The Variable Fate of Ag and TiO2 Nanoparticles in Natural Soil SolutionsSorption of Organic Matter and Nanoparticle Stability

Engineered nanoparticles (NP) like Ag and TiO2 offer unique properties for various applications. Thus, the entry of the NP in soil environments is expected to increase in the future due to their growing industrial use. To avoid potential hazards due to these anthropogenic products, NP behavior in the environment should be well understood. In natural soil solutions, we investigated NOM adsorption onto Ag and TiO2 NP and its influence on NP colloidal stability. Therefore, we extracted soil solutions from a floodplain soil (Fluvisol) and a farmland soil (Cambisol) differing in NOM quality and inorganic ion concentration. We measured the amount of adsorbed organic carbon as well as changes in aromaticity and molecular weight of NOM upon adsorption onto NP. Additionally, the size and zeta potential of NP in both soil solutions were investigated. We observed that the highly hydrophilic NOM of floodplain soil solution rich in inorganic ions strongly adsorbed to Ag but not to TiO2 NP. Instead, sorption to TiO2 NP was observed for the more hydrophobic NOM of the farmland soil with low ionic strength which did not sorb to Ag NP. These differences had a strong effect on NP stability, leading to Ag NP destabilization in case of floodplain soil solution and TiO2 NP stabilization in the presence of farmland soil solution. Our results point out the necessity of studies in more complex systems and suppose that oxic and metallic NP might show very different fate depending on the environment they are exposed to.