Effect of aging on desorption kinetics of sediment-associated pyrethroids

Pyrethroids are insecticides commonly used in both agricultural and urban environments. Residues of pyrethroids frequently are found in bed sediments of regions such as California (USA), and as such, sediment toxicity from pyrethroid contamination is an emerging concern. Pyrethroids are highly hydrophobic, and toxicity from sediment-borne pyrethroids is expected to depend closely on their desorption rate. In the present study, we evaluated the effect of aging on desorption kinetics of sediment-borne pyrethroids. Two sediments spiked with four pyrethroids were incubated for 7, 40, 100, and 200 d at room temperature. Desorption measured using sequential Tenax extractions was well described by a three-compartment model. The estimated rapid desorption fraction (F(rapid)) decreased quickly over time and was accompanied by an increase of the very slow desorption fraction. The aging effect on desorption kinetics followed a first-order model, with half-lives for the decrease in F(rapid) for all four pyrethroids in both sediments ranging from two to three months. When coupled with degradation, the estimated half-lives of the rapidly desorbing fraction (and, thus, the potentially bioavailable concentration) were two months or less for all four pyrethroids. Two field-contaminated sediments displayed distinctively different desorption kinetics. The sediment with fresh residues exhibited rapid desorption, and the sediment containing aged residues was highly resistant to desorption. The observation that desorption of pyrethroids decreased quickly over contact time implies that the bioavailability of sediment-borne pyrethroids may diminish over time and that use of nonselective extraction methods may lead to overestimating the actual sediment toxicity from pyrethroid contamination.