Toxicokinetics of Ag (nano)materials in the soil model Enchytraeus crypticus (Oligochaeta) - impact of aging and concentration

Silver (Ag) nanomaterials (NMs) are used in many products, eventually reaching the environment at some life stage and as they can be harmful their impact should be assessed. Although research has focused on Ag NM toxicity, less focus has been on toxicokinetics. The aim of this study was to assess the kinetics of Ag nanomaterial (Ag NM300K) and AgNO3 in the soil invertebrate Enchytraeus crypticus. Tests followed OECD guideline 317, with 14 days uptake followed by 14 days elimination in LUFA 2.2 soil. Two sub-lethal concentrations were selected based on enchytraeid sensitivity in a reproduction test (6 and 60 mg Ag per kg for Ag NM300K, and 5 and 45 mg Ag per kg for AgNO3), and spiked soil aged for 3 and 14 days after spiking. Total and 0.01 M CaCl2 extractable soil concentrations were evaluated at day 0, 1 and 14 for all the exposures. Overall, enchytraeids showed increasing Ag uptake with time, followed by a decrease when transferred to clean soil. For the lowest exposure concentrations, the difference in Ag uptake rate constants between 3 and 14 days aging was larger (10-fold) for AgNO3 than for NM300K (uptake rates being highest for soil aged for 3 days), which was in line with the higher CaCl2-extractable Ag concentrations in AgNO3 spiked soil. At the higher exposure concentrations, for AgNO3 the difference in Ag uptake rate constants between 3 and 14 days aged soils was 2-fold, with the bioaccumulation factor (BAF) being highest at 3 days aging. For Ag NM300K, the uptake rate constant was low with virtually no elimination, suggesting that body Ag concentrations may keep on increasing with time leading to a higher risk of longer-term exposure compared to the Ag ions. These findings show the importance of understanding the toxicokinetics of ionic and nano forms of silver and other elements, and the key role of aging in determining NM bioavailability.