Photocatalytic oxidation of herbicides in single-component and multicomponent systems: Reaction kinetics analysis

The photocatalytic oxidation (PCO) of the herbicides isoproturon, simazine and propazine over irradiated TiO2 suspensions was studied in single-component and in multicomponent systems. The initial herbicide concentration ranged from 70 μg L−1 to 3 mg L−1 in order to approach typical concentrations found in contaminated ground- and surface waters. The time-dependent degradation profiles of each herbicide were successfully modelled using an approximation of the Langmuir–Hinshelwood (L–H) rate equation, which takes into account the direct effect of the intermediate reaction products. The L–H rate equations were successfully extended to represent the time-dependent degradation profiles of multicomponent systems of herbicides, using the kinetic parameters determined in single-component experiments. A direct comparison of the binding constants of the herbicides observed under dark adsorption and under PCO shows that these are very similar suggesting that the degradation of isoproturon, simazine and propazine mixtures follows a surface or near-surface reaction according to a competitive L–H mechanism. The above findings were observed only at herbicide concentrations less than approximately 1 mg L−1 when monolayer coverage of TiO2 is attained. Above 1 mg L−1 there is a departure from the L–H mechanism due to multilayer adsorption of the herbicides leading to faster herbicide degradation kinetics.