Fipronil, an increasingly popular insecticide used for urban pest control, is known to readily transform into several degradates that generally have similar or greater toxicity to aquatic organisms than the parent compound. However, knowledge on the fate of these degradates in the environment is obscure. In the present study, degradation kinetics and sorption of desthiofipronil, fipronil sulfide, and fipronil sulfone were investigated in urban stream sediments. All degradates showed enhanced persistence in sediments compared to fipronil under facultative or anaerobic conditions. Under facultative conditions, the estimated half-lives of desthiofipronil, fipronil sulfide, and fipronil sulfone were 217 to 497, 195 to 352, and 502 to 589 d, respectively. Under anaerobic conditions, the corresponding half-lives were over one year in one sediment, while no detectable degradation occurred in the other two sediments after 280 d. Sorption isotherms of fipronil and its degradates in the sediments were linear, with mean KOC values of 802, 1,296, 3,684, and 3,543 L/kg for fipronil, desthiofipronil, fipronil sulfide, and fipronil sulfone, respectively, suggesting that the degradates generally have a higher sorption capacity than fipronil. Sorption coefficient Kd increased up to fourfold over 280 d, suggesting an aging effect on sorption. The inherent toxicity, long persistence, and strong sorption potential highlight the importance for a better understanding of the sediment toxicity of fipronil degradates in surface water bodies.
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