Fabrication , characterization , and adsorption capacity of Fe 3 O 4 / graphene oxide nanocomposites for nickel removal

In this research, graphene oxide (GO) was synthesized via modified Hummers’ method and for the preparation of Fe3O4/GO nanocomposites by impregnation method. Characterization of the nanocomposites was performed by X–ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscope, specific surface area, and vibrating sample magnetometer. The concentration of Ni (II) ion in solutions was determined using UV-Visible spectrophotometer. The adsorption capacity for Ni (II) removal was examined with respect to pH effect, kinetic data and equilibrium isotherms in batch experiments. The maximum adsorption capacity of the Fe3O4/GO estimated with the Langmuirisotherm model for Ni (II) was 27.62 mg/g at room temperature.


INTRODUCTION
Graphene (GE) is a two dimensional material that has between one and ten layers of sp 2 -hybridized carbon atoms arranged in sixmembered rings.The length of bonds of GE is 1.42 Å [1].Single layer GE nanosheet was first obtained by mechanical exfoliation ("Scotchtape" method) of bulk graphite [2].Besides, GE sheets have also been fabricated by other methods such as metal ion intercalation, liquid phase exfoliation of graphite, chemical vapor deposition, chemical reduction-oxidation of graphite.Graphene oxide (GO) is a product of graphite oxidation, is often used to make GE.GO refers to GE with oxygen-containing functional groups as epoxy (C-O-C), hydroxyl (OH), carbonyl (C-O) groups on basal planes and carboxyl (COOH) groups on edges [3].Therefore, it can be easily exfoliated and functionalized to form homogeneous suspensions in both water and organic solvents.The existence of oxygen functional groups and aromatic sp 2   domain allows GO to participate in a wide range of bonding interactions.GO has attracted significant attention because of its advantages, such as a large surface area, more activated functionalized sites, easy preparation, and good biocompatibility.These features ensure that GO can be rapid and efficient removal heavy metal ions such as Ni 2+ , As 2+ , Cd 2+ , etc.However, separating and recycling of GO turn out to be a challenge because of their small size.In addition, the π-π interactions between neighboring sheets might lead to serious agglomeration and restacking, which result in the loss of effective surface area and low adsorption capacity [4].In order to solve these problems, Fe 3 O 4 was added into GO sheets for the efficient removal of heavy metal ions due to the high loading capacity and easy manipulation by external magnets.The magnetic property, 2D structure, and existence of active sites make Fe 3 O 4 /GO nanocomposites a potential adsorbent for treatment of heavy metal contaminated wastewater.
In this work, Fe 3 O 4 /GO nanocomposites were synthesized, characterized, and investigated the adsorption capacity for Ni 2+ ions.

Chemicals
Graphite was purchased from Sigma

Synthesis of Fe 3 O 4 /GO nanocomposites
GO was synthesized by modified Hummers' method [5].Fe 3 O 4 nanoparticles were prepared according to the modified Massart's method [6] via the co-precipitation of a mixture of FeCl 3 .6H 2 O and FeCl 2 .4H 2 O.After that, GO dispersion (0.3 g GO in 300 ml distilled water) was sonicated for 30 min.An amount of Fe 3 O 4 nanoparticles (0.3 g) was added to the dispersion.After 30 min of sonication, to obtain an homogeneous suspension, the resulted nanocomposites were collected by magnet and then freeze-dried [7].

Characterization
X-ray diffraction (XRD) patterns were observed on a Bruker D8 Advanced powder diffractometer system using Cu-K radiation (λ = 1.54 Å).Fourier transform infrared (FTIR) spectra were recorded in the 400-4000 cm -1 range at a resolution of 4 cm −1 with a Bruker FTIR Alpha-E spectrometer.The morphology of the nanocomposite was investigated using a transmission electron microscope (TEM) JEOL JEM 1010 operating at 100 kV and equipped with a Gatan Orius SC600 CCD camera for digital imaging.

Langmuir isotherm for the adsorption of Ni 2+
A typical adsorption experiment was carried out by adding 0.02 g Fe 3 O 4 /GO to a 20 ml Ni 2+ solution at room temperature (25C) under suitable pH and contact time.After that, Fe 3 O 4 /GO was removed by using a magnet.Then, the residual solution was collected and analyzed.The initial concentration of Ni 2+ solution was changed from 5 mg/l to 250 mg/l.
The Langmuir isotherm relationship is of a hyperbolic form: where q is sorption uptake; q m is the maximum sorbate uptake under the given conditions; C f is final equilibrium concentration of the residual sorbate remaining in the solution; b is a coefficient related to the affinity between the sorbate and sorbate.111), ( 311), ( 400), ( 422), ( 511), (440), and ( 533) respectively [10].Furthermore, the absence of the (001) reflection of the GO in XRD pattern of Fe 3 O 4 /GO showed that GO layers were exfoliated completely.

TEM image and BET surface area
TEM observation was also undertaken to characterize the morphologies of the Fe 3 O 4 /GO nanocomposite.As shown in Figure 3  The BET specific surface area of Fe 3 O 4 /GO was about 72.9 m 2 /g.

Magnetization
It can be seen at Figure 4 that Fe 3 O 4 /GO nanocomposite could be easily separated under an external magnetic field.Without the magnet, the nanocomposite was dispersed in water.
Using VSM method, the magnetic behaviors of Fe 3 O 4 /GO were further investigated at room temperature in the field range of -15 < H < +15 kOe. Figure 5 shows magnetic hysteresis loops for Fe 3 O 4 /GO.The saturated magnetization for Fe 3 O 4 /GO was 27.1 emu/g.This result is good compared with Fe 3 O 4 /GO nanocomposites reported in references [12,13].

Effect of contact time
The effect of contact time on Fe 3 O 4 /GO adsorption from solution is given in Figure 6.It can be seen that Ni (II) adsorption increases with increase of contact time, and a rapid adsorption is observed in 200 min.Based on these results, a contact time of 500 min was assumed to be suitable for the sorption experiments.
In order to determine Ni (II) adsorption kinetics, the pseudo-second-order kinetic model was investigated as follows: where q t and q e are total adsorbed amounts at time t and at equilibrium, respectively; k 2 he pseudo-second order constant.
According to this equation, the factors of adsorption kinetic of Fe 3 O 4 /GO for nickel were revealed: As seen from Table I, the correlation coefficients (R 2 ) given by the pseudo-secondorder kinetic is 0.9649.High regression correlation coefficient is suggesting that the adsorption nickel by Fe 3 O 4 /GO was fitted with pseudo-second-order kinetic model.
Based on above discussion, the pseudosecond-order adsorption mechanism is predominant, meaning that chemical sorption takes part in the adsorption process.

Langmuir isotherm model for the adsorption of Ni 2+
Adsorption isotherm is of fundamental importance in the design of adsorption system, which indicates how Ni (II) ions is partitioned between the adsorbent and liquid phases at equilibrium as a function of increasing ions concentration.
TEM sample was prepared by dropping ethanol dispersion of Fe 3 O 4 /GO on carbon-coated copper grids (200 mesh).The surface area of the nanocomposite was characterized by isothermal adsorption method (BET).The superparamag-netism of Fe 3 O 4 /GO was presented by vibrating sample magnetometry (VSM).The adsorption capacity for Ni 2+ ions was investigated by Langmuir model.The concentration of residual Ni 2+ ions was measured by ultraviolet and visible spectra (UV-Vis).

A
typical adsorption experiment was carried out by adding 0.02 g Fe 3 O 4 /GO to a 20 ml Ni 2+ solution (C o = 250 mg/l) at room temperature (25C).The pH of solution was adjusted in the range of 2 to 8.After proper time, Ni (II) solution was collected with 0.5 ml.This specimen was measured by UV-Vis to determine Ni (II) concentration.

Figure 2 .
Figure 2. FTIR spectrum of Fe 3 O 4 /GO Additionally, according to Figure 2, the spectrum of Fe 3 O 4 /GO presented the broad band around 3380 cm -1 is assigned to O-H stretching vibration due to the method of sample preparation.The band at 1399 cm -1 was attributed to C=C stretching mode of the sp 2 carbon skeletal network.Carbonyl groups of GO were observed as bands at 1700 cm -1 , while the band at 1053 cm -1 was attributed to the stretching vibrations of C-O of epoxy groups.The spectrum of Fe 3 O 4 /GO nanocomposite additionally presented the characteristic stretching vibration peak 596 cm -1 which proved that Fe 3 O 4 nanoparticles were successfully anchored onto GO sheets.These results were proper with the prehistoric research [11].
, Fe 3 O 4 particles are agglomerated, evidenced by formation of large clusters.It can be distinctly seen that the Fe 3 O 4 clusters were deposited onto GO surfaces of the nanocomposites.Moreover, no isolated Fe 3 O 4 clusters were observed beyond the GO, suggesting a strong interaction between the Fe 3 O 4 clusters and GO sheets.

Figure 3 .
Figure 3. TEM images of Fe 3 O 4 /GO nanocomposites Additionally, TEM image also revealed that the size of the Fe 3 O 4 nanoparticles was approximately 20-25 nm.

Figure 4 .Figure 5 .
Figure 4. Digital photos of Fe 3 O 4 /GO suspension (a) with and (b) without exterior magnetic field

Figure 6 .
Figure 6.Effect of contact time on adsorption of Ni 2+ by Fe 3 O 4 /GO 3.4.2Effect of pH Because hydrogen atom will compete with the positively charged metal ions on the active sites of the adsorbent in the solution, pH is considered as the most important parameter affecting metal ion adsorption [14].The pH effects related to the sort and behavior of the adsorbent in the solution, together with the adsorbed ions sorts.It is observed that the adsorption of Ni (II) is strongly dependent on pH value.At pH 3-7, the sorption ability for all samples is low, meaning the competition of an excess of hydrogen ions with Ni (II) for bonding sites.At pH 2 and 8, the sorption increases sharply.The effect of pH can be explained by considering the surface charge of the Fe 3 O 4 /GO and the degree of ionization and the species of nickel.It is well known that Ni (II) can present in the forms of Ni 2+ , Ni(OH) + , Ni(OH) 2 o , Ni(OH) 3- in the solution.At pH < 7, the predominant nickel species is Ni 2+ .

Figure 7 .
Figure 7. Effect of pH on adsorption of Ni 2+ by Fe 3 O 4 /GO Adsorption functional groups such as carboxyl or hydroxyl are negatively charged.Consequently, the electrostatic attraction of positively charged Ni (II) onto the adsorbents enhances the capacity greatly.At pH > 8.2, the maximum Ni (II) removal is attributed to the formation of hydrolysis species i.e.Ni(OH) + , Ni(OH) 2 o [15].

Table 1 .
The factors of adsorption kinetic

pH Pseudo-second-order equation q e (mg/g) k 2 (min -1 ) R 2
Table II shows the Langmuir model for the nickel adsorption of Fe 3 O 4 /GO, Fe 3 O 4 , and GO.