Magnetically Separable Fe3O4/SnO2/Graphene Adsorbent for Waste Water Removal

Our previous study conducted the SnO2 and SnO2/graphene adsorption efficiency in Methylene Blue removal from aqueous solution, however, the difficulty of adsorbent separation from the methylene blue solution limits its efficiency. Therefore, in this work, SnO2 and SnO2/graphene was combined with Fe3O4 to improve the separation process and adsorption performance for removing the organic dyes. Fe3O4/SnO2/grapheme were synthesized by using the co-precipitation method. The graphene content was varied from 1, 3, and 5 weight percent (wt%). The crystalline phase and thermal stability of the samples were characterized by using X- ray Diffraction (XRD) and Thermal Gravimetric Analysis (TGA). The adsorption ability of the samples was investigated by using significant adsorption degradation of MB observed when the graphene in Fe3O4/SnO2 nanocomposite was added. The other parameters such as pH and initial concentration have also been investigated. The reusability was also investigated to study the stability of the samples. The fitting of equilibrium adsorption capacity result indicates that the adsorption mechanism of Fe3O4/SnO2 nanocomposite with graphene tends to follow the Langmuir adsorption isotherm model.


Introduction
The environmental pollution has increased public concern nowadays because of its problem for aquatic life and environment. This problem must be decreased by using the suitable methods and technologies [1]. Methylene blue (MB) dye is one of the cationic dyes used as a material for dyeing wood, cotton, and silk [2]. A variety of methods such as adsorption, coagulation and flocculation, oxidation, chemical precipitation, and so forth, have been reported for wastewater treatment [3]. Among these, adsorption is an attractive and favorable technique because of their high efficiency, cost effectiveness, and simple operation [4]. A variety of sorbents have been tested to remove dyes in aqueous solution, and some metal oxides exhibit prominent potentials for adsorptive removal of dye waste [5]. One of metal oxide that is widely used in the degradation process of dye wastes is a tin oxide (SnO2). SnO2 has various advantages such as its sensitivity when it is exposed to the light and thermal energy, its numbers of an intrinsic defect in its structure caused by low formation energy and strong attraction between the tin sites and oxygen 2

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The 4th International Conference on Advanced Materials Science and Technology IOP Publishing IOP Conf. Series: Materials Science and Engineering 202 (2017) 012009 doi:10.1088/1757-899X/202/1/012009 vacancies [6]. Graphene, a novel one-atom-thick two-dimensional graphitic carbon system with sp2bonded carbon localized on the honeycomb lattice, has been under intense multidisciplinary study attribute to its extraordinary electrical and mechanical properties, large surface area, an excellent mobility of charge carriers, and good thermal conductivity [7]. Furthermore, combining the graphene material into the metal oxide can enhance the adsorption performance of the sample due to their functional groups and defect sites of graphene extending the lifetime of the adsorbent material by acting as support material which inhibits leaching of fine metal oxide nanoparticles into the treated water [8]. The coupling SnO2 and graphene with magnetic adsorbent were investigated widely to overcome the disadvantage for difficult separation. Several researchers have reported on the use of Fe3O4 for removal and separation of dyes [9][10]. Therefore, it is believed that combining SnO2, Fe3O4 and graphene will exhibit excellent adsorptive with highly effectively recovery by magnetic separation technique.
In this study, Fe3O4/SnO2 nanoparticle will be combined with various weight percentages of graphene (1, 3, and 5 wt%) that are applied as adsorbents to degrade the methylene blue (MB) dye and the adsorption mechanism, pH, initial concentration, and temperature of these composites . The sample was characterized by using X-ray Diffraction (XRD) and Thermal Gravimetric Analysis (TGA).

Preparation of SnO2 nanoparticle and Fe3O4/SnO2 nanocomposites
The Fe3O4 nanoparticles were synthesized by using the same method used in our previous study [11]. The SnO2 nanoparticles were synthesized by using a modification of the method reported by Yue Li and co-workers [12]. First, SnCl2 was dissolved in a mixture of ethanol and aqueous solution, which was then added into the NaOH solutions by using magnetic stirrer. Then, the mixed solutions were heated at 180 °C for 3 h, then chilled to room temperature. The precipitate was obtained by centrifugation and washed by using aqueous solutions and ethanol several times. The precipitated particles were dried under vacuum at 80 °C, and SnO2 particles obtained by calcination for 3 h at 700 °C. The Fe3O4/SnO2 nanocomposites were synthesized by using the sol-gel method. First, SnO2 nanoparticles were mixed with Fe3O4 in a mixture of ethanol and aqueous solution. Each mixture was ultrasonicated for 2 h and then centrifuged to obtain the precipitate. The resulting product was then dried under vacuum at 80 °C for 12 h to obtain Fe3O4/SnO2 nanocomposites.

Preparation of Fe3O4/SnO2/graphene composites
Fe3O4/SnO2/graphene was synthesized by using the co-precipitation method. First, graphene with various weight percentage (1, 3, 5 wt%) was dissolved in distilled water and ethanol through the ultrasonic treatment for 2 hours, then Fe3O4/SnO2 nanocomposites were poured into the solution and stirred magnetically. The mixed solution was then heated at 120 °C for 3 hours. The result of a solution was then centrifuged and dried at 70 °C under vacuum condition.

Characterization
The sample was characterized by XRD using a Rigaku Miniflex 600 (Rigaku, Tokyo, Japan) with a Cu K-α radiation source ( =1.5406 Å) and thermal gravimetric analysis (TGA) of the sample obtained by using Rigaku TG8121 with the temperature from 27 o C until 900 o C.

Adsorption processes
Adsorption experiment was carried out by mixing Fe3O4/SnO2/graphene into methylene blue (MB) solution used as pollutant model with the concentration of 20 mg/L, and the pH solution was adjusted By using NaOH. The concentration of dye was analyzed by using UV-Vis Spectrometer. The maximum degradation and the amount of MB adsorbed onto sample were calculated by using the following equation: where C0 and Ct (mg L -1 ) are initial and real-time concentration of organic pollutant, respectively. q t (mg g -1 ) is the amount of adsorbed per unit mass of the absorbent at time t. V (L) is the volume of the absorbed solution and m (g) is the mass of the absorbent. Furthermore, adsorption experiment is carried out again by varying MB concentration for sample with best adsorption activity.

Results and Discussion
The crystalline structure of the Fe3O4/SnO2/graphene composite with various weight percentages (1, 3, 5 wt% labeled as FS-g 1wt%, FS-g 2wt% and FS-g 5wt%, respectively) were analyzed by XRD and showed in Fig. 1. All of the Fe3O4/SnO2/graphene composites displayed peaks corresponding to the tetragonal structure of SnO2 and cubic spinel structure from Fe3O4. In Fig. 1 (533), respectively. For graphene, there is only a broad peak at around of 2θ of 25°, which shows the amorphous structure of graphene. As can be seen, the diffraction peak of the SnO2 and graphene were overlapped at 2θ around 26 o at Fe3O4/SnO2/graphene composites. Figure 2 shows the weight loss curve for Fe3O4/SnO2/graphene composites with different weight percentages (1, 3, and 5 wt%) obtained from TGA measurement. As can be seen, Fe3O4/SnO2 nanoparticle did not exhibit the significant percentage loss in the result of TGA measurement which indicated that Fe3O4/SnO2 nanoparticle without any addition of graphene was stable until 1000 o C. The weight loss started around 100 o C was due to the volatilization of adsorbed water [13]. The weight loss stage from 400 o C decomposition took place thereafter [14]. The percentage loss of the samples could be identified as the total amount of graphene in the samples. Furthermore, the weight loss of graphene at TGA measurement could confirm the existence of the graphene in Fe3O4/SnO2/graphene composite samples.  Figure 3 shows the adsorption ability of the Fe3O4/SnO2/graphene composites with various weight percentages (1, 3, and 5 wt%). The results show that Fe3O4/SnO2/graphene 5 wt% composites exhibited the highest adsorption ability than Fe3O4/SnO2/graphene of 1 wt%, Fe3O4/SnO2/graphene of 3 wt%, and Fe3O4/SnO2 composite. Fig. 3a shows the color change (decolorization) of MB during the adsorption process. The decolorization process increased simultaneously with the addition of weight percentage of graphene at Fe3O4/SnO2 from 33.94% for Fe3O4/SnO2 nanoparticle until 51.25%, 60.13%, and 66.29% for Fe3O4/SnO2/graphene of 1, 3, 5 wt%, respectively. The certain contact time during an adsorption process was performed to identify the adsorption capability of the adsorbent to adsorb the pollutants on its surface per unit of time [15]. Fig. 3b shows the adsorption capacity against time up to 250 minutes. As can be seen in Fig. 3b, Fe3O4/SnO2/graphene 5 wt% showed the highest adsorption capacity (44.19 mg/g) then followed by Fe3O4/SnO2/graphene 3 wt% (40.08 mg/g), Fe3O4/SnO2/graphene 1 wt% (34.16 mg/g) and Fe3O4/SnO2 (22.62 mg/g). The maximum adsorption capacity of the samples reached at amm ount graphene material in Fe3O4/SnO2/graphene was 5 wt%. It may due to the increase of the surface area because the addition of graphene was proportional to the mass of graphene in the solution [16].   Figure 4 shows the influence of pH in the adsorption ability of Fe3O4/SnO2/graphene of 5 wt% with broad range of pH from pH 3 until pH 13. All measurements were made at room temperature and for the contact time of 4 hours. The result showed the adsorption ability was found to increase with the increasing solution pH. The lower value of adsorption ability at acidic pH may be due to the presence of excess H + ions from the acid solution competing with the charge MB dye cations for vacant adsorption site [3,17]. That phenomenon indicated the maximum adsorption capacity was obtained at the pH value of 13 for Fe3O4/SnO2/graphene 5 wt% composite. To investigate the influence of the initial concentration of MB dye, Figure 5 shows the adsorption capacity of Fe3O4/SnO2/graphene of 5 wt% composite with the variation of MB concentration from 20 mg/L-150 mg/L with the adsorbent dosage of 0.3 g/L. The result shows, the adsorption capacity of Fe3O4/SnO2/graphene decreased with the increase of MB concentration with the equilibrium adsorption capacity reach its maximum of 105.48 mg/g.
The pseudo-first-order ( Figure 6a) and pseudo-second-order (Figure 6b) models were examined in this study to analyze the kinetics of MB adsorption. The selection of the best applicable adsorption kinetics model for the adsorption of MB onto the samples was done on the basis of the values of the Log(q e -q t ) = log q e -(k 1 /2.303)t (3) where qe and qt are the adsorption capacity of Fe3O4/SnO2 nanocomposite and Fe3O4/SnO2/graphene with various weight percentages (1, 3, 5 wt%) for MB at equilibrium (mg g -1 ) and at given time (min), respectively. While k1 is the rate constant for the pseudo-first-order of the adsorption (g mg -1 min -1 ).  where k2 is the rate constant for the pseudo-second-order adsorption process [4]. The value of the R 2 for the pseudo-first-order and pseudo-second-order models are shown in Table 1. The low value of the correlation coefficient R 2 indicated that the adsorption of MB onto samples poorly fitted with pseudo first-order kinetic model. However, the adsorption of MB onto all samples shows a better fit to the correlation coefficient R 2 of the pseudo-second-order model. That case indicated the adsorption process were confirmed to the pseudo-second-order kinetics.Two adsorption kinetics models were applied to know the mechanism of the adsorption process. Figure 7 shows the Freundlich (Figure 7a) and Langmuir (Figure 7b) adsorption kinetics models from adsorption process of Fe3O4/SnO2/graphene of 5 wt%. The figure shows the Langmuir isotherm is a better fit than the Freundlich isotherm kinetic model, with the value of regression coefficient (R 2 ) was 0.99 and 0.88 for Langmuir isotherm and Freundlich adsorption isotherm model, respectively.

Conclusion
The Fe3O4/SnO2 nanocomposite and the different weight percentages of graphene (1, 3, 5 wt%) in Fe3O4/SnO2/graphene have been successfully synthesized by using ultrasonic assisted and coprecipitation method, respectively. The structural characterization of the samples was studied by using XRD measurement, which revealed a combination phase from the tetragonal phase of SnO2 and cubic spinel phase of Fe3O4. The TGA measurement confirmed the existence of graphene in Fe3O4/SnO2/graphene composite by showing the weight loss at around 400oC which attributes to combustion effect from graphene. The maximum equilibrium adsorption capacity reached Fe3O4/SnO2/graphene of 5wt% with the value of 44.19 mg/g. The maximum adsorption capacity was obtained at the pH value of 13 for Fe3O4/SnO2/graphene of 5 wt% composite. The adsorption process of Fe3O4/SnO2/graphene 5 wt% follows the Langmuir adsorption isotherm model.