Kinetics and mechanism of thiamethoxam abatement by ozonation and ozone-based advanced oxidation processes

In this study, the abatement of neonicotinoid insecticide, thiamethoxam, by single ozonation, ozone/ultraviolet (O₃/UV) and electro-peroxone (EP) process was evaluated. The second-order rate constants for the reaction of thiamethoxam with O₃ and hydroxyl radical (OH) at pH 7 were determined to be 15.4 M^-1 s^-1 and 3.9 × 10⁹ M^-1 s^-1, respectively. The degradation pathways of thiamethoxam were proposed based on quantum chemical calculations and transformation products were identified using chromatographic and mass-spectrometric techniques. The acute and chronic toxicity of thiamethoxam and its major TPs to various aquatic organisms were assessed. With typical ozone doses applied in water treatment (≤5 mg/L), thiamethoxam was abated by only ∼16-32 % in two real water matrices (groundwater and surface water) during single ozonation, but by ∼100 % and >70 % during the O₃/UV and EP treatment, respectively. The energy demand to abate 90 % thiamethoxam in the two water matrices was generally comparable for single ozonation and the EP process (∼0.14 ± 0.03 kW h/m³), but higher for the O₃/UV process (0.21-0.22 kW h/m³). These results suggest that single ozonation is unable to sufficiently abate thiamethoxam under typical conditions of water treatment. Therefore, ozone-based advanced oxidation processes are needed to enhance thiamethoxam abatement.