(462d) Effect of the Aggregation of TiO2 Nanoparticles On Their Fate and Transport In Natural Waters

Nanotechnology is a growing area with many promising and revolutionary advances in a variety of fields, from medicine to sporting good design and personal care products. One of the most widely applied nanomaterial is nanoTiO₂. Although it is a well-known material at the macroscale, the environmental implications of the special properties exhibited by TiO₂ in its nanoform are not fully understood. Of particular interest are the aggregation processes in natural waters, the interaction with natural organic matter, and the effect of the presence of divalent ion such as calcium and magnesium. The different conditions of the water matrix that TiO₂ nanoparticles may encounter in surface and groundwater will influence their physico-chemical properties, and thus modify their surface charge, aggregation, and attachment characteristics. These properties are key to determine the fate and transport of nanomaterials in the environment, and therefore a complete understanding of the transformations that TiO₂ undergo in a natural setting is necessary to assess the risk posed to the environment by products containing this nanomaterial.

The size and surface charge of TiO₂ nanoparticles was investigated as a function of pH for a variety of relevant conditions, including different levels of ionic strength, concentration of natural organic matter and non-indiferent cations by Dynamic Light Scattering (DLS) using a Nanosizer Z90 (Malvern, UK) and by means of settling column experiments for those cases when DLS was not applicable. By size the authors refer to the measured equivalent hydrodynamic diameter of the aggregates formed by the nanoparticles. DLVO theory was applied to model the experimental conditions explored, and generally good agreement was observed when aggregate size is taken as effective particle size for the calculations.

Aggregate size and morphology were investigated by Scanning Electron Microscopy using a FESEM (JEOL JSM-6701F). To preserve the aggregates conditions as close as possible to their original form in suspension, a substrate was placed vertically in each suspension and the sample to be imaged was formed by aggregates that attached during the descent of the liquid level as the suspension evaporates. Special care was taken so that the change in concentration of the constituents in the solutions did not vary significantly during this process, taking into account the ratio of the relative change of volume to the initial volume of sample suspension and interrupting the process accordingly. Prior to imaging, the samples were gold coated with a sputter coater (Denton Vacuum Desk IV) to provide a conductive surface.

Sand column transport experiments were conducted for some characteristic conditions to investigate the mobility of nanoTiO₂ in porous media. The results were modeled applying filtration theory. 

It was observed that aggregation is dependent on pH, shear (mixing conditions), presence of natural organic matter and divalent ions through their effect in the zeta potential of the particles although this parameter alone was insufficient to explain the different size of the aggregates. Attachment to the sand grains during flow in the porous media followed a similar trend.

This work provides critical insights on the physico-chemical properties of TiO₂ nanoparticles suspended in environmentally relevant aqueous matrixes. The information presented is key to understand the interaction with the natural environment: their fate and transport on groundwater and surface water, their bioavailability and their subsequent effect on the biota.