Removal of anionic dyes from wastewater using silica supported TiO2 or ZnO colloidal nanoparticles at equal conditions

Andraž Šuligoj, Department of Inorganic Chemistry and Technology, National Institute of Chemistry, Ljubljana, Slovenia
Darja Maučec, Department Of Inorganic Chemistry And Technology, National Institute Of Chemistry, Ljubljana, Slovenia
Albin Pintar, Department Of Environmental Sciences And Engineering, National Institute Of Chemistry, Ljubljana, Slovenia
Nataša Novak, Department Of Inorganic Chemistry And Technology, National Institute Of Chemistry, Ljubljana, Slovenia

Different types of supported TiO2 and ZnO nanoparticles have been reported as efficient photocatalysts for removal of organic dyes in wastewater at different pH values and conditions. Herein, we evaluate for the first time mesoporous silica supported TiO2 versus ZnO photocatalysts at neutral pH and equal conditions.
Photocatalysts were prepared from colloidal solutions of TiO2 and ZnO nanoparticles and ordered mesoporous silica (SBA-15) compared to disordered mesoporous silica (KIL-2).
TiO2 composites exhibited lower adsorption and slower degradation kinetics of anionic RB19 as a model dye when grafted onto disordered KIL-2 silica than on ordered SBA-15, while opposite trends were observed for ZnO-based solids. Anionic RB19 dye was adsorbed onto silica supported ZnO nanoparticles at neutral pH value in much higher manner than on TiO2 nanoparticles, even though the surfaces of ZnO– and TiO2–silica composite catalysts were negatively charged in both cases (ca. -20 mV of zeta potential). This was ascribed to the strong positive zeta potential of naked ZnO nanoparticles at neutral pH value (ca. +37 mV) and consequently to more mixed oxide (spinel-like) ZnO-silica composite behaviour if compared to physical mixture oxide TiO2-silica composites. Pure TiO2 or silica supported TiO2 nanoparticles were stable under UV irradiation. On the other hand, interaction between SiO2 and ZnO resulted in increased resistance to photocorrosion of ZnO, which was proven to be an issue in pure ZnO nanoparticles.
The potential of silica supported TiO2 and ZnO photocatalysts for anionic or cationic dyes in industrial wastewater treatment is finally discussed.