Bioavailability of phthalate congeners to earthworms ( Eisenia fetida) in artificially contaminated soils

Abstract

Bioavailability of phthalate congeners, dimethyl phthalate, diethyl phthalate, di- n-butyl phthalate (DBP), di-(2-ethylhexyl) phthalate (DEHP), and dioctyl phthalate, to earthworms ( Eisenia fetida) were studied when earthworms were exposed to two artificially contaminated agricultural and forest soils. Only DBP and DEHP were detected in earthworms. The uptake kinetics of DBP and DEHP in earthworms was fast within the initial 10 days followed by a nearly steady state for the subsequent 20 days. An equilibrium partitioning model could be used to describe the uptake kinetics of DBP and DEHP by earthworm in two types of soils ( r = 0.709 – 0.864 ). The average biota-to-soil accumulation factors (BSAFs) of DBP and DEHP at 5 mg kg −1 in soil were 0.27±0.07 and 0.17±0.03, respectively, in agricultural soil, while the BSAFs were 0.21±0.06 and 0.07±0.02, respectively, in forest soil. The concentrations of phthalates in earthworms increased with increasing concentrations of phthalates in soil. There was a significant correlation between log C soil and log C worm , with r = 0.999 – 0.993 , demonstrating a single linear partitioning of phthalates between soil and earthworms. The bioavailability of DBP and DEHP was assessed by Soxhlet, methanol, and methanol–water (1:1) extraction methods. Our results indicated that the extractable amounts of freshly added DBP and DEHP in soils by these extraction methods were significantly correlated with those in earthworms. It was observed that the extractable DBP and DEHP by the methanol and methanol–water (1:1) extraction methods decreased with their increasing residence time in soil. In contrast, the amount extracted by the Soxhlet extraction method did not show a similar decline. Therefore, Soxhlet extraction was a poor indicator of the bioavailability of DBP and DEHP to earthworms in soil, which could lead to overestimation of the risk of soil-associated DBP and DEHP. The extractable DBP and DEHP by methanol and methanol–water (1:1) significantly decreased over 440 days. Compared with the methanol–water (1:1) extraction method, the methanol extraction method was preferred for its ability to predict the bioavailability of DBP and DEHP in aged soils.