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

Abstract

Abstract 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.