From commercial products to subsurface wetlands: a mesocosm approach examining the impacts of silver nanomaterials

There is an increasing potential for toxic effects on the environment from the release of weathered silver nanomaterials (Ag NMs) from commercial products. In the case of wetlands, their ecosystems have an innate ability to transform a wide range of pollutants into harmless by-products mainly through their microbial communities and could be impaired by the release of Ag NMs. This project endeavours to examine the fate and effects of both pristine and weathered Ag NMs through a mesocosm-scale wetland design. Twenty-four batch-fed subsurface flow planted wetland mesocosms were developed for this study; twelve were intensified with artificial aeration and twelve were non-aerated. The experiment consisted of four design types including a positive control, a negative control, a pristine Ag NM and an artificially weathered Ag NM exposure, all conducted in triplicate. Two separate in-situ exposures were conducted and monitored using a characterization suite consisting of five assessment categories (water quality, hydrological, microbial, plant health, and NM assessment) to profile the impacts on treatment performance and microbial function. Finally, the overall fate of silver within the mesocosms was determined through destructive sampling of the constructed wetland biofilm, plant roots, and aboveground plant biomass. Overall, there was no discernible difference in microbial activity between the silver treatments and their respective controls. Total silver data from the deconstruction indicates that the highest concentrations of silver were located in the gravel-associated biofilm; in particular in the 30 - 60 cm layer. This research demonstrates the robustness of wetland mesocosms in the presence of Ag NMs at environmentally relevant concentrations. As the Ag NMs sequester in the biofilm, it is likely to transform into less toxic silver species, such as silver sulfide, which would result in reduced toxicity to downstream environmental receptors.