Consideration on degradation kinetics and mechanism of thiamethoxam by reactive oxidative species (ROSs) during photocatalytic process

• OH radicals functioned as the main ROSs for the degradation of thiamethoxam.
• Oxidative ions promote the degradation rate, while reductive anions inhibit it.
• The thiophene ring is the most active site which reacts with ROSs.
• 7 Photocatalytic intermediates were separated and identified by HPLC and HPLC/MS/MS.
• Cleavage of oxadiazinane and hydroxylation of thiophene were led to degradation.

Photocatalytic degradation kinetics of thiamethoxam was optimized by using both single-variable-at-a-time and central composite design based on the response surface methodology. Four factors, such as catalyst dosages, pH values, temperatures and substrates concentration, affect the photocatalytic degradation rates significantly. Moreover, surface reactions with photoholes and OH on the surface of TiO2 are responsible for the photocatalytic degradation of thiamethoxam. The effects of inorganic anions, metal ions and the contribution of reactive oxidation species on the degradation kinetics were also discussed. It could be concluded that the oxidative metal ions such as Cu2+ and Fe3+ promoted the degradation of thiamethoxam, while highly reductive anions such as I−, Br−, S2O32-, NO2- and CrO32- inhibited the photocatalytic activity. The results also indicated that OH radicals functioned as the main active species for the photocatalytic degradation of thiamethoxam. Seven photocatalytic intermediates were separated and identified by HPLC and HPLC/MS/MS. The active sites of thiamethoxam were predicted by frontier electron densities and chemisorption pattern. The activation barriers Ea were calculated by the transition state of intermediates. Two degradation pathways were thereby proposed as the cleavage of 1,3,5-oxadiazinane ring and the hydroxylation of thiophene rings attacked by OH radicals on the initial stage of photocatalytic process.

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