Retention and release of TiO2 nanoparticles in unsaturated porous media during dynamic saturation change

The retention and release of TiO2 nanoparticles in porous media (packed glass beads) were studied under transient unsaturated conditions as the media were taken through multiple drainage/imbibition (drying/wetting) cycles at three different pH values. The focus of the work was to better understand the role of changing water table levels and rainfall infiltration events on the ultimate mobility of TiO2 nanoparticles. Results indicate that retention during saturated transport varied considerably, from very strong retention at pH 5 (likely due to electrostatic interactions), to no measurable retention at pH 10. During primary drainage, additional retention (i.e., beyond what was retained during initial saturation) was observed at all pH values. During subsequent imbibition/drainage cycles where nanoparticle-free water was imbibed into the porous medium prior to drainage, the mass of retained TiO2 remained nearly constant at all three pH values. Final imbibition/drainage and subsequent flushing, both using solution conditions adjusted to favor high mobility, showed very little additional nanoparticle release. These results indicate that the release of TiO2 nanoparticles following retention by either saturated or unsaturated packed glass beads was difficult to achieve, regardless of the likely initial mechanisms of retention, even when solution conditions were changed to those that should favor high mobility.