Photocatalytic degradation of the herbicides propanil and molinate over aqueous TiO2 suspensions: identification of intermediates and the reaction pathway

The light-induced degradation of propanil and molinate under simulated sola r irradiation has been investigated in aqueous solutions containing TiO2 su spensions as photocatalysts. The study focus on the identification of possi ble intermediate products and the determination of inorganic ions formed du ring the process, using several powerful analytical techniques such as gas chromatography-mass spectrometry (GC-MS) and ion chromatography (IC). The p rimary degradation of propanil and molinate has been a fast process with ha lf-lives varied from 4.3 to 2.9 min, respectively, and followed pseudo-firs t-order kinetics according to the Langmuir-Hinshelwood model. The stoichiom etric transformation of organic chlorine into chloride ion and organic sulf ur to sulfate ions was observed for propanil and molinate, respectively, wh ereas oxidation of nitrogen to nitrate ions took place at delayed irradiati on times for both herbicides. The mineralization of the organic carbon to C O2 after 240 min of irradiation was found to be greater than or equal to 95 % for both herbicides. Various organic intermediates detected during the tr eatment have been identified by GC-MS techniques. Based on this by-product identification, a possible multi-step degradation scheme was proposed for e ach herbicide including hydroxylation, dechlorination, dealkylation and oxi dation steps that lead to the mineralization of the starting molecule. This work points to the necessity of extended knowledge of the successive steps in a solar-assisted detoxification process. (C) 2001 Elsevier Science BN. All rights reserved.