ZnO supported on zeolite pellets as efficient catalytic system for the removal of caffeine by adsorption and photocatalysis

The combination of adsorption and photocatalytic processes (adsorption/photocatalysis) was studied in the removal of the psychoactive substance caffeine from aqueous solutions at initial pollutant concentration in the range 6–25 mg/L. For this purpose, photoactive ZnO semiconductor was supported on commercial zeolite pellets (ZnO/ZEO) using wet impregnation method. From an engineering point of view, the use of pellets in a packed-bed reactor permits to overcome the limitation of slurry systems that need the expensive post-treatment steps for the separation of particles from treated wastewater. X-ray diffraction, UV–Vis reflectance spectroscopy, scanning electron microscopy and N2adsorption-desorption measurements at −196 °C were performed to analyze the characteristics of the samples. In particular, it was found that ZnO nanoparticles are in wurtzite phase and incorporated in the mesoporous structure of the zeolite pellets. The final ZnO loading in ZnO/ZEO composites was found to be 6.1 wt%. The comparison between the adsorption and adsorption/photocalytic tests performed with ZnO/ZEO pellets showed that in dark conditions, the caffeine removal was 60% after 120 min of run time, while, in presence of UV light, the almost total removal of caffeine was achieved after the same treatment time. Moreover, also after several reuse cycles of adsorption/photocalytic tests, the removal rate of caffeine did not change, meaning that the photocatalytic oxidation process is able to degrade the caffeine adsorbed on ZnO/ZEO pellets surfaces resulting in the recovery of the adsorption capacity.