Adsorption of organic dyes from aqueous solutions using surfactant exfoliated graphene

In this study, graphene exfoliated in the presence of surfactants was investigated as an adsorbent for the removal of organic dyes from aqueous solution. The resultant graphene particles were shown using zeta potential measurements to possess effective surface charges dominated by the charge of the adsorbed exfoliating surfactant. In this way, the overall charge on the particles, and hence the ability to adsorb dyes, was significantly enhanced compared to previously explored surfactant free systems. Furthermore, the graphene particles could be produced to have either positive or negative charge allowing the selective removal of anionic or cationic species from water respectively. The adsorption of the dyes from solution was demonstrated to be driven by electrostatic interactions with the adsorbed surfactant. The maximum removal of dye was achieved when cationic methylene blue was exposed to graphene exfoliated using the anionic surfactant, sodium dodecyl sulfate (SDS). Adsorption of methylene blue on SDS exfoliated graphene was strongly influenced by contact time and temperature, while variations in pH were shown to have a minor effect on adsorption. Indeed, the rate of adsorption was faster than previous studies due to the inherent 2D nature of the highly exfoliated graphene particles. The adsorption data was modelled using the Freundlich and Langmuir adsorption isotherms, whilst the pseudo-second order model and the intraparticle diffusion models were used to model the kinetics of the adsorption process. SDS exfoliated graphene particles exhibited a maximum adsorption capacity of 782.3 mg/g at 25 °C, greater than that of many other graphene-based materials. Thus, surfactant exfoliated graphene particles not only demonstrated excellent adsorption characteristics, but also the ability to maximize the amount of dye adsorbed based on solution conditions or the exfoliating surfactant.