Visualization and Modeling of Nanoparticle Transport in Porous Media

Abstract:

We examine transport of nanoparticles (NPs) in a refraction index-matched, three-dimensional, water-saturated porous medium, consisting of polyacrylamide beads which carry a negative surface charge. Flow cell experiments involved use of synthesized, negatively-charged gold and silver nanoparticles (AuNPs, AgNPs) and positively-charged AgNPs, to determine the effect of surface charge on NP transport behavior. From sequential imaging of the flow cell and detailed image analysis, we obtained measurements of the temporally evolving spatial NP concentration distribution. The current experiments focused on point source, pulse injection of AuNPs and AgNPs into a uniform flow field. Parallel experiments employing a conservative (Congo red) tracer demonstrated the transport to be mildly non-Fickian. A continuous time random walk (CTRW) particle tracking (PT) model was then used to quantify the spatial and temporal migration of both the conservative and reactive tracer plumes; model parameters related to the non-Fickian transport were determined from conservative (column and flow cell) tracer experiments. The results show that the negatively-charged NPs behaved similarly to the conservative tracer. These results are in accord with previous measurements of transport of negatively-charged AgNPs in sand columns. The positively-charged AgNPs, in contrast, displayed a decreasing tendency over distance to attach to the negatively-charged porous medium. Further analysis showed that the decreasing tendency of these NPs to attach to the porous medium may be correlated to the particle residence time. This transport behavior is understood by DLS and ζ potential measurements, which showed that aggregation processes and inversion in particle surface charge occurred during the transport of the positively-charged NPs. Modeling results indicated the transport to be more non-Fickian for the positively-charged AgNPs, highlighting a coupling between attachment and transport processes. We conclude that interactions between migrating particles and the porous medium may alter NP chemical properties and lead to a significant change in their mobility. We further emphasize the ability of the CTRW-PT modeling approach to quantify various geochemical processes that occur during non-Fickian transport in porous media.