Green Synthesis of ZnO Nanoparticles and Conversion of Post-extraction Residue of Canna indica L. Leaf into ZnO/Activated Carbon to Remove Malachite Green from Water

During the synthesis of metal/metal oxide nanoparticles using plant extracts, a significant amount of post-extraction residue is often discarded. To take advantage of this valuable carbonaceous source, converting post-extraction residue into high-value adsorbents such as activated carbon and carbon-based composite, is essential for the valorization of bio-wastes. Here, we synthesized ZnO nanoparticles (ZnO-NPs) utilizing Canna indica L. (CIL) leaf extract and ZnO/activated carbon (ZnO-AC) obtained from post-extraction residue of CIL leaves through ZnCl₂ activation for removal of malachite green dye from water. The ZnO-AC exhibited a remarkable BET surface area of up to 718.7 m²/g, 129 folds higher than that of ZnO-NPs. As a result, ZnO-AC exhibited an adsorption capacity (118.9 mg/g), approximately 2.5 folds higher than that of ZnO-NPs. Regarding photocatalytic activity, the degradation efficiency of ZnO-NPs under solar light irradiation was slightly higher than that of ZnO-AC. Thus, CIL leaf extract and the post-extraction residue can be utilized as potential raw materials to produce ZnO-NPs and ZnO-AC for water treatment applications.