Dye wastewater degradation by the synergetic effect of an atmospheric pressure plasma treatment and the photocatalytic activity of plasma-functionalized Cu‒TiO nanoparticles.

In this paper, the photocatalytic activity of plasma-functionalized Cu-doped TiO nanoparticles (NPs) and the oxidization process of atmospheric pressure plasma jet were combined for the degradation of reactive red-198 (RR-198) in aqueous solution. The first part of the study was thus devoted to subject Cu-‒TiO NPs synthetized by the sol-gel method to various plasma treatments operating in air, argon, oxygen and nitrogen to improve their degradation efficiency. The physicochemical properties of the NPs were then assessed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) measurements. XRD results indicated the predominant presence of the anatase phase which is the most photoactive form of TiO. The XPS analysis revealed that the different plasma treatments triggered the formation of oxygen vacancies, Ti oxidation state and Cu oxidation state on the surface of Cu-‒TiO NPs. These changes, known to prevent the recombination of electron-hole pair, have led to a reduction in the bandgap that was more pronounced for the N plasma-treated NPs. The second part of the paper explored the actual degradation of RR-198 in aqueous solution by an Ar plasma treatment alone or combined with the plasma pre-treated Cu-‒TiO NPs. Optical emission spectroscopy (OES) and spectrophotometric analyses showed that the synergetic effect of Ar plasma and N plasma-treated NPs produced the highest concentration of OH radicals and HO species which led to the highest RR-198 degradation efficiency. This was further confirmed by pH, electrical conductivity and total organic carbon (TOC) removal measurements. The degradation of RR-198 was determined using UV-Vis spectroscopy and high-performance liquid chromatography (HPLC). Overall, it can be concluded that plasma-assisted processes illustrated by a combination of a direct plasma treatment with plasma-functionalized Cu-‒TiO NPs can be used in various textile and pharmaceutical industries as a highly effective treatment of their effluents before discharging.