Biodegradable poly(butylene succinate-co-terephthalate) nanofibrous membranes functionalized with cyclodextrin polymer for effective methylene blue adsorption

Abstract

The burgeoning electrospinning technology brings about a diversity of nanofibers and promotes their applications in various fields including wastewater treatment. Herein, we fabricated biodegradable poly(butylene succinate-co-terephthalate) (PBST) nanofibrous membranes by electrospinning for the first time and functionalized them with β-cyclodextrin polymer (CDP) through the in situ polymerization of CDP on the surface of PBST nanofibrous membranes. Furthermore, there was much higher efficiency in removing methylene blue (MB) dye on the resultant PBST/CDP nanofibrous membranes than that on pure CDP, or PBST nanofibrous membrane. The adsorption performance of the composite membranes to MB molecules was influenced greatly by the amount of CDP that dominated the surface area and surface morphologies of the prepared membranes. The adsorption kinetics of PBST/CDP nanofibrous membrane fitted well with the pseudo-second order model and Langmuir isotherm model well described the relationship between the adsorption capacity and initial MB solution concentration, exhibiting the maximum adsorption capacity of 90.9 mg g⁻¹, which was much higher than the adsorption capacity of the other adsorbents. Considering the excellent adsorption performance of the as-prepared material, the PBST/CDP nanofibrous membranes is a promising candidate in environmental purification applications.

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Introduction

During the past three decades, a large number of researchers have been investigating the removal of dyes in wastewater, which still is a matter of concern for the long-term development of industries and the sustainability of the environment at present. A variety of techniques, including physical, chemical and biological methods, have been reported to decolorize water and capture the containments.^1–3 However, among them, adsorption is one of the most attractive routes owing to its simplicity, ease of operation and high efficiency, leading to numerous natural and synthetic materials being explored as potential adsorbents.^4–6 With the rapid development of electrostatic spinning technology, the resultant nanofibrous membranes have been applied successfully as alternative adsorbents to purify wastewater, achieving promising adsorption performance because nanofibrous mats possess a large surface area and porous structure.^7–9

It is widely known that cyclodextrin (CD) has a unique structure of a hydrophobic cavity and hydrophilic shell, endowing it with outstanding capability to form inclusion complexes with various molecules through host–guest interactions,^10,11 which further makes CD quite applicable in cosmetics, food, pharmacy, etc.^12–14 In fact, it has been reported that CD can be incorporated into nanofibers by electrospinning a mixed polymer/CD solution for the removal of organic molecules from liquid media, which integrates the strengths of CD and nanofibers.^15,16 However, the capacity to remove pollutants from water is still confined due to the solubility of CD in water. Therefore, various water-insoluble CD-based polymers were synthesized through different ways, such as crosslinking^17,18 and chemical grafting,^19,20 with an objective of expanding the application of CD, particularly for seizing organic molecules from wastewater.

Poly(butylene succinate-co-terephthalate) (PBST), one type of aliphatic–aromatic copolymers, has been intensively investigated on its synthesis, molecular structure, biodegradability, thermal and mechanical properties for application in melt-spinning fibers and yarns.^21–23 However, it has been mature in the production and manufacture of conventional spinning methods; on the other hand, electrospinning technology also endows PBST with new possibilities in developing and extending its application. The electrospun PBST fibers have never been reported, nor have their composite materials with CD for the purpose of wastewater adsorption been reported. PBST nanofibrous membrane, as a substrate to adsorb dye molecules, can be biodegraded into CO₂ and H₂O. This also provides opportunity for concentrating the adsorbed dyes, which is beneficial to further degradation or recycling of the dyes.

In this contribution, we present the fabrication of PBST nanofibrous membrane via electrospinning for the first time and its complexes with cyclodextrin by the combination of in situ polymerization cyclodextrin polymer (CDP) in the presence of a PBST nanofibrous membrane. The adsorption performance of PBST/CDP nanofibrous membranes to methylene blue in terms of the effects of the CDP amount, adsorption time, initial solution concentration, and other factors was investigated intensively.

Experimental

Materials

PBST pellets (M_n = 50 600 g mol⁻¹, polydispersity index = 1.92) were provided by Jiangsu Heshili New Materials Co., Ltd, China. Trifluoroacetic acid (TFA) and dichloromethane (DCM) were supplied by Aladdin Chemical Regents Co., Ltd., China. β-Cyclodextrin (CD), citric acid (CTR), sodium hypophosphite hydrate (SHPI) and methylene blue (MB) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used as received without further purification.

Preparation of PBST nanofibrous membranes

PBST nanofibrous membranes were fabricated by electrospinning. First, a 25 wt% PBST solution was prepared by dissolving PBST pellets into a mixed solvent of TFA and DCM at a volume ratio of 1 : 1, and stirring vigorously for 12 h. The resulting solution was loaded into a syringe with a metallic needle (inner diameter 0.5 mm) to perform the electrospinning process. A high voltage of 20 kV, feed rate of 1.5 mL h⁻¹ and distance between needle tip and collector of 15 cm were applied. The relevant temperature and humidity during electrospinning were kept at 25.0 °C and 45%, respectively.

Preparation of cyclodextrin polymer (CDP)

15 g of CD was mixed in 150 mL of deionized water at 50 °C, and 15 g of CTR and 1.8 g of SHPI as a catalyst were then added to the CD solution and stirred for 30 min at 50 °C. The solution was then heated to 150 °C for 1 h and washed to remove the extra CD and CTR if any present and dried at 50 °C.

Fabrication of PBST/CDP nanofibrous membranes

PBST nanofibrous membranes functionalized with CDP were prepared via in situ synthesizing the CDP onto PBST nanofibers, as shown in Fig. 1. Typically, 15 g of CD, 15 g of CTR and 1.8 g of SHPI were mixed in 150 mL of deionized water and stirred for 30 min at 50 °C. After all the chemicals were dissolved completely, PBST nanofibrous membrane with a mass of W₁ was immersed into the resulting solution for 3 h at 50 °C. The nanofibrous membrane was dried at 150 °C for 1 h. Finally, the membrane was washed for the removal of extra CD and CTR and dried again. The mass of resulting membrane was marked W₂ and the mass ratio of CDP to PBST was equal to (W₂ − W₁)/W₁. Herein, PBST/CDP nanofibrous membranes with the mass ratio of 1.2, 2.0 and 3.2 were prepared and abbreviated as PBST/CDP-1.2, −2.0 and −3.2, respectively.

Fig. 1 Schematic of the preparation of PBST/CDP nanofibrous membranes.

Adsorption experiments

To compare the adsorption performance of CDP, PBST and PBST/CDP-1.2 nanofibrous membranes, 50 mg of the three materials were immersed individually in 40 mL of a MB aqueous solution at a concentration of 10 mg L⁻¹ for a certain time; 50 mg of PBST/CDP membranes with various mass ratios were placed into 40 mL of a MB solution (10 mg L⁻¹) to examine the effect of the mass ratio on the adsorption amount; 50 mg of PBST/CDP-2.0 membranes were added to 50 mL of a MB solution at different initial concentrations (5–100 mg L⁻¹) to examine the adsorption isotherms; 50 mg of PBST/CDP-2.0 membrane was immersed in 50 mL of MB solution (10 mg L⁻¹) and 2 mL of the MB solution was withdrawn at predetermined times to study its adsorption kinetics. The abovementioned adsorption experiments were performed in a thermostatic shaker bath at 30 °C. The concentrations of MB solution before and after adsorption were measured by UV-vis spectroscopy (Shimadzu UV-2550) at a wavelength of 665 nm. The dye removal efficiency and adsorption amount at time t were calculated using the following equations,

R_t = 100 × (C₀ − C_t)/C₀

and

q_t = V × (C₀ − C_t)/m

where R_t is the dye removal efficiency at time t (%), and q_t is the adsorption amount at time t (mg g⁻¹); C₀ is the initial concentration (mg L⁻¹), and C_t is the concentration at time t (mg L⁻¹); V is the volume of MB solution (L), and m is the mass of absorbent (g).

Characterization

The Fourier transform infrared spectra (FTIR) of CD, CDP, PBST and PBST/CDP nanofibrous membranes were recorded by a FT-IR analyzer (Nicolet 6700, Thermo Fisher) equipped with the Smart iTR operated on the attenuated total reflectance (ATR) mode in the wave-number range of 4000–600 cm⁻¹. The morphologies of PBST and PBST/CDP nanofibrous membranes were examined by scanning electron microscopy (SEM) with a JSM-5600LV (Japan) at an accelerating voltage of 10 kV. The nanofibers were coated with gold prior to the SEM examination. The surface area of the PBST and PBST/CDP nanofibrous membranes were determined by Brunauer–Emmett–Teller (BET) analysis using a Micromeritics Gemini 2360 Surface Area Analyser. The water contact angles of the nanofibrous membranes were measured using a contact angle goniometer Kino SL200B. The resulting nanofibrous membranes were also analyzed by differential scanning calorimetry (Pyris-1 DSC, Perkin-Elmer) to calculate the crystallinity, which equals to melting enthalpy of the samples divided by the enthalpy of a 100% crystalline sample, which is 142 J g⁻¹.²⁴ Around 7 mg of sample sealed in an aluminium pan was heated to 210 °C at a rate of 10 °C min⁻¹.

Results and discussion

PBST nanofibrous membranes functionalized with CDP

Fig. 2 presents the infrared spectra of CD, CDP, PBST and PBST/CDP nanofibrous membranes, which show the main chemical group differences and allow us to understand the synthesis and modification mechanism. As shown in this figure, compared to CD, there are two evident adsorption bands (1712 and 1272 cm⁻¹) on the spectrum of CDP, which should be assigned to the C=O stretching vibration and C–O stretching vibration in the ester group, respectively.¹⁰ These newly emerging bands indicated that the hydroxyl groups of CD reacted with the carboxyl groups of citric acid in a way of forming ester groups. Fig. 3 displays a schematic for the chemical reaction. The water generated during the reaction was driven away quickly and successively by N₂ flow, so that the reaction was pushed forward. Moreover, due to the extra hydroxyl groups and carboxyl groups in CD, citric acid and the intermediate products, the esterification and polymerization reactions progressed and finally CDP with a three-dimensional network structure was formed, which also was the reason why the CDP was water-insoluble.¹¹

Fig. 2 Infrared spectra of CD, CDP, PBST and PBST/CDP nanofibrous membranes.

Fig. 3 Schematic of the reaction between β-cyclodextrin and citric acid.

In comparison with CDP and PBST, the spectrum of PBST/CDP in Fig. 2 exhibits no newly forming adsorption bands, suggesting that no chemical reaction occurred between CD/CTR and the PBST nanofibrous membrane due to the lack of free reactive groups on PBST macromolecules. Therefore, we successfully modified the electrospun PBST nanofibrous membrane during the process of in situ polymerization of CDP and the CDP was physically adhered to the PBST nanofibers instead of being covalently fixed.^11,25 In terms of the physical adhesion between the PBST nanofibrous membrane and CDP, the photographs of membrane immersed in pure water and MB solution after vigorous shaking for 2 h (Fig. S1, ESI†) both showed that no evident CDP residues were found in water, indicating that the physical adhesion was firm enough for the adsorption of this material.

The representative SEM images of PBST and PBST/CDP nanofibrous membranes with different mass ratios of CDP to PBST shown in Fig. 4 illustrate that the PBST nanofibers were oriented randomly with an average diameter of 651 nm (Table 1). Both the average diameters and standard deviations of the PBST/CDP nanofibrous membranes were larger than those of pure PBST nanofibrous membrane, which should be attributed to the cladding of CDP and its uneven distribution on the surface of fibers and membrane. It can be seen from Fig. 4b–d that the CDP was not only deposited on the top of nanofibers and wrapped them, but also the area of CDP adhered to the surface of membranes increased with increasing mass ratio. The entire surface of the PBST/CDP-3.2 membrane was covered by the CDP (Fig. 4d), whereas the surfaces of the other membranes with a lower CDP content were only partly covered and holes connected by CDP were also formed on the top of PBST nanofibers. Furthermore, the surface areas (Table 1) of the PBST/CDP nanofibrous membranes were reduced with increasing amount of CDP, whereas the crystallinity (Table 1 and Fig. S2, ESI†) of the composite membranes was almost unchanged compared to pure PBST. It is known that the properties of materials are dependent on the crystal structure and crystallinity, and thus the unchanged crystallinity to some extent indicated that the thermo-mechanical properties of the PBST membranes did not change significantly after being immersed in the hot solution (150 °C) for 1 h.¹¹ The morphologies of the PBST/CDP membranes must affect their adsorption performance, which will be discussed later.

Fig. 4 SEM images of nanofibrous membranes: (a) pure PBST, (b) PBST/CDP-1.2, (c) PBST/CDP-2.0, (d) PBST/CDP-3.2 (the insets are images of their water contact angle).

Table 1 Average diameter, surface area and crystallinity of PBST and PBST/CDP nanofibrous membranes

Sample	PBST	PBST/CDP-1.2	PBST/CDP-2.0	PBST/CDP-3.2
a Determined by BET.b Calculated according to the melting enthalpy in DSC data (shown in Fig. S2, ESI).
Diameter (nm)	651 ± 34	675 ± 58	683 ± 75	688 ± 70
Surface area^a (m^2 g^−1)	24.4	22.0	19.8	15.7
Crystallinity^b (%)	24.1	23.8	24.5	22.5

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Furthermore, it was worth noting from the inset of Fig. 4 that the hydrophobic PBST nanofibrous membrane with a water contact angle of 136° (Fig. 4a) transferred to hydrophilic nanofibrous membranes (Fig. 4b–d) after being modified by CDP, which is consistent with the FTIR result (Fig. 2), where a broad adsorption band (3330 cm⁻¹) of CD, which belongs to the –OH stretching vibration, also turns up in the spectra of CDP and PBST/CDP, indicating that the as-prepared CDP and PBST/CDP membranes are hydrophilic and consequently are more suitable for the dye adsorption in an aqueous solution.

Adsorption performance

It is well-known that cyclodextrins have outstanding capability to form inclusion complexes with a variety of molecules, and electrospun nanofibrous membranes have been widely reported on the application of adsorption, which prompted us to investigate the adsorption performance of PBST/CDP nanofibrous membranes. Fig. 5a shows the UV-vis spectra of MB solution adsorbed by CDP, PBST and PBST/CDP-1.2 membranes, in which the concentration of MB is in proportion to the adsorption peak intensity, and the corresponding dye removal efficiency and adsorption amount are listed in Table 2. As displayed in Fig. 5a, the concentrations of MB aqueous solutions all decreased within the contact time. It was evident that when the MB solution was adsorbed for 12 h, the adsorption peak treated by PBST/CDP became almost flat and the value of R_t reached 98.8%, whereas the adsorption amounts (q_t) of both CDP and PBST membranes were much lower than that of the PBST/CDP membrane. As the adsorption time was increased to 24 h, the adsorption peaks for PBST and PBST/CDP membranes were almost unchanged, and q_t and R_t of CDP membrane kept increasing, but were still smaller than that of PBST/CDP membrane. As a result, a brief conclusion could be drawn that the PBST/CDP nanofibrous membrane exhibits excellent adsorption capability, even though the composite membrane possesses less mass of PBST and CDP compared with the equal mass of pure PBST and CDP. Based on the results of the BET measurement (Table 1), the surface area of the PBST/CDP-1.2 membrane is smaller than that of PBST nanofibrous membrane; however, the adsorption capacity is much better than that of pure PBST, which indicates that CDP plays a crucial role in capturing the dye molecules.¹¹

Fig. 5 UV-vis spectra of methylene blue solutions adsorbed with different materials: (a) PBST, CDP and PBST/CDP-1.2; (b) PBST/CDP nanofibrous membranes with various mass ratios.

Table 2 Adsorption amount and dye removal efficiency of CDP, PBST and PBST/CDP membranes for different adsorption time

Sample adsorption time (h)	PBST		CDP		PBST/CDP-1.2				PBST/CDP-2.0		PBST/CDP-3.2
	12	24	12	24	12	24	0.5	1	0.5	1	0.5	1
a The maximum adsorption amount in theory, i.e. dyes were completely adsorbed.
Maximum^a (mg g^−1)	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00
qt (mg g^−1)	5.87	5.93	7.23	7.61	7.90	7.91	7.27	7.61	7.66	7.81	7.73	7.73
Rt (%)	73.4	74.1	90.4	95.1	98.8	98.9	90.9	95.1	95.8	97.6	96.6	96.6

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Fig. 5b presents the UV-vis spectra of the MB solution adsorbed by the PBST/CDP nanofibrous membranes with various mass ratios and the q_t and R_t values are also tabulated in Table 2. As can be seen, as the MB solution was exposed to PBST/CDP membranes for 0.5 h, the adsorption peak intensities decreased greatly and the larger the mass ratio (more CDP), the more significant the decrease was. Moreover, these membranes showed noticeable MB removal capacity for the first 0.5 h with more than 90% removal efficiency (Table 2). As the adsorption time was increased to 1 h, the adsorption amounts of PBST/CDP-1.2 and −2.0 membranes kept increasing, whereas that of PBST/CDP-3.2 was almost constant, and also the values of q_t and R_t for PBST/CDP-2.0 were higher than those for PBST/CDP-3.2, which should be attributed to the amount of CDP in the membrane that dominates the surface area and surface morphology. Specifically, PBST/CDP-3.2 has much more CDP than PBST/CDP-2.0, but a lower surface area since there are many holes on the surface of the PBST/CDP-2.0 membrane (Fig. 4), which allows the dye molecules to flow into membrane, and thus the PBST nanofibers and inner CDP can adsorb more dye.

Adsorption kinetics and isotherms

To deeply investigate the adsorption performance of the nanofibrous membrane, the effects of the adsorption time and initial concentration of the MB solution on the adsorption behavior were examined. From the results in Fig. 5b, PBST/CDP-2.0 nanofibrous membrane exhibited the best adsorption performance among all these membranes, so it was selected as a typical sample to explore the adsorption properties of the composite membrane. As shown in Fig. 6, when the PBST/CDP-2.0 nanofibrous membrane was immersed in the MB solution, the blue color of MB gradually faded with the immersing time (Fig. 6a) and the characteristic adsorption peak intensity also decreased (Fig. 6b). The blue color was very light after exposure of the membrane to the MB solution for 15 min, indicating that most of MB molecules were adsorbed onto the membrane during the short 15 min period. The adsorption amount of the PBST/CDP-2.0 membrane as a function of the adsorption time is shown in Fig. 7a (square dots). It can be seen that the adsorption amount is increased rapidly with increasing adsorption time and reached a plateau within 30 min, demonstrating the high adsorption efficiency of the PBST/CDP membrane.

Fig. 6 Photographs (a) and UV-vis spectra (b) of MB solution with different concentrations adsorbed by PBST/CDP-2.0 nanofibrous membrane at different time intervals.

Fig. 7 Kinetics plots of the adsorption of PBST/CDP nanofibrous membranes in the MB solutions: (a) pseudo-first order fitting; (b) pseudo-second order fitting.

Quantitatively, the adsorption kinetics was analyzed using pseudo-first order and pseudo-second order models,^26,27 which were expressed as follows:

Pseudo-first order:

Integrated and simplified:

q_t = q_e(1 − e^−k₁t)

Pseudo-second order:

Integrated and simplified:

where k₁ is the pseudo-first order rate constant (min⁻¹), and k₂ presents the pseudo-second order rate constant (g mg⁻¹ min⁻¹). q_t and q_e are the adsorption amount at time t and equilibrium state (mg g⁻¹), respectively.

The experimental data and fitting analyses are also plotted in Fig. 7. The calculated results (insets of Fig. 7a and b) indicated that the pseudo-second order model was more suitable for fitting the experimental values with a higher correlation coefficient (R-square) and closer calculated equilibrium adsorption amount value compared to the pseudo-first order model. This also suggested that the adsorption kinetics of the PBST/CDP-2.0 nanofibrous membrane followed the pseudo-second order model, i.e., the adsorption rate was directly proportional to the square of the available surface sites.²⁷

The initial concentration of the MB solution is another important factor dramatically affecting the adsorption capacity of the membrane; thus the static adsorption isotherms towards the MB solutions at different concentrations varying from 5 to 100 mg L⁻¹ were tested. Fig. 8a shows the color changes of the MB solution adsorbed by PBST/CDP-2.0 membrane with increasing contact time and corresponding UV-vis spectra for the initial and final solutions are shown in Fig. 8b. The MB molecules in solution with a concentration less than 50 mg L⁻¹ were almost completely adsorbed in the first 1 h, however 12 h later, the concentration of solution with an initial concentration of 100 mg L⁻¹ still was high. The adsorption amount and dye removal efficiency for the adsorption time of 12 h are listed in Table 3. When the initial concentration of the solution was increased from 5 mg L⁻¹ to 100 mg L⁻¹, the removal efficiency of the PBST/CDP-2.0 membrane was still very high (higher than 97% at an initial concentration of less than 50 mg L⁻¹), and the adsorption capacity increased monotonically from 4.94 mg g⁻¹ to 85.4 mg g⁻¹, indicating that the increasing initial concentration was an external driving force to enhance the adsorption capacity.²⁷

Fig. 8 Photographs (a) and UV-vis spectra (b) of MB solutions adsorbed by PBST/CDP-2.0 nanofibrous membrane at different times.

Table 3 Adsorption capacity of the PBST/CDP-2.0 membrane to MB solutions with different initial concentrations

Initial concentration (mg L^−1)	5	10	20	50	100
a The maximum adsorption capacity in theory.b The solution concentration after adsorption for 12 h.
Maximum^a (mg g^−1)	5	10	20	50	100
Ce^b (mg L^−1)	0.06	0.16	0.38	3.20	14.6
qt (mg g^−1)	4.94	9.84	19.62	48.80	85.40
Rt (%)	98.8	98.4	98.1	97.6	85.4

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Herein, Langmuir²⁸ and Freundlich models²⁹ were both used to quantitatively analyze the adsorption isotherm, and the two models were expressed as follows:

Langmuir model:

C_e/q_e = 1/(bq_m) + C_e/q_m

Freundlich model:

log q_e = log K_f + (log C_e)/n

where C_e is the concentration at the adsorption equilibrium (mg L⁻¹), q_e is the adsorption amount at equilibrium (mg g⁻¹), b is the equilibrium constant that is associated with the energy of adsorption (L mg⁻¹), q_m is the maximum adsorption amount that is used to compare with other materials (mg g⁻¹), and K_f and n are the Freundlich constants.

The experimental data and linear fitting based on the two models are shown in Fig. 9. Both models appeared to fit the data well within the concentration range investigated. According to the correlation coefficient (R-square, inset of Fig. 9), the Langmuir isotherm equation was more suitable for describing the adsorption process of PBST/CDP nanofibrous membrane, demonstrating that the adsorption of MB molecules was monolayer adsorption and the interactions between the MB molecules were quite weak.²⁷ Furthermore, the maximum MB adsorption capacity (q_m) of the PBST/CDP-2.0 nanofibrous membrane was estimated to be about 90.9 mg g⁻¹, which is comparatively higher than the values reported for other adsorbents (Table 4), further indicating that PBST/CDP nanofibrous membrane is an efficient adsorbent to remove dyes from aqueous solutions.

Fig. 9 Adsorption isotherms for the adsorption of the PBST/CDP-2.0 membrane to the MB solution: (a) Langmuir isotherm, (b) Freundlich isotherm.

Table 4 Adsorption capacities of the other materials to MB molecules, as reported in the literature for comparison

Absorbents	Absorption capacity (mg g^−1)	References
PBST/CDP	90.9	This study
Raw chitin	12.52	30
Chitosan	30.1	31
Lignite	41.49	32
Commercial activated carbon	14	33
Modified cyclodextrin	56.5	34
Clay	58.2	35

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Conclusions

We successfully prepared PBST/CDP nanofibrous membranes with excellent adsorption performance via the combination of in situ polymerization of water-insoluble CDP with the incorporation of PBST nanofibrous mats. The results showed that the composite membranes exhibited a larger adsorption amount than either pure PBST nanofibrous membrane or pure CDP with equal mass. In addition, the adsorption performance was greatly dependent on the amount of CDP, which should be attributed to the domination of CDP to the surface area and surface morphologies of the membranes. The more the amount of CDP, the less the surface area and also the entire membrane surface would be covered completely by CDP, while there were pores and holes distributed in the surface of membrane with less CDP, which further allowed the inner PBST nanofibers and CDP to adsorb more dye. Furthermore, the adsorption kinetics of the PBST/CDP nanofibrous membrane followed a pseudo-second order model and the maximum adsorption capacity was estimated to be about 90.9 mg g⁻¹, which was higher than many other adsorbents that consequently provided more choices for the removal of dyes in wastewater.

Acknowledgements

This work has been financially supported by Shanghai Natural Science Fund 14ZR1400250.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Photograph of PBST/CDP nanofibrous membrane immersed in water after shaking (Fig. S1); DSC curves of PBST/CDP nanofibrous membranes (Fig. S2). See DOI: 10.1039/c6ra22941g

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