Uniform quartz - Silver nanoparticle injection experiment

• http://onlinelibrary.wiley.com/doi/10.1002/2015WR017167/full
• doi:10.1002/2015WR017167

Abstract — Nanoparticle transport and retention within porous media is treated by conceptualizing the porous media as a series of independent collectors (e.g., Colloid Filtration Theory). This conceptualization assumes that flow phenomena near grain-grain contacts, such as immobile zones (areas of low flow), exert a negligible influence on nanoparticle transport and assumes that retention and release of particles depends only on surface chemistry. This study investigated the impact of immobile zones on nanoparticle transport and retention by employing synchrotron X-ray computed microtomography (SXCMT) to examine pore-scale silver nanoparticle distributions during transport through three sand columns: uniform iron oxide, uniform quartz, and well-graded quartz. Extended tailing was observed during the elution phase of all experiments suggesting that hydraulic retention in immobile zones, not detachment from grains, was the source of tailing. A numerical simulation of fluid flow through an SXCMT data set predicted the presence of immobile zones near grain-grain contacts. SXCMT-determined silver nanoparticle concentrations observed that significantly lower nanoparticle concentrations existed near grain-grain contacts throughout the duration of all experiments. In addition, the SXCMT-determined pore-scale concentration gradients were found to be independent of surface chemistry and grain size distribution, suggesting that immobile zones limit the diffusive transport of nanoparticles toward the collectors. These results suggest that the well-known overprediction of nanoparticle retention by traditional CFT may be due to ignoring the influences of grain-grain contacts and immobile zones. As such, accurate prediction of nanoparticle transport requires consideration of immobile zones and their influence on both hydraulic and surface retention.

• http://pubs.acs.org/doi/abs/10.1021/acs.langmuir.6b01233
• DOI: 10.1021/acs.langmuir.6b01233

Abstract — This study evaluates the pore-scale distribution of silver nanoparticles during transport through a sandy porous medium via quantitative Synchrotron X-ray Computed MicroTomography (qSXCMT). The associated distribution of nanoparticle flow velocities and mass flow rates were obtained by coupling these images with Computational Fluid Dynamic (CFD) simulations. This allowed, for the first time, the comparison of nanoparticle mass flow with that assumed by the standard Colloid Filtration Theory (CFT) modelling approach. It was found that (i) 25% of the pore space was further from the grain than assumed by the CFT model; (ii) Average pore velocity agreed well between results of the coupled qSXCMT/CFD approach and the CFT model within the model fluid envelope, however, the former were 2 times larger than the latter in the centers of the larger pores and individual velocities were upwards of 20 times those in the CFT model at identical distances from grain surfaces ; and (iii) Approximately 30% of all nanoparticle mass and 38% of all nanoparticle mass flow occurred further away from the grain surface than expected by the CFT model. This work suggest that a significantly lower fraction of nanoparticles will contact a grain surface by diffusion than expected by CFT models, likely contributing to inadequate CFT model nanoparticle transport predictions.