Pore-Scale Investigation of Nanoparticle Transport in Saturated Porous Media Using Laser Scanning Cytometry

Knowledge of nanoparticle transport and retention mechanisms is essential for both the risk assessment and environmental application of engineered nanomaterials. Laser scanning cytometry, an emerging technology, was used for the first time to investigate the transport of fluorescent nanoparticles in a microfluidic flow cell packed with glass beads. The laser scanning cytometer (LSC) was able to provide the spatial distribution of 64 nm fluorescent nanoparticles attached in a domain of 12 mm long and 5 mm wide. After 40 pV of injection at a lower ionic strength condition (3 mM NaCl, pH 7.0), fewer fluorescent nanoparticles were attached to the center of the flow cell, where the pore-scale velocity is relatively higher. After a longer injection period (300 PV), more were attached to the center of the flow cell, and particles were attached to both the upstream and downstream sides of a glass bead. Nanoparticles attached under a higher ionic strength condition (100 mM NaCl, pH 7.0) were found to be mobilized when flushed with DI water. The mobilized particles were later reattached to some favorable sites. The attachment efficiency factor was found to reduce with an increase in flow velocity. However, torque analysis based on the secondary energy minimum could not explain the observed hydrodynamic effect on the attachment efficiency factor

Impacts of Future Climate Variability on Atrazine Accumulation and Transport in Corn Production Areas in the Midwestern United States

Atrazine is one of the most prevalent herbicides that has been widely applied to agricultural lands in the U.S. Understanding the transport and accumulation of atrazine in the subsurface under future climate scenarios is essential for future agriculture and water management. Here, we predict atrazine transport and accumulation under an intensive corn production land based on 20 projected global climate model (GCM) realizations, while considering uncertainties of transport parameters. Our study predicted continuous groundwater table declination and atrazine mass accumulation on the study site. We show that atrazine mass accumulation in corn production areas is subject to total precipitation in the atrazine application season, whereas atrazine plume movement is controlled by the sequence of annual precipitation. Atrazine mass transport and accumulation are more sensitive to climate variation on the field sites with low sorption and atrazine degradation rate. Under the extreme condition, the atrazine plume can migrate as far as five meters from the ground surface in only three years. While annual mean precipitation in the Midwestern U.S. is projected to increase in the future, groundwater vulnerability to atrazine and associated water quality impacts may rise in the U.S. Corn Belt, especially in sites with low atrazine degradation and sorption

Transport and Retention of Colloids in Porous Media: Does Shape Really Matter?

The effect of particle shape on its transport and retention in porous media was evaluated by stretching carboxylate-modified fluorescent polystyrene spheres into rod shapes with aspect ratios of 2:1 and 4:1. Quartz crystal microbalance with dissipation (QCM-D) experiments were conducted to measure the deposition rates of spherical and rod-shaped nanoparticles to the collector (poly-l-lysine coated silica sensor) surface under favorable conditions. The spherical particles displayed a significantly higher deposition rate compared with that of the rod-shaped particles. Theoretical analysis based on Smoluchowski–Levich approximation indicated that the rod-shaped particles largely counterbalance the attractive energies due to higher hydrodynamic forces and torques experienced during their transport and rotation. Under unfavorable conditions, the retention of nanoparticles in a microfluidic flow cell packed with glass beads was studied with the use of laser scanning cytometry (LSC). Significantly more attachment was observed for rod-shaped particles than spherical particles, and the attachment rate of the rod-shaped particles showed an increasing trend with the increase in injection volume. Rod-shaped particles were found to be less sensitive to the surface charge heterogeneity change than spherical particles. Increased attachment rate of rod-shaped particles was attributed to surface heterogeneity and possibly enhanced hydrophobicity during the stretching process

Investigation of the Transport and Deposition of Fullerene (C60) Nanoparticles in Quartz Sands under Varying Flow Conditions

A coupled experimental and mathematical modeling investigation was undertaken to explore nanoscale fullerene aggregate (nC60) transport and deposition in water-saturated porous media. Column experiments were conducted with four different size fractions of Ottawa sand at two pore-water velocities. A mathematical model that incorporates nonequilibrium attachment kinetics and a maximum retention capacity was used to simulate experimental nC60 effluent breakthrough curves and deposition profiles. Fitted maximum retention capacities (Smax), which ranged from 0.44 to 13.99 μg/g, are found to be correlated to normalized mass flux. The developed correlation provides a means to estimate Smax as a function of flow velocity, nanoparticle size, and mean grain size of the porous medium. Collision efficiency factors, estimated from fitted attachment rate coefficients, are relatively constant (∼0.14) over the range of conditions considered. These fitted values, however, are more than 1 order of magnitude larger than the theoretical collision efficiency factor computed from Derjaguin−Landau−Verwey−Overbeek (DLVO) theory (0.009). Data analyses suggest that neither physical straining nor attraction to the secondary minimum is responsible for this discrepancy. Patch-wise surface charge heterogeneity on the sand grains is shown to be the likely contributor to the observed deviations from classical DLVO theory. These findings indicate that modifications to clean-bed filtration theory and consideration of surface heterogeneity are necessary to accurately predict nC60 transport behavior in saturated porous media

A Pivotal New Approach to Groundwater Quality Assessment

Areas of intensive agriculture and irrigation are prone to groundwater nitrate contamination, which can threaten drinking water supplies. Irrigation center pivots are a common feature in heavily irrigated regions and have the potential to provide insight into subsurface redox chemistry. In this study, we hypothesized that the same geochemical condition(s) that causes rust staining on center pivot systems will strongly influence groundwater nitrate concentrations. In south central Nebraska, 700 center pivot irrigation systems were classified by appearance of iron staining (full rust, part rust, or no rust) using Google Earth imagery and/or ground-based surveys. Ground-based observation of 270 center pivots yielded the same classifications as Google Earth imagery 83% of the time. Groundwater nitrate concentrations correlated with pivot classifications show lower nitrate concentrations in full rust and part rust pivots when compared with no rust pivots. The novelty of this work is to provide a framework for understanding groundwater quality using an inexpensive method applicable to both established and developing agricultural communities

Improving the Sweeping Efficiency of Permanganate into Low Permeable Zones To Treat TCE: Experimental Results and Model Development

The residual buildup and treatment of dissolved contaminants in low permeable zones (LPZs) is a particularly challenging issue for injection-based remedial treatments. Our objective was to improve the sweeping efficiency of permanganate into LPZs to treat dissolved-phase TCE. This was accomplished by conducting transport experiments that quantified the ability of xanthan-MnO₄^– solutions to penetrate and cover (i.e., sweep) an LPZ that was surrounded by transmissive sands. By incorporating the non-Newtonian fluid xanthan with MnO₄^–, penetration of MnO₄^– into the LPZ improved dramatically and sweeping efficiency reached 100% in fewer pore volumes. To quantify how xanthan improved TCE removal, we spiked the LPZ and surrounding sands with ¹⁴C-lableled TCE and used a multistep flooding procedure that quantified the mass of ¹⁴C-TCE oxidized and bypassed during treatment. Results showed that TCE mass removal was 1.4 times greater in experiments where xanthan was employed. Combining xanthan with MnO₄^– also reduced the mass of TCE in the LPZ that was potentially available for rebound. By coupling a multiple species reactive transport model with the Brinkman equation for non-Newtonian flow, the simulated amount of ¹⁴C-TCE oxidized during transport matched experimental results. These observations support the use of xanthan as a means of enhancing MnO₄^– delivery into LPZs for the treatment of dissolved-phase TCE

Enhanced Mobility of Fullerene (C₆₀) Nanoparticles in the Presence of Stabilizing Agents

Experimental and mathematical modeling studies were performed to examine the effects of stabilizing agents on the transport and retention of fullerene nanoparticles (nC₆₀) in water-saturated quartz sand. Three stabilizing systems were considered: naturally occurring compounds known to stabilize nanoparticles (Suwannee river humic acid (SRHA) and fulvic acid (SRFA)), synthetic additives used to enhance nanoparticle stability (Tween 80, a nonionic surfactant), and residual contaminants resulting from the manufacturing process (tetrahydrofuran (THF)). The results of column experiments demonstrated that the presence of THF, at concentrations up to 44.5 mg/L, did not alter nC₆₀ transport and retention behavior, whereas addition of SRHA (20 mg C/L), SRFA (20 mg C/L), or Tween 80 (1000 mg/L) to the influent nC₆₀ suspensions dramatically increased the mobility of nC₆₀, as demonstrated by coincidental nanoparticle and nonreactive tracer effluent breakthrough curves (BTCs) and minimal nC₆₀ retention. When columns were preflushed with surfactant, nC₆₀ transport was significantly enhanced compared to that in the absence of a stabilizing agent. The presence of adsorbed Tween 80 resulted in nC₆₀ BTCs characterized by a declining plateau and retention profiles that exhibited hyperexponential decay. The observed nC₆₀ transport and retention behavior was accurately captured by a mathematical model that accounted for coupled surfactant adsorption–desorption dynamics, surfactant–nanoparticle interactions, and particle attachment kinetics

Experimental and Numerical Validation of the Total Trapping Number for Prediction of DNAPL Mobilization

The total trapping number (NT), quantifying the balance of gravitational, viscous, and capillary forces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet, was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual, flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 × 10−3–1.10 × 10−2), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing

Experimental and Numerical Validation of the Total Trapping Number for Prediction of DNAPL Mobilization

The total trapping number (NT), quantifying the balance of gravitational, viscous, and capillary forces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet, was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual, flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 × 10−3–1.10 × 10−2), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing

Experimental and Numerical Validation of the Total Trapping Number for Prediction of DNAPL Mobilization

The total trapping number (NT), quantifying the balance of gravitational, viscous, and capillary forces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet, was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual, flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 × 10−3–1.10 × 10−2), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing