Toxicokinetics of zinc-oxide nanoparticles and zinc ions in the earthworm Eisenia andrei

The toxicokinetics of zinc in the earthworm Eisenia andrei was investigated following exposure for 21 days to ionic zinc (ZnCl2) or zinc oxide nanoparticles (ZnO-NPs) in Lufa 2.2 soil, followed by 21 days elimination in clean soil. Two concentrations were tested for both ZnCl2 (250 and 500 μg Zn g−1) and ZnO-NPs (500 and 1000 μg Zn g−1), corresponding to EC25 and EC50 for effects on reproduction. Based on the measured internal Zn concentrations in the earthworms over time of exposure, the kinetics parameters ka - assimilation rate constant (gsoil g−1body weight day−1) and ke - elimination rate constant (day−1) were estimated using a one-compartment model for either total Zn concentrations in the soil or porewater Zn concentrations. In the ZnCl2 treatments, ka was higher for total Zn concentrations in soil, whereas in the ZnO-NP treatments, ka was higher for porewater Zn concentrations. The value of ke did not differ between the two Zn forms (ZnCl2 vs ZnO-NPs) for either EC50 or EC25 when related to total Zn concentrations in soil, but for EC50, ke related to porewater Zn concentrations was significantly higher for ZnCl2 than for ZnO-NPs. It is concluded that differences in kinetic parameters between treatments were connected with exposure concentrations rather than with the form of Zn. Zinc was efficiently regulated by the earthworms in all treatments: a 2-fold increase in exposure concentration resulted in a less than 2-fold increase in internal concentration, and after transfer to uncontaminated soil the internal Zn concentrations in the earthworms returned to ca 111 μg g−1 dw in all treatments.