The role of magnetite/graphene oxide nano-composite as a high-efficiency adsorbent for removal of phenazopyridine residues from water samples, an experimental/theoretical investigation

Presence of phenazopyridine residues in water resources can be harmful to human body, hence it is very important to remove them from aqueous media. In this study, a magnetic nano-composite adsorbent, i.e. reduced graphene–iron oxide (rGO–Fe3O4) was synthesized and used for removal of phenazopyridine residues from wastewater samples. Experimental parameters influencing adsorption process were optimized, and results showed a sorption capacity of 14.064 mg g−1 at pH=6.0 for phenazopyridine. Under the best experimental condition, rGO–Fe3O4 nanocomposite showed an efficiency of 91.4% for removal of phenazopyridine in aqueous solution. Adsorption of phenazopyridine on flat and parallel oxygen and iron–terminated rGO–Fe3O4 surfaces was studied by density functional theory calculations. It was found that, in case of both oxygen and iron–terminated rGO–Fe3O4 surfaces, phenazopyridine prefers to interact with aromatic ring and amino flat to oxygen–terminated rGO–Fe3O4 surface, as opposed to iron–terminated sites due to enhanced charge migration at the interface. Negative adsorption energy revealed that formation of phenazopyridine@rGO–Fe3O4 is an exothermic process, confirming stability of these species.