Norfloxacin Sorption and Its Thermodynamics on Surface-Modified

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Environ. Sci. Technol. 2010, 44, 978–984

Norfloxacin Sorption and Its Thermodynamics on Surface-Modified Carbon Nanotubes Z H E N Y U W A N G , †,‡ X I A O D O N G Y U , †,‡ B O P A N , ‡,§ A N D B A O S H A N X I N G * ,‡ College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China, Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, and College of Environmental Science & Technology, Kunming University of Science & Technology, Yunnan, 650093, China

Received September 14, 2009. Revised manuscript received November 6, 2009. Accepted December 3, 2009.

Adsorption on carbon nanotubes (CNTs) may affect the environmental behavior of organic contaminants including antibiotics. In this study, sorption of norfloxacin (NOR) onto graphitized multiwall CNTs (G-CNTs), carboxylated multiwall CNTs (C-CNTs), hydroxylated multiwall CNTs (H-CNTs), and activated carbon (AC) was investigated. All sorption isotherms were highly nonlinear and were fitted well by Freundlich and Polanyi-Manes models. AC showed the highest NOR sorption capacity because of its highest surface area. H-CNTs had much higher NOR sorption than C-CNTs, and the π-π electron donor-acceptor (EDA) interactions could explain the distinction between the two types of CNTs. Comparison of sorption coefficients at different pHs indicates that hydrophobic and electrostatic interaction also played major roles in sorption of NOR on CNTs. Furthermore, high sorption capacity and hysteresis of NOR on CNTs were demonstrated in this study, which needs to be considered for predicting environmental risks of CNTs and NOR. The results from thermodynamic analysis show that sorption of NOR on AC and CNTs was thermodynamically favorable and generally endothermic. Sorption site energy analysis illustrates a distribution of sorption energy, consistent with nonlinear isotherms, which indicates the heterogeneous sites on CNTs for NOR adsorption.

Introduction Increasing concern has been raised regarding the potential risks of antibiotics to human and ecological health because of their extensive use. For example, antibiotic-resistant genes could be induced among microorganisms via prolonged exposure to relatively low antibiotic concentrations (1). Carbon nanotubes (CNTs) as novel materials have strong sorption affinity for organic compounds (2-9). CNTs may have significant impacts on the fate and transport of antibiotics if they are released to the environment. Norfloxacin (NOR) is a fluoroquinolone antibacterial agent with high antibacterial activity against both Gram-negative and Gram-positive bacteria through inhibition of DNA gyrase * Corresponding author phone: (413) 545-5212; fax: (413) 5453958; e-mail: [email protected]. † Ocean University of China. ‡ University of Massachusetts. § Kunming University of Science & Technology. 978

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(10). It is widely used in human and veterinary medicines. NOR has been found in a wide range of environmental samples around the world (11, 12). It cannot be removed completely during wastewater treatment using current technologies (13), thus, enters the environment. Application of manure as fertilizer and leaking from septic systems can also lead to the direct release of NOR into surface water. Previous research has examined NOR sorption by different environmental matrices, including alluvium, silica, alumina, and soils (14-17), but not on CNTs. Hydrophobic effect, electrostatic interaction, and ionic exchange have been proposed as sorption mechanisms. However, the properties of CNTs are significantly different from those materials and the sorption mechanisms can be different. Furthermore, sorption properties of CNTs could be modified by surface chemistry changes (18, 19). Presently, sorption capacity and exact sorption mechanisms of NOR by CNTs are unknown. The effect of surface oxygen-containing functional groups of CNTs on NOR sorption needs to be studied as well. Understanding the thermodynamics of the sorption process is not only critical for interpretation of sorption mechanism but also for CNTs practical applications. Although the sorption thermodynamics of metals is widely studied, little research has been conducted to determine the thermodynamic parameters of organic compounds sorption (19). Thermodynamics of sorption can be concentration-dependent (20); however, the current approach obtains thermodynamic information by conducting sorption experiments only at a single concentration point, which may be of limited use. Hence, it is important to study the thermodynamics of NOR sorption on CNTs using different sorbate loadings. Therefore, the main objectives of the present work were to (1) investigate the sorption capacity and desorption hysteresis of NOR on surface-modified CNTs; (2) examine the possible major mechanisms in the NOR-CNTs sorption process; and (3) reveal the effect of oxygen-containing functional groups on the sorption of NOR by CNTs. The results from this study will provide useful information for potential NOR removal by CNTs as well as their environmental risk assessment.

Materials and Methods Materials. The CNTs used in this study were graphitized multiwall CNTs (G-CNTs), carboxylated multiwall CNTs (C-CNTs), and hydroxylated multiwall CNTs (H-CNTs). CNTs were synthesized by the chemical vapor deposition method. G-CNTs were produced by treating synthesized CNTs under inert gas at 2800 °C for about 240 h. C-CNTs and H-CNTs were produced by KMnO4 oxidation in HCl solution at different temperatures and different KMnO4 concentrations. All the CNTs were purchased from Chengdu Organic Chemistry Co., Chinese Academy of Sciences and used as-received. Activated carbon (AC) from wood charcoal was from Fisher Scientific. NOR with purity of 99.7% was purchased from MP Biomedicals, its properties are listed in Table S1 (Supporting Information). Selected physicochemical properties of the CNTs and AC are listed in Table S2. Sorption-Desorption Experiments. The isotherms were obtained by batch experiments (21) performed at 288 ( 1 K, 298 ( 1 K, and 310 ( 1 K, respectively. Briefly, NOR was dissolved in a background solution using 0.01 mol L-1 CaCl2 and 200 mg L-1 NaN3 in deionized distilled water. A weighed quantity of sorbents (2.0 mg for AC, 3.0 mg for G-CNTs and C-CNTs, and 2.5 mg for H-CNTs) and the aqueous solution (15 mL) of NOR with a series of initial concentrations (10-40 mg L-1) were added into 16-mL Teflon-lined screw cap glass 10.1021/es902775u

 2010 American Chemical Society

Published on Web 12/23/2009

vials. The initial concentrations of NOR were chosen based on preliminary experiments, so that sorbed amount of NOR would be controlled between 20 and 80% of the initial amounts. The vials were placed on a rotary shaker in the dark for 72 h (the time as determined by the preliminary study) at set temperatures. After centrifugation (3000 rpm for 15 min), the concentration of NOR in the supernatant was analyzed by an Agilent 8453 UV spectrophotometer at the maximum adsorption wavelength of 272 nm. Samples for each sorption isotherm data point were conducted in duplicate. The pH of the system started at 6.7 and changed little during the whole sorption process. The same concentration series of NOR solutions without sorbent were run under the same condition as the controls, showing that the loss of the initially added amounts of NOR was less than 3%. Therefore, the amount of NOR adsorbed by sorbents was calculated by the mass difference. The desorption experiment was conducted immediately after sorption (22) at the same temperatures as the sorption experiments. Twelve mL of supernatant was removed from the vials, and the same volume of background solution was added. The vials were sealed and shaken for an additional 72 h. Then the vials were centrifuged, the concentration of NOR in the supernatant was determined, and the sorbed amount of NOR was calculated by mass difference. Repeated procedures were conducted for the second and third desorption cycles. To compare the pH effect, single concentration point sorption experiments were conducted at different pHs using initial concentrations between 25 and 26 mg L-1 at 298 K. Various amounts of NaOH or HCl solution were used to adjust the pH of the system. The same process as above was followed. Infrared Analysis. Weighed quantity of sorbents and aqueous solution of NOR with a same initial concentration (25 mg L-1) was shaken in the dark for 72 h at neutral pH. Supernatant were removed carefully from the vials after being centrifuged. The sorbents were washed 3 times with deionized distilled water to remove NOR which remained in solution. The freeze-dried sorbents with NOR, without NOR, and pure NOR (5 mg) were each mixed gently with 95 mg of KBr using a mortar and pestle and analyzed with diffuse reflectance infrared Fourier transform spectroscopy (DRIFT; PerkinElmer, USA). The blank was collected with pure KBr. DRIFT spectra were recorded from 450 to 4000 cm-1 over 200 scans. The DRIFT spectra of the sorbed NOR were obtained by subtracting the spectrum of the corresponding adsorbent from the complex spectrum.

Data Analysis Sorption Isotherm Fitting. Three different models were used in this work to fit the sorption isotherms. The equations are described as follows: Freundlich model (FM): Se ) KF × (Ce)n

(1)

where Se (mg kg-1) is the solid-phase concentration, Ce (mg L-1) represents the aqueous-phase concentration, and KF and n are the Freundlich sorption parameters. Langmuir model (LM): Se )

SL0 × b × Ce 1 + b × Ce

(2)

where SL0 (mg kg-1) is the LM sorption capacity, and b (L mg-1) is the LM sorption affinity parameter. Polanyi-Mane model (PMM):

( ( ( )) ) Cs Ce

Se ) Sp0 × exp Z × RTln

d

(3)

where Sp0 (mg kg-1) is the PMM sorption capacity, Cs stands for the solubility of NOR at 20 °C (400 mg L-1 at pH ) 7), Z and d are PMM sorption parameters, R is the universal gas constant (8.314 × 10-3 kJ mol-1 K-1), and T is absolute temperature (K). The standard coefficient of determination (r2) is influenced by both the number of data points and the number of fitting parameters; therefore, the adjusted coefficient of determination (radj2) was calculated to investigate the fitting results of the three models mentioned above: radj2 ) 1 -

(m - 1)(1 - r2) m-p-1

(4)

where m is the number of data points used for fitting, and p is the number of parameters in the fitting equation. Thermodynamic Analysis. Study of the temperature dependence of sorption provides valuable information regarding the energetic changes during sorption. The sorption isotherms of NOR on AC, G-CNTs, C-CNTs, and H-CNTs at 288, 298, and 310 K were obtained to determine the thermodynamic parameters. The sorption coefficient K0 for the sorption was defined as: K0 )

Se Ce

(5)

The standard Gibbs free energy change (∆G0), standard enthalpy change (∆H0), and standard entropy change (∆S0) were determined from K0 by the following equations: ∆G0 ) -RT × ln K0

(6)

∆G0 ) ∆H0 - T∆S0

(7)

Rearrangement of eqs 6 and 7 yields: ln K0 ) -

∆H0 ∆S0 + RT R

(8)

The ∆H0 and ∆S0 were obtained from the slope and intercept of the linear plot of lnK0 against 1/T. To investigate sorption/desorption hysteresis, the thermodynamic index of irreversibility (TII) was calculated using the equation proposed by Sander et al. (23). In this study, a Freundlich form of TII from their paper was adopted. TIIFreundlich ) 1 -

ndesorb nsorb

(9)

where ndesorb and nsorb are the nonlinear factors for desorption and sorption isotherms, respectively. Site Energy Distribution. The relationship between the sorption energy and the equilibrium aqueous solute concentration can be described as below:

(

Ce ) Cs × exp -

E* RT

)

(10)

where E* is the difference of sorption energy at Ce and Cs. Rearrangement of eq 1 and eq 10 yields:

(

Se ) KF × (Cs)n × exp -

nE* RT

)

(11)

F(E*) was derived from the Freundlich model (24) as: VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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F(E*) )

-dSe(E*) dE*

(12)

Differentiating eq 11 with respect to E* and combining it with eq 12 yields F(E*) )

KFn(Cs)n nE* × exp RT RT

(

)

(13)

Equation 13 was used for site sorption energy distribution analysis for sorption data obtained in this study.

Results and Discussion Sorption Isotherms. Sorption isotherms of NOR on CNTs and AC at different temperatures are presented in Figure 1. Figure 1A compares the sorption of NOR between different sorbents on a unit mass basis while Figure 1B compares the sorption on a unit surface area basis. LM failed to describe the isotherms, but FM and PMM fitted the sorption isotherms well (Table 1). Only the results from FM fitting are discussed here because sorption nonlinearity and desorption hysteresis can be easily compared based on this isotherm model. In addition, FM-based thermodynamics and energy distribution analysis are widely available in literature and thus the comparison with the literature data could be readily made. For all sorbents, sorption of NOR was highly nonlinear (n ) 0.11-0.18), suggesting the highly heterogeneous distribution of sorption energy. The sorption coefficient (K, Table 1, calculated at Ce ) 0.01Cs) at a same temperature followed the order of AC > H-CNTs > G-CNTs > C-CNTs on the unit mass basis. However, the order changed to G-CNTs > H-CNTs > C-CNTs > AC on the unit surface area basis (K′, Table 1). This result can be understood from the following two aspects: (1) the availability of surface area on AC and CNTs is different. AC had a much larger BET surface area (664 m2g-1) than CNTs (117-228 m2g-1). However, not all the surfaces of AC were accessible to NOR because NOR molecules are larger than N2 molecules, which were used to measure surface area. Therefore, although AC could sorb a larger number of NOR molecules than CNTs on the unit mass basis, the efficiency of NOR occupying AC sorption sites was low. (2) NOR molecules could be better packed on CNT surface than on AC because CNTs had more regular surface morphology than AC. Previous studies indicate that molecules could not get into the inner pores of the CNTs (21); and there were three types of sorption sites available on CNTs (Figure S1A): the surface, groove areas, and interstitial pores (8). It is also known that the sites on CNT surfaces are highly ordered (20). NOR molecule has a nearly planar structure (25) (Figure S2), with one benzene ring and one heterocyclic hexa-ring, so that NOR could have high packing efficiency on the axis direction of CNTs (Figure S1B). Thus, CNTs had higher surface area-normalized K′ than AC. In addition, the higher peaks in DRIFT spectra could indicate higher sorbed quantity; the DRIFT peak intensity in this study (Figure S5) followed the same order with K′, providing further data for understanding the higher K′ of NOR on CNTs. To compare our results with the published data, the following discussion is based on the unit mass basis. Sorption Mechanisms. Oxygen-containing functional groups including -OH and -COOH groups could change the sorption properties of organic molecules (5, 7, 19) via reducing the hydrophobicity of CNT surface. For example, sorption of naphthalene and nitroaromatic compounds (7, 19) was depressed after CNT oxidation. An interesting observation in our study is that, even though both H-CNTs and C-CNTs had oxygen-containing functional groups, H-CNTs sorbed more NOR molecules than G-CNTs, while C-CNTs had lower sorption than G-CNTs (Figure 1 and Table 1). 980

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These results suggest that the surface chemistry was an important factor influencing the sorption process. As shown in Figure 2A, sorption coefficients of three different CNTs had a similar trend with pH changes. In the range of pH < 7.2, K of NOR increased with increasing pH, while at pH > 7.2, K declined with increasing pH. NOR has two pKa values and can be positively charged (cationic), negatively charged (anionic), and/or zwitterionic. NOR is mostly cationic at pH < 6.2, while anionic NOR is the dominant species at pH > 8.5 (Figure S3). The zwitterionic form of NOR dominates at pH 6.2-8.5. The surface of all CNTs was positively charged at pH lower than pHPZC (around 4) and negatively charged at pH higher than pHPZC. Thus, electrostatic interaction between charged NOR and charged CNT surface is expected to dominate in the sorption process. Taking H-CNTs for an example, at pH < 4.0 and pH > 8.5, sorption of NOR on H-CNTs is expected to be depressed, because NOR molecules and CNT surfaces had a same sign of charge and could repel each other. At pH between 4.0 and 6.2, the NOR molecules and CNT surface had opposite charges, thus sorption was expected to be enhanced. These are consistent with the experimental data (Figure 2A). Therefore, electrostatic interaction between NOR and the CNTs was one of the major factors controlling the sorption process. As the outer surface of CNTs provides evenly distributed hydrophobic sites (20), hydrophobic interaction is another mechanism for NOR sorption on CNTs. This hypothesis was verified by the observation that the sorption coefficient of NOR reached maximum at pH ) 7.2 (Figure 2A). At this pH, the lowest NOR solubility was observed (26), indicating its highest hydrophobicity compared to other pHs. Although K of three different CNTs had a trend similar to the function of pH, the difference among CNTs was obvious (Figure 2A). Even though the hydrophobicity of H-CNTs is reduced by surface functional groups, the K value was still higher than that of G-CNTs. Therefore, there could be additional mechanisms contributing to the NOR sorption process. The π-π electron donor-acceptor (EDA) interaction has been considered as one of the predominant driving forces for the sorption of chemicals with benzene rings on CNTs (5, 27-30). At a neutral pH, the benzene ring on NOR can function as a π-electron-acceptor due to the strong electronwithdrawing ability of the fluorine group. Carboxyl groups on CNTs make C-CNTs electron acceptors, while hydroxyl groups make H-CNTs electron donors (31). As proposed in the EDA theory, the interactions between a π-donor compound and a π-acceptor compound are much stronger than that between donor-donor or acceptor-acceptor pairs. Therefore, the force of the EDA interaction between C-CNTs and NOR decreased compared to G-CNTs. For the same reason, the sorption of NOR on H-CNTs significantly increased relative to G-CNTs. Hydrogen bonds (H-bonds) have been proposed as a mechanism for understanding sorption of aromatics on AC (32). Similarly, NOR molecule with two CdO and one O-H groups might form H-bond with the surface-oxygen of AC, as well as with the functional groups on CNT surface. In addition, the benzene ring on CNT surface may act as H-bond donor (33) and form H-bonds with oxygen-containing functional groups on NOR molecules. Clearly, further study is required to evaluate the importance of H-bonds in the NOR-CNTs sorption process though an insignificant role of H-bonds was reported for substituted aromatics and CNTs (27). The DRIFT measurements were performed to compare the different sorption of NOR on CNTs and AC. Three strong absorption peaks were observed in the spectrum of free NOR (Figure S4). The peaks at 1731 and 1619 cm-1 correspond to the stretching vibration of the carboxylic (υCOOH) and

FIGURE 1. Sorption of NOR on different sorbents at 288 K (O), 298 K (1), and 310 K (0). Lines stand for the Freundlich model fitting of AC (s), C-CNTs (- -), G-CNTs ( · · · ), and H-CNTs (- · · -) respectively. A (left): solid phase concentration on unit mass basis. B (right): solid phase concentration on unit surface area basis. carbonyl (υCO) groups (34), and the peak at 1484 cm-1 conforms to CH2 deformation vibration (35). It is clear that there were two peaks in the spectra of H-CNTs and G-CNTs (Figure S5). One is observed at 1743 cm-1, which may be the shift of the characteristic peak of NOR (υCOOH). The other one is observed at 1634 cm-1, which could be the shift of υCO groups in NOR. These two shifts (from 1731 to 1743 cm-1 and from 1619 to 1634 cm-1) indicate stronger bonding (perhaps EDA interaction) of NOR on H-CNTs and G-CNTs as compared to C-CNTs. The high sorption on AC may be explained by pore-filling (36-38) which may not result in any significant DRIFT peak shift. Desorption Hysteresis. Sorption-desorption hysteresis has been widely observed for organic chemicals, such as polycyclic aromatic hydrocarbons (PAHs) sorption on geosorbents (39), benzenes on charcoal (40), and pesticides on sediments (41). In this work, desorption hysteresis was generally observed for all sorbents at the three temperatures. Desorption isotherms clearly shifted to the upper-left direction from the sorption isotherms (Figure S6), indicating that NOR molecules tend to remain adsorbed with sorbents during desorption. Most previous studies focused on the sorption process of CNTs.Onlylimitedworkhasaddressedthesorption-desorption hysteresis of organic compounds from CNTs. Two endocrinedisrupting chemicals (EDCs) showed hysteresis on CNTs (8), but no significant hysteresis was observed for atrazine (42) or PAHs (22). Both the rearrangement of aggregates (22) and

the penetration of sorbate into closed interstitial spaces (43) were proposed to explain the desorption hysteresis from fullerene. Similarly, rearrangement of CNTs bundles or aggregates could occur after sorption, causing the different pathways between adsorption and desorption (8), which may be the case for NOR and CNTs. TII values were calculated according to eq 9 and listed in Table S3. For AC, the effect of temperature is not obvious. When the temperature increased from 288 to 310 K, the TII value changed little (0.988) at the low initial NOR concentration (10 mg L-1), and from 0.797 to 0.751 at a higher initial NOR concentration (40 mg L-1). However, NOR desorption from CNTs was influenced greatly by temperature change. For example, TII of C-CNTs changed from 0.881 to 0.340 (less hysteretic) at the low initial NOR concentration (10 mg L-1), and 0.619 to 0.292 at a higher initial NOR concentration (40 mg L-1). The CNTs used in present study were not sonicated and most of the CNTs were present as bundles and aggregates in solution. The bundles of CNTs might get relatively loose when the temperature of the sorption system increased, which might affect the TII values of CNTs at different temperatures. The structure of AC aggregates might not be influenced much by temperature, thus the difference among TII values had no obvious change. Exact mechanism needs further investigation. Thermodynamic and Site Energy Distribution Analysis. Temperature can influence the sorption process, and there are two major effects. One is that the rate of molecular VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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a

288 298 310 288 298 310 288 298 310 288 298 310

temperature (K)

106.59 112.48 123.18 56.86 57.55 56.66 51.18 54.44 48.70 75.32 76.34 72.04

4.40 4.43 4.45 4.12 4.14 4.12 4.05 4.06 4.06 4.21 4.24 4.24

radj2 0.778 0.733 0.792 0.826 0.763 0.686 0.819 0.949 0.631 0.758 0.886 0.703

b (L mg-1) 4.64 5.50 2.53 2.95 5.47 3.49 1.68 1.32 5.07 1.55 2.48 7.01

KF 81.31 85.69 88.12 39.09 42.30 39.08 32.90 33.73 36.08 46.52 51.87 54.91

logK′ (Ce ) 0.01Cs) (L m-2) -1.07 -1.05 -1.03 -0.95 -0.93 -0.95 -1.17 -1.16 -1.20 -1.17 -1.14 -1.14

KF [(mg kg-1)/(mg L-1)n] sorption capacity coefficient from Freundlich model.

H-CNTs

C-CNTs

G-CNTs

AC

sorbent

SL0 (g kg-1)

logK (Ce ) 0.01Cs) (L kg-1)

Langmuir model

TABLE 1. Fitting Results of Isotherms for NOR on CNTs and ACa

FIGURE 2. (A) Sorption of NOR on AC, G-CNTs, C-CNTs, and H-CNTs as a function of pH. (B) Dependence of sorption energy on different sorbate loading at 288 K. (C) Plots of standard enthalpy change (∆H0) of NOR sorption on different sorbents. Symbols stand for AC (]), G-CNTs (∆), C-CNTs (0), and H-CNTs (*).

diffusion increases with increasing temperature, meanwhile the viscosity of solution decreases. Thus, it can be easier for sorbate molecules to cross the external boundary layer and move into the internal pores of sorbents. The second effect is that the sorption capacity of sorbents might change as the temperature changes. Thermodynamic parameters, standard Gibbs free energy change (∆G0), standard enthalpy change (∆H0), and standard entropy change (∆S0), can provide information on the inherent energetic changes involved during the sorption process. As can be seen from Figure 1, when Ce was in the range of 0-25 mg L-1 for AC, 0-15 mg L-1 for H-CNTs, and 0-8 mg L-1 for C-CNTs, the sorption increased with increasing temperature. Because the sorption site energy distribution

0.11 0.12 0.13 0.15 0.14 0.16 0.16 0.17 0.13 0.18 0.15 0.13

n 0.974 0.983 0.987 0.970 0.980 0.991 0.973 0.937 0.937 0.979 0.959 0.991

radj2 4.37 4.40 4.42 4.08 4.11 4.08 4.01 4.03 4.04 4.18 4.20 4.22

logK (Ce ) 0.01Cs) (L kg-1)

Freundlich model

-1.10 -1.07 -1.06 -0.99 -0.96 -0.98 -1.20 -1.19 -1.18 -1.20 -1.17 -1.16

logK′ (Ce ) 0.01Cs) (L m-2) 156.85 142.42 165.03 65.50 77.40 98.54 67.57 58.55 106.79 123.62 89.00 104.63

Sp0 (g kg-1) 0.971 0.987 0.988 0.969 0.973 0.989 0.983 0.984 0.991 0.978 0.990 0.989

radj2

4.37 4.41 4.42 4.10 4.12 4.08 4.02 4.04 4.03 4.18 4.23 4.22

logK (Ce ) 0.01Cs) (L kg-1)

Polanyi-Mane model

-1.11 -1.07 -1.06 -0.97 -0.95 -0.98 -1.20 -1.17 -1.18 -1.20 -1.15 -1.15

logK′ (Ce ) 0.01Cs) (L m-2)

(heterogeneity) was different at different temperatures, values of n varied with temperatures in this study. Consequently, the isotherms at three temperatures obtained from the Freundlich model crossed each other when aqueous-phase concentration increased. As discussed above, thermodynamic parameters could provide meaningful information on sorption process, and can be concentration-dependent. However, only limited research has examined the thermodynamics at different sorbate loadings, and no previous study has reported the crossing of fitting isotherms at different temperatures. Therefore, to make this discussion useful for comparing with literature, the following thermodynamic analysis is discussed in the specific Ce ranges of sorbates before isotherm crossing point (i.e., 0-25 mg L-1 for AC, 0-15 mg L-1 for H-CNTs, and 0-8 mg L-1 for C-CNTs). The plots in Figure 2 and Figure S7 were determined by eqs 7 and 8. Because the plot of lnK0 versus 1/T was not linear for G-CNTs, thermodynamic parameters are not available for G-CNTs. The negative value of ∆G0 for AC, C-CNTs, and H-CNTs indicates that the sorption process was thermodynamically favorable and spontaneous. The less negative the ∆G0, the weaker the driving force of sorption. As shown in Figure S7B, ∆G0 became more negative (absolute value of ∆G0 increased) as temperature increased at the same sorbate loading, suggesting that the driving force of sorption increased with increasing temperature. The affinity of the solute for the sorbent surface was related to the position of the energy distribution mean on the energy axis (44), the higher the value of mean energy, the higher sorption affinity. As shown in Figure S8B, the average sorption energy (E*) for all the sorbents follows an order of 310 K > 298 K > 288 K, suggesting that higher temperatures were favorable for the NOR sorption. The ∆G0 analysis was consistent with energy analysis data below, confirming that the affinity of all the sorbents increased with increasing temperature. For a given sorbent at a certain temperature, ∆G0 became less negative when the amount of sorbed NOR increased, meaning the driving force became weaker. This is in keeping with the calculation of E* (calculated from eq 11). As illustrated in Figure 2B and Figure S8A, E* decreased dramatically for all the sorbents with increasing NOR loading, revealing that NOR molecules occupied the high-energy sorption sites first at low concentration and then spread to low-energy sorption sites. This conclusion is also supported by the decreased TII with NOR loading (Table S3). Thus larger E* and TII could be related to stronger sorption affinity. As shown in Figure 2C, the ∆H0 values for AC, C-CNTs, and H-CNTs were positive at three temperatures, indicating that sorption of NOR on these sorbents was endothermic. ∆H0 increased with increasing NOR loading on AC, but decreased for CNTs, reflecting that the sorption process became less endothermic for CNTs but more endothermic for AC with increasing solid-phase loading. Environmental Implication. CNTs are considered as a special group of sorbents because of their strong sorption affinity and capacity for organic contaminants. As observed in this study, CNTs showed high NOR sorption capacity and adsorption/desorption hysteresis; the surface properties of CNTs played important roles in sorption process. Thus, organic contaminants could be selectively removed by specially functionalized CNTs. Additionally, CNTs may potentially affect the environmental fate and transport of NOR (e.g., solid wastes from wastewater treatment plants) and possibly their toxicity and risks. CNTs also have the potential to affect aquatic life (45). CNTs with adsorbed NOR may exhibit the toxicity of both the NOR and CNTs once they are in contact with living organisms. The synergistic or antagonistic effect of toxic chemicals and CNTs have received little attention up to date. Therefore, it is important to

consider the coexistence of CNTs and toxic chemicals as the potential risk of CNTs is evaluated.

Acknowledgments This research was supported by the USDA Hatch program (MAS 000978) and the Cheung Kong Scholar Program, Education Ministry of China.

Supporting Information Available NOR properties (Table S1); selected properties of carbon nanotubes and activated carbon (Table S2); thermodynamic index of irreversibility (Table S3); sorption coefficients of NOR on environmental matrices in literature (Table S4); schematic diagrams for sorption of NOR on CNTs (Figure S1); optimized 3-dimensional structures of NOR (Figure S2); NOR solution speciation (Figure S3); DRIFT spectra of NOR and NOR after sorbed on CNTs and AC (Figure S4, S5); desorption isotherms of NOR from CNTs and AC (Figure S6); plots of ∆S0 and ∆G0 of NOR sorption (Figure S7), and sorption energy and site energy distribution of NOR sorption (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

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