Photocatalytic Systems Based on ZnO Produced by Supercritical Antisolvent for Ceftriaxone Degradation

Abstract

Emerging contaminants are a significant issue in the environment. Photocatalysis is proposed as a solution for the degradation of pollutants contained in wastewater. In this work, ZnO-based photocatalysts have been produced and tested for the photocatalytic degradation of an antibiotic; specifically, ceftriaxone has been used as a model contaminant. Moreover, there is particular interest in combining small-size ZnO particles and β-cyclodextrin (β-CD), creating a hybrid photocatalyst. Zinc acetate (ZnAc) (subsequently calcinated into ZnO) and β-CD particles with a mean diameter of 0.086 and 0.38 µm, respectively, were obtained using the supercritical antisolvent process (SAS). The produced photocatalysts include combinations of commercial and micronized particles of ZnO and β-CD and commercial and micronized ZnO. All the samples were characterized through UV–Vis diffuse reflectance spectroscopy (DRS), and the band gap values were calculated. Raman and FT-IR measurements confirmed the presence of ZnO and the existence of functional groups due to the β-cyclodextrin and ZnO combination in the hybrid photocatalysts. Wide-angle X-ray diffraction patterns proved that wurtzite is the main crystalline phase for all hybrid photocatalytic systems. In the photocatalytic degradation tests, it was observed that all the photocatalytic systems exhibited 100% removal efficiency within a few minutes. However, the commercial ZnO/micronized β-CD hybrid system is the photocatalyst that shows the best performance; in fact, when using this hybrid system, ceftriaxone was entirely degraded in 1 min.

Photocatalytic Degradation of Ceftriaxone Using TiO2 Coupled with ZnO Micronized by Supercritical Antisolvent Route

“…The micronization of zinc acetate dihydrate (ZnAc) powders was carried out using the SAS process [32]. This process exploits the peculiar characteristics of carbon dioxide in the supercritical state: diffusivities comparable to those of gases and densities of an order of magnitude similar to those of liquids [29].…”

“…After each SAS experiment, the powd was collected in the precipitator, and DMSO was extracted using supercritical carbon d oxide (scCO2). The obtained powder was annealed at 500 °C for 2 h in air with a heati rate equal to 2 °C min −1 to obtain zinc oxide nanoparticles (named ZnOSAS) [29]. Micronization conditions, including a pressure equal to 150 bar, temperature equal to 40 • C, and concentration of 15 mg mL −1 for the micronization of ZnAc, were chosen following a previous work [29].…”

“…The obtained powder was annealed at 500 °C for 2 h in air with a heati rate equal to 2 °C min −1 to obtain zinc oxide nanoparticles (named ZnOSAS) [29]. Micronization conditions, including a pressure equal to 150 bar, temperature equal to 40 • C, and concentration of 15 mg mL −1 for the micronization of ZnAc, were chosen following a previous work [29]. Before each experiment, the solution was prepared by dissolving the solute in 100 mL of DMSO, which served as the solvent, to achieve the desired concentration and maximum sample yield.…”

“…After each SAS experiment, the powder was collected in the precipitator, and DMSO was extracted using supercritical carbon dioxide (scCO 2 ). The obtained powder was annealed at 500 • C for 2 h in air with a heating rate equal to 2 • C min −1 to obtain zinc oxide nanoparticles (named ZnOSAS) [29].…”

“…They proved that the coupling of ZnO and TiO 2 led to an increase in the lifetime of the photogenerated charges, allowing more efficient separation of electron-hole pairs due to the electron transfer from the conduction band of ZnO to the conduction band of TiO 2 with the simultaneous transfer of positive holes from the valence band of TiO 2 to the valence band of ZnO [21]. In recent years, ZnO-based photocatalysts in the form of nanoparticles were prepared by the supercritical antisolvent (SAS) precipitation route [29][30][31][32]. They showed better photocatalytic performances than those prepared by drying-precipitation and sol-gel methods [32].…”