Adsorption removal of ciprofloxacin by multi-walled carbon nanotubes with different oxygen contents from aqueous solutions

Adsorption removal of ciprofloxacin by multi-walled carbon nanotubes with different oxygen contents from aqueous solutions

Chemical Engineering Journal 285 (2016) 588–595 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevie...

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Chemical Engineering Journal 285 (2016) 588–595

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Adsorption removal of ciprofloxacin by multi-walled carbon nanotubes with different oxygen contents from aqueous solutions Fei Yu a,b,1, Sainan Sun a,1, Sheng Han a,⇑, Jie Zheng c, Jie Ma b,⇑ a

School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Hai Quan Road, Shanghai 201418, China State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China c Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Carbon nanotube adsorbents with

different oxygen content were synthesized.  Oxidized carbon nanotube have excellent adsorption properties.  Investigated relationship between adsorption properties and oxygen content.  Adsorption mechanism and effect of environmental factors were discussed.

a r t i c l e

i n f o

Article history: Received 5 September 2015 Received in revised form 14 October 2015 Accepted 18 October 2015 Available online 20 October 2015 Keywords: Adsorption Removal Ciprofloxacin Multi-walled carbon nanotubes Oxygen content

a b s t r a c t The oxidized multi-walled carbon nanotubes (MCNTs) were used as adsorbents to investigate the effects of oxygen contents on adsorption properties of ciprofloxacin (CPX). With the oxygen content increasing from 2.0% to 5.9%, the normalized maximum adsorption capacity of CPX appeared growing state. However, the increment rate became slower, which is mainly attributed to p–p electron donor–acceptor interaction. The promotion of hydrophilicity and dispersibility, and the inhibition of water cluster had played a coordinate role in the whole adsorption process of CPX onto MCNTs. The isotherm adsorption data were more appropriate to Dubinin–Radushkevich and Langmuir model. Moreover, the MCNTs with the best adsorption properties was chosen to investigate adsorption kinetics and the effect of environmental factors (dosage, and pH, ionic strength) on CPX adsorption. The experimental kinetic data showed that intra-particle diffusion and outer diffusion may both present in the removal process to control the adsorption rate. CPX adsorption strongly depended on the pH of the solution. The alkaline condition was not conducive to the adsorption of CPX on MCNTs. However, the ionic strength had no significant effect on the adsorption capacity of CPX onto MCNTs. Therefore, the electrostatic interaction may be the main adsorption mechanism in the adsorption process. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction With the widespread use of antibiotics, pharmaceutical effluents containing antibiotics have recently attracted wide attentions ⇑ Corresponding authors. Tel./fax: +86 21 60873182 (S. Han), +86 21 65981831 (J. Ma). E-mail addresses: [email protected] (S. Han), [email protected] (J. Ma). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.cej.2015.10.039 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

since it has potential adverse effects. Although the injuriousness of antibiotics is not so intuitive like other environmental pollution, residues of the antibiotic drug have become a seriously ignored problem. The abuse of antibiotics can damage the immune function of animals, when again infected, and then they need more antibiotics to treat. Thus, it could sink into a vicious cycle [1]. Ciprofloxacin (CPX, C17H18FN3O3), one of the extensively used antibiotics in the world, can lead to the growing of

F. Yu et al. / Chemical Engineering Journal 285 (2016) 588–595

antibiotic-resistant bacteria even if the concentration is lower. Meanwhile, CPX is difficulty biodegradable substance. Once released into the aquatic environment, CPX easily enriches to induce resistant strains, which will have a serious impact on the ecological environment. Therefore, before being discharged into the drainage system, CPX must be removed to a permissible level from the wastewater [2]. For antibiotic contaminants with such characteristics, relatively stable, non-biodegradable, highly toxic, and cumulative effect, adsorption may be an effective way to remove antibiotics due to low cost, high efficiency, and good feasibility. For MCNTs have the unique structure of the porous and hollow, large specific surface area, and exist multiple interactions between the contaminants and MCNTs, which has an excellent potential for the removal of organic contaminants in aqueous phase, causing widespread concerns [3–14]. Moreover, surface chemical functionalization of MCNTs can have an impact on adsorption of target pollutants [15–18]. Carabineiro et al. [4] selected three types of carbon-based materials: activated carbon, carbon nanotubes, and carbon xerogel as adsorbents for the removal of CPX, the above results showed that MCNTs exhibited the most excellent adsorption properties for per specific surface area (SSA). Sheng et al. [17] conducted a compared adsorption experiment of asprepared and oxidized MCNTs, finding that the adsorption capacity became lower after oxidation treatment. In addition to the repulsion of negative charges, p–p dispersion forces caused by the introduction of carboxyl group between MCNTs and the aromatic ring of ionizable aromatic compounds, which may also obstruct adsorption. Recently, the adsorption of organics on MCNTs was mainly focused on improving the adsorption performance, exploring divergence caused by different physical and chemical properties of organic pollutant itself. However, few studies had reported the influence of MCNTs’ properties on organics adsorption. Carbon nanotubes were oxidized and modified by sodium hypochlorite, which can effectively improve the adsorption performance of organic pollutants on MCNTs [19]. However, relatively few studies had reported effects and mechanisms of oxygen content and surface functional groups of carbon nanotubes on adsorption performance [20,21]. MCNTs as adsorbents and released to the environment after adsorption process, which may be contacted with some oxidant, and then functionalized and purified. Therefore, a clear understanding of adsorption properties and the effect of oxygen content on the surface of oxidized carbon nanotubes play a significant role in assessing the environmental risks. MCNTs with different oxygen contents were obtained by regulating the concentration of NaClO solution, and then they were chosen as adsorbents to study adsorption characteristics of CPX on MCNTs with different oxygen contents. From the perspective of surface oxygen content and oxygen-containing functional groups of the adsorbent material, we studied the intrinsic link between characteristics of MCNTs with different oxygen contents and adsorption properties of CPX. Moreover, we investigated the interaction mechanism between carbon nanotubes and CPX. To ensure the accuracy of the adsorption properties of the adsorbent in actual wastewater treatment, we chose MCNTs with 4.7% oxygen contents (CNTs-4.7%O), which exhibit the best adsorption properties, as an adsorbent to investigate the effect of different environmental factors (pH, ionic strength, and dosage) on CPX adsorption.

2. Materials and methods MCNTs were oxidized by different concentrations of sodium hypochlorite (NaClO) to be loaded different oxygen contents. The oxygen content were measured by X-ray photoelectron spectro-

589

scopic (XPS), XPS analysis was carried out in a Kratos Axis Ultra DLD Spectrometer, using monochromated Al Ka X-rays, at a base pressure of 1  109 Torr. The MCNTs with different oxygen content, which are prepared in the previous studies [19], used in this study were named CNTs-2.0%O, CNTs-3.2%O, CNTs-4.7%O, and CNTs-5.9%O. Ciprofloxacin, of assay >90% were purchased from Sigma, Yanyu (Shanghai) Chemical Reagent Co., Ltd. All other reagents were analytical grade and were used as received. All adsorption experiments were carried out in a series of 150 ml flasks containing 20 mg MCNTs and 40 ml CPX solutions. To obtain adsorption isotherms, 20 mg MCNTs were contacted with 40 ml of different concentrations of CPX solutions (10– 160 mg/L). The mixtures were then shaken at 180 r/min for 24 h at 298 K. When reaching adsorption equilibrium, the appropriate solution was gathered to filter using a syringe with 0.45 lm filter membrane and then determine by UV–Vis spectrophotometer (UV759UV-VIS, Shanghai Precision & Scientific Instrument Co. Ltd.) at 275 nm. Simultaneously conducting blank experiments was CPX solution without adsorbent to eliminate the effects caused by agents. In the adsorption kinetics experiment, 20 mg of MCNTs were added to 40 ml of 150 mg/L constant concentration CPX solution at 298 K. At predetermined time intervals, the MCNTs were collected through a 0.45 lm membrane filter and the filtrate was analyzed by UV–Vis method. Furthermore, additional experiments were also carried out to detect the influence of dosage, pH, and ionic strength on the adsorption properties. The effect of dosage was performed to select an adsorbent dose of 10, 20, 30, and 40 mg. The effect of pH on CPX sorption by MCNTs was evaluated by adjusting the pH values of the solutions to the designated values. Used in the experiment was 0.01 M HCl or 0.01 M NaOH with the initial concentration of 150 mg/L of CPX. To study the effect of ionic strength, predetermined amounts of NaCl were added to obtain 0.05 M, 0.1 M, 0.2 M, and 0.4 M ionic strength solutions with a CPX concentration of 150 mg/L at 298 K. All adsorption experiments were performed to ensure a single variable. The equilibrium adsorption amount of CPX was calculated as follows:

qe ¼

ðC 0  C e Þ  V m

ð1Þ

where C0 and Ce are the CPX concentrations (mg/L) in the initial solution and at equilibrium, respectively; V is the volume of the solution and m represents the weight of the MCNTs used for adsorption studies. The nonlinear forms of Langmuir and Freundlich models were expressed as follows:

qe ¼

qm K l C e 1 þ K lCe 1

qe ¼ K f C en

ð2Þ ð3Þ

where Ce and qe are the concentrations of adsorbate in water, and the amount of adsorbate adsorbed to adsorbent when the adsorption equilibrium is reached, respectively. qm is the maximum adsorption capacity, and Kl is Langmuir constant (L/mg) related to the energy of adsorption and the affinity of the binding sites. Moreover, Kf is Freundlich constant, also known as a capacity factor associated with adsorption capacity ((mg/g)(L/mg)1/n). 1/n related to adsorption intensity, which is a dimensionless empirical parameter. To deepen the understanding of adsorption mechanism, the Dubinin–Radushkevich (D–R) isotherm model was chosen to apply on adsorption study. The linear form of D–R model was expressed as follows:

590

ln qe ¼ ln qm  Be2

F. Yu et al. / Chemical Engineering Journal 285 (2016) 588–595

300

where B is a constant related to the mean free energy of adsorption (mol2/kJ2), qm is the theoretical saturation capacity (mg/g), and e is the Polanyi potential, which can be calculated from Eq. (5)

200

ð5Þ

where R (J/(mol K) is thermal equilibrium constant and T (K) is the absolute temperature. The slope of the plot of ln qe versus e2 gives B (mol2/kJ2), and the intercept yields the theoretical saturation capacity, qm. The pseudo-first-order model was represented by the following equation:

log ðqe  qt Þ ¼ log qe  k1 t

t 1 1 ¼ þ t qt k2 q2e qe

ð7Þ

where k2 (g/mg min) is the rate constant of pseudo-second-order model. Similarly, values of k2 and qe can be calculated from the intercept and slope of the linear plot of t/qt versus t, respectively. Intra-particle diffusion model was represented by the following equation:

qt ¼ kid t 0:5 þ C

ð8Þ

where kid (mg/g min0.5) is the rate constant of intra-particle diffusion model. As before, Value of kid was obtained from the slope of the linear plot of qt versus t0.5 and C (mg/g) is the intercept. 3. Results and discussion 3.1. Effect of dosage In general, the dose of the adsorbent has an impact on overall adsorption properties in adsorption process, the optimal dose is the basis of reaching maximum adsorption performance. Adsorption experiments were carried out by various amounts of CNTs4.7%O from 10 to 40 mg, and the results were shown in Fig. 1. The adsorption capacity of CPX decreased slowly from 209.6 to 167.7 mg/g with the increase of adsorbent dosage from 10 to 40 mg. Clearly, the maximum adsorption capacity of CPX on CNTs-4.7%O was reached at an adsorbent dosage of 10 mg, but the weighing error is relatively large. Taking into account the adsorption capacity and weighing error, an adsorbent dosage of 20 mg was selected finally as the optimal dose for further study. 3.2. Effects of solution pH The solution pH that can affect the physiochemical properties of the adsorbate is a major factor controlling the CPX molecule adsorption process. Furthermore, changes in pH can alter the surface electric charge of adsorbent and adsorbate, which affects the electrostatic interactions between the adsorbent and adsorbate, thereby changing the adsorption behavior. To further investigate the adsorption process, the pH was adjusted in the range of 2–10 and the effects of pH on the adsorption capacity of CPX onto CNTs-4.7%O were explored. The results were presented in Fig. 2. It proved the fact that the CPX adsorption strongly depended on the pH of the solution.

0

10

20

30

40

Fig. 1. Effect of adsorbent dose on CPX adsorption by CNTs-4.7%O.

200

ð6Þ

where qt and qe (mg/g) are the amounts of CPX adsorbed per unit mass of adsorbent at any time t (min) and equilibrium, respectively. k1 (min1) is the rate constant of pseudo-first-order model. Values of k1 and qe can be calculated from the slope and intercept of the linear plot of log ðqe  qt Þ versus t, respectively. The pseudosecond-order kinetic model can be expressed as follows:

100

Dosage (mg)

160 ( / ) qe(mg/g)

e ¼ RT ln ð1 þ 1=C e Þ

qe (mg/g)

ð4Þ

120 80 40 0 2

4

6

8

10

pH Fig. 2. Effect of pH on CPX adsorption by CNTs-4.7%O.

As shown in Fig. 2, the maximum adsorption capacity of 192.4 mg/g for CPX was obtained at pH 4. The alkaline condition was not conducive to the adsorption of CPX on CNTs-4.7%O. CNTs-4.7%O has an overall negative surface charge at pH 2–10 [22]. CPX has two pKa values of 6.1 and 8.7, respectively. CPX can exist as a cationic, zwitterionic and anionic form in three regions divided by the two pKa. When the pH value was less than 6.1, CPX in the cationic form of existence generated electrostatic attraction with negatively charged CNTs-4.7%O on the surface, accordingly facilitating the adsorption. While at low pH, the competitive inhibition of excessive hydrogen ions and other coexistence cations will reduce the adsorption effect. Therefore, adsorption capacity at pH = 2 was slightly lower than that at pH = 6. CPX was zwitterionic and anionic form at 6.1 < pH < 8.7 and pH > 8.7, respectively. In both cases, part of anionics in former and all anionics in latter can engender electrostatic repulsion with negatively charged CNTs-4.7%O, so that the adsorption capacity continued to decrease. Hence, the electrostatic interaction is the main adsorption mechanism in the adsorption process. 3.3. Effects of ionic strength Ionic strength is one factor controlling the adsorption process. The influence of salinity on CPX adsorption on CNTs-4.7%O was shown in Fig. 3. The adsorption capacity increased slightly with increasing salt concentration from 0.0 to 0.05 M, then slowly decreased from 0.05 to 0.4 M. In general, the salt concentration had no significant effect on the adsorption capacity of CPX on CNTs-4.7%O, indicating the high stability of interaction between CNTs-4.7%O and CPX in a certain range of salt concentration. 3.4. Adsorption kinetics This section selected CNTs-4.7%O with the best CPX adsorption properties as an adsorbent to study the adsorption kinetics. The resulting kinetic plot was shown in Fig. 4a. It showed that CPX

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240 200

qe(mg/g)

160 120 80 40 0 0.0

0.1

0.2 0.3 Ionic strength

0.4

Fig. 3. Effect of ionic strength on CPX adsorption by CNTs-4.7%O.

adsorption on CNTs-4.7%O increased sharply at initial 10 min, and slowed down gradually, then reached an equilibrium value at nearly 60 min. To evaluate the adsorption kinetics of CPX, experimental kinetic data were analyzed by fitting the pseudo-firstorder model, the pseudo-second-order model, and the intraparticle diffusion model. The linear plots were shown in Fig. 4b–d. The kinetic parameters calculated from the pseudo-first-order model, the pseudo-second-order model, and the intra-particle diffusion model were given in Table 1. It can be seen that the experiment data showed better compliance with the pseudosecond-order kinetic model based on the highest correlation

coefficient value (R2 = 0.9998). Furthermore, the q value (qe,cal) calculated from the pseudo-second-order model was more identical with the experimental q value (qe,exp) than that calculated from pseudo-first-order model. The result was consistent with that obtained by criteria of R2. Hence, the experimental data of CPX adsorbed by CNTs-4.7%O were well described by pseudo-secondorder kinetic model. Considering that pseudo-second-order kinetic model can not determine the diffusion mechanism, the intra-particle diffusion model proposed by Weber and Morris [23] was used to fit the experimental data from the adsorption process of CPX onto CNTs-4.7%O to appraise the nature of adsorption. The overall adsorption process can pass through the following three stages: (1) outer diffusion: external mass transfer of the adsorbate can bechance to the exterior surface of adsorbent across the liquid film, which is also called boundary layer diffusion or film diffusion; (2) inner diffusion: adsorbate is transported from the exterior adsorbent surface to the pores or capillaries of the internal adsorbent structure, which is also called intra-particle diffusion; (3) the adsorbate is adsorbed onto the active sites on outer surface and inner of the adsorbent [14]. The third step is considered to be very fast and thus cannot be treated as rate-limiting step. Given that the overall adsorption process may be controlled by one or more of the steps, the adsorption rate was controlled by outer diffusion or inner diffusion or both. If the regression of qt versus t0.5 was linear, and the straight line passed through the origin, then the rate of adsorption was controlled by intra-particle diffusion only. Otherwise, the other diffusion mechanism will be accompanied by intra-particle diffusion. As

200

2.4 pseudo-first-order kinetic model Linear Fit of log(qe-qt)

150 1.6

t

log(qe-qt)

100

50

0

60

120

0.0

(a)

adsorption kinetic

0

180

0.8

240

(b) 0

50

100

t(min)

200

250

180

1.6 pseudo-second-order kinetic model Linear Fit of t/qt

intra-particle diffusion model Linear Fit of qt

178 qt(mg/g)

1.2

qt

150 t(min)

0.8

176

174

0.4

(d)

(c) 172

0.0 0

60

120 t(min)

180

240

0

4

8 0.5

12

16

0.5

t (min )

Fig. 4. Adsorption kinetics (a) of CPX on CNTs-4.7%O (pH 6.0, 298 K). Linear regression kinetics plots: pseudo-first-order model (b); pseudo-second-order model (c); intraparticle diffusion model (d).

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Table 1 Kinetic parameters of CPX adsorption on CNTs-4.7%O (at 298 K). Model

Parameters

CNTs-4.7%O

C0 (mg/L) qe,exp (mg/g)

13.97 177.8

k1 (min1) qe,cal (mg/g) R2

0.0028 7.47 0.0149

Pseudo-first-order model

Pseudo-second-order model K2 (g/mg min) qe,cal (mg/g) R2 Intra-particle diffusion model kid (g/mg min0.5) C R2

0.018 176.1 0.9998 0.198 173.0 0.1338

shown in Fig. 4d, the plot of qt versus t0.5 was liner but not passing through the origin. Thence, intra-particle diffusion was not the unique rate-controlling step.

3.5. Adsorption isotherms Adsorption isotherms can provide effective information about adsorption mechanisms, the affinity and surface properties. The experimental equilibrium adsorption data were simulated with three well-known isotherm models, Langmuir, Freundlich, and Dubinin–Radushkevich (D–R). Curves fitted by the three isotherm models were shown in Fig. 5a–c, and the relative parameters calcu-

lated from the three models for all adsorbents were listed in Table 2. With the high correlation coefficient (R2), it can be seen that among the three isotherms tested, D–R and Langmuir model was revealed as the best model for describing the adsorption process. This situation was rare since most isotherm data were more appropriate to Freundlich model [2,3,24–27]. D–R model was often used to distinguish the adsorption mechanism. The average free energy of adsorption, Ea is an important parameter to determine the adsorption mechanism. The value of Ea can be calculated by Ea ¼ ð2BÞ1 0:5 . Generally, it is believed that physical adsorption exists in the adsorption process at the value of Ea < 8 kJ/mol [28], while chemical adsorption such as ion exchange, etc. is dominant at value of Ea 8–16 kJ/mol, while its value in the range of 20–40 kJ/mol is indicative of chemisorption [29]. As the oxygen content increased, the value of Ea changed without the certain rule in Table 1. However, the values of Ea for CNTs-2.0%O, CNTs-3.2%O, CNTs-4.7%O, CNTs-5.9%O were 1.78–3.97 kJ/mol, which were all less than 8 kJ/mol. Therefore, the CPX adsorption process on MCNTs should be a physical adsorption. Further studies were carried out simultaneously to investigate the adsorption mechanism. It is known that the surface physical and chemical properties of carbon nanotubes have a large influence on adsorption [15,30–34]. This work focused on the impact of physical and chemical properties for adsorption. From the analysis of Table 3, it was shown that the diversification in the adsorption properties of CPX was associated with the change of surface oxygen contents for the MCNTs. Hence, it was deduced that the variance in adsorption properties of CPX may be caused by the

250

250

(b) 200

150

150

100

CNTs-4.7% O

qe(mg/g)

200

100

CNTs-4.7% O

CNTs-5.9% O

50

CNTs-5.9% O

50

CNTs-3.2% O

CNTs-3.2% O CNTs-2.0% O

CNTs-2.0% O

0

0 0

15

30

45

60

0

75

15

Ce(mg/L)

30

45

60

Ce(mg/L) 6

(c)

CNTs-4.7% O CNTs-5.9% O

5 lnqe

qe(mg/g)

(a)

CNTs-3.2% O CNTs-2.0% O

4

3 0.00E+000

2.00E+007

4.00E+007

ε

6.00E+007

2

Fig. 5. Langmuir (a), Freundlich (b) and D–R (c) model fitting for CPX adsorption on MCNTs with different oxygen contents.

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F. Yu et al. / Chemical Engineering Journal 285 (2016) 588–595 Table 2 Parameters of the Langmuir, Freundlich, and D–R models for the adsorption of CPX on MCNTs with different oxygen contents at 298 K. Parameters

Langmuir D-R

Adsorbents

200

CNTs2.0%O

CNTs3.2%O

CNTs4.7%O

CNTs5.9%O

Langmuir model

qm (mg/g) Kl (L/mg) R2

150.6 4.144 0.981

178.9 1.428 0.954

206.0 1.231 0.931

181.2 4.527 0.954

Freundlich model

Kf (mg/g) 1/n R2

90.90 0.143 0.778

97.51 0.172 0.804

119.7 0.149 0.774

116.8 0.135 0.730

D–R model

qm (mg/g) B (mol2/kJ2) Ea (kJ/mol) R2

146.6 0.033 3.92 0.994

168.6 0.069 2.692 0.975

194.2 0.158 1.78 0.961

176.2 0.032 3.97 0.985

qm(mg/g)

Model

220

180

160

140 2

3

4

5

6

O(%) Fig. 6. Maximum adsorption capacity (qm) of CPX from Langmuir and D–R model plotted against surface oxygen content.

Table 3 Physicochemical properties of MCNTs with different surface oxygen contents and the maximum adsorption capacity (qm, and qm normalized by SSA) of CPX fitted from Langmuir and D–R model. Adsorbent

CNTs2.0%O

CNTs3.2%O

CNTs4.7%O

CNTs5.9%O

SSA [19] PV [19] APD [19] O% [19] O%/SSA [19] qm (Langmuir) qm (D–R) qm (Langmuir)/SSA qm (D–R)/SSA

471 0.64 5.4 2.0 4.25 150.6 146.6 0.32 0.31

381 0.58 6.0 3.2 8.40 178.9 168.6 0.47 0.44

382 0.58 6.0 4.7 12.30 206.0 194.2 0.54 0.51

327 0.49 5.9 5.9 18.04 181.2 176.2 0.55 0.54

Note: SSA = specific surface area (m2/g); PV = pore volume (cm3/g); APD = average pore diameter (nm); O%: surface oxide (at.%) by XPS; O%/SSA means that surface oxides values normalized specific surface area.

surface chemical properties of MCNTs after oxidized, specifically the variation of surface oxygen content. From Table 1, the Langmuir and D–R model were both suitable for fitting isotherm data based on R2 (0.931–0.981, 0.961–0.994, respectively). The relationship of different oxygen contents and maximum adsorption capacity (qm) can reflect the adsorption properties of CPX on MCNTs. As shown in Fig. 6, with the increase of the oxygen content, the qm of CPX fitted from Langmuir and D–R model both decreased after the first rise, not showed a tendency of continued increase or decrease, which is according with the previous literatures [19,35]. Adsorption properties of MCNTs depend on not only its surface chemical properties, but also the surface physical properties; wherein the SSA is one of the important factors affecting MCNTs adsorption. Analyzing the data, we found that whether the qm, or qm normalized by SSA was not significantly relevant to SSA, pore size, and pore volume of adsorbent. Further analysis revealed that the SSA of MCNTs with different oxygen contents can alter, as shown in Table 3. The results indicated that the surface oxygen content of MCNTs had a greater influence on adsorption, but SSA were different. Therefore, to more accurately analyze the influence of oxygen content on adsorption properties of CPX, qm was taken normalized to exclude the impact of specific surface area, as shown in Fig. 7. The relationship between qm/SSA of CPX calculated from Langmuir and D–R model, phenolic hydroxyl content, and oxygen content was presented in Fig. 7. Obviously, with the oxygen content increasing from 2.0% to 5.9%, qm/SSA of CPX appeared growing state. It has different results with the Fig. 3; there was the emergence of a turning point at

CNTs-4.7%O. When 3.9% increase of oxygen content from CNTs2.0%O to CNTs-5.9%O led to a 72%, 74% increase in qm/SSA from Langmuir and D–R model, respectively. Increasing surface oxygen content of MCNTs can significantly improve hydrophilicity, dispersibility of MCNTs in aqueous solution, thereby increasing the active sites to promote aqueous phase adsorption. The results in Fig. 7 and Table 3 reveal that a 1.2% increase in oxygen concentration (from 2.0% to 3.2%) leads to a 47% and 42% increase in qm/SSA for CPX adsorption from Langmuir and D–R model, respectively. Compared with the first stage, the same 1.2% increase of oxygen content (from 4.7% to 5.9%) leaded to the 2% and 6% mere increase in qm/SSA from the two models, respectively. Obviously, the increment rate of the maximum adsorption capacity became slower with the rise of oxygen content, which is not according with the TEX adsorption in the previous literatures [19]. It can be deduced that hydrophilicity play an adverse effect on adsorption of pollutants. Strong hydrophilicity may result in the formation of water cluster on the surface of MCNTs, thereby inhibiting the adsorption process [36,37]. Therefore, within the described range of oxygen content, hydrophilicity and dispersibility promote the adsorption process, but not the main adsorption mechanism. The promotion of hydrophilicity and dispersibility, and the inhibition of water cluster had played a coordinate role in the whole adsorption process. After oxidation treatment by sodium hypochlorite, the MCNTs surface was introduced numerous phenolic hydroxyl groups, as shown in Table S1, which had the ability of electron-donating to make carbon ring become a p-electron donor. Judging from the molecular structure of CPX, a fluorine atom connected to benzene ring had strong electronegativity, so that the benzene ring linked to it became strong p-electron acceptor, resulting in p–p electron donor–acceptor (EDA) interaction, and then, the adsorption increased. An 181% increase of phenolic hydroxyl content from CNTs-2.0%O to CNTs-3.2%O, led to a 47%, 42% increase in qm/SSA from Langmuir and D–R model, respectively. A 147% increase of phenolic hydroxyl content from CNTs-3.2%O to CNTs-4.7%O, led to a 15%, 16% increase in qm/SSA from two models, respectively. A 141% increase of phenolic hydroxyl content from CNTs-4.7%O to CNTs-5.9%O, led to a 2%, 6% increase in qm/SSA from two models, respectively. Obviously, with the decline of the growth rate of phenolic hydroxyl content, the growth rate of qm/SSA also declined. Therefore, within the described range of oxygen content, p–p EDA was the main adsorption mechanism. The above results also confirm the above conclusion that the CPX adsorption process was physical and chemical adsorption.

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0.3 0.55

0.50

0.45

(

qm/SSA mg/m

2

)

0.2

0.40

0.1

0.35

0.30

Phenolic hydroxyl content (mmol/g)

Langmuir D-R Phenolic hydroxyl

0.0 2

3

4

5

6

O(%) Fig. 7. qm of CPX from Langmuir and D–R model normalized by SSA (qm/SSA) and phenolic hydroxyl content plotted against surface oxygen concentration.

4. Conclusions MCNTs were oxidized by different concentrations of sodium hypochlorite to obtain those with different oxygen contents, and then used as adsorbents to remove the CPX from aqueous solutions. This paper focused on the effect of various oxygen contents on CPX adsorption performance. The results showed that the maximum adsorption capacity (qm) decreased after the first increase with increasing oxygen contents, but qm/SSA continued to increase with increasing oxygen contents. Moreover, the increment rate became slower. This was mainly attributed to p–p electron donor–acceptor interaction. Obviously, surface physical and chemical properties of MCNTs had a significant impact on CPX adsorption. The isotherm adsorption data were more appropriate to D–R model. The experimental kinetic data showed that intra-particle diffusion and outer diffusion may both present in the adsorption process to control the adsorption rate. The alkaline condition was not conducive to the adsorption of CPX on CNTs-4.7%O. However, the ionic strength had no significant effect on the adsorption capacity of CPX onto MCNTs. Therefore, the electrostatic interaction may be the main adsorption mechanism in the adsorption process. Acknowledgment This research was supported by the National Natural Science Foundation of China (Nos. 21577099, 51408362), China, and State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRF14021), China. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.10.039. References [1] J. Ma, M.X. Yang, F. Yu, J. Zheng, Water-enhanced removal of ciprofloxacin from water by porous graphene hydrogel, Sci. Rep.-Uk 5 (2015). [2] S. Wu, X. Zhao, Y. Li, C. Zhao, Q. Du, J. Sun, Y. Wang, X. Peng, Y. Xia, Z. Wang, L. Xia, Adsorption of ciprofloxacin onto biocomposite fibers of graphene oxide/calcium alginate, Chem. Eng. J. 230 (2013) 389–395.

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