Metolachlor photocatalytic degradation using TiO2 photocatalysts

The photocatalytic degradation of the herbicide metolachlor is Studied, using TiO2 photocatalysts. Apart from the titania aqueous dispersions, immobilized semiconductor thin films were used in order to evaluate their effectiveness. The films exhibit a rough, nanocrystalline configuration, ideal for heterogeneous processes. Additionally the thin films were silver modified to determine if noble metal nanoclusters affect the photocatalytic efficiency. The kinetics obtained demonstrated that powder Titania (t(1/2) = 21 min) was more efficient for the photocatalytic decomposition of metolachlor compared to pure titania thin film catalysts (t(1/2) = 52 min), while the presence of silver ions in TiO2 particles modification did not prove to promote the solar photocatalytic process. Intermediate products from the slurry system were isolated and identified by means of solid-phase extraction (SPE) coupled to gas chromatography-mass spectroscopy techniques (GC-MS). A proposed degradation pathway of metolachlor is presented, involving mainly hydroxylation and dealkylation reactions. In addition, toxicity studies based on the luminescence of the bacteria Vibrio fischeri before and after the photocatalytic treatment were performed. The results demonstrated that the transient intermediates formed were more toxic compared to parent compound. At low photocatalyst loads, the toxicity of the irradiated solution remains relatively high even though 88% of metolachlor is decomposed. (C) 2003 Elsevier B.V. All rights reserved.