Fabrication and adsorption properties of magnetic graphene oxide nanocomposites for removal of arsenic (V) from water

In this work, magnetic graphene oxide nanocomposites were synthesized by co-precipitation method and used as an adsorbent for removal of arsenic (V) ions from water. The structure and morphology of magnetic graphene oxide nanocomposites were studied by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, Brunauer–Emmett–Teller specific surface area, and vibrating sample magnetometry. Fourier transform infrared spectroscopy, X-ray diffraction, and transmission electron microscopy results of magnetic graphene oxide presented that the Fe3O4 nanoparticles in the size range of 10–25 nm were decorated on graphene oxide nanosheets. The adsorption properties of magnetic graphene oxide nanocomposites for arsenic (V) from water were investigated to study the effects of magnetic graphene oxide mass ratio, contact time, pH, and initial concentration. The suitable magnetic graphene oxide mass ratio of nanocomposites for arsenic (V) adsorption was determined to be 4:1 (FG2). The adsorption process on FG2 followed a pseudo-second-order kinetic and well fitted in to Langmuir isotherm model with the maximum adsorption capacity of 69.44 mg/g at pH 3. Accordingly, FG2 could be used as an effective adsorbent for removal of arsenic (V) from water.


Introduction
Arsenic (As) is considered as one of the most toxic chemicals and a carcinogenic element (Chandra et al., 2010). The most common forms of arsenic species in aqueous environments are arsenite As(III) (as H 3 AsO 3 and H 2 AsO 3 À ) in ground water system and arsenate As(V) (as H 2 AsO 4 À and HAsO 4 2À ) in surface water. Arsenic compounds easily accumulate in human organs and cause various health problems (Feng et al., 2012;Vaclavikova et al., 2008). Thus, the removal of As from water has been investigated.
Currently, the research and development of new adsorbent materials is being promoted. Graphene (Gr) with unique characteristics, such as large surface area, mechanical strength, and thermal conductivity, has attracted great interest of many researchers. Gr is a monolayer of carbon atoms, which is packed firmly, forming a two-dimensional structure of honeycomb lattice (Meyer et al., 2007;Zhu et al., 2010). Graphene oxide (GO), a derivation of Gr, is fabricated by exfoliation of graphite oxide, which is synthesized from graphite by oxidizing. GO contains a variety of oxygen-containing groups on the surface like hydroxyl (-OH), epoxy (-O-), carbonyl (-C ¼ O), and carboxylic (-COOH). These groups provide GO with negative charged surface and an ability to interact with positive ions such as heavy metals, dyes, and organic compounds (Stankovich et al., 2006;Zhao et al., 2011). However, the nano size, high dispersion, and difficult separation of GO prevent the direct use of GO as adsorbents. To solve these problems, the magnetic graphene oxide (Fe 3 O 4 /GO) nanocomposite has been developed. The unique advantages of Fe 3 O 4 /GO such as high adsorption capacity and easy separation of Fe 3 O 4 /GO make it a potential adsorbent for the removal of pollutants from water (Fan et al., 2016).
In this work, the Fe 3 O 4 /GO nanocomposites were synthesized by co-precipitation method. The characterization of nanocomposites was examined by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) specific surface area, and vibrating sample magnetometry (VSM). Effects of Fe 3 O 4 :GO mass ratio, contact time, pH, and initial concentration on the As(V) adsorption capacity of Fe 3 O 4 /GO nanocomposites were studied.

Synthesis of Fe 3 O 4 /GO nanocomposites
GO was prepared by improved Hummers' method (Marcano et al., 2010). Fe 3 O 4 /GO was synthesized by co-precipitation method (Fan et al., 2016). Briefly, 10 ml of FeCl 3 Á6H 2 O and FeCl 2 Á4H 2 O solution was slowly added into 50 ml of GO suspension (6 mg/ml). The mixture was stirred and heated to 80 C at pH 10 for 2 h. The black precipitation was collected by using a magnet, then washed with water and ethanol. After drying at 50 C for 24 h, the nanocomposites were obtained. The nanocomposites with different Fe 3 O 4 :GO mass ratios of 8:1, 4:1, 2:1, and 1:1 were marked as FG1, FG2, FG3, and FG4, respectively.

Characterizations
The crystal state was analyzed by XRD D2 Phaser machine (Bruker, Germany) with Cu Ka radiation (k ¼ 0.154 nm). FTIR spectra (Bruker FTIR Alpha-E, Germany) are used to analyze the functional groups on the surface of materials. The morphology of the materials was investigated by TEM (JEM-1400 microscope, Japan). The BET specific surface areas of materials were determined by Nova 3200e through nitrogen adsorption-desorption isotherm method (Quantachrome Instruments, USA). The elemental composition of materials was specified by using scanning electron microscope along with energy-dispersive X-ray analyzer (SEM/EDX) (Jeol JMS 6490, JEOL, Japan). Magnetic properties of the nanocomposites were performed using a MicroSense Easy VSM version 9.13 L machine (MicroSense, USA).

Adsorption experiments
Twenty milligrams of adsorbent was poured into 50 ml of As(V) solution. Effects of mass ratios of Fe 3 O 4 to GO (FG1, FG2, FG3, and FG4), contact time (30, 60, 120, 240, 480, and 1440 min), pH (3, 5, 7, 9, and 12), and initial As(V) concentration (25, 50, 100, 150, and 200 mg/l) were studied. The residual As(V) concentration was determined by inductively coupled plasma mass spectrometry (ICP-MS 7500, Agilent, USA). All experiments were triplicated to estimate the error. The adsorption efficiency (H, %) and adsorption capacity (q, mg/g) were calculated using the following equations (1) and (2) where C o and C e (mg/l) are the initial and equilibrium concentration of As(V), respectively; V (ml) is the volume of As(V) solution; and m (mg) is the mass of adsorbent. The kinetic of adsorption process was investigated by pseudo-first-order and pseudosecond-order models. These models can be expressed as equations (3) and (4) ln q e À q t ð Þ ¼ lnq e À k 1 t (3) where q e and q t are the adsorption capacity at equilibrium and at time t (mg/g), respectively, and k 1 (min À1 ) and k 2 (g/mg min) are the pseudo-first-order and pseudo-second-order rate constants (Chandra et al., 2010). The zeta potential (pH pzc ) values of materials were determined by pH drift method (Martinez-Vargas et al., 2018). Besides, to investigate adsorption capacity, the experimental data were applied to the Langmuir and Freundlich isotherm models. The linear equations for isotherm models are expressed as follows (equations (5) and (6) where q m (mg/g) is the maximum adsorption capacity, k 1 (l/mg) is the Langmuir constant, and n and k f (l/mg) are the Freundlich constants (Hosseini et al., 2011).

Results and discussion
Characterization XRD patterns. The XRD patterns of GO, Fe 3 O 4 , and Fe 3 O 4 /GO are shown in Figure 1. The diffraction peak at 2h ¼ 11.12 , ascribed to the crystallographic planes (002), indicates the distance of 0.795 nm between GO sheets. This distance was higher than the interlayer spacing of graphite (0.340 nm) since the oxygen-containing groups were inserted between GO sheets (McAllister et al., 2007).  (Metin et al., 2014;Yang et al., 2009). The peaks at 630.98 and  584.13 cm À1 can be ascribed to Fe-O bonds of Fe 3 O 4 (Sheng et al., 2012). This result indicates the existence of distinctive functional groups of GO and Fe 3 O 4 .
TEM images. The morphologies of the materials were analyzed by TEM images as shown in Figure 3. GO shows high transparency with small wrinkles, which indicates effective exfoliation during sonication. The effect of Fe 3 O 4 :GO mass ratio on the size and the distribution of Fe 3 O 4 on GO surface is shown in Figure 3  BET specific surface areas. Table 1 shows the BET surface areas of Fe 3 O 4 /GO nanocomposites and other materials. The Fe 3 O 4 nanoparticles were attached on the surface of GO, decreasing the aggregation of Fe 3 O 4 and stacking of GO, leading to the increase in specific surface area. Besides, the surface area of FG2 was higher than that of other nanocomposites due to a larger number of active sites on the surface. FG3 and FG4 had fewer Fe 3 O 4 nanoparticles, leading to the reduction in the number of sites, which resulted in the decrease in surface area.
SEM/EDX. The composition of elements in Fe 3 O 4 /GO was determined through SEM/EDX data as shown in Figure 4 and    Table 3. The M s values of nanocomposites are less than that of the   electrostatic interaction between the negative charged surface of GO and As(V) ions, and (2) the complex interaction between metallic ions and oxygen-containing groups on the surface of Fe 3 O 4 /GO (Wang et al., 2013). The FG2 has the highest As(V) adsorption capacity due to its largest surface area and the uniform distribution of Fe 3 O 4 nanoparticles on the surface of GO. Therefore, FG2 was selected as an optimal adsorbent for the following experiments.

Effects of factors on the As(V) adsorption capacity of Fe 3 O 4 /GO nanocomposites
Contact time. Figure 6 shows the increase in As(V) adsorption capacity of FG2 as contact time increased. The As(V) adsorption process occurred rapidly due to a substantial amount  of active sites. After 480 min, As(V) ion concentration slightly changed due to the reduction of empty active sites on the surface of FG2. Therefore, the adsorption time of FG2 for As (V) was 480 min. The adsorption process of As(V) by FG2 well fitted to the pseudo-secondorder model, with a correlation coefficient value close to 1 (R 2 ¼ 0.9987) (Figure 7).
pH. The increase of pH values inhibited the As(V) adsorption capacity of FG2 ( Figure 8). As(V) ions exist in different forms including H 3 AsO 4 (pH < 2.1), H 2 AsO 4 À (2.1 < pH < 6.9), HAsO 4 2À (6.9 < pH < 11.5), and AsO 4 3À (pH > 11.5) (Zhu and Bates, 2013). Based on the experimental data, the pH of zero-point charge (pH ZPC ) of FG2 was measured to be 5.2. When the pH was lower than 5.2, the surface of FG2 was positively charged, making -OH and -COOH groups on FG2 surface become -OH 2 þ and -COOH 2 þ cations, which increased the number of active sites to interact with As(V) ions (Saadi et al., 2015;Zhu and Bates, 2013), leading to the increase in the adsorption capacity of FG2. As pH was increased, the surface of FG2 became less positively charged, the interactions between surface charges and As(V) declined gradually and changed into repulsive forces, decreasing the adsorption capacity. The experimental data showed that the effective removal conditions of FG2 for As(V) adsorption was achieved at pH 3.  Initial concentration. Figure 9 shows the relationship between the adsorption capacity of FG2 and initial As(V) concentration. The uptake positively correlated with the initial concentration of As(V). The parameters of Langmuir, and Freundlich isotherm models are showed in Table 4 and Figure 10. The correlation coefficient of Langmuir model (R 2 ¼ 0.9059) was higher than that of Freundlich model (R 2 ¼ 0.8199). The adsorption of FG2 followed the Langmuir isotherm model with q m ¼ 69.44 mg/g. Besides, the As(V) adsorption capacity of FG2 was in comparison with other materials ( Table 5). The result could be explained as the linkage of Fe 3 O 4 nanoparticles on the GO surface, leading to an increase in active sites and surface area, resulting in the increase in adsorption capacity.
Besides, the As(V) adsorption capacity of FG2 was compared with other materials as shown in Table 5. The result can be elaborated that Fe 3 O 4 nanoparticles linked on the GO surface, leading to an increase in active sites and surface area, resulting in the adsorption capacity being increased.
Adsorption mechanism. The As(V) adsorption mechanism of Fe 3 O 4 /GO was interpreted via the surface complexation between As(V) ions and functional groups on nanocomposite surface, consisting of two types: outer-sphere and inner-sphere. For outer-sphere complexation, the interaction between As(V) ions and adsorption sites on nanocomposite surface depended on the electrostatic interaction. The in-sphere complexation was formed in Fe 3 O 4 / GO structures, the As-O-Fe bonds were formed between As-O(H) groups from H 2 AsO 4 À and HAsO 4 2À as ligands and -OH, -COOH groups of Fe 3 O 4 /GO structure (Huong et al., 2018;Kumar et al., 2014).
Based on the results of the survey of pH effects, the adsorption capacity of Fe 3 O 4 /GO reduced while pH values increased. At low pH, the concentration of H þ ions in the solution increases, -OH and -COOH groups become -OH 2 þ and -COOH 2 þ cations, which is advantageous for the adsorption of As(V) (Huong et al., 2018). The main mechanism of adsorption depends on the outer-sphere complex as follows

Conclusions
Fe 3 O 4 /GO nanocomposites were successfully fabricated by co-precipitation method. XRD, FTIR, TEM, and SEM/EDX results showed that the Fe 3 O 4 particles were anchored on the surface of GO sheets. The Fe 3 O 4 /GO with mass ratio of 4:1 (FG2) showed highest adsorption capacity of As(V) compared with other nanocomposites. The adsorption process of FG2 well fitted to the pseudo-second-order and Langmuir isotherm models with the maximum adsorption capacity of 69.44 mg/g at pH 3. Therefore, FG2 could be applied as an adsorbent to remove As(V) from water.

Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is funded by Ho Chi Minh City University of Technology, VNU-HCM, under grant number BK-SDH-2020-1880319.