Effects of aging and soil properties on zinc oxide nanoparticle availability and its ecotoxicological effects to the earthworm Eisenia andrei

To assess the influence of soil properties and ageing on the availability and toxicity of Zn applied as nanoparticles (ZnO NPs) or as Zn2+ ions (ZnCl2), three natural soils were individually spiked with either ZnO NPs or ZnCl2 and incubated for up to 6 months. Available Zn concentrations in soil were measured by pore water extraction (ZnPW), while exposures of earthworms (Eisenia andrei) were performed to study Zn bioavailability. ZnPW was lower when Zn was applied as nanoparticles than as ionic form, and decreased with increasing soil pH. ZnPW for both Zn forms were affected by ageing, but varied among the tested soils, highlighting the influence of soil properties. Internal Zn concentration in the earthworms (ZnE) was highest for the soil with high organic carbon content (5.4%) and basic pH (7.6) spiked with ZnO NPs, but the same soil spiked with ZnCl2 showed the lowest increase in ZnE compared to the control. Survival, weight change, and reproduction of the earthworms were affected by both Zn forms, but differences in toxicity could not be explained by soil properties or ageing. This shows that ZnO NPs and ZnCl2 behave differently in soils depending on soil properties and ageing processes, but differences in earthworm toxicity remain unexplained.