Sonocatalytic and photocatalytic efficiency of transition metal-doped ZnO nanoparticles in the removal of organic dyes from aquatic environments

The present study investigated the efficiency of transition metal-doped ZnO nanoparticles (NPs) in the removal of Direct Blue 71 from aqueous solutions through photocatalytic (UV and visible light) and sonocatalytic processes. The ZnO particles were synthesized and analyzed by the SEM, XRD, FTIR, AFM, DLS, and zeta potential. The reaction conditions were optimized by concerned catalyst (i.e., dopant percentage and catalyst amount), solution pH, catalyst dose and initial dye concentration. The results demonstrated that the Ag-doped ZnO NPs could provide the highest UV light-based photocatalytic efficiency, while the Cu-doped NPs present the greatest sonocatalytic and visible light-based photocatalytic efficiencies. In all processes, the dye removal efficiency was better in acidic pH. Based on the insignificant difference of efficiency over the acidic range of pH, the natural pH of the dye, i.e., 5.6, was considered as the optimal pH value. Also, the increase of dopant percentage enhanced the decolorization efficiency of the catalysts. However, 2.5% dopant amount was selected as the optimal dopant content due to the negligible difference in the decolorization percentages observed using the 2.5 and 5% dopant percentages. Furthermore, the increase of catalyst dose and contact time increased the removal efficiency, while the increase of initial dye concentration resulted in a lower extent of dye decolorization.