Cyanobacterial extract mediated biogenic synthesis of NiO/CoO nanocomposites for enhanced photodegradation of crystal violet dye

Abstract The continuous discharge of synthetic dyes into the aquatic environment poses a serious ecological threat, particularly biodegradable toxic cationic dyes such as crystal violet (CV). Consequently, developing effective and sustainable wastewater treatment methods is essential. To address the current research gap in green synthesis of multi-metal oxides heterostructures, this study provides a novel green synthesis route for nickel oxide/cobalt oxide (NiO/CoO@CM) nanocomposites using cyanobacterial extract, where the bioactive extracts act as a natural reducing and stabilizing agent for highly efficient synthesis. Both chemically synthesized NiO/CoO and biogenically NiO/CoO@CM nanocomposites were characterized using Brunauer-Emmet-Teller (BET), ultraviolet-visible diffuse reflectance (UV-Vis DRS), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transformation infrared spectroscopy (FTIR). Characterization findings revealed that the biogenic NiO/CoO@CM showed higher surface area (38.97 m 2 /g), smaller crystallite size (27.05 nm), a narrowed band gap energy (2.9 eV) and abundant biofunctional groups. Under the optimized conditions (pH = 9, 90 min contact time, 0.1 g/50 mL catalyst dosage), the biogenic NiO/CoO@CM catalyst achieved 96.5% crystal violet (CV) degradation, significantly outperforming the 79.2% removals efficiency observed for chemically synthesized NiO/CoO. Isotherm and kinetic modeling reveled that Freundlich isotherm and pseudo-second order kinetics models best described the degradation process, indicating heterogeneous surface adsorption dominated by chemisorption. These findings confirm the novelty and potential of cyanobacterial extract-mediated synthesis as a cost-effective, eco-friendly and highly efficient technique for sustainable remediation of dye contaminated wastewater.