Green Synthesis of CuO Nanoparticles Using Beta vulgaris L. Extract for Removal of Methyl Violet Dye

Wastewater discharge containing organic dyes may pose a hazard to the environment, which necessitates that dye removal must occur prior to wastewater release into water bodies. Herein, copper oxide nanoparticles (CuO NPs) were prepared by a green precipitation method to enable decolorization of a cationic dye (methyl violet; MV) from aqueous media. Complementary tools were employed to characterize the CuO NPs adsorbent: spectroscopy (FTIR and UV-VIS), microscopy (FESEM and TEM), XRD, BET surface area analysis, and point of zero charge (pH_PZC) via potentiometry. The FTIR bands at 722, 663, 569, and 465 cm^-1 correspond to the vibrational modes of CuO NPs, along with the optical absorbance band at 275 nm that supports the formation of CuO NPs. The XRD and TEM analyses predicted single-phase CuO NPs with a monoclinic framework. BET was employed to assess the textural characteristics and accounted for the specific surface area (12.97 m²·g^-1). Batch adsorption studies were carried out to assess the role of initial pH (3.58-10.53), CuO NPs dose (0.02-0.25 g/L), initial MV concentration (20-140 mg/L), contact time (5-90 min), and temperature (298, 308, and 318 K) on the dye removal efficiency. The adsorption capacity of CuO NPs for MV was determined to be 5.06 mg/g at 45°C. The pseudo-second-order (PSO) model described kinetic isotherms, and equilibrium adsorption data were adequately fitted by the Freundlich model. Thermodynamic results revealed that adsorption was spontaneous, endothermic, and entropy driven at the solid-liquid interface. The CuO NPs further displayed good reusability with high efficiency for six successive cycles of adsorption-desorption using 0.1 M HCl as a desorbing agent. These findings validate the efficacy of CuO NPs as a green and effective adsorbent for wastewater treatment processes for cationic dye removal.