Synthesis of β-Cyclodextrin-Based Electrospun Nanofiber Membranes

Nov 16, 2015 - strength increases significantly and the BET surface area is. 19.49 m2/g. The cross-linked fibers exhibited high adsorption capacity fo...
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Synthesis of β‑Cyclodextrin-Based Electrospun Nanofiber Membranes for Highly Efficient Adsorption and Separation of Methylene Blue Rui Zhao, Yong Wang, Xiang Li,* Bolun Sun, and Ce Wang* Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: Water-insoluble β-cyclodextrin-based fibers were synthesized by electrospinining followed by thermal cross-linking. The fibers were characterized by field-emission scanning electron microscopic (FE-SEM) and Fourier transformed infrared spectrometer (FT-IR). The highly insoluble fraction obtained from different pH values (3−11) indicates successful cross-linking reactions and their usability in aqueous solution. After the cross-linking reaction, the fibers’ tensile strength increases significantly and the BET surface area is 19.49 m2/g. The cross-linked fibers exhibited high adsorption capacity for cationic dye methylene blue (MB) with good recyclability. The adsorption performance can be fitted well with pseudo-second-order model and Langmuir isotherm model. The maximum adsorption capacity is 826.45 mg/g according to Langmuir fitting. Due to electrostatic repulsion, the fibers show weak adsorption toward negatively charged anionic dye methyl orange (MO). On the basis of the selective adsorption, the fiber membrane can separate the MB/MO mixture solution by dynamic filtration at a high flow rate of 150 mL/min. The fibers can maintain good fibrous morphology and high separation efficiency even after five filtration−regeneration cycles. The obtained results suggested potential applications of β-cyclodextrin-based electrospun fibers in the dye wastewater treatment field. KEYWORDS: β-cyclodextrin, electrospinning, dye, adsorption, separation Many kinds of materials (such as chitosan,19 carbon-based materials,20,21 zeolite,22 polymer,23 cyclodextrins,24 etc.) have been prepared into adsorbents and reported to be able to remove organic dye contaminants. Among these adsorbents, cyclodextrin-based materials have received much attention for removing dye contaminants in recent years. Cyclodextrins (CDs) are cyclic oligosaccharides of α(1,4)-linked glucopyranoside units. There are three main types of CDs that have six, seven, or eight glucopyranoside units and are named α-, β-, and γ-CD, respectively.25 The most notable feature of CD is its ability to form host−guest complexes with a wide range of molecules due to its special molecular structure (hydrophobic internal cavity and hydrophilic external surface), making them quite applicable in various fields such as chemical separations, functional foods, adsorbents, textiles, and pharmaceuticals.26 By contrast, β-CD attracts more attention in applications due to its lower cost and higher productivity.27 Recently, β-CD-functionalized nanoparticles,24 electrospun nanofibers,28 carbon nanotubes,29 graphene,17 and hydrogels30 have been studied widely as adsorbents for removing dye applications. By comparison with these kinds of materials, one-

1. INTRODUCTION Organic dye contaminants have received increased concerns in recent years because of the fast development of industry, such as textile finishing, plastics, paper, cosmetics, pharmaceuticals, and food processing.1,2 Among these pollution sources, textile wastewaters play a major role in dye pollutants. The discharge of dye pollutants into the aqueous environment can cause severe health and ecological problems to human beings.3−5 Most of these dyes possess highly complex structures, high molecular weight, and low biodegradability; meanwhile, the presence of these dyes in water is highly observable even at very low concentrations.6,7 Moreover, they are stable toward heat, light, and oxidizing agents; thus, they have the potential danger of bioaccumulation.8,9 In addition, they are mutagenic and carcinogenic.10 Hence, removal of the dye contaminants prior to their discharge into the environment is necessary. This situation requires researchers to develop environmentally friendly and cost-effective wastewater treatment techniques for reducing the concentration of the contaminants to permissible levels. Many physiochemical and biochemical treatment processes, including biological processes,11 coagulation−flocculation,12 chlorination,13 electrochemical,14 photocatalytic degradation,15 and adsorption,16 have been widely studied to treat dye contaminants. Because of its low cost, easy regeneration, and effectiveness, adsorption is regarded as a useful technique.16−18 © 2015 American Chemical Society

Received: September 7, 2015 Accepted: November 16, 2015 Published: November 16, 2015 26649

DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657

Research Article

ACS Applied Materials & Interfaces

Sinogharm Chemical Reagent Co., Ltd. All reagents were used without further purification. 2.2. Electrospinning. The homogeneous clear solution was prepared by dissolving 15.6 wt % β-CD, 3.1 wt % PAA, and 3.1 wt % citric acid in DMF at room temperature. The mass ratio of β-CD/PAA/ citric acid was 10:2:2. The solution was loaded into a glass syringe and connected to a positive electrode of high-voltage power supply (Gamma High Voltage Research, Ormond Beach, FL, USA; dc power supply). Fifteen kilovolts was provided between the positive syringe and a negative aluminum foil at a distance of 15 cm to prepare the fibers. The resultant nanofibers were transferred into an electric thermostatic oven at 170 °C for 2 h to conduct the thermal cross-linking. 2.3. Characterization. The morphology of the obtained fibers was characterized by a field-emission scanning electron microscopy (SEM, FEI Nova NanoSEM). The pH values of the reaction solutions were measured using a pH meter (Starter 2100, Ohaus Instruments Co., Ltd.). FT-IR spectra were recorded on a Bruker Vertex80 V spectrometer from 4000 to 400 cm−1 using powder-pressed KBr pellets at room temperature. The mechanical properties of the fiber membrane before and after cross-linking were performed by assembling the membranes (dimensions: length = 60 mm, width = 30 mm) between two stainless steel clamps with a tensile speed of 20 mm·min−1 on a mechanical strength microtest device (410R250, Test Resources, Shakopee, MN, USA). The values were averaged over five measurements. The N2 adsorption−desorption isotherms were recorded on an ASAP 2020 instrument at 77 K. Specific surface areas were calculated according to the Brunauer−Emmett−Teller (BET) method. Pore size and total pore volume were calculated on the basis of the Brrett− Joyner−Halenda (BJH) method. For solubility experiments, squares of 1 cm2 of the fiber mats were immersed in deionized water with different pH values ranging from 3 to 11. After 24 h, the fiber mats were removed from the deionized water and placed in a vacuum oven to dry until constant weight. The insoluble fraction (%) is determined by eq 1

dimensional continuous nanofiber membrane by electrospinning technique has become a research focus due to its relatively simple manufacturing process and cost-effectiveness.31 Electrospun nanofibers show high porosity, large surface-to-volume ratio, easy recyclability, and good film-forming and facile modification properties, which have found wide applications in energy storage, catalyst, environmental remediation, protective clothing, sensors, and biological medicine.32−34 Owing to their remarkable feature, various electrospinning-based nanofiber adsorbents, such as single or composite inorganic nanofibers,35 functionalized organic nanofibers,36 and inorganic/organic composite nanofibers,37 have been investigated for treating dye wastewater pollution. β-CD-based electrospun nanofibers have also been researched extensively in the past few years. Pure cyclodextrin nanofibers from nonpolymeric systems have been electrospun using NaOH aqueous solution38 and 1,1,1,3,3,3-hexafluoro-2propanol39 as solvents. β-CD/polymer blend electrospun nanofibers have also been fabricated and used for filtering organic molecules in solution and capturing organic vapors.40,41 Because CDs are water-soluble, the outflow of cyclodextrin molecules from the blend fibers during adsorption in the liquid media always happens, resulting in the reduction of the adsorption capacity, so the potential applications can be limited without a cross-linking or modification process when cyclodextrin-functionalized nanofibers are used in aqueous solution media. Recently, the Uyar group reported polymer electrospun nanofibers that had been surface modified with cyclodextrin to avoid the leaching of cyclodextrin molecules from the fiber surface. The functionalized nanofibers were used for removal of organic pollutants from water and wastewater.42 Nonetheless, the cyclodextrin molecules are on the surface of the fibers, and the relative content of cyclodextrin is lower than the single native cyclodextrin nanofibers, which can influence the adsorption ability. In the present study, we used a relatively high content of β-CD toward helping polymer poly(acrylic acid) (PAA) in the electrospinning solution using common N,N-dimethylformamide (DMF) as solvent, to increase the content of β-CD in the electrospun nanofibers. Cross-linking reagent citric acid was also added into the electrospinning solution. Composite nanofibers were successfully achieved by electrospinning technique. After the in situ thermal cross-linking of the composite nanofibers, the self-standing and water-insoluble β-cyclodextrin-based nanofiber membrane was investigated to evaluate the adsorption property toward cationic dye methylene blue (MB). The results show that the obtained β-cyclodextrin-based nanofibers have high adsorption capacity for MB with good recyclability and promise great potential application for efficient wastewater treatment. Interestingly, the existence of carboxyl groups (−COOH) in the obtained nanofibers made the nanofibers adsorb little anionic dye methyl orange (MO) due to the electrostatic repulsion. On the basis of the selective adsorption of cationic dye MB, a dynamic filtration separation of MB/MO mixture solution was also studied.

insoluble fraction (%) =

Wi × 100% W0

(1)

where W0 and Wi are the weights of the initial samples and the samples after drying in a vacuum oven, respectively. 2.4. MB Adsorption Study. Batch adsorption experiments were performed on a model BETS-M1 shaker (Kylin-Bell Lab Instruments Co., Ltd., China) with a shaking speed of 120 rpm. The initial pH of the MB solution was adjusted to values in the range of 2−11 by dropwise adding 0.1 mol/L NaOH or 0.1 mol/L HCl solutions to study the effects of initial solution pH. Kinetic experiments were performed by mixing 14 mg of adsorbent into 80 mL of MB solution with a known initial concentration (40 mg/L) at 20 ± 1 °C and at the optimal pH value. The concentration of MB in solution was determined by using a Shimadzu UV-2501 UV−vis spectrophotometer based on the standard curve. The adsorption capacity (q) of MB adsorbed onto the cross-linked fibers was calculated on the basis of the equation43

q (mg/g) =

(C0 − Ce)V W

(2)

where C0 and Ce are the initial and the equilibrium concentrations of dye MB in the test solution (mg/L), V is the volume of the testing solution (L), and W is the weight of the adsorbent (g). Adsorption isotherms were conducted with initial concentrations ranging from 80 to 800 mg/L (initial pH 9). For the desorption experiment, the MB-adsorbed β-cyclodextrin-based fibers were washed thoroughly with deionized water. Then the fibers were put into methanol solution containing 5% (v/v) HCl. After desorption equilibrium, the fibers were washed several times with deionized water and were reused in adsorption experiments (adsorbent, 6 mg; MB, 40 mL, 40 mg/L; initial pH, 9) and the process was repeated five times. 2.5. Comparison of MO Adsorption. The adsorption study of MO was carried out in the same mode as the MB adsorption study. The pH effect of MO solution for adsorption ranged from 4 to11. Adsorption kinetics was performed at the initial concentration of 40 mg/L. The concentration of MO in solution was also determined by using a

2. 2. MATERIALS AND METHODS 2.1. Materials. PAA (average Mv = 450 000) was purchased from Sigma-Aldrich. β-Cyclodextrin (98%) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Citric acid (99%) was purchased from Beijing Chemical Works. DMF was purchased from Tiantai Chemical Corp. Methylene blue (MB) (C16H18clN3S·3H2O) and methyl orange (MO) (96%) (C14H14N3SO3Na) were purchased from 26650

DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657

Research Article

ACS Applied Materials & Interfaces

Figure 1. SEM images of β-cyclodextrin-based electrospun fibers before (A, a) and after (B, b) thermal cross-linking and of the cross-linked fibers after soaking in water (pH 7) for 24 h (C, c).

fibers kept good fiber morphology and their fibrous structure (Figure 1B). The fiber surface is smooth through observing the high-magnification images and is randomly oriented (Figure 1b). Furthermore, to analyze their surface area, pore volume, and pore diameter, BET adsorption−desorption isotherms (Figure S1) of the fibers before and after cross-linking were characterized. The corresponding results are summarized in Table 1. The BET surface area decreases to 19.49 from 34.88 m2/g after the thermal cross-linking, whereas cumulative pore volume and average pore diameter also show a similar decrease for the cross-linked fibers (shown in Table 1). The cumulative pore volume is reduced to 0.0398 from 0.112 cm3/g, and the average pore diameter is reduced to 14.53 from 20.56 nm. This is because the junction among the fibers becomes closer and the fiber mat is denser after the cross-linking. This variation tendency is consistent with previous reports.43,44 The cross-linked fibers showed a good resistance to water, and the insoluble fractions after soaking in water for 24 h were all >95% when the pH values ranged from 3 to 11 (Figure S2). High water insoluble level makes the fibers useful for aqueous medium applications. After soaking in water (pH 7) for 24 h, the fibers became intertwined and slightly swelled as displayed in Figure 1C,c. The SEM and insoluble fraction study suggest that waterinsoluble β-cyclodextrin-based electrospun fibers (CDBEF) are prepared successfully. 3.2. FT-IR and Mechanical Test Results. To further explore the structural components of CDBEF, FT-IR spectra of the fibers before (a) and after (b) thermal cross-linking were recorded and are shown in Figure 2A. The strong and broad band at ∼3400 cm−1 in (a) and (b) corresponded to the O−H stretching vibration of the hydroxyl groups.45 An adsorption band in (a) and (b) appearing at 1738 cm−1 is due to the CO stretching vibration of carboxyl groups and ester groups.46 After thermal cross-linking, the band at 1650 cm−1 owing to the O−H deformation vibration47 and the band at 1332 cm−1 owing to the carboxylate groups48 decreased, suggesting the cross-linking reaction had occurred. The reaction formed an ester bond between the hydroxyl groups from β-cyclodextrin and the carboxyl group from PAA or citric acid (inset of Figure 2). After the cross-linking reaction, the absorption bands46 including 1414, 1038, 943, 854, 750, and 582 cm−1 for β-CD all appeared in (b), indicating that the structural characteristics of β-CD are essentially maintained in CDBEF.

Shimadzu UV-2501 UV−vis spectrophotometer based on the standard curve. 2.6. Filtration Separation of MB/MO Mixed Dyes. The equipment used for filtration separation is the general silica sand filtration unit. A piece of β-cyclodextrin-based nanofiber membrane with a diameter of 6 cm and mass of 50 mg was placed in the unit. The 100 mL mixed dye solution of MB/MO with the concentrations of MB 4.27 mg/ L and MO 16.03 mg/L was forced to pass through the nanofiber membrane at a flow rate of 150 mL/min using a vacuum pump. The separation process was taken as a video by a camera. After the separation experiment, the MB-loaded nanofiber membrane was eluted by methanol solution containing 5% (v/v) HCl at the flow rate of 150 mL/min for its regeneration. After being washed by deionized water, the regenerated β-cyclodextrin-based nanofiber membrane was again used to conduct the above separation experiment to evaluate recycle ability. The recycle was repeated five times. The separation efficiency (S) is used to evaluate the separation ability by the equation

S (%) = c(MO)f /[c(MB)f + c(MO)f ] × 100%

(3)

where c(MB)f and c(MO)f are the concentrations of MB and MO in the filtrate, respectively.

3. RESULTS AND DISCUSSION 3.1. Morphology and BET Analysis of the Fibers and the Insoluble Fraction after Cross-Linking. To obtain high Table 1. BET Analysis and Mechanical Property Results for βCyclodextrin-Based Electrospun Fibers before and after Cross-Linking sample before crosslinking after crosslinking

surface area (m2/g)

pore volume (cm3/g)

pore diameter (nm)

tensile strength (MPa)

elongation at break (%)

34.88

0.112

20.56

0.68 ± 0.08

5.77 ± 0.23

19.49

0.0398

14.53

2.28 ± 0.12

9.19 ± 0.31

content of β-cyclodextrin-based electrospun fibers, an electrospinning solution of 500 wt % β-cyclodextrin with respect to polymer PAA was prepared. Cross-linking agent citric acid, rich in carboxyl groups, was also added into the solution; in addition, PAA also played a role as cross-linking agent due to the presence of carboxyl groups. After the electrospinning process, long and continuous fibers were obtained and the surface of the fibers is smooth, as shown in Figure 1A,a. After thermal cross-linking, the 26651

DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657

Research Article

ACS Applied Materials & Interfaces

Figure 2. (A) FT-IR spectra of β-cyclodextrin-based electrospun fibers before (a) and after (b) thermal cross-linking (inset, structure of the generated ester bond); (B) stress−strain curves of β-cyclodextrin-based electrospun fibers before and after thermal cross-linking.

Figure 3. (A) Effect of pH on the adsorption of MB or MO onto CDBEF and (B) adsorption kinetics curve of the adsorption of MB onto CDBEF (inset, pseudo-second-order kinetic plots).

Table 2. Kinetics Parameters for the Adsorption of MB on CDBEF experimental adsorbent CDBEF

qexp 119.84

pseudo-first-order model qe (mg/g) 139.89

−1

k1 (min )

pseudo-second-order model R

0.0162

2

0.9761

The mechanical properties of the β-cyclodextrin-based electrospun fiber mats before and after thermal cross-linking were evaluated using tensile testing. The stress−strain curves are shown in Figure 2B, and the corresponding results are listed in Table 1. It is evident that the tensile strength increases remarkably after the cross-linking. As presented in Table 1, the tensile strength increases from 0.68 to 2.28 MPa, and the elongation at break increases from 5.77 to 9.19%. This is because that the ester bonds form between the molecules after the crosslinking. The small molecules (β-cyclodextrin and citric acid) are combined onto the long chain of flexible polymer PAA, so the cross-linked fiber membrane shows better flexibility (Figure S3) and satisfactory mechanical strength for its usability in adsorption and filtration applications. 3.3. MB Adsorption Study. 3.3.1. Effect of Solution pH on Adsorption. The effect of pH values (ranging from 2 to 11) on the adsorption capacity of MB onto CDBEF is shown in Figure 3A with an initial concentration of 40 mg/L MB. The adsorption capacity of MB increased with increasing of pH values between 2 and 9 and decreased between pH 9 and 11. The CDBEF exhibited maximum adsorption capacity at pH 9. This variation tendency is consistent with a previous report. 49 This

qe (mg/g) 149.70

k2 (g/mg·min) −5

8.753 × 10

R2 0.9949

phenomenon can be explained by the electrostatic action. At low pH values, the electrostatic repulsion between protonated adsorption sites on CDBEF and protonated dimethylamine group from MB restricted the interaction between MB molecules and CDBEF, so the adsorption capacity is less at low pH values. With increasing pH values, the level of protonation decreased and the electrostatic repulsion weakened, so more MB could be adsorbed onto the CDBEF. At pH 9, the CDBEF showed maximum adsorption capacity toward MB. At higher pH value (>9), some of the hydroxy and carboxyl groups became deprotonated and the density of electron clouds for groups containing nitrogen MB increased, resulting in the increase of the repellent force. Therefore, the adsorption capacity decreased with continued increase of the pH value when it is >9. The results indicate that the solution pH value is a significant influencing factor during the adsorption process. The adsorption of MB contributes not only to the host−guest inclusion with βcyclodextrin but also to the electrostatic interaction with carboxyl groups from CDBEF. Therefore, a pH value of 9 was selected for subsequent adsorption study. 3.3.2. Adsorption Kinetics. To get a better understanding of the adsorption property of MB onto CDBEF, the adsorption 26652

DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657

Research Article

ACS Applied Materials & Interfaces

Figure 4. Adsorption isotherms (A) and the corresponding Langmuir plot (B) and Freundlich plot (C) for MB adsorption onto CDBEF; (D) adsorption−desorption cycles.

Table 3. Summary of the Langmuir and Freundlich Isotherm Fitting Parameters for the Adsorption of MB onto CDBEF Langmuir isotherm qmax (mg/g)

adsorbent CDBEF

Freundlich isotherm R

b (L/mg) −3

4.02 × 10

826.45

0.9915

qm

pH

carboxymethyl-β-cyclodextrin modified magnetic nano adsorbent β-cyclodextrin polymer cross-linked by citric acid magnetic β-cyclodextrin grafted carbon nanotube composites hydroxypropyl-β-cyclodextrin cyclodextrin modified polyester cyclodextrin-based material containing carboxylic groups β-cyclodextrin grafting wood flour copolymer β-cyclodextrin/poly(acrylic acid)/grapheme oxide nanocomposites oxime-grafted-polyacrylonitrile electrospun nanofiber sericin/β-cyclodextrin/PVA composite electrospun nanofibers ethylenediamine-grafted polyacrylonitrile electrospun nanofibers β-cyclodextrin-based electrospun fibers

277.8

12.00

a

a

ref 24

105.00

NR

51

196.5

NR

29

23.00

NR

52

56.50

8.00

53

86.21

6.89

54

247.99

7.89

55

102.10

NR

56

187.27

8.00

9

94.07

NR

36

826.45

9.00

this work

KF

n

R2

20.02

1.857

0.9552

process was observed in the initial 100 min owing to the existence of many available adsorption sites. After that, MB molecules have to traverse the surface and get deeper into the fibers, where they encounter much greater resistance, thus resulting in a slowing of the adsorption rate. The adsorption reached saturation at 360 min. Therefore, 360 min was selected as equilibrium time for subsequent experiments. Two kinetic models, that is, the pseudofirst-order and pseudo-second-order models, are ued to explore the adsorption kinetics behavior as shown in eqs 4 and (5), respectively50

Table 4. Comparison of the Maximum Adsorption Capacity (qm) of MB on CDBEF with Other Cyclodextrin-Based Adsorbents adsorbent

2

log(qe − qt ) = log qe − k1t

(4)

t 1 t = + 2 qt qe k 2qe

(5)

where qt and qe (mg·g−1) are the adsorption capacity at time t and equilibrium time, respectively, and k1 (min−1) and k2 (g·mg−1· min−1) are the pseudo-first-order model rate constant and the pseudo-second order model rate constant, respectively. The corresponding kinetic adsorption parameters calculated from the above two kinetic models are listed in Table 2. Compared with the correlation coefficient R2, the adsorption process can be described well with the pseudo-second-order model. The pseudo-second-order kinetic plot t/qt versus t is shown in the inset of Figure 3Bm and the pseudo-first-order kinetic plot is shown in Figure S4.

NR, not reported.

kinetics was conducted with the initial concentration of 40 mg/L MB, at an initial pH of 9. As Figure 3B shows, a fast adsorption 26653

DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657

Research Article

ACS Applied Materials & Interfaces

Figure 5. Separation mechanism of MB/MO mixture dyes (inset, self-standing β-cyclodextrin-based electrospun fiber membrane).

3.3.3. Adsorption Isotherm and Cycle Adsorption. Furthermore, the equilibrium isotherm data are described by two well-known Langmuir and Freundlich isotherm models.50 Langmuir isotherm Ce C 1 = + e qe bqm qm

known that the reusability of an adsorbent is important for reducing the overall cost in practical applications. Thus, the regeneration and recycling experiments were conducted to study the reusability of CDBEF. As the pH effects study showed that the CDBEF could adsorb few MB molecules at a low pH value, a methanol solution containing 5% (v/v) HCl was used to regenerate the dye-adsorbed CDBEF. The desorption− adsorption cycle was repeated five times. As shown in Figure 4D, the removal efficiency remained at 90% after five regeneration cycles, indicating the CDBEF has a good recyclability. On the basis of the above results on the adsorption of MB, the prepared β-cyclodextrin-based electrospun fiber membrane could be a good candidate for removing MB dye application in wastewater treatment. 3.4. Separation Experiment. Because the carboxyl groups (−COOH) existed in the nanofibers, we consider the CDBEF as not conducive to adsorbing anionic dye due to the electrostatic repulsion, although MO molecules can act as a guest to form noncovalent complexes with β-cyclodextrin. To explore this, anionic dye methyl orange (MO) was selected as the model to study its adsorption onto CDBEF at different pH values. As the results show in Figure 3A, the CDBEF can adsorb much less MO than MB at the selected pH range (from 4 to 11). At an optimal pH value (5) of MO adsorption, the adsorption capacity for MO can only reach 12.41 mg/g. At an optimal pH value (9) of MB adsorption, the adsorption capacities of MB and MO differed greatly. The difference value is 134 mg/g. For the comparison, the kinetics of MO adsorption onto CDBEF at initial pH 9 was conducted with the same experiment parameter of MB adsorption kinetics, and the results are shown in Figure S5 and Table S1. The equilibrium adsorption capacity is 7.78 mg/g according to the pseudo-second-order kinetic fitting. On the basis of the unique selective adsorption between MB and MO, the CDBEF is expected to be used in the separation of dye mixture (MB/MO). The separation mechanism is shown in Figure 5. The positively charged MB molecules can be adsorbed onto the surface of CDBEF and then form the host−guest

(6)

Freundlich isotherm ln qe = ln KF +

1 ln Ce n

(7)

where qe is the equilibrium adsorption capacity of the MB adsorbed onto the CDBEF (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), and qm and b are Langmuir constants related to maximum adsorption capacity and binding energy, respectively; KF and n are empirical constants that indicate the Freundlich constant (L/mg) and heterogeneity factor, respectively. The isotherms of the above two models are displayed in Figure 4A. The related equilibrium parameters can be determined from the corresponding linear fitting (Figure 4B,C), and the results are listed in Table 3. The higher correlation coefficient (R2 = 0.9915) of the Langmuir model suggests that the adsorption data of MB onto CDBEF fit better with the Langmuir isotherm, indicating that the adsorption process is mainly a monolayer adsorption and relatively homogeneous. The obtained maximum adsorption capacity (qm) is 826.45 mg/g, which is higher than that for the available cyclodextrin-based adsorbent materials and other electrospun fiber adsorbents, which were reported in the literature. The comparative results are shown in Table 4. The high adsorption capacity of our prepared CDBEF contributes to the higher content of β-cyclodextrin in the nanofibers, which can capture MB molecules by host−guest complexes. Moreover, the carboxyl groups (−COOH) in the fibers can also adsorb the MB molecules by electrostatic interaction. The synergy of the two aspects is responsible for the high adsorption capacity. It is 26654

DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657

Research Article

ACS Applied Materials & Interfaces

membrane, the MB dye will be captured onto the CDBEF membrane and the MO dye will remain in the filtrate. For separation study, 100 mL of green MB/MO mixture solution (concentration of MB = 4.27 mg/L, MO = 16.03 mg/L; optimal pH value of MB adsorption, 9) was forced to filter through the CDBEF separation membrane at a high flux of 150 mL/min. The separation process was done after 40 s. As Figure 6A and movie S1 show, the MB/MO mixture solution was green before the filtration and the color of filtrate turned to yellow for MO after separation; in addition, the CDBEF membrane became blue for MB, suggesting a good separation phenomenon. The UV−vis spectra of the solution before and after separation were used to monitor the concentrations of MB and MO (Figure 6B). The concentration of MB in the filtrate decreased from the initial 4.27 to 0.07 mg/L and the concentration of MO decreased little from 16.03 to 15.36 mg/L. The concentration ratio of MB and MO in initial mixture solution was 0.266, and it decreased to 0.005 in the filtrate. Thus, the separation efficiency was 99.55%. The obtained results indicate that the CDBEF membrane can successfully separate the MB/MO mixture dyes by dynamic filtration at a high flow rate of 150 mL/min. The recyclability was also explored. The MB-loaded nanofiber membrane was eluted by methanol solution containing 5% (v/v) HCl at a flow rate of 150 mL/min for its regeneration. After five filtration−regeneration cycles, the separation efficiency was maintained at 92.51% (Figure 7A). The SEM image of CDBEF after five cycles (Figure 7B) showed that the fibers kept good fibrous morphology, suggesting their potential practical applications.

4. CONCLUSIONS We demonstrated the facile fabrication of β-cyclodextrin-based electrospun fibers (CDBEF). After the subsequent thermal cross-linking, the β-cyclodextrin-based fibers become stable and resistant to aqueous solution and show a higher tensile strength. Due to the high content of β-cyclodextrin and the existence of carboxyl groups, the fibers show excellent adsorption toward cationic dye MB with good recyclability. MB adsorption isotherms onto CDBEF can be well-fitted with the Langmuir model, and the adsorption kinetics follow the pseudo-secondorder model. The maximum adsorption capacity (qm) is 826.45 mg/g, which is much higher than that of many other cyclodextrin-based adsorbents and other electrospun fiber adsorbents. On the other hand, CDBEF can adsorb little anionic dye MO because of the electrostatic repulsion between the carboxyl groups of CDBEF and the negative electricity of anionic

Figure 6. Separation of MB/MO mixture dyes: (A) equipment for dynamic filtration and photographs before and after filtering MB/MO mixture solution (inset, photograph of CDBEF filtering membrane after adsorbing MB); (B) UV−vis spectra of MB/MO solution before and after filtration.

inclusion with β-cyclodextrin. However, the negatively charged MO molecules cannot be adsorbed due to the electrostatic repulsion, so the separation process can be achieved. Because the prepared β-cyclodextrin-based electrospun fibers exhibited selfstanding and good film-forming properties, the separation of dye mixture was conducted by dynamic filtration. When the MB/MO mixture aqueous solution passes through the CDBEF separation

Figure 7. Separation efficiency for different filtration−regeneration cycles (A) and SEM image of CDBEF after five cycle experiments (B). 26655

DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657

Research Article

ACS Applied Materials & Interfaces

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dye. On the basis of this selective adsorption, the CDBEF membrane can successfully separate MB/MO mixture dyes by dynamic filtration at a high flow rate of 150 mL/min. After five filtration−regeneration cycles, the fibers even maintain high separation efficiency. The experimental results imply that the βcyclodextrin-based electrospun fibers are expected to have very promising adsorption and separation applications for dye wastewater treatment.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08403. Nitrogen adsorption−desorption isotherms of β-cyclodextrin-based electrospun fibers before and after crosslinking, insoluble fraction of cross-linked β-cyclodextrinbased electrospun fibers after soaking in water (pH range from 3 to 11) for 24 h, pseudo-first-order kinetic plots of MB adsorption, adsorption kinetics curve of the adsorption of MO onto CDBEF, comparison of kinetics parameters for MB, and MO adsorption (PDF) Separation movie S1 for MB/MO mixture dyes (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*(X.L.) Phone: +86-431-85168292. Fax: +86-431-85168292. Email: [email protected] *(C.W.) E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by research grants from the National Key Technology Research and Development Program (2013BAC01B02), the National Natural Science Foundation of China (Nos. 21274052, 51303060, and 21474043), Jilin Provincial Science and Technology Department Project (No. 20130206064GX), Research Fund for the Doctoral Program of Higher Education of China (No. 20120061120017), and Changchun City Science and Technology Department Project (No. 13KG32).

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REFERENCES

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DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657

Research Article

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DOI: 10.1021/acsami.5b08403 ACS Appl. Mater. Interfaces 2015, 7, 26649−26657