| Abstract
| - In the use of desorption to estimate bioavailability, short desorption intervals give a measurement similar to the freely dissolved concentration, while longer intervals approximate bioaccessibility.
- Availability is an important factor regulating the fate and toxic effects of hydrophobic organic compounds (HOCs) in soil and sediment. Many methods have been proposed for measuring HOC availability, but ambiguity exists in the selection of methods or method conditions. In this study, using pyrethroid insecticides as model HOCs, we measured their desorption kinetics from black carbon (BC)-amended sediments and used comprehensive statistical analysis to understand the dependence of the derived parameters on desorption intervals. Fitting of data from Tenax-aided depletive desorption to a three-phase model gave estimates of 11−13, 28−33, and 57−60% pyrethroid distribution in the rapid (Frapid), slow (Fs), and very slow (Fvs) desorption fractions, respectively. The desorbed fraction after 24 h, or F24h, essentially equaled to Frapid, while the desorbed fraction after 6 h (F6h) was only about half of Frapid, suggesting that the practice of using F6h in lieu of Frapid would lead to inaccurate assessment of availability. In contrast, Pearson correlation coefficients for the desorbed fractions and uptake into polydimethylsiloxane (PDMS) fibers decreased with increasing desorption intervals, with F6h giving the most agreeable measurements. Therefore, while Frapid estimated from depletive desorption reflects the total chemical accessibility, desorbed fractions after short intervals likely provide a measure for the immediate availability, much like PDMS fibers. The use of desorbed fractions after short intervals (e.g., F6h) to approximate Frapid may give estimates substantially different from Frapid and therefore should be avoided.
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