Adsorption removal of Congo red from aqueous solution by polyhedral Cu₂O nanoparticles: Kinetics, isotherms, thermodynamics and mechanism analysis

Highlights: • Polyhedral Cu₂O NPs with rough surfaces were prepared by a one-pot sonochemical precipitation method. • The Cu₂O NPs show unprecedented adsorption capability toward Congo red. • CR adsorption onto Cu₂O is a spontaneous, endothermic and chemisorption process. • The Cu₂O adsorbent can be photocatalytically regenerated by visible light irradiation. - Abstract: Polyhedral cuprous oxide nanoparticles (Cu₂O NPs) with rough surfaces were prepared by a one-pot sonochemical precipitation method. The products were characterized by SEM, XRD, EDS, XPS, and UV–Vis DRS, respectively. The adsorption behavior of Congo red (CR) from aqueous solution onto the as-prepared Cu₂O NPs was systematically investigated. The equilibrium and kinetic studies suggested that the adsorption process followed Freundlich isotherm and pseudo-second order model, respectively. The as-prepared Cu₂O NPs exhibited remarkable adsorption properties toward CR. The maximum adsorption capacity at 20 °C was 3904 mg g⁻¹, which was the highest reported value so far in adsorption removal of CR. Together with the evaluation of the thermodynamic parameters such as Gibbs free energy, enthalpy and entropy change, our results show that the adsorption of CR onto Cu₂O is a spontaneous, endothermic and chemisorption process. A putative interaction model between CR and Cu₂O NPs was proposed. Moreover, the Cu₂O adsorbent could be photocatalytically regenerated and reused without significant loss of its adsorption capability