Measurement and modeling of engineered nanoparticle transport and aging dynamics in a reactive porous medium

A continuous time random walk particle tracking (CTRW-PT) method was employed to model flow cell experiments that measured transport of engineered nanoparticles (ENPs) in a reactive porous medium. The experiments involved a water-saturated medium containing negatively charged, polyacrylamide beads, resembling many natural soils and aquifer materials, and having the same refraction index as water. Negatively and positively charged ENPs were injected into a uniform flow field in a 3-D horizontal flow cell, and the spatial and temporal concentrations of the evolving ENP plumes were obtained via image analysis. As a benchmark, and to calibrate the model, Congo red tracer was employed in 1-D column and 3-D flow cell experiments, containing the same beads. Negatively charged Au and Ag ENPs demonstrated migration patterns resembling those of the tracer but were slightly more dispersive; the transport was well represented by the CTRW-PT model. In contrast, positively charged AgNPs displayed an unusual behavior: establishment of an initial plume of essentially immobilized ENPs, followed by development of a secondary, freely migrating plume. The mobile plume was found to contain ENPs that, with aging, exhibited aggregation and charge inversion, becoming negatively charged and mobile. In this case, the CTRW-PT model was modified to include a probabilistic law for particle immobilization, to account for the decreasing tendency (over distance and time) of the positively charged AgNPs to attach to the porous medium. The agreement between experimental results and modeling suggests that the CTRW-PT framework can account for the non-Fickian and surface-charge-dependent transport and aging exhibited by ENPs in porous media.