Electrospun H₄SiW₁₂O₄₀/chitosan/polycaprolactam sandwich nanofibrous membrane with excellent dual-function: adsorption and photocatalysis

Abstract

For comprehensive treatment of wastewater, an electrospun H₄SiW₁₂O₄₀ (SiW₁₂)/chitosan (CS)/polycaprolactam (PA6) sandwich nanofibrous membrane (SNM) with dual-functions of adsorption and photocatalysis was prepared to remove Cr(VI) and methyl orange (MO) from aqueous solution. The three layers of the SNM had good compatibility and thus no visible stratification phenomenon was observed in the SNM. Taking advantage of the merits of high porosity, good hydrophilicity, robust mechanical strength and superior water tolerance, the obtained SNM presented an excellent comprehensive performance with high adsorption capacity toward Cr(VI) (78.5 mg g⁻¹), high photodegradation efficiency toward MO (96.7%) and good reusability. Based on the images of the SNM after Cr(VI) adsorption, the SiW₁₂/PA6 layer (upper and bottom layer) was not affected in the adsorption process, which was consistent with our initial design. XPS analysis was carried out to investigate the mechanism of adsorption and photocatalysis. In the adsorption process, Cr(VI) was initially adsorbed on the CS/PA6 layer (intermediate layer), followed by the reduction of Cr(VI) to Cr(III) by the –NH₂ of CS. The photocatalytic process was driven by the reductive pathway with faster degradation rate due to the electron donation from PA6 to SiW₁₂. Although Cr(III) at the layer boundary was involved in the photocatalytic process, the photocatalytic activity of the SiW₁₂/PA6 layer was almost not influenced. This work may provide a new and effective strategy to construct multifunctional nanofibrous membranes for the comprehensive treatment of wastewater.

---

1 Introduction

Water pollution with toxic inorganics and organics is of major concern for human health and environmental quality, and heavy metals and dyes are considered to be highly dangerous.¹ Heavy metals may accumulate to toxic levels under certain environmental conditions and cause adverse effects to living organisms. Azo dyes, which are used extensively and occupy around 70% of the world's dye market,² may generate toxic substances through chemical reactions occurring in the wastewater phase³ and thus influence the photosynthetic activity of aquatic biota. Therefore, the efficient removal of heavy metals and dyes from wastewater has become a critical issue. Numerous physical, chemical and biological methods⁴ have been developed for removing heavy metals and/or dyes from wastewater, such as adsorption, electrochemical precipitation, ion exchange, membrane filtration, photodegradation and flocculation.^5–9 Amongst them, adsorption and photodegradation have been shown to be promising treatments due to their remarkable advantages: high removal efficiency, low-cost, ease of operation and absence of harmful by-products.^10,11 However, most studies were devoted to the elimination of a sort of pollutant, whereas heavy metals and dyes often coexist in practical waste effluents. For that reason, developing versatile materials for comprehensive treatment of wastewater attracts increasing attentions.

Min et al. fabricated a novel micro–nano structure PES/PEI nanofibrous affinity membrane for adsorption of anionic dyes and heavy metal ions in aqueous solution.¹² Aziz et al. reported a low-cost sorbent, chemically modified olive stone, for cadmium and safranine removal from their respective aqueous solutions.¹ Rosales et al. investigated the application of zeolite–Arthrobacter viscosus system for the removal of Cr(VI) and azure B.¹³ Kyung et al. studied simultaneous and synergistic conversion of dyes and heavy metal ions using TiO₂ as photocatalyst.¹⁴ From the standpoint of separation and recovery of sorbents or catalysts in practical wastewater treatment, preparation of functional membranes is an effective way because no separation process is needed for the membranes. Electrospinning is a simple and effective method to prepare nanofibrous membranes with many remarkable characteristics, such as large specific surface area, high porosity and high permeability.¹⁵ Moreover, electrospun nanofibrous membrane could present much improved performance by blending two or more materials in which advantage of each component are synergized. Although the aforementioned materials could treat both heavy metals and dyes, they all possessed only a single function of adsorption or photocatalysis.

In this work, electrospun H₄SiW₁₂O₄₀ (SiW₁₂)/chitosan (CS)/polycaprolactam (PA6) sandwich nanofibrous membranes (SNMs) with dual-function of adsorption and photocatalysis were prepared for the removal of Cr(VI) and methyl orange (MO) from aqueous solution. According to the reported studies^16,17 and our previous experiments,¹⁸ although CS with large amounts of hydroxyl (–OH) and amino (–NH₂) groups exhibited a high metal binding capacity, it dissolved in highly acidic solution which limited its application in adsorption. SiW₁₂, as photocatalyst for the degradation of organic pollutants in water, has received increasing attentions due to its non-toxicity and photostability.¹⁹ SiW₁₂ with Keggin structure has photocatalytic properties comparable to the best-known semiconductors such as TiO₂, ZnO, CdS, etc.²⁰ The main drawback of SiW₁₂ is its high solubility in aqueous solution which makes it impossible to be separated for reuse.²¹ For the purpose of practical applications, it is desirable to combine CS and SiW₁₂ with suitable supporting materials to make them more recoverable. In consideration of the membrane compatibility of the three layers, PA6 with high mechanical strength, good spinnability and excellent water tolerance was chosen as the support for both CS and SiW₁₂.

Scheme 1 shows the structure of the SNM, the treatment of Cr(VI) and MO in aqueous solutions, and the regeneration process. MO is an azo dye that has been widely used as a model compound for the research on photoreactions. More importantly, in the presence of sacrificial electron donor, MO could undergo photoreductive decomposition with much faster degradation rate using POMs as photocatalysts. Sandwich structure was designed for avoiding mutual interference between adsorption and photocatalysis. CS/PA6 layer acted as adsorbent and SiW₁₂/PA6 layer acted as photocatalyst. Considering the contact between SiW₁₂ and MO in the photocatalytic process, SiW₁₂/PA6 layers were placed at the top and bottom of the sandwich membrane. Because both SiW₁₂O₄₀⁴⁻ and Cr₂O₇²⁻ were anions, the SiW₁₂/PA6 layer had little effect on Cr(VI). Therefore, SiW₁₂/PA6 layer would not be affected in adsorption process. On the other hand, by means of filtration, the Cr(VI)-containing solution was forced to flow through the whole membrane, so adsorption would occur in the intermediate layer. In view of this, CS/PA6 layer was placed at the intermediate of the sandwich membrane. The removal of Cr(VI) and MO was conducted successively. To the best of our knowledge, this is the first report on the preparation of a SNM with dual-function of adsorption and photocatalysis for the removal of heavy metals and dyes. The compatibility of the three layers, the interrelationship of the SiW₁₂/PA6 layer and CS/PA6 layer in adsorption and photocatalysis, and the effect of pH values on the adsorption and photocatalysis were mainly discussed. Meanwhile, by means of the XPS analysis, the possible mechanism of treating mixed wastewater containing both Cr(VI) and MO with the SNM was proposed. The successful preparation of such hybrid membrane provides new insight into development of multifunctional nanofibrous membrane applicable to comprehensive treatment of wastewater.

Scheme 1 The structure of the SNM, the treatment of Cr(VI) and MO in aqueous solutions, and the regeneration process.

2 Experimental

2.1 Materials

Polycaprolactam (PA6) (M_w = 30 000) was purchased from Hunan Yueyang Baling Petrochemical Co., Ltd. (Hunan, China). H₄SiW₁₂O₄₀, formic acid (HCOOH), perchloric acid (HClO₄) and potassium dichromate (K₂Cr₂O₇) were supplied by Sinopharm Chemical Reagent Co., Ltd. Chitosan (90.58% deacetylated) was obtained from Aoxing Biotechnology Co., Ltd (Zhejiang, China). Diphenylcarbazide, hexafluoroisopropanol (HFIP) and methyl orange (MO) were purchased from Aladdin Industrial Corporation. All chemicals were of analytical grade and were used without further purification. All the aqueous solutions were prepared with deionized water.

2.2 Preparation of the SiW₁₂/CS/PA6 SNMs

The SNM was fabricated by electrospinning from two solutions. One was prepared by dissolving 2.0 g PA6 in 7 mL HCOOH, after PA6 completely dissolved, 3 mL HCOOH containing 0.75 g H₄SiW₁₂O₄₀ was added into the solution in a dropwise way with vigorous stirring. It was used for fabricating the upper and bottom layers. The other was prepared by dissolving 1.5 g PA6 in 10 mL mixture solvent of HCOOH and HFIP (7/3, v/v), after stirring for 6 h, 0.3 g chitosan was added into the solution and the mixture was stirred for another 2 h. It was used for fabricating the intermediate layer.

The two as-prepared electrospinning solutions were transferred to 5 mL glass syringes with metal needle tips (0.5 mm diameter), respectively. For the upper and bottom layers, the electrospinning process was implemented by utilizing spinning equipment supplied with an applied voltage of 20 kV, a controllable propulsion velocity of 0.5 mL h⁻¹, and a collection distance of 17 cm from the needle tip to aluminum foil. With regard to the intermediate layer, the used voltage was 20 kV, the flow rate was 0.4 mL h⁻¹, and the collection distance was 15 cm. The three-layer nanofibrous membranes were compressed for two minutes at room temperature before being peeled off from the aluminum foil. Because the upper and bottom layers were very thin and the good compatibility of the three layers, the layer thickness of the upper and bottom layers were difficult to define accurately. Therefore, the volume (mL) of electrospinning solutions consumed was selected as the index to the character of the layer thickness. According to the volume (mL) of electrospinning solutions that the upper, intermediate and bottom layers consumed, SNMs are referred to as 0.6/1.6/0.6, 0.6/2.0/0.6, 0.6/2.4/0.6, respectively.

2.3 Cr(VI) removal via suction filtration

Stock solution (1000 mg L⁻¹) of Cr(VI) was prepared by dissolving 2.829 g K₂Cr₂O₇ in 1000 mL deionized water. Solutions of different Cr(IV) concentrations used in various experiments were obtained by diluting the stock solution.

Cr(VI) filtration was conducted on the SiW₁₂/CS/PA6 SNM with a radius of 27 mm in a suction filter, as shown in Scheme 1. 50 mL Cr(VI) solution (80 mg L⁻¹) was placed at top of the setup. With the application of vacuum pressure, the solution was drawn to pass through the SNM. The flow rate of Cr(VI) solution could be adjusted by the throttle and each filtration lasted for around 6 h. The Cr(VI) solution was filtered three times continuously and the filtrate was collected in the bottom flask. The concentration of Cr(VI) before and after filtration were determined spectrophotometrically at 540 nm using diphenylcarbazide as the complexing agent. The samples (1 mL each) were withdrawn and diluted to 50 mL by deionized water for measurement. The amount of Cr(VI) adsorbed, q_t (mg g⁻¹), at time t and the percentage removal of Cr(VI) were calculated with the following equations:

C₀ (mg L⁻¹) is the initial concentration of Cr(VI), C_t (mg L⁻¹) is the concentration of Cr(VI) at time t, W is the weight of the adsorbent used (g), and V is the volume of Cr(VI) solution (L).

2.4 Photocatalytic activity test

The photocatalytic activity of the SNMs were evaluated for decomposition of harmful methyl orange (MO) aqueous. A 300 W high pressure mercury lamp was suspended vertically, and the distance between quartz glass reactor and lamp was 10 cm. The reaction temperature was kept around 20 °C. In a typical experiment, the SNM after saturation adsorption of Cr(VI) was immersed in 50 mL MO aqueous solution (10 mg L⁻¹, pH was adjusted to 2.0 using HClO₄). Before irradiation, the mixture was shaken in the dark until the adsorption equilibrium was reached. The concentrations of MO were determined using an UV-vis spectrophotometer (λ = 510 nm). At given intervals of illumination, the samples (3.0 mL) of MO solution were taken out and analyzed.

2.5 Regeneration and reusability of the SNMs

To evaluate the reusability of the SNMs, the adsorption–desorption experiments were conducted for 3 cycles. The SNM after adsorption and photocatalysis was treated with 0.1 mol L⁻¹ H₂SO₄ (ref. 22) through filtering. Then the SNM was washed with deionized water for three times and dried in vacuum drying oven. The regenerated SNM was repeatedly used in several cycle experiments to clarify the long-term usability.

2.6 Characterization and instrumentation

The morphologies of the samples were observed by using a Hitachi S-4800 field emission-scanning electron microscope (FE-SEM). Fourier transform infrared spectroscopy (FT-IR) was performed with a VETERX70 spectrometer in the range 4000–400 cm⁻¹. The spectra of Cr(VI) solutions were recorded by a 8000S UV-vis spectrophotometer (Shanghai Metash, China) over the wavelength range of 190–800 nm. The N₂ adsorption–desorption isotherms and BET surface area were characterized by a 3H-2000PS1 analyzer (Beishide Instrument Co., China). The pore-size distribution of the membrane was characterized by 3H-2000PB full-automatic bubble pressure method filter membrane pore size analyzer (Beishide Instrument Co., China). The tensile mechanical properties of SNMs were measured on a Shimadzu tensile tester (AGS-X, Japan). The specimens were 60 mm in length and 10 mm in width. XPS measurement was carried out on an ESCALAB 250Xi spectrometer with the non-monochromatised Al Kα X-radiation (hν = 1486.6 eV) and a power of 150 W (10 mA × 15 kV). The water contact angles were measured by a Harke-Space contact angle measuring device.

3 Results and discussion

3.1 Characterization of SiW₁₂/CS/PA6 SNMs

The representative SEM image of PA6 membrane was shown in Fig. 1a. The membrane indicated a nonwoven-like structure piled up by randomly deposited nanofibers with an average diameter of 161 nm. After the introduction of SiW₁₂ (Fig. 1b) and CS (Fig. 1c), the structure and surface morphology of the membranes were essentially unchanged comparing with PA6 membrane. However, the average diameter of SiW₁₂/PA6 and CS/PA6 nanofibers decreased to 120 nm and 105 nm, respectively. As shown in the EDS spectrum (inset of Fig. 1b), C, O, Si and W elements were detected in the SiW₁₂/PA6 layer, providing preliminary evidence that SiW₁₂ has been loaded into the upper and bottom layers of the SNM.

Fig. 1 SEM images of PA6 (a), SiW₁₂/PA6 (b) and CS/PA6 (c) nanofibers. The inset shows the EDS spectrum of SiW₁₂/PA6 nanofibers. (d) FT-IR spectra of PA6, SiW₁₂, CS, SiW₁₂/PA6 and CS/PA6.

The effective preparation of SiW₁₂/PA6 and CS/PA6 layers were also confirmed by the analysis of FT-IR spectra (Fig. 1d). The FT-IR spectrum of SiW₁₂/PA6 composite membrane had nearly all the key features of PA6 and SiW₁₂. The presence of the characteristic absorption peaks at 790–1100 cm⁻¹ supported that the Keggin structure of SiW₁₂ was not destroyed in the composite membrane.²³ The peaks corresponding to ν_as (W=O_d) and ν_as (W–O_c–W) shifted from 980 cm⁻¹ to 970 cm⁻¹, and 792 cm⁻¹ to 802 cm⁻¹, respectively, while the intensity of the peak related to N–H stretching of amide group weakened significantly. All the changes indicated that there may be hydrogen bonds between PA6 and SiW₁₂, which would inhibit the leakage of SiW₁₂ from the membrane during the photocatalytic process. The FT-IR spectrum of CS/PA6 composite membrane also had all the characteristic peaks of PA6 and CS with intensity changes of some peaks (N–H of PA6 and O–H of CS), indicating the existence of weak interaction between PA6 and CS.

Fig. 2a shows the N₂ adsorption–desorption isotherms of the SNM. The BET surface area of the SNM was 13.1 m² g⁻¹. A sharp increase in the isotherm at high pressure from 0.9 to 1.0 (p/p₀) indicated the existence of macropores.²⁴ Fig. 2b shows the pore size of the SNM measured by bubble pressure method. The pore size was around 310 nm with the narrow pore size distribution, and the most probable pore size was 313 nm. The porosity of the SNM was 89.5%, facilitating the exposure of active sites for adsorption and photocatalysis. As shown in Fig. 2c, the water contact angle (WCA) of PA6 was 91.4° owing to the rough surface of electrospun nanofibers.²⁵ By introducing SiW₁₂ or CS into PA6, the composite membrane exhibited a relatively low WCA of 56.9° and 34.7°, respectively. The high hydrophilicity of the membranes contributed to the contact of the SNM and pollutants. Superior mechanical strength and flexibility are critical properties of membranes for long term use. As shown in Fig. 2d, the tensile strength of PA6 nanofibrous membrane was 27.51 MPa, supporting that PA6 was an excellent support. The tensile strength of the nanofibrous membranes decreased with the introduction of SiW₁₂ and CS, 13.79, 13.28 and 14.0 MPa were observed for SiW₁₂/PA6, CS/PA6 and SNM, respectively. The as-prepared SNM inherited the excellent flexibility of PA6, as shown in Fig. 2e and f and Movie S1.† Bending, folding, or twisting of the SNM would not destroy its morphology and performance, making it an outstanding nanofibrous material in the application of wastewater treatment.

Fig. 2 N₂ adsorption–desorption isotherms (a) and pore size distribution (b) of the SNM (0.6/2.4/0.6). (c) WCAs and the corresponding optical profiles of water droplets of PA6, SiW₁₂/PA6 and CS/PA6 nanofibrous membrane. (d) The tensile strength of PA6, SiW₁₂/PA6, CS/PA6 membrane and the SNM (0.6/2.4/0.6). (e) and (f) Photographs presenting the robust flexibility the SNM (0.6/2.4/0.6).

3.2 Adsorption performance of the SNM

To demonstrate the adsorption effect, Cr(VI) removal with the SNM was conducted via suction filtration. Fig. 3a is the image of the SNM before filtration, while Fig. 3b–d are the images of the SNM in different light after filtration. From Movie S2,† it can be seen that there is no visible stratification phenomenon in the membrane, supporting that the SNM is a whole and the three layers has good compatibility. However, as shown in Fig. 3b and c, it is clearly that Cr(VI) existed in the intermediate layer after filtration. Under the strong light, the color (yellow) of the intermediate layer was more distinguishable (Fig. 3d). This result was consistent with our initial design that the CS/PA6 layer acted as the adsorbent and the SiW₁₂/PA6 layer was not affected in adsorption process. Both SiW₁₂O₄₀⁴⁻ and Cr₂O₇²⁻ were anions, so the SiW₁₂/PA6 layer had little effect on Cr(VI). Moreover, by means of filtration, the Cr(VI)-containing solution was forced to flow through the whole membrane, hence adsorption could occur in the intermediate layer.

Fig. 3 (a) The image of the SNM before filtration. The images of the SNM after filtration in different light: (b) natural light, (c) fluorescent light and (d) strong light.

Fig. 4a shows the percentage removal of Cr(VI) with SNMs in which the volume (mL) of electrospinning solutions consumed for preparing the intermediate layers were different. The percentage removal of Cr(VI) were 43.6%, 74.9% and 78.0% for the 0.6/1.6/0.6, 0.6/2.0/0.6 and 0.6/2.4/0.6 SNMs, respectively. The mass of CS increased with the increase of solution volume studied here, resulting in the increase of the percentage removal. However, the percentage removal barely changed when the solution volume exceeded 2.4 mL, so 0.6/2.4/0.6 SNMs were used to conduct the following experiments. The UV-vis absorption spectra of Cr(VI) vs. filtering times are depicted in Fig. 4b. The major absorption peak of Cr(VI) at around 540 nm declined sharply after the first filtration, indicating that most of Cr(VI) was removed at the time of the first filtration. The color of Cr(VI) solution faded gradually during the three times filtration, suggesting an increase of Cr(VI) on the SNM. Adsorption capacity of the SNM and PA6 nanofibrous membrane at varying pH values (2.0–6.0) are shown in Fig. 4c and d, respectively. As presented in Fig. 4c, the adsorption capacity of Cr(VI) increased with the decrease of pH value, and reached a maximum value at pH 3.0 (78.5 mg g⁻¹). This phenomenon may be explained by the fact that the adsorption behavior was controlled by electrostatic forces. The positive charges of the SNM would increase with the decrease of pH value owing to the elevated protonation of –NH₂.²⁶ As Cr₂O₇²⁻ was anion, the electrostatic forces between Cr(VI) and SNM gradually enhanced with the decrease of pH value, thereby leading to the increase of adsorption capacity. By contrast, the adsorption capacity of Cr(VI) on PA6 nanofibrous membrane was all around 2.9 mg g⁻¹ in a pH range from 2.0 to 6.0 (Fig. 4d), supporting that CS was the essential adsorbent for Cr(VI) adsorption.

Fig. 4 (a) The percentage removal of Cr(VI) on the SNMs in which the volume (mL) of electrospinning solutions consumed for preparing the intermediate layers were different (pH = 2.0). (b) UV-vis absorption spectra variation of Cr(VI) vs. filtering times on the SNM (0.6/2.4/0.6, pH = 3.0). The adsorption capacity of Cr(VI) on the SNM (0.6/2.4/0.6) (c) and PA6 nanofibrous membrane (d) at pH 2.0–6.0.

3.3 Photocatalytic activity of the SNM

The photocatalytic activities of the SNMs were investigated by degrading aqueous methyl orange (MO) under UV irradiation and the results are presented in Fig. 5. With increasing the irradiation time, as shown in Fig. 5a, the maximum absorption peak (around 510 nm) of MO declined remarkably. From the inset, it was apparent that MO solution was decolorized after 150 min irradiation, indicating a nearly complete degradation of MO. As a comparison, the control experiments were carried out (Fig. 5b). It is easy to find that no obvious degradation of MO was observed in the absence of photocatalyst under UV irradiation. The degradation efficiency of MO with CS/PA6 composite nanofibrous membrane was 17.7%, maybe due to the physical adsorption. With equal amounts of SiW₁₂ (around 29 mg), the degradation efficiency of MO after 150 min UV light irradiation were calculated as about 50.4%, 95.1% and 96.7% for SiW₁₂, SiW₁₂/PA6 composite membrane and SNM, respectively. Our previous studies found that PA6 could change the mechanism of MO photodegradation with POMs and enhance the degradation rate (see below).²⁷ There was almost no difference between SiW₁₂/PA6 membrane and SNM in the degradation efficiency of MO, suggesting that the intermediate layer after saturation adsorption of Cr(VI) did not influence the photocatalytic activity of the SiW₁₂/PA6 layer.

Fig. 5 (a) UV-vis spectra of MO vs. photoreaction time catalyzed by the SNM. (b) The control experiments. The degradation efficiency (c) and kinetics linear simulation curves (d) of MO photocatalytic degradation over SNMs after saturation adsorption of Cr(VI) in various pH values.

Fig. 5c and d show the degradation efficiency and kinetics linear simulation curves of MO photocatalytic degradation over SNMs after saturation adsorption of Cr(VI) in various pH values, respectively. As presented in Fig. 5c, the degradation efficiency of MO slightly decreased with the adsorption pH value increased. It might be because SiW₁₂O₄₀⁴⁻ is stable when the pH of solution is lower than 2.0. However, at higher pH value, it is partly converted to SiW₁₁O₃₉⁸⁻, whose photocatalytic activity is remarkable lower than that of the saturated one.²⁸ As can be seen from Fig. 5d, the photodegradation reactions follow a Langmuir–Hinshelwood apparent first-order kinetics model which can be expressed by the following equations:

where C₀ is the initial concentration of the reactant (mg L⁻¹), C is the concentration of the reactant at time t (mg L⁻¹), t is the UV-light illumination time, and k_app is the apparent first-order rate constant (min⁻¹).²⁹ The determined k_app are 0.022, 0.023, 0.018, 0.016 and 0.013 for the SNMs after saturation adsorption of Cr(VI) in a pH range from 2.0 to 6.0, respectively.

3.4 Reusability of the SNM

Taking economic plausibility into consideration, it is very important that the SNM is able to reuse.³⁰ Desorption and the regeneration of adsorption sites was carried out with H₂SO₄ (0.1 mol L⁻¹). The adsorption capacity of Cr(VI) and the degradation efficiency of MO within three successive cycles are presented in Fig. 6. As shown in Fig. 6a, comparing with the initial capacity, 18.7% decrease was observed after three cycles. A slightly higher loss of the adsorption capacity may be because the adsorption layer with Cr(VI) was not on the surface of the SNM, influencing the desorption efficiency to a certain extent. By contrast, the degradation efficiency of MO kept relatively stable values within the three cycles, as displayed in Fig. 6b. Such excellent reusability may be attributed to the hydrogen bonds between PA6 and SiW₁₂, which would inhibit the leakage of SiW₁₂ from the support during the photocatalytic process. To investigate the compatibility between SiW₁₂, CS and PA6 after long time use, we prepared a double layer membrane (DLM) in which the upper layer was SiW₁₂/PA6 nanofibrous membrane, while the bottom layer was CS/PA6 nanofibrous membrane. Fig. 6c and d show the FT-IR spectra of the CS/PA6 layer and SiW₁₂/PA6 layer after a three-cycle experiment, respectively. It is clear that there was almost no difference in the FT-IR spectra of the CS/PA6 layer and SiW₁₂/PA6 layer before and after cycling uses, supporting the good compatibility between SiW₁₂, CS and PA6. On the whole, the SNM presented good cycling performance, thereby could greatly meet the requirement of long service life in its practical application.

Fig. 6 Performance of the SNM (0.6/2.4/0.6) for the adsorption (a) and photocatalysis (b) with three times of cycling uses. FT-IR spectra of the CS/PA6 layer (c) and SiW₁₂/PA6 layer (d) before and after a three-cycle experiment.

3.5 Possible mechanism of the adsorption and photocatalysis

Photocatalytic degradation of azo dyes with POMs can be driven by either oxidative or reductive pathway. Our previous work has found that PA6, as sacrificial electron donor, could make MO undergo photoreductive decomposition with much faster degradation rate.²⁷ To further investigate the interaction of the CS/PA6 layer and SiW₁₂/PA6 layer in the photocatalytic process, the DLM was prepared and both the two layers were very thin for the later test. XPS analysis was carried out on the DLM before adsorption, after adsorption (before photocatalysis) and after photocatalysis. The results are presented in Fig. 7. As shown in Fig. 7a, the survey spectrum of the DLM after Cr(VI) adsorption indicated the presence of C, N, O, W, Si and Cr. From Fig. 7b, it is obvious that the binding energy attributed to C–N increased from 399.2 eV to 399.6 eV after Cr(VI) adsorption, suggesting that –NH₂ may be the primary adsorption site. As we can see from Fig. 7c, the binding energy peaks of Cr 2p after adsorption were observed at 576.7 and 586.3 eV, supporting that chromium adsorbed on the DLM existed only in the form of Cr(III).³¹ It was reported that amino groups of the adsorbent was related to the redox process.³² It is noteworthy that after photocatalysis, two different chemical states of Cr were observed. Besides Cr(III), a second doublet at 574.0 and 583.1 eV were assigned to Cr metal, supporting that Cr(III) was partly reduced in the photocatalytic process. Fig. 7d shows the W 4f spectrum of the DLM before and after photocatalysis. It is apparent that there was only one chemical state of W. The peaks with binding energies for the W 4f_7/2 core level of 35.2 eV (before) and 35.4 eV (after) demonstrated the presence of W(VI) centers, which represented the full oxidation state in SiW₁₂.³³

Fig. 7 (a) XPS survey spectrum of the DLM after Cr(VI) adsorption. (b) N 1s narrow scan before (A) and after (B) Cr(VI) adsorption. Cr 2p (c) and W 4f (d) spectrum before (A) and after (B) photocatalysis. The binding energy of C–C is 284.8 eV.

According to the literature^34,35 and the experimental results, the possible mechanism of Cr(VI) adsorption and MO photodegradation with the SNM was proposed, as shown in Fig. 8. In adsorption process, Cr(VI) was initially adsorbed on the intermediate layer due to the electrostatic attraction, followed by the reduction of Cr(VI) to Cr(III) by the amino groups of CS. In the following photocatalytic process, H₄SiW₁₂O₄₀ absorbed light and was reduced to H₅SiW₁₂O₄₀ via accepting electron donated by PA6, and then MO was reductive degradation with H₅SiW₁₂O₄₀ which had strong reducibility. At the same time, Cr(III) at the layer boundary was also reduced. However, Cr(III) reduction had little effect on the degradation rate of MO, maybe due to the significant excess of PA6 in terms of stoichiometry.

Fig. 8 Adsorption and photocatalytic mechanism for the Cr(VI) removal and MO degradation on the SNM.

4 Conclusions

In conclusion, electrospun SiW₁₂/CS/PA6 SNM with dual-function of adsorption and photocatalysis was prepared for the removal of Cr(VI) and MO from aqueous solution. The three layers of the SNM had good compatibility and thus no visible stratification phenomenon was observed in the SNM. Taking advantage of the unique integrated characteristics of the CS/PA6 layer and the SiW₁₂/PA6 layer, the resultant SNM exhibited an excellent adsorption capacity of 78.5 mg g⁻¹ toward Cr(VI) and high degradation efficiency of 96.7% toward MO under UV irradiation. It is noteworthy that the SiW₁₂/PA6 layer had little effect on Cr(VI) adsorption, while the CS/PA6 layer after saturation adsorption of Cr(VI) did not influence the photocatalytic activity of the SiW₁₂/PA6 layer. These results were consistent with our initial design. More importantly, the SNM presented an excellent reversibility, which can meet well with the requirements of practical applications. This work has provided a new and effective strategy to construct multifunctional nanofibrous membrane for comprehensive treatment of wastewater.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51472074), Natural Science Foundation of Hebei Province (E2016209202), the Scientific Research Foundation for Colleges and Universities of Hebei Province under grant for Project (QN2014051), the Hundred Talents Program of Hebei Province of China (E2012100005), the Graduate Student Innovation Fund of North China University of Science and Technology (2016S08) and the Engagement Fund of North China University of Science and Technology (GP201517), to which the authors would like to express grateful appreciation.

Notes and references

1. A. Aziz, M. S. Ouali, E. H. Elandaloussi, L. C. De Menorval and M. Lindheimer, J. Hazard. Mater., 2009, 163, 441–447.
2. A. Bafana, S. S. Devi and T. Chakrabarti, Environ. Rev., 2011, 19, 350–371.
3. T. Robinson, G. McMullan, R. Marchant and P. Nigam, Bioresour. Technol., 2001, 77, 247–255.
4. R. Khan, P. Bhawana and M. H. Fulekar, Rev. Environ. Sci. Bio/Technol., 2013, 12, 75–97.
5. T. Yao, S. Guo, C. Zeng, C. Wang and L. Zhang, J. Hazard. Mater., 2015, 292, 90–97.
6. I. Ali and V. K. Gupta, Nat. Protoc., 2007, 1, 2661–2667.
7. G. Kyzas and D. Bikiaris, Mar. Drugs, 2015, 13, 312.
8. G. Jayanthi Kalaivani and S. K. Suja, Carbohydr. Polym., 2016, 143, 51–60.
9. Y. Tian, B. Ju, S. Zhang and L. Hou, Carbohydr. Polym., 2016, 136, 1209–1217.
10. M. I. Litter, R. J. Candal and J. M. Meichtry, Advanced Oxidation Technologies: Sustainable Solutions for Environmental Treatments, Taylor & Francis, 2014.
11. G. Zhao, X. Wu, X. Tan and X. Wang, Open Colloid Sci. J., 2010, 4, 19–31.
12. M. Min, L. Shen, G. Hong, M. Zhu, Y. Zhang, X. Wang, Y. Chen and B. S. Hsiao, Chem. Eng. J., 2012, 197, 88–100.
13. E. Rosales, M. Pazos, M. A. Sanromán and T. Tavares, Desalination, 2012, 284, 150–156.
14. H. Kyung, J. Lee and W. Choi, Environ. Sci. Technol., 2005, 39, 2376–2382.
15. F. E. Ahmed, B. S. Lalia and R. Hashaikeh, Desalination, 2015, 356, 15–30.
16. R. Nithya, T. Gomathi, P. N. Sudha, J. Venkatesan, S. Anil and S.-K. Kim, Int. J. Biol. Macromol., 2016, 87, 545–554.
17. W. S. Wan Ngah, L. C. Teong and M. A. K. M. Hanafiah, Carbohydr. Polym., 2011, 83, 1446–1456.
18. Z. Li, T. Li, L. An, P. Fu, C. Gao and Z. Zhang, Carbohydr. Polym., 2016, 137, 119–126.
19. A. Corma, H. García and F. X. Llabrés i Xamena, Chem. Rev., 2010, 110, 4606–4655.
20. C. Leal Marchena, L. Lerici, S. Renzini, L. Pierella and L. Pizzio, Appl. Catal., B, 2016, 188, 23–30.
21. I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171–198.
22. V. M. Boddu, K. Abburi, J. L. Talbott and E. D. Smith, Environ. Sci. Technol., 2003, 37, 4449–4456.
23. C. Feng and H. Shang, Chem. Res. Chin. Univ., 2012, 28, 366–370.
24. F. Miao, C. Shao, X. Li, K. Wang and Y. Liu, J. Mater. Chem. A, 2016, 4, 4180–4187.
25. T. Sun, L. Feng, X. Gao and L. Jiang, Acc. Chem. Res., 2005, 38, 644–652.
26. P. Khare, A. Yadav, J. Ramkumar and N. Verma, Chem. Eng. J., 2016, 293, 44–54.
27. W. Li, T. Li, X. Ma, Y. Li, L. An and Z. Zhang, RSC Adv., 2016, 6, 12491–12496.
28. H. Hori, Y. Takano, K. Koike, K. Takeuchi and H. Einaga, Environ. Sci. Technol., 2003, 37, 418–422.
29. M. S. Lee, S. S. Park, G.-D. Lee, C.-S. Ju and S.-S. Hong, Catal. Today, 2005, 101, 283–290.
30. Y. Li, Y. Wen, L. Wang, J. He, S. S. Al-Deyab, M. El-Newehy, J. Yu and B. Ding, J. Mater. Chem. A, 2015, 3, 18180–18189.
31. L. Sun, Z. Yuan, W. Gong, L. Zhang, Z. Xu, G. Su and D. Han, Appl. Surf. Sci., 2015, 328, 606–613.
32. M. Gan, S. Sun, Z. Zheng, H. Tang, J. Sheng, J. Zhu and X. Liu, Appl. Surf. Sci., 2015, 356, 986–997.
33. C. Sui, C. Li, X. Guo, T. Cheng, Y. Gao, G. Zhou, J. Gong and J. Du, Appl. Surf. Sci., 2012, 258, 7105–7111.
34. B. Qiu, C. Xu, D. Sun, Q. Wang, H. Gu, X. Zhang, B. L. Weeks, J. Hopper, T. C. Ho, Z. Guo and S. Wei, Appl. Surf. Sci., 2015, 334, 7–14.
35. A. Troupis, T. M. Triantis, E. Gkika, A. Hiskia and E. Papaconstantinou, Appl. Catal., B, 2009, 86, 98–107.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18920b

---