The photocatalytic performance of graphene oxide/zinc oxide (GO/ZnO) nanocomposite was investigated for degradation of methylene blue (MB) from wastewater and was compared with that of zinc oxide (ZnO), graphene oxide (GO) and carbon nanotube/zinc oxide (CNT/ZnO). The properties of the GO/ZnO nanocomposite were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and TEM results revealed the nanostructure of the composite. The photocatalytic process was modeled by response surface methodology (RSM), considering four independent factors. The significance of the model was approved by analysis of variance (ANOVA) (determination coefficient R 2 = 0.95, R 2 adj = 0.91). The optimum conditions of the process were found to be at photocatalyst dosage, initial pH, H 2 O 2 concentration and irradiation time of 2.1 g L -1 , 4.5, 3.5 × 10 -4 mol L -1 and 140 min, respectively. The removal of MB over GO/ZnO was 99% under the optimum conditions and was superior than that over GO and CNT/ZnO. According to Pareto analysis of the modeling, the irradiation time and photocatalyst dosage were the most effective factors on the decolorization efficiency of methylene blue, respectively.

The graphene oxide was prepared from graphite powder by a modified Hummers method. 29 Briefly, 1 g graphite powder, 6 g KMnO 4 and 1 g NaNO 3 were mixed with 46 mL concentrated sulphuric acid (97 %). The mixture was stirred for 3 h within an ice-bath. After having been removed from the ice-bath, 280 mL deionized water was added and stirred at room temperature for 2 h. Then the sample was stirred at 89°C for 2 h, followed by 6 mL H 2 O 2 which was added dropwise until the colour of the solution turned from dark brown to yellow. The solid product was separated by centrifugation and washed several times with distilled water until the pH of the solution became neutral. The residue was dried at 60°C overnight. The resulting graphene oxide was suspended in water and exfoliated through sonication for 3 h to obtain GO nano sheets. Preparation of Zinc OxideFor the ZnO, 2g ZnSO 4 was dissolved in sodium hydroxide solution and stirred for 12 h. The obtained white product was washed several times with distilled water and was collected by filtration. Finally, it was dried under vacuum to get zinc nano oxide. Preparation of GO/ZnO HybridFor the composite, 5g graphene oxide was dissolved in 20 mL ethanol and kept under ultrasonication for 2 h; it was denoted as GO solution. The as-prepared ZnO nanoparticles (5 g) were dissolved into the mixture of 18 mL methanol and 2mL chloroform and kept under ultrasonication for 2 h; it was denoted as ZnO solution. The GO solution and the ZnO solution were mixed and kept under constant stirring for 24 h. The collected precipitate was obtained by centrifugation and washed with methanol several times. The product was freeze-dried overnight and denoted as GO-ZnO nano composites. The ZnO/CNT composite was synthesized in the same way in order to compare its adsorption performs with GO/ZnO composite. Characterization of the Nano CompositeThe structure and crystal phase of adsorbent was investigated by a powder X-ray diffraction system (XRD6000, Shimadzu) equipped with Cu Ka radiation with a 2 q scanning range of 5-40°. Fourier transform infrared (FT-IR) spectra of samples were recorded by Bruker 27 in the range 400-4000 cm -1 . The morphology of the adsorbents was investigated with a scanning electron microscope (model EQ-C1-1). Determination of pH PZC of the AdsorbentsThe point of zero charge (pH PZC values) is the point at which the net charge on the adsorbent surface is zero. The pH PZC values of GO and GO/ZnO was determined by pH drift method. 30 Briefly, several solutions containing 0.01 M NaCl were supplied. The initial pH (pH i ) of the solutions was adjusted to a value between 2 and 11 using 0.1 M HCl or 0.1 M NaOH. 0.02 g adsorbent was added to the solution. The electrolyte solution with adsorbent was equilibrated for 24 h. After equilibrium, the final pH (pH f ) was recorded. The pH f was plotted against the initial pH (pH i ) values. The pH at which pH i crossovered the (pH f ) was referred to as the pH PZC . The pH PZC of GO and GO/ZnO was determined 7.5 and 8.2, respectively. Adsorpt...

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