Novel magnetic Fe₃O₄@rGO@ZnO onion-like microspheres decorated with Ag nanoparticles for the efficient photocatalytic oxidation of metformin: Toxicity evaluation and insights into the mechanisms

Emerging water contaminants, including pharmaceutical and personal care products, have become a major concern in water pollution, and several efforts have been made for the efficient removal of these contaminants. To address this concern, this study reports the synthesis of a new nanostructured hybrid onion-like spherical catalyst, Fe₃O₄@rGO@ZnO/Ag-NPs (FGZAg), and investigates its ability to degrade metformin (MTF) under both ultraviolet- and visible-light irradiation. The catalysts were prepared using functionalized Fe₃O₄ microspheres as the templates, successively coated with graphene oxide and ZnO shells followed by Ag nanoparticle (NP) decoration, which were finally annealed under a N₂ atmosphere. The prepared samples were characterized using XRD, N₂ adsorption-desorption, SEM, HRTEM, UV-vis absorption spectroscopy, FTIR spectroscopy, Raman spectroscopy, VSM, XPS, EIS, and PL measurements. The results showed that the prepared onion-like spheres with a hexagonal wurtzite crystal structure had a mesoporous texture, and Ag NPs were loaded on the surface of the catalyst. As compared to plain ZnO, the as-made FGZAg catalyst significantly enhanced the photocatalytic activity toward MTF degradation under visible-light irradiation. The complete degradation and 60% mineralization of 20 mg L^-1 MTF was achieved with FGZAg within 60 min in the visible-light photocatalytic process. The interaction between rGO and Ag NPs exhibited a beneficial synergistic effect and reduced electron-hole recombination, thereby facilitating charge transport. The possible mechanism for MTF degradation was the generation of OH and O₂^- radicals. Furthermore, the magnetic characteristic of the prepared catalyst facilitated its separation from the treated solution. The as-prepared magnetic FGZAg catalyst could preserve its catalytic activity during four consecutive cycles of the MTF degradation process. The cytotoxicity of the untreated and photocatalytic oxidation (PCO)-treated MTF solutions was evaluated using cultured human embryonic kidney (HEK) cells, which revealed that the MTF solution in the developed PCO system could be significantly detoxified.