Sorption and Competition of Aromatic Compounds and Humic Acid on

Jul 13, 2009 - Sorption of humic acid (HA) and aromatic compounds by multiwalled carbon nanotubes (MWCNTs), and their competition on MWCNTs were exami...
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Environ. Sci. Technol. 2009, 43, 6214–6219

Sorption and Competition of Aromatic Compounds and Humic Acid on Multiwalled Carbon Nanotubes X I L O N G W A N G , †,‡ S H U T A O , † A N D B A O S H A N X I N G ‡,* Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China, and Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003

Received April 23, 2009. Revised manuscript received June 22, 2009. Accepted June 24, 2009.

Sorption of humic acid (HA) and aromatic compounds by multiwalled carbon nanotubes (MWCNTs), and their competition on MWCNTs were examined. HA sorption by MWCNTs was regulated by their surface area (SA). Hydrophobic and π-π attractions of HA with MWCNTs were main driving forces for their interactions. Kd/Kow values of phenanthrene (Phen), naphthalene (Naph), and 1-naphthol (1-Naph) by individual MWCNTs were positively correlated with their molecular size, suggesting that micropore-filling could not be a predominant mechanism. HA had the lowest competition with Phen and 1-Naph on MWCNT20, due to its greatest abundance of sorption sites. Competition between HA and 1-Naph followed an order MWCNT40 < MWCNT60 < MWCNT100, due to their reduction in SA and porosity. Micropore blockage and direct competition by HA increased with deceasing SA and porosity of MWCNTs. MWCNT20 had much more sorption sites than other MWCNTs, leading to insignificant difference in competition between 1-Naph and Phen with HA. Also, HA had higher competition with Phen on MWCNT40, MWCNT60, and MWCNT100 than 1-Naph. Our results highlight the significance of MWCNT SA for HA sorption and the associated influence on sorption of aromatic compounds. Further, molecular size and hydrophobicity of aromatic compounds strongly affected their competition with HA on MWCNTs.

Introduction Carbon nanotubes (CNTs) have gained increasing attention since their discovery in 1991 (1) due to their unique structure, outstanding electronic and mechanical properties, as well as thermal and chemical stability (2). They have widely been used as catalyst supports and components for DNA and protein biosensors, and have potential applications as electrodes in batteries and supercapacitors (2-4). CNTs have also been proposed as a new sorbent to remove environmental contaminants due to their large SA. A better understanding of the underlying sorption mechanisms of hydrophobic organic compounds (HOCs) by CNTs is thus essential for environmental applications of CNTs and health risk * Corresponding author (Dr. Baoshan Xing). Tel: (413) 545-5212; Fax: (413) 545-3958; Email: [email protected]. † Peking University. ‡ University of Massachusetts. 6214

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assessment of both HOCs and CNTs once they are released into the environment (5). It was reported that CNTs had much higher efficiency to remove dioxin than activated carbon (5). Sorption of 1,2dichlorobenzene by CNTs was pH dependent (6). Hydrophobic interactions played an important role in sorption of HOCs (i.e., Naph) by CNTs (7). In addition, π-π interactions between aromatics and graphene sheets of CNTs were demonstrated by a few studies (8). Chen et al. (9) further observed that sorption of nitroaromatics by CNTs increased with the number of substituent nitro-groups, which the authors ascribed to the π-π interactions of nitroaromatics (electron acceptors) with highly polarizable graphene sheets (electron donors) of CNTs. Pan and Xing (10) recently reviewed that sorption of HOCs by CNTs was regulated by multiple mechanisms. Moreover, Yang et al. (11) reported that Polanyi model was the most appropriate to depict sorption of aromatics by CNTs among several models used for data fitting. These studies are helpful for elucidating sorption mechanisms of HOCs by CNTs. However, the relative importance of external surface and pores of CNTs is largely unclear, which needs to be further addressed. Natural organic matter (NOM) is ubiquitous in the environment, and it would interact with CNTs upon contact and their interactions may change environmental behaviors of CNTs. It was observed that HAs of higher molecular weight and hydrophobicity had higher sorption capacity by CNTs than fulvic acids. Furthermore, among various moieties present in NOM, aromaticity of NOM was linearly correlated with their sorption by CNTs (12). Stability of CNTs was also enhanced by NOM isolated from Suwannee River, and microscopic analyses clearly showed that the suspension consisted primarily of individually dispersed CNTs (13). To date, most current studies focus on the interactions of single HOC or NOM with CNTs (14). However, singlesolute systems may not adequately represent the real environmental systems. Our previous study showed that HA coating (85.5 mg/g) did not make any significant change on sorption of Phen, Naph, and 1-Naph by one MWCNT (i.e., sorption coefficients of phenanthrene (Phen), naphthalene (Naph), and 1-naphthol (1-Naph) by the HA-coated MWCNT were in the similar ranges of these compounds by the original MWCNT) although the HA used had much lower sorption for these three compounds, suggesting that humic acid coating vastly altered physical form and surface properties of the MWCNT. A recent study showed that incorporation of O-containing functionalities (10%) created polar regions that dramatically reduced Naph sorption (70%) by MWCNTs (15). It was hypothesized that NOM coating would have distinct effects on HOC sorption by MWCNTs from that HOC and NOM coexist in a bisolute sorption system (competition system). The overall goal of this work was to examine the underlying sorption mechanisms of NOM and HOCs on MWCNTs and the competitive adsorption of NOM and HOCs on MWCNTs. Our specific objectives were to elucidate the relative importance of external surface and pores of MWCNTs in their sorption for HA and HOCs, and determine the effect of molecular size and hydrophobicity of HOCs as well as the properties of MWCNTs on HOC-HA competition on MWCNTs. Difference in competitive adsorption of polar and nonpolar HOCs with HA on MWCNTs was also investigated.

Materials and Methods Sorbates and Sorbents. MWCNTs of various outer diameters with purity >95% were purchased from Shenzhen Organic 10.1021/es901062t CCC: $40.75

 2009 American Chemical Society

Published on Web 07/13/2009

TABLE 1. Selected Properties of Multiwalled Carbon Nanotubesa MWCNTs

C (%)

H (%)

MWCNT20 MWCNT40 MWCNT60 MWCNT100

98.09 98.15 97.68 98.01

0.25 0.19 0.14 0.11

N (%)

O (%)

ash (%)

SA (m2/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

purity

0.90 0.02 0.09

0.76 1.64 2.09 2.06

126 86 73 58

0.051 0.034 0.029 0.023

0.364 0.285 0.162 0.114

>95 >95 >95 >95

a SA, surface area; Vmicro: micropore volume; Vmeso: mesopore volume. The numbers after MWCNTs are their outer diameters.

Chemistry Co., China. A chemical vapor deposition approach with Ni as catalyst was employed to synthesize MWCNTs from a CH4/H2 mixture at 700 °C. A HNO3/H2SO4 mixture was used to purify the synthesized MWCNTs. Humic acid was isolated from a peat soil collected from Amherst, Massachusetts using 0.1 M Na4P2O7. The extracted HA was treated with 0.1 M HCl/0.3 M HF to reduce ash content (16). The HA was then rinsed five times with deionized water, freeze-dried, and finally ground to pass through a 250 µm sieve and stored for characterization and sorption experiments. 14C-labled and nonlabeled Phen, Naph, and 1-Naph were purchased from Sigma-Aldrich Chemical Co and used as sorbates. Selected properties of these compounds are presented in Supporting Information (SI) Table S1. MWCNT and HA Characterization. The C, H, N contents of MWCNTs were determined using a Carlo Erba 1110 CHN elemental analyzer. Ash contents of the MWCNTs were measured by heating them at 900 °C for 10 h (17), and the O content was calculated by mass difference. Surface area, micro-, and mesopore volume of MWCNTs were calculated from N2 adsorption-desorption isotherms at 77 K using multipoint BET method (17). Solid-state cross-polarization magic angle-spinning and total-sideband-suppression 13C NMR spectrum of HA was obtained using a Bruker DSX-300 spectrometer (Karlsruhe, Germany) operated at 13C frequency of 75.5 MHz. The NMR running parameters and chemical shift assignments are described elsewhere (18) and the integrated NMR data of HA are summarized in SI Table S2. Elemental composition of HA is presented in SI Table S2. HA Sorption. Sorption isotherms of HA by MWCNTs were obtained by batch experiments in screw cap vials. Solid to solution ratio was adjusted to obtain 30-70% solute uptake by MWCNTs. The background solution contained 200 mg/L NaN3 to minimize bioactivity. Stock solution of HA was prepared by dissolving it with 0.1 M NaOH, followed by adjusting pH to 7. Test solutions of HA at various concentrations were shaken for 1 h before adding to vials with a desired amount of MWCNTs. The vials were then sealed with Teflonscrew caps and placed on a rotary shaker to mix for 9 days at room temperature (23 ( 1 °C). After mixing, the vials were centrifuged at 3000 rpm for 1 h, followed by filtering the supernatants through 0.2 µm poly(tetrafluoroethylene) (PTFE) filter (Whatman) (17). Finally, HA concentrations in the supernatant were measured using a UV-visible spectrometer. The same method was used to determine HA concentrations in HA-activated carbon sorption systems (19). All samples including blanks were run in duplicate. Uptake of HA by MWCNTs was calculated by mass difference because their mass loss was negligible for blank samples. HOC Sorption. Sorption isotherms of Phen, Naph, and 1-Naph by MWCNTs were also obtained using a batch equilibration technique in screw cap vials with aluminum foil-Teflon linears. The background solution contained 200 mg/L NaN3 to minimize bioactivity and 0.01 M CaCl2 to maintain a constant ionic strength. Stock solutions of 14C labeled Phen, Naph, and 1-Naph as well as nonlabeled Phen and Naph were made by dissolving them in methanol, whereas that of nonlabeled 1-Naph was prepared in water. Test solutions of Phen, Naph, and 1-Naph at various

concentrations were made by spiking 14C labeled and nonlabeled stock solutions to the background solution. They were sealed and placed on a shaker to mix for 1 h before adding to the vials that contained a preweighed amount of sorbents until a minimum headspace was reached. The vials were immediately sealed with aluminum-foil-lined Teflon screw caps and placed on a rotary shaker to mix for five days at room temperature (23 ( 1 °C). Our preliminary test showed that equilibrium was reached within four days. All samples, including blanks, were run in duplicate. Solid-to-solution ratios were adjusted to attain 30-80% uptake of the test compounds at equilibrium. Methanol content in the test solutions was controlled below 0.1% by volume to minimize cosolvent effect. After mixing, the vials were centrifuged at 3000 rpm for 30 min, after which 1.8 mL of supernatant was sampled and added to scintiverse cocktail (6 mL) (Fisher Scientific Co.) for scintillation counting. Due to negligible mass loss of solutes, their uptake by MWCNTs was calculated by mass difference. HOC-HA Competitive Sorption. Competitive sorption experiment was performed using Phen or 1-Naph as primary solutes and HA as competitor. A stock solution of HA (120 mg/L) with pH of 7 was prepared using the identical method as stated above. Stock solution containing 14C labeled and nonlabeled Phen or 1-Naph was prepared using the same approach as described above for single HOC sorption experiment. Test solutions contained 200 mg/L NaN3 to inhibit biodegradation. Initial HA concentrations in the test solutions varied from 0 to 90 mg/L, whereas that of Phen and 1-Naph was constant (0.26 and 60 mg/L, respectively). All samples including blanks were run in duplicate. All vials were shaken for five days at room temperature (23 ( 1 °C). After mixing, the vials were centrifuged at 3000 rpm for 1 h, and 1.8 mL of the supernatant was sampled and added to scintiverse cocktail (6 mL) for scintillation counting. The same method was successfully used to separate natural organic matter (i.e., HA, peptone and R-phenylalanine)-coated MWCNTs (4). Also, the presence of dissolved HA up to 80 mgOC/L did not affect scintillation counting of organic compounds (i.e., pyrene) (20). Due to negligible mass loss of solutes in both single and competitive sorption systems, their uptake by MWCNTs was calculated by mass difference. Sorption Isotherm Models. The logarithmic form of the Freundlich model was used for data fitting in this work. log Q ) log Kf + nlog Ce where Q and Ce are equilibrium solid- (mg/kg) and liquid phase (mg/L) concentrations, respectively. Kf is the sorption coefficient ((mg/kg)/(mg/L)n), and n is often used as an indicator of isotherm nonlinearity.

Results and Discussion Elemental Composition of MWCNTs. All MWCNTs had very high and comparable organic carbon content and low oxygen content, reflecting their high purity and hydrophobicity (Table 1). Among all MWCNTs tested, MWCNT20 had the highest polarity as indicated by its highest oxygen content, thus the highest abundance of polar functionalities. Its polar moieties could primarily be a result of oxidation in the purification process. In comparison, only little amount of polar functionalities was introduced to MWCNT40, MWCNT60, and VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Sorption isotherms of HA by MWCNTs: MWCNT20 (]); MWCNT40 (0); MWCNT60 (∆), and MWCNT100 (O). MWCNT100 in their purification process. MWCNTs could be functionalized by treating with strong oxidizing agents (e.g., concentrated HNO3 or H2SO4 or their mixture). However, the MWCNTs used in this work were not highly functionalized as indicated by their low oxygen contents (Table 1). Surface Area and Porosity of MWCNTs. MWCNT20 had much larger SA and porosity than MWCNT40, MWCNT60, and MWCNT100, suggesting more sorption sites for sorption of HA and HOCs. Surface area and porosity of MWCNTs decreased with an increase in their outer diameters, implying that MWCNTs of larger outer diameter would have a reduced number of sorption sites (Table 1). Humic Acid Sorption. All MWCNTs had nonlinear sorption isotherms for HA, due to their heterogeneous sorption sites (Figure 1). No clear difference in the sorbed amount of HA to MWCNTs was observed at low HA concentrations (i.e., e 6 mg/L), due to an ample amount of sorption sites on MWCNTs. With increasing HA concentration, sorbed amount of HA by individual MWCNTs reached a plateau, indicating that HA sorption reached the maximum capacity (Figure 1). The maximum sorption capacities of HA by MWCNT20, MWCNT40, MWCNT60, and MWCNT100 were approximately 79, 82, 65, and 53 mg/g, respectively. NMR and elemental composition data showed that HA was rich in O-containing functionalities (57.9%) (SI Table S2). These moieties can interact with polar functionalities (e.g., COOH and OH groups) on the graphene surfaces of MWCNTs via hydrogen bonding (21). Such interaction would be more evident for MWCNT20 than other MWCNTs due to its higher oxygen content (0.9%) (SI Table S2), which can in part interpret its higher sorption for HA relative to MWCNT60 and MWCNT100. Hydrogen bonding between HA molecules and MWCNT40, MWCNT60, or MWCNT100 surfaces should be lower as indicated by their low oxygen content thus very few O-containing functionalities on the MWCNT surfaces. As reported, π-π interaction is an important mechanism that governs sorption of both π-acceptors and π-donors to graphene surfaces (10, 22). Hyung et al. (12) further stated that sorption of NOM and other HOCs containing aromatic functionalities by CNTs was largely driven by π-π interactions. HA had 26.3% of aromatic carbon (Table S2), which may interact with electron-rich sites of the graphene surfaces of MWCNTs via π-π interactions. It was reported that micropore-filling is a key mechanism affecting sorption of small molecules such as HOCs by chars (23). A greater number of smaller molecules (i.e., Naph) were sorbed to biopolymer-derived chars at the maximum sorption capacity in comparison with larger ones (i.e., Phen) (23). However, due to much larger molecular size of HA compared to HOC molecules, it would be difficult for HA molecules to penetrate into micropores in MWCNTs due to steric hindrance. Therefore, micropore-filling would not be a predominant mechanism for HA sorption by MWCNTs. Fulvic acid generally has smaller molecular size than HA, and it is 6216

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able to penetrate into pore throats in wood chars, whereas most HA molecules are too large to get deeply into pores in these sorbents (24). MWCNTs were extremely hydrophobic although MWCNT20 had 0.9% of oxygen, because such a low oxygen content may not decrease its hydrophobicity significantly. The hydrophobic surfaces of MWCNTs favor HA sorption by hydrophobic interactions (12). MWCNTs and activated carbons are composed mainly of graphitic sheets. It was reported that hydrophobic and π-π interactions were responsible for NOM sorption by activated carbon (25). Such mechanisms would also operate for HA sorption by MWCNTs. Although π-π and hydrophobic interactions played an important role in HA sorption by MWCNTs, a single interaction cannot explain the difference between MWCNTs of various outer diameters. It is evident that, sorption capacity of HA was linearly correlated with the SA of MWCNT40, MWCNT60, and MWCNT100 (SI Figure S1), suggesting that SA of MWCNTs was a dominant factor. However, π-π and hydrophobic attractions between HA and MWCNT surfaces would be the principal driving forces for their approaching and further interactions. Surface area of MWCNT20 (126 m2/ g) was much larger than MWCNT40 (86 m2/g) (Table 1). However, sorption capacity of HA by these two MWCNTs was comparable (SI Figure S1). This could be a result of lower dispersion of MWCNT20 relative to MWCNT40 in the HAMWCNT sorption systems as visually observed in our experiments. Relatively a greater portion of sites on MWCNT40 surfaces would thus be effective for HA sorption compared to MWCNT20. Overall, comparable sorption of HA by MWCNT40 and MWCNT20 implied that elevated sorption of MWCNT20 for HA due to its larger SA and porosity and more hydrogen bonds relative to MWCNT40 was offset by increased site availability of MWCNT40 because of higher dispersion in the presence of HA. Similar to our observation, MWCNTs of larger outer diameter had greater dispersion as compared to that of smaller outer diameter at a same tannic acid concentration in the aqueous phase (17). HOC Sorption. All MWCNTs had nonlinear sorption isotherms for Phen, Naph, and 1-Naph (Table 2), suggesting the heterogeneous sorption sites. Due to high hydrophobicity of MWCNTs, they can readily aggregate and form bundles in aqueous phase due to strong van der Waals forces along their length axis, which has been observed by SEM and TEM (17, 26). Entanglement of individual MWCNTs and their bundles would create heterogeneous sites for HOC sorption (10, 17), and these heterogeneous sites can be (i) in the hollow space inside nanotubes and interstitial space between neighboring nanotubes; and (ii) on the grooves and curved surface of the periphery of nanotube bundles (4, 10). The external curved surface of CNTs was the primary destination for HOC (i.e., Naph) sorption, while the hollow space inside nanotubes was inaccessible to Naph molecules due to steric restrictions (7). Sorption of Phen, Naph, and 1-Naph by individual MWCNTs followed an order: Phen > Naph >1-Naph, in line with linear free energy relationship (SI Figure S2, Tables 2 and SI Table S1). This suggests that sorption of HOCs by individual MWCNTs was dependent on their hydrophobicity. To probe the role of sorbate hydrophobicity in their sorption by MWCNTs, Kd values of Phen, Naph, and 1-Naph were normalized with their respective Kow values. After normalization, sorption of these chemicals by individual MWCNTs followed an order: Phen >1-Naph > Naph (Table 2). Thus, when the molecular number of sorbate in aqueous phase was identical, a greater number of sorbate molecules of larger molecular size would be sorbed to MWCNTs provided that HOCs had no hydrophobicity difference. A positive correlation between Kd/Kow values of Phen, 1-Naph, and Naph by individual MWCNTs and their molecular size demonstrated that micropore-filling could not

TABLE 2. Parameters of Freundlich Model-Based Sorption Isotherms Fitting for Phen, Naph, and 1-Naph on MWCNTsa chemicals

sorbents

log Kf

Phen

MWCNT20 MWCNT40 MWCNT60 MWCNT100

4.735 ( 0.011 4.388 ( 0.011 4.399 ( 0.006 4.290 ( 0.005

Naph

MWCNT20 MWCNT40 MWCNT60 MWCNT100

4.046 ( 0.008 3.744 ( 0.008 3.656 ( 0.010 3.646 ( 0.008

1-Naph

MWCNT20 MWCNT40 MWCNT60 MWCNT100

4.552 ( 0.017 4.382 ( 0.021 4.432 ( 0.018 4.446 ( 0.023

Freundlich R 2

Kd (0.5 Sw)

Kd /Kow (0.5 Sw)

0.994 0.974 0.992 0.989

75100 36900 37100 29800

2.60 1.28 1.29 1.03

0.375 ( 0.006 0.443 ( 0.007 0.453 ( 0.010 0.448 ( 0.007

0.995 0.995 0.992 0.995

2000 1210 1000 975

1.03 0.62 0.52 0.50

0.315 ( 0.008 0.335 ( 0.010 0.339 ( 0.008 0.310 ( 0.011

0.990 0.986 0.990 0.978

557 425 489 423

1.11 0.85 0.98 0.84

n b

0.416 ( 0.008 0.254 ( 0.011 0.292 ( 0.007 0.236 ( 0.006

c

a Kf ((mg/kg)/(mg/L)n); Kd: distribution coefficient (L/kg); Sw: aqueous solubility (mg/L). respectively. c Standard errors of logKf and n, respectively.

b

Standard errors of logKf and n,

FIGURE 2. Sorption isotherms of Phen, Naph and 1-Naph by MWCNTs: MWCNT20 (]); MWCNT40 (0); MWCNT60 (∆); and MWCNT100 (O). be a major sorption mechanism for these compounds. Otherwise, their Kd/Kow values by individual MWCNTs should be reverse to our observation because steric hindrance of these chemicals follows an order: Phen >1-Naph > Naph. Zhu et al. (22), however, observed that chemicals with four substituents or those with three substituents were not able to penetrate into a portion of pores in wood chars that was accessible to smaller substituted benzene compounds due to their larger molecular size, thus greater steric hindrance. This work along with others suggests that, different from MWCNTs, HOC sorption by black carbons (e.g., chars) and activated carbons was dominated by a micropore-filling mechanism (23, 27). Phen and Naph have similar molecular structure, but compared to Naph, Phen would be more polarizable due to its larger molecular weight thus having greater dispersive force with highly polarizable graphene sheets of individual MWCNTs (SI Table S1). Therefore, Phen had greater Kd/Kow values by individual MWCNTs relative to Naph. Both Naph and 1-Naph had two benzene rings in their molecular structure, and the only difference in their structure is that 1-Naph has an additional hydroxyl group at R position. The hydroxyl group on 1-Naph can interact with O-containing groups at the surfaces of MWCNT20 via hydrogen bonding thus enhancing their interaction. However, due to very low oxygen content of MWCNT40, MWCNT60, and MWCNT100, such an interaction could be low. It was reported that stacking of benzene rings to MWCNT surfaces greatly increased surface coverage of surfactants and polymers (28). Due to hydrophobic nature of benzene ring in 1-Naph’s structure, its benzene ring would preferentially be attracted and stacked to MWCNT surfaces via hydrophobic interactions while leaving its hydrophilic hydroxyl group facing the aqueous phase. The outward hydroxyl group of 1-Naph thus may have created “hydrogen bonding sorption sites”, so a second layer of 1-Naph would be sorbed to the initially sorbed 1-Naph

molecules through hydrogen bonding between hydroxyl groups, π-π and hydrophobic interactions between benzene rings. The same mechanism was employed to interpret the cooperative sorption of 1-Naph and 1-naphthylamine on macroreticular adsorbents (29), and that of 1-Naph and lipid on lipid-removed soil, HA fractions and humin (30). Compared to 1-Naph, Naph molecules can only be sorbed to individual MWCNTs through π-π and hydrophobic interactions. Hence, 1-Naph had greater Kd/Kow values by individual MWCNTs than Naph. MWCNT20 had higher sorption for Phen, Naph, and 1-Naph than MWCNT40, MWCNT60, and MWCNT100 (Figure 2, Table 2), which can be ascribed to its much larger SA and porosity (Table 1). Relative to other MWCNTs, MWCNT20 can provide a greater number of sites for HOC sorption. Compared to other MWCNTs, MWCNT20 may offer sites on its surfaces that are capable of forming hydrogen bonding and dipole-dipole interactions with polar compound (i.e., 1-Naph) due to its polar functionalities, thus increasing the driving force for sorption of this compound (31). Although SA and porosity of MWCNT40, MWCNT60, and MWCNT100 decreased with increasing outer diameter, they had comparable sorption for Phen, Naph, and 1-Naph. This was mostly due to aggregation difference of MWCNT particles at different diameters. Particularly, MWCNTs of smaller diameter had greater and tighter aggregation relative to that of larger diameter (17). Aggregation and entanglement of MWCNT particles would reduce their effective SA and porosity for HOC sorption. HOC-HA Competition. Phen and 1-Naph had high competition with HA at low HA concentrations, and the competition was less pronounced with increasing HA concentrations. Since a given sorbent has a limited number of high-energy sorption sites, both HA and Phen or 1-Naph molecules would preferentially occupy these sites at low HA concentrations, thus having higher competition. With inVOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effect of humic acid on 1-Naph (A) and Phen (B) sorption by MWCNTs. MWCNT20 (]); MWCNT40 (0); MWCNT60 (∆); and MWCNT100 (O). The y-axis represents the ratio of 1-Naph or Phen sorbed by MWCNTs in the presence of HA to that in the absence of HA at various initial HA concentrations. creasing HA concentrations, both HA and Phen or 1-Naph molecules would be forced to occupy low-energy sorption sites, thereby having lower competition on MWCNTs. Humic acid had much lower Kd values for Phen and 1-Naph than MWCNTs (30). However, HA-coating did not clearly suppress Phen and 1-Naph sorption by MWCNTs, due to the possible dispersion of MWCNTs as induced by the sorbed HA molecules thus creating new sites for HOC sorption (4). Due to a much larger molecular size of HA relative to Phen and 1-Naph, the number of sorption sites occupied by a HA molecule on MWCNT surfaces could be much more than that by one Phen or 1-Naph molecule. Sorption of HA would mask a great number of sorption sites on MWCNTs, thereby reducing their sorption for Phen and 1-Naph. Humic acid sorption would deplete a great number of sorption sites on MWCNT surfaces due to surface coverage and pore blockage, which would reduce sorption of Phen and 1-Naph to MWCNTs. In addition, HA is more hydrophobic than water, thereby having higher sorption affinity for Phen and 1-Naph than water. Sorption of Phen and 1-Naph to the HA phase in the test solution would also reduce their sorption to MWCNTs. Similarly, high competition of dissolved natural organic matter with atrazine on activated carbon was also observed in a previous study (32). Natural organic matter such as HA may compete with HOC molecules via two major mechanisms (i.e., direct site competition and pore blockage) on MWCNTs. On the one hand, HA molecules may compete with HOC molecules for sorption sites on MWCNT surfaces through direct site competition. On the other hand, a portion of large HA molecules would be sorbed to the entrances to pores of MWCNTs that is large enough to accommodate HOC molecules, thus reducing accessibility of HOC molecules to sorption sites in pores by pore blockage. Since MWCNT20 had the highest abundance of sorption sites among all MWCNTs tested, both pore blockage by HA sorption and direct site competition of HA and HOC molecules would be lower for MWCNT20 than other MWCNTs. Due to larger SA and porosity of MWCNT20 than other MWCNTs, HA would have relatively lower steric restriction to prevent Phen and 1-Naph molecules from approaching this sorbent, which may facilitate sorption of these two compounds by MWCNT20. Therefore, both Phen and 1-Naph had the lowest competition with HA molecules on MWCNT20 among all MWCNTs examined (Figure 3). Competition of 1-Naph and HA on MWCNTs gradually increased with increasing outer diameter of MWCNTs. An increase in competition of 1-Naph and HA on MWCNT20, MWCNT40, MWCNT60, and MWCNT100 can be a result of reduction in their SA and porosity (Table 1). Our previous study showed that HA sorption decreased SA and microporosity of MWCNTs (4). With decreasing SA and porosity of MWCNTs, micropore blockage by HA sorption and direct competition of HA and 1-Naph molecules for sorption sites on MWCNTs would increase proportionally. Relative steric 6218

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restriction of HA molecules to 1-Naph sorption would also increase with decreasing SA of MWCNTs, thus more substantially decreasing accessibility of 1-Naph molecules to MWCNT surfaces. Although SA and porosity of MWCNT20, MWCNT40, MWCNT60,andMWCNT100decreasedinanorder:MWCNT20 > MWCNT40 > MWCNT60 > MWCNT100, competition between HA and Phen on MWCNT40, MWCNT60, and MWCNT100 was comparable and it was higher than that on MWCNT20. Due to much larger SA and porosity, thus a greater number of sorption sites on MWCNT20 relative to other MWCNTs, no clear difference in competition of 1-Naph and Phen with HA on MWCNT20 was observed. However, competition between HA and Phen on MWCNT40, MWCNT60, and MWCNT100 was higher than 1-Naph (Figure 3). Due to smaller SA and porosity of MWCNT40, MWCNT60, and MWCNT100 relative to MWCNT20, HA molecules would have relatively higher steric hindrance to inhibit Phen from coming closer and being sorbed onto these MWCNTs. Micropore blockage by HA for Phen sorption to pores of MWCNT40, MWCNT60, and MWCNT100 would be higher than MWCNT20. Due to larger molecular size of Phen relative to 1-Naph, HA would have relatively higher steric restriction to prevent Phen molecules from approaching and further interacting with MWCNTs in comparison with 1-Naph. In addition, since Phen is more hydrophobic than 1-Naph, it would have a higher sorption by the HA dissolved in the test solution than 1-Naph, which would more strongly reduce Phen sorption to MWCNTs in comparison with 1-Naph. As a result, HA had higher competition with Phen on MWCNT40, MWCNT60 and MWCNT100 than 1-Naph. Environmental Implications. Sorption capacity of HA by MWCNTs was regulated by their SA, suggesting that MWCNTs of smaller outer diameter would more strongly affect transport and recycling of organic carbon in the environment as compared to those of larger outer diameters. Micropore-filling could not be a dominant sorption mechanism of HOCs by MWCNTs, noting the importance of MWCNT SA in their sorption for HOCs. This can be a key point for understanding the underlying sorption mechanisms of HOCs by MWCNTs. The competitive sorption results indicate that HOC sorption by MWCNTs would be greatly suppressed in the presence of NOM. Molecular size and hydrophobicity of primary solute appear to be important factors affecting competition. Polar and nonpolar compounds have dissimilar competition characteristics with DOM on MWCNTs, and their competition is affected by the outer diameter of MWCNTs.

Acknowledgments This work was supported in part by the Massachusetts Agricultural Experiment Station (MAS00973 and MAS90), Massachusetts Water Resources Research Center (2007MA73B), the Startup Fund for the Peking University 100-Talent Program, and National Natural Science Foundation of China (40730737).

Supporting Information Available Physicochemical properties of Phen, Naph and 1-Naph (Table S1); integrated solid-state 13C NMR and elemental composition data of HA (Table S2); relationship between SA of MWCNTs and their sorpiton capacities for HA (Figure S1); relationship between distribution coefficient and reduced concentration of Phen, Naph, and 1-Naph on individual MWCNTs (Figure S2).This material is available free of charge via the Internet at http://pubs.acs.org.

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