Transport of two metal oxide nanoparticles in saturated granular porous media: role of water chemistry and particle coating

The growing use of nanosized titanium dioxide (nTiO₂) and zinc oxide (nZnO) in a large number of commercial products raises concerns regarding their release and subsequent mobility in natural aquatic environments. Laboratory-scale sand-packed column experiments were conducted with bare and polymer-coated nTiO₂ and nZnO to improve our understanding of the mobility of these nanoparticles in natural or engineered water saturated granular systems. The nanoparticles are characterized over a range of environmentally relevant water chemistries using multiple complimentary techniques: dynamic light scattering, nanoparticle tracking analysis, transmission electron microscopy, and scanning electron microscopy. Overall, bare (uncoated) nanoparticles exhibit high retention within the water saturated granular matrix at solution ionic strengths (IS) as low as 0.1 mM NaNO₃ for bare nTiO₂ and 0.01 mM NaNO₃ for bare nZnO. Bare nTiO₂ and nZnO also display dynamic (time-dependent) deposition behaviors under selected conditions. In contrast, the polymer-coated nanoparticles are much less likely to aggregate and exhibit significant transport potential at IS as high as 100 mM NaNO₃ or 3 mM CaCl₂. These findings illustrate the importance of considering the extent and type of surface modification when evaluating metal oxide contamination potential in granular aquatic environments.