Fate of ZnO and Ag Nanoparticles During the Anaerobic Process and Post-Processing of Wastewater.

The rapid development of manufactured nanomaterials has sustained the growth of the nanotechnology industry over the last decade. However, this rapid technological development has translated in an increase concern regarding the fate, behaviour and possible toxicity of manufactured nanoparticles (NPs) in the environment. In this study we have investigated the transformation of metallic Ag and ZnO NPs during the anaerobic digestion of wastewater, which is thought to be the primary exposure pathway of these NPs to the environment. Three different ZnO and 3 Ag NPS were added, at environmentally relevant concentrations, to anaerobic reactors containing primary and secondary wastewater sludge which were run for 30 days. The NPs were chosen to cover a range of coatings/formulations and the experimental setup was complemented by a control and a Ag/Zn metal salt control were soluble salts of these elements were added to the same levels used for the NPs. Furthermore, the resulting sewage sludges (biosolids) were subjected to aerobic post-processing simulating composting/stockpiling. Metal speciation throughout the experiment was investigated using X-ray Absorption Near Edge Spectroscopy (XANES). The results indicate that both the ZnO and Ag NPs undergo a rapid transformation in the anaerobic digestors leading to the formation of sulfides. However, hydrophobic coatings significantly retarded this transformation process indicating that degradation processes are influenced by the commercial formulations in which the NPs are dispersed. Furthermore, especially in the case of Zn, aerobic post-processing of the sewage sludge has dramatic implications in terms of metal speciation. These results suggest that a significant part of the environmental risk assessment for metallic NPs, through this pathway of exposure, can rely on the knowledge gained over decades of investigation of metal behaviour in sewage sludge/biosolids. However, the effect of commercial formulation/coating on the behaviour of metallic NPs requires further investigation.