pH-dependent contribution of chlorine monoxide radicals and byproducts formation during UV/chlorine treatment on clothianidin

Combining UV and free chlorine (UV/chlorine) is an efficient advanced oxidation process for the abatement of recalcitrant organic compounds in drinking water and wastewater. This study investigated the degradation of a neonicotinoid insecticide, clothianidin (CTD), by UV/chlorine treatment. The free chlorine concentration was optimized at 160 mu M, and 90.1 +/- 0.4% of 40 mu M CTD was degraded after 300 s of treatment. Radical quenching tests using tert-butyl alcohol, Cl-, HCO3-, and N3- indicated that chlorine monoxide (ClO center dot) was the main radical species for CTD degradation. The second-order rate constants of CTD reacting with ClO center dot (kClO center dot,CTD = 7.3 +/- 0.1 x 109 M- 1s- 1) was 4.3 times higher than that for center dot OH (k center dot OH,CTD = 1.7 +/- 0.2 x 109 M- 1s- 1). The presence of humic acid inhibited CTD degradation by filtering UV and scavenging ClO center dot. The pH was optimized at 7, and the overall reaction rate constant (k ') was 2.35 +/- 0.02 x 10-2 s- 1 (half-life = 0.49 min). Degradation products identified during the UV/chlorine treatment were 1-methyl-3-nitroguanidine ([M + H]+ = 118.9), nitroguanidine ([M + H]+ = 105.1), methylguanidine ([M + H]+ = 74.3), and clothianidin urea ([M + H]+ = 206). The detailed timedependent concentrations of the generated products under different pH conditions were also provided. The results suggest that the UV/chlorine treatment can be an efficient strategy for CTD degradation.

Automated summary

Combining UV and free chlorine is an efficient method for degrading recalcitrant organic compounds in water. This study found that chlorine monoxide was the main radical species responsible for the degradation of the neonicotinoid insecticide clothianidin, and the reaction rate constants indicated a faster degradation rate compared to other radical species. The presence of humic acid was found to inhibit the degradation process.