The impact of immobile zones on the transport and retention of nanoparticles in porous media

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.

Colloid Transport in Porous Media: A Review of Classical Mechanisms and Emerging Topics

“…Experimental retention profiles in unfavorable conditions are reported to be hyper-exponential, linearly decreasing or non-monotonic [113][114][115][116][117][118][119][120][121]. Breakthrough curves may be non-symmetric, a slow increase in concentration over time [122][123][124], a slow decrease (i.e., filter ripening\clogging) [125], or there may be a slow decrease in colloid concentration during elution (i.e., tailing) (illustrated in Figure S2 in the supplementary information) [126][127][128][129][130]. The application of to calculate in unfavorable conditions does not resolve the anomaly between experimentally observed anomalous behaviours and the behaviour predicted by the ADE/CFT approach.…”

The development and numerical modelling of a Representative Elemental Volume for packed sand

“…In the segmented image approach, a set of selected images are mainly scanned by applying a specific imaging procedure required for SXCMT (Synchrotron X-ray Computed Microtomography) technique of uniform quartz in a porous medium [58,59]. Moreover, it is about a 3D dataset of X-ray image averaged over a 9.87×9.87×9.87 μm for each direction, where the voxel resolution is 9.87 μm/voxel [19,58]. Each image needs roughly 40 minutes of imaging time in order to finalize the scan.…”

“…Imaging was completed at the Argonne National Lab, Advanced Photon Source Synchrotron, 13-BM-D beamline with the GSECARS research group [19,58]. The imaging reconstruction procedure and equipment used are detailed in [19].…”

A binomial modeling approach for upscaling colloid transport under unfavorable conditions: Emergent prediction of extended tailing

“…While much research has been devoted to understand better the influence of secondary minimum attraction on colloid residence in the NSFD at the pore‐scale [ Elimelech , ; Hahn et al ., ; Bradford and Torkzaban , ; Bradford et al ., ; Molnar et al ., ], little work has examined how residence of colloids in the NSFD influences the transport behavior of colloids at the assemblage scale, e.g., extended tailing [ Schijven et al ., ; Harter et al ., ; Molnar et al ., ], as well as hyperexponential [ Simoni et al ., ; Baygents et al ., ] and nonmonotonic [ Li and Johnson , ; Bradford et al ., ; Bolster et al ., ; Shani et al ., ] retention profiles. To examine these issues, we have begun developing a model that uses Monte‐Carlo simulations of colloid transport in a dual network of pores and grains that is three‐dimensional and heterogeneous.…”