Keywords: 
• pyrethroids /
• carbon sorbents /
• Tenax extraction /
• bioavailability /
• sediment

Abstract: Pyrethroids are a group of widely used synthetic insecticides with high hydrophobicity and aquatic toxicity. Previous studies have indicated that organic carbon (OC) especially black carbon (BC) plays an important role in the bioavailability of hydrophobic organic contaminants (HOCs) in sediments. However, the effect of OC on the bioavailability of pyrethroids in sediments is less konwn. In this study, the effect of four different types of carbon sorbents (peat soils, biochar, charcoal and activated carbon) on the bioavailability of sediment-associated pyrethroids was studied based on the bioaccumulation in Venerupis philippinarum and the Tenax desorption kinetics. The results showed that the carbon sorbents could decrease the fraction of rapid desorption (F_rap) and increase the fraction of very slow desorption (F_vs) of pyrethroids in sediment. The sequestration capacity of four sobents were similar except for peat soils as the sorption capacity to HOCs of BC sorbents (biochar, charcoal and activated carbon) was found much stronger than that of humic substances (peat soils). Although the three BC sorbents have different specific surface area (SSA) and pore volumes, they still had a similar sequestration capacity to the bioavailability of pyrethroids, mainly due to the large molecular sizes and the nonplanar structure of pyrethroids. In addition, the efficiency of the carbon sorbent treatment was found to increase had the activated carbon in sediments increased. After 672 h Tenax extraction, about 77.5%-91.6%, 71.9%-87.4%, 68.7%-82.6%, 65.6%-78.0%, and 47.8%-70.5% pyrethroids was desorbed for the 0, 0.7%, 2%, 3%, and 5% activated carbon amended sediments, respectively. Significant correlations were observed between the F_rap of pyrethroids with Tenax 6 h and with 24 h (P<0.0001) in sediment (F₆/F_rap =0.95, F24/F_rap=1.18). Abnormal points were found when the sediments were amended with 5% activated carbon. It demonstrated that the single time-point Tenax 6 h or 24 h, instead of the whole desorption kinetics can be used to evaluate the bioavailability of sediment-associated pyrethroids. However a large amendment of BC might make the method instability. Further, the relationship between the OC-normalized concentration of rapid desorption (C_s,rap) and the lipid-normalized concentration of pyrethroids in Venerupis philippinarum (C_b,lip) was analyzed. There was a siginificant correlation between C_s,rap and C_b,lip(log C_s,rap=1.5 log C_b,lip-0.49，R²=0.38, P<0.0001), indicating that Tenax extraction not only can predict the concentrations of prethroids in benthic organisms in complex matrix, but also can provide the basis for evaluating the bioavailability of sediment-associated HOCs in aquaculture pond and the quality of aquatic products.