Adsorption of anionic azo-dyes from aqueous solutions onto graphene oxide: Equilibrium, kinetic and thermodynamic studies. 
In the present study, graphene oxide (GO) was used for the adsorption of anionic azo-dyes such as Acid Orange 8 (AO8) and Direct Red 23 (DR23) from aqueous solutions.  GO was characterized by Fourier Transform-Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), Thermogravimetric Analysis (TGA), Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HRTEM) and zeta potential measurements.  The influence of dye initial concentration, temperature and pH on AO8 and DR23 adsorption onto GO was investigated.  Equilibrium data were analyzed by model equations such as Langmuir Freundlich, Temkin, Dubinin-Radushkevich and Redlich-Peterson isotherms and were best represented by Langmuir and Redlich-Peterson isotherm model.  Kinetic adsorption data were analyzed using the pseudo-first-order kinetic model, the pseudo-second-order kinetic model and the intraparticle diffusion model.  The adsorption kinetics well fitted using a pseudo-second-order kinetic model.  Thermodynamics parameters, ΔG°, ΔH° and ΔS°, were calculated, indicating that the adsorption of AO8 and DR23 onto GO was spontaneous process.  The adsorption process of AO8 onto GO was exothermic, while the adsorption of DR23 onto GO was endothermic in nature.