Earthworm Uptake Routes and Rates of Ionic Zn and ZnO Nanoparticles at Realistic Concentrations, Traced Using Stable Isotope Labeling

The environmental behavior of ZnO nanoparticles (NPs), their availability to, uptake pathways by, and biokinetics in the earthworm Lumbricus rubellus were investigated using stable isotope labeling. Zinc isotopically enriched to 99.5% in Zn-68 (Zn-68-E) was used to prepare (ZnO)-Zn-68 NPs and a dissolved phase of Zn-68 for comparison. These materials enabled tracing of environmentally relevant (below background) NP additions to soil of only 5 mg Zn-68-E kg(-1). Uptake routes were isolated by introducing earthworms with sealed and unsealed mouthparts into test soils for up to 72 h. The Zn isotope compositions of the soils, pore waters and earthworms were then determined using multiple collector inductively coupled plasma mass spectrometry. Detection and quantification of Zn-68-E in earthworm tissue was possible after only 4 h of dermal exposure, when the uptake of Zn-68-E had increased the total Zn tissue concentration by 0.03 parts per thousand. The results demonstrate that at these realistic exposure concentrations there is no distinguishable difference between the uptake of the two forms of Zn by the earthworm L. rubellus, with the dietary pathway accounting for similar to 95% of total uptake. This stands in contrast to comparable studies where high dosing levels were used and dermal uptake is dominant.