Role of the air-water interface in the retention of TiO2 nanoparticles in porous media during primary drainage

The increasing use of nanomaterials in commercial products has resulted in increased concerns about their potential environmental impacts. The overall mobility of nanomaterials in the environment may depend in part on their mobility in the unsaturated zone of the subsurface, which may provide a sink for nanomaterials, preventing their spread, or a long-term contaminant source. The objective of this work was to study the dynamic unsaturated transport of titanium dioxide (TiO2) during primary drainage to examine the role of air-water interface formation on nanomaterial retention. A specialized automated system was used to track depletion of TiO2 in the pore solution of a porous medium during dynamic drainage, while simultaneously measuring capillary pressure (Pc) and saturation (S). A continuous mass balance was used to calculate the mass of retained TiO2 nanoparticles. Experiments were specifically designed to minimize TiO2 interactions with solid surfaces to allow direct assessment of the role of the air-water interface. Results indicate that the mass of retained TiO2 increases as saturation decreases at all drainage rates, with slower drainage rates corresponding to greater retention at a given saturation. Normalizing the retained mass (M) bythe measured air-water interfacial area (A) shows near-constant M/A values at high saturations (S > 0.4) and increasing M/A values with decreasing saturation as saturation drops below 0.4. This result may indicate air-water interfacial adsorption at high saturations, with increasing contributions from film straining at lower saturations.