Effect Of Environmental Factors On The Fate And Transport Of TiO₂ Nanoparticles In Saturated Porous Media

As an important inorganic functional material, titanium dioxide nanoparticle（TiO₂NPs） has good application in the fields. However, during their industrial productions and/or applications, nanoparticles may inevitably enter soils leading to the potential risk to human health and living environments. In addition, due to excess of fertilizers in cropland, a large number of nutrient ions are discharged into water body causing eutrophication. Thus, it is important to investigate the effect of environmental factors on the fate and transport of TiO₂ NPs and phosphate for predicting and evaluating the damage of them in the subsurface and groundwater environments.In this study, the dispersion and transport of TiO₂ NPs through the saturated sand columns were observed under different environmental conditions, such as particle size,solution pH, ionic strength, flow velocity and organic acids. The hydrodynamic radius,zeta potential and adsorption ability of phosphate and humic acid onto TiO₂ NPs were employed to investigate the mechanisms of effect of phosphorus on the transport of TiO₂ NPs.（1） Dispersion of titanium dioxides nanoparticles in the presence of phosphate:Rutile TiO₂ NPs were more stable than anatase in solution and weak alkaline（pH 7.5）favored the dispersion of TiO₂ NPs. In addition, the aggregation of 20 nm TiO₂ NPs tended to be easier than 50 nm TiO₂ NPs due to their stronger Van der Waals force on the surface. Especially, the dispersion of TiO₂ NPs was facilitated by the presence of phosphate due to more negative zeta potential and the stronger repulsive force between nanoparticles.（2） Facilitated transport of anatase titanium dioxides nanoparticles in the presence of phosphate in saturated sands: Phosphate favored the dispersion of TiO₂ NPs, and consequently improved their transport patterns. The likely mechanism was that phosphate adsorption increasing the negative charge on the surface promoted the transportability of TiO₂ NPs resulting from the low deposition rate and attachment efficiency of NPs. In particular, the facilitated transport of TiO₂ NPs（50 nm） was greater than those relative smaller as 20 nm. This enhancement of TiO₂ NPs transportability by phosphate at pH 6.5 was increased at higher pH of 7.5 due to the more negative zeta potential of surface. In addition, with the increasing NaNO3 concentration, the zeta potentials of TiO₂ NPs becomed less negative duo to the compression of the electrostaticdouble layer of Ti O2 NPs. Consequently, Ti O2 NPs aggregate and their transport were inhibited.（3） Transport of anatase titanium dioxides nanoparticles in saturated sands: Effect of phosphate and humic acid: The increase of phosphate and humic acid concentration promoted the transport of TiO₂ NPs through the saturated sand columns at pH 6.0, while negligible influence at pH 9.0. The enhancement of TiO₂ NPs transport by phosphate was higher than that by humic acid. Especially, slight change could be found with the increase of humic acid concentration at pH 6.0 in the coexistence of phosphate and humic acid solution. Due to hydraulic shear force, the increase of flow velocity reduced the retention of TiO₂ NPs in sand column, and consequently favored their transport patterns. In addition, the order of effectiveness of five organic acids in enhancing the transport of TiO₂ NPs was humic acid > gallic acid > oxalic acid > tannic acid > fulvic acid.