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