Green-synthesized Plectranthus amboinicus derived Z-scheme rGO-based CeO2/g-C3N4 nanocomposite for visible-light-driven dye degradation, antimicrobial activity and ecological safety

Synthetic dyes such as methylene blue (MB) are persistent and not effectively removed by conventional adsorption or biological treatment processes, motivating the development of advanced oxidation technologies. Herein, an rGO/CeO 2 /g-C 3 N 4 ternary heterostructure was synthesized via an eco-friendly route, in which Plectranthus amboinicus leaf extract was used to reduce graphene oxide prior to the incorporating CeO 2 and g-C 3 N 4 . Structural, morphological, and optical characterization confirmed successful formation of the ternary heterostructure, with UV-Vis DRS analysis specifically revealing an optical band gap of 2.50 eV. Process optimization using response surface methodology with central composite design (RSM-CCD) yielded a maximum experimental degradation efficiency of 98.6% within 60 min. Scavenger experiments identified photogenerated holes as the principal oxidizing species, with a secondary contribution from superoxide radicals and only minor involvement of hydroxyl radicals. PL and EIS results, together with XPS and band-position analysis, support the proposed rGO-assisted direct Z-scheme charge-transfer pathway, which facilitates charge separation while retaining strong redox potentials. GC-MS profiling detected partially oxidized aromatic intermediates consistent with stepwise degrdation rather than complete mineralization. The catalyst showed a gradual decrease in activity over five consecutive reuse cycles and concentration-dependent antibacterial activity but no antifungal effect, and reduced MB-associated phytotoxicity from a highly to a moderately phytotoxic range, indicating substantial but incomplete detoxification. These findings demonstrate the potential of the green ternary photocatalyst for visible-light-driven dye wastewater treatment.