A Novel Fe3O4/Graphene Oxide Composite Prepared by Click Chemistry for High-Efficiency Removal of Congo Red from Water

In this paper, a magnetic graphene oxide (MGO) composite was prepared by the click reaction between the alkyne-modified Fe3O4 nanoparticles and the azide-modified graphene oxide for the purpose of removing the Congo red (CR) dye from water. The deposition of the Fe3O4 nanoparticles on the graphene oxide to successfully prepare the MGO composite was evidenced by the Fourier-transform infrared spectrometer, wide-angle X-ray diffraction equipment, scanning electron microscope, thermal gravimetric analyzer, and Raman spectrometer. The value of saturation magnetization for the MGO composite was 34.9 emu/g. The CR absorption capacities of the MGO composite increased first and then decreased as the pH value increased. It was found that the maximum adsorption capacity of the MGO composite for the CR was as high as 769.2mg/g. In the absorptiondesorption experiment, the CR absorption capacities of the MGO composite from the second cycle to the fifth cycle remained stable to be about 130mg/g. Moreover, both the Langmuir model for the adsorption isotherm and the pseudo-second-order kinetic model could be used to describe the CR absorption behaviors of the MGO composite.


Introduction
Recently, the problem of wastewater caused by organic dyes has aroused great concern and became an urgent event to be solved. So far, the commonly used technologies for the dye treatment of wastewater include biological treatment, coagulation/flocculation treatment, ozone treatment, chemical oxidation, membrane separation, and photocatalytic degradation [1][2][3][4][5][6]. Although the above methods have some advantages such as good decontamination ability, high efficiency, and convenient operation, they will consume enormous energy or have poor repeatability. For example, the degradation process needs not only exciting energy (e.g., gamma radiation or ultraviolet light) but also some additional chemicals (e.g., S 2 O 8 2-or/and H 2 O 2 ) [7][8][9]. On the other hand, the adsorption method exhibited great potential for the treatment of wastewater containing various dyes [10][11][12]. Ahmed et al. have deposited magnesium/iron-layered double hydroxide nanoparticles on waste foundry sand. The adsorption capacity of the resultant composite for the Congo red (CR) dye reached as high as 9127.08 mg/g [10]. Zubair et al. have prepared the starch-NiFe-layered double hydroxide composite by the coprecipitation method, which possessed a high adsorption capacity of 387.59 mg/g for the methyl orange dye [11].
Graphene is a new material with many excellent properties such as its huge surface area and the possession of a number of free π electrons, which can be used as a good adsorbent for some dyes [13][14][15]. As a graphene derivative, graphene oxide (GO) has a two-dimensional planar structure similar to that of the graphene. GO contains a number of functional groups including carboxyl, hydroxyl, and epoxy on its surface, making it has a wide application prospect in the field of adsorbents with good absorption ability [16]. Magnetic nanometal oxides have been extensively studied and applied in magnetic fluid, data storage, catalysis, biomedicine, and pollutant treatment [17]. Graphene combined with magnetic nanomaterial can be used as an adsorbent to treat wastewater contaminants and then easily separated from an aqueous solution under external magnetic field conditions. It has been reported that the above magnetic GO composites simultaneously have the characteristics of GO and the magnetic property, which are new materials widely used in environmental treatment in recent years [18][19][20][21][22][23][24].
"Click chemistry" is an advanced synthesis method discovered by Sharpless et al., which can complete the chemical synthesis of the molecules quickly and reliably by the conjunction of small units [37]. This synthesis method has mild experimental conditions and usually can be operated at room temperature. In recent years, the synthetic method is widely used in the functionalized biological drug, the modification of nanoparticle surface, the application of complex drugs, the magnetic nanocomposites, and so on [38][39][40][41][42][43][44].
In recent years, the adsorption behaviors of the graphene and its derivatives were extensively studied. Graphene and its derivatives have a remarkable adsorption capacity on methylene blue (MB), fluoride, and other substances [41][42][43][44][45][46][47][48][49][50]. Chen et al. prepared the GO by the solvothermal method for the absorption of the MB in the aqueous solution [44]. Leng et al. prepared the GO by the modified Hummers method and then used it to absorb the toxic metal ion antimony with a high adsorption rate of 99.5% [49]. The adsorption of GO and reduced GO (RGO) for the acridine orange dye was also investigated [49]. Furthermore, the research studies on the adsorption of the organic dyes by the low-dimensional magnetic carbon nanomaterials have been increased. The magnetic Fe 3 O 4 /GO composites were used to absorb the MB and CR dyes with the adsorption capacities of 190.14 mg/g and 140.79 mg/g, respectively [42]. The magnetic iron nanoparticles/graphene (Fe/G) composites were also prepared for the absorption of MB [43].
In the current work, we had prepared the alkynylated Fe 3 O 4 nanoparticles and then used the click chemistry to effectively attach them onto the azide-modified GO surface for the fabrication of a novel magnetic graphene oxide (MGO) composite, as shown in Figure 1. The resultant MGO composite was characterized by the Fourier-transform infrared (FTIR) spectrometer, the wide-angle X-ray diffraction (WAXD) equipment, the Raman spectrometer, the vibrating sample magnetometer (VSM), the X-ray photoelectron spectrometer (XPS), the thermogravimetric analyzer (TGA), and the scanning electron microscope (SEM). Furthermore, the absorption behavior of the MGO composite for the CR dye was studied in detail.

Preparation of the MGO Composite by the Click Reaction
. 20 mg GO was well dispersed in 8 mL SOCl 2 and then stirred at 65°C for 24 h. After the excessive SOCl 2 was removed, the resulting SOCl 2 -modified GO was dispersed in 10 mL anhydrous DMF by ultrasonication. Next, 0.58 g DCC, 30 mg DMAP, and 150 mg N 3 -PEG-NH 2 were added into the above SOCl 2 -modified GO/DMF dispersion under nitrogen. The resultant mixture was magnetically stirred at room temperature for 2 h and 60°C for 48 h. After the reaction was completed, the resultant GO-N 3 was thoroughly washed with deionized water and ethanol and finally dried under vacuum at 50°C overnight.

Preparation of Propiolic Acid-Functionalized Fe 3 O 4
Nanoparticles (Alkyne-Fe 3 O 4 ). The mixture of 60 mg Fe 3 O 4 , 20 mL toluene, and 1.5 mL APTES was mechanically stirred at room temperature for 5 h under the protection of nitrogen. The resultant APTES-modified Fe 3 O 4 (Fe 3 O 4 -NH 2 ) nanoparticles were washed with C 2 H 5 OH and CH 2 Cl 2 . The mixture of the above as-prepared Fe 3 O 4 -NH 2 nanoparticles, 20 mL anhydrous CH 2 Cl 2 , 0.75 g DCC, 68 mg DMAP, and 0.43 g propiolic acid was mechanically stirred for 1 h under the protection of an ice bath. Next, after the removal of the ice bath, the reaction was further carried out for 24 h at room temperature. The resulting alkyne-Fe 3 O 4 black solid was magnetically separated, washed with C 2 H 5 OH for several times, and dried at 50°C under vacuum for 12 h.

Characterization Techniques.
FTIR results were obtained from the Bruker Vertex 70 instrument. X-ray diffraction (XRD) measurements were carried out on a Bruker D8 ADVANCE equipment. SEM images were obtained from the Hitachi/SU8010 equipment. Raman spectra were obtained from an HORIBA Jobin-Yvon LabRAM spectrometer. TGA results were obtained from the PerkinElmer Diamond equipment. Ultraviolet and visible (UV-Vis) absorption spectra were obtained from a WFXX-26A UV-Vis spectrophotometer. The magnetic property was characterized using a VSM magnetic measuring instrument (Quantum Design, model MPMS, SQUID, US). XPS spectra were obtained from an X-ray photoelectron spectrometer in the Axis Ultra DLD model.

Adsorption Experiments.
After the absorption by the MGO composite and the following separation of the MGO composite by a magnet, the equilibrium concentration (C e ) of the CR in the aqueous solution was measured with a UV-Vis spectrophotometer at the wavelength of 497 nm. The absorption capacity and the removal rate of the MGO composite for the CR were calculated by where C 0 was the initial concentration of the CR in the aqueous solution before the absorption, V was the CR aqueous solution volume, and m was the MGO composite mass.

Results and Discussion
3.1. Characterization of the MGO Composite. It could be clearly observed from the SEM images of the MGO composite (see Figure 2) that a lot of Fe 3 O 4 nanoparticles had been homogeneously attached on the surface of GO, which intuitively confirmed the formation of the magnetic Fe 3 O 4 /GO composite after the click reaction. The FTIR spectra of the GO and the GO-N 3 are shown in Figure 3. The FTIR spectrum of the GO displayed the C=O stretching vibration of the carboxyl groups, the O-H stretching vibration of the hydroxyl groups in both the graphene skeleton and the carboxyl groups, the C=C stretching vibration of the graphene skeleton, and the C-O stretching 3 Journal of Nanomaterials vibration of the epoxy groups on the graphene skeleton at 1722 cm -1 , 3436 cm -1 , 1624 cm -1 , and 1062 cm -1 , respectively. After the modification of the GO, the resultant GO-N 3 exhibited a series of FTIR peaks at 1547 cm -1 for the N-H stretching vibration of the amide groups (band II), 1463 cm -1 for the C-N stretching vibration of the amide groups, 2912 cm -1 as well as 2843 cm -1 for the C-H stretching vibration of the methylene groups, and 2092 cm -1 for the N≡N stretching vibration of the azide groups, respectively. Moreover, compared with the FTIR spectrum of GO, both the higher peak intensity of the C-O stretching vibration for epoxy groups at 1052 cm -1 and the significantly decreased peak intensity of the O-H stretching vibration for carboxyl groups at 3436 cm -1 could be found in the FTIR spectrum of GO-N 3 . These FTIR results confirmed the successful amidation reaction between the GO and the N 3 -PEG-NH 2 , indicating that the N 3 -PEG-NH 2 macromolecules were linked to the GO by the covalent amide bonding.
The   The Raman spectra of the graphite, the GO, and the GO-N 3 are shown in Figure 5. The Raman shifts of the D peak and the G peak for the graphite were at 1324 cm -1 and 1572 cm -1 , respectively, and the intensity ratio of the D peak to the G peak (I d /I g ) for the graphite was 0.25. For the GO obtained from the oxidation of the graphite, its Raman shifts of the D peak and the G peak were at 1324 cm -1 and 1601 cm -1 , respectively, along with a I d /I g value of 1.36. These results indicated that there were many defections on the GO surface due to the breaking of carbon-carbon double bonds and the attachment of a lot of the oxygenating groups during the oxidation process. For the GO-N 3 , its Raman shifts of the D peak and the G peak were at 1336 cm -1 and 1593 cm -1 , respectively, along with a I d /I g value of 1.48. This indicated that, after the grafting of the N 3 -PEG-NH 2 on the graphene surface by the amidation reaction, the surface structure of the resultant GO-N 3 had been changed in comparison with that of the GO.
The WAXD patterns of the GO and the MGO composite are shown in Figure 6. In the WAXD pattern of the GO, the diffraction peak appearing at 2θ = 10:  Figure 7. There were two significant weight loss stages in the TGA curve of the GO with a weight loss of 63.1 wt.% at 800°C. The first thermal decomposition stage of the GO below 100°C with a weight loss of 15.2 wt.% was mainly due to the evaporation of the adsorbed water while the second one between 150°C and 230°C with a weight loss of 29.6 wt.% could be ascribed to the decomposition of oxygen-containing functional groups. Similarly, the GO-N 3 also had two main thermal decomposition stages with a weight loss of 41.2 wt.% at 800°C and exhibited much more stability than the GO because the molecular weight of the

Adsorption Studies of the MGO Composite.
According to the relationship between the contact time (t) and the absorption capacity within t minute (q t ) of the MGO composite (see Figure 10) at the t moment, the MOG composite quickly absorbed the CR dye in the aqueous solution in 10 minutes and the absorption equilibrium was reached within 30 minutes. The CR absorption capacity of the MGO composite at equilibrium (q e ) was about 490 mg/g with a removal rate of about 98%. We believed that the good CR absorption ability of the MGO composite could be explained by two reasons: (1) both the high-surface area MGO composite and the CR had the benzene ring structure, leading to the mutual attraction by the π-π interaction, and (2) the MGO composite containing the hydroxyl group and the imide group could    Journal of Nanomaterials effectively absorb the CR containing the amine group by the hydrogen bonding and the dipole-dipole mutual interaction [52][53][54][55]. Moreover, the MGO composite after the CR absorption could be recollected by a magnet, as shown in the inset of Figure 9. Furthermore, the mechanism of the CR absorption kinetic model for the MGO composite was studied by the quasi-first-order model and the quasi-second-order model by the fitting of the t and the q t as follows.  Pseudo-first-order kinetics Pseudo-second-order kinetics Quasi-second-order kinetic model: where k 1 and k 2 were the constant. The experimental fitting results (see Figure 11 and Table 1) showed that the CR adsorption process of the MGO composite could be well fitted by the quasi-second-order kinetic model with R 2 = 0:9998, indicating that the CR adsorption kinetic model of the MGO composite was accurately simulated by the quasisecond-order kinetic model. Both the Langmuir isotherm model (see Equation (4)) and the Freundlich isotherm model (see Equation (5)) were used to study the absorption isotherm of the MGO composite for the CR.
Langmuir isotherm model: Freundlich isotherm model: where both b and K f were the constant, and q mL was the maximum CR absorption capacity of the MGO composite basing on the Langmuir isotherm model. As shown in Figure 12 and listed in Table 2, the adsorption process of the MGO composite was more consistent with the uniform monolayer CR adsorption described by the Langmuir model with the R 2 = 0:949. The q mL value was calculated to be 769.2 mg/g, which was much higher than many other reported absorbents (see Table 3) [54][55][56][57][58][59][60]. Figure 13 shows the effect of the pH value on the CR absorption capacities of the MGO composite, which increased first and then decreased as the pH value increased. It is well known that the concentration of the H + decreases as the pH value increases. When the pH value was low, there was a competition between the protonated positive CR molecules and there was a lot of H + on the active adsorption site of the negative MGO composite. On the other hand, when the pH value was high, the CR became negatively charged, which resulted in the reduction of the mutual interaction with the negative MGO composite. Therefore, the maximum CR absorption capacity by the MGO composite could be obtained at pH = 7 with the maximum CR absorption capacity of 442.8 mg/g and the removal rate of 88.6%.
The MGO composite was subjected to the absorptiondesorption experiment for the CR in five cycles (see Figure 14), and the resultant CR absorption capacities of the MGO composite were 442.8 mg/g with the removal rate of 88.6%, 133.3 mg/g with the removal rate of 26.7%, 128.9 mg/g with the removal rate of 25.8%, 137.7 mg/g with the removal rate of 27.5%, and 122.7 mg/g with the removal rate of 24.5% in sequence. This result was probably caused    Journal of Nanomaterials by the strong mutual interaction between some absorption sites of the MGO composite and the absorbed CR molecules, which were difficult to desorb. Although the regenerated ability of the MGO composite for the CR absorption was not ideal, the CR absorption capacities of the MGO composite from the second cycle to the fifth cycle remained stable to be about 130 mg/g. These values were still obviously higher than those (113.2 mg/g and 33.66 mg/g) of the other reported magnetic graphene-based absorbents [54][55][56][57].

Conclusion
Both the alkyne-modified Fe 3 O 4 nanoparticles and the azidemodified GO were prepared and then attached together by the click chemistry to obtain the MGO composite. The successful attachment of the Fe 3 O 4 nanoparticles onto the GO surface was testified by FTIR, WAXD, SEM, and Raman spectrum, and VSM. The optimized condition of the CR absorption by the MGO composite was at pH = 7. The MGO composite possessed a high CR absorption capacity of 769.2 mg/g and could be magnetically recollected after absorption. Moreover, the CR absorption behaviors of the MGO composite could be explained by the pseudo-secondorder kinetic model and the Langmuir model. All of the experimental results indicated that the MGO composite could be a potential adsorbent to purify the wastewater containing the CR dye.

Data Availability
Any data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.