Discovery and Characterization of Nano Ag and Zn Sulfides and Nano TiO2 in Sewage Sludge Products, and Soils Amended with Sewage Sludge.

The increasing production and extensive, existing and new application of engineered nanoparticles (NPs) for commercial use will likely increase their release to the environment. Yet, our understanding of their fate and behavior in the soil environment is quite limited. In this study we examined the most likely route for engineered NPs to enter soils – the application of sewage sludge products as a soil amendment – by focusing on Ag-, Zn-, and Ti-containing NPs, the most widely used engineered NPs to date.     

     First, we examined several sewage sludge products collected as part of the US EPA Targeted National Sewage Sludge Survey for their occurrence, abundance, and nature. Second, we conducted a field-scale mesocosm experiment to investigate the fate and behavior of TiO₂ NPs-bearing sewage sludge in soils. In both cases, we employed transmission electron microscopy (including scanning and high resolution TEM, as well as selected area electron diffraction) combined with energy dispersive X-ray spectroscopy for direct visualization and thorough characterization of the NPs within the highly complex and heterogeneous sewage sludge matrices. 

     In all sludge samples tested, both Ag and Zn are always associated with sulfur, and form sulfide nanocrystals (5 – 20 nm) with the crystal structure of α-Ag₂S (acanthite) and ZnS (sphalerite). It is likely that they form in situ in anoxic, sulfur and organic-rich sewer and/or wastewater treatment systems. In contrast, we only found oxides of Ti (in all cases rutile, TiO₂) from sewage sludge products in a similar size range as the sulfides or larger. Our mesocosm study also shows for the first time that nano-TiO₂ can sorb toxic trace metals like Ag, similarly to other metal oxides, and then enter the soil. We therefore believe that our findings will be useful to predict the long-term behavior, fate, and impact of engineered NPs in soil environment.