Adsorption of methylene blue using Fe3O4/CuO/ZnO/ nanographene platelets (NGP) composites with various NGP concentration

This study will examine the use of Fe3O4/CuO/ZnO nanocomposites that have been modified by Nanographene Platelets (NGP) as an adsorbent to degrade organic dye waste Methylene Blue (MB). The nanocomposites were synthesized using the sol-gel method then combined with three variatons of NGP weight percents by simple hydrothermal method. The Fe3O4/CuO/ZnO/NGP composites were characterized using the X-Ray Diffraction (XRD) spectroscopy, Fourier Transform Infrared (FTIR), Energy Dispersive X-Ray (EDX), Thermogravimetric Analysis (TGA) and Vibrating Sample Magnetometer (VSM). The composites exhibit ferromagnetic behaviour. The presence of hexagonal wurtzite of ZnO, monoclinic of CuO and cubic spinel of Fe3O4 were found in the composites. The graphitic-like structure represents the presence of the NGP in the composites. However, the addition of NGP weight percent reduces the thermal stability of the composites. The adsorption capability of the composites are analyzed by observing the degradation of organic dye Methylene Blue (MB) under dark condition. The NGP addition of 15 wt% show the best result of the composites to degrade Methylene Blue in alkaline condition. Adsorption mechanism of the composites with NGP addition tend to follow the model Langmuir adsorption kinetic models.


Introduction
Each year, the total amount of dye wastes disposed into the environment from the food and textile industry kept increasing which will cause more water pollution [1][2]. To cope with the problem, an effective waste water treatment is required to reduce the content of environmental pollution. There are various methods for dye wastes treatment such as adsorption, coagulation, flocculation, reverse osmosis, biological treatment, and catalytic activity [1][2][3][4][5]. Adsorption is one of reliable methods in overcoming water pollution because of its several advantages such as low cost, simple design, and easy operations [2,6]. However there are constraints applying it since its adsorption capacity is not too high. In recent years, semiconductor material draws an attention because of its capability in degrading wastes [7][8]. ZnO is a semiconductor that is widely used because of its low cost, high photochemical, biological stability, and low toxicity. However, the use of single material such as ZnO still has shortage in term of efficiency in waste degradation. Combination of two or more semiconductors has been proven to improve the process of waste degradation because of the synergistic effect of each semiconductor [8][9][10]. CuO is one of semiconductors with relatively small band gap hence it is expected that by combining it with ZnO we could modify the band gap of the combination these two semiconductor to improve the ability of waste degradation. Another problem in using ZnO semiconductor in the degradation process of wastes is the difficulty in separating the ZnO material from the solution after the reaction process. Therefore further modification is required to facilitate the separation process of the adsorbent material from the waste solution. Using magnetic materials such as Fe 3 O 4 has been proven to facilitate the separation process of adsorbent materials from the solution [11]. Some researchers have also reported that carbon materials such as graphene could be an ideal supporting material for nanoparticles or composites because of its unique properties such as high surface area, high carrier mobility, and high electrical conductivity [6,12]. High surface area of graphene is considered able to increase the adsorption capacity of nanomaterials. Therefore, in this paper, NGP will be combined with Fe 3 O 4 /CuO/ZnO nanocomposites with several variation of NGP weight percent (wt%). Here the influence of NGP addition into nanocomposites in degrading Methylene Blue (MB) as the model pollutant was examined under the adsorption method, and the influence of initial pH and MB concentration were also investigated.
Nanocomposites Synthesis: Fe 3 O 4 /CuO/ZnO is synthesized using the method that was applied in the previous study by Taufik, et al. [13][14] without modification. Fe 3 O 4 /CuO/ZnO/NGP composites were made using simple hydrothermal method [15]. In brief, a certain amount of NGP was first added into aquades solution and ethanol before being sonicated for 2 hours. After sonication process, Fe 3 O 4 /CuO/ZnO was poured and stirred for 2 hours to get homogeneous suspension. The resulting solution was further heated at the temperature of 120 °C for 3 hours, and then centrifuged and vacuumed at the temperature of 70 °C for 12 hours. The amount of NGP addition into Fe 3 O 4 /CuO/ZnO nanocomposites was varied from 5wt%, 10wt%, and 15wt%.
Characterizations: The samples were characterized by X-Ray Diffraction (XRD) using Raigaku Miniflex 600 monochromatic Cu-K α (λ = 1.5 Å) radiation operated at 30 kV and 15 mA at the span angle of 10 o to 80 o and the absorption spectrum were characterized using Shimadzu FTIR spectrophotometer in the range of 400-4000 cm -1 . Thermogravimetric Analysis (TGA) was measured at the heating rate of 10 °C/min in N 2 atmosphere, and magnetic measurement was performed using Oxford type 1.2 T vibrating sample magnetometer (VSM).
Adsorption measurement: A certain amount of samples that have been synthesized were dissolved into Methylene Blue (MB) solution with the concentration of 20 mg/L at pH 13. The solution was stirred using magnetic stirrer. At a determined time, 5 mL of solution was collected and centrifuged to separate it from Fe 3 O 4 /CuO/ZnO nanocomposites and then the concentration was measured using UV-VIS spectroscopy. The efficiency of the adsorption process can be calculated using the equation of C t /C 0 , where C t is the MB concentration at a certain interval and C 0 is the initial concentration of MB. Subsequently, several parameter measurements are carried out in the form of initial dye concentration: (20-150 mg/L) and initial pH of the solution: (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13).  Figure 1 indicates the XRD spectrum of Fe 3  An increased intensity is observed at NGP peak by increasing the wt% of NGP given in composites. This is because of increased amount of NGP in the composites. The average particle size (D) is calculated using Scherer formulation D = 0.9λ/(β cosƟ) where λ is the wavelength of incident X-Ray, β is the full width at half maximum and Ɵ as the Bragg angle. The result of average particle size analysis is tabulated in        Fig.3(B). As comparison, the magnetic saturation curve of Fe 3 O 4 nanoparticles is also presented in Fig. 3 causes an increase of the magnetic saturation value compared to Fe 3 O 4 /CuO/ZnO nanocomposites and addition of 15wt% NGP lowers the magnetic saturation value but still higher than Fe 3 O 4 /CuO/ZnO. Increase of magnetic saturation value when NGP material is added can be caused by Fe 3 O 4 material sticking on the NGP surface which causes defect and stabilize NGP-based magnetic material [21]. The adsorption capacity measurement of Fe 3 O 4 /CuO/ZnO and Fe 3 O 4 /CuO/ZnO/NGP nanocomposites is analyzed by observing the methylene blue degradation represented by C t /C 0 (C t =MB concentration at certain interval, C 0 =initial concentration) as seen in Fig 4(A). It can be seen in the figure an increase in the adsorption capacity simultaneously with NGP material addition. The qualitative data from the adsorption process for all samples is represented in the form of adsorption capacity as indicated in Fig. 4(B). The maximum value of adsorption capacity from all samples is shown in Table 2. The maximum adsorption capacity of Fe 3 O 4 /CuO/ZnO increases simultaneously with increasing the wt% of NGP.

Results and discussion
The adsorption process is also influenced by the acidity of the waste solution being used; therefore in this study we also analyze the influence of methylene blue solution acidity on the adsorption capacity of Fe 3 O 4 /CuO/ZnO/NGP. The acidity of methylene blue solution is varied from 3, 5, 7, 9, 11, and 13 using NaOH and CH 3 COOH. The result obtained can be seen in Fig. 5  Based on the study carried out using several parameters, the optimum condition obtained to degrade MB from aqueous solution (MB concentration of 20 mg/L) using the adsorption method is obtained for Fe 3 O 4 /CuO/ZnO nanocomposites with 15wt% NGP addition, for the dosage of 0.3 mg/L and pH 13. In this study, maximum condition of the degradation and adsorption capacity is reached 43% and 29 mg/g.

Conclusion
In this study, Fe 3 O 4 /CuO/ZnO/NGP composites with different added amount of NGP has been successfully synthesized using two step method, sol-gel and simple hydrothermal method. The XRD and FTIR characterization result indicate the existence of structure and vibration of Fe 3 O 4 , CuO, ZnO and NGP in the composites. The result also indicates the amount of NGP addition in accordance with the result of TGA measurement. Addition of NGP may improve efficiency in the adsorption process of methylene blue in aqueous solution, with the addition of 15wt% the degradation efficiency and adsorption capacity increased by 15% and 11 mg/g. The experimental result follows properly the adsorption of Freundlich dan Langmuir kinetic model, where the conformity value of Langmuir is higher than the Freundlich.