Composite TiO2-SiO2-REOs photocatalysts for water treatment: Degradation kinetics of monuron and its intermediates

Modifying a hydrophilic TiO2 photocatalyst by hydrophobic rare-earth oxides (REOs) may promote affinity to hydrophobic non-polar species, thus improving photocatalytic activity. The present work was focused on the kinetics of photocatalytic degradation of the polar Acid Orange 7 (AO7) and of the non-polar herbicide monuron (3-(4-chlorophenyl)-1, 1-dimethylurea) and its degradation intermediates in the presence of composite TiO2 films containing SiO2 binder and different REOs (CeO2, La2O3, Er2O3, Yb2O3). The addition of REOs particles into the TiO2/SiO2 composite matrix resulted in an increase in the initial degradation rate of monuron from 2.1 nmol min-1 cm-2 to 2.7-3.0 nmol min-1 cm-2. Furthermore, the ratios between the initial degradation rates of monuron to those of AO7 were significantly higher for composite films containing REOs showing a positive effect on the removal of the non-polar monuron molecules for all REOs. A comparison between the kinetics with the various types of photocatalysts reveals that the positive impact of SiO2 and that of SiO2/REOs on the kinetics was not limited to the conversion of monuron but to the overall mineralization. The main novelty represents the fact although the rate of TOC decrease varied among photocatalysts, the kinetics of intermediates is not significantly influenced.