Article pubs.acs.org/est
Pore-Scale Investigation of Nanoparticle Transport in Saturated Porous Media Using Laser Scanning Cytometry Ryan May,† Simin Akbariyeh,‡ and Yusong Li*,‡ †
Mechanical Sales, Inc., 7222 South 142nd Street, Omaha, Nebraska 68138, United States Department of Civil Engineering, University of Nebraska-Lincoln, 2200 Vine Street, Lincoln, Nebraska 68583, United States
‡
S Supporting Information *
ABSTRACT: 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.
1. INTRODUCTION
To better reveal the mechanisms controlling colloid transport, various noninvasive visualization techniques, such as light transmission technique,15 UV−fluorescence technique,16 magnetic resonance imaging,17 and X-ray tomography18 were developed. While those efforts provided improved insights, they targeted at a continuum scale, so that the influence of pore-scale hydrodynamics and porous medium geometry on particle deposition are not directly revealed. In recent years, pore-scale experimental techniques have been very successful in directly observing and visualizing the transport and retention of colloidal particles.19−21 Such techniques generally require very high resolution to observe particles at the nanoscale. Typically, only several pore spaces can be observed if particles are smaller than one micrometer. Observation at a larger scale is necessary to quantitatively simulate nanoparticle transport in porous media. In this work, we explored the possibility of using laser scanning cytometry to obtain the spatial distribution of nanoscale particles in a porous medium domain at the centimeter scale. Laser scanning cytometry is an emerging technology used in the biomedical field to image and quantitatively analyze individual cells in tissues in situ.22 A laser scanning cytometer (LSC) comprises an optics unit that
The recent revolution of nanotechnology has brought more than 1300 nanotechnology-enabled consumer products into the marketplace.1,2 Strong evidence3−7 has suggested that nanomaterials may enter the subsurface environment via direct leaking from underground waste sites, reuse of wastewater, and agricultural use of biosolids containing engineered nanomaterials. The concerns and also demonstrated evidence suggested that some engineered nanomaterials may impose negative impacts to the environment and human health. Understanding the fate and transport of nanomaterials in the subsurface porous media will provide critical information for risk assessment and regulation policy development.8 Nanoparticles are defined as a subclass of colloid particles that have at least one dimension less than 100 nm. Traditional column experiments were conducted to investigate the transport and retention of different types of nanomaterials (e.g., nano zerovalent iron,9 fullerene C60,10−12 carbon nanotubes,13 and nano titanium dioxide14) in porous media. Typical data from a column experiment include (i) breakthrough curves representing averaged aqueous concentration of particles in the column effluent over time and (ii) retention profiles representing averaged attached phase concentration along the column. Although these two pieces of information can provide valuable insights, they do not clearly distinguish the ways that spatial and temporal changes to the hydrodynamic conditions affect particle transport. © 2012 American Chemical Society
Received: Revised: Accepted: Published: 9980
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using a syringe pump (KD Scientific, Inc., Holliston, MA) through TYGON 3350 sanitary silicone tubing with a 1/16 in. inner diameter (Saint-Gobain Performance Plastics Corporation, Beaverton, MI). A fluorescent nanosphere suspension with a solids content of 0.0025% in 3 mM or 100 mM NaCl was pumped into the flow cell at a Darcy velocity of 0.02, 0.04, 0.06, or 0.08 cm/s. After injecting 40 PV of the nanosphere suspension, 10 PV of a background solution with same electrolyte composition and concentration was injected. Experiments were conducted in duplicate under various velocity conditions. 2.3. Laser Scanning Cytometry Scanning. A LSC instrument (CompuCyte Corporation, Westwood, MA) equipped with an Olympus BX-50 fluorescence microscope was used to scan the nanospheres attached to the glass beads in the flow cell. The LSC was equipped with a violet laser (405 nm), 20 mW argon ion laser (488 nm, Cyonics Uniphase model 2014A-20SL), and a 5 mW red HeNe laser (633 nm, Cyonics Uniphase), each of which provides a single wavelength to excite fluorescently dyed nanospheres. Four photomultiplier tube (PMT) detectors are available on the LSC, each of which measures a different range of wavelengths. The nanospheres were excited with an argon ion laser (488 nm, 20 mW) and detected through the D: 530/30 (FITC, Green Fluorescent Protein) optical filter. A schematic diagram of the LSC setup is provided in Figure S2 of the Supporting Information. The ibidi μ-Slide I0.8 luer flow cell was mounted on the motorized stage of the fluorescence microscope. The microscope, operated under a 20× magnification, was slowly brought into focus until the top of the glass beads just come into focus. A scanning area was then determined as the middle section of the flow channel measuring 12 mm long and 5 mm wide. The restriction of the scanning length occurred because the upward protrusion of the elbow luer adapters from the flow cell was high enough to contact the microscope objective. The LSC scan occurred in 0.5 μm steps, and it took about 20 min to finish scanning the defined scan area. For each scan, the LSC measured the emitted fluorescence of the nanospheres and also precisely recorded the x and y coordinates of each recording event. 2.4. Interaction Energy Profile. Electrostatic interactions and van der Waals interactions were considered when calculating the interaction energy profile. The sphere−plane interaction expressions developed by Guzman et al.24 using a surface element integration technique that excludes the Derjaguin approximation were used for the system of nanospheres and glass beads in this study. The Hamaker constant of 1 × 10−20 J was used for the glass−water−latex system based on previous studies25,26 of similar systems. Equations used for the interaction energy calculation are provided in the Supporting Information. 2.5. Flow Field Simulation. Computational fluid dynamics (CFD) simulations were performed to mimic the threedimensional (3D) flow fields in the flow cell packed with glass beads. The purpose of the CFD simulations was to reveal the influence of the pore-scale 3D fluid flow fields on nanosphere transport and attachment. A glass bead packing was constructed for CFD simulation, based on one real flow cell experiment. The detailed description on flow cell packing generation and CFD simulation domain are provided in the Supporting Information. Due to the computational intensity of the CFD simulation, only the middle 5 mm of the flow cell was simulated. Flow fields with Reynolds numbers (Re = ρlqd50/μ,
generates the laser scanning beam, an upright epifluorescence microscope with a motorized stage to allow for the generation of sample scan images, and a computer to acquire and analyze scan data (ComputCyte Corporation, Westwood, MA). Compared with traditional flow cytometry technology, which focuses on measuring particles suspended in a stream of liquid, a LSC is capable of quantifying fluorescent cells or particles fixed on a solid substrate, which makes it possible to detect nanoparticles attached to the collector surface. Compared with laser-based confocal microscopy, a LSC is able to scan relatively large sample areas without needing to refocus the instrument, which makes this technology suitable for investigating nanoparticle distributions in a porous medium domain at the centimeter scale. In this study, we demonstrated the possibility of using a LSC as an effective tool for investigating the retention of nanoparticles in porous media. Nanoparticles were injected into a microfluidic flow cell packed with glass beads. The spatial distributions of attached nanospheres in porous media under various solution chemistries and flow velocities were developed based on the LSC scanning results. The mechanisms controlling nanoparticle transport were analyzed based on the LSC scanning results.
2. MATERIALS AND METHODS 2.1. Model Particles and Porous Media. Carboxylmodified polystyrene latex fluorescent nanospheres (Phosphorex, Inc., Fall River, MA) with a manufacturer-reported mean diameter of 57 nm were used as model particles in this study. The fluorescent dye used to mark the particles was green with an excitation maximum at 480 nm and an emission maximum at 520 nm. The density of the particles was 1.06 g/cm3. The particle-size distributions were measured using a 90Plus particle size analyzer (Brook-haven Instruments Corporation, Holtsville, NY) at all solution chemistry investigated. Glass beads conforming to a 20−30 U.S. mesh size (Potters Industries, Inc., Valley Forge, PA) were selected as the model porous medium. The glass beads were further sieved using a size 25 U.S. sieve corresponding to 710 μm, resulting in particles with diameters ranging from 600 to 710 μm. Prior to use, the glass beads were washed to remove impurities from their surfaces, following the procedure reported in Wang et al.12 The electrophoretic mobilities of the nanospheres and glass beads were determined using a ZetaPALS analyzer (Brookhaven Instruments Corporation, Holtsville, NY). The procedure followed to measure the electrophoretic mobilities of the glass beads is the same as that reported by Kuznar and Elimelech.23 2.2. Flow Cell Experiment. An ibidi μ-Slide I0.8 luer flow cell was purchased from ibidi LLC, Verona, WI; the apparatus consisted of a hydrophobic plastic designed for high-resolution microscopic analysis. The optical quality of the material is comparable to that of glass and exhibits low birefringence and autofluorescence. The flow cell is 25.5 mm wide and 75.5 mm long. A rectangular flow channel is located at the center of flow cell. The flow channel measures 5 mm in width, 50 mm in length, and 0.8 mm in height. The inlet and outlet of the flow channel are connected to tubes and pumps via luer adapters. The design of the flow cell is shown in Figure S1 of the Supporting Information. One layer of cleaned glass beads was packed in the flow channel, which resulted in a porous medium with porosity in the range 0.45−0.49. Before each experiment, the flow cell was saturated with a desirable electrolyte injected 9981
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expected that the characteristics of the flow field in this particular glass bead packing should represent a typical flow field because the key parameters, such as the mean diameter of the glass beads, the width and depth of the flow cell, and the Reynolds number of the flow field, were consistent between the experiments and the model. The flow velocity field revealed by CFD simulation was generally nonuniform in three dimensions. As shown in Figure 1A, flow velocity close to the top of the flow cell is obviously higher than that close to the flow cell bottom, which is due to a gap between flow cell (0.8 mm high) and the top of glass beads with an average diameter of 0.64 mm. Figure 1B, C showed plane views of flow streamline and velocity distribution at z = 0.29 and 0.59 mm, respectively. Clearly, velocity in the upper plane (Figure 1C) is generally higher than the velocity in the lower plane (Figure 1B), both of which are highly nonuniform along the length of flow cell and in the transverse-flow direction. Several apparent preferential flow pathways were noted by flow streamlines. In the pore spaces, velocity was highest at the pore centers and vanished along the collector surfaces due to the nonslip boundary conditions. It is expected that the variation of velocity field will have significant impacts on nanoparticle transport. 3.3. Distribution of Attached Nanospheres. Representative distributions of the detected fluorescence emitted from the attached nanospheres are shown in Figure 2. The
where ρl is the liquid density, q is the Darcy velocity, d50 is the mean grain diameter, and μ is the dynamic viscosity of the fluid) of 0.51, 0.39, 0.25, and 0.13, corresponding to Darcy velocities of 0.08, 0.04, 0.06, and 0.02 cm/s, respectively, were simulated. The COMSOL Multiphysics 4.2a CFD module (COMSOL, 2012) was used for these simulations.
3. RESULTS AND DISCUSSION 3.1. Electrokinetic Properties of Latex Particles and Porous Media. Dynamic light scattering measurements revealed that the mean diameter of the fluorescent microspheres changed from 64.2 to 59.8 nm when the ionic strength increased from 3 to 100 mM (NaCl, pH 7.0), as shown in Figure S5 of the Supporting Information. The electrophoretic mobility values of the nanospheres and glass beads as a function of ionic strength (NaCl, pH 7.0) are shown in Figure S6 of the Supporting Information. Both the nanosphere and glass bead surfaces were negatively charged. The electrophoretic mobility increased from −3.3 to −2.5 (μm/s)/(V/cm) and from −4.2 to −1.5 (μm/s)/(V/cm) for the nanospheres and glass beads, respectively, when the ionic strength increased from 3 to 100 mM (NaCl, pH 7.0). Although a clear increase in the electrophoretic mobility occurred between ionic strengths of 20 and 50 mM, the electrophoretic mobility of the nanospheres was not as sensitive to the ionic strength as that of the glass beads, which may be attributed to the resistance of a carboxyl layer on the particle surface to the double-layer compression effect.27 The interaction energy profile calculations shown in Figure S7 of the Supporting Information revealed the presence of a repulsive energy barrier and a small secondary energy well minimum at ionic strengths of 3−50 mM NaCl for the nanosphere−glass bead system. The energy barrier and secondary minimum energy disappeared at ionic strengths of 100 mM NaCl, which indicated favorable conditions for particle deposition. 3.2. Flow Field in Flow Cell. Figure 1 illustrates the simulated velocity field mimicking the flow field in the flow cell packed with glass beads. Although the glass bead configuration in the flow cell may be different for each experiment, it is
Figure 2. Distributions of detected fluorescence emitted from attached nanospheres at 3 mM NaCl, pH 7.0, Darcy velocity 0.08 cm/s after injection of (A) 40 PV and (B) 300 PV nanosphere suspension followed by injection of 10 PV of background solution.
experiments were conducted in duplicate to ensure the reproducibility of the data. Figure S8 in the Supporting Information provides example LSC scans for two flow-cell experiments conducted as duplicates. Figure 2A represents an LSC scan of a 12 mm long section in the middle of the flow cell (i.e., from 19 to 31 mm away from the inlet) after the injection of 40 PV nanospheres followed by the injection of 10 PV of background solution (i.e., 3 mM NaCl, pH 7.0) at a Darcy velocity of 0.08 cm/s. The color bar represents the amount of detected fluorescence, for which a higher value corresponds to more nanospheres attached to that location. Clearly shown here, the distribution of attached
Figure 1. (A) Domain dimension and velocity distribution for CFD simulation at a Darcy velocity of 0.08 cm/s; (B) velocity distribution at a plane z = 0.29 mm; and (C) velocity distribution at a plane z = 0.58 mm. 9982
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3.5. Mobilization and Reattachment of Nanospheres. Experiments were conducted to investigate the attachment of nanospheres under higher ionic strength (100 mM, NaCl, pH 7.0). After a typical attachment experiment, 10 PV of DI water was sequentially pumped into the flow cell to investigate any potential mobilization and reattachment. The LSC scans taken before and after the DI water elution are provided in Figure 3.
nanospheres along the transverse direction was not uniform. Obviously, fewer nanospheres were detected in the central region of the flow cell. Examining the flow field simulation (Figure 1) showed several higher flow velocity regions in the domain, which are located close to the center region of the flow cell. Correspondingly, flow velocity adjacent to the lateral boundaries of the flow cell was relatively lower due to wall effects. The flow velocity at the pore scale directly influences the single collector efficiency factor η0, which is a parameter reflecting the frequency of particle collisions with the grain surface.28 Although η0 is typically considered to be influenced by the combined effects of sedimentation, interception, and diffusion, diffusion is expected to be the controlling factor for nanoparticles. For nanospheres with an average diameter of 64.2 nm (3 mM NaCl, pH 7.0), the diffusion efficiency ηD accounts for 99.8% of η0 and has a reverse power-law relationship with velocity, that is, ηD ≈ v−0.75, according to a correlation developed by Long and Hilpert.29 Therefore, it is reasonable to believe that the lower degree of attachment in the central part of the flow cell is due to the relatively higher local velocity around the glass beads in the region. For the middle 12 mm section of the flow cell, the distribution of attached nanospheres did not have an apparent change along the flow direction, which however, does not necessarily indicate a uniform distribution along the entire length of flow cell. In the current experimental setup, the LSC was not able to scan flow cell inlet due to the restriction of an upward protrusion of the elbow luer adapter (Supporting Information, Figure S1), although previous research30,31 have reported significant amount of deposition can occur close to the inlet. 3.4. Distribution of Attached Nanospheres after Longer Injection Duration. Figure 2B presents the distribution of the attached nanospheres after injecting 300 PV of nanospheres followed by 10 PV of background solution (i.e., 3 mM NaCl, pH 7.0) at a Darcy velocity of 0.08 cm/s. Compared with the 40 PV injection experiment (Figure 2A), the nanospheres became attached to the entire surface of the glass beads, as indicated by the many closed circles detected by the LSC. Based on the simulated flow streamline (Figure 1B, C), the nanospheres need to migrate across the streamlines and reach the rear stagnation zones of the glass beads to produce the detected circled distribution. This phenomenon further indicates that diffusion is a dominating factor for nanosphere transport. As shown in Figure 2B, this particular LSC experiment was able to delineate the silhouette of packed glass beads in the scan area, allowing us to generate the glass bead packing for CFD simulations illustrated in Figure 1. Compared with the shorter injection duration (40 PV) experiment (Figure 2A), significantly more nanospheres were attached in the center area of flow cell after 300 PV of injection. Furthermore, more red dots were present in Figure 2A in the center of each glass bead, indicating more attachment in the upper hemisphere of glass beads. The locations of these enhanced attachment events are corresponding with a preferential flow pathway as illustrated in Figure 1C, a horizontal plane view of the flow field located in the upper hemisphere of glass beads. Higher velocity in the flow preferential pathway will generate lower ηD as previously discussed. However, after a longer injection period, the preferential flow pathway will bring in more nanoparticle mass than other areas, leading to more attachment in this region.
Figure 3. Distributions of attached nanospheres (A) before and (B) after 10 PV of DI was pumped into the flow cell following a typical attachment experiment, where 40 PV of nanospheres was injected followed by the injection of 10 PV of 100 mM NaCl at a Darcy velocity of 0.04 cm/s.
Apparently, the nanosphere distribution patterns are different before and after flushing with DI water, which indicates that the attached nanospheres were mobilized when DI water was injected. The observed mobilization suggested that some nanoparticles were previously not attached in the primary energy minimum, although the calculated energy barrier disappeared for the nanosphere−glass bead system when the ionic strength reached 100 mM (Supporting Information, Figure S7). The interaction energy calculation was based on an averaged surface potential of the glass beads. The actual surface charge distribution of a glass bead however very possibly was not homogeneous. Most solid surfaces in aqueous media are heterogeneously charged due to either the complexity of the crystalline structure of solids or due to their complex chemical composition. On the well-cleaned glass bead surfaces, surface charge heterogeneity is possible due to the presence of heterogeneously distributed hydroxyl groups.32,33 Based on Figure S7 of the Supporting Information, the interaction energy profile just transitioned from repulsive to attractive when the ionic strength increased to 100 mM. Sensitivity analysis showed that a slight decrease (5%) of glass bead ζ potential value used for the calculation will lead to a low energy barrier (0.54 kT at 1.15 nm) and a secondary energy minimum (−1.74 kT at 2.62 nm). Thus, some nanospheres may be just loosely associated with the glass bead surface at areas slightly more negatively charged, so that change of solution to DI water will lead to their mobilization. The total amount of fluorescence detected after flushing with DI water was approximately 1.36 times that detected before flushing with DI water. Given the fact that the scan area is 9983
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located in the middle along the length of flow cell, it is possible that some nanospheres mobilized in the upstream of the flow cell may reattach in this area, resulting in increased fluorescence. Furthermore, the nanosphere distribution after flushing with DI water (Figure 3B) showed much more scattered red spots compared with the distribution observed before DI water flushing (Figure 3A). The red spots corresponded to greater amounts of detected fluorescence at the given locations, indicating that multiple nanospheres were detected in those locations, which was favorable under DI water flushing. The observed preferential deposition onto certain favorable spots likely reflected the influence of surface charge heterogeneity.34−36 Although the glass beads used in the experiments were cleaned by acids, it is extremely difficult to completely rule out the contributions from impurities (e.g., metal oxide). Previous work36 has shown that the amount of latex colloidal particle deposition onto glass beads was sensitive to the glass bead clean methods. Typically, surface charge heterogeneity was considered to be negligible at high pH conditions (e.g., pH 10).37 In our experiments (pH 7), it is highly possible that nanoscale metal oxide impurity on the glass bead surfaces will form local favorable spots, attracting the mobilized nanospheres. 3.6. Influence of Darcy Velocity. The distributions of nanospheres at Darcy velocities of 0.02, 0.04, and 0.06 cm/s are presented in Figure 4. As in the experiment conducted at 0.08 cm/s (Figure 2A), the attached nanosphere distributions are generally uniform along the longitudinal direction, but fewer are attached to the center than close to the lateral boundaries. It seems that more full circles were formed around the glass bead surfaces at lower velocities and that more scattered red spots were observed at higher velocities. This observation could also
be attributed to the diffusion-control mechanism. The porescale velocity is generally lower at a lower Darcy velocity, so that nanospheres diffuse more easily across the flow streamlines and reach every point on the surfaces of the glass beads. However, a relatively higher Darcy velocity mitigates this effect. Recent studies38−41 have reported that hydrodynamic forces can influence not only the transport of a particle to the vicinity of a collector but also the particle attachment. To evaluate this effect in the studied system, we estimated the attachment efficiency factor α based on LSC scanning at different flow velocities. By definition, α reflects the fraction of collisions that lead to successful attachment. Under favorable conditions, all of the collisions lead to attachment, such that α equals 1. Assuming a linear relationship between fluorescent signal and attached nanoparticle concentration, α can be estimated as the ratio of the detected amount of fluorescence between unfavorable I and favorable conditions If corrected by flow velocity: I uf α= If u (1) where u and uf are the Darcy velocities for the unfavorable and favorable experiments, respectively. Favorable attachment conditions were achieved using 100 mM NaCl as the background solution (Figure 3A), which has no energy barrier according to the interaction energy calculation (Supporting Information, Figure S2). Our calculation showed that α gradually increased from 0.31 to 0.41 when the flow velocity decreased from 0.08 to 0.04 cm/s; α then sharply increased to close to 1.0 when the flow velocity reached 0.02 cm/s (Supporting Information, Figure S9). Previous studies have also reported a decrease in the attachment rate with an increase of flow rate38,41,42 under unfavorable conditions. In these studies, the influences of hydrodynamic forces on particle attachment were incorporated using a torque analysis approach. Successful attachment was considered to occur when the adhesive torque acting on the attached particles was greater than the hydrodynamic torque. We conducted torque analysis for two glass beads adjacent to the wall of the flow cell and two glass beads in the center of the flow cell, following the procedures by Torkzaban et al.,39 which is detailed in the Supporting Information. Briefly, the hydrodynamic torque was calculated based on the hydrodynamic shear obtained from the computational fluid dynamic simulation of porous media in the flow cell. The adhesive torque was estimated based on the secondary energy minimum well and the corresponding separation distance. About 5.9% and 9.3% of the surfaces were found to be hydrodynamically favorable for attachment (or the adhesive torque is greater than the hydrodynamic torque) for glass beads located in the center of the flow cell and adjacent to the flow cell lateral boundary, respectively. The lower hydrodynamically favorable surface area of the glass beads in the center of the flow cell is consistent with the previous observation that fewer nanospheres were detected in the center of the flow cell. However, the calculated percentage of hydrodynamically favorable surface areas (Sf) is not sensitive to the flow velocities investigated here. Although the hydrodynamic torque acting on a 64.2 nm diameter nanosphere is very small, it is generally larger than the adhesive torque (6.29e−26 N·m) based on the very small secondary minimum energy (ca. 0.03 kT) exerted rather far away from the collector surface (i.e., 52.2 nm). Only in the flow stagnation regions is the adhesive torque larger than the hydrodynamic
Figure 4. Distributions of nanospheres after injection of 40 PV of nanosphere suspension followed by injection of 10 PV of 3 mM NaCl background solution at Darcy velocities of (A) 0.02 cm/s, (B) 0.04 cm/s, and (C) 0.06 cm/s. 9984
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under grant CBET 1133528. We thank the Biomechanics, Biomaterials and Biomedicine (BM3) Instrumentation Facility at the University of Nebraska-Lincoln for providing access to the laser scanning cytometer.
torque and thus hydrodynamically favorable. Although particle deposition in the secondary minimum has been widely considered as a key mechanism to explain deposition under unfavorable conditions,42−44 it seems insufficient to explain the observed declining trend in the collision efficiency factor with flow velocity. Sensitivity analysis (Supporting Information, Figure S9) indicated that, if the adhesive torque was approximately 10 times higher than the secondary minimum and located closer (e.g., 15 nm away) to the collector surface, the dependency of α on the velocity predicted by torque analysis would be similar to that observed in this study. We again speculated that the surface charge heterogeneity of glass beads played an important role here. Presence of metal impurities may lead to less negative surface potentials on certain locations of the glass beads, which will lead to higher adhesive torques.
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ASSOCIATED CONTENT
S Supporting Information *
Additional experimental details, interaction energy calculations, figures, and tables. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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4. ENVIRONMENTAL IMPLICATION In this work, we demonstrated the capability of a LSC to investigate nanosphere deposition and mobilization at pore scale. Most importantly, the LSC provided detailed distributions of attached nanoparticles in a pore space in the scale of several centimeters, which allows directly linking porous medium configuration and pore-scale hydrodynamics with nanoparticle deposition. A LSC scan revealed nanosphere mobilization and reattachment under DI water flushing following a deposition experiment at higher ionic strength. The column breakthrough curve cannot reflect the mobilization process discovered here, because most mobilized nanoparticles were reattached when moving to the subsequent part of the column, leading to minimal influence on the nanoparticle concentration in the column effluent. The similar comparison of retention profile before and after DI water flushing is not possible using a column experiment, because the retention profile of a column experiment is typically obtained by destructively measuring the amount attached in a different section of the column. The LSC served as a novel technique to allow the investigation of these phenomena for particles at the nanoscale. This work only provided proof-of-concept by using fluorescent polystyrene nanoparticles under simple solution chemistry and pore structures. It can be easily extended to study more realistic nanomaterials with natural or artificial fluorescent dyes. Further investigation on quantitatively linking the detected fluorescence with attached nanoparticle concentration is warranted.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (402) 472-5972; fax: (402) 472-8934; e-mail: yli7@ unl.edu. Notes
The authors declare no competing financial interest. 9985
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dx.doi.org/10.1021/es301749s | Environ. Sci. Technol. 2012, 46, 9980−9986
