Comparative study on photocatalytic degradation of methomyl and parathion over UV-irradiated TiO2 particles in aqueous solutions

• Photodegradation of methomyl and parathion by UV/TiO2 was investigated and compared.
• Alkaline solutions (e.g., pH 10) were favorable for such TiO2-mediated degradation.
• Parathion is hard to be degraded than methomyl due to its more complicated structure.
• The initial stage of the degradation process well follows the first-order kinetics.
• Degradation paths could be inferred from time change of final product concentration.

Photodegradation of methomyl and parathion, the representatives of carbamate and organophosphorus pesticides, respectively, over TiO2 particles under 400-W UV irradiation was investigated and compared at 40 °C. All experiments were conducted at different initial pH values (4.0–10.0), initial pesticide concentrations (10–30 mg/L), and TiO2 doses (0–1.5 g/L). The Langmuir–Hinshelwood kinetic model was used to evaluate the apparent first-order rate constants of both pesticides at their initial stages of degradation processes. It was seen that methomyl was degraded much faster than parathion under comparable conditions, primarily due to more complicated structures of parathion molecules. The mineralization processes of methomyl and parathion by TiO2/UV were also studied on the basis of the measured total organic carbon (TOC) data. Finally, the possible degradation paths of methomyl and parathion were inferred by following the variations of the concentrations of final mineralized products with time.