Exchange of TiO2 Nanoparticles Between Streams and Streambeds

The rapid growth of the nanotechnology industry and the expanding usage of manufactured nanoparticles have increased the potential for their release into the natural environment. Particularly, titanium dioxide nanoparticles pose significant exposure risk to humans and other living species due to their extensive usage in a wide range of fields. In order to better understand the environmental and health risks associated with the environmental release of TiO₂ nanoparticles, knowledge on fate and transport of this type of nanomaterial is needed. This study is intended to evaluate the transport of two different TiO₂ nanoparticles, one commercially available (P25 TiO₂), and the other synthesized at a lab scale (synthesized TiO₂). Laboratory flume, settling column, and batch experiments were conducted to investigate the processes dominating the transport of TiO₂ nanoparticles between stream and streambed and to characterize the properties of these nanoparticles under different physicochemical conditions. Results show that the synthesized TiO₂ was more stable compared with the P25 TiO₂, which underwent significant aggregation under the experimental conditions tested. As a result, P25 TiO₂ was deposited at a significantly faster rate than the synthesized TiO₂ in the streambed. Both types of TiO₂ nanoparticles deposited in the streambed were easily released when the stream velocity was increased. Also, the deposition of P25 TiO₂ was highly dependent on stream pH. A process-based stream-subsurface colloid exchange model was also applied to interpret the observed transport behavior of the TiO₂ nanoparticles.