Different dynamic accumulation and toxicity of ZnO nanoparticles and ionic Zn in the soil sentinel organism Enchytraeus crypticus

There is still no consensus over the specific effects of metal-based nanoparticles when compared with the conventional metal salts. Here, the accumulation and toxicity of ZnO-NPs and ZnCl2 in Enchytraeus crypticus over time (1–14 d) were investigated using a sand-solution exposure medium and applying a toxicokinetics and toxicodynamics approach. For both Zn forms, body Zn concentration in the organisms was dependent on both the exposure concentration and exposure time, with equilibrium being reached after 7–14 days of exposure. Generally, the uptake and elimination rate constants (Ku and Ke1) were smaller for ZnO-NPs (5.74–12.6 mg kg−1d−1 and 0.17–0.39 d−1) than for ZnCl2 (8.32–40.1 mg kg−1d−1 and 0.31–2.05 d−1), suggesting that ionic Zn was more accessible for E. crypticus than nanoparticulate Zn. Based on external exposure concentrations, LC50s for ZnO-NPs and ZnCl2 decreased with time from 123 to 67 Zn mg L−1 and from 86 to 62 Zn mg L−1, reaching an almost similar ultimate value within 14 d. LC50s based on body Zn concentrations were almost constant over time (except for 1 d) for both ZnO-NPs and ZnCl2, with overall LC50body of Zn being 1720 and 1306 mg kg−1 dry body weight, respectively. Body Zn concentration, which considers all available pathways, was a good predictor of dynamic toxicity of ZnCl2, but not for ZnO-NPs. This may be attributed to the specific internal distribution and detoxification mechanisms of ZnO-NPs. The particles from ZnO-NPs dominated the accumulation (>75%) and toxicity (∼100%). Our results suggest that dynamic aspects should be taken into account when assessing and comparing NPs and metals uptake and consequent patterns of toxicity.