Nanoscale titanium dioxide (nTiO2) aggregation and transport in the co-presence of dissolved phosphate, illite colloid, and Fe oxyhydroxide coating

Abstract

Many studies have been conducted to understand the fate and transport of nanoscale titanium dioxide (nTiO2) in subsurface environments. However, most of these studies are under simplified conditions which do not represent the complex physicochemical conditions in the natural environment, where nTiO2 simultaneously encounter various groundwater/aquifer components. To better understand the mechanisms that control the transport of this emerging contaminant in groundwater, column experiments were conducted under low ionic strength conditions at pH 5 and 9 to clarify the influence of co-present dissolved component, suspended natural particle, and medium geochemical heterogeneity on nTiO2 transport through water-saturated porous media. Results showed that when using nTiO2 as influent at pH 5, Fe oxyhydroxide coating increased the nTiO2 mobility owing to the repulsive forces between positively charged nTiO2 and the Fe oxyhydroxide coating. Conversely, illite inhibited nTiO2 transport due to illite straining and nTiO2 entrapment. Phosphate served as a bridge between nTiO2 particles at pH 5, resulting in large aggregates and consequent straining, and therefore inhibited transport. At pH 9, the mobility of nTiO2 was high in quartz sand columns, regardless of the presence of phosphate/illite colloids, due to the repulsive forces between negatively charged nTiO2, phosphate, illite colloids, and quartz surface. However, Fe oxyhydroxide coating reduced nTiO2 mobility, attributable to pH buffering, which resulted in attractive electrostatic forces between the Fe oxyhydroxide coating and the negatively charged nTiO2. Findings from this study showed that under complex physicochemical conditions nTiO2 transport is controlled by the interactions between nTiO2 and common groundwater/aquifer components, as well as the interactions between these components.