Systematic study of graphene oxide production using factorial design techniques and its application to the adsorptive removal of methylene blue dye in aqueous medium

Water pollution is a serious environmental problem and coloured pigments are considered one of the greatest concerning contaminants. Graphene, a new two-dimensional material presenting high theoretical specific surface area, has been widely studied for applications in wastewater treatment. It was the aim of this work to study the feasibility of producing Graphene Oxide (GO) under different conditions, using Factorial Design techniques, so that it could be successfully applied in the adsorptive removal of Methylene Blue (MB) dye in aqueous medium. For GO production, the best conditions were achieved within 3 h of reaction and no ultrasonic bath, for a percentage removal of MB over 99%. Surprisingly, it was verified that ultrasonic bath and the increase in reaction time had negative effects under GO production process. For the adsorptive studies using the best samples produced, it was observed that the best pH value was 5.5, and the maximum adsorptive capacities predicted by the Langmuir model were approximately 365 and 504 mg.g(-1), although this was not the model that best fit the experimental data. Unlike most previous works which reported MB adsorption onto GO occurs in mono-layers, in this study it was proved the process happens with formation of multiple layers, as the equilibrium data best fit the Temkin model. Both the physical nature and the spontaneity of the process were verified by the Gibbs free energy of adsorption. Equilibrium was reached very quickly in less than 10 min, and kinetics data were properly fitted to pseudo-second order model.