The effect of evaporation-induced flow at the pore scale on nanoparticle transport and deposition in drying unsaturated porous media

An understanding of nanoparticle interactions with solid surfaces in unsaturated porous media is important both for understanding nanoparticle fate in the environment, and for the design of environmental applications that depend on the delivery of nanoparticles. While surface-chemical interactions can have a strong influence on nanomaterial attachment to surfaces in environmental porous media, the hydraulics of water flow can also play an important role in attachment. In the unsaturated zone, naturally-occurring evaporation is a major source of water flow. The purpose of this work was to examine how evaporation-induced water flow at the pore scale impacts the transport and deposition of negatively-charged sulfate-modified polystyrene nanoparticles. Evaporation experiments were conducted by initially saturating small clusters of sand grains with a suspension of nanoparticles. Confocal microscopy was then used to track the changing water surface profile, as well as to track the transport and deposition of nanoparticles in the grain clusters. Confocal data showed that nanoparticles tended to deposit on sand grains near the receding air-water interface, an expected behavior. This process led to attachment on grain surfaces as they were exposed by the receding interface. Evaporation was found to produce complex flow patterns with temporally-changing flow directions at the pore scale. A finite difference model developed to explore the link between evaporation and water flow in pore spaces was able to duplicate many of the observed phenomena. Simulations suggest that distinct differences in deposition mechanisms should be expected for porous media undergoing evaporation compared with porous media experiencing drainage.