Magnetically separable functionalized graphene oxide decorated with magnetic cyclodextrin as an excellent adsorbent for dye removal

Abstract

Magnetic cyclodextrin/graphene oxide (MCGO) materials were fabricated through a facile chemical route and their application as excellent adsorbents for methylene blue (MB) was also demonstrated. The results from FTIR, SEM, TEM, VSM and XRD showed that MCGO was prepared. The large saturation magnetization (51.13 emu g⁻¹) of the synthesized nanoparticles allows fast separation of the MCGO from liquid suspension. The MCGO demonstrate extremely fast MB-removal from wastewater with a high removal efficiency within 50 min. The adsorption kinetics, isotherms and thermodynamics were investigated and indicate that the kinetics and equilibrium adsorption are well-described by pseudo-second-order kinetics and the Langmuir isotherm model, respectively. The results showed that, benefiting from the surface properties of graphene oxide, the hydrophobicity of cyclodextrin, and from the magnetic properties of Fe₃O₄, the adsorbent possesses quite a good and versatile adsorption capacity towards the dye under investigation. Most importantly, the adsorbent can be easily and efficiently regenerated for reuse with hardly any compromise of the adsorption capacity. The inherent advantages of the nano-structured adsorbent, such as high adsorption capacity, easy operation, rapid extraction, and regeneration, may pave a new, efficient and sustainable way towards highly-efficient dye pollutant removal in water and wastewater treatment.

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1. Introduction

Dyes are important chemicals widely used in pharmaceuticals, rubbers, pesticides, varnishes, dyestuffs and so on.^1–4 Consequently, much attention should be paid to treat dyes before discharge. Accordingly, several conventional technologies have been proposed for separation of dyes from wastewater, including liquid–liquid extraction, membrane filtration, and adsorption.^5–8 Among these methods, adsorption is the most widely used method because of the ease of operation and comparable low cost of application. Various adsorbents, such as bamboo activated carbon,⁹ cedar sawdust and crushed brick,¹⁰ jute fiber carbon,¹¹ unburned carbon,¹² carbon nanotube,¹³ bentonite,¹⁴ and garlic peel¹⁵ have been studied for adsorption of MB from aqueous solutions.

However, these processes vary in their effectiveness, cost and environmental impact.¹⁶ Adsorption is a more competitive treatment process for dye removal because of its simplicity, high efficiency, and wide-ranging availability.^17,18 Hence, there is a need to search for adsorbents that are more effective, since the water treatment industry requires ecofriendly, highly effective and reusable adsorbents that are available in tonnage quantities.

β-Cyclodextrin (β-CD) is a cyclic oligosaccharide consist of seven α-D-glucose units connected through α-(1,4) linkages, which has a hydrophobic inner cavity and a hydrophilic exterior. Cyclodextrin can enable them to bind selectively various organic, and inorganic compound into their cavities to form stable host–guest inclusion complexes¹⁹ or nanostructured supramolecular assemblies in their hydrophobic cavity, showing high molecular enantioselectivity and selectivity.^20–22 There are some reports on environmental applications of cyclodextrin for dyes. Magnetic cyclodextrin have high surface area, high adsorption capability, strong acid-resistant and can be easy separation, which can facilitate the removal of dyes from wastewater.^23–25 We have explored the magnetic cyclodextrin in the application of adsorption metal before.²⁶

Graphene and graphene oxide (GO) based nanocomposites have attracted tremendous attentions due to their many excellent properties such as mechanical flexibility, thermal and chemical stability, and large surface area.^27,28 Compared with conventional adsorbents, the merits of GO as adsorbents are their structure types and diverse compositions, tunable pore size, large surface area, and coordinatively unsaturated or saturated metal sites to regulate the adsorption ability.^29–32

Cyclodextrins are environmentally friendly and can improve the stability of functional materials. If graphene oxide (GO) is modified with magnetic cyclodextrin, it is possible to obtain new materials simultaneously possessing the unique properties of GO (large surface area and good mechanical properties) and magnetic cyclodextrin (strong acid-resistant, superparamagnetism and high adsorption capability) through combining their individual characteristics. The magnetic cyclodextrin benefiting from the core–shell structure are uniformly monodispersed on the graphene oxide sheets and are stable against oxidation/dissolution in HCl acid (1 M). Furthermore, fabrication of magnetic β-cyclodextrin/graphene oxide nanocomposite, especially those with great potential, can be used in environmental remediation with fast treatment.

The objectives of this study were (1) to prepare MCGO composites and to characterize them by FTIR, SEM, VSM and XRD; (2) to simulate MB adsorption isotherms and to estimate the adsorption capacities; (3) to study the effects of the treatment time and pH values on the MB removal; and (4) to evaluate the possibility of regeneration and reusability of MCGO as an adsorbent. In previous study, our laboratory have explored magnetic chitosan/graphene oxide as dye-adsorbent,³³ but the cyclodextrins is better than chitosan serve as a dye remover in aqueous solutions because of envelope interaction, so decorated the GO with magnetic cyclodextrins could improve the adsorptive property of the material. The MCGO are found to possess unique capability to remove MB very quickly and efficiently from wastewater. The application of MCGO for removal of MB with the help of an external magnetic field is shown in Scheme 1.

Scheme 1 Synthesis of MCGO and their application for removal of MB with the help of an external magnetic field.

2. Experimental section

2.1 Materials

Fe₃O₄ (average particle diameter of 10 nm, 99.9% purity, Boyu Company, Beijing). All reagents were analytical-grade and were used as received.

Graphene oxide nanosheets were prepared by using the modified Hummers method from the natural flake graphite (average particle diameter of 20 mm, 99.95% purity, Qingdao Tianhe Graphite Co. Ltd., China) using concentrated H₂SO₄, HNO₃ and KMnO₄ to oxidize the graphite layer. With the aid of ultrasonication, the oxidized graphite layers were exfoliated from each other. Then 20% H₂O₂ was added in the suspension to eliminate the excess MnO₄⁻. The desired products were rinsed with deionized water.

2.2 Preparation of MCGO

0.5 g of Fe₃O₄ was mixed with 20.0 mL of 80 mg mL⁻¹ NH₂-β-CD³⁴ aqueous solution and 100.0 mL of ammonia solution. 5.0 mL of pure glutaraldehyde was added into the above solution and was stirred at 50 °C for 0.5 h. Then, a 20.0 mL portion of the homogeneous graphene oxide dispersion (0.5 mg mL⁻¹) was added. After being vigorously shaken or stirred for a few minutes, the vial was put in a water bath (60 °C) for 3.5 h. The obtained product was collected by the aid of an adscititious magnet and dried in a vacuum oven at 50 °C. The obtained product was MCGO. The carboxyl group of GO chemically reacts with the amine group of NH₂-β-cyclodextrin with consequent formation of chemical bond between GO and magnetic cyclodextrin.

2.3 Adsorption experiments

All batch adsorption experiments were performed on a model KYC-1102 C thermostat shaker (Ningbo, China) with a shaking speed of 150 rpm. Simulated wastewater with different MB concentrations (40.0, 60.0, 80.0, 100.0, 120.0, 140.0, 160.0, 180.0 mg L⁻¹) were prepared. MCGO (0.10 g) were added to 100 mL of the above MB solution under mechanical agitation. For all adsorption tests, the initial pH values of the MB solutions were adjusted with 0.1 mol L⁻¹ HCl solution or 0.1 mol L⁻¹ NaOH solution measured by laboratory pH meter (MP511, China). After the adsorption processes, MCGO was conveniently separated by magnetic separation and the supernatant was immediately analyzed by atomic absorption spectrometry (WFX-1F2, China). To study the influence of initial pH on the removal of MB, the initial pH values of the solutions were adjusted to 2, 3, 4, 8, 9, 10 and 11. The concentration of MCGO was 1.0 g L⁻¹. The initial MB concentration was 100.0 mg L⁻¹. For the kinetic study, the MCGO concentration was maintained at 1.0 g L⁻¹ in the neutral solution for different adsorption time such as 20, 30, 40, 50, 60, 70, 90 and 110 min. For the regeneration, aqueous solutions adjusted to pH 2 were used as the eluant for MB. To test the reusability of the beads, this adsorption–desorption cycle was repeated five times by using the same affinity adsorbents. The adsorption amount and adsorption rate are calculated based on the difference in the MB concentration in the aqueous solution before and after adsorption, according to the following equation:

Q = (C₀ − C_e)V/W, E = (C₀ − C_e)/C₀ × 100%

where, C₀ and C_e are the initial and equilibrium concentrations of MB in milligrams per liter, respectively, V is the volume of MB solution, in liters, and W is the weight of the MCGO used, in grams.

2.4 Replication of batch experiment

Each batch adsorption experiment above was conducted twice to obtain reproductive results with error <5%. In the case of deviation larger than 5%, more tests were carried out. The experimental data could be reproduced with an accuracy greater than 95%. All the data of batch adsorption experiments are the average values of two tests.

2.5 Characterizations

Microscopic observation of samples was carried out by using a scanning electron microscope (S-2500, Japan Hitachi). Transmission electron microscopy was conducted on a JEOL-2011 operated at 200 kV. FT-IR spectra of samples were recorded with KBr pellet in the range of 4000–400 cm⁻¹ on FTIR spectra were measured on a Nicolet, Magna 550 spectrometer. Wide angle X-ray diffraction (WAXRD) patterns were recorded by a D8 ADVANCE X-ray diffraction spectrometer (Bruker, German) with a Cu Kα target at a scan rate of 0.02° 2θ s⁻¹ from 10° to 80°. The Brunauer–Emmett–Teller (BET) surface area and the pore size distribution of the composite were measured using N₂ adsorption and desorption (QUADRASORBSI, Quantachrome, USA) at 77 K over a relative pressure ranging from 0.0955 to 0.993. The magnetic property of the MCGO at room temperature was measured in a 9 T physical properties measurement system (PPMS) by quantum design.

3. Results and discussion

3.1 Characterization of MCGO

The results of SEM, XRD, FTIR, and VSM characterization of the prepared MCGO are shown in Fig. 1. XRD patterns of pure GO (A) and MCGO (B) are shown in Fig. 1a. As shown in Fig. 1a(A), the broad and relatively weak diffraction peak at 2θ = 10.03° (d = 0.87 nm), which corresponds to the typical diffraction peak of graphene oxide nanosheets, is attributed to the (002) plane. The c-axis spacing increases from 0.34 to 0.87 nm after the graphite is modified to graphene oxide nanosheets, which is due to the creation of the abundant oxygen-containing functional groups on the surfaces of graphene oxide nanosheets. The XRD analysis results of pure Fe₃O₄ (Fig. S1†) and MCGO (B) were mostly coincident. Six characteristic peaks for Fe₃O₄ (2θ = 30.1, 35.5, 43.3, 53.4, 57.2 and 62.5), marked by their indices ((220), (311), (400), (422), (511), and (440)), were observed in MCGO.

Fig. 1 Characterization of samples: (a) XRD patterns of graphene oxide (A) and MCGO (B); (b) the absence of external magnetic field (A) and the presence of additional magnetic field (B); (c) SEM image of graphene oxide (d) SEM image of MCGO; (e) room temperature hysteresis loop of the MCGO; (f) FT-IR spectrum of GO and MCGO.

Fig. 1c shows the typical SEM images of GO obtained by a modified Hummers method. It presents the sheet-like structure with the large thickness, smooth surface, and wrinkled edge (Fig. S2†). After the combination with magnetic cyclodextrin to form the MCGO composite (Fig. 1d and S2†), the MCGO had a much rougher surface, which reveals that many small magnetic cyclodextrin had been assembled on the surface of GO layers.

The magnetization property of the MCGO composite was investigated at room temperature by measuring the magnetization curve (Fig. 1e). The saturation magnetization (Ms) of the MCGO composite is 51.13 emu g⁻¹, indicating that the MCGO composite has a high magnetism. The inset of Fig. 1e shows that the MCGO composite are attracted quickly toward a magnet (Fig. 1b).

The FTIR pattern of GO, as shown in Fig. 1f, reveals the presence of the oxygen-containing functional groups. The peaks at 1070, 1380, 1630 cm⁻¹ correspond to C–O–C stretching vibrations, C–OH stretching, C–C stretching mode of the sp2 carbon skeletal network, respectively, while peaks located at 1730 and 3440 cm⁻¹ correspond to C–O stretching vibrations of the –COOH groups, and O–H stretching vibration, respectively. As shown in MCGO, except characteristic peaks (1070, 1630 cm⁻¹) of GO, 890 cm⁻¹ is the characteristic band of β-(1,4) glucopyranose in cyclodextrin. In addition, 580 cm⁻¹ is the characteristic peak of Fe₃O₄. These indicated that magnetic cyclodextrin was successfully grafted on the surface of GO.

3.2 Effect of initial solution pH

The pH value of the solution has been identified as the most important variable governing dyes adsorption on the adsorbent. The effect of initial solution pH on the removal of MB by MCGO was investigated (Fig. 2). The maximum adsorption value for MB onto MCGO was 273.41 mg g⁻¹ at pH 11. The graphene sheets are negatively charged in solution due to the presence of the oxygen containing groups.^35,36 The decrease in the adsorption capacity of dye at lower pH values may be due to the protons competition with the dye molecules for the available adsorption sites.³⁷ As the pH of the MB solution increases, the electrostatic attraction between the negatively charged surface of the MCGO and cationic MB molecule increases, resulting in an increase in the adsorption capacity of MB.

Fig. 2 The effect of solution pH on MB removal efficiency of MCGO (initial concentration, 100 mg L⁻¹; contact time, 50 min; temperature: 30 °C).

3.3 Adsorption kinetics

In order to study the mechanism of adsorption kinetics, models have been used to test experimental data. The kinetics of the adsorption describing the MB uptake rate is one of the important characteristics which control the residence time of adsorbate uptake at the solid–liquid interface. The effect of the contact time for MCGO on the adsorption capacity for MB is described in Fig. 3a. Obviously, adsorbent showed a good performance in adsorption during the first 50 min. Quantifying the changes in adsorption with time requires an appropriate kinetic model, pseudo-first-order and pseudo-second-order are investigated and compared.

Fig. 3 (a) Time profile of MB removal with MCGO (pH 10; initial concentration, 100 mg L⁻¹; temperature: 30 °C); (b) pseudo-second-order kinetics of MB adsorption on the MCGO composite.

The pseudo-first-order kinetic model is expressed by the following equation:

ln(Q_e − Q_t) = ln Q_e − K₁t

where Q_e and Q_t are the amount adsorbed in mg g⁻¹ at equilibrium, time ‘t’ in min, and K₁ is the rate constant of adsorption (min⁻¹). The values of correlation coefficient (R² = 0.36) are relatively low. This shows that the adsorption process may not be the correct fit to the first-order rate equation.

Another kinetic model is pseudo-second-order model, which is expressed by:

t/Q_t = 1/(K₂Q_e²) + t/Q_e

where K₂ is the equilibrium rate constant of pseudo-second-order adsorption (g mg⁻¹ min⁻¹). The slope and intercept of the plot of t/Q_t versus t were used to calculate the second-order rate constant, K₂. The comparison between the experimental adsorption capacity (Q_exp: 226.8 mg g⁻¹, 100 mg L⁻¹) value and the calculated adsorption capacity (Q_cal: 228.5 mg g⁻¹, 100 mg L⁻¹) value shows that Q_cal value is very close to Q_exp value for the pseudo-second-order kinetics. Moreover, the adsorbent system can be well described by pseudo-second-order kinetic model (Fig. 3b), which also is confirmed according to the correlation coefficient value (R² = 0.992) for pseudo-second-order model, much higher than that of pseudo-first-order (R² = 0.36), suggesting that the adsorption may be the rate-limiting step involving valence forces through sharing or exchange of electrons between the adsorbent and the adsorbate.

3.4 Adsorption isotherms

Several mathematical models have been used for describing equilibrium studies for the adsorption of dyes on solid surfaces. The Langmuir and Freundlich models are frequently utilized to fit the experimental data. In this study, both models were applied to describe the experimental data obtained at 303 K.

The Langmuir adsorption isotherm can be expressed as:

C_e/Q_e = 1/(K_LQ₀) + C_e/Q₀

where C_e is the equilibrium concentration of MB in solution (mg L⁻¹), Q_e is the adsorbed value of MB at equilibrium concentration (mg g⁻¹), Q₀ the maximum adsorption capacity (mg g⁻¹), and K_L is the Langmuir binding constant, which is related to the energy of adsorption. Plotting C_e/Q_e against C_e gives a straight line with slope and intercept equal to 1/Q₀ and 1/(K_LQ₀), respectively. It is described in Fig. 4a.

Fig. 4 The equilibrium isotherm for MB adsorbed by MCGO: (a) the Langmuir isotherm, (b) the Freundlich isotherm (pH 10; contact time, 50 min; temperature: 30 °C).

By calculating, the results are as follows:

C_e/Q_e = 0.00382C_e + 0.00416, R² = 0.996, Q₀ = 261.78 mg g⁻¹, K_L = 0.91 L mg⁻¹

The MB maximum adsorption capacity of MCGO was 261.78 mg g⁻¹, which was higher than the reported values of pyrophyllite (4.2 mg g⁻¹),³⁸ carbon nanotube (46.2 mg g⁻¹)³⁹ and graphene⁴⁰ indicating that MCGO is a good adsorbent to remove dyes from aqueous solutions. In addition, the coefficients of determination R² of the Langmuir equation demonstrated that the adsorption of MB onto MCGO follows the Langmuir's model.

Freundlich isotherm is an empirical equation based on adsorption on a heterogeneous surface. The equation is commonly represented by:

ln Q_e = ln K_F + (1/n)ln C_e

K_F (mg g⁻¹ (L mg⁻¹)^1/n) and n are the Freundlich constants characteristics of the system, indicating the adsorption capacity and the adsorption intensity, respectively. If the value of 1/n is lower than 1, it indicates a normal Langmuir isotherm; otherwise, it is indicative of cooperative adsorption. It is described in Fig. 4b.

By calculating, the results are as follows:

ln Q_e = 0.08281 ln C_e + 5.21264, R² = 0.91, 1/n = 0.08281, R² = 0.96

The Langmuir and Freundlich adsorption constants and the corresponding correlation coefficients are got. The adsorption of MB was well fitted to the Langmuir isotherm model with the higher R² (0.996). It indicated the adsorption took place at specific homogeneous sites within the adsorbent forming monolayer coverage of MB at the surface of the MCGO. The Freundlich constant 1/n was smaller than 1, indicating a favorable process.

Furthermore, the essential characteristics of the Langmuir isotherm can be described by a separation factor, which is defined by the following equation:^41,42

R_L = 1/(1 + K_LC₀)

The value of R_L indicates the shape of the Langmuir isotherm and the nature of the adsorption process. It is considered to be a favorable process when the value is within the range 0–1. In the study, the values of R_L calculated for the initial concentrations of MB were 0.01. Since the result is within the range of 0–1, the adsorption of MB onto adsorbent appears to be a favorable process.

3.5 Thermodynamic study

To evaluate the effect of temperature on adsorption process of MB onto MCGO, the thermodynamic parameters such as Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) are calculated using the following equation:

ln (Q_e/C_e) = −ΔH/(RT) + ΔS/R

ΔG = ΔH − TΔS

R is the universal gas constant (8.314 J mol⁻¹ K) and T is the absolute temperature (in Kelvin). Plotting ln(Q_e/C_e) against 1/T gives a straight line with slope and intercept equal to −ΔH/R and ΔS/R, respectively. It is described in Fig. 5a.

Fig. 5 (a) Van't Hoff plots for the adsorption of MB onto MCGO (pH 10; initial concentration, 100 mg L⁻¹; contact time, 50 min); (b) effect of recycling adsorbents on MB adsorption (pH 10; initial concentration, 100 mg L⁻¹; temperature, 30 °C; contact time, 50 min).

The negative value of ΔH (Table 1) shows exothermic nature of adsorption process. The adsorption was favored at lower temperature and MB molecules were orderly adsorbed on the surface of MCGO. The negative value of ΔG suggests the feasibility and the spontaneous nature of the adsorption.

Table 1 Thermodynamic parameters at different temperatures

T (K)	ΔG (kJ mol^−1)	ΔH (kJ mol^−1)	ΔS (J mol K^−1)
303	−7.157	−4.595	−8.455
313	−7.241	—	—
323	−7.326	—	—

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3.6 Cycles of sorption–desorption

The stability and regeneration ability of the adsorbent is crucial for its practical application. The sorption experiments revealed that strongly basic (pH 10) conditions were necessary for the binding of MB. Thus, it would be logical that the sorbed dyes could be recovered under reversely conditions i.e. strongly acid for MB.⁴³ Therefore, aqueous solutions adjusted to pH 2 were used as the eluant for MB. The effect of six consecutive adsorption–desorption cycles was studied, and the results are shown in Fig. 5b. The adsorption capacity of MB on the adsorbents decreased slowly with increasing cycle number. The percentage adsorption remained steady at about 90% in the first three cycles, and then the adsorption capacity of MB decreased. At the fifth regeneration cycle, the adsorption remained at 75%. These results show that the adsorbents can be recycled for MB adsorption, and the adsorbents can be reused.

4. Conclusions

MCGO is prepared via a simple chemical synthesis method. MCGO composites are superparamagnetic at room temperature and can be separated by an external magnetic field. In batch adsorption experiments, the adsorption kinetics, isotherms and thermodynamics were investigated in detail. The kinetic study revealed the adsorption process was well fitted the pseudo-second-order kinetic model. The equilibrium data were well-modeled by the Langmuir isotherm model. The thermodynamic parameters suggested that the adsorption process was spontaneity and endothermic in nature. This work showed that the MCGO composite could be utilized as a magnetically separable and efficient adsorbent for the environmental cleanup.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC, nos 21345005 and 21205048), the Shandong Provincial Natural Science Foundation of China (no. ZR2012BM020) and the Scientific and technological development Plan Item of Jinan City in China (no. 201202088).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06292b

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