Cotransport of titanium dioxide nanoparticles and formaldehyde in saturated and unsaturated columns packed with quartz sand

Laboratory-scale experiments were conducted to investigate the simultaneous transport of titanium dioxide (TiO₂) nanoparticles and formaldehyde (FA) in columns packed with quartz sand under water saturated and unsaturated flow conditions. The effects of interstitial velocity and solution ionic strength on the TiO₂ and FA cotransport were examined. The experimental results indicated that substantial retention of TiO₂ nanoparticles occurs in both saturated and unsaturated porous media. The solution ionic strength was found to have a noticeable effect on the retention of TiO₂ nanoparticles in the packed columns. Moreover, the results from the TiO₂ nanoparticle transport experiments in water-saturated packed columns suggested that the TiO₂ nanoparticle mass recoveries increased with increasing flow rate. The results from the TiO₂ nanoparticles and FA cotransport experiments in both water saturated and unsaturated packed columns did not reveal a distinct relationship between mass recoveries and flow rate. The transport of FA in both saturated and unsaturated packed columns was hindered in the presence of TiO₂ nanoparticles, especially at high ionic strength. This work provides useful insights into fate and transport of TiO₂ nanoparticles and FA in saturated and unsaturated porous media.