Dissolution of metal and metal oxide nanoparticles under natural freshwater conditions

The dissolution of some widely used nanoparticles (NPs), Ag (citrate coated), ZnO, CuO and Cu-carbon coated (Cu/C), has been studied over a period of 9 days in five different natural waters: wastewater treatment plant effluent (WWTP Dubendorf) and lakes Greifen, Lucerne, Gruere and Cristallina. These waters differ in ionic strength, pH and dissolved organic carbon (DOC). The dissolved fraction of metals from NPs was determined using DGT (diffusion gradients in thin films) and ultrafiltration (UF). ZnO-NPs and CuO-NPs dissolved to a large extent in all waters, whereas the dissolved fraction was much smaller in the case of Cu/C and Ag-NPs. All NPs dissolved to a larger extent in water from Lake Cristallina with low pH, low ionic strength and low DOC. Ag-NP dissolution was favoured at low ionic strength and low pH, whereas dissolution of CuO-NPs was mostly dependent on pH. Cu/C-NPs strongly agglomerated and sedimented and yielded low dissolved Cu concentrations. DGT and UF produced similar results, although these two methods differ in the measurement time scale. The results of this study indicate that dissolution is an important process for these NPs under conditions of natural waters or wastewaters.