ZnO–CuO nanocomposites for sustainable water treatment linking photocatalytic efficiency with ecotoxicological safety

The development of sustainable water treatment technologies requires a balance between high pollutant removal efficiency and environmental safety across multiple trophic levels. In this study, ZnO-CuO nanocomposites were prepared by mechanical mixing of commercially available ZnO and CuO nanopowders, providing a scalable route. Structural and optical analyses confirmed the formation of p-n heterojunctions, enhancing charge separation and photocatalytic performance. Photocatalytic activity was assessed via Rhodamine B degradation under simulated solar irradiation, with ZnO97%-CuO3% showing the highest efficiency. To assess sustainability beyond contaminant removal, ecotoxicological assays were performed across multiple trophic levels, including a long-term 7-day assay on microalgae (Scenedesmus obliquus) and short-term tests involving plants (Ocimum basilicum), bacteria (Aliivibrio fischeri, Microtox®, 30 min), crustaceans (Daphnia magna, 48 h immobilization), and fish embryos (Danio rerio, up to 120 h post-fertilization). No adverse plant physiological effects were observed after exposure to the nanocomposite or treated water, unlike untreated dye solutions, which induced significant toxicity. Aquatic bioassays revealed organism-specific responses linked to particle-associated effects. Quantification of metal ion release and nanoparticle dissolution in aqueous matrices enabled differentiation between particulate and ion-mediated toxicity. Overall, the findings demonstrate that efficient photocatalytic water treatment can be achieved while maintaining a favorable ecotoxicological profile.