Electrochemically enhanced adsorption of nonylphenol on carbon nanotubes: Kinetics and isotherms study

Journal of Colloid and Interface Science 415 (2014) 159–164

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Electrochemically enhanced adsorption of nonylphenol on carbon nanotubes: Kinetics and isotherms study Xiaona Li, Shuo Chen ⇑, Liying Li, Xie Quan, Huimin Zhao Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, PR China

a r t i c l e

i n f o

Article history: Received 25 July 2013 Accepted 14 October 2013 Available online 24 October 2013 Keywords: Multi-walled carbon nanotubes Nonylphenol Electrosorption

a b s t r a c t Removal of nonylphenol (NP) from aqueous solution has attracted widely attention due to its aquatic toxicity and potential to disrupt the endocrine system. In an effort to develop the effective and environmentfriendly treatment method for NP, adsorption of 4-n-nonylphenol (4-NP) on multi-walled carbon nanotubes (MWCNTs) under electrochemical assistance was studied. The adsorption kinetics and isotherms were investigated at different polarization potentials and compared with those of open circuit (OC) and powder MWCNTs adsorption. The adsorption kinetics was simulated by the model including pseudo-first-order model, pseudo-second-order model and intraparticle diffusion model. The isotherm was simulated with Langmuir model and Freudlich model, respectively. Experimental results indicated that 4-NP is able to be efficiently removed at a potential of 0.6 V. Comparing with that of powder MWCNTs adsorption, the initial adsorption rate t0 at 0.6 V increased 7.9-fold according to pseudo-second-order model and the maximum adsorption capacity qm improved 1.7-fold according to Langmuir model. The improved adsorption effect at negative potential was ascribed to enhanced p–p electrondonor–acceptor (EDA) interaction between 4-NP and MWCNTs under electrochemical assistance. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction The environmental pollution caused by organic chemicals is getting serious with the development of industrialization. Thus, development of pollution control technology toward contaminants removal is highly desired. Nonylphenol (NP) as an anaerobic biodegradation by-product of non-ionic surfactant nonylphenol ethoxylates (NPEOs) is recognized as an endocrine disruptor due to its property of mimicing the action of natural hormones [1]. In addition, NP distributes widely in aquatic environments, soil and sediment via the application of sludge, industrial effluents or atmospheric deposition [2–4]. Despite relatively low concentration of NP was detected in natural water, it still affects water environment and is toxic to aquatic organisms [5,6]. Therefore, removal of NP has received much environmental concern. Although numerous treatment processes including biodegradation [7], photochemical oxidation [8,9] or electrochemical decomposition [10,11] were developed to remove NP, highly efficient and environment-friendly approach is lacking. Electrosorptive removal technique has the advantages such as low energy cost, high removal efficiency and environmental friendliness [12–14]. In general, electrosorption is defined as a

⇑ Corresponding author. Fax: +86 411 84706263. E-mail address: [email protected] (S. Chen). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.021

potential-induced adsorption on the surface of charged electrodes [15], where the adsorption rate and capacity of the adsorbent could be improved by electrochemical polarization. Porous carbon materials such as activated carbon and carbon aerogel, have been used as electrosorption electrodes in the literature [15–17]. However, the high electron transfer resistance and low mechanical strength limit their widely application [18]. On the other hand, carbon nanotubes (CNTs) as a promising adsorbent have been applied in pollutants removal by electrochemical assistant adsorption because of the large surface area, high conductivity and superb electrochemical stability [19]. Our previous work indicated that multi-walled carbon nanotubes (MWCNTs) showed high removal efficiency in electro-assistant adsorption of typical pollutants perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) [20]. To further extend the application of CNTs in water treatment, the electrosorption behavior of 4-NP on MWCNTs and the adsorption kinetics were investigated in this work. MWCNTs electrodes were prepared by electrophoretic deposition (EPD); the morphology and characteristics of which were described by scanning electron microscopy (SEM), infrared spectra (IR) and cyclic voltammetry (C–V) curves. Electrosorption kinetics and isotherms under different polarization potentials were simulated by various models and compared with respect to open circuit (OC) and powder MWCNTs adsorption. Based on these studies, the mechanism of electrosorption was proposed.

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2. Experimental 2.1. Materials The pristine MWCNTs (purchased from Chengdu Organic Chemicals Co. Ltd.) were synthesized by a chemical vapor deposition method using nickel as catalysts (purity > 95%, out diameters < 8 nm, inner diameters < 5 nm, length: 10–30 lm). The MWCNTs were pretreated with the method described in our previous work [21]. Simply, MWCNTs were heated at 350 °C for 30 min to remove amorphous carbon, and added into concentrated HNO3 and H2SO4 (1:3) stirring for 24 h at room temperature to remove metal catalysts. Then, MWCNTs were washed with Milli Q water and filtered until the pH was nearly neutral. Resulted MWCNTs were dried and stored in desiccator. The specific surface area (SBET), average pore diameter (Daverage) and total pore volume (Vtotal) of pristine and treated MWCNTs were determined by nitrogen adsorption at 77 K using a surface area analyzer (Quadrasorb SI4, Quantachrome, USA). The average length of the MWCNTs was measured with transmission electron microscopy (TEM). The elemental compositions were obtained from an elemental analyzer (vario EL III, elementar, Germany). 4-n-nonylphenol (4-NP) used in this research (purity > 98%) were purchased from Alfa Aesar chemical Co., Ltd. The typical physicochemical properties were listed in Table 1. Actonitrile (99.8% HPLC grade), obtained from Fisher Scientific (USA), and Milli Q water were used for HPLC analysis. Standard stock solution of 4-NP (200 mg/L) was prepared by dissolving solid standards in methanol, and stored in 4 °C. 2.2. Fabrication of MWCNTs electrodes MWCNTs based electrodes were prepared with EPD method developed by Wu et al. [25], and depicted simply as follows: The MWCNTs was added into anhydrous isopropanol (250 mg/L) and sonicated for 1 h to disperse MWCNTs. Sonication continued for another 15 min to get Mg2+-adsorbed MWCNTs suspension by adding Mg(NO3)26H2O. Then, the suspension of 10 mL was put into a quartz cell, where a platinum foil was used as anode and a polished titanium sheet was used as cathode keeping a distance of 10 mm between two electrodes. EPD was performed at 160 V potential and sustained for 3 min. The amount of MWCNTs coated on titanium sheet was determined as the difference between MWCNTs electrode and blank Ti plate, which was weighted to be 1 mg for each electrode.

Electrosorption experiments were conducted in conventional three electrodes cell configuration, which was sealed with quartz cover in order to avoid volatilization. An electrochemical workshop (CH instruments Model 611D, Shanghai Chenhua Co., China) was applied to supply potential. The distance between working Table 1 Properties of 4-NP. Properties

Specification

Molecular formula Structure

C15H24O

Molecular formula (g/mol) Solubility in water (mg/L) pKa log Kow Vapor pressure (mPa) Henry’s law constant (Pa m3/mol)

2.4. Quantitative analysis of 4-NP The concentrations of 4-NP were quantified by Waters 2695 high-performance liquid chromatography (HPLC) with waters 2475 fluorescence detector (Waters company, USA) equipped with a SunFire ODS reverse–phase column (150  4.6 mm, 5.0 lm). The mobile phase was acetonitrile/water (80:20, v/v) with flow rate of 1.0 mL/min, and the Ex/Em wavelength was 228 nm and 305 nm, respectively. 2.5. IR spectra and C–V analysis The FT-IR spectra of pristine MWCNTs and MWCNTs coated on electrode were measured by an infrared spectrometer from Bruker Co., Ltd. The C–V curves of blank solution and 100 lg/L 4-NP solution in three electrodes cell configuration were recorded in the range of 1.2 to 1.2 V at a scan rate of 10 mV/s by an electrochemical workshop. 3. Results and discussion 3.1. Characterization of MWCNTs electrodes

2.3. Adsorption/electrosorption experiments

CH3

electrode (MWCNTs electrode) and counter electrode (platinum foil) was about 18 mm. The saturated calomel electrode (SCE) served as reference electrode. The electrosorption kinetics of 4-NP (100 lg/L) were carried out in 20 mL aqueous solution containing 1 mM Na2SO4 as electrolyte at different potentials of 0.6, 0.3, 0.3, 0.6 V and open circuit potential. For comparison, adsorption kinetics of 4-NP (100 lg/L) on powder MWCNTs were performed in 20 mL aqueous solution containing 1 mM Na2SO4 in a quartz cell. Isotherm experiment condition of electrosorption (0.6 V), OC adsorption and powder MWCNTs adsorption was the same as that of kinetics, and 4-NP concentration was in the range of 100 lg/L–4 mg/L. All the adsorption experiments were performed at room temperature (25 °C). The pH value of 4-NP solution was nearly neutral, and kept unchanged after electrosorption or powder MWCNTs adsorption. To investigate the experimental uncertainty resulted from volatilization of 4-NP or adsorption on reactor cell, recovery experiments were conducted in quartz cells containing 20 mL 4-NP solution in the concentration range of 100 lg/L–4 mg/L. The solutions were stirred at the same rate as the adsorption batch experiments for 5 h. The recovery was determined to be 98.9 ± 8.4% (n = 6), and the adsorbed amounts of 4-NP was calculated directly according to the mass loss.

CH2

220.36 5.4–8.0 [22] 10.7 [22] 5.8 [23] 4.55 [24] 11.2 [24]

8

OH

Fig. 1 shows the N2 adsorption–desorption isotherms and pore size distribution curves of pristine and treated MWCNTs. It was observed from Fig. 1(a) that there is a closed adsorption–desorption hysteresis loop with the relative pressure above 0.2, which is suggested to be due to the mesopores with a capillary condensation [26]. The pore size distribution curves shown in Fig. 1(b) also indicate the mesopore structure of MWCNTs. SBET and Daverage of treated MWCNTs are determined to be 519 m2/g and 4.5 nm, respectively (Table 2). It was speculated that large surface area and mesoporous structure are advantageous for high adsorption capacities [27,28]. In addition, it was found that the SBET and Daverage of treated MWCNTs decreased compared with those of pristine MWCNTs. Also, the C, H and N contents decreased and the oxygen content increased according to the result of elemental analysis. It is obvious that introduction of oxygen functional groups lead to the SBET and Daverage decreased, and the polarization of MWCNTs strengthened, which contributed to fabricate stable MWCNTs electrode.

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(a)

600 pristine MWNTs

1050

1640

T (%)

400

3

V/cm (STP)g

-1

pristine MWCNTs treated MWCNTs

treated MWNTs 1710 1570 1060 1640 1180

200 3450

0.0

0.2

0.4

0.6

0.8

1.0

4000

3000

1000 -1

(b)

Fig. 2. FT-IR spectra of MWCNTs.

0.6 dV (logd) (cm3/g)

2000

Wavenumbers cm

RelativePressure(P/P0)

pristine MWCNTs treated MWCNTs

0.4

0.2

0.0 0

30

60

90 120 150 Pore Diameter (nm)

180

210

Fig. 1. N2 adsorption–desorption isotherms (a) and pore size distribution curves (b) of pristine and treated MWCNTs.

The FT-IR spectra of MWCNTs in Fig. 2 shows the presence of hydroxy groups (3450 cm1), carboxyl (1640 cm1) groups and lactonic groups (1050 cm1) on pristine MWCNTs. After treatment with acid, stronger peaks were shown at 1640 and 1060 cm1, indicating more carboxyl and lactonic groups were formed on the surface of MWCNTs. In order to characterize the functional groups more clearly, the FT-IR spectra of pristine and treated MWCNTs at 3700-2700 cm-1 and 2000-800 cm-1 were given in Fig. S1 separately (shown in supplementary material). Consistent with the results from FT-IR analysis, the decrease in C content and increase in H and O contents were found for the acid treated MWCNTs compared with pristine MWCNTs (Table 2). The SEM images of MWCNTs coated electrode were shown in Fig. 3. It was found that the MWCNTs were arranged uniform over the whole Ti sheet. The length was about 100–300 nm without obvious defect. The thickness of MWCNTs films on Ti sheet is about 15 lm by profile measurement of electrode. The surface area and the interstitial and groove areas among carbon nanotubes are assumed to be the main adsorption sites [29]. The cyclic voltammetry curves of blank solution and 4-NP solution are shown in Fig. 4. There is no obvious oxidation or reduction peaks appeared in the scan range of 1.2 to 1.2 V, indicating that 4-NP and MWCNTs are chemically stable, and no chemical reaction happened when the applied voltage on electrode surface was in the range of ±1.2 V.

3.2. Electrosorption kinetics

potentials of 0.6, 0.3, 0.3 and 0.6 V are shown in Fig. 5(a) and (b). Adsorption kinetics of 4-NP on powder MWCNTs is also given in Fig. 5 for comparison. It was found that adsorption rates dramatically increase in the first half hour for all electrosorptions at different potentials and reach apparent equilibrium within 2 h. For the powder MWCNTs, a longer period at least 3 h was needed to reach equilibrium. Compared with the common adsorbent for example granular activated carbon, the equilibrium time for 4-NP electroassistant adsorption on MWCNTs was also short [30,31]. The removal efficiencies (RE) under different conditions shown in Fig. 6 were sequenced in the order of: 0.6 V > 0.3 V OC > 0.3 V > 0.6 V > powder MWCNTs. The highest RE at 0.6 V is 98.6%, which is better than those of OC adsorption (RE: 92%) or powder MWCNTs adsorption (RE: 62.6%), indicating that 4-NP could be removed efficiently by imposing a low potential on MWCNTs. To gain insight into the kinetics, Lagergren’s pseudo-first-order kinetic model and pseudo-second-order model were both used for kinetic data fitting. Their equations are described below: Pseudo-first-order model:

lgðqe  qt Þ ¼ lgqe 

k1 t 2:303

ð1Þ

Pseudo-second-order model:

t 1 1 1 t ¼ þ t¼ þ qt k2 q2e qe m0 qe

ð2Þ

where qe and qt (mmol/g) are the amounts of contaminants adsorbed on the adsorbents at equilibrium and a given time (h), respectively. k1 (1/h) and k2 (g/mmol/h) are the adsorption rate constants of the pseudo-first-order model and the pseudo-second-order model, respectively. v0 is the initial adsorption rate (mmol/h/ g). Since our data were found to best match the pseudo-second-order model with correlation coefficients (R2) in the range of 0.9814– 0.9997, thus, the adsorption rate is assumed to be controlled by chemical adsorption and the adsorption capacity is correlated to the numbers of active sites on adsorbents [32]. Pollutant adsorption on porous adsorbent is generally described in three stages: external diffusion, intraparticle diffusion and adsorption on active sites [33]. In order to clarify the mechanism, the intraparticle diffusion model in Eq. (3) was used to fit the adsorption kinetics: 1

The adsorption rate and efficiency were determined from adsorption kinetics. The amounts of adsorbed 4-NP under OC condition and under polarization conditions with various

qt ¼ kd t 2

ð3Þ

where qt (mmol/g) is the amount of contaminants adsorbed on the adsorbents at time t (h) and kd (mmol/g/h1/2) is the adsorption rate

Table 2 Selected physicochemical parameters of MWNTs. Adsorbent

SBET (m2/g)

Daverage (nm)

Vtotal (cm3/g)

N (%)

C (%)

H (%)

O (%)

Pristine MWNTs Treated MWNTs

562 519

5.7 4.5

0.80 0.57

0.18 0.16

95.36 89.40

0.22 0.35

3.84 10.07

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Fig. 3. SEM images of MWCNTs electrode of low-magnification (a) and high-magnification (b) prepared by EPD.

100

RE (%)

80 60 40 20 0 -0.6 V -0.3 V Fig. 4. C–V curves of blank solution and 4-NP solution (working electrode: MWCNTs, counter electrode: Pt, reference electrode: SCE, electrolyte: 1 mM Na2SO4).

OC

0.3 V 0.6 rVMWNTs e powd

Fig. 6. The removal efficiency of 4-NP reaching adsorption equilibrium at 0.6, 0.3, 0.3, 0.6 V, OC adsorption and powder MWCNTs adsorption, respectively.

dramatically when t1/2 is lower than 1, then approaches adsorption equilibrium gradually. Furthermore, the liner curve simulation by intraparticle diffusion model show positive intercepts, indicating that external diffusion participates in the electrosorption process and intraparticle diffusion is not the only rate-controlling step. The polarization potential imposed on the electrode has an important influence on the adsorption of 4-NP. According to Table 3, initial adsorption rate t0 under different conditions is sequenced in the order of 0.6 V > 0.3 V > OC > 0.3 V > 0.6 V > powder MWCNTs, indicating that both the positive and negative potentials are able to improve t0. Notably, t0 at 0.6 V (0.182 mmol/h/g) was improved by 1.3-fold and 7.9-fold compared to those of OC adsorption (0.135 mmol/h/g) and powder MWCNTs adsorption (0.023 mmol/h/g), respectively. 3.3. Electrosorption isotherms Since the highest t0 was obtained from the electrosorption of 4-NP at 0.6 V, electrosorption isotherm experiments were

Fig. 5. Adsorption kinetics curves of 4-NP under different conditions. (a) Simulated with pseudo-first-order kinetics model. (b) Simulated with pseudo-second-order kinetics model.

constant. Fig. 7 shows the modeling result of 4-NP adsorption on MWCNTs under different conditions. According to Boyd’s report [34], linear relationship can be obtained between qt and t1/2 with the line passing through the origin in case that intraparticle diffusion is the only rate-controlling step. As shown in Fig. 7, qt increases

Fig. 7. Adsorption kinetics curves of 4-NP were simulated with intraparticle diffusion model.

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X. Li et al. / Journal of Colloid and Interface Science 415 (2014) 159–164 Table 3 The parameters of 4-NP adsorption on MWNTs simulated by pseudo-first-order and pseudo-second-order models. Volt (V)

Pseudo-first-order parameter

0.6 V 0.3 V OC 0.3 V 0.6 V Powder MWNTs

Pseudo-second-order parameter 2

k1 (1/h)

R

13.3 14.1 10.8 10.5 8.8 2.7

0.9821 0.9767 0.9642 0.9730 0.9286 0.9788

conducted at 0.6 V compared with OC adsorption and powder MWCNTs adsorption. The isotherms are depicted in Fig. 8. All isotherms show non-linear characters and were simulated by both Langmuir model and Freundlich model. The two model equations are expressed below:

qe ¼

bqm C e 1 þ bC e

ð4Þ

qe ¼ K F C ne

ð5Þ

where Ce and qe are the concentration of contaminants in water and adsorbed on adsorbent reaching adsorption equilibrium, respectively. KF and n are Freundlich constants related to adsorption capacity and adsorption intensity of the adsorbents, respectively. qm is the maximum adsorption capacity, and b is the adsorption equilibrium constant of Langmuir model related to the affinity of binding sites. The corresponding parameters and regression coefficients R2 are listed in Table 4. On the basis of regression constant R2, Langmuir model showed better simulation of electrosorption performance than Freundlich model under all experimental conditions. According to Table 4, the adsorption capacity of 4-NP was notably improved by electro-assistant adsorption on MWCNTs. For example, qm at 0.6 V is 0.55 mmol/g, which is 1.6-fold and 1.7-fold relative to those of OC adsorption (0.35 mmol/g) and powder MWCNTs adsorption (0.32 mmol/g), respectively. Furthermore, in comparison with the adsorption of 4-NP on granular activated carbon [30] and marine sediments [35], the adsorption capacity were also increased. Considering that Langmuir model is mainly suitable for monolayer adsorption on smooth and homogeneous surface and Freundlich model is mainly suitable for adsorption on surfaces with nonuniform energy distribution, our results indicate the possible monolayer adsorption on MWCNTs [33]. 3.4. Adsorption mechanisms Various mechanisms such as hydrophobic interaction, p–p interaction, hydrogen bond and electrostatic interaction have been proposed to simultaneously act on the adsorption of organic chemical on CNTs. The adsorption of 4-NP on MWCNTs is similar to that

0.5

-0.6 V OC powder MWNTs

qe (mmol/g)

0.4 0.3

Langmuir Freudlich

0.2 0.1 0.0 1E-6

1E-5

1E-4

1E-3

0.01

Ce (mmol/L) Fig. 8. Adsorption isotherms of 4-NP under conditions of 0.6 V potential, OC potential and powder MWCNTs adsorption.

t0 (mmol/h/g)

k2 (g/mmol/h)

R2

0.182 0.163 0.135 0.122 0.090 0.023

2197.8 2262.4 1920.0 1771.8 1325.6 651.9

0.9997 0.9979 0.9988 0.9967 0.9814 0.9924

of the reported system by Chen et al. [36] and Yang et al. [37]. A proposed mechanism was suggested in Fig. 9. First, carbon nanotubes contain polarized electron-rich and electron-depleted sites. For example, the surface defects in the vicinity of the edges or some replaced sites of functional groups of CNTs are often electron-rich, while the regions in the center of the graphene surface are typically electron-depleted. On the other hand, –OH group as a strong electron-donating substituent makes the benzene ring of 4-NP electron-rich, thus the p–p electron-donor–acceptor (EDA) interaction between p-electron-rich benzene ring of 4-NP and p-electron-depleted regions on graphene sheet of MWCNTs was assumed to play a domain role on 4-NP’s adsorption [38]. p–p interaction also occurs between carbonyl groups on MWCNTs and the benzene ring of 4-NP. Second, –OH and –COOH groups observed on acid-treated MWCNTs can interact with –OH groups of 4-NP through hydrogen bond. Therefore, hydrogen bonding is one mechanism for the adsorption of 4-NP on MWCNTs. Especially, the H and O contents increased of treated MWCNTs compared with that of pristine MWCNTs, resulting in the strengthened hydrogen bonds interaction. Third, since the pKa value of 4-NP is 10.7, 4-NP is a neutral compound with low water solubility and highr Kow value (n-octanol– water partition coefficient) under our working conditions at pH 7 (Table 1). Thus, hydrophobic interaction may also contribute to the adsorption. On the basis of above analysis, we proposed that once applying a negative potential on MWCNTs coated electrode, the electron donating ability of MWCNTs could be improved due to the accumulation of negative charges. Accordingly, the p–p interaction between 4-NP and MWCNTs is strengthened because of improved p electron polarization, leading to higher t0 and qm at 0.6 V in comparison with those of OC adsorption and powder MWCNTs adsorption. Furthermore, compared t0 at the potential of 0.3 V and 0.6 V simulated by pseudo-second-order model (shown in Table 3), improving t0 was observed with increasing negative potential, which is also the result of strengthening p–p interaction. In addition, one might suspect that molecular attraction existing between the surface-adsorbed and dissolved 4-NP molecules through hydrogen bonding may contribute to the observed adsorption [39]. For example, molecular attraction for chlorinated phenols has been reported [40]. However, this effect was not likely the factor for enhanced adsorption under polarization of negative potential. Since the qm value of 4-NP on MWCNTs is 0.55 mmol/g as simulated by Langmuir model, which was equivalent to a 24% surface coverage calculated using the effective diameter of 4-NP of 0.69 nm (calculating method for the effective diameter was shown in supplementary material). Therefore, the adsorption was single-layered at negative potential. Regarding the enhancement of t0 at positive potential of 0.3 V (5-fold) and 0.6 V (4-fold) compared with that of powder MWCNTs, it is probably due to the interaction between positively charged MWCNTs surface [20] and p-electrons of benzene ring in 4-NP. In fact, similar behaviors have been reported for the other aromatic chemicals [15,41].

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Table 4 Langmuir and Freundlich model parameters under different experimental conditions. Experimental conditions

0.6 V OC Powder MWNTs

Langmuir constants

Freundlich constants

qm (mmol/g)

b (L/mmol)

R

KF (mmol(1-n) Ln/g)

n

R2

0.55 ± 0.07 0.35 ± 0.04 0.32 ± 0.04

3.3E4±1.1E4 2.9E3±0.9E3 3.7E2±0.9E2

0.9740 0.9623 0.9793

51.73 ± 51.47 3.53 ± 2.15 5.08 ± 2.87

0.52 ± 0.10 0.39 ± 0.09 0.60 ± 0.10

0.9505 0.9102 0.9283

OH

2

OH HO

OH Hydrogen bond

Hydrogen bond

HO

HO HO

HO

OH C

OH

HO

O

(1)

+

C

(2)

O

π

HO HO

C O

OH

C O

π

π – π interaction

OH

OH HO

OH

π

C O

π

OH

C O

π

OH

OH

OH

π

π

Hydrophobic interaction

π – π interaction

Hydrophobic interaction

Fig. 9. A diagram of mechanism for adsorption/electro-assistant adsorption of 4-NP on MWCNTs. (1) Adsorption and (2) electro-assistant adsorption.

4. Conclusions In summary, we showed that electrochemical assistant adsorption on MWCNTs is able to efficiently remove 4-NP. The initial adsorption rate t0 and the maximum adsorption capacity qm were greatly increased with respect to OC adsorption and powder MWCNTs adsorption. Especially, t0 and qm at 0.6 V enhanced by 7.9-fold and 1.7-fold with respect to those of powder MWCNTs, respectively. According to the structural properties of MWCNTs and 4-NP, the enhanced adsorption rate and adsorption capacity of 4-NP on MWCNTs by electro-assistant adsorption was ascribed to the improvement of p–p electron-donor–acceptor interaction between 4-NP and MWCNTs. This study provides not only an effective energy-saving approach for 4-NP removal, but also a promising strategy for application of carbon nanotubes in water treatment. Acknowledgments The work was supported by the National Basic Research Program of China (2011CB936002) and the Fundamental Research Funds for the Central Universities (DUT13LK19). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.021. References [1] M. Sole, M.J.L. de Alda, M. Castillo, C. Porte, K. Ladegaard-Pedersen, D. Barcelo, Environ. Sci. Technol. 34 (2000) 5076–5083. [2] M. Ahel, W. Giger, M. Koch, Water Res. 28 (1994) 1131–1142. [3] J. Dachs, D.A. Van Ry, S.J. Eisenreich, Environ. Sci. Technol. 33 (1999) 2676– 2679. [4] A.M. Soto, H. Justicia, J.W. Wray, C. Sonnenschein, Environ. Health Perspect. 92 (1991) 167–173. [5] E. Argese, A. Marcomini, P. Miana, C. Bettiol, G. Perin, Environ. Toxicol. Chem. 13 (1994) 737–742.

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