Statistical Optimization and Modeling of Methylene Blue Adsorption Onto Graphene Oxide in Batch and Fixed-Bed Column

The batch and fixed-bed column adsorption of methylene blue (MB), a widely used toxic dye, onto graphene oxide (GO) was investigated in this study. GO was synthesized using modified Hummers method and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Response surface methodology (RSM) was employed to optimize batch and fixed-bed column adsorption of MB. Batch adsorption experiments were carried out by central composite design (CCD) with three input parameters including initial MB concentration (C0: 50-350 mg/L), GO dosage (D: 0.05-0.7 g/L), and pH (pH: 3-9). The adsorption capacity of GO for MB removal in the optimum level of factors (C0: 50 mg/L, D: 0.05 g/L, and pH: 8.5) was predicted by the model to be 700 mg/g. Adsorption kinetic data were tested using pseudo-first order, pseudo-second order, and intraparticle diffusion models. The kinetic experimental data was well fitted with pseudo-second order kinetic model (R2=1). The adsorption of MB onto GO demonstrated that Langmuir model (R2=0.999) could better fit the adsorption data than the Freundlich model (R2=0.914). Thermodynamic parameters including enthalpy (ΔH), Gibbs free energy (ΔG), and entropy (ΔS) were also investigated. Positive value of ΔH and negative value of ΔG indicated the endothermic and spontaneous nature of the adsorption. The positive value of ΔS also showed increased randomness at the solid/liquid interface during the adsorption of MB onto GO. The real wastewater experiment at optimum conditions showed high performance of adsorbent in the presence of other ions. Fixed-bed column experiments were designed using a three-factor, three-level Box-Behnken design (BBD) to investigate the single and combined effects of influent concentration (Cinf: 50-200 mg/L), flow rate (Q: 0.25-0.8 mL/min), and bed height (BH: 3-7 cm). MB removal from GO in the optimum levels of factors (Cinf: 51 mg/L, BH: 5.7 cm, and Q: 0.25 mL/min) was predicted by the model to be 86% (qe=459.3 mg/g). Fixed-bed experimental data were also fitted well to the Thomas and BDST models. The results showed that GO can be used as an efficient adsorbent for batch and fixed-bed adsorption of cationic dyes from synthetic and real wastewater.