The phototransformation of carbaryl was investigated upon solar light exposure on three surfaces, silica, kaolin and sand, as soil models. By excitation with a Suntest set up at the surface of the three solid supports, the degradation of carbaryl followed first-order kinetics with a rate constant of 0.10 h−1. By using the Kubelka Munk model, the quantum yield disappearance at the surface of kaolin was evaluated to 2.4 × 10−3. Such a value is roughly one order of magnitude higher than that obtained in aqueous solutions. The results indicated that the particle size and the specific surface area of the various models have significant effects. The photo-oxidative properties as well as the byproduct elucidation by liquid chromatography combined with diode arrays (LC-DAD) and liquid chromatography coupled mass spectrometry (LC-MS) analyses allowed us to propose the degradation mechanism pathways. The main products were 1-naphtol and 2-hydroxy-1,4-naphthoquinone, which arise from a photo-oxidation process together with products from photo-Fries, photo-ejection and methyl carbamate hydrolysis. The toxicity tests clearly showed a significant decrease of the toxicity in the early stages of the irradiation. This clearly shows that the generated products are less toxic than the parent compound.

The indirect photochemical degradation of mesotrione induced by humic substances was studied by excitation with a solar light simulator. The disappearance rate constant increased by increasing the concentration of molecular oxygen. Three main byproducts were observed. They all arise from the dissociation of the non-aromatic cycle. Collision-induced dissociation tandem mass spectrometry (CID-MS/MS) of the deprotonated molecule was carried out and the effect of the collision energy as well as the elemental compositions of the products ions was used to propose the chemical structures. The presence in the chemical structure of the SO2-Me moiety permitted several types of homolytic scissions leading to the formation of radical anions.

LC/ESI-Q-TOFMS appeared to be a valuable and precise tool for structural elucidation of the unknown by-products that were generated during hydroxyl radical reactions with Chloridazon. Several hydroxylated and dihydroxylated isomers were identified together with dechlorinated and bridge opening products.