Investigating the fate of manufactured nanoparticles in waste water treatment by the use of radiolabeled nanoparticles

Waste water treatment plants (WWTP) are considered one of the largest sinks for manufactured nanoparticles (NPs), as well as a potential source for the reintroduction of NPs into the environment via the WWTP effluents. Consequently, the fate of NPs in WWT is an important factor for the risk assessment of the impact of manufactured NPs on Nature and human health. However the investigation of NP fate in WWT is hindered by large elemental and particulate backgrounds. To overcome these problems we used the radiolabeling of NPs as a means of sensitive selective detection of NPs in the complex media of activated sludge and cleared waste water in a model WWTP study. TiO2, CeO2, MWCNTs and quantum dots (QDs) were radiolabeled either by activation ([48V]TiO2), in-diffusion ([139Ce]CeO2), recoil labeling ([7Be]MWCNT) or radiosynthesis ([75Se]CdSe/[65Zn]ZnS QDs). The radiolabeling allowed us to quantify NP distribution between sludge and water phase in the WWTP and in the WWTP effluents. A distribution of about 10000 : 1 between sludge-associated NPs and free NPs in water is reached in the WWTP already shortly after injection of the NPs. Thus the elimination of the NPs from the WWTP is mainly controlled by the removal of surplus sludge taking place every day of operation. The NPs are eliminated from the WWTP with a half-life of about 6 days reflecting the pre-set sludge age. After about 22 days of operation 10 % of the initial NPs remain in the WWTP. Approximately 1 % of the NPs leave the WWTP via the cleared waste water, mainly associated with non-sedimented sludge particles, such that only about 1 ‰ of the NPs leave the WWTP as free particles via the cleared water. An impact of the NPs on the clearing process, as monitored by chemical oxygen demand of the inflow vs. the outflow, was not observed.