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 K(OC) 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 K(d) 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.
Abstract Considerable attention has been paid to the enantiomeric resolution and toxicity of some chiral organophosphorous pesticides (OPs) with one asymmetric center, but research concerning chiral OPs with two asymmetric centers is still limited. In the present study, the stereoisomeric separation and toxicity of fosthiazate, a chiral OP with two asymmetric centers on phosphorus and carbon atoms, was investigated. All four stereoisomers of fosthiazate were separated successfully with a Chiralpak® AD [amylase tris(3,5‐dimethyl‐phenyl carbamate)] column on high‐performance liquid chromatography. The resolved isomers and the pairs of enantiomers were further distinguished using a circular dichroism detector and an optical rotation detector, designating the first (pk1) and third (pk3) eluted peaks as one pair of enantiomers and the second (pk2) and fourth (pk4) peaks as the other pair. The developed method was used to prepare microquantities of individual stereoisomers that were used for in vitro and in vivo bioassays. The inhibition potencies of the stereoisomers against acetylcholinesterase of Electrophorus electricus were slightly stereoselective, with a maximum difference of 1.4‐fold among the isomers. A 3.1‐fold difference, however, was observed in the acute toxicity of isomers to Daphnia magna. The 48‐h toxicity of isomers to D. magna followed the order pk1 > pk2 > pk4 > racemate ≈ pk3. The stereoselective toxicity to D. magna found in fosthiazate suggests that the environmental safety of fosthiazate should be evaluated on the basis of its individual isomers.
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