The effect of soil properties on the toxicity and bioaccumulation of Ag nanoparticles and Ag ions in Enchytraeus crypticus

Standard natural Lufa soils (2.2, 2.3 and 5 M) with different organic carbon contents (0.67–1.61%) and pH_CaCl2 (5.5–7.3) were spiked with ionic Ag (AgNO₃) and polyvinyl pyrrolidone (AgNP-PVP) and citrate (AgNP-Cit) coated Ag nanoparticles (NPs). Enchytraeus crypticus were exposed for 21 days to assess effects on survival and reproduction. Soil, pore water and animals were analyzed for Ag. AgNP-Cit had a strong increasing effect on soil pH, leading to high enchytraeid mortality at concentrations higher than 60–100 mg Ag/kg dry soil which made it impossible to determine the influence of soil properties on its toxicity. LC50s were lower for AgNO₃ than for AgNP-PVP (92–112 and 335–425 mg Ag/kg dry soil, respectively) and were not affected by soil properties. AgNO₃ and AgNP-PVP had comparable reproductive toxicity with EC50s of 26.9–75.2 and 28.2–92.3 mg Ag/kg dry soil, respectively; toxicity linearly increased with decreasing organic carbon content of the soils but did not show a clear effect of soil pH. Ag uptake in the enchytraeids was higher at higher organic carbon content, but could not explain differences in toxicity between soils. This study indicates that the bioavailability of both ionic and nanoparticulate Ag is mainly affected by soil organic carbon, with little effect of soil pH.