Effects of the amendment of biochars and carbon nanotubes on the bioavailability of hexabromocyclododecanes (HBCDs) in soil to ecologically different species of earthworms

•Biochar and CNT amendment reduced the bioavailability of HBCDs to earthworms.•The reduction in HBCD bioaccumulation was greater in E. fetida than M. guillelmi.•The amendments did not alter the isomer selectivity in HBCD bioaccumulation.•HBCDs in soils resisted desorption, with Frap < Fslow.•24 h-desorption is a good proxy for the bioavailability of HBCDs to earthworms.

Biochar is a promising material used in soil amendment and carbon nanotubes may enter soil due to its increasing application. These carbonaceous materials may change the bioavailability of pollutants in soil. In this concern, 0.5% w/w multi-walled carbon nanotubes (MWCNTs) and 3 corn-straw biochars acquired at different pyrolyzing temperatures were used in soil amendment and their influences on the bioavailability of hexabromocyclododecanes (HBCDs), a brominated flame retardant, to 2 ecologically different earthworm species were studied. The amendment of 4 carbonaceous materials all reduced the bioaccumulation of HBCDs in earthworms by 18.2%-67.3%, which varied depending on the type of carbonaceous materials and the pyrolyzing temperature of biochars. The reduction in HBCDs uptake by Eisenia fetida (an epigeic species) was greater than by Metaphire guillelmi (an anecic species). The 2 earthworm species both showed bioaccumulative selectivity on certain HBCD diastereoisomer and enantiomer in the amended soils, which was similar to that in the control soil. Moreover, Tenax-assisted HBCDs desorption test was carried out for the simulation of their bioavailability. The rapid desorption fraction (Frap), total desorption (15 d), and 24 h desorption all correlated well with the uptake of HBCDs in the earthworms, suggesting that the 24 h-desorption, due to its easy availability, can be a good proxy to predict the bioavailability of HBCDs to earthworms in soil.

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