Effect of intensifying additives on the degradation of thiamethoxam using ultrasound cavitation

The present study has investigated the degradation of thiamethoxam using ultrasound cavitation (US) operated at a frequency of 20 kHz and its combination with intensifying additives viz. hydrogen peroxide, Fenton and photo-Fenton reagent. At the outset, the performance of US (20 kHz) has been maximised by the optimization of process parameters. Highest rate of degradation of thiamethoxam was observed at the optimum ultrasonic power density of 0.22 W/mL, thiamethoxam concentration of 10 ppm and the pH of 2. The established optimum values of operating parameters were used further in case of combined treatment approaches. The effect of concentration of H₂O₂ on the rate of degradation of thiamethoxam in the case of US + H₂O₂ process has confirmed the existence of optimum concentration of H₂O₂ with the ratio of thiamethoxam: H₂O₂ as 1:10. US + Fenton process indicated the optimal molar ratio of FeSO₄·7H₂O:H₂O₂ as 1:15. The combined processes of US + H₂O_2, US + Fenton and US + photo-Fenton have resulted in the extent of degradation of 20.47 ± 0.61%, 34.41 ± 1.03% and 85.17 ± 2.56% respectively after 45 min. of operation. These combined processes lead to the synergistic index of 2.04 ± 0.06, 2.26 ± 0.07 and 2.42 ± 0.07 in case of US + H₂O₂, US + Fenton and US + photo-Fenton processes respectively over only US/stirring treatment with the additive. Additionally, the extent of mineralization and the energy efficiency of individual and combined processes have been compared. US + photo-Fenton process has been found to be the best strategy for effective degradation of thiamethoxam with a significant intensification benefit. The by-products formed during the ultrasonic degradation of thiamethoxam have been identified by using LC-MS/MS analysis.