Competitive Adsorption of Naphthalene with 2,4-Dichlorophenol and 4

Mar 4, 2010 - Competitive adsorption between nonpolar organic compounds and polar ionic organic compounds (IOCs) on carbon nanotubes (CNTs) is essenti...
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Environ. Sci. Technol. 2010, 44, 3021–3027

Competitive Adsorption of Naphthalene with 2,4-Dichlorophenol and 4-Chloroaniline on Multiwalled Carbon Nanotubes K U N Y A N G , * ,†,‡ W E N H A O W U , † QINGFENG JING,† WEI JIANG,‡ AND BAOSHAN XING‡ Department of Environmental Science, Zhejiang University, Hangzhou 310028, China, and Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts, 01003

Received January 4, 2010. Revised manuscript received February 22, 2010. Accepted February 23, 2010.

Competitive adsorption between nonpolar organic compounds and polar ionic organic compounds (IOCs) on carbon nanotubes (CNTs) is essential for application of CNTs as superior sorbents and for environmental risk assessment of both CNTs and organic contaminants. It was observed in this study that adsorption of neutral and dissociated species of polar 2,4-dichlorophenol (DCP) and 4-chloroaniline (PCAN) on a multiwalled CNT sample (MWCNT15) can be suppressed by nonpolar naphthalene. Naphthalene adsorption can also be suppressed by neutral DCP/PCAN, but not dissociated DCP/ PCAN. Moreover, competition of naphthalene decreased the adsorption affinity of neutral DCP/PCAN, but not their adsorption capacity because of the formation of solute bilayer on MWCNT15. For dissociated DCP/PCAN, naphthalene not only decreased their adsorption affinity but also their adsorption capacity because no solute bilayer was formed. Neutral DCP/ PCAN also decreased the adsorption affinity and adsorption capacity of naphthalene. These observations indicate that competitive adsorption of naphthalene with DCP/PCAN depends on the dissociation of DCP/PCAN, as interpreted by (i) the different sites on CNTs for adsorption of organic chemicals (i.e., naphthalene, and the neutral and dissociated species of DCP/ PCAN), (ii) the interactions between organic chemicals, and (iii) the interactions of organic chemicals with CNT surface.

Introduction Since their first discovery (1, 2), carbon nanotubes (CNTs) have attracted special attention to their potential applications in areas such as purification of air and water as adsorbents due to their large surface area (3, 4). However, the rapid growth in production and applications of CNTs will inevitably increase their release and exposure into the environment, and thus has recently raised serious concerns over their toxicities and potential environmental risks (5, 6). One of the concerns is that the significant adsorption of toxic pollutants * Corresponding author phone: 86-571-88273190; fax: 86-57188273693; e-mail: [email protected]. † Zhejiang University. ‡ University of Massachusetts. 10.1021/es100018a

 2010 American Chemical Society

Published on Web 03/04/2010

on CNTs in the environment would alter the environmental behavior and bioavailability of the pollutants, add additional toxicities to the CNTs, and thus complicate the risk assessment of CNTs simultaneously (5-12). In this regard, knowledge of adsorption behavior of toxic chemicals on CNTs is essential not only to promote the application of CNTs as adsorbents but also to determine the environmental and health risks of both toxic chemicals and CNTs. A number of studies have been conducted to examine the adsorption of organic chemicals on CNTs in aqueous systems for the application of CNTs as adsorbents in removal of organic pollutants from wastewater and for the prediction of the potential behavior of CNTs with organic pollutants in the environment (5-17). These organic chemicals include nonpolar aliphatics (e.g., cyclohexane), polar aliphatics (e.g., trihalomethanes), nonpolar aromatics (e.g., polycyclic aromatic hydrocarbons (PAHs) and benzene), polar and nonionic aromatics (e.g., toluene, chlorobenzenes, and nitrobenzenes), and polar and ionizable aromatics (e.g., aniline derivates, phenolic chemicals, amino-substituted aromatics, atrazine, and natural organic matter). In these studies, only singlesolute sorption on CNTs was examined, which may not be useful for predicting contaminant sorption in real environments and wastewater because multiple contaminants and competitive sorption among them are generally present (18-22). The competition of an organic contaminant by others can result in significant sorption decrease of the contaminant (18-20), and thus, may decrease the removal efficiency of CNTs for the contaminant and potentially alter the health and environmental risks of CNTs and organic contaminants. Therefore, competitive sorption data and mechanisms are crucial for predicting contaminant sorption on CNTs and their environmental implications. Moreover, competitive sorption data would give important information to help understand the adsorption sites and underlying mechanism of organic chemicals on CNTs (7, 18). To date, few studies have dealt with competitive sorption of organic chemicals on CNTs in aqueous phase. In our previous study (18), significant competition between nonpolar PAHs (i.e., naphthalene, phenanthrene, and pyrene) was observed upon their adsorption on CNTs. It was observed that the competition between nonpolar PAHs decreased their adsorption affinity but not their adsorption capacity, i.e., the adsorption capacity of a given PAH chemical on CNTs could not be decreased by the addition of other PAHs, which was attributed to the possible formation of multilayer adsorption in the multiple solute system (18). Competition of organic chemicals (i.e., 2,4,6-trichlorophenol, naphthalene, atrazine, 1,3-dinitrobenzene, and 1,3,5-trinitrobenzene) by metal ions (i.e., Cu2+, Pb2+, and Cd2+) on CNTs was also observed (12, 19, 20). This competitive sorption was attributed to the formation of surface complexes of metal ions with oxygen functional groups on CNTs and is dependent on the oxygenated functional group densities (20). In addition, natural organic matter (NOM) and surfactants were reported to suppress the adsorption of organic chemicals such as PAHs and nitroaromatics on CNT surfaces (12, 21, 22). However, information about competitive sorption between nonpolar and polar organic chemicals on CNTs and the influences of dissociation of ionzable organic chemicals (IOCs) on their competitive sorption with nonpolar organic chemicals is still scarce. IOCs such as aniline derivates and phenolic chemicals are polar, and can exist either as neutral or dissociated species in aqueous phase. These chemicals are environmentally significant because they could have notable concentration higher than nonpolar organic chemicals in the environment VOL. 44, NO. 8, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Selected Properties of 2,4-Dichlorophenol, 4-Chloroaniline, and Naphthalene chemical

Csa

MWb

Gc

pKad

VI/100e

π*f

βmg

rmh

2,4-dichlorophenol (DCP) 4-chloroaniline (PCAN) naphthalene

4600 2755 31.7

163.00 127.58 128.20

1.383 1.170 0.997

7.90 4.15

0.720 0.653 0.753

0.87 0.73 0.70

0.18 0.40 0.15

0.78 0.31 0

a Cs: water solubility (mg/L). b MW: molecular weight (g/mol). c F: density (g/cm3). d pKa: dissociated constant (mL/mol). VI: intrinsic molar volume (mL/mol). f π*: polarity/polarizability parameter. g βm: hydrogen-bonding acceptor parameter. h Rm: hydrogen-bonding donor parameter. Data of pKa were obtained from ref 9. Data of VI, π*, βm, and Rm were obtained from refs 23 and 24. e

due to their solubility. It has been observed that their adsorption on CNTs is significantly different from that of nonpolar organic chemicals because of their dissociation and the functional groups in their molecules (9), thus, their competition with a nonpolar organic chemical on CNTs can be quite different from that between nonpolar organic chemicals. Although cationic and anionic surfactants and NOM are also IOCs, the reported competition of these IOCs with nonpolar organic chemicals on CNTs is commonly accompanied with the influences of aliphatic chains in their molecules (4, 21, 22). These influences include (i) the sorption of adsorbed NOM and surfactants for organic chemicals, which could promote the adsorption of organic chemicals on CNTs and could be altered by the configuration changes of the adsorbed NOM and surfactants (21); (ii) the dispersion of CNTs by NOM and surfactants, which could lead to more CNT surface sites exposed for organic chemical adsorption (4, 22); and (iii) solubility enhancement of organic chemicals by dissolved NOM and surfactants, which could decrease chemical adsorption (4). These influences, observed for surfactants and NOM, may not be applied for other simple IOCs such as substituted anilines and phenols because their molecules have no aliphatic chains, which allows one to explore the influences of dissociation and functional groups of IOCs on their competition with nonpolar organic chemicals. In this study, experiments regarding the competitive sorption of naphthalene with two IOCs (i.e., 2,4-dichlorophenol and 4-chloroaniline) on a multiwalled CNT sample (MWCNT15) were conducted to investigate (i) the competitive sorption mechanism between nonploar organic chemicals (i.e., naphthalene) and polar organic chemicals (i.e., 2,4dichlorophenol and 4-chloroaniline) and (ii) the influences of dissociation and functional groups of IOCs on their competition with nonpolar organic chemicals. 2,4-Dichlorophenol and 4-chloroaniline were selected because (i) 2,4dichlorophenol can be changed to be anion in alkaline solution, while 4-chloroaniline can be dissociated to be cation in acidic solution, and (ii) their pKa values are close to 7.0 (Table 1) and thus they can exist either as neutral or dissociated species in natural environments.

Materials and Methods Chemicals. 2,4-Dichlorophenol (DCP, +99.5%) was purchased from Shanghai Reagent Co. 4-Chloroaniline (PCAN, +98%) was purchased from Pharmacy Factory of China Second Military Medical University. Naphthalene (+98%) was purchased from Acros Organics Co. Selected properties of these chemicals are listed in Table 1. Carbon Nanotube. Multiwalled CNT sample (MWCNT15) was from the same batch as the one used in our previous studies (9, 18), which has a purity of +95%, elemental carbon content of +99.8%, length of 10-50 µm, outer diameter of 8-15 nm, and inner diameter of 3-5 nm. Its surface area, mesopore, and micropore volumes are approximately 174 m2/g, 0.597 cm3/g, and 0.068 cm3/g, respectively (9, 18). The surface oxygen and carbon atomic percents of MWCNT15, 3022

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determined by X-ray photoelectron spectroscopy (XPS), are approximately 1.6% and 98.4%, respectively. Adsorption Experiments. All isotherms in single- and bisolute systems were obtained based on duplicate tests using a batch equilibration technique at 25 ( 1 °C. DCP and PCAN were dissolved in a background solution containing 0.01 mol/L CaCl2 and 200 mg/L NaN3 (as a biocide) in deionized distilled water. Naphthalene in methanol or solid naphthalene was added into the background solution for sorption. The solution pH was adjusted using HCl or NaOH. Aqueous solution of DCP, PCAN, and naphthalene was mixed with MWCNT15 in 8- or 22-mL screw cap vials. Ratios of MWCNT15 to water, adjusted to achieve the removal of organic chemical more than 20%, were 50 mg:8 mL for the system of naphthalene with neutral DCP and PCAN, 300 mg:8 mL for the system of naphthalene with dissociated DCP, and 500 mg:8 mL for the system of naphthalene with dissociated PCAN. The vials were sealed and shaken at 100 rpm for 5 days to reach apparent equilibrium. The solid and aqueous phases were separated by centrifugation (3500g, 20 min). Then, the pH of supernatant was determined and adjusted to pH 10.0 using HCl or NaOH for the analysis of DCP and PCAN in the supernatant. In single-solute systems, DCP and PCAN were determined by a UV-spectrophotometer (Shimadzu, UV-2450) at 245 and 238 nm respectively, while naphthalene was determined by a fluorospectrophotometer (Shimadzu, RF-5301PC) at the excitation and emission wavelengths of 280 and 328 nm, respectively. In bisolute systems of naphthalene and PCAN, solute concentrations were determined using the same methods as in single-solute systems. In bisolute systems of naphthalene and DCP, however, solutes in the supernatant were determined by a reverse-phase HPLC (Agilent 1200 series, XDB-C18, 4.6 × 150 mm) equipped with both UV detector and fluorescence detector at a UV-wavelength of 245 nm for DCP and at the fluorescence excitation and emission wavelengths of 220 and 325 nm, respectively, for naphthalene. Methanol and water mixture at a volume ratio of 85% to 15% was used at a flow rate of 1 mL/min as the mobile phase of HPLC. The HPLC retention time for DCP and naphthalene is 2.3 and 3.3 min, respectively. Experimental uncertainties were evaluated in vials without MWCNT15, which showed that total uncertainty was less than 4% of the initial solute concentrations. Therefore, adsorbed solute amount by MWCNT15 was calculated directly by the difference of solute in initial and equilibrium solutions. Sorption Models and Regression Analysis. The DubininAshtakhov (DA) model (eq 1), used successfully in our previous studies (9, 18), was also employed here to fit the experimental data. log qe ) log Q0 + (ε/E)b

(1)

where qe [mg/g] is the adsorbed solute amount in equilibrium; Q0 [mg/g] is the adsorption capacity of solute; ε [KJ/mol], ε ) -RTln(Ce/Cs), is the effective adsorption potential; Ce [mg/ L] is the equilibrium aqueous concentration of solute; Cs

FIGURE 1. qe as a function of pH for 2,4-dichlorophenol, 4-chloroaniline, and naphthalene adsorption by MWCNT15 at given concentrations (C0).

FIGURE 2. Single-solute isotherms of naphthalene (pH 6.0), neutral DCP (pH 4.0), neutral PCAN (pH 6.0), dissociated DCP (pH 12.0), and dissociated PCAN (pH 1.0) on MWCNT15. Dotted lines represent the isotherm fitting of DA model. [mg/L] is the water solubility of solute; R [8.314 × 10-3 KJ/ (mol K)] is the universal gas constant; and T [K] is the absolute temperature; E [KJ/mol] is the “correlating divisor”; and b is a fitting parameter.

Results and Discussion Influences of Solution pH on Adsorption. Adsorption of DCP and PCAN on MWCNT15 at relatively low and high initial concentrations is varied with solution pH around their pKa but remains relatively constant at solution pH away from their pKa (Figure 1), as could be attributed to their dissociation (9). Moreover, neutral species of either DCP or PCAN have higher adsorption than the dissociated ones (Figure 1). However, adsorption of naphthalene is independent of solution pH in the pH region from 2.0 to 12.0 (Figure 1b) because naphthalene is a nonionizable chemical. Thus, competitive adsorption experiments were conducted in this study at pH 4.0 and 12.0 for the bisolute system of DCP and naphthalene, and at pH 6.0 and 1.0 for the bisolute system of PCAN and naphthalene, to examine the influences of neutral and dissociated DCP/PCAN separately on their competitive adsorption with naphthalene. Isotherms of naphthalene (at pH 6.0), neutral DCP (at pH 4.0), neutral PCAN (at pH 6.0), dissociated DCP (at pH 12.0) and dissociated PCAN (at pH 1.0) are nonlinear (Figure 2). For a given concentration, adsorption of investigated chemicals on MWCNT15 is in the order of naphthalene > neural DCP > neutral PCAN > dissociated DCP > dissociated PCAN (Figure 2). Isotherms of naphthalene, neutral DCP/PCAN can be fitted well by the DA model (Figure 2 and Table 2), which is in agreement with the reported results (9, 18). We

did not fit the isotherms of dissociated DCP/PCAN using the DA model because of the lack of the solubility data of dissociated DCP/PCAN. Neutral DCP has the highest adsorption capacity (Q0 ) 138 ( 1 mg/g) on MWCNT15 (Table 2), followed by neutral PCAN (Q0 ) 100 ( 1 mg/g) and naphthalene (Q0 ) 69.2 ( 1.1 mg/g). Neutral DCP also has the highest E (18.6 ( 0.5) and b (1.56 ( 0.09) values of the DA fitted isotherm (Table 2), followed by neutral PCAN (E ) 13.4 ( 0.4 and b ) 1.25 ( 0.09) and naphthalene (E ) 11.9 ( 0.5 and b ) 1.01 ( 0.04), as could be attributed to the interactions and the interaction strength between chemicals and MWCNT15 (9): (i) Hydrophobic interaction and π-π bonding interaction are responsible for naphthalene adsorption on MWCNT15, while hydrogen-bonding interaction, in addition to hydrophobic interaction and π-π bonding interaction, is responsible for the adsorption of neutral DCP/ PCAN; and (ii) The strength of hydrophobic interaction depends on the ratio of the concentration of a solute to its solubility, while that of π-π bonding interaction and hydrogen-bonding interaction depend on the solute π-polarity ability (π*) and hydrogen-bonding donor ability (Rm) respectively. Therefore, the highest adsorption of naphthalene at a given concentration (Figure 2) could be attributed to the highest hydrophobic interaction of naphthalene because the ratio of the given concentration to naphthalene solubility is higher than that to DCP/PCAN solubility (Table 1). The higher adsorption of neutral DCP than neutral PCAN (Figure 2) could be attributed to the higher π-polarity ability (π*) and hydrogen-bonding donor ability (Rm) of DCP than PCAN (Table 1). Dissociation of DCP/PCAN results in the disappearance of their hydrogen-bonding donor ability and the increase in their solubility (9), and thus, dissociated DCP/ PCAN have lower adsorption than their neutral species respectively. Maximum surface coverage of MWCNT15 by neutral DCP, neutral PCAN, and naphthalene, calculated by dividing their adsorption capacity (Q0) with the calculated monolayer adsorption capacity (Qm, Table 2), were 119%, 105%, and 80%, respectively. The 80% surface coverage of MWCNT15 by naphthalene indicates that a fraction (20%) of surface sites of MWCNT15 (type I in Figure 3) cannot be occupied by naphthalene. However, this fraction of surface sites could be occupied by neutral DCP/PCAN due to their higher surface coverage on MWCNT15. The hydrophilicity of these sites (4, 25), derived from the functional groups such as -OH and -COOH on MWCNT15 surface, could be responsible for the selected adsorption of neutral DCP/PCAN rather than naphthalene (26, 27) because neutral DCP/PCAN have higher solubility than naphthalene (Table 1). Thus, these sites could also be occupied by dissociated DCP/PCAN because they are more hydrophilic and have higher solubility than their neutral species. The 119% surface coverage of MWCNT15 by VOL. 44, NO. 8, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Results of DA Model Fits to Sorption Data of Naphthalene and Neutral DCP and PCAN on MWCNT15a primary adsorbate (competitor)

pH

C0b (mg/L)

Q0 (mg/g)

E (KJ/mol)

b

r2

Nc

Qmd (mg/g)

DCP DCP (naphthalene) PCAN PCAN (naphthalene) PCAN (naphthalene) PCAN (naphthalene) naphthalenef naphthalene (PCAN) naphthalene (PCAN) naphthalene (PCAN) naphthalene (DCP) naphthalene (DCP) naphthalene (DCP)

4 4 6 6 6 6 6 6 6 6 4 4 4

0 200 0 20 120 200 0 230 850 1500 180 350 660

138 ( 1e 138 ( 1 100 ( 1 87.1 ( 1.0 91.2 ( 1.0 91.2 ( 1.0 69.2 ( 1.1 63.1 ( 1.0 53.7 ( 1.0 40.7 ( 1.0 46.8 ( 1.0 41.7 ( 1.0 28.8 ( 1.0

18.6 ( 0.5 15.6 ( 0.2 13.4 ( 0.4 12.7 ( 0.1 10.5 ( 0.1 9.45 ( 0.09 11.9 ( 0.5 8.96 ( 0.23 7.42 ( 0.14 7.53 ( 0.08 10.1 ( 0.2 7.90 ( 0.11 6.91 ( 0.10

1.56 ( 0.09 1.14 ( 0.04 1.25 ( 0.09 1.23 ( 0.03 1.17 ( 0.02 1.12 ( 0.02 1.01 ( 0.04 0.976 ( 0.029 0.972 ( 0.021 1.02 ( 0.01 1.04 ( 0.05 0.933 ( 0.022 0.937 ( 0.021

0.992 0.998 0.994 0.999 0.999 1.000 0.999 0.999 0.999 1.000 0.997 0.999 0.999

20 20 16 16 15 16 12 18 19 18 17 17 15

116 95

86

a All estimated parameter values and their standard errors were determined by a commercial software program (origin 7.0) with a nonlinear regression program. b C0 is the initial concentration of competitors. c N ) number of observations. d Qm is the monolayer adsorption capacity (mg/g), calculated by the method described in ref 13. e Mean ( standard deviation. f Isotherm of naphthalene at pH 4.0 is the same with that at pH 6.0 because adsorption of naphthalene on MWCNT15 is pH-independent.

FIGURE 3. Schematic diagram of possible adsorption and competition of naphthalene (shaded circle), neutral DCP or PCAN (X), and dissociated DCP or PCAN (x) on MWCNT15 surface with different potential adsorption sites ((b) site I, (gray circle) site II, and (O) site III). The heterogeneity nature of the MWCNT15 surface is presented here by three different adsorption sites due to their hydrophilicity: site I > site II > site III. Site type I can be occupied by neutral and dissociated DCP/PCAN but not naphthalene because these sites are hydrophilic. Site type II can be occupied by neutral and dissociated DCP/PCAN and naphthalene. Site type III can be occupied by naphthalene and neutral DCP/PCAN but not dissociated DCP/PCAN because these sites could be too hydrophobic. neutral DCP indicates that adsorption of neutral DCP is above a simply monolayer adsorption, possibly due to that the adsorbed molecules are oblique to the surface of MWCNT15 with an angle (Figure 3) since the monolayer-adsorption capacity (Qm, Table 2) was calculated with an assumption (i.e., the adsorbed molecules are parallel to the surface of MWCNT15). The -OH group of neutral DCP and the -NH2 group of neutral PCAN could be responsible for this oblique attachment because these groups can form hydrogenbonding interactions with MWCNT15 surface for adsorption of neutral DCP/PCAN (9), in addition to the hydrophobic and π-π interactions of naphthalene with MWCNT15. The lower adsorption of dissociated DCP/PCAN than that of naphthalene and neutral DCP/PCAN (Figure 2) indicates that a fraction of surface sites of MWCNT15 (type III in Figure 3) could not be occupied by dissociated DCP/PCAN although they can be occupied by naphthalene and neutral DCP/PCAN. Competition between Naphthalene and Neutral DCP/ PCAN. Adsorption coefficients (Kd ) qe/Ce) of neural DCP (pH 4.0) at an initial concentration of 350 mg/L and neutral PCAN (pH 6.0) at an initial concentration of 230 mg/L decrease significantly with naphthalene as the competitor 3024

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(Figure 4A). Adsorption coefficients (Kd) of naphthalene at an initial concentration of 200 mg/L also decrease significantly with addition of DCP (pH 4.0) and PCAN (pH 6.0) as the competitors (Figure 4B). The addition of naphthalene decreases the E and b values of DA model-fitted neutral DCP/ PCAN isotherms but almost not their Q0 values (Table 2), in agreement with the reported results for PAHs (18). The relatively invariant adsorption capacity (Q0) of neutral DCP/ PCAN on MWCNT15 is a result of the adsorption of DCP/ PCAN on adsorbed naphthalene and the formation of bilayer of adsorbates on MWCNT15 surface in bisolute systems (Figure 3). The decrease of E and b values of neutral DCP/ PCAN isotherms could be attributed to the weaker interactions of DCP/PCAN with naphthalene relative to that with MWCNT15 because E and b are parameters accounting for the interactions and the interaction strength between chemicals and adsorbents (9). According to the π* and βm (hydrogen-bonding acceptor parameter) values of aromatic hydrocarbons (23), that are dependent on the number of aromatic rings in their molecules and in an order of benzene (π* ) 0.59 and βm ) 0.10) < naphthalene (π* ) 0.70 and βm ) 0.15) < phenanthrene (π* ) 0.80 and βm ) 0.20) < pyrene

FIGURE 4. Competition between naphthalene and neutral DCP (at pH 4.0) and PCAN (at pH 6.0) on MWCNT15: (A) Kd of 350 mg/L DCP (O) or 230 mg/L PCAN (×) as a function of equilibrium naphthalene concentration; (B) Kd of 200 mg/L naphthalene as a function of equilibrium DCP (]) or PCAN (4) concentration. (π* ) 0.90 and βm ) 0.25), it is reasonable that MWCNT15 has higher π* and βm values than naphthalene because the graphite sheets of MWCNT15 are made up with plenty of aromatic rings. Therefore, the strength of naphthalene (hydrogen-bonding acceptor) to form hydrogen-bonding interaction and π-π bonding interaction with neutral DCP/ PCAN (hydrogen-bonding donor) for DCP/PCAN adsorption should be weaker than that of MWCNT15. The addition of neutral DCP/PCAN decreases not only the E and b values of DA model fitted naphthalene isotherms but also the Q0 values (Table 2). The decrease of E and b values of naphthalene isotherms could also be attributed to the weaker π-π bonding interaction of naphthalene with neutral DCP/PCAN than that with MWCNT15 as mentioned above. The decrease of naphthalene adsorption capacity (Q0) indicates that the adsorbed DCP/PCAN on MWCNT15 is at least partly unavailable for naphthalene adsorption, as is different from the reported invariant adsorption capacity of naphthalene with other PAHs (18). As compared with the molecular structure of PAHs, functional groups of neutral DCP/PCAN (i.e., -OH group of neutral DCP and -NH2 group of neutral PCAN) should be responsible for the decreased naphthalene adsorption capacity because these groups prefer to form hydrogen-bonding interaction with water molecules rather than naphthalene molecules. Two possible types of attachment of neutral DCP/PCAN molecules could occur on MWCNT15: (i) DCP/PCAN molecules are attached on MWCNT15 via the π-π interaction of aromatic rings, which results in the -OH and -NH2 groups of DCP/PCAN molecules far away from the MWCNT15 surface to form hydrogenbonding interaction with water and thus prohibit naphtha-

FIGURE 5. Competition between naphthalene and dissociated DCP (at pH 12.0) and PCAN (at pH 1.0) on MWCNT15: (A) adsorbed amounts of dissociated DCP (at an given concentration of 480 mg/L) or PCAN (at an given concentration of 200 mg/L) as a function of equilibrium naphthalene concentration; (B) adsorbed amounts of naphthalene (at given concentrations of 700 or 300 mg/L) as a function of equilibrium concentration of dissociated DCP; (C) adsorbed amounts of naphthalene (at given concentrations of 1200 or 200 mg/L) as a function of equilibrium concentration of dissociated PCAN. lene adsorption (Figure 3); (ii) DCP/PCAN molecules are attached on MWCNT15 via the hydrogen-bonding interaction of the -OH and -NH2 groups, which results in the aromatic ring of DCP/PCAN molecules being exposed to water and possibly forming π-π bonding interaction with naphthalene because the aromatic rings are hydrophobic (Figure 3). The latter is reasonable because of the relatively invariant adsorption capacity of neutral DCP/PCAN (Table 2). Without the π-π interaction of DCP/PCAN with naphthalene, DCP/ PCAN would not adsorb on adsorbed naphthalene since the -OH and -NH2 groups of neutral DCP/PCAN prefer to form hydrogen-bonding interaction with water molecules rather than naphthalene, in conflict with the invariant adsorption capacity of neutral DCP/PCAN. Competition between Naphthalene and Dissociated DCP/PCAN. Adsorption of dissociated DCP (at pH 12.0) and PCAN (at pH 1.0) can be suppressed by added naphthalene as the competitor (Figure 5A), as shown by the decrease in both adsorption affinity and capacity of dissociated DCP/ PCAN (Figure 6). However, adsorption of naphthalene could not be suppressed by the dissociated DCP/PCAN (Figure 5B and C), i.e., both adsorption affinity and capacity of naphthalene are constant. The invariant adsorption of naphthalene in the presence of dissociated DCP/PCAN suggests that (i) no bilayer of adsorbed molecules can form in the bisolute systems of naphthalene with dissociated DCP/PCAN and (ii) the MWCNT15 surface sites occupied by adsorbed naphVOL. 44, NO. 8, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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contaminant sorption and thus the removal of contaminants from wastewater by CNTs. Moreover, the observed competitive adsorption of nonpolar naphthalene with polar DCP/ PCAN depends on the dissociation of DCP/PCAN, which is different from the competition between nonpolar PAHs (18). Thus, the observed competition data between nonpolar PAHs are also not useful for the prediction of the competitive adsorption of polar IOCs on CNTs. The observed dependence of competitive adsorption between nonpolar organic compounds and IOCs on the dissociation of IOCs is environmentally significant because IOCs can exist either as neutral or dissociated species in the environment having notable concentration higher than nonpolar organic chemicals due to their solubility. The competitive adsorption of nonpolar naphthalene with polar DCP/PCAN on CNTs is also different from that of organic chemicals by metal ions (12, 19, 20), NOM (12, 22) and surfactants (21). In the assessment of the application of CNTs as sorbents and the environmental risks of CNTs and organic contaminants, therefore, competitive adsorption and the influence of species of organic contaminants on competition should be accounted for. The competitive adsorption of organic chemicals, including at least nonpolar aliphatics, polar aliphatics, and polar aromatics, with other chemicals is still poorly undstood, thus should address this important issue.

Acknowledgments This work was supported by NSF of China (20737002, 40973065 and 20777065), the Program for New Century Excellent Talents in University of China (NCET-08-493) and Zhejiang Provincial NSF of China (Z507093). FIGURE 6. Sorption isotherms of dissociated DCP at pH 12.0 (A) and dissociated PCAN at pH 1.0 (B) on MWCNT15 without or with naphthalene as the competitor. thalene molecules are not be available for the adsorption of dissociated DCP/PCAN via displacement. Otherwise, naphthalene adsorption affinity should be decreased if naphthalene can adsorb on adsorbed molecules of dissociated DCP/ PCAN to form bilayer because the interactions of naphthalene with dissociated DCP/PCAN are weaker than that with MWCNT15, as has been observed above for the adsorption of naphthalene with neutral DCP/PCAN (Table 2). Moreover, the adsorption capacity of dissociated DCP/PCAN should not be decreased by the addition of naphthalene if molecules of dissociated DCP/PCAN can adsorb on adsorbed naphthalene molecules to form bilayer, as has been observed above for neutral DCP/PCAN in the presence of naphthalene (Table 2) but not for dissociated DCP/PCAN (Figure 6). The decreased adsorption of dissociated DCP/PCAN (Figures 5A and 6) could be explained by the displacement of adsorbed molecules of dissociated DCP/PCAN with naphthalene molecules since no bilayer of adsorbed solutes can form in the bisolute systems. However, not all of the adsorbed molecules of dissociated DCP/PCAN can be displaced by naphthalene because the adsorption amounts of dissociated DCP/PCAN on MWCNT15 decrease to a constant value with the increase of added naphthalene dose (Figure 5A), i.e., there are some surface sites (type I in Figure 3) of MWCNT15 that can be occupied by the dissociated DCP/PCAN molecules but not the naphthalene molecules because of the hydrophilicity of these sites due to their oxygen containing functional groups such as -OH and -COOH (26, 27). Environmental Implications. The competition between naphthalene and the neutral and dissociated species of DCP/ PCAN for CNT surface observed in this study suggests that using single-solute sorption data may overpredict the contaminant sorption on CNTs in the environment and wastewater because multiple contaminants are generally present there. The competition results in the decrease of 3026

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Supporting Information Available Figure of DA model-fitted isotherms for naphthalene and neutral DCP/PCAN and table of Freundlich model-fitted parameters of isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

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