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.