Fabrication of Amino Functionalized Magnetic Expanded Graphite Nanohybrids for Application in Removal of Ag ( I ) from Aqueous Solution

Ethylenediamine functionalized magnetic expanded graphite decorated with Fe3O4 nanoparticles (MEG-NH2) was fabricated by one-pot solvothermal method. The as-prepared MEG-NH2 nanohybrids were characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), and Zeta potential analyzer. The effects of Fe3O4 content in MEG-NH2 nanohybrids, pH, initial concentration, contact time, and dosage on adsorption properties of the MEG-NH2 nanohybrids for Ag(I) from aqueous solution were investigated by batch experiments. The pseudo-first-order and the pseudosecond-order kinetic models were utilized to study adsorption kinetics. The experimental data was also analyzed with Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isothermmodels.The results show that Ag(I) was reduced to silver in the process of the adsorption byMEG-NH2 nanohybrids; the experimental data was better fitted to pseudo-second-order model and Langmuir isotherm model which revealed that the adsorption process was a chemical adsorption by the formation of silver on the surface of MEG-NH2 nanohybrids.


Introduction
In recent years, numerous water bodies such as rivers, lakes, and ponds have accommodated a great deal of waste water composed of industrial and domestic sewage.Among various contaminants, heavy metal ions occupy a large proportion.Drainage of high concentration of heavy metal ions is extremely deleterious to living organism and the environment, especially to the existence and development of human beings, which has aroused global concerns.Among heavy metals, sliver is not dietary requirement for organic survival.Englobement of high level of silver is toxic to human cells.Ag(I) is more fatal for fish than copper or mercury [1].On the other hand, as a precious metal, silver possesses extensive commercial value in various fields for its unique ornament and decoration performance and highest electrical and thermal conductivity along with excellent machinability [2].
Given the above notable properties, the widely technical and industrial usage of silver in photography, brazing, electronic products, electroplation, and so forth heavily accelerates its requirements.However, with surging demand of silver, silver bearing mines that are available for exploitation have reached a steep reduction.Moreover, substantial employment of silver in various industries inevitably creates mass production of silver-containing effluent.To dispose the thorny issue, a large number of measures have been taken, including adsorption, cathodic reduction [3], ion-exchange [4], solvent extraction [5], and so on.During the past decade, adsorption method has attracted much attention in heavy metal removal and recycling [6] for its energy saving, high efficiency, and outstanding selectivity, especially its superiority in disposal of trace metal ions.Currently, various adsorbents have been utilized to treat waste water containing silver ions, such as chitosan [7], ion-imprinted polymers [8], zeolite, chemically 2 Journal of Nanomaterials modified melamine resins [9], and bioadsorbent [10].It is somewhat difficult for these traditional adsorbents to separate for resource recycling after adsorption.The magnetic adsorption materials solve the practical problem.
Magnetic expanded graphite (MEG) holds lower price and easier preparation properties compared with graphene, carbon nanotubes, and other carbonaceous adsorbents, arousing researchers' extensive interests [11,12].Generally, the synthesis process of MEG with a majority of macropores and favorable magnetic response is composed of oxide intercalation, high temperature expansion, and magnetic loading.The resulting macropores formed in the interlayers, providing passageway for ion diffusion.Acquisition of magnetic performance avoids centrifugation, decompressed filter, and slather usage of expensive percolators.With a weak magnet, it is very simple and effective to separate the material along with target contaminant.Thus, secondary pollution will never occur, and the recovery and recycling of silver are ready to be realized [13].Nevertheless, it is not ideal for pure MEG to be applied to adsorb silver ions owing to lack of appropriate functional groups.These materials mostly need modification to improve their physicochemical properties.Accordingly, it is necessary to graft suitable functional groups onto surface of MEG to introduce active sites for silver ion adsorption.
In this study, using ethylenediamine functionalized magnetic expanded graphite (MEG-NH 2 ) nanohybrids with desirable performance composed of magnetism of Fe 3 O 4 , chemical adsorption of amino groups, and unique ion diffusion channels of EG were synthesized by one-pot solvothermal reaction.In contrast to chemical coprecipitation method [14], the approach prevented oxidation and aggregation of nano-Fe 3 O 4 .The effects of Fe 3 O 4 content in MEG-NH 2 nanohybrids, pH, initial concentration, contact time, and dosage on the adsorption properties of the MEG-NH 2 nanohybrids for Ag(I) were taken into consideration during adsorption experiments.Adsorption kinetics and isotherms were also studied to ascertain the adsorption process.

Preparation of Expanded
Graphite.Expanded graphite (EG) was prepared according to our previous work [15].NFG (20 g) was added to the mixture mingled with 70% H 2 SO 4 (45 g) and 68% HNO 3 (15 g) in a 250 mL threeneck flask.Subsequently, KMnO 4 (2.2 g) was put into the mixture within several times under vigorous stirring.After reaction for 50 min, the products were collected with highspeed centrifuge and washed with deionized water till the filtrate turned into neutrality.Then, the as-prepared product was dried in vacuum at 50 ∘ C for 24 h.Finally, the above dried sample was heat treated abruptly at 600 ∘ C for 15 s to obtain EG.

Preparation of MEG-NH 2
Nanohybrids.MEG-NH 2 nanohybrids were synthesized by using one-pot solvothermal method as follows.Firstly, different contents of FeCl 3 ⋅6H 2 O (0.8 g, 1.0 g, and 1.2 g) and sodium acetate (3.0 g) were fully dissolved into a 100 mL beaker containing ethanediol (20 mL), and then ethylenediamine (10 mL) was added under stirring.The mixture turned into transparent yellowish color after ultrasonic oscillation was exerted.The above solution was transferred into a Teflon lined stainless steel autoclave containing EG (0.15 g) with vigorous agitation.The container was sealed and heated to 200 ∘ C for 8 h in a muffle furnace and then cooled to room temperature.The black product was washed several times with ethanol by applying an external magnetic field.Ultimately, the obtained different contents of Fe 3 O 4 of MEG-NH 2 nanohybrids were dried in vacuum at 50 ∘ C for 24 h.Fe 3 O 4 -NH 2 was prepared by the process mentioned above in the absence of EG.

Characterization.
The scanning electron microscopy (SEM) images taken with a JEOL JSM-6701 field emission scanning electron microscope (FESEM, 5 kV) were applied to observe the external morphology of samples.X-ray diffraction patterns (XRD, Rigaku D/MAX-2400 X-ray diffractometer with Ni-filtered Cu K radiation ( = 1.54056 >)) were performed to detect the crystal structure of samples.Fourier transform infrared spectroscopy spectra (FTIR) were recorded on a NEXUS 670 spectrometer with samples pressed into KBr pellets.X-ray photoelectron spectroscopy (XPS) analysis was carried out to detect the adsorbents before and after adsorption.Thermogravimetry analyzer (TGA, STA449C, Netzsch, Germany) was performed to investigate the thermal stability of samples.Hysteresis loops were obtained by a vibrating sample magnetometer (VSM) which was employed to perform magnetic properties of as-synthesized samples.Zeta potential was measured in a Malvern Zetasizer by dispersing the sorbent in deionized water with different pH values.Ultraviolet spectrophotometer (UV-752N) was used to measure the absorbance of Ag(I) at 365 nm.

Adsorption Experiments.
Adsorption experiments were carried out with a typical batch approach.AgNO 3 was used as source of Ag(I).Various parameters including pH, contact time, dosage, and temperature were investigated.The pH was adjusted to certain values by dropping different concentrations of HNO 3 and NaOH aqueous solution.The concrete procedure was according to the following method: a certain amount of adsorbent was added to a 250 mL conical flask containing AgNO 3 aqueous solution of 50 mL at a known concentration accompanied with manual shaking rather than ultrasonic dispersion to keep the original morphology of samples.After that, the aforementioned conical flask sealed with preservative film was fastened to a platform constant rate of 120 rpm in a thermostat oscillator under dark environment.Magnet was employed to pull the adsorbent down to acquire supernatant to test absorbance with an ultraviolet-visible spectrophotometer when the platform constant shaking incubator was reached at the given time.
In order to reduce detection errors, absorbance of each sample was under triplicate measurements, the mean value of which was used to calculate adsorption capacity of adsorbent and removal efficiency according to the following equations [16,17]: where  is adsorption capacity of Ag(I) on MEG-NH 2 nanohybrids (mg⋅g −1 ),  is removal efficiency,  0 and  are initial and outlet concentration of Ag(I) in solution (mg⋅L −1 ), respectively,  is the volume of the added Ag(I) aqueous solution (mL), and  is dosage of the adsorbent (mg).

Morphological and Structural Studies of MEG-NH 2
Nanohybrids.SEM analysis was performed on the asobtained samples to determine the features under micronano-scale.Figure 1  Figure 3 shows the FTIR spectra of EG and MEG-NH 2 nanohybrids.The spectrum shows that the major FTIR adsorption peaks of EG are at 3434 cm −1 , 1630 cm −1 , and 1408 cm −1 , which are attributed to stretching vibration of the -OH group on the surface of EG, aromatic C=C, and methylene, respectively [18].The formation of EG is due to high temperature expansion of graphite oxide and so are oxygen-containing functional groups on the surface of EG.The peak at 1110 cm −1 is assigned to stretching vibration of C-O-C [23].
However, in comparison with EG, MEG-NH 2 shows totally different FTIR vibrations.The peaks at 2922 cm −1 and 2853 cm −1 correspond to asymmetric and symmetric stretching of -CH 2 group of ethylenediamine, which provide the presence of ethylenediamine on MEG surface.The appearance of peaks at 1621 cm −1 (bending vibration of N-H), 1574 cm −1 (deformation vibration of N-H), and 1193 cm −1 (stretching vibration of C-N) further indicate that the ethylenediamine was successfully grafted on the surface of MEG nanohybrids.
XPS analysis was initially conducted for adsorbents prior to adsorption in order to characterize the available functional groups and to characterize the functional groups involved in the adsorption mechanisms [24].XPS spectra of both survey of MEG-NH 2 nanohybrids (before and after Ag(I) adsorption) and high-resolution scans for Ag3d, C1s, and N1s were measured, as shown in Figure 4. From the survey spectra (Figure 4(a)), it can be seen that the chief elements in MEG-NH 2 nanohybrids include C, N, O, and Fe, which is also in agreement with FT-IR spectra.After adsorption, the presence of Ag3d on the spectrum of MEG-NH 2 nanohybrids clearly confirms the successful adsorption of Ag(I).shows Ag3d spectrum, where a pair of doublet-peaks appear, which could be assigned to Ag3d 3/2 and Ag3d 5/2 of silver with binding energies at 374.2 eV and 368.2 eV, respectively.This result is in accordance with the XRD analysis that Ag(I) was reduced to elemental silver in the process of the adsorption by MEG-NH 2 nanohybrids.
As shown from Figures 4(c) and 4(d), the C1s spectra do not show any significant changes before and after adsorption.But the N1s spectra changed obviously.Before adsorption, the peaks at 399.9 eV and 400.6 eV are ascribed to -NH 2 and NH 3 + which is the outcome generated from -NH 2 protonation [25].After adsorption, the two peaks are transformed to 397.7 eV and 399.8 eV, respectively.Besides, the peak area ratio between -NH 2 and NH 3 + is changed from 1 : 0.33 to 1 : 2.40.This phenomenon can be attributed to the formation of NH-Ag complexes; in detail a lone pair of electrons in the nitrogen atom is provided to the shared bond between the N atom and Ag.As reported [26,27], heavy metal ions like Ag + with a high standard reduction potential can be reduced to metals by some functional groups like -NH 2 , -CN, and so forth.
To investigate the thermal stability of synthesized samples, TGA was performed in nitrogen atmosphere with a heating rate of 10 ∘ C⋅min −1 .Figure 5 shows the TGA curves of EG (a), Fe 3 O 4 -NH 2 (b), MEG-NH 2 nanohybrids prepared with 1.2 g (c), 1.0 g (d), and 0.8 g (e) FeCl 3 ⋅6H 2 O.As shown in Figure 5(a), the TGA curve of EG presented three stages: the first stage is due to the loss of adsorbed water and gas molecules from 50 ∘ C to 150 ∘ C; the second stage is attributed to the decomposition of oxygen-containing functional groups from 150 ∘ C to 430 ∘ C [28]; the third stage is associated with the removal of more stable oxygen functionalities [15].The weight loss throughout the process was 12%.The TGA curve of Fe 3 O 4 -NH 2 (Figure 5(b)) presented two stages: the first stage is due to the removal of adsorbed water below 200 ∘ C; the second stage is associated with the degradation of amino functional groups from 200 ∘ C to 340 ∘ C. The weight loss throughout the process was closed to 12%.For MEG-NH 2 nanohybrids, the mass loss below 135 ∘ C was related to the evaporation of absorbed water and gas molecules.With the increasing of temperature, they displayed a gradual  Figure 7.It can be seen clearly that the adsorption capacity increased with the pH increasing from 1.0 to 4.0 and reached a plateau value with pH range from 5.0 to 7.0.This phenomenon could be explained by the complex mechanism between Ag(I) and the donor atom (N) on surface of sample [30], which is relevant to interface protonation of the -NH 2 groups of MEG-NH 2 nanohybrids in the aqueous solution.From Figure 7 we can also see that the surface of MEG-NH 2 nanohybrids   + made the electrostatic attraction between MEG-NH 2 nanohybrids and Ag(I) was relatively weak and hindered the generation of metal complex.With the pH increasing, the number of positive charges on MEG-NH 2 nanohybrids decreased and the repulsion between Ag(I) and positive charge on MEG-NH 2 nanohybrids weakened, which contributed to -NH 2 groups on MEG-NH 2 nanohybrids coordination with Ag(I) and led to adsorption capacity increase.Further increasing the pH value from 5.0 to 7.0, the surface of MEG-NH 2 nanohybrids is negatively charged and the adsorption capacity reached a plateau value, which is possibly due to the fact that AgOH species formed have a higher formation constant and can steadily stay in the equilibrium solutions.Hence, pH 5.0 was determined as the optimal pH value in subsequent investigations.

Effect of Different Initial Concentrations.
The effect of initial Ag(I) concentration (10,20,30,40, and 50 mg⋅L −1 ) in the solution on the adsorption of MEG-NH 2 nanohybrids is shown in Figure 8.With the increase of initial concentration, the adsorption capacity and removal rate presented opposite variation tendency.At the lowest Ag(I) concentration, the maximum removal rate was obtained, while the adsorption capacity was the minimum.The high removal rate was inclined to be acquired in the process of water treatment compared with adsorption capacity.So, the MEG-NH 2 nanohybrids are an ideal adsorbent to treat low Ag(I) concentration in aqueous solution. of MEG-NH 2 nanohybrids for Ag(I) increased slowly till reaching equilibrium.

Effect of Dosage.
In order to investigate the effect of dosage on the removal rate and adsorption capacity of MEG-NH 2 nanohybrids for Ag(I), different mass of MEG-NH 2 nanohybrids was added during the process of adsorption.
The results are presented in Figure 10, from which, with the increase of the dosage of MEG-NH 2 nanohybrids, the removal rate and adsorption capacity presented opposite variation tendency.When the adsorbent mass was assigned to 5 mg, the adsorption capacity reached the maximum of 100.24 mg⋅g −1 , while the removal rate achieved the minimum value.Interestingly, when the dosage of adsorbent increased to 30 mg, the removal rate ascended gradually to the maximum of 97.4%, which was an ideal result.In contrast, the adsorption capacity descended to the minimum value.This meaningful phenomenon was generated because the quantity of Ag(I) in the aqueous solution was a constant, and, with the increase of adsorbent mass, the available active sites were also bound to increase to a considerable amount.Thus, the finite Ag(I) could be adsorbed efficiently and led to the removal rate increase and adsorption capacity decrease.

Adsorption Kinetics.
In this study, in order to ascertain the most controlling mechanism for Ag(I) removal, pseudofirst-order and pseudo-second-order kinetics models were in application to fit the experimental data.Pseudo-first-order and pseudo-second-order kinetics were expressed as follows [6]: where  1 (min −1 ) and  2 (g⋅mg −1 ⋅min −1 ) are the rate constant of adsorption of pseudo-first-order model and pseudosecond-order model, respectively.  (mg⋅g −1 ) is the equilibrium adsorption capacity and   (mg⋅g −1 ) is the adsorption capacity at time t.
The fitting curves are shown in Figure 11.The parameters and the correlation coefficients ( 2 ) of the two models are listed in Table 2.As shown in Figure 11 and Table 2; the adsorption of Ag(I) using MEG-NH 2 nanohybrids was well fitted by pseudo-second-order kinetics model with much higher correlation coefficient (0.999) than the pseudo-firstorder kinetics (0.912) at the same conditions.The result indicates that the process controlling the rate was a chemical adsorption.

Adsorption Isotherms.
To study the adsorption isotherms, the adsorption data in the concentration range from 10 to 50 mg⋅L −1 were fitted to Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models (Figure 12).
Langmuir isotherm was often applied to homogeneous adsorption surface with all the adsorptive sites qualified with equal affinity to adsorbate.The equation can be described as follows [31]: where   (mg⋅g −1 ) and   (mg⋅L −1 ) are the equilibrium adsorption capacity and the equilibrium concentration of the adsorbate, while  max and   (mg⋅g −1 ) represent the maximum adsorption capacity of adsorbent and the Langmuir adsorption constant (L⋅mg −1 ), respectively.The values of  max and   are calculated from the slope and intercept of the linear plot of   /  versus   .
Freundlich isotherm model assumes heterogeneity of adsorption surface, which is expressed as follows [32]: where   (mg⋅g −1 ) and   (mg⋅L −1 ) are the amount of adsorbed Ag(I) and Ag(I) concentration at equilibrium, respectively.  and  are the Freundlich isotherm constants.

1 )Figure 7 :
Figure 7: Effect of aqueous solution pH on the adsorption of MEG-NH 2 nanohybrids for Ag(I) and Zeta potentials of MEG-NH 2 nanohybrids.

Figure 11 :
Figure 11: The kinetic models of pseudo-first-order (a) and pseudo-second-order (b) for adsorption of Ag(I) onto MEG-NH 2 nanohybrids.

Table 1 :
[29]ct of Fe 3 O 4 content in MEG-NH 2 nanohybrids on the adsorption of MEG-NH 2 nanohybrids for Ag(I).With the temperature increasing sequentially, there was further mass loss due to the bulk pyrolysis of the stable functional groups[29].The weight loss of MEG-NH 2 nanohybrids increased with the decreasing of FeCl 3 ⋅6H 2 O addition.From Figure5, we calculate that the Fe 3 O 4 contents in MEG-NH 2 nanohybrids prepared with 1.2 g, 1.0 g, and 0.8 g FeCl 3 ⋅6H 2 O were 43.2%, 35.2%, and 32.9%, respectively.Effect of Fe 3 O 4 Content in MEG-NH 2 Nanohybrids.In order to optimize the preparation condition of MEG-NH 2 nanohybrids, Fe 3 O 4 content in MEG-NH 2 nanohybrids on the adsorption of MEG-NH 2 nanohybrids for Ag(I) was studied at initial Ag(I) concentration of 40 mg⋅L −1 .As shown in Table 1, it can be clearly seen that the adsorption capacity of Ag(I) increased firstly and then decreased with the increasing of Fe 3 O 4 content in MEG-NH 2 nanohybrids; it reached maximum when Fe 3 O 4 content in MEG-NH 2 nanohybrids was 35.2%.Thus MEG-NH 2 nanohybrids with Fe 3 O 4 content of 35.2% were used in the subsequent adsorption process.
3.2.VSM.The vibrating sample magnetometer (VSM) was applied to test the magnetic property of pure Fe 3 O 4 nanoparticles and MEG-NH 2 nanohybrids, as shown in Figure6.The saturation magnetizations of Fe 3 O 4 nanoparticles and MEG-NH 2 nanohybrids are 59.6 emu⋅g −1 (Figure6(a)) and 34.6 emu⋅g −1 (Figure6(b)), respectively.Obviously, the presence of EG should be responsible for the decrease of overall magnetic performance.Coercivities of the two samples are approximate to zero, which shows that Fe 3 O 4 nanoparticles and MEG-NH 2 nanohybrids have superparamagnetism.Figure6(c) shows a photograph of MEG-NH 2 nanohybrids dispersed in the water (left) and exerting an external magnetic field (right).In the presence of external magnetic field, MEG-NH 2 nanohybrids are attracted to the wall of the bottle swiftly, which caters separation demand from aqueous solution.3.3.2.Effect of pH.The pH value of aqueous solution is the premier parameter in adsorption studies and strongly affects the adsorption property of MEG-NH 2 nanohybrids.The effect of solution pH on the adsorption Ag(I) of MEG-NH 2 nanohybrids from aqueous solutions was investigated in the pH ranges of 1.0-7.0 at 25 ∘ C for 12 h as shown in