Engineered silver nanoparticles in terrestrial environments: a meta-analysis shows that the overall environmental risk is small

Abstract

Engineered silver nanoparticles (Ag-ENPs) are widely used in a range of biocidal and consumable products, raising concerns regarding their potential risks upon release into the environment. Both Ag and Ag-ENPs have a long history of usage (e.g. at least 12 decades for Ag-ENPs). This historical usage of Ag, together with its concomitant release into the environment, provides a source of information for assessing whether newly emerging Ag-based nanotechnology constitutes an environmental risk. Here, we have utilized a meta-analysis to examine the behavior of Ag along the ‘source – waste water treatment plant (WWTP) – soil – receptor’ pathway. This is because, when total Ag is considered (i.e. bulk Ag plus Ag-ENPs), the majority of what is released into the environment accumulates in agricultural soils due to the application of sludge from WWTPs. However, the mass flow of Ag-ENPs entering soils only represents a small fraction of the total Ag, with the concentrations of Ag-ENPs in soil generally three orders of magnitude lower than the total Ag concentrations for field-measured values in sludge-treated soils. Furthermore, most Ag-ENPs lose their engineered nano-properties and are sulfidated to Ag₂S within WWTPs before being applied to agricultural soils. Importantly, the Ag₂S produced is very stable in soils, with the soluble Ag concentration being very low (generally ≤6 μg L⁻¹) – this being at least six orders of magnitude lower than the no-observed-effect concentrations reported for plant growth and terrestrial fauna in soils. Accordingly, the bioaccumulation of Ag in edible plant tissues in sludge-treated soils is also much lower than the maximum permissible levels of Ag in diets for animals. Taken together, the risk of Ag-ENPs to terrestrial plants and fauna (including humans through trophic transfer) is small. However, the usage of Ag-ENP products may potentially pose a risk to soil microbial communities and in natural waters, and more research is needed in these areas.