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 solar irradiation has been investigated in aqueous solutions containing TiO2 suspensions as photocatalysts. The study focus 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 (IC). The primary degradation of propanil and molinate has been a fast process with half-lives varied from 4.3 to 2.9 min, respectively, and followed pseudo-first-order kinetics according to the Langmuir–Hinshelwood model. The stoichiometric transformation of organic chlorine into chloride ion and organic sulfur to sulfate ions was observed for propanil and molinate, respectively, whereas oxidation of nitrogen to nitrate ions took place at delayed irradiation times for both herbicides. The mineralization of the organic carbon to CO2 after 240 min of irradiation was found to be ≥95% for both herbicides. Various organic intermediates detected during the treatment have been identified by GC–MS techniques. Based on this by-product identification, a possible multi-step degradation scheme was proposed for each herbicide including hydroxylation, dechlorination, dealkylation and oxidation 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.