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 ZnCl 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 (K and K) were smaller for ZnO-NPs (5.74-12.6 mg kgd and 0.17-0.39 d) than for ZnCl (8.32-40.1 mg kgd and 0.31-2.05 d), suggesting that ionic Zn was more accessible for E. crypticus than nanoparticulate Zn. Based on external exposure concentrations, LC50s for ZnO-NPs and ZnCl decreased with time from 123 to 67 Zn mg L and from 86 to 62 Zn mg L, 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 ZnCl, with overall LC50 of Zn being 1720 and 1306 mg kg dry body weight, respectively. Body Zn concentration, which considers all available pathways, was a good predictor of dynamic toxicity of ZnCl, 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.