Solvent-dependent tannic acid modification of copper(II) oxide for cationic and anionic dye decolorization

Abstract

Copper(II) oxide is an attractive photocatalyst for dye-contaminated wastewater treatment, but its performance is limited by charge-carrier recombination and weak dye-surface interaction. In this study, copper(II) oxide was synthesized in ethanol, acetone, water, and dimethylformamide/water, then modified with tannic acid to tune morphology, surface charge, and interfacial properties. Structural and surface analyses confirmed phase-pure monoclinic copper(II) oxide and revealed solvent-dependent differences in particle size, dispersion, surface area, and zeta potential. The ethanol-derived tannic-acid-modified sample showed small particles of 20–40 nm, the highest surface area of 25.85 m² g⁻¹, and the most negative zeta potential, giving 90.1% methylene blue decolorization under ultraviolet C irradiation for 180 min without hydrogen peroxide. Adding hydrogen peroxide only slightly increased removal to 92.4%, indicating reduced dependence on external oxidants. The ethanol-derived sample favored cationic methylene blue removal, whereas the dimethylformamide/water-derived sample showed higher activity toward anionic Sunset Yellow and Alizarin Red S, achieving 77.1% and 85.9% decolorization, respectively. Kinetic analysis after the initial adsorption-dominated stage confirmed the highest apparent methylene blue decolorization rate for the ethanol-derived sample, and scavenger experiments identified hydroxyl radicals as the dominant reactive species. The simple precipitation-based synthesis and room-temperature tannic acid modification suggest potential applicability for dye wastewater treatment; however, recycling tests showed gradual activity loss, indicating that coating stability and possible copper or tannic acid leaching require further evaluation.