Influence of Solution Chemistry on the Release of Multiwalled Carbon

Sep 30, 2013 - Zhiwei Wang , Xueye Wang , Junyao Zhang , Xueqing Yu , and Zhichao .... Chongyang Shen , Mengjia Zhang , Shuzhen Zhang , Zhan Wang ...
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Influence of Solution Chemistry on the Release of Multiwalled Carbon Nanotubes from Silica Surfaces Peng Yi and Kai Loon Chen* Department of Geography and Environmental Engineering, Johns Hopkins University, Baltimore, Maryland 21218-2686, United States S Supporting Information *

ABSTRACT: The release of multiwalled carbon nanotubes (MWNTs) that were deposited on silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were deposited on silica surfaces at elevated NaCl and CaCl2 concentrations before being rinsed with eluents of different solution chemistries to induce their remobilization. Energetically speaking, the MWNTs were released from the primary energy minimum when the background NaCl or CaCl2 concentrations were decreased at pH 7.1. The increase in electrostatic repulsion between MWNTs and silica likely caused a reduction in the energy barrier, which enabled the release of MWNTs. The degree of release increased in a stepwise fashion when the nanotubes were sequentially exposed to eluents of decreasing electrolyte concentrations, possibly due to the heterogeneity in nanotube surface charge densities. The degree of release via a successive reduction in NaCl concentration was lower at pH 4.0 than at 7.1 due to MWNTs and silica surfaces exhibiting a less negative surface charge at pH 4.0. Most of the deposited MWNTs were released when the pH was decreased from 7.1 to 4.0 at 1.5 mM CaCl2. This was attributed to the elimination of calcium bridging between the carboxyl groups on MWNTs and silanol groups on silica surfaces.

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INTRODUCTION Carbon nanotubes (CNTs) are increasingly used in the fields of materials science1−4 and electronics engineering5,6 due to their extraordinary stiffness and tensile strength3,7 and chiralitydependent electrical conductivities.7,8 With the production of CNTs escalating, it is anticipated that CNTs will be released into natural aquatic systems during the manufacture, use, and disposal of CNT-containing products.9 Some studies have reported that CNTs can exhibit toxic effects on human and mammalian cell lines, as well as on microorganisms.10−16 Therefore, it is important to understand the fate and transport of CNTs in aquatic environments to assess the risk of human exposure to CNTs. Deposition and release (or remobilization)17 of CNTs on environmental surfaces are two key processes that control the fate and transport of CNTs in surface waters and groundwaters. CNTs suspended in the aqueous phase are likely to deposit on the surfaces of rocks, sand grains, and soils under high ionic strength conditions.18,19 However, when the solution chemistry changes substantially, for instance, during a heavy rainfall or flooding event, the deposited CNTs may be released back into the aqueous phase. The deposition of CNTs on model environmental surfaces has been studied by several groups20−23 and is shown in these studies to be in qualitative agreement with the Derjaguin− Landau−Verwey−Overbeek (DLVO) theory, which states that the interaction between a colloid and a collector is the sum of their electrical double layer and van der Waals interactions.24 Jaisi et al.20 conducted column filtration experiments using © 2013 American Chemical Society

single-walled CNTs (SWNTs), and their results showed that the deposition rate coefficients of SWNTs on quartz sand increased with increasing KCl concentrations. Liu et al.21 reported that the mobility of multiwalled CNTs (MWNTs) in a quartz sand column increased when the ionic strength was decreased. In our earlier study using a quartz crystal microbalance with dissipation monitoring (QCM-D),22 the deposition kinetics of MWNTs on silica surfaces was found to increase with increasing NaCl and CaCl2 concentrations. Similarly, Hwang et al.23 observed using a QCM-D that the deposition kinetics of UV-irradiated MWNTs on silica surfaces increased when the NaCl concentration was raised. While the deposition of CNTs has been well-studied, investigations on the release of deposited CNTs are still limited. Jaisi et al.20 observed that SWNTs that were deposited in porous sand media in the presence of KCl were mobilized when eluted with deionized (DI) water. Tian et al.25 reported that less than 27% of MWNTs deposited in both quartz sand and glass bead media at 10 mM KCl in their column filtration experiments were released when the deposited MWNTs were exposed to DI water. In both studies, it was suggested that the CNTs were released from the secondary energy minimum while the possibility of CNTs being released from the primary Received: Revised: Accepted: Published: 12211

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energy minimum was not discussed.20,25 Several other studies, however, have shown that nanoparticles deposited in a primary minimum can be remobilized.26,27 These studies were performed using QCM-D systems that have flow cells with geometries that are similar to that of a parallel-plate flow chamber which allows for nanoparticle deposition to take place in the primary energy minimum and not in the secondary minimum.24 Conversely, retention of nanoparticles in porous media in column filtration experiments can occur through deposition in either the primary or secondary minimum. In the QCM-D study of Chen and Elimelech,26 fullerene (C60) nanoparticles, another class of carbonaceous nanomaterials, that were originally deposited on a flat silica surface were shown to be released from the primary energy minimum when exposed to an elevated pH of 12.3. Also, our recent investigation using the QCM-D provided evidence that MWNTs deposited on model biological membranes can be released from the primary energy minimum when the electrolyte concentration was decreased or when the solution pH was raised.27 In light of these findings, the reversibility of CNT deposition in the primary minimum on naturally occurring surfaces should be examined. In this study, the influence of electrolyte concentrations and cation types (Na+ and Ca2+), as well as solution pH, on the release of deposited MWNTs from silica, a ubiquitous surface in the environment, was investigated using a QCM-D system that only allows for nanotube deposition in the primary energy minimum. MWNTs were first deposited on silica surfaces under favorable deposition conditions and then exposed to eluents of different solution chemistries to induce the release of MWNTs. The deposited MWNTs were found to be released from the primary energy minimum when the NaCl or CaCl2 concentrations were sufficiently decreased. In addition, the effects of solution pH on the release of MWNTs were examined in the presence of NaCl and CaCl2.

dissipation response of the crystal sensors at nth harmonics (n = 1, 3, 5, 7, 9, 11, 13) as deposition and remobilization of MWNTs occurred on silica-coated crystal surfaces. Two of the four flow modules in the E4 system were arranged in a parallel configuration and used to obtain duplicate data for each experimental condition. A silica-coated 5 MHz AT-cut quartz crystal sensor (QSX303, Q-Sense) was mounted in each flow module. Before each QCM-D experiment, the crystal sensors were carefully cleaned as described in our previous publication.22 The flow modules were also thoroughly rinsed with 2% Hellmanex II cleaning solution (Hellma GmbH & Co. KG, Müllheim, Germany) and DI water (Millipore, MA). All electrolyte solutions were degassed through ultrasonication for 10 min (Branson 5510R-DTH, output power 135 W, frequency 40 kHz) and stored in a water bath at 27 °C, 2 °C above the experimental temperature, before use. Release of Deposited MWNTs from Silica Surfaces. To investigate the reversibility of MWNT deposition, MWNTs were first deposited on silica-coated crystal surfaces in the QCM-D system at either 1.50 mM CaCl2 or 600 mM NaCl, both at pH 7.1. Since these concentrations are higher than the critical deposition concentrations (CDC) in CaCl2 and NaCl (or the minimum electrolyte concentrations that allow for favorable deposition) which were determined to be 1.1 mM CaCl2 and 330 mM NaCl in our previous study,22 MWNTs underwent favorable deposition on silica surfaces under both conditions. These electrolyte concentrations are high enough to sufficiently neutralize (in the case of CaCl2) or screen (in the case of NaCl) the surface charges of the MWNTs and silica surfaces such that every close approach between a nanotube and silica surface will result in a permanent attachment (i.e., attachment efficiency = 1). After stable frequency and dissipation baselines had been obtained by rinsing the crystals successively with DI water and a 1.50 mM CaCl2 or 600 mM NaCl solution, MWNTs were deposited on the crystal surfaces in the same electrolyte solution for 80−120 min. During this deposition stage, a MWNT suspension prepared in DI water and the electrolyte solution were withdrawn separately and combined in a T-junction before they were introduced into flow modules using a peristaltic pump (ISM935C, Ismatec SA, Zürich, Switzerland). This approach was shown in our earlier study22 to significantly reduce the degree of concurrent aggregation of MWNTs. The flow rate of the combined suspension entering each flow module was 0.60 mL/min (±0.03 mL/min), which results in a laminar flow in the flow module.22 The flow rates of the branches delivering the MWNT suspension and electrolyte solution were equal (within 5% deviation from the average of both flow rates). The MWNT concentration in the combined suspensions entering the flow modules was ca. 0.5 mg/L. As MWNTs deposited on the crystal surface, the frequencies at all overtones of the crystal decreased, while the dissipations at all overtones increased simultaneously. Images of deposited MWNTs on silica surfaces were captured using a scanning electron microscope (SEM) (FEI Quanta 200 ESEM). The procedure for the preparation of MWNT samples for SEM imaging is provided in the SI. Following the deposition stage, the deposited MWNTs were first rinsed with the same electrolyte solution used for MWNT deposition and then rinsed with the eluents at the same flow rate to investigate the effects of solution chemistry on the release of deposited MWNTs. Depending on the ionic composition and pH of the rinse solutions, various degrees of MWNT release occurred, resulting in an increase in frequency

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MATERIALS AND METHODS Preparation and Characterization of MWNTs. The MWNTs used in this study were identical to the ones used in our previous studies.22,27,28 Briefly, the MWNTs were prepared by refluxing pristine MWNTs (NanoLab, Inc.) in a concentrated 3:1 (volume ratio) mixture of sulfuric acid (98% H2SO4 by mass) and nitric acid (69% HNO3 by mass) at 70 °C for 8 h. The oxidized MWNTs were cleaned through repeated cycles of ultracentrifugation and decantation of the supernatant and then dried overnight in an oven at 100 °C.22 Through Xray photoelectron spectroscopy (XPS) analysis,29 the predominant oxygen-containing surface functional groups of the oxidized MWNTs were determined to be the carboxyl groups.22 More details on MWNT characterization and preparation of MWNT stock suspensions are provided in the Supporting Information (SI). Solution Chemistry. ACS-grade NaCl and CaCl2 were used for preparing the stock solutions in this study. All stock solutions were filtered with 0.1 μm syringe filters (Anotop 25, Whatman, Middlesex, UK) before use. Most experiments were performed at pH 7.1 ± 0.2 (buffered with 0.15 mM NaHCO3). A few experiments were conducted at pH 4.0 ± 0.1 (adjusted with HCl) and at pH 10.0 ± 0.1 (adjusted with NaOH). The temperature for all experiments was maintained at 25 °C. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). A QCM-D system (E4, Q-Sense, Västra Frölunda, Sweden) was used to monitor the frequency and 12212

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response and concurrent decrease in dissipation response. The rinse processes were stopped when the shifts in the normalized frequency (Δf) and dissipation response (ΔD) at the fifth overtone were less than 0.3 Hz and 0.15 × 10−6, respectively, over a time period of 10 min. Quantifying Deposited and Released MWNTs with the Voigt-Based Model. In all our experiments, Δf at the different harmonics, as well as ΔD at the different harmonics, deviated considerably from one another when MWNTs were deposited on the crystal surface. Also, the ratios of dissipation shift to normalized frequency shift, ΔD/Δf, at all harmonics were considerably large (ca. 0.6 × 10−6/Hz). These two features have been observed for viscoelastic films, such as protein and DNA films.30,31 Hence, these observations suggest that the deposited MWNTs exhibited some characteristics of a viscoelastic layer. Therefore, instead of using the Sauerbrey relation32 that is valid only for rigid films, the Voigt-based model33 was employed to quantify the mass of MWNT layers on crystal surfaces. Experimentally obtained Δf and ΔD at the 5th, 7th, 9th, and 11th harmonics were fitted with the Voigtbased model using the mass, viscosity, and shear modulus of the MWNT layers as the fitting parameters (QTools 3 software, QSense). Since all of the experiments were conducted in aqueous solutions, the fluid density and viscosity were fixed at 1.00 × 103 kg/m3 and 1.00 × 10−3 kg/(m·s), respectively. The density of the MWNT layers was fixed at 1.05 × 103 kg/m3, which is a reasonable assumption based on the study of Lee and Cui.34

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RESULTS AND DISCUSSION Release of Deposited MWNTs at Decreased Electrolyte Concentrations. Figure 1a shows the modeled mass of deposited MWNTs when MWNTs were first deposited on a silica surface at 1.5 mM CaCl2 and then exposed to a 1 μM CaCl2 eluent, both at pH 7.1. The normalized frequency and dissipation responses at the 5th, 7th, 9th, and 11th harmonics, as well as the corresponding best-fitting frequency and dissipation shifts generated by the QTools 3 software, are presented in SI Figure S1. The mass of deposited MWNTs increased from 0 to 1.46 μg/cm2 when deposition took place at 1.50 mM CaCl2 (from 11 to 92 min). The deposited MWNTs were then rinsed with a 1.50 mM CaCl2 solution (from 92 to 122 min), during which no release of deposited MWNTs was observed. The inset of Figure 1a presents a SEM image of MWNTs deposited on a silica surface at 1.50 mM CaCl2 and pH 7.1. The image shows that, while the deposited MWNTs were mostly dispersed as individual strands, some surfacebound MWNT aggregates were present. When the MWNT layer was subsequently rinsed with a 1 μM CaCl2 eluent (from 122 min onward), however, 92.8% (standard deviation = 0.2%) of the deposited MWNTs was released from the silica surface. To determine if the release of MWNTs was due to the detachment of MWNTs from the silica surface or disaggregation (or breakage) of surface-bound MWNT aggregates, an experiment was conducted by first depositing a similar amount of MWNTs on a positively charged poly-L-lysine (PLL)-modified surface at 1.50 mM CaCl2 and pH 7.1 and then rinsing the deposited MWNTs with low ionic strength eluents that do not contain calcium ions, namely, 1 mM NaCl solution followed by DI water, both at pH 7.1 (Figure 2). The preparation of these PLL-modified surfaces has been described in our previous study.22 Since the MWNTs and PLL-modified surface are oppositely charged, MWNTs are not expected to detach from the PLL-modified surface under low

Figure 1. (a) Mass of deposited MWNTs on a silica surface during deposition at 1.50 mM CaCl2 and release at 1 μM CaCl2, both at pH 7.1. The inset shows a representative SEM image of MWNTs on a silica surface after deposition at 1.50 mM CaCl2 and pH 7.1. (b) Viscosity and shear modulus of the MWNT layer during the deposition and release processes. The inset illustrates the change in the conformation of deposited MWNTs when exposed to 1 μM CaCl2.

ionic strength conditions. On average, only 3.9% (standard deviation = 3.7%) of deposited MWNTs were released from the PLL-modified surfaces when the MWNTs were exposed to the eluents. This contrasting degree of MWNT release demonstrates that most of the MWNTs originally deposited on the silica surface were released through the detachment from the silica surface when exposed to the 1 μM CaCl2 eluent. The slight release from the PLL-modified surface is attributed to a small degree of disaggregation of the surface-bound MWNT aggregates. A similar deposition and release experiment was conducted in the presence of NaCl at pH 7.1, as shown in Figure 3. MWNTs were first deposited on a silica surface at 600 mM NaCl and pH 7.1 (from 11 to 84 min). The deposited MWNTs were then rinsed with the same electrolyte solution (from 84 to 125 min) and a 1 mM NaCl and pH 7.1 eluent (from 125 min onward). The significant differences in the densities and viscosities between the 600 and 1 mM NaCl solutions resulted in considerable shifts in both frequency and dissipation response when the 600 mM NaCl eluent was replaced by the 12213

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concentration (CCC), that is, the minimum electrolyte concentration that allows for favorable aggregation, of MWNTs in NaCl (210 mM) while the CaCl2 concentration used (1.50 mM) is comparatively closer to the CCC of MWNTs in CaCl2 (0.9 mM),22 the degree of later-stage aggregation that MWNTs underwent at 600 mM NaCl may be higher than that at 1.50 mM CaCl2, despite the similar early stage aggregation kinetics in both solution chemistries. The greater degree of later-stage aggregation of the MWNTs at 600 mM NaCl can result in a smaller diffusion coefficient of the MWNTs and, hence, slower nanotube deposition compared to deposition at 1.50 mM CaCl2. When the deposited MWNTs were rinsed with a 600 mM NaCl eluent, no significant change in deposited mass was observed, indicating that no release of MWNTs occurred under such conditions. When the MWNTs were subsequently rinsed with a 1 mM NaCl eluent, however, 91.0% (standard deviation = 8.5%) of the deposited MWNTs were released from the silica surface. Release of MWNTs from the Primary Energy Minimum. Since the geometry of the flow modules in our QCM-D E4 system is similar to that of a parallel-plate channel which does not allow for deposition in the secondary energy minimum,24 MWNTs were expected to undergo deposition in the primary energy minimum prior to the release process. Furthermore, since the deposition of MWNTs occurred at 1.50 mM CaCl2 and 600 mM NaCl, both concentrations higher than the CDCs of their respective electrolytes,22 no secondary minimum should exist at these favorable deposition conditions.24 Thus, during the release processes, the MWNTs were released from the primary minimum. In the theoretical work by Ruckenstein and Prieve,35 the authors considered Born repulsion and DLVO interactions, namely, electric double-layer and van der Waals interactions, in their calculation of the total interaction energy between a spherical colloid and a planar surface. Born repulsion is a shortrange interaction that originates from the interpenetration of electron clouds surrounding the atoms on a colloid and a planar surface when the separation between the colloid and the planar surfaces is less than ca. 0.5 nm.24,35,36 The authors showed that the inclusion of Born repulsion can result in a finite primary minimum in the interaction energy profile between a colloid and a planar surface,35 as illustrated in the inset of Figure 3. Conversely, exclusion of Born repulsion will result in an infinite primary minimum.37 In the presence of Born repulsion, the difference in energy level between the primary minimum and energy maximum is known as the energy barrier for the release of deposited colloids (indicated as A and B in the inset of Figure 3).35 Colloids deposited in the primary minimum will need to overcome this energy barrier to be released from the planar surface. For colloids and planar surface with charges of the same sign, as the surface charges or surface potentials of the colloids and planar surface increase in magnitude, the energy barrier for colloid release was predicted by Ruckenstein and Prieve35 to decrease (as illustrated by the change from A to B in the inset of Figure 3). A decrease in ionic strength was also calculated to reduce the energy barrier for colloid release if a constant surface charge assumption is made.35 In both scenarios, when the energy barrier for colloid release becomes sufficiently small, the colloids trapped in primary minimum can be readily released from the planar surface. The experimental results of this study are consistent with the predictions of Ruckenstein and Prieve.35 In the release

Figure 2. Mass of deposited MWNTs on a PLL-modified surface during deposition at 1.50 mM CaCl2 and release at 1 mM NaCl followed by DI water, all at pH 7.1.

Figure 3. Mass of deposited MWNTs on a silica surface during deposition at 600 mM NaCl and release at 1 mM NaCl, both at pH 7.1. The inset illustrates the variation of interaction energy, V, between a spherical colloid and a planar surface as a function of separation distance, s, based on the work of Ruckenstein and Prieve.35 The energy barrier for colloid release will decrease from A to B when the surface potentials or charges of the colloid and planar surface increase or when the ionic strength decreases if a constant surface charge assumption is made.

1 mM NaCl eluent. This buffer effect22,27 was thus corrected for, as described in the SI, before the Voigt-based model was used to analyze the experimental frequency and dissipation response. The experimental frequency and dissipation responses after the correction for the buffer effect and the best-fitting curves are presented in SI Figure S2. During the deposition stage at 600 mM NaCl, the mass of deposited MWNTs increased from 0 to 0.3 μg/cm2. Since the NaCl concentration employed for MWNT deposition (600 mM) is substantially higher than the critical coagulation 12214

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manner. Figure 4a shows the mass of deposited MWNTs when the MWNTs were first deposited on a silica surface at 1.50 mM

experiment in the presence of CaCl2 (Figure 1a), a decrease in CaCl2 concentration from 1.50 mM to 1 μM reduced the charge neutralization effect of Ca2+ cations and hence increased the surface charge densities of both MWNTs and silica surface. The enhancement of surface potentials of both MWNTs and silica surface is likely to significantly decrease the height of energy barrier for MWNT release and thus result in 93% of deposited MWNTs to be released from the silica surface. In the release experiment in the presence of NaCl (Figure 3), likewise, a reduction in NaCl concentration from 600 to 1 mM may have resulted in a decrease in the energy barrier, as predicted by Ruckenstein and Prieve,35 to cause a release of 91% of deposited MWNTs. Since the origin of surface charge of MWNTs and silica is the dissociation of surface functional groups (carboxyl and silanol groups, respectively) and since both carboxyl (pKa ∼ 3−422) and silanol groups (pKa ∼ 4−538) are fully dissociated at pH 7.1, the constant surface charge assumption is appropriate for this system.39,40 The qualitative agreement between the experimental results and theoretical predictions, despite the difference in the shape of colloids for both systems, suggests that a nanotube can potentially be modeled as a string of spherical beads (or nanoparticles) in the calculation of nanotube−planar surface interactions. Influence of Decreasing Electrolyte Concentrations on Viscoelastic Properties of Deposited MWNTs. The viscosity and shear modulus of the MWNT layers during the deposition and release of MWNTs from silica surfaces in the presence of CaCl2 and NaCl are presented in Figure 1b and SI Figure S2c, respectively. In both cases, the viscosity and shear modulus of MWNT layers remained constant during the deposition processes, indicating that the viscoelastic properties of MWNT layers were independent of the MWNT coverage on the crystal surface. During the release processes under reduced electrolyte concentrations, however, both the viscosity and shear modulus decreased, suggesting that the MWNT layers became less compact compared to the layers at high electrolyte concentrations. Since the carboxyl groups may not be uniformly distributed along a MWNT, the surface charge density may vary spatially along a nanotube. When the electrolyte concentration is high, it is likely that every segment along a MWNT strand can adhere firmly to the silica surface due to strong charge neutralization or charge screening effects, resulting in the MWNT layer to take a compact structure, as illustrated in the inset of Figure 1b. When the electrolyte concentration is decreased, only the nanotube segments with low surface charge densities are fully neutralized or screened, and they remained attached to the silica surface. Conversely, the segments with high surface charge densities were only partially neutralized or screened, thus resulting in the highly charged segments to be detached from the silica surface and become suspended in the aqueous phase (inset of Figure 1b). Hence, the MWNT layer takes a more extended conformation at low electrolyte concentrations. Other than the nonuniform distribution of nanotube surface charges, it is plausible that the change in the size distribution of the deposited MWNTs and their aggregates during the release process may also contribute to the drop in both the viscosity and shear modulus of the MWNT layers. Sequential Release of MWNTs from Silica Surfaces. To further investigate the influence of electrolyte concentration, as well as the effects of the type of counterions and solution pH, on the degree of MWNT release from silica surfaces, release experiments were performed by rinsing the deposited MWNTs with eluents of different solution chemistries in a sequential

Figure 4. (a) Mass of deposited MWNTs on a silica surface during deposition at 1.50 mM CaCl2 and pH 7.1 (Stage B) and sequential release at different solution chemistries. The deposited MWNTs were rinsed with a series of eluents (pH of all eluents is 7.1, unless otherwise indicated)1.50 mM CaCl2 (Stage C), 0.70 mM CaCl2 (Stage D), 0.40 mM CaCl2 (Stage E), 0.10 mM CaCl2 (Stage F), 0.01 mM CaCl2 (Stage G), 1 mM NaCl (Stage H), and DI water at pH 10 (Stage I). The inset shows the viscosity, η, and shear modulus, μ, of the MWNT layer on the silica surface during the deposition and release processes. (b) Cumulative fractions of deposited MWNTs that were released from the silica surface at the various stages of elution. Error bars represent standard deviations. *The cumulative fractions at Stages C and D are not detectable.

CaCl2 (Stage B) and then exposed to a series of eluents. The deposited MWNTs were first rinsed with 1.50 mM CaCl2 (Stage C) and 0.70 mM CaCl2 (Stage D) solutions, during which no release was observed. The MWNTs were subsequently rinsed with 0.40 mM CaCl2 (Stage E), 0.10 mM CaCl2 (Stage F), 0.01 mM CaCl2 (Stage G), and 1 mM NaCl (Stage H) solutions, followed by DI water at pH 10.0 12215

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in Figure 5, the cumulative fractions of deposited MWNTs that were released at each elution step (shaded bars) increased in a

(Stage I), during which release of MWNT was observed to occur in a stepwise fashion. The cumulative fractions of deposited MWNTs that were released from the silica surface increased from 0% in Stage C to 100% in Stage I, as presented in Figure 4b. There are three types of transitions in solution chemistry during the release process in Figure 4decreasing CaCl2 concentrations (Stage C to G), transitioning from CaCl2 to NaCl (Stage G to H), and increasing pH (Stage H to I). All three transitions were expected to lead to an increase in the magnitude of surface charge of both MWNTs and silica surface. From Stage C to G, decreasing CaCl2 concentrations from 1.50 mM to 0.01 mM reduced the charge neutralization effect by Ca2+ cations and thus enhanced the negative surface charge of both MWNTs22 and silica. When the 0.01 mM CaCl2 eluent was replaced by the 1 mM NaCl eluent (Stage G to H), the charge neutralization effect was eliminated when Ca2+ ions in the solution were replaced by Na+ ions, further increasing the surface charge of both MWNTs22 and silica. Finally, a rise in the solution pH from 7.1 to 10.0 (Stage H to I) caused an additional enhancement in the surface charge of both MWNTs and silica surface due to the increase in both deprotonated carboxyl groups on the MWNTs and deprotonated silanol groups on the silica surface. It is conceivable that the partial release of MWNTs in Stages E−H may be due to the MWNTs having a distribution of surface densities of the carboxyl groups.27 The released MWNTs may have higher surface densities of carboxyl groups compared to the MWNTs that remain deposited to the silica surface. The MWNTs that remained deposited on the silica surface during each switch in eluent may require a stronger surface charge enhancement in order for the energy barrier to decrease sufficiently for the nanotubes to be released from the primary minimum. Nonetheless, we do not rule out other factors, such as the heterogeneity in the size and conformation of MWNT aggregates and uneven distribution of silanol groups on silica surfaces, which may also contribute to the different propensities for the MWNTs to be released. The inset of Figure 4a presents the viscosity and shear modulus of the MWNT layer during the sequential release experiment. Both the viscosity and shear modulus remained reasonably constant from Stage B to E, indicating that the compactness of the MWNT layer did not change significantly when the CaCl2 concentrations were decreased from 1.50 to 0.40 mM. The viscosity and shear modulus, however, decreased noticeably in a stepwise fashion from Stage F to H, suggesting that the structure of the remaining MWNT layer became looser with each variation in solution chemistry. The decreasing viscosity and shear modulus were probably due to the enhanced electrostatic repulsion between the deposited MWNTs and silica surface, which resulted in the MWNT layer to take a more extended conformation. To investigate the influence of ionic strength in the absence of charge neutralization effects (i.e., in the absence of Ca2+ ions) on the degree of release of deposited MWNTs, the MWNTs were first deposited on silica surfaces at 600 mM NaCl, and the deposited MWNTs were rinsed with a 600 mM NaCl solution. The deposited MWNTs were then successively rinsed with 300 mM NaCl, 100 mM NaCl, and 1 mM NaCl solutions, all prepared at pH 7.1. The mass of deposited MWNTs during the release processes was derived through Voigt-based modeling after corrections for the buffer effects were made using the method described in the SI. As presented

Figure 5. Influence of pH of elution solutions on the cumulative fractions of deposited MWNTs that were released from a silica surface at the various stages of release after deposition at 600 mM NaCl and pH 7.1. Error bars represent standard deviations. *For both pH 4.0 and 7.1, the cumulative fractions at 600 mM NaCl are not detectable.

stepwise manner as the NaCl concentrations were decreased from 600 to 1 mM. The partial release of MWNTs at NaCl concentrations of 300, 100, and 1 mM may be due to the heterogeneity in surface charge densities of the MWNTs. The MWNTs that remained deposited during each switch in eluent may require a more drastic decrease in surface charge screening through the reduction in ionic strength in order for the energy barrier to be small enough for them to be released. Similar to earlier results, this observation is consistent with the predictions by Ruckenstein and Prieve35 which showed that a decrease in ionic strength can result in a reduction in the height of the energy barrier for colloid release for colloids that carry a constant surface charge. Influence of pH on the Degree of MWNT Release from Silica Surfaces. To investigate the effects of the degree of dissociation of surface functional groups on the release of MWNTs, the degrees of MWNT release from silica surfaces with decreasing NaCl concentrations were compared at pH 4.0 and pH 7.1 (Figure 5). The procedure for the release experiment conducted at pH 4.0 was similar to that for the release experiment at pH 7.1 (presented in preceding section). The MWNTs were first deposited on a silica surface at 600 mM NaCl and pH 7.1. The deposited MWNTs were then rinsed consecutively with 600 mM NaCl (pH 7.1) and 600 mM NaCl (pH 4.0) solutions, and no release of deposited MWNTs was observed with the drop in pH. Afterward, the deposited MWNTs were successively rinsed with 300 mM NaCl, 100 mM NaCl, and 1 mM NaCl solutions, all prepared at pH 4.0. The cumulative fractions of deposited MWNTs that were released at the various elution stages at pH 4.0 was compared to the fractions determined at pH 7.1 (Figure 5). For all elution stages, the degree of MWNT release was smaller at pH 4.0 (black bars) than at pH 7.1 (shaded bars). We determined the electrophoretic mobility (EPM) of the MWNTs (ZetaPALS, Brookhaven Instruments Corp., Holtsville, NY) at 1 mM NaCl and pH 4.0 to be −1.40 × 10−8 m2/(V s), which is noticeably less negative than their EPM at 1 mM NaCl and pH 7.1 (= 12216

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−3.36 × 10−8 m2/(V s)).22 This result shows that considerably fewer carboxyl groups on the MWNTs were deprotonated at pH 4.0 than at pH 7.1. The weaker electrostatic repulsion between the less negative charged MWNTs and silica surfaces at pH 4.0 compared to pH 7.1 expectedly resulted in a larger energy barrier for colloid release and therefore in a smaller degree of nanotube release. Conversely, the degree of dissociation of surface functional groups had a dissimilar effect on the release behavior of deposited MWNTs in the presence of CaCl2. As shown in Figure 6, MWNTs were first deposited on a silica surface at

electrostatic attraction or ionic bonding between Ca2+ cations and negatively charged functional groups.41 Thus, Ca2+ cations may bridge the deprotonated carboxyl groups on deposited MWNTs and ionized silanol groups (Si−O−) on silica surface at pH 7.1. When the solution pH was decreased from 7.1 to 4.0, a considerable fraction of the silanol groups became protonated, as shown by the almost neutral EPM measured for the silica colloids at pH 4.0. As a result, the Ca2+ cations cannot form complexes with the un-ionized silanol groups, and the loss of the bridging effect may lead to the detachment of MWNTs from the silica surface at pH 4.0. In addition, the lack of bridging between MWNTs and silica surfaces at pH 4.0 may have resulted in the MWNT layer to become less compact when the pH was decreased, as indicated by the decline in shear modulus and viscosity (inset of Figure 6). Environmental Implications. To investigate the propensity for MWNTs to be released back into the bulk solution once they have been deposited on silica surfaces, the deposited MWNTs are exposed in this study to eluents of varying NaCl and CaCl2 concentrations and pH conditions, and the degree of MWNT release is determined using a QCM-D. MWNTs that are initially deposited in a primary energy minimum can be released from a silica surface when the NaCl and CaCl2 concentrations in the bulk solution are reduced or when the solution pH is elevated (pH = 10). Both transitions in solution chemistry are thought to reduce the energy barrier for nanotube release. The results in this study will enable a better prediction of the mobility and transport of these carbonaceous nanomaterials in freshwater systems and subsurface environments. These findings will also allow for a better understanding of the fate of retained MWNTs in granular media filters in water treatment plants during the backwashing process. In addition to solution chemistry, further studies are needed to investigate the roles of surface chemistry and surface charge of the environmental surface, surface functionality of MWNTs, and surface coatings on the minerals or MWNTs (e.g., natural organic matter and synthetic stabilizing agents, such as polyelectrolytes) on the remobilization of deposited MWNTs.

Figure 6. Mass of deposited MWNTs on a silica surface during deposition at 1.50 mM CaCl2 and pH 7.1 (Stage B) and release at 1.50 mM CaCl2 and pH 4.0 (Stage D). The system was rinsed with a 1.50 mM CaCl2 and pH 7.1 solution in Stages A and C. The inset shows the viscosity, η, and shear modulus, μ, of the MWNT layer on the silica surface during the deposition and release processes.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Additional figures and details for materials and methods. This material is available free of charge via the Internet at http:// pubs.acs.org.

1.50 mM CaCl2 and pH 7.1. The deposited MWNTs were then rinsed with a 1.50 mM CaCl2 and pH 7.1 solution, during which no release of MWNTs took place. When the deposited MWNTs were subsequently rinsed with a 1.50 mM CaCl2 and pH 4.0 eluent, however, 89.9% (standard deviation = 6.0%) of deposited MWNTs was released, in contrast to the absence of observable release when the pH was reduced from 7.1 to 4.0 in the presence of 600 mM NaCl. While the EPMs of MWNTs at pH 7.122 and pH 4.0, both in 1.50 mM CaCl2, were determined to be very similar (ca. −1.2 × 10−8 m2/(V s)), the EPMs of silica colloids (Bangs Laboratories, Inc., IN), which were used as surrogates for the QCM-D silica surfaces, increased dramatically from −3.15 × 10−8 m2/(V s) at pH 7.1 to −0.17 × 10−8 m2/(V s) at pH 4.0. Thus, the significant release of deposited MWNTs when the pH was decreased from 7.1 to pH 4.0 in the presence of 1.50 mM CaCl2 was not expected to be due to the enhancement of electrostatic repulsion between the MWNTs and the silica surface. Calcium cations have been reported to bridge deprotonated carboxyl groups on organic matter and ionized OH groups (−O−) on mineral surfaces (e.g., goethite, aluminosilicates, and kaolinite) through complex formation that involve either

Corresponding Author

*E-mail: [email protected]. Phone: (410) 516-7095. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Semiconductor Research Corporation (award 425-MC-2001, project 425.041). We acknowledge Dr. Michael McCaffery from the Integrated Imaging Center at the Johns Hopkins University for performing the SEM imaging.

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REFERENCES

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