Photocatalytic degradation of propachlor in aqueous TiO2 suspensions. Determination of the reaction pathway and identification of intermediate products by various analytical methods.

The light-induced degradation of propachlor (2-chloro-N-isopropylacetanilide) under simulated solar irradiation has been investigated in aqueous solutions containing TiO2 suspensions as photocatalysts. The study focuses on the identification of possible intermediate products and the determination of inorganic ions formed during the process, using several powerful analytical techniques such as gas chromatography mass-spectrometry (GC-MS) and ion chromatography. The primary degradation of propachlor has been a fast process with a half-live time of 2.3 min and followed pseudo-first-order kinetics according to the Langmuir-Hinshelwood model. The mineralization of the organic carbon to CO2 after 240 min of irradiation was found to be < 63%. The corresponding stoichiometric transformation of organic chlorine into chloride ion was reached in 30 min, whereas oxidation of nitrogen to nitrate ions took place at delayed irradiation times. Various aromatic intermediates have been identified by GC/MS techniques during the treatment, which were involved into three tentative degradation routes. One, was based on the dechlorination through positive holes or solvated electrons to form the hydroxy or aliphatic derivative. The second route consists of the rupture of amide bond or the N-C bond via nitrogen ionization by positive holes and through the ring hydroxylation, respectively. Finally, the third possible degradation route was based on hydrogen abstraction by the hydroxyl radicals or positive holes, followed either by the addition of oxygen to the alkyl radical or the decomposition of the peroxyl radical formed. From the analytical data, a possible multi-step degradation scheme was proposed. This work points to the necessity of extended knowledge of the successive steps in a solar-assisted detoxification process.