<p>In this study, Eu-doped CuO nanoparticles were synthesized through a fully green and waste-derived route using a composite extract prepared from banana peels, orange peels, and used green tea leaves. Unlike previous green synthesis approaches relying on a single plant source or undoped CuO structures, this work introduces a multi-biomass extract–assisted method combined with Eu³⁺ incorporation, enabling simultaneous control over crystallite size, surface hydroxylation, oxygen vacancy formation, and band–gap modulation. This integrated green synthesis strategy provides a novel pathway for tailoring the structural and optical properties of CuO, resulting in significantly enhanced photocatalytic degradation of MB and MO under visible light.</p><p>Structural and surface analyses confirm the successful Eu incorporation, resulting in lattice distortion, reduced crystallite size, and enhanced surface hydroxylation and oxygen vacancies. These features significantly boosted photocatalytic degradation of methylene blue (78%) and methyl orange (85%) under visible light. Additionally, the Eu–CuO catalyst retained over 80% activity after five reuse cycles. The novelty of this study lies in the synergistic use of a complex biomass extract for rare-earth doping, and the systematic dual-dye evaluation under identical visible-light conditions, which is rarely addressed in the literature. This work demonstrates a viable route for developing eco-friendly and reusable nanocatalysts from food waste for practical wastewater remediation.</p> Graphical Abstract <p></p>