Environ. Sci. Technol. 2009, 43, 8153–8158
Mobility of Multiwalled Carbon Nanotubes in Porous Media X U E Y I N G L I U , † D E N I S M . O ’ C A R R O L L , * ,† ELIJAH J. PETERSEN,‡ QINGGUO HUANG,§ AND C. LINDSAY ANDERSON| Department of Civil & Environmental Engineering, The University of Western Ontario, London, ON, Canada N6A 5B8, Department of Chemical Engineering, University of Michigan, 2200 Bonisteel Boulevard, Ann Arbor, MI 48109, Department of Crop and Soil Sciences, University of Georgia, Griffin Campus, 1109 Experiment Street, Griffin, GA 30223, and Biological and Environmental Engineering, Cornell University, 328 Riley-Robb Hall, Ithaca, NY 14853
Received May 5, 2009. Revised manuscript received September 22, 2009. Accepted September 25, 2009.
Engineered multiwalled carbon nanotubes (MWCNTs) are the subject of intense research and are expected to gain widespread usage in a broad variety of commercial products. However, concerns have been raised regarding potential environmental and human health risks. The mobility of MWCNTs in porous media is examined in this study using onedimensional flow-through column experiments under conditions representative of subsurface and drinking water treatment systems. Results demonstrate that pore water velocity strongly influenced MWCNT transport, with high MWCNT mobility at pore water velocities greater than 4.0 m/d. A numerical simulator, which incorporated a newly developed theoretical collector efficiency relationship for MWCNTs in spherical porous media, was developed to model observed column results. The model, which incorporated traditional colloid filtration theory in conjunction with a site-blocking term, yielded good agreement with observed results in quartz sand-packed column experiments. Experiments were also conducted in glass beadpacked columns with the same mean grain size as the quartz sand-packed columns. MWCNTs were more mobile in the glass bead-packed columns.
Introduction Nanotechnology focuses on the investigation and application of materials with at least one characteristic dimension less than 100 nm (1). The unique physical, chemical, and electronic properties of nanomaterials make them a highly promising novel class of materials for a variety of potential applications. Carbon nanotubes (CNTs), for example, have significant potential in drug delivery, electric and optical devices, and energy storage (2, 3). In the environmental science and engineering community, there is concern related to the potential human health and ecological consequences after possible dispersal of nanomaterials in environmental systems (4). Research into the environmental impacts of * Corresponding author phone: (519)661-2193; fax: (519) 661-3942; e-mail:
[email protected]. † The University of Western Ontario. ‡ University of Michigan. § University of Georgia. | Cornell University. 10.1021/es901340d CCC: $40.75
Published on Web 10/06/2009
2009 American Chemical Society
single and multiwalled carbon nanotubes (SWCNT and MWCNT, respectively) to date has revealed variable toxicity of these two types of nanotubes. For example, one study found that SWCNTs were more toxic to microorganisms than MWCNTs (5), another found that neither had an effect on earthworm masses or lipid contents (6), and another study reported that double-walled carbon nanotubes, a form of MWCNTs, impacted earthworm reproduction (7). In order to determine the risks nanoparticles may pose to the environment, it is essential to understand the factors that govern their toxicity and mobility. An understanding of mobility will also help predict the ability of traditional water treatment plants to remove nanoparticles and facilitate the application of engineered nanoparticles for site remediation. The mobility of colloids in porous media are governed by interception, Brownian diffusion, and gravitational sedimentation (8) as well as nonphysicochemical removal mechanisms (9-11). These include adsorption site blocking (12, 13), straining (11, 14, 15), porous media charge heterogeneities (13), colloid surface charge variability (16, 17), and secondary energy minimum deposition of colloids (18-21). The relatively few experimental studies that have investigated engineered nanoparticle mobility found that colloid filtration theory (CFT) did not fully explain observed behavior (12, 14, 20-23). For example, Lecoanet and Wiesner (21) investigated the mobility of a range of nanoparticles in glass bead column experiments and found that nanoparticle mobility varied greatly. These differences could not be readily related to measured parameters. In addition, CFT assumes spherical collectors and colloids, but many colloids of environmental relevance and many nanoparticles are nonspherical (24). Studies have also suggested that SWCNTs are mobile in porous media under the conditions studied (14, 21), but the mobility of MWCNTs is not yet well understood and may vary substantially given that their diameters may be an order of magnitude or two larger than those for SWCNTs. Given that SWCNTs and MWCNTs are being investigated for a variety of different potential applications, the fate of both nanoparticles needs to be investigated (25). In this work, a series of one-dimensional (1D) column experiments were conducted to assess the mobility of MWCNTs in porous media. Experiments were conducted for pore water velocities ranging over 2 orders of magnitude to represent conditions that may be found in slow sand filters at drinking water treatment plants, sites undergoing pump and treat remediation, and sites with undisturbed groundwater conditions. The ability of a numerical simulator, incorporating colloid filtration theory, to simulate observed MWCNT transport behavior is assessed, and the inclusion of an adsorption blocking term is evaluated. To accomplish this aim, a new relationship is developed for the collector efficiency of MWCNTs (cylindrical particles) in a spherical collector system. Experiments were also conducted in columns packed with glass beads, in addition to quartz sand, to investigate the impact of collector shape and pore size distribution on MWCNT removal.
Materials and Methods Porous Medium. Quartz sand (d50 ) 476 µm, UI ) 1.5; Barco 32, BEI Pecal, Hamilton, ON, CA) and soda-lime glass beads (class V; diameter, 425-500 µm; MO-SCI Corporation, Rolla, MO) were used in the column experiments. The quartz sand and glass beads were repeatedly washed, alternating between 0.1 M HCl and 5% H2O2 solution. Deionized water was used to rinse the porous media between steps and after the wash to remove impurities. Washed sand and glass beads were VOL. 43, NO. 21, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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oven-dried (105 °C) for over 24 h. The sand was then sieved to remove particles less than 152 µm. This cleaning procedure was developed to remove organic and metal oxide impurities from the sand surface, ensuring that the sand surface had uniform surface chemistry. Nanomaterials. Multiwalled carbon nanotubes were synthesized using a chemical vapor deposition (CVD) method with nickel and magnesium catalysts (26). After purification with concentrated HCl for 1 h, these nanotubes were treated with an aggressive acid treatment using a 3 to 1 ratio of sulfuric and nitric acids to enhance the stability of the MWCNTs in water through the addition of hydroxyl and carboxyl functional groups on the MWCNT surface (27). Additional characterization and aqueous suspension preparation information can be found in the Supporting Information. Column Experiments. Two sets of column experiments were conducted in custom designed 10 cm columns in this study (Supporting Information). In the first set, quartz sand was used as the porous medium, and MWCNTs were dispersed in stock solution (SS) I (2.8 mM NaHCO3, 2.1 mM Na2CO3, and 1 mM NaBr). In this set of experiments, at least 10 pore volumes of SS I (2.8 mM NaHCO3, 2.1 mM Na2CO3, and 1 mM NaCl) were flushed upward through the column following the deionized water flush. This flow rate was then adjusted to the experimental design flow rate, and the flow direction was switched to vertically downward for at least one pore volume (PV) prior to injection of the MWCNT suspension. In the low ionic strength column experiments, conservative tracer experiments were conducted with SS I, and the column was then flushed with 5 to 6 pore volumes of SS II prior to the initiation of the MWCNT column experiment. The stock solution was delivered to the column via a gear pump for experiments with pore water velocities of 43 m/d, 21 m/d, and 4.0 m/d, while a syringe pump was used for experiments with a pore water velocity of 0.42 m/d. The MWCNT suspensions were delivered by two or three 60 ml plastic syringes and injected downward through the column at the design pore water velocity. Throughout the entire experiment, 3.5 ml effluent samples were collected to quantify concentrations of the conservative tracer and MWCNTs. Chloride and bromide concentrations were measured using a HPLC and conductivity detector (Waters Company).
Mathematical Model Governing MWCNT Transport In order to simulate the transport of MWCNTs in these column experiments, a one-dimensional finite element code was developed to solve the mass balance equations of MWCNTs associated with the aqueous and solid phases (Supporting Information). Use of the numerical simulator for parameter estimation made use of the entire MWCNT effluent concentration breakthrough curve and eliminated the need for steady-state effluent concentrations. Traditional colloid filtration theory assumes the theoretical single collector efficiency is the sum of the contact efficiencies due to interception (ηI), sedimentation (ηG), and diffusion (ηD) or η0 ) ηI + ηG + ηD
(1)
These efficiencies have been determined for spherical particles flowing through a system with spherical collectors (8, 28), however, they have not been determined for cylindrical particles (i.e., MWCNTs) flowing through a system with spherical collectors. As a result, new relationships were derived for ηI, ηG, and ηD for MWCNTs in a spherical collector system (Supporting Information). A relatively simple approach was used to develop the collector efficiency for cylindrical particles. As with spherical particles, trajectory analysis (29) or solution of the convective-diffusive equation 8154
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(28) would facilitate a more comprehensive approach and inclusion of additional retention mechanisms to determine the collector efficiency. Unlike spherical particles flowing through a system with spherical collectors, the orientation of the cylindrical particles as they approach the spherical collector is important for the determination of the interception contact efficiency. In this analysis, two types of MWCNT/ collector contacts were assumed, the MWCNT end contacting the spherical collector (i.e., the circular end of the MWCNT contacting the spherical collector) and the MCWNT side contacting the collector (i.e., the length of the MWCNT contacting the spherical collector). The collection efficiency due to “end contact” can be defined as follows η0 ) ηI + ηG + ηD )
[ ( )( 1 l 2 dc
2
l l + dc
3-
)]
+ ηG + ηD (2)
Where l is the length of the MWCNT and dp is the MWCNT diameter. The collector efficiency for “side-contact” is defined as η0 ) ηI + ηG + ηD )
[ ( )( 1 dp 2 dc
2
3-
dp dp + dc
)]
+ ηG + η D (3)
where dp
ηG ) ( /l)
2/ 3
(
2
0.146(Fp - F)g(dp2l) /3 1
µνo[1 - (dp/l)2] /2
)( ln
1
1 + [1 - (dp/l)2] /2 dp
/l
)
(4a)
and
[ (
ηD ) 4.03 kT ln
1
1 + (1 - (dp/l)2) /2 dp
/l
(3πµd v (23d l) c o
2 p
1/ 3
)]
2
2/ 3
× 1
(l/dp) /3(1 - (dp/l)2) /2
)
-2/ 3
(4b)
where Fp is the MWCNT density, F is the fluid density, µ is the fluid viscosity, T is the absolute temperature, and k is the Boltzmann constant. In order to determine the collector efficiency due to gravity sedimentation and diffusion, a friction factor is required. Because a friction factor has not been developed for a cylindrical particle, to our knowledge, the friction factor developed for a prolate ellipsoid has been employed instead. It is not anticipated that this assumption will impact the magnitude of the theoretical contact efficiency as the MWCNT length is much larger than MWCNT diameter. The numerical model has been validated using an analytical solution of eqs S1 and S2 of the Supporting Information. The analytical solution does not incorporate the Smax term (i.e., Smax is assumed to very large in the analytical solution) or MWCNT detachment from the solid surface.
Results and Discussion Impact of Flow Rate on MWCNT Removal in Sand-Packed Columns. A series of column experiments were conducted to investigate the impact of pore water velocity on the transport of MWCNTs. Experiments were conducted using sand-packed columns and MWCNT suspensions in SS I (I ) 10 mM, pH 10) at four different pore water velocities (43 m/d, 21 m/d, 4.0 m/d, and 0.42 m/d), representing the upper range of velocities that may be found in slow sand filters, at pump and treatment groundwater remediation sites, or under typical natural groundwater conditions. MWCNT breakthrough curves for the three fastest pore water velocities are similar (Figure 1).
FIGURE 1. Observed and model fit breakthrough curves of MWCNTs at different pore water velocities conducted in quartz sand-packed columns. Ionic strength of the aqueous phase is 10 mM. (Only representative conservative tracer curves are presented to limit the number of data points.) The MWCNTs break through with the conservative bromide tracer and MWCNT concentrations are similar up to a normalized MWCNT concentration of 0.4. Conservative tracers were injected with all MWCNT suspensions in this phase of the study and yielded very similar breakthrough results. Above a normalized MWCNT concentration of 0.4, it is evident that some of the MWCNTs are retained in the column as the breakthrough characteristics of the MWCNTs start to depart from that of the conservative bromide tracer. In all cases, the normalized effluent MWCNT concentrations were slowly approaching 1.0 at the completion of each of these injections, indicating that MWCNT retention in the porous media system had not reached steady state. The results determined here differ from those previously determined for SWCNTs in significant ways (14, 20, 21). Lecoanet and Wiesner (21), who used dodecylbenzene sulfonic acid, a sodium salt as a surfactant to disperse their SWCNTs, reported greater retention of SWCNTs at the initial stages of SWCNT injection at a pore water velocity of 273 m/d in comparison to that at a pore water velocity of 82 m/d in glass bead column experiments. This behavior was also observed for three fullerene nanoparticles transport experiments in their study. The reason for this dip was unclear and not observed in their later nC60 transport study, suggesting that this could be an experimental artifact (30). This dip behavior differs from that reported by Jaisi et al. (14) for carboxyl functionalized SWCNT transport experiments at 18 m/d and from those obtained here. Jaisi et al. (14) observed a maximum normalized concentration of approximately 0.25 at an ionic strength of 10 mM (KCl), which is less than that observed by Lecoanet and Wiesner (21) at higher flow rates and also less than the maximum observed normalized effluent concentration observed at 4.0, 21, and 43 m/d in this study. Both studies used SWCNTs with similar diameters; however, the SWCNT length used in the Jaisi et al. study (14) may have been longer. There are numerous differences among these studies that could have impacted the transport results. First, different methods were used to form stable carbon nanotube aqueous phase suspensions, which could potentially yield different carbon nanotube surface charges in each study. In addition, the diameter range of the MWCNTs (7- 70 nm) used in this study is much larger than those for the SWCNTs used in other studies (∼1 nm), and the length of the MWCNTs is also slightly larger. Yet another important potential difference among these studies is that Lecoanet and Wiesner (21) used silicate glass beads, while natural quartz sand was used here and by Jaisi et al. (14). The surface characteristics (e.g., roughness and chemistry) of quartz sand and glass beads
FIGURE 2. Observed and model fit breakthrough curves of MWCNTs at a pore water velocity of 0.42 m/d conducted in quartz sand-packed columns (I ) 10 mM). likely differ. Finally, the average grain size diameter used in the two SWCNT transport studies were slightly smaller than the average grain size diameter used in this study (i.e., 355 µm for Lecoanet and Wiesner (21), 263 µm for Jaisi et al. (14), and 476 µm in this study). MWCNT transport behavior significantly differs at a lower flow pore water velocity of 0.42 m/d (Figure 2). Unlike MWCNT breakthrough at the higher velocities, the MWCNTs do not break through with the conservative tracer but are instead retained in the porous media to a more significant extent. For example, at 1.65 PV, normalized MWCNT concentrations ranged from 0.60 to 0.67 at the slowest flow rate and from 0.88 to 0.93 at the faster flow rates. In each of the four experiments at this low flow rate, the concentration of MWCNTs monotonically increased, similar to the behavior observed at higher flow rates. Similar to breakthrough curves at the faster flow rates, the normalized MWCNT effluent concentrations were gradually increasing at the completion of the injection, indicating that steady state was not achieved in the system. On the basis of these results, it appears that above a critical pore water velocity the MWCNTs are quite mobile, as MWCNT mobility was similar and quite good above 4 m/d but decreased for 0.42 m/d. On the basis of a mass balance of MWCNTs injected into the column and MWCNT concentration in the effluent, mass retention is similar for the three fastest pore water velocities. At the completion of the lowest flow rate experiments, 3.3-3.9 mg of MWCNTs or 30 to 35% of the injected mass was retained in the porous media. In contrast, at the higher flow rates between 0.65 and 1.5 mg of MWCNT or 4% to 9% of the injected mass was retained. At the termination of the experiments, MWCNTs were not detected in the effluent. This suggests that some fraction of the MWCNTs is retained irreversibly in the columns. Jaisi et al. (14) also report irreversible retention of SWCNTs in their study and attribute this to retention of SWCNTs in the primary energy minimum and also to SWCNT straining. They postulate that highly bundled SWCNTs would be susceptible to straining. SEM images of the MWCNTs would suggest limited bundling in this study. Simulation of MWCNT Transport. The numerical model was used to simulate the transport of MWCNTs in the quartz sand-packed column at high ionic strength (IS ) 10 mM) for all four pore water velocities (i.e., 43 m/d, 21 m/d, 4 m/d, and 0.42 m/d) in conjunction with the newly derived collector efficiency for MWCNTs in a spherical collector system. Known parameters were used in the numerical model when possible (e.g., pore water velocity, MWCNT dimensions, and porosity), and key unknown parameters in eqs S1 and S2 of the Supporting Information were estimated by minimizing the VOL. 43, NO. 21, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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root-mean-square error (RMSE) between simulated and observed MWCNT effluent concentration as a function of pore volume eluted. It should be noted that use of the numerical simulator for parameter estimation permitted the use of the entire MWCNT effluent concentration breakthrough curve and eliminated the need for steady-state effluent concentrations. An unconstrained nonlinear optimization routine was used to minimize the RMSE. Experimental results from seven breakthrough curves, at all four pore water velocities (i.e., 43 m/d, 21 m/d, 4 m/d, and 0.42 m/d for experiments A-D), were used to calculate the RMSE. In this calculation, the RMSE of the four low flow rate experiments was averaged so that the RMSE at each flow rate had equal weight. Initially, only mechanisms associated with traditional colloid filtration theory were employed in the fitting routine (i.e., Rl in eq S1 of the Supporting Information and R in eq S4 of the Supporting Information were fit; kdet and Ψ in eq S2 of the Supporting Information were set to 0 and 1, respectively). In these simulations, it was assumed that attachment of MWCNTs was through side MWCNT contact with the porous media and not MWCNT end contact. This assumption is required to determine the interception collector efficiency as it is not possible to determine the fraction of collisions attributed to end or side contact. In this system, the end and side contact theoretical single collector efficiencies differed by less than 9% because the diffusion collector efficiency, which is independent of orientation, dominated. The simulated MWCNT breakthrough curve at the lowest flow rate (0.42 m/d), using only mechanisms traditionally associated with colloid filtration theory, suggests that MWCNTs exit the column before they were observed experimentally and reach a plateau lower than the maximum normalized concentration observed experimentally (Figure 2). In this series of simulations, only the attachment efficiency, R in eq S4 of the Supporting Information, and the MWCNT dispersivity, Rl in eq S1 of the Supporting Information (D ) v × Rl), were fit (Table S1 of the Supporting Information). It was assumed that the attachment efficiency is independent of pore water velocity (i.e., one attachment efficiency value was fit to all four velocities) and that the dispersivity of the MWCNT could differ from that of the conservative tracer. The dispersivity fit to the conservative tracer data at all pore water velocities was 5.9 × 10-4 m, which is similar to the mean grain size. The dispersivity value fit to the MWCNT data was larger (1.12 × 10-3 m) but is on the same order of magnitude as the conservative tracer dispersivity. Differences between observed and model fit MWCNT breakthrough curves were less pronounced at the higher flow rates as observed MWCNT effluent concentrations approach 1.0 faster at the higher flow rates. However, the model fits do approach 1.0 faster than observed experimentally at these flow rates (results not shown). Becasue the traditional colloid filtration theory does not adequately predict MWCNT transport in this system, particularly at the lowest flow rate, additional transport and retention mechanisms were considered in the numerical simulator to determine if they improve agreement between observed and modeled MWCNT effluent concentrations. The first additional transport mechanism considered was detachment of adsorbed MWCNTs from the sand surface (i.e., kdet in eq S2 of the Supporting Information is also fit). The inclusion of detachment does not substantially improve agreement between observed and simulated MWCNT effluent concentrations (Figure 2 and Table S1 of the Supporting Information). Because the model was unable to yield an adequate representation of observed behavior by fitting MWCNT dispersivity, Rl (eq S1 of the Supporting Information), kdet (eq S2 of the Supporting Information), and R (eq S4 of the 8156
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FIGURE 3. Fitted and observed breakthrough curves of MWCNTs at a pore water velocity of 0.42 m/d in quartz sand and glass bead columns. Ionic strengths of the aqueous phase are 10 mM and 0.1 mM. Supporting Information), a site-blocking term was added given the slow rise to the effluent plateau concentration observed in the experimental results. Such experimentally observed behavior has previously been attributed to the blocking of adsorption sites (13). The inclusion of the adsorption site-blocking term dramatically improves agreement between observed and simulated MWCNT effluent concentrations with the simulated effluent MWCNT concentration slowly increasing at the slowest flow rate (0.42 m/d) as observed experimentally (Figure 2). This mechanism has been included in eq S2 of the Supporting Information through the addition of the Ψ term, which is initially 1.0 and decreases to 0.0 as all of the adsorption sites are occupied. It should be noted that these experiments were conducted at an ionic strength of 10 mM; with increased ionic strength, the energy barrier to MWCNT deposition would decrease, which could increase the Smax term to a greater extent than the fit in this study. As shown in Table S1 of the Supporting Information, the RMSE, summed over all pore water velocities, decreases from 2.18 when only mechanisms traditionally associated with colloid filtration theory are included in the numerical simulator to 1.36 when the adsorption siteblocking term is included. This suggests that there are only a finite number of adsorption sites available for MWCNT adsorption. The fit dispersivity for this case is similar to other cases and slightly larger than the average grain size. The rate constant for MWCNT detachment (kdet) is essentially 0. This analysis suggests that use of CFT, with the inclusion of a site-blocking term, yields model results that are in agreement with observed behavior. Significance of Ionic Strength on MWCNT Removal. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory is commonly applied in traditional filtration theory to estimate the particle-grain interaction energy profiles (18, 31). The large repulsive barrier makes attachment to the collector due to interception, Brownian diffusion, and gravitational sedimentation at unfavorable deposition sites even more unfavorable at lower ionic strengths. To investigate the importance of ionic strength and repulsive electrostatic MWCNT-sand particle interactions, we conducted a second set of column experiments with a low ionic strength aqueous phase (0.1 mM) in sand- and glass bead-packed columns at a pore water velocity of 0.42 m/d. At the low ionic strength conditions (0.1 mM), MWCNTs break through with the conservative tracer and rapidly approach a steady-state normalized effluent concentration (Figure 3). At both ionic strengths, the ultimate steady-state normalized effluent concentrations appear to be similar. However, the rate at which steady state is achieved is faster
in the lower ionic strength experiments in comparison to the higher ionic strength experiments. These observations confirm the hypothesis of a higher repulsive electrostatic energy barrier at a lower ionic strength given the higher MWCNT mobility at the lower ionic strength. An additional simulation was run to assess the ability of the numerical simulator to model MWCNT mobility at the low ionic strength. In this simulation MWCNT dispersivity, Rl (eq S1 of the Supporting Information), kdet (eq S2 of the Supporting Information), R (eq S4 of the Supporting Information), and Smax (eq S3 of the Supporting Information) were fitted to the observed low ionic strength breakthrough curve. The upper value of the Smax search range was constrained by the value fitted to the higher ionic strength data because the number of available sites on the collector surface for MWCNT attachment will decrease with decreasing ionic strength due to the increased repulsive energy barrier at a lower ionic strength. No such constraints were imposed on finding optimum values for Rl, kdet, or R. It is therefore assumed that the attachment efficiency, R, at low ionic strength could be larger than the value fitted at higher ionic strength. This is conceivable because a larger fraction of collisions at the limited number of attachment sites (i.e., lower Smax) could result in successful attachment of MWCNTs at these sites. The fits in the glass bead experiments were good (Figure 3). However, the fitted breakthrough curve for the MWCNTs at the lower ionic strength in the quartz sand yields a break through at the correct pore volume, but the simulated breakthrough curve does not rapidly achieve the effluent plateau as observed experimentally. While the reason for this discrepancy is unclear, if the fit Smax was unconstrained, it is likely a near perfect fit would have been obtained as a blocking term approaching 1.0 could lead to a fit resembling simulated MWCNT transport with a first-order sink term. As discussed above, more adsorption sites at the lower ionic strength system, in comparison to the higher ionic strength system, would not be possible given the larger electrostatic energy barrier at the lower ionic strength. It is also possible that other MWCNT removal mechanisms, which were not incorporated in the numerical simulator, could reduce discrepancies between model and observed results. A number of physicochemical and nonphysicochemical removal mechanisms, which are not accounted for in CFT, have been proposed to explain the transport of colloids and nanoparticles in porous media. For example, nonuniform collector surface charges could yield a distribution of favorable and unfavorable deposition sites, while CFT assumes uniform collector surface characteristics (13, 31). However, the surface charges of the quartz sand and glass bead collectors are negative in the pH range of this study as is the surface charge of the MWCNTs suggesting that on average deposition of the MWCNTs on the collector surface is unfavorable. Furthermore, the aqueous phase pH of 10 used in this study would yield a negative surface charge because most sand impurities present (e.g., metal impurities) and organic impurities on the sand surface would have been removed via oxidation when the sand was cleaned. Therefore, sand surface charge heterogeneity is likely only to have a limited impact on observed results. However, if sand surface heterogeneities do exist, nanoparticles may be more sensitive to them because of their small size in comparison to larger colloids, which may experience a more averaged collector surface charge distribution. There is also the potential for MWCNT retention in a secondary energy minima at the collector surface (18), although the secondary energy minima would not be present in the series of low ionic strength experiments that exhibited significant MWCNT retention, thus, making this removal mechanism unlikely. Finally, the nonphysicochemical removal mechanism straining, where the MWCNTs are retained at grain-grain intersections, may
also account for observed discrepancies at the low ionic strength (31-33). In a recent study by Jaisi et al. (14), they explain observed behavior by proposing that straining was enhanced because of bundled SWCNTs. CFT assumes a monodisperse suspension of MWCNTs, which is supported by TEM images of the nanotubes used in this study. However, there is likely some degree of aggregation and bundling of the MWCNTs as they flow through the porous media. This analysis suggests that the use of CFT with the inclusion of the blocking term (eq S3 of the Supporting Information) yields good fits to observed MWCNT breakthrough curves, with the exception of the low ionic strength quartz sand experiment. Given the results at low ionic strength, it may be necessary to consider additional nonphysicochemical removal mechanisms for the prediction of MWCNT transport in porous media systems. It should be noted that the MWCNTs have been idealized as rigid rods with a uniform diameter and length in the development of the collector efficiency relationship. MWCNTs are not rigid rods and span a range of diameters and lengths potentially contributing to differences between observed behavior and model results. Maximum normalized effluent concentrations in the glass bead-packed column experiments exceed those of the quartz sand column experiments. In both cases, the mean particle diameters are similar and collector surface charges negative. Glass beads are, however, smoother than quartz sand, and given the uniformity coefficient of the quartz sand (UI ) 1.5), the pore size distribution in the quartz sand-packed columns is wider. The average pore diameter is also expected to be smaller because of the angularity of the quartz sand (9). The larger number of smaller pores in the quartz sand-packed columns will yield more favorable locations for MWCNT physical removal mechanisms such as straining, which could explain the observed lower maximum normalized effluent concentrations and the difference between the experimentally observed data and model fits. Environmental Implications. This study has important implications related to the mobility of MWCNTs in sand filtration drinking water treatment systems as well as in subsurface porous media. At the higher flow rates employed in this study, similar to those that would be encountered in slow sand filters, the MWCNTs were quite mobile, suggesting that traditional filtration systems that do not incorporate additional treatment steps such as coagulation may not adequately remove MWCNTs. Under natural subsurface conditions, where pore water velocities would be in the lower range of those used in this study, the MWCNTs are substantially less mobile. It should be noted that the MWCNTs employed here were specifically engineered to be stable in aqueous solutions and thus possessed different physicochemical properties than unmodified nanotubes. Such nanotubes will likely be found in environmental systems after various applications that require higher stability in water such as those in the biomedical field (34) or also after nanotube interactions with humic substances, which have been shown to disperse nanotubes (35, 36).
Acknowledgments This research was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada and Ontario Ministry of the Environment, University of Michigan Graham Environmental Sustainability Institute, and U.S. EPA Grant RD833321 and R834094. Such support does not indicate endorsement by any sponsor. The authors also thank Nathalie Tufenkji, Scott Bradford, and Yusong Li for helpful advice.
Supporting Information Available MWCNT characterization and aqueous suspension preparation, column experiments, numerical model development, derivation of ηI, ηG, and ηD for MWCNTs and parameter fits. VOL. 43, NO. 21, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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This material is available free of charge via the Internet at http://pubs.acs.org.
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