Fe3O4/Reduced Graphene Oxide Nanocomposite: Synthesis and Its Application for Toxic Metal Ion Removal
The synthesis of reduced graphene oxide modified by magnetic iron oxide (Fe3O4/rGO) and its application for heavy metals removal were demonstrated. The obtained samples were characterized by X-ray diffraction (XRD), nitrogen adsorption/desorption isotherms, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and magnetic measurement. The results showed that the obtained graphene oxide (GO) contains a small part of initial graphite as well as reduced oxide graphene. GO exhibits very high surface area in comparison with initial graphite. The morphology of Fe3O4/rGO consists of very fine spherical iron nanooxide particles in nanoscale. The formal kinetics and adsorption isotherms of As(V), Ni(II), and Pb(II) over obtained Fe3O4/rGO have been investigated. Fe3O4/rGO exhibits excellent heavy metal ions adsorption indicating that it is a potential adsorbent for water sources contaminated by heavy metals.
The content of heavy metals in supplied water has steadily increased over the last years as a result of overpopulation and industrialization. Heavy metals are a well-known highly toxic and carcinogenic element. Its contamination in aqueous system has been a serious concern throughout the world. Arsenic (As), one of the trace elements in drinking water, is toxic to living organism when its concentration exceeds 10 μg/L [1, 2]. Arsenic is introduced into aquatic environment from both natural and man-made sources. Typically, arsenic occurrence in groundwater is caused by the weathering and dissolution of arsenic-bearing rocks, minerals, and ores . Besides, nickel is a nutritionally essential trace metal for at least several animal species, microorganisms, and plants, and therefore either deficiency or toxicity symptoms can occur when too little or too much nickel is taken up. Nickel is an important metal, heavily utilized in industry mainly due to its anticorrosion properties. As a consequence, nickel containing wastes such as spent batteries and catalysts, wastewater, and bleed-off electrolytes are generated in various processes. The environment is polluted by nickel through water, air, and soil and food can be containing it [4, 5]. In addition, pollution of water and soil by lead has been recognized as a serious threat to human health. Pb(II) is known to damage kidney, liver, reproductive system, basic cellular processes, brain functions, and so forth. Waste from metallurgy, electroplating, storage battery, paint, electronics, petroleum refining industry, and so forth released into water bodies is a major source of Pb(II) pollution .
A number of techniques including ion exchange , floatation , and adsorption [1, 2] are prevalent for removal of heavy metal ions from aqueous solutions. As an efficient separation technique, adsorption has been in use for a long time to remove heavy metal ions. Large-scale wastewater treatment systems are employing it now as a cost effective technique . Numerous materials from natural ones to the specially designed have already been proposed for the adsorption of heavy metals from water . However, such adsorbents can suffer from low adsorption capacities and separation inconveniences. Therefore, the exploration of new promising adsorbents is still desirable.
Graphene and reduced graphene oxide (rGO) are kinds of novel and interesting carbon materials and have attracted tremendous attentions from both the experimental and theoretical scientific communities in recent years [9, 10]. In addition to being the principle component of the most carbon-based nanomaterials, rGO also exhibits extraordinary properties, such as excellent mechanical, electrical, thermal, and very high specific surface area, and it might be also a good candidate as an adsorbent. Recently, graphene-based composites have been applied for the extraction of polycyclic aromatic hydrocarbons  and parathyroid pesticides  with excellent results. However, to separate the adsorbent of graphene-based composites from the aqueous solution, high-speed centrifugation was needed. Magnetic separation has been one of the promising techniques for environmental water purification because no contaminants such as flocculants and the capability of treating large amount of wastewater within a short time can be obtained .
In the past few years, magnetic separation technology has been widely used in the fields of separations and adsorptions [13, 14]. The introduction of magnetic properties into graphene or rGO will combine their high adsorption capacity and the separation convenience of the magnetic materials. The preparations of graphene-based magnetic nanocomposites and their application arsenic removal have been reported recently [15–19]. However, the applications of graphene-based magnetic nanoparticles as the adsorbents for the extraction of Pb(II) and Ni(II) are still very few in the literatures [20–23]. In this work, magnetic reduced graphene oxide nanocomposite (Fe3O4/rGO) has been synthesized by a solvothermal method, in which, in addition to providing the magnetic property, Fe3O4 can exhibit adsorption sites. The practical application potential for this material in the removal of arsenate, nickel, and lead from aqueous solution has been investigated.
Graphite powder, potassium permanganate (KMnO4), iron(III) chloride hexahydrate (FeCl3·6H2O), iron(II) sulfate heptahydrate (FeSO4·7H2O), sodium nitrate (NaNO3), sodium monohydrogen arsenate (Na2HAsO4·2H2O), and ethanol (C2H5OH) were purchased from Merck. Sulfuric acid (H2SO4), hydrogen peroxide (H2O2, 30%), ammonia solution (NH4OH, 25%), hydrochloric acid (HCl), lead nitrate (Pb(NO3)2), and nickel nitrate (Ni(NO3)2) were purchased from Sigma-Aldrich. All chemicals are of analytical grade.
2.2. Preparation of GO, rGO, and Fe3O4/rGO
The preparation of graphene oxide (GO) was carried out by a modified Hummer’s method . In a typical synthesis, 1.0 g of graphite powder was added into 2.5 g of NaNO3 and 100 mL of concentrated H2SO4 under stirring. Then 3.0 g of KMnO4 was added gradually to this mixture at 10°C under stirring for 2 h. The resulting mixture was added to 100 mL of distilled water and then heated to 98°C. The obtained mixture was continued to be stirred for 2 h. After that, 10 mL of H2O2 was added in the mixture with stirring for 2 h. The color of the mixture changed to bright yellow. Finally, the mixture was filtered and washed with a 5% HCl aqueous solution to remove metal ions, followed by distilled water for removal of the acid. The resulting solid with brown black color was separated by ultrasonic treatment in water and dried at 60°C for 12 h.
To prepare reduced graphene oxide (rGO), as-prepared GO (1.0 g) was ultrasonically dispersed into 300 mL of distilled water in a flask for 1 h under nitrogen atmosphere. The temperature of mixture was then elevated to 80°C, and 10 g of ascorbic acid was added under gradual stirring for 20 minutes. A brown solid was obtained by filtering and washing with ethanol and dried at 80°C in vacuum condition for 10 h.
The Fe3O4/rGO nanocomposite, in which the rGO was anchored by Fe3O4 nanoparticles, was prepared by a direct method according to the reports [25, 26]. In a typical procedure, rGO (500 mg) was dispersed in distilled water under ultrasonication for 2 h, and then a mixture of FeSO4·7H2O (0.002 mol) and FeCl3·6H2O (0.004 mol) was added under ultrasonication for 1 h. A solution of NH4OH (1.65 M) was taken in dropping under vigorous stirring for 2 h. The obtained black solid was filtered and washed with water and then with ethanol several times to remove residue acid and dissociative Fe(II). The solid was soaked in anhydrous ethanol for 60 min, filtered, and then dried in vacuum at 333 K.
2.3. Characterization of Materials
X-ray diffraction (XRD) analysis was carried out on a D8 Advanced Bruker anode X-ray Diffractometer with Cu Kα (.5406 Å) radiation. Transmission electron microscope (TEM) images were obtained by JEOL JEM-2100F. Infrared (IR) spectra for the samples were recorded on an IRPrestige-21 spectrophotometer (Shimadzu). X-ray photoelectron spectrometry (XPS) was conducted by ESCALAB 250. The magnetic property was analyzed using a vibrating sample magnetometer (VSM, MicroSence Easy VSM 20130321-02) at room temperature. Nitrogen physisorption measurements were conducted using a Micromeritics ASAP 2020 analyzer. Samples were pretreated by heating under vacuum at 130°C for 10 h. Specific surface area values of samples were calculated by the Brunauer-Emmett-Teller (BET) method. Metal ions were determined on a Perkin Elmer 3110 spectrometer.
The point of zero charge () of Fe3O4/rGO was evaluated by the solid method . According to that, a series of 0.1 M KNO3 solutions (25 mL) in 250 mL conical flasks was prepared. The initial pH value () of the solution was adjusted from 2 to 10 by adding either 0.1 M HCl or 0.1 M NaOH. Then, 0.05 g of Fe3O4/rGO was added to each flask and mixtures were agitated. After 24 h, the final pH () of solution was measured. The pH variation between the initial solution and final solution was plotted against the . The point of intersection of curve with abscissa, at which pH = 0, provided .
2.4. Adsorption Experiments
2.4.1. Equilibrium Experiments
The equilibrium adsorption of As(V), Pb(II), and Ni(II) on Fe3O4/rGO was carried out at various metal ion concentrations and pH values with a constant temperature of 25°C. In this experiment, 250 mL of solution of the metal ion was taken in 500 mL conical flask and the initial pH was adjusted to 5.5–6 for As(V) and Pb(II) and 4.7–5.2 for Ni(II) using HCl or NaOH solutions. 0.1 g of Fe3O4/rGO was then added to one of the seven flasks and kept in the shaking incubator for 24 h to ensure equilibrium adsorption at the 25°C. The flask was kept sealed to minimize metal ion concentration change due to water evaporation. The solution was analyzed for metal ion concentration at the equilibrium time, , by AAS. The amount of metal ion adsorbed on the adsorbent at equilibrium time, , was calculated as follows:where is the initial concentration (mg/L) of metal ion, is the concentrations (mg/L) of metal in solution after the adsorption, is the volume of solution (mL), and is the weight () of Fe3O4/rGO.
2.4.2. Effect of pH on As(V), Ni(II), and Pb(II) Adsorption
To investigate pH effect, a series of 50 mL (50 mg/L) of metal ions was put in flasks. The pH of solution was carefully adjusted to the range 2.2–12.4 for As(V) solution and 2–7 for the other metals by adding a small amount of dilute HCl or NaOH solution using a pH meter. Then 0.1 g of Fe3O4/rGO was put into each flask and the mixtures were shaken for 24 h. The ion metal concentration of solutions was measured before and after adsorption by AAS. The capacity of adsorbed metal ion is calculated using (1).
2.4.3. Adsorption Kinetics and Isotherms
Kinetics study was conducted by varying initial concentration of the metal ions (As(V), Pb(II), and Ni(II)). 100 mL of metal ion solution with pH 5.5–6 for As(V) and Pb(II) and 4.7–5.2 for Ni(II) was taken into a 500 mL conical flask that was placed in a thermostatically controlled water bath at 25°C with stirring. 0.1 g of Fe3O4/rGO was added to the flask. During the experiment, small samples of the mixtures were taken out for analysis at predetermined intervals and then were centrifuged for 10 min to separate the supernatant and the adsorbent. The metal ion concentration, , in the clear supernatant at different time intervals was then determined by AAS. The initial concentration of metal ion was varied from around 20.00 to 100.00 mg/L for As(V), 11.69 to 51.12 mg/L for Ni(II), and 13.10 to 75.80 mg/L for Pb(II).
3. Results and Discussion
3.1. Characterization of Materials
XRD measurements were employed to investigate crystalline phase and structure of the synthesized samples. Figure 1 shows the XRD patterns of GO, rGO, and Fe3O4/rGO. The peak at around 11.4° of GO was observed, which is attributed to the introduction of oxygen-containing functional groups into the graphite sheets in the formation of GO [18, 19]. However, broad peaks at around 26°, which can be ascribed to the natural graphite, show the incomplete oxidation of graphite. In fact, the similar results are also obtained in the reported papers [19, 28]. After the reduction with ascorbic acid, the peak at 11.4° disappeared and the weak and broad reflection peak at 25.8° corresponding to the relative short-range order structures in disordered stacked rGO arose , which indicates the successful reduction of GO. Figure 1 also shows the XRD pattern of the Fe3O4/rGO nanocomposites. The characteristic diffractions of iron oxide match the face-centered cubic crystal Fe3O4 (JCPDS file card number 19-0629) implying the existence of magnetic oxide.
The morphologies of GO and Fe3O4/rGO were observed as shown in Figure 2. The TEM image of GO shows a stacked and crumpled morphology, revealing deformation because of the exfoliation and restacking process  (Figure 2(a)). The Fe3O4 particles with average size of 20 nm dispersed well over the rGO sheet were observed (Figure 2(b)). However, it is difficult to obtain monodisperse Fe3O4 particles due to their inherent magnetism.
FT-IR spectra of GO, rGO, and Fe3O4/rGO are shown in Figure 3. Upon oxidation of graphite to GO, the characteristic peaks of the presence of oxygen in carbon frameworks which include the band at 1730 c (C=O stretching of carbonyl and carboxyl groups), 1412 c (tertiary C-OH groups stretching), and 1060 cm−1 (C-O stretching vibration of epoxide) were observed. However, all these absorption bands related to oxidized groups disappear in the FT-IR spectrum of rGO, indicating the reduction of the groups containing oxygen by ascorbic acid. The FT-IR spectrum of Fe3O4/rGO differed from that of GO as evidenced by the weakening of the peaks of C=O and 1730 and 3430 c, respectively. The band around 578 c was attributed to Fe-O, replying the existence of Fe3O4 .
The XPS spectrum in Figure 4(a) shows the wide spectrum of Fe3O4/rGO nanocomposite which indicates the presence of the three elements at 285, 530, and 710 eV corresponding to carbon (C 1s), oxygen (O 1s), and Fe 2p, respectively. The high-resolution XPS for C 1s of GO and rGO is shown in Figures 4(b) and 4(c), respectively. The deconvoluted C 1s spectrum demonstrates that GO consists of functional groups including sp2 (C=C, 284.8 eV), hydroxyl and epoxy (C-O, 286.8 eV), carbonyl (C=O, 287.9 eV), and carboxylates (O=C-O, 288.8 eV) . The high-resolution XPS of C 1s for rGO shows that the intensity of functional groups of rGO remarkably reduced compared with those of GO, indicating an efficient deoxidization. Since XRD patterns of magnetite Fe3O4 and maghemite Fe2O3 are very similar, XPS Fe 2p core level can be conducted to confirm magnetite. Figure 4(d) shows XPS Fe 2p core level of Fe3O4/rGO. It can be seen that characteristic peaks around 719 eV for maghemite are absent, while the broad peaks around 710,9 and 724,5 eV for magnetite are present. This reflects a successful loading of Fe3O4 .
The VSM magnetic characterization of Fe3O4/rGO was determined at room temperature, where the magnetization hysteresis loops appear S-like, and saturation magnetization is 59.4 emu as shown in Figure 5. This value is rather lower than the value of the pure nanomagnetic Fe3O4  but higher than those of Fe3O4/rGO in the reports [31, 32]. This low saturation magnetization value of Fe3O4/rGO in comparison with pure magnetic Fe3O4 may be attributed to the presence of reduced graphene oxide nanosheets and the relatively lower density of magnetic components in the nanocomposites. The magnetic coercivity was nearly zero, evidencing no remaining magnetization upon removal of the external magnetic field. Therefore, superparamagnetic behavior of nanocomposites Fe3O4/rGO was established.
The nitrogen adsorption/desorption isotherms of the obtained samples corresponding to typical type IV are shown in Figure 6. The presence of hysteresis loop at highly relative pressure region indicates that the mesopore may be formed from the void between the primary particles. The specific surface areas of GO, rGO, and Fe3O4/rGO are 318 m2/g, 386 m2/g, and 109 m2/g, respectively. Two reasons for the surface area reduction of Fe3O4/rGO can be considered. First, some micropores of rGO were occupied by Fe3O4 particles during the synthesis. Second, some parts of the composite powders of Fe3O4/rGO were aggregated after Fe3O4 particles were incorporated to the composite. All the above data demonstrate that the Fe3O4/rGO can be successfully achieved by a simple approach. The formation mechanism of the nanocomposite can be explained as follows :
3.2. Adsorption Test
The point of zero charge for Fe3O4/rGO determined by solid addition method is around 6.3. It is well known that solution pH plays a significant influence on the adsorption process because it can affect the degree of ionization of pollutants as well as the ionic state of the functional groups on the surface of adsorbent . To investigate the effect of pH on toxic metal ions adsorption, the solution pH was adjusted with HCl and NaOH solutions. To avoid precipitation of lead (II) and nickel (II) hydroxides, the experiments were performed in the pH range from around 2.0 to 7.0, whereas the solution pH for adsorption of As(V) was used at the range of 2.0–12.0. As shown in Figure 7, the adsorption amount of As(V), Ni(II), and Pb(II) increased with an increase of pH from 2 to 6.27, 5.02, and 5.65, respectively, while it decreased with a further increased pH. This phenomenon can be explained by two effects: (i) Protonation under acidic condition or ionization under neutral and basic conditions for the surface hydroxyl functional groups of Fe3O4/rGO can take place, following (4) and (5) ; (ii) the electrostatic repulsion between metal species and charges of the sorbent surface would also lead to a decrease in the removal extent of metal ions at stronger acidic conditions and .
Figure 8 shows the adsorption data of different concentrations for As(V), Ni(II), and Pb(V) with time. It can be seen that the adsorption capacity of Fe3O4/rGO for As(V) increases with the initial As(V) concentration from 13.1 ppm to 75.8 ppm. This observation can be attributed to the enhanced interaction between the arsenate and adsorbent with accreting initial As(V) concentration . Moreover, the mass transfer driving force becoming larger as an increase of the initial concentration is attributed to higher uptake of As(V) from aqueous solution . The process shows a very fast adsorption rate, which can be verified by the fact that the amount of adsorbed As(V) on Fe3O4/rGO composite in As(V) solution is around 60% within 25 minutes. The time required to reach the adsorption equilibrium between the Fe3O4/rGO and As(V) in the solution was less than 170 min. The similar trends occurred in the cases of Ni(II) and Pb(II) adsorption (Figures 8(b) and 8(c)).
The pseudo-first-order and pseudo-second-order kinetic models were used to investigate the kinetics of metal adsorption on the Fe3O4/rGO composite. The pseudo-first-order model in linear form could be expressed  aswhere and are the amounts of metal ion adsorbed on adsorbent (mg·g−1) at equilibrium and at time , respectively, and is the rate constant of first-order adsorption (min−1). Straight-line plots of against were used to determine the rate constant, .
The pseudo-second-order model in linear form may be expressed as where is the rate constant of second-order adsorption (g mg−1 min−1). Straight-line plots of against were tested to obtain rate parameters.
A comparison of pseudo-first-order and pseudo-second-order kinetic models of metal adsorption onto Fe3O4/rGO at various initial concentrations is illustrated in Tables 1, 2, and 3. The results show that the coefficients of determination () for pseudo-second-order model are higher compared with those of pseudo-first-order one and the calculated values () from this model agree well with the experimental data (). These results suggest that the kinetics of As(V), Ni(II), or Pb(II) adsorption on the Fe3O4/rGO follow the pseudo-second-order model.
The adsorption isotherm models of Langmuir and Freundlich were applied to fit the adsorption equilibrium data of As(V), Ni(II), and Pb(II) on Fe3O4/rGO composite. According to the Langmuir isotherm model, adsorption takes place at specific homogeneous sites on a sorbent and the linear form can be written as where and are the equilibrium metal ion contents on the sorbent (mg g−1) and in the solution (mg L−1), respectively, is the maximum monolayer adsorption capacity of the sorbent (mg g−1), and is the Langmuir adsorption constant (L mg−1).
Freundlich isotherm model assumes a heterogeneous adsorption surface and active sites with different energy, and the Freundlich isotherm model is given in linear form as where (mg g−1) is the Freundlich constant, which is a measure of adsorption capacity, and is an empirical parameter related to the nature and strength of the adsorption process and the distribution of the active sites. Low values of mean that the surface is heterogeneous. For values in the range , adsorption is favorable . The Langmuir and Freundlich isotherm parameters for metal ion adsorption onto the Fe3O4/rGO were calculated by plotting of versus and the results are presented in Table 4. The equilibrium data of metal ion adsorption onto the Fe3O4/rGO can be well fitted by the two adsorption isotherm models since the coefficients of determination () in the two models are very close. The high correlation to both Langmuir and Freundlich isotherms implies a monolayer adsorption and the existence of heterogeneous surface in the adsorbents, respectively. This observation is similar to the paper reported by Kong et al. , in which both Langmuir and Freundlich models were well fitted to describe the adsorption of As(V) on nanoscale Fe-Mn binary oxides loaded on zeolite. The maximum adsorption amounts of As(V), Pb(II), and Ni(II) over Fe3O4/rGO calculated by Langmuir model are 58.48 mg/g for As(V), 65.79 mg/g for Pb(II), and 76.34 mg/g for Ni(II), rather higher than the other reports [43–46]. This result demonstrates that the obtained Fe3O4/rGO nanocomposite is a potential adsorbent for treating water sources contaminated by heavy metals.
Fe3O4/rGO nanocomposite was synthesized by a facile one-step process. The iron oxide in magnetic iron oxide is highly dispersed over rGO. The morphology of Fe3O4/rGO consists of very fine spherical particles in nanoscales. Fe3O4/rGO exhibits superparamagnetic properties at room temperature and saturation magnetization approaching 59 emu . The adsorption of As(V), Ni(II), and Pb(II) on the Fe3O4/rGO was fitted well to the pseudo-second-order kinetic model and obeyed both Langmuir and Freundlich models, which indicates surface heterogeneity and monolayer adsorption of the adsorbents. The Fe3O4/rGO exhibits excellent heavy metals adsorption. The maximum monolayer adsorption capacities calculated by Langmuir equation are 58.48 mg/g for As(V), 65.79 mg/g for Pb(II), and 76.34 mg/g for Ni(II). The Fe3O4/rGO nanocomposite can be used as a potential absorbent for removing toxic ion metals from aqueous solution.
The authors declare that they have no competing interests.
This work was funded by the project of Vietnamese Ministry of Education and Training (no. B2014-28-39).
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