Towards a Definition of Harmless Nanoparticles from an Environmental and Safety Perspective

The rapid development of nanoparticles (NPs), such as silicon nanoparticles (Si NPs) and ferric oxide nanoparticles (Fe 2 O 3 NPs), and their use in myriad commercial applications have raised questions of their potential impacts on wastewater treatment systems. In this study, we investigated the consequences of the presence of Si NPs and Fe 2 O 3 NPs in the denitrification of anoxic sludge. Fe 2 O 3 NPs, at a concentration up to 50 mg/L, had no significant impact on nitrate removal, whereas Si NPs, at concentrations up to 50 mg/L, increased the rate of nitrate removal. We used transmission electron microscopy (TEM) to investigate the effect of Si NPs and Fe 2 O 3 NPs. Si NPs exposure enhanced the abundance of narG -1 gene, which might promote nitrate removal process directly. Finally, we reviewed and identified the specific properties of a variety of NPs responsible for toxicity and found NPs larger than about 100 nm and without ion release in general possible to energy safety and nontoxic or low toxic to environment. Our results provide useful information to understand the response of anoxic sludge to Si NPs and Fe 2 O 3 NPs in complex environmental matrix as well as potent support for wide use of the environmentally friendly NPs.