adsorption at the initial stage. The kinetic models including pseudo-first (Eq. 2) and pseudo-second (Eq. 3) orders were selected to gain more insights into the adsorption mechanism. The kinetic equations for the above models are written as follows:

  $ \lg \left(Q_{\mathrm{e}}-Q_t\right)=\lg Q_{\mathrm{e}}-\frac{k_1}{2.303} $

  (2)

  $ \frac{t}{Q_t}=\frac{1}{k_2 Q_{\mathrm{e}}^2}+\frac{t}{Q_{\mathrm{e}}} $

  (3)

  Where k₁ and k₂ denote the rate constant of pseudo-first and pseudo-second order adsorption, respectively; Q_t (mg·g^-1) represents the MB sorption capacity at time t. The results are illustrated in Fig. 5c, 5d, and Table 2. The pseudo-second-order model was relatively fitter to describe appropriately the adsorption of MB on Fe₃O₄/CS (R²=0.995 9), A_NaOH-Fe₃O₄/C (R²=0.999 8), and A_KOH-Fe₃O₄/C (R²=0.997 1). Furthermore, the above results also suggest that chemisorption occurs.

  Table 2

  

  Table 2.  Kinetic parameters of the adsorption of MB on the samples

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  Sample	ρ0 /((mg·L-1))	Qe/ (mg·g-1)	Pseudo-first-forder				Pseudo-second-order
  			k1/min-1	Qe/(mg·g-1)	R2		k2/(g·mg-1·min-1)	Qe/(mg·g-1)	R2
  Fe3O4/CS	30	59.68	0.011 2	31.62	0.998 9		0.000 9	61.05	0.995 9
  ANaOH-Fe3O4/C	50	94.73	0.007 8	33.82	0.941 8		0.000 9	94.34	0.999 8
  AKOH-Fe3O4/C	80	159.5	0.0174	120.6	0.984 6		0.005 6	177.6	0.997 1

  The effects of the initial concentration of MB on the adsorption capacity of prepared three samples at three different temperatures (298, 308, and 318 K) were presented in Fig. 6. As depicted in Fig. 6a-c, the adsorption capacities (Q_e) increased with the increase of the dye concentration (ρ₀), and the increase of Q_e was slowed down at higher values of ρ₀. Moreover, the adsorption amount of the three adsorbents increased with the increase in the temperature, suggesting that increasing the temperature can facilitate the adsorption of MB molecules by the adsorbents. The adsorption amount of A NaOH-Fe₃O₄/C and A_KOH-Fe₃O₄/C was significantly higher than that of Fe₃O₄/CS, and A_KOH-Fe₃O₄/C had the highest adsorption amount. The maximum adsorption capacity for MB was examined as 300.6 mg· g^-1. The Langmuir and Freundlich isotherms models were adopted to analyze the adsorption isotherms^{[22, 31]}. The Langmuir adsorption isotherm is expressed by Eq.4, and the linear Freundlich adsorption model formula is written as Eq.5:

  $ \frac{\rho_{\mathrm{e}}}{Q_{\mathrm{e}}}=\frac{1}{Q_{\mathrm{m}} k_{\mathrm{L}}}+\frac{\rho_{\mathrm{e}}}{Q_{\mathrm{m}}} $

  (4)

  $ \ln Q_{\mathrm{e}}=\ln k_{\mathrm{F}}+\frac{1}{n} \ln \rho_{\mathrm{e}} $

  (5)

  Figure 6

  

  Figure 6.  (a‐c) Adsorption isotherms for MB of the samples at different temperatures; (d‐f) Langmuir and (g‐i) Freundlich adsorption model curves for MB adsorption of the samples

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  Where Q_m (mg·g^-1) denotes the maximum adsorption capacity; k_L (L·mg^-1) represents the Langmuir isothermal constant; k_F (mg^1-1/n·L^1/n·g^-1) is Freundlich adsorption constant; n represents the empirical constant of the adsorption process, and n > 1 indicates a strong interaction between adsorbent and MB^[32]. As depicted in Fig. 6d-6i and Table 3, the correlation coefficients of the Langmuir and Freundlich isotherm adsorption models were nearly higher than 0.9 at three experimental temperatures. The comparison of the correlation coefficient parameters of the two models suggests that the Langmuir model is more suitable for describing the prepared three samples. Fitted by the Langmuir model, the maximum adsorption capacities for MB of Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C were obtained as 176.0, 246.8, 300.6 mg·g^-1 at 308 K, respectively, consistent with the experimental results. The activation step increased the micropores on the surface of the A_NaOH - Fe₃O₄/C and A_KOH - Fe₃O₄/C, so more active sites were created on the surface of them, thus they more significantly adsorbed MB.

  Table 3

  

  Table 3.  Langmuir and Freundlich adsorption isotherm parameters for MB adsorption of the samples

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  Sample	T/K	Langmuir				Freundlich
  		Qm/(mg·g-1)	kL/(L·mg-1)	R2		n	kF/(mg1-1/n·L1/n·g-1)	R2
  Fe3O4/CS	298	80.26	0.200 7	0.990 0		3.462	19.41	0.947 3
  	308	108.1	1.223	0.999 4		11.65	72.45	0.9123
  	318	119.6	2.049	0.999 7		10.17	78.70	0.920 2
  ANaOH-Fe3O4/C	298	175.1	0.250 0	0.999 2		12.50	109.4	0.977 2
  	308	184.8	0.387 5	0.999 4		10.79	113.1	0.854 0
  	318	227.3	1.043	0.999 4		8.432	127.0	0.937 9
  AKOH-Fe3O4/C	298	176.0	0.870 2	0.999 8		15.17	123.0	0.939 8
  	308	246.8	0.173 3	0.979 6		6.824	145.7	0.942 5
  	318	300.6	0.493 4	0.998 6		7.761	151.5	0.991 0

  Reusability and stability are a prerequisite for the adsorbent to be used in practical applications^[33]. After reaching saturated adsorption of the dye, the adsorbents were collected with a magnet and heated in a nitrogen atmosphere at 700 ℃ for 2 h. Six consecutive MB adsorption experiments were examined under the same experimental conditions to investigate the adsorption activity of the adsorbent after using repeatedly. As depicted in Fig. 7, the removal efficiency of A_KOH-Fe₃O₄/C was reduced slightly, and a removal rate of about 86% for MB was observed after six successive cycles. This result indicates that activated magnetic porous carbon microspheres exhibit high reusability and can serve as a potential candidate for industrial wastewater treatment.

  Figure 7

  

  Figure 7.  Six cycles of adsorption-desorption of MB on Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C

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  3.   Conclusions

  In brief, a new and effective method based on the electrostatic spraying strategy and the combination method of high-temperature carbonization and alkali activation was developed to prepare A-Fe₃O₄/C, a magnetic porous carbon adsorbent. The adsorption properties of A-Fe₃O₄/C for MB were investigated. The results suggest that A - Fe₃O₄/C has better adsorption ability than Fe₃O₄/CS under the effect of the larger specific surface area. The adsorption process of MB on A-Fe₃O₄/C was systematically illustrated based on the investigations of isotherms and kinetics. The experiments and corresponding calculations suggest that the adsorption of MB on A-Fe₃O₄/C is consistent with the Freundlich isotherm model and the pseudo-second-order kinetic model. Furthermore, A-Fe₃O₄/C can be easily separated from the solution and efficiently regenerated after reaching saturated adsorption. After six cycles, its adsorption capacity for MB was still nearly 86%. In general, a new type of environment-friendly, highly efficient, recyclable adsorbent material was proposed in this study for the removal of dye from wastewater.

  
  
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  Figure 1  SEM images (a-c) and particle size distribution (d-f) of Fe₃O₄/CS (a, d), A_NaOH-Fe₃O₄/C (b, e), and A_KOH-Fe₃O₄/C (c, f)

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  Figure 2  EDS representative SEM image (a-c) and corresponding elemental mappings (d-f) of Fe₃O₄/CS (a, d), A_NaOH-Fe₃O₄/C (b, e), and A_KOH-Fe₃O₄/C (c, f)

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  Figure 3  FT-IR spectra of Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C

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  Figure 4  Nitrogen adsorption-desorption isotherms at 77 K (a) and pore size distributions (b) of Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C

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  Figure 5  (a) Effect of pH on the adsorption of MB (ρ₀ of MB: 100 mg·L^-1, adsorbent dosage: 10 mg, sample volume: 20 mL, t: 8 h); (b) Effect of contact time on the adsorption of MB; (c) Pseudo‐first‐order and (d) pseudo‐second‐order kinetic models for the adsorption of MB by the adsorbents

  Inset: image showing the separation of the sample by a magnet

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  Figure 6  (a‐c) Adsorption isotherms for MB of the samples at different temperatures; (d‐f) Langmuir and (g‐i) Freundlich adsorption model curves for MB adsorption of the samples

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  Figure 7  Six cycles of adsorption-desorption of MB on Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C

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  Table 1.  Texture properties of Fe₃O₄/CS, A_NaOH-Fe₃O₄/C, and A_KOH-Fe₃O₄/C

  Sample	SBET/(m2·g-1)	VM/(cm3·g-1)
  Fe3O4/CS	286	0.38
  ANaOH-Fe3O4/C	350	0.48
  AKOH-Fe3O4/C	368	0.52

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  Table 2.  Kinetic parameters of the adsorption of MB on the samples

  Sample	ρ0 /((mg·L-1))	Qe/ (mg·g-1)	Pseudo-first-forder				Pseudo-second-order
  			k1/min-1	Qe/(mg·g-1)	R2		k2/(g·mg-1·min-1)	Qe/(mg·g-1)	R2
  Fe3O4/CS	30	59.68	0.011 2	31.62	0.998 9		0.000 9	61.05	0.995 9
  ANaOH-Fe3O4/C	50	94.73	0.007 8	33.82	0.941 8		0.000 9	94.34	0.999 8
  AKOH-Fe3O4/C	80	159.5	0.0174	120.6	0.984 6		0.005 6	177.6	0.997 1

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  Table 3.  Langmuir and Freundlich adsorption isotherm parameters for MB adsorption of the samples

  Sample	T/K	Langmuir				Freundlich
  		Qm/(mg·g-1)	kL/(L·mg-1)	R2		n	kF/(mg1-1/n·L1/n·g-1)	R2
  Fe3O4/CS	298	80.26	0.200 7	0.990 0		3.462	19.41	0.947 3
  	308	108.1	1.223	0.999 4		11.65	72.45	0.9123
  	318	119.6	2.049	0.999 7		10.17	78.70	0.920 2
  ANaOH-Fe3O4/C	298	175.1	0.250 0	0.999 2		12.50	109.4	0.977 2
  	308	184.8	0.387 5	0.999 4		10.79	113.1	0.854 0
  	318	227.3	1.043	0.999 4		8.432	127.0	0.937 9
  AKOH-Fe3O4/C	298	176.0	0.870 2	0.999 8		15.17	123.0	0.939 8
  	308	246.8	0.173 3	0.979 6		6.824	145.7	0.942 5
  	318	300.6	0.493 4	0.998 6		7.761	151.5	0.991 0

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