Photocatalytic oxidation of multicomponent solutions of herbicides: Reaction kinetics analysis with explicit photon absorption effects

The intrinsic reaction kinetics of the photocatalytic oxidation (PCO) of the herbicides isoproturon, simazine and propazine over irradiated TiO2 (Degussa P25) suspensions was studied in single-component and in multicomponent systems. Experiments were carried out at different photon fluxes and different herbicide concentrations in the range below 1 mg L-1. The results were analyzed in terms of a Langmuir-Hinshelwood (L-H) kinetic model previously established for multicomponent solutions of herbicides, but including the explicit effect of photon absorption. This was accomplished by modeling the radiation field in the reactor with the Six-Flux Absorption-Scattering Model (SFM) (i.e., scattered photons follow the route of the six directions of the Cartesian coordinates) using optical parameters averaged across the spectrum of the incident radiation. The intrinsic reaction kinetic constants of isoproturon, simazine and propazine independent of the radiation field in the reactor were determined. The oxidation rate of the herbicides was found to follow half-order dependence with respect to the local volumetric rate of photon absorption (LVRPA), in the range of photon fluxes investigated. The established model was found to be appropriate to predict the time-dependent degradation profiles of the herbicides in single- and multicomponent systems not only at all radiation intensities investigated in this study but also when one or two herbicides were present in excess of the others in the mixture. Using this simple approach, intrinsic kinetic data can be obtained. © 2006 Elsevier B.V. All rights reserved.