Single-Walled Carbon Nanotube Transport in Representative

Article pubs.acs.org/est

Single-Walled Carbon Nanotube Transport in Representative Municipal Solid Waste Landfill Conditions Iftheker A. Khan,† Nicole D. Berge,†,* Tara Sabo-Attwood,‡ P. Lee Ferguson,§ and Navid B. Saleh† †

Department of Civil and Environmental Engineering, University of South Carolina, Columbia, South Carolina 29208, United States Department of Environmental and Global Health, University of Florida, Gainesville, Florida 32610, United States § Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina 27708, United States ‡

S Supporting Information *

ABSTRACT: Single-walled carbon nanotubes (SWNTs) are being used in many consumer products and devices. It is likely that as some of these products reach the end of their useful life, they will be discarded in municipal solid waste landfills. However, there has been little work evaluating the fate of nanomaterials in solid waste environments. The purpose of this study is to systematically evaluate the influence of organic matter type and concentration in landfill-relevant conditions on SWNT transport through a packed-bed of mixed municipal solid waste collectors. The influence of individual waste materials on SWNT deposition is also evaluated. Transport experiments were conducted through saturated waste-containing columns over a range of simulated leachate conditions representing both mature and young leachates. Results indicate that SWNT transport may be significant in mature waste environments, with mobility decreasing with decreasing humic acid concentration. SWNT mobility in the presence of acetic acid was inhibited, suggesting their mobility in young waste environments may be small. SWNTs also exhibited collector media-dependent transport, with greatest transport in glass and least in paper. These results represent the first study evaluating how leachate age and changes in waste composition influence potential SWNT mobility in landfills.

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INTRODUCTION

To date, transport and deposition of carbonaceous nanomaterials (e.g., SWNTs,9 MWNTs,10 fullerenes,11 and their derivatives,12 graphene and its oxides13) have mostly focused on relatively controlled systems containing various singular porous media (e.g., sands, glass beads, and/or a selected set of soils)14 and in presence of a narrow range of organic matter (≤25 mg/L)14 and background electrolyte concentrations (≤100 mM ionic strength).14 However, such results cannot be effectively used to assess potential nanomaterial fate in MSW landfills. Solid waste environments differ substantially from those evaluated in these previously conducted studies. Typical MSW is heterogeneous, containing both organic and inorganic constituents and encompass a large range of particle sizes and surface chemistries that change with waste age and decomposition. Landfill leachate composition is also inherently more complex than solutions used in previously conducted transport experiments. Leachate is generally high in both organic matter content (ranging from 30−29 000 mg/L as TOC) and salt concentrations (e.g., chloride: 150−4500 mg/L), each changing with time and waste degradation.15

Single-walled carbon nanotubes (SWNTs) and their derivatives are emerging as one of the most commercially important and technologically relevant engineered nanomaterial (ENM) in research and consumer industries. Their unique physicochemical properties allow for molecular level manipulation and thus have enabled them to be used in a variety of common consumer products, including sporting goods (e.g., tennis rackets, bicycle parts), compact discs (CDs), cookware, water filters, and plastics.1−3 As these products reach the end of their useful life, it is likely that they will be discarded with household garbage and ultimately be placed within municipal solid waste (MSW) landfills. Nanomaterial release from these products under conditions typically found in landfills is likely.4,5 Life cycle assessment studies estimate that approximately 40%6 to 78%7 of carbon nanotubes (CNTs) produced will ultimately be discarded in MSW landfills. Implications associated with potential SWNT releases from landfills are significant. SWNTs are known to have deleterious environmental impacts, as demonstrated via decreases in bacterial viability in aquatic organismal uptake and toxicity studies.8 A current lack of understanding associated with SWNT transport in solid waste environments represents a critical data gap that requires careful evaluation. © 2013 American Chemical Society

Received: Revised: Accepted: Published: 8425

April 21, 2013 June 27, 2013 July 1, 2013 July 1, 2013 dx.doi.org/10.1021/es401748f | Environ. Sci. Technol. 2013, 47, 8425−8433

Environmental Science & Technology

Article

mM NaCl. Additional details associated with the procedures used to prepare the synthetic leachates can be found in the SI. Porous Media. Representative waste materials were mixed at proportions typically found in MSW landfills.5 Composition of the mixed MSW is (% wt.): 45.5% paper (punched foam board with 7 mm diameter and 5 mm thickness), 9.6% glass (2.5 mm diameter glass beads), 10.9% metal (shredded discarded aluminum cans of 5 mm length and 3 mm width), 16.4% plastic (6 mm diameter plastic beads), and food surrogate (rabbit food pellets, 10 mm long and 4 mm dia.). The paper material is clay-coated and likely contains appreciable cationic polyelectrolytes that are often used to increase clay deposition (e.g., poly(ethylene imine)).26 Because the paper material is fibrous in nature, it is inherently rougher than the other media used. The glass beads are smooth and made from soda-lime glass. The metal cans are also smooth and composed of typical aluminum alloy used in the United States for beverage cans (∼97% aluminum, with smaller percentages of iron (∼0.4%), magnesium (1%), and manganese (∼1%)). The plastic beads are also smooth. The exact chemical composition of the beads is unknown, but it is likely they contain amino resins or cellulosics (e.g., cellulose acetate, cellulose nitrate). The food surrogate (rabbit food) is comprised of 16% protein, 2.5% crude fat, 20% fiber, 2.5% calcium, 0.5% phosphorus, 0.75% salt, and 0.3% sodium. The representative waste materials are shown in Figure S1 of the SI. Column experiments were conducted with both mixed MSW and individual waste materials except food. A column of only rabbit food presented significant operational issues. The food cemented when exposed to water, reducing achievable water flow through the column and prevented the ability to conduct transport experiments (Figure S2 of the SI). Transport Experiments. One-D column experiments were conducted in glass chromatography columns of 2.5 cm inner diameter (Kimble Chase, NJ, U.S.) to evaluate SWNT transport. The column packing, saturation, and subsequent tracer and SWNT transport experiments were conducted following that described by Lozano and Berge.5 Specific details associated with column packing and saturation, as well as the tracer tests can be found in the SI. Prior to introduction to the column, SWNTs were dispersed in the appropriate background leachate solution and subjected to sonication for 1 h at an amplitude of 50 (Misonix S-4000, Farmingdale, NY) with a 3/4 in. horn in an ice-bath. An average ultrasonication output energy of 115 kJ (equivalent to 54.9 kJ actual system input) was maintained for 150 mL stock solution.27 HA and AA concentrations prior to and following sonication were measured and indicate that there was no measureable change in the organic concentration as a result of sonication. Prior to the start of all column experiments (and subsequent to tracer tests, see SI for additional details), the columns were flushed with the leachate background solution (e.g., HA or AA concentration with 200 mM NaCl) for a period of at least 4 h. Column experiments were not begun until the effluent HA (or AA) concentrations were constant and equal to that of the influent HA (or AA) concentrations (see Figure S3 of the SI). This procedure resulted in the coating of all waste materials with the organic present in the synthetic leachate solution, and served to minimize desorption of HA from the SWNTs, likely eliminating interaction with the bound organics on the SWNTs and the collector media. The transport experiments were performed via a three-step procedure9 (Table S1 of the SI) to

A systematic evaluation of the role of changing landfill conditions on nanomaterial transport is currently lacking. Smaller molecular weight organics, such as volatile fatty acids (>95% of the dissolved organic matter), characterize young leachates, whereas high concentrations of refractory dissolved organic matter (30−60% of the dissolved organic matter), usually in the form of humic and/or fulvic acid, are predominant in mature leachates.15 To date, the only study evaluating mobility of pristine SWNTs in mixed MSW under mature leachate conditions (400 mg TOC/L humic acid and 200 mM NaCl) showed high SWNT mobility, indicating a need for further systematic evaluation.5 It is well-known that type and composition of organic matter influences stability of carbonaceous nanomaterials due to their unique interfacial interaction.16−18 Properties of collector surfaces, which present heterogeneity in electrokinetics,12,19,20 surface roughness,21 and pore structure22,23 also influence the fate of nanomaterials through packed-bed systems. A systematic evaluation of SWNT fate in the unique organic composition found in leachate and through complex MSW mixed-media is imperative to better understand material fate upon release in end-of-life environmental scenarios. This work presents a systematic study of SWNT transport through representative MSW (a complex collector system) and individual waste materials under saturated conditions. Acid functionalized SWNTs were dispersed in solutions representative of young (acetic acid) and mature (humic acid) leachates, with an ionic strength of 200 mM NaCl. Detailed characterization of SWNTs was performed using Raman spectroscopy, transmission electron microscopy (TEM), and electrophoretic mobility (EPM) measurements. Saturated column experiments containing representative MSW collectors were performed to evaluate and quantify the: (1) influence of organic type (humic and acetic acid) and concentration on SWNT transport and (2) influence of collector type (e.g., individual waste material) on SWNT deposition. Results from this work will provide key insights into potential SWNT mobility in landfills and environmental implications associated with these end-of-life conditions.

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MATERIALS AND METHODS Preparation and Characterization of SWNTs. Pristine CoMoCat SWNTs (CG200: lot no. 400) was purchased from SouthWest NanoTechnologies Inc. (SWeNT, Norman, OK, U.S.). The manufacturer reports an average diameter of 1.012 nm. The SWNTs were covalently functionalized via acid etching;24 details associated with the etching are described in the Supporting Information, SI. TEM and Raman spectroscopy were performed to study SWNT morphology changes upon covalent functionalization. Specific details associated with the SWNT preparation and associated characterization techniques can also be found in the SI. Synthetic Leachate. Humic acid (HA, Acros Organics) and acetic acid (AA, Acros Organics) were used as surrogates for organics present in mature and young leachate, respectively. HA concentrations ranging from 10 to 400 mg TOC/L and one AA concentration (400 mg TOC/L) were evaluated to assess the role of organic type (correlating to differences in leachate age) and concentration on SWNT transport. Acetic acid was chosen to represent the organics found in young leachates because it is often the dominant acid, and it has been previously used to simulate organics in young leachate.25 The ionic strength of all synthetic leachates was maintained at 200 8426

dx.doi.org/10.1021/es401748f | Environ. Sci. Technol. 2013, 47, 8425−8433


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Table 1. Conditions Employed for SWNT Dispersion Transport Experiments Experiment no. Porous media Organic Organic conc. (mg TOC/L) Ionic strength (mM NaCl) Dispersion EPM (10−8 m2 V−1 S−1) Packed length (cm) SWNT conc. added (mg/L) Column pore volume (mL) Porosity Avg. travel time (h) Pore velocity (m/h) SWNT introduced (PV) Effluent SWNT recovery (% wt) SWNT retained (% wt)a SWNT release (Step C) (% wt. of eluted particles)b

I

II

III

IV

V

VI

VII

VIII

IX

Mixed MSW HA 400 200 −2.95 ± 0.54 8.02 22.20 11.40 0.29 0.19 0.42 7.89 69.46 30.54 BDL

Mixed MSW HA 200 200 −2.64 ± 0.44 8.02 22.13 11.40 0.29 0.19 0.42 7.89 62.62 37.38 BDL

Mixed MSW HA 50 200 −2.94 ± 0.56 8.02 21.80 11.40 0.29 0.19 0.42 7.89 49.39 50.61 0.15

Mixed MSW HA 10 200 −2.93 ± 0.54 8.02 21.73 11.40 0.29 0.19 0.42 7.89 8.19 91.81 18.57

Mixed MSW AA 400 200 −1.81 ± 0.39 8.02 22.33 11.40 0.29 0.19 0.42 7.89 5.75 94.25 6.30

Paper

Metal

Plastic

Glass

HA 50 200 −2.66 ± 0.41 8.02 21.73 11.20 0.28 0.19 0.43 8.04 19.51 80.49 4.73

HA 50 200 −2.72 ± 0.39 8.02 23.53 35.40 0.90 0.59 0.14 2.54 76.67 23.33 BDL

HA 50 200 −2.67 ± 0.42 8.02 21.07 20.10 0.51 0.34 0.24 4.48 87.65 12.35 BDL

HA 50 200 −2.63 ± 0.40 8.02 21.60 19.00 0.48 0.32 0.25 4.74 92.61 7.39 4.47

a Calculated based on effluent and influent SWNT mass; brelease is based on recovery; BDL=below detection limit; HA=humic acid; AA=acetic acid; TOC=total organic carbon.

simulate SWNT deposition and release. During the first step (Step A: Deposition), the leachate solution (at the desired concentration of either HA or AA) containing 200 mM NaCl and 22 ± 0.6 mg/L SWNTs was introduced at the bottom of the column at a flow rate of 1 mL/min with a syringe pump (NE-300, New Era Pump Systems, Inc.). The effluent SWNT concentration was monitored every 30 s via UV−vis spectroscopy at a wavelength of 800 nm. Previous studies have confirmed linear UV-response of SWNTs in HA at this wavelength.5,16,18 Next (Step B: Rinse), a SWNT free leachate solution (at the composition added during step A) was passed through the column for approximately 2 h. To assess potential release of deposited SWNTs (Step C: Release), the HA or AA solution with no background NaCl was subsequently passed through the column for at least 1 h. Further details are listed in Table S1 of the SI. The percentage of SWNTs retained and eluted from the column were determined via mass balances. Table 1 contains a detailed list of all experimental conditions, as well as results from the column experiments. Characterization of Influent and Effluent SWNTLeachate Dispersions. Relative stability and EPM of all SWNT-leachate dispersions passed through the solid waste columns were evaluated. In addition, both influent and effluent SWNT hydrodynamic radii from the column were evaluated over time using dynamic light scattering technique. Details associated with these techniques can be found in the SI.

quantified via the ratio of the peak responses of the defect band or “D-band” (near 1310 to 1330 cm−1) and the characteristic graphitic or “G-band” (near 1590 cm−1) (Figure S5 of the SI). The results confirm that surface oxidation was successfully achieved via acid etching, as represented by the increase in D/G from 0.11 ± 0.02 to 0.17 ± 0.08 for pristine and functionalized SWNTs, respectively. Literature findings for mechanochemically or strong oxidant treated SWNTs concur with the D/G presented here.17,24 Increase in D/G upon functionalization indicates C−C bond breakage and subsequent formation of C− O and O−CO functional groups. Such functional group presence was similarly observed for MWNTs.28 SWNT Stability in Representative Young and Mature Leachate. SWNT stability is highly influenced by leachate chemical composition as well as its relative concentration. The settling profiles (Figure S6 of the SI) demonstrate that the presence of HA in the range of 50−400 mg TOC/L effectively stabilizes the SWNTs, as shown by the least amount of settling over the 12 h period. Significant SWNT settling was observed within the first 2 h when suspended in a solution of 10 mg TOC/L of HA and 400 mg TOC/L of AA. These settling profiles indicate a lack of SWNT stability. At HA concentrations ranging from 50−400 mg TOC/L, influent SWNTs possess similar average hydrodynamic radii over time, ranging from 245 ± 14 to 316 ± 51 nm (Figure 1, Table S2 of the SI). A stable average hydrodynamic radii (Figure S7 of the SI) was not achieved in the presence of 400 mg TOC/L AA for a range of electrolyte concentrations (10− 200 mM NaCl), as evidenced by the large variation in SWNT hydrodynamic radii (0.8−48.2 μm) and significant scatter. However, in the presence of 200 mg TOC/L of HA, enhanced SWNT stability was observed (Figure S7f of the SI) with a stable average hydrodynamic radius of ∼250 nm. The inability of AA to effectively disperse SWNTs is consistent with previous literature findings.29 Such a lack in stability and higher settling in the presence of AA is likely due to lower electrokinetic interaction as observed from EPM measurements (Figure S8 of the SI and Table 1). Changes in HA concentration did not alter EPM values significantly.

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RESULTS AND DISCUSSIONS Morphological and Chemical Characteristics of SWNTs. Representative TEM micrographs are presented in Figure S4 of the SI showing bundling and clustering state of the SWNTs. The pristine SWNTs were observed to be heavily bundled (Figure S4a of the SI). Significant debundling occurred upon surface oxidation (Figure S4b of the SI). Moreover, dark spherical features are observed for pristine case (Figure S4a of the SI), which appear to be absent for the acid treated SWNTs (Figure S4b of the SI). Similar debundling has been observed upon acid functionalization in previously reported studies.17,24 Raman spectroscopic measurements show higher extent of defects on SWNTs upon functionalization. Extent of defect is 8427



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Figure 1. Average hydrodynamic radius (HR) of influent and effluent SWNTs from experiments with mixed MSW (Experiments I−V, Table 1). DLS experiments were conducted at humic acid (HA) concentrations of (a) 400, (b) 200, (c) 50, and (d) 10 mg TOC/L and at an acetic acid (AA) conc. of (e) 400 mg TOC/L. The bar chart indicates eluted SWNTs from column; sample was collected for every 10 min (equivalent to 0.9 PV) and DLS was run within 10 min of sample collection; details in Table S2 of the SI. The red line indicates stock SWNTs dispersion injected to column; DLS was run at regular intervals during Step A. HR of background HA alone was in the range of 33 ± 4 to 45 ± 2 nm while for AA alone was 0.0 nm. Error bar indicates one standard deviation considering DLS run of 5−7 min with 15 s interval for every data point. DLS measurements were carried out at a room temperature of 20 °C under solution conditions of 200 mM NaCl and an adjusted pH of 7.0 ± 0.01.

Effect of HA Concentration on SWNT Transport. Results from SWNT transport experiments conducted for a range of HA concentrations (10−400 mg TOC/L) in mixed MSW are presented in Figure 2 and Table 1. Overall, HA concentration significantly influences particle mobility. SWNT breakthrough profiles at HA concentrations ranging from 50− 400 mg TOC/L follow those of the conservative tracer. Effluent SWNT concentrations never reach that of the influent. Particle retention decreases with increasing HA concentration (Figure 2a). SWNT mobility shows a notable decrease at 10 mg TOC/ L HA (Figure 2b). The maximum recovery of SWNTs in the column effluent was measured at a HA concentration of 400 mg TOC/L (69%) and the lowest at 10 mg TOC/L (8%).

These results are supported by the SWNT stability and settling profiles. A notable observation associated with results at HA concentrations of 50 and 10 mg TOC/L is that the SWNTs deposited during the breakthrough experiments were remobilized or released when the background electrolyte concentration decreased (Step C: HA only, salt- and SWNT-free, Figure 2). A larger SWNT release or remobilization occurs for 10 mg TOC/L (i.e., 19% of recovered SWNTs), when compared to the release from the 50 mg TOC/L case (i.e., 0.2% of recovered SWNTs). Such release signifies weak attachment of a small fraction of the SWNTs to the collector surfaces that can be released upon perturbation of the 8428



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Figure 2. SWNT breakthrough profiles from experiments in mixed MSW at various humic acid concentrations: (a) 400, 200, and 50 mg and (b) 10 mg TOC/L. Red circles indicate tracer (LiBr) breakthrough curves. C/C0 indicates the normalized SWNT concentration; C0 and C represent influent and effluent SWNTs concentrations, respectively. Column saturation was done before PV = 0.0; details in the SI Table S1. Step A: HA + 200 mM NaCl + SWNTs, Step B: HA + 200 mM NaCl, and Step C: HA only. Insets depict SWNT release following elution with only HA solution. Experimental parameters relevant to column experiments are listed in Table 1.

graphene oxide) was reported.13 The retained particles in the inlet were found to be larger when compared to the particles deposited along the rest of the column.13 The filtration mechanism at the inlet was identified as physical straining.13 Similarly, size analyses of influent and effluent titania were used to decipher roles of particle flow and physical straining.37 Filtration mechanisms were further probed by analyzing the size data of the effluent SWNTs. Careful evaluation of the eluted SWNT sizes (Table S2 of the SI) provides key indirect insight into the filtered fraction of the samples. Figure 1 illustrates an interesting trend of aggregated eluted SWNTs when compared to the influent size data (red reference lines). As the HA concentration decreases from 400 to 200 mg TOC/ L and lower, the eluted SWNTs are relatively larger than the influent SWNTs. Effluent SWNTs smaller than those entering the column were only observed at a HA concentration of 400 mg TOC/L. The observation of larger effluent SWNTs at HA concentrations of 200 and 50 mg TOC/L was unexpected. It is important to note that the EPM values (Figure S8 of the SI) show little difference in the electrostatic contribution at these different HA concentrations. We hypothesize that although SWNTs are stable at lower HA concentrations (as low as 50 mg TOC/L), the HA is not sufficient to preserve the stability during transport through the tortuous packed-bed of collectors. Either higher collisions in the mixed MSW matrix or presence of cationic polyelectrolytes associated with the paper (will be discussed in more detail in the subsequent sections) initiated SWNT aggregation during transport and ultimately resulted in larger effluent SWNTs. The SWNT effluent size data for 10 mg TOC/L HA shows a bimodal distribution. In the early stages of transport, smaller sized fractions were eluted; whereas larger fractions eluted from the column at later times. The effluent SWNT size data corroborates the bimodal breakthrough profile at this condition (Figure 2b). The increase in aggregate size for the latter time period may be associated with the aforementioned aggregation mechanism of the SWNTs. Steric stabilization of SWNTs in presence of HA has been previously reported in the literature.17,38−41 A significant reduction in attachment efficiency with only 2.5 mg TOC/L Suwannee River HA with 300 mM NaCl was attributed to steric repulsion.17 Similar stability enhancement at 1−5 mg TOC/L HA with 160 mM NaCl was observed elsewhere.42

background electrolyte concentrations. No observable SWNT release was detected at HA concentrations ranging from 200− 400 mg TOC/L. Weak attachment of SWNTs in controlled single-collector systems has been previously reported in the literature. Jaisi et al.9 report that SWNT release increases with increases in ionic strength. Similarly, the observation of enhanced SWNT mobility in presence of organic matter is consistent with the previously published reports.9,30 Transport Mechanisms. The key filtration mechanism when in the presence of HA is likely electrosteric stabilization of SWNTs, leading to combined physical and chemical filtration of the tubes by the complex MSW media collectors. During pre-equilibration of the column with HA/AA, the organic molecules likely coat the MSW media surfaces and thus mostly control the electrostatic behavior of the collector surfaces. The supporting SWNT characterization data clearly indicate a relative inadequacy of HA concentration, at or below 10 mg TOC/L, that causes a decrease in stability (Figure S6 of the SI) and results in substantial SWNT filtration. It is acknowledged that deconvoluting physical from chemical filtration mechanisms at such a high background electrolyte concentration is difficult. However, it can be concluded that most, if not all of the tubes filtered chemically at HA concentrations ranging from 200−400 mg TOC/L were at their primary minima and thus irreversibly deposited to the collectors. Partial release (Step C) of SWNTs only occurred at HA concentrations of 50 and 10 mg TOC/L, indicating a reversible deposition of a fraction of the SWNTs. Such deposition occurred under secondary minima interaction energies of the tubes and the collector surfaces.31,32 The mechanisms of SWNT filtration have previously been probed for low electrolyte conditions (0.1 mM NaCl and 1 mM KCl), where filtration due to electrostatic screening is negligible.9,33 The key filtration mechanism in these studies was identified as physical removal.9 However, deciphering mechanisms of filtration at elevated electrolyte levels is difficult and thus mostly unexplored, except for a few studies attempting to probe with retention profile along the column length13,34−36 and eluted size analyses.37 Greater retention of particles (e.g., graphene oxide,13 ferihydrite nanoparticle,35 iron nanoparticle,34 and latex colloids36) at the column inlet under higher ionic strength have been reported. In similar studies, overall cluster size of influent materials (i.e., 8429



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the AA failed to provide electrostatic stabilization as observed from EPM values (Figure S8 of the SI). Moreover, the bound fraction of AA (smaller organic molecules) only provides a thin surface layer and thus results in negligible to no steric hindrance to aggregation and deposition. The ineffectiveness of AA in rendering stability to SWNTs has observed via both aggregation profiles (Figure S7 of the SI) for a range of electrolyte conditions (DI water and 10−200 mM NaCl) and the settling profile with 200 mM NaCl (Figure S6 of the SI). The size data of effluent SWNTs also reveals an interesting filtration mechanism in the presence of AA (Figure 1). The size measurements for the AA case show that large size SWNTs are eluted and size specific filtration likely has not occurred. A combination of the large effluent SWNT average hydrodynamic radii (on the order of several micrometers, 5−20 μm) and lower EPM values signify that the majority of SWNTs were likely filtered via physical mechanisms. It is likely that larger cluster sized SWNTs will have higher propensity for interception by the collector pores as well as by the initially retained SWNTs clusters and thus might have contributed to a higher physical removal of these tubular structures.37 However, it is also important to note that substantial reversible chemical attachment occurred, as observed via the release profile (inset of Figure 3). Moreover, the bimodal breakthrough profile could not be assigned to any specific effluent SWNT size, as most eluted size fractions were in multimicron range with high standard errors. Weak stabilization with smaller organic molecules has been reported previously.17 However, the influence of acetic acid on SWNTs and other carbon allotrope transport has not been previously studied. Role of Collector Type on SWNT Transport. The effect of individual collector type (e.g., paper, metal, plastic, and glass) on SWNT transport under a HA concentration of 50 mg TOC/L is presented in Figure 4. The breakthrough profiles associated with most collectors closely follow the tracer profiles. An exception to this is the response associated with paper. Results from the column containing paper show a sharp and early end to transport. SWNT recovery through metal, plastic, and glass was significant (i.e., 77, 88, and 93% effluent mass recovery, respectively). The greatest retention was observed in paper, with only 20% effluent mass recovery. Moreover, it is noteworthy that weak attachment of SWNTs is only observed in the paper and glass collectors. These collector-dependent releases indicate that alteration of background electrolyte concentrations initiate electrokinetic responses from the collector surfaces and likely not from the filtrate SWNTs. Results also highlight that paper may be responsible for the majority of SWNT retention in the mixed MSW columns, as discussed earlier. Collector media effects on SWNT mobility have been previously reported. SWNTs have showed limited transport in natural soil at low electrolyte conditions (i.e., 1 mM KCl) SWNTs;33 however, changing the porous media to sand enhanced mobility under similar electrolyte conditions.9 MWNTs10 and fullerenes11 were found to be more mobile in columns packed with glass beads than that with sand. Greater retention of SWNTs is reported in mixed MSW compared to sand columns for HA stabilized SWNT dispersions (400 mg TOC/L) under 200 mM NaCl.5 No study to date has reported transport of SWNTs in plastic, metal, or paper. Transport Mechanisms. SWNT filtration mechanisms associated with mixed MSW studies can be probed by studying the physicochemical properties of the MSW collectors. The transport data for the individual collector surfaces also provide

Moreover, SWNT deposition was observed to be hindered in presence of HA in a previously conducted study evaluating landfill related conditions.5 Fullerenes also demonstrated enhanced mobility in presence of HA, likely attributed to steric interaction between fullerenes and collector surfaces.39 Effect of Acetic Acid on SWNT Transport. The breakthrough profile associated with SWNT transport with 400 mg TOC/L AA indicates significant reduction in SWNT mobility (Figure 3). Moreover, a bimodal breakthrough is

Figure 3. SWNT breakthrough profile at 400 mg TOC/L of acetic acid (AA) in the experiment with mixed MSW. Red circles indicate tracer (LiBr) breakthrough curves. C/C0 indicates the normalized SWNT concentration; C0 and C represent influent and effluent SWNTs concentration, respectively. Column saturation was done before PV = 0.0; details in SI Table S1. Step A: AA + 200 mM NaCl + SWNTs, Step B: AA + 200 mM NaCl, and Step C: AA only. Inset depicts SWNT release following elution with only AA solution. Experimental parameters relevant to this column experiment are listed in Table 1.

observed, indicating size-dependent transport of SWNTs. The biomodal breakthrough does not directly corroborate with sizedependent filtration of SWNTs. It is to be noted that the influent SWNTs possessed multimicron sized clusters, on average. Thus SWNT transport did not have a preference for elution of smaller clusters. The second breakthrough peak is thus not a result of size-dependent elution, but rather is likely caused by internal mobility of the large and unstable SWNT clusters, which might have initially been retained by the heterogeneous porous MSW collectors. SWNT retention in presence of AA (400 mg TOC/L) was significantly higher (∼94%) than that observed with HA concentrations ranging from 50−400 mg TOC/L. Transport behavior in the presence of AA is similar to that observed at the lowest HA concentration evaluated (10 mg TOC/L, Figure 2b). The AA case also demonstrates weak attachment of SWNTs, as observed by the release profile. Effect of Leachate Type on Transport Mechanisms: AA vs HA. Electrostatic and steric repulsion between the SWNTs were likely altered due to the change in stabilizer chemistry. This change likely resulted in SWNT aggregation and subsequent attachment to the MSW collectors. The AA molecular structure possesses a single carboxylic group with a molecular weight of 60 Da, compared to multiple carboxyl and hydroxyl ligand presence in HA with a much higher average molecular weight (6000 to 10 000 Da).43 Although AA was added at 400 mg TOC/L concentration, the bound fraction of 8430



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Figure 4. SWNT breakthrough profiles at 50 mg TOC/L of humic acid (HA) with individual porous media of (a) paper, (b) metal, (c) plastic, and (d) glass. Red circles indicate tracer (LiBr) breakthrough curves. C/C0 indicates the normalized SWNT concentration; C0 and C represent influent and effluent SWNTs concentration, respectively. Column saturation was done before PV = 0.0; details in SI Table S1. Step A: HA + 200 mM NaCl + SWNTs, Step B: HA + 200 mM NaCl, and Step C: HA only. Insets depict SWNT release following elution with only HA solution. Experimental parameters relevant to column experiments are listed in Table 1.

from silanol groups in glass resulted in lower SWNT attachment in both of these collector systems. The results from the individual collectors and analyses of filtration mechanisms help postulate the overall filtration process in the mixed MSW media conditions. The lower porosity of the column containing the mixed MSW and/or the rough surface of paper likely contribute to particle retention. It is possible that the presence of cationic polyelectrolytes from the paper cause SWNT aggregation during transport. Such aggregation is likely more pronounced at lower HA concentrations. It is important to note that the role of individual collectors relevant to MSW landfill conditions has not been previously systematically evaluated. Environmental Implications. Understanding such transport is an important first step to assessing the suitability of placing nanomaterial-laden wastes in MSW landfills. If nanomaterials are mobile in landfills, then the potential for release to the surrounding environment increases. Understanding the potential for nanomaterial transport through the solid waste is thus important when considering appropriate leachate treatment options. Currently, landfill leachate (from Subtitle D landfills) is collected, pumped from the landfill, and ultimately treated either on-site, in situ, or at a wastewater treatment plant. Understanding the magnitude of nanomaterial contamination in the collected leachate is critical when making such treatment decisions.

insight into the mixed MSW transport results discussed earlier. Nanomaterial transport is known to be influenced by media physiochemical properties as well as packed bed porosity.44 The packed bed porosities differ substantially between tests (Table 1). The column containing metal has the largest porosity (0.90), followed by similar porosities for glass and plastics (0.48 and 0.51, respectively), and paper possesses the lowest (0.28). Changes in porosity likely play a role in SWNT transport. Media physiochemical properties also likely influenced nanomaterial transport. Among the four collectors analyzed, paper (80% retention) and metal (23% retention) possessed unique transport behavior when compared to plastic (12%) and glass (7%). Paper not only possesses surface roughness (due to fibrous nature), but also likely contains bound cationic polyelectrolytes that are commonly used during pulp processing.45 The cationic polyelectrolytes that are typically added in paper processing industries include: polyallylamine,46 cationic polyacrylamide,47 and polyethyleneimine.48 Increased surface roughness of the paper may have resulted in favorable SWNT attachment12 onto the paper surfaces, while the cationic polyelectrolytes may have initiated SWNT aggregation and subsequent filtration during the transport experiments. Plastic and glass collectors, however, are relatively smooth. Glass collectors contain silanol surface groups.49,50 The reduction in surface roughness and the unfavorable electrostatic repulsion 8431



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Research Foundation. We are grateful to USC Electron Microscopy Center for its assistance in TEM imaging.

Predicting nanomaterial transport in landfills is difficult due to variable flow conditions, waste heterogeneity and preferential flow in such environments. Moreover, the temporal and spatial changes in waste composition also complicate such predictions. The model landfill system used in this study attempts to capture such complex variations and to systematically evaluate SWNT fate in such environments. It is important to note that the presence of preferential flow (e.g., cracks, fissures) in landfills is likely and will significantly impact nanomaterial mobility, potentially enhancing mobility. Waste heterogeneity may also influence SWNT transport in such conditions as demonstrated by the media-specific study results (Table 1). Results from this work also indicate that changes in ionic strength at relatively low HA or at high AA conditions may result in a release of SWNTs from attached surfaces, which is important when considering long-term nanomaterial releases from landfills. It should be noted that the experimental conditions evaluated in this work represent rather simplified MSW landfill conditions. To date, there are no data that provide a clear picture of the concentration and form (e.g., type of surface functionalization) of nanomaterials present in landfills. In addition, landfills are generally dominated by unsaturated flow conditions and waste materials are significantly more complex than the materials used in this study. Leachate composition is also more complex, containing a range of different salts, organics, and metals. Despite being conducted in simplified conditions, results of this work provide important information concerning nanomaterial transport and deposition. Overall, the study highlights that SWNTs may be mobile in landfills and that accurate determination of SWNT fate requires additional systematic studies in more realistic landfill conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Details of SWNT functionalization and characterization, details of synthetic leachate preparation and characterization, and details of transport experiment (Section S1). Representative column packed with mixed MSW (Figure S1). PV for transport experiments (Table S1). Column packed with food (Figure S2). Column saturation with background humic acid (Figure S3). TEM images of SWNTs (Figure S4). Raman spectra of SWNTs (Figure S5). SWNT-leachate settling profiles (Figure S6). Aggregation profiles of SWNT-AA (Figure S7). EPM of SWNTs (Figure S8). Data for Figure 1 (Table S2). This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: (803) 777-7521; fax: (803) 777-0670; e-mail: berge@ cec.sc.edu Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation (under Grant No. 0933484) and the South Carolina Research Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or the South Carolina 8432



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Single-Walled Carbon Nanotube Transport in Representative

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