Abstract

The increasing discharge of toxic azo dyes like Congo red (CR) into aquatic environments poses serious ecological and health risks, necessitating the development of efficient and sustainable remediation technologies. In this study, ZnFe2O4/g–C3N4 (ZFOCN) nanocomposites were synthesized via a simple sol–gel method combined with ultrasonication. The resulting composite exhibited a reduced crystallite size (29 nm), strong interfacial bonding, and improved visible-light absorption with a bandgap of ~2.6 eV. SEM and XPS analyses confirmed the successful anchoring of ZnFe2O4 on g–C?N? sheets and the presence of mixed-valence Fe species, respectively. Under visible-light irradiation, the ZFOCN nanocomposite achieved 79% degradation of CR dye within 160 minutes, outperforming the individual components (ZFO and CN). The formation of a heterojunction structure enabled effective charge separation, which significantly enhanced photocatalytic activity. Furthermore, the composite demonstrated excellent reusability over five cycles. This work highlights the potential of ZFOCN as a cost-effective, magnetically recoverable, and visible-light-responsive photocatalyst for practical wastewater treatment applications. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2025.