Soil species sensitivity distributions for terrestrial risk assessment of silver nanomaterials: the influence of nanomaterial characteristics and soil type†

Abstract

Silver nanomaterials (AgNMs) are released into the soil through various anthropogenic activities, including as biocides and in biosolid amendments. There is an abundance of toxicity data available for AgNMs and soil organisms, yet the assessment of their ecological risk and the influence of NM characteristics and exposure conditions on AgNM hazard in soils are not well elucidated. In this study, available soil ecotoxicology data for AgNMs and other Ag forms were collated from literature into a database. Using this database, species sensitivity distributions (SSDs) for soil biota were constructed. From these SSDs we calculated hazard concentrations for 50% of species (HC₅₀) that would allow us to robustly compare effects on soil organisms soil or liquid media and to assess relationships to NM properties (coating) and major soil properties. For all AgNMs, the calculated HC₅₀ value was 3.09 (1.74–5.21) mg kg⁻¹ for studies conducted with soil dwelling species in soils and 0.70 (0.32–1.64) mg L⁻¹ for liquid exposures. In comparison, the HC₅₀ value for Ag salt (silver nitrate, AgNO₃) was 2.74 (1.22–5.23) mg kg⁻¹ for soil and 0.01 (0.01–0.03) mg L⁻¹ for liquid-based exposures. At a detailed level, the Ag salt was more toxic than the NMs across most soil species and endpoints. Further analyses indicated that both NM surface coating and soil type influence AgNM toxicity. In soil exposures SSDs indicated similar effects across differently coated NM forms, however, in liquid-based assays both uncoated and PVP-coated AgNMs were more toxic to soil tested organisms than citrate-coated AgNMs. Soil cation exchange capacity (CEC) and organic carbon (OC) also influenced AgNM toxicity, with AgNMs being more toxic in soils with higher CEC and lower OC. Our study provides a data resource of toxicity data for soil species and the first hazard thresholds for risk assessment of AgNMs in soils and provides new insights into the factors driving AgNM hazard for soils species.