Nanoscale Titanium Dioxide (nTiO2) Transport in Natural Sediments: Importance of Soil Organic Matter and Fe/Al Oxyhydroxides.

Many engineered nanoparticle (ENP) transport experiments use quartz sand as the transport media; however, sediments are complex in nature, with heterogeneous compositions that may influence transport. Nanoscale titanium dioxide (nTiO2) transport in water-saturated columns of quartz sand and variations of a natural sediment was studied, with the objective of understanding the influence of soil organic matter (SOM) and Fe/Al-oxyhydroxides and identifying the underlying mechanisms. Results indicated nTiO2 transport was strongly influenced by pH and sediment composition. When influent pH was 5, nTiO2 transport was low because positively charged nTiO2 was attracted to negatively charged minerals and SOM. nTiO2 transport was slightly enhanced in sediments with sufficient SOM concentrations due to leached dissolved organic matter (DOM), which adsorbed onto the nTiO2 surface, reversing the zeta potential to negative. When influent pH was 9, nTiO2 transport was generally high because negatively charged medium repelled negatively charged nTiO2. However, in sediments with SOM or amorphous Fe/Al oxyhydroxides depleted, transport was low due to pH buffering by the sediments, causing attraction between nTiO2 and crystalline Fe oxyhydroxides. This was counteracted by DOM adsorbing to nTiO2, stabilizing it in suspension. Our research demonstrates the importance of SOM and Fe/Al oxyhydroxides in governing ENP transport in natural sediments.