Relationships between desorption intervals and availability of sediment-associated hydrophobic contaminants

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 (F(rapid)), slow (F(s)), and very slow (F(vs)) desorption fractions, respectively. The desorbed fraction after 24 h, or F(24h), essentially equaled to F(rapid), while the desorbed fraction after 6 h (F(6h)) was only about half of F(rapid), suggesting that the practice of using F(6h) in lieu of F(rapid) 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 F(6h) giving the most agreeable measurements. Therefore, while F(rapid) 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., F(6h)) to approximate F(rapid) may give estimates substantially different from F(rapid) and therefore should be avoided.