A new composite membrane based on Keggin polyoxotungstate/poly(vinylidene fluoride) and its application in photocatalysis

Abstract

Polyoxometalate (POM) is a promising candidate as a photocatalyst for the removal of organic pollutants from water, showing striking similarities to TiO₂. However, the drawback of its high solubility in solution makes it difficult to recover and recycle, which impedes its potential applications in wastewater treatment. We have developed a new composite membrane based on Keggin POM and poly(vinylidene fluoride) (PVDF) via combining a doctor-blade method, immersion precipitation and colloid interface techniques. The as-prepared composite membrane was characterized by FTIR, SEM and X-ray EDS, revealing that the Keggin POM could be homogenously intermingled in the composite membrane. The composite membrane not only exhibits good degradability on methyl orange in the presence of H₂O₂ and excellent photocatalytic repeatability by maintaining 97.36% of the initial decolorization efficiency even after 8 cycles, but also can be easily handled and recycled.

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1. Introduction

With the rapid development of the economy and society, serious environmental problems, such as the increasing contamination of water and air, have attracted more and more scientists’ attention.^1,2 To reduce the impact of environmental pollution, photocatalysts have been extensively studied as excellent materials for the elimination of hazardous organic compounds in contaminated water and air.

Polyoxometalate (POM), as a large variety of an oxygen-bridged metal cluster with unique structural characteristics, has been a promising green and cheap photocatalyst for the removal of organic pollutants or transition metal ions from water.^2–9 POM, as a photocatalyst, exhibits striking similarities to TiO₂ in terms of the overall mechanism of photodecomposition of organic compounds, intermediate species and final photodegradation products.^10–12 Ultraviolet light induces POM to produce oxygen-to-metal charge transfer (OMCT) with promoting the electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).^13–17 The charge-transfer excited state (POM*), which has an oxidising property, can directly oxidize the target pollutant, or react with water or another electron acceptor to generate an ˙OH radical.^2,11 But the drawback of the high solubility in solution makes it difficult to recover and recycle, which impedes its potential applications in wastewater treatment. Many researchers have made efforts in order to resolve this problem. Incorporation of homogeneous POM with ESI,† such as TiO₂, Ta₂O₅, ZrO₂, and SiO₂, has been extensively investigated.^18–20 Although those hybrids overcome some difficulties in recovering and recycling, these methods have other shortcomings, such as a limited activity or stability during the reaction.²¹ Conversely, the polymer membrane as a support may have more advantages, such as high stability, low cost and much easier separation from the reaction solution if the organic polymers can avoid UV light degradation. Many studies have focused on the use of an organic polymer as a matrix support for photocatalytic materials, especially in the preparation of heterogeneous catalysts. Some organic polymers with UV light resistance, such as poly tetrafluoroethylene,²² cellulose microspheres,²³ polystyrene beads,²⁴ cationic exchange resin,²⁵ polyethylene oxide,²⁶ chitosan,²⁷ and poly(vinylidene fluoride) (PVDF)^28,29 have been used as the support for photocatalysts. The TiO₂/poly(dimethylsiloxane) (PDMS) composite membrane directly coated on poly(methyl methacrylate) (PMMA) substrates by a sol–gel method showed good photocatalytic activity.³⁰ PDMS-W10, polyether ether ketone (PEEK)-W10 and PVDF-W10 polymer membranes, as heterogeneous photocatalysts, for the oxidation of a series of alcohols were also fabricated.²⁸ However, a composite membrane as a photocatalyst based on Keggin POM and PVDF for decomposing organic dyes has not been reported yet.

In this paper, we will report a new composite membrane constructed from Keggin POM and PVDF by combining a doctor-blade method, immersion precipitation and colloid interface techniques. The composite membrane not only exhibits good degradability on methyl orange in the presence of H₂O₂ under UV light irradiation and excellent photocatalytic repeatability, but also can be easily handled and recycled.

2. Materials and methods

2.1. Materials

Poly(vinylidene fluoride) (PVDF, with the trade name Kinar 460), N,N′-dimethylacetamide (DMA), 12-tungstophosphoric acid (H₃PW₁₂O₄₀, PW),12-tungstosilicatic acid (H₄SiW₁₂O₄₀, SiW), and 12-tunstogermanic acid (H₄GeW₁₂O₄₀, GeW) were used in this work. Tetrabutylammonium bromide (C₁₆H₃₆NBr, TBA), as a counter cation, was reacted with PW, SiW, GeW to get the lipophilic salt of dodecatungstate (PWT, SiWT, GeWT). Methyl orange (MO) was used as a photocatalytic degradation target. The above materials were all purchased from Shanghai Chemical Reagents Co., China and used without purification.

2.2. Preparation of the PWT/PVDF composite

PWT was prepared by the exchange of the counter cation:³¹ a TBA aqueous solution (7.22 × 10⁻² M, 5 ml) was dropped into a PW aqueous solution (8.02 × 10⁻³ M, 15 ml), and the reaction mixture was then refluxed under a nitrogen atmosphere at 75 °C for at least 24 hours. Finally, the product was filtered and washed with ethanol, distilled water, and dried at 60 °C for 24 hours in vacuum (yield: 35%). The obtained PWT can be dissolved in common organic solvents, such as DMA, ethanol, chloroform, etc.

The PWT/PVDF composite membrane was prepared as following:^32,33 a suitable polymer PVDF (15.0 wt%) was dissolved in DMA (80.0 wt%) by magnetic stirring at room temperature, then the lipophilic salt of PWT (5.0 wt%) was added and was stirred in a closed flask at room temperature for at least 24 hours to form a sol–gel solution. A plastic adhesive tape was fixed to the four sides of the cleaned glass sheet to restrict the thickness and area of the composite membrane. The colloid was spread on the glass sheet with a glass rod using a doctor-blade method. The membrane was immersed for 2 hours in a coagulation bath containing deionized water at 25 °C, and then was separated and dried in a vacuum oven at 60 °C for 24 hours. Thus, the PWT/PVDF composite product was obtained. A similar way was also used to prepare the SiWT/PVDF and GeWT/PVDF composite membranes.

2.3. Photocatalytic experiments

The photocatalytic reaction was carried out in a Pyrex reactor of 250 ml capacity attached to an inner radiation type with a 100 W high-pressure mercury lamp as a UV light source. The inner cell had thermostated water flowing through a jacket between the mercury lamp and the reaction chamber, and the system was open to the air. The photocatalytic activities of the samples were evaluated by measuring the degradation efficiency of the methyl orange solution after a regular interval UV light irradiation. The steps were as follows:³⁴ a single piece of composite membrane (size: 2.0 × 5.0 cm, weight: 0.2 g) was placed on the internal wall of the cell opposite to the light source to collect all the focused radiation. A 100 ml 15 mg l⁻¹ methyl orange solution, containing 1.5 mmol l⁻¹ H₂O₂ (pH = 2.5), was placed in the cell under magnetic stirring. Prior to irradiation, the mixed suspension was allowed to stand for 30 min to reach absorption equilibrium due to the methyl orange concentration difference between the solution and the inside of the PWT/PVDF composite. After a fixed irradiation time, 3 ml solution was drawn from the reaction chamber. The MO concentration (C) was determined by measuring the maximum absorbance at 505 nm as a function of irradiation time using a Lambda35 spectrophotometer (Perkin-Elmer, USA). The decolorization efficiency (D) of the methyl orange solution was calculated by the following formula:where D is the decolorization efficiency (%), C₀, A₀ and C₁, A₁ are the concentration and absorbency of the methyl orange solution at the peak of 505 nm before and after irradiation, respectively.

The reproducibility of the photocatalytic degradation activity of the PWT/PVDF composite was studied using 100 ml MO solution (15 mg l⁻¹, pH = 2.5) containing 1.5 mmol l⁻¹ H₂O₂ and a catalyst dosage of 2 g l⁻¹ (effective dosage: 0.5 g l⁻¹) in each cycle after two hour irradiation. At the end of irradiation, the composite membrane was separated from the reaction mixture, washed with water, exposed to oxygen atmosphere overnight and then was dried under vacuum. Before cycling, the weight of the PWT/PVDF composite was measured using a Mettler Toledo Balance (MS105DU). Then the recovered photocatalyst was reused in the next cycle. The PWT concentration in the PWT/PVDF composite was analyzed by a Jobin Yvon Ultima2 ICP atomic emission spectrometer before the 1^st and after the 8^th cycle. For comparison, 0.2 g polymer PVDF membrane was placed in the concentrated POM solution (10 g l⁻¹) for two hours of UV irradiation. After it was separated, washed and dried in vacuum, the weight of PVDF was also measured and used in the next cycle.

2.4. Characterizations

Elemental analyses (C, H, and N) for PWT, SiWT and GeWT were carried on an Elementar Vario EL III analyzer. W is determined by a Jobin Yvon Ultima2 ICP atomic emission spectrometer. (1) For PWT: (C₁₆H₃₆N)₃(PW₁₂O₄₀) anal. calcd: C, 16.00%; H, 3.02%; N, 1.17%; W, 61.21%. Found for (C₁₆H₃₆N)_2.91H_0.09(PW₁₂O₄₀): C, 15.61%; H, 2.94%; N, 1.14%; W, 61.56%. (2) For SiWT: (C₁₆H₃₆N)₄(SiW₁₂O₄₀) anal. calcd: C, 20.00%; H, 3.78%; N, 1.46%; W, 57.39%. Found for (C₁₆H₃₆N)_3.87H_0.13(SiW₁₂O₄₀): C, 19.51%; H, 3.69%; N, 1.42%; W, 57.86%. (3) For GeWT: (C₁₆H₃₆N)₄(GeW₁₂O₄₀) anal. calcd: C, 19.77%; H, 3.73%; N, 1.44%; W, 56.74%. Found for (C₁₆H₃₆N)_3.88H_0.12(GeW₁₂O₄₀): C, 19.32%; H, 3.65%; N, 1.41%; W, 57.17%.

Scanning electron microscopy (SEM) micrographs and X-ray elemental analysis were performed by field emission SEM (JSM-6700F). Fourier transform infrared (FTIR) spectra were recorded in the range of 400–4000 cm⁻¹ on a Perkin Elmer Spectrum using KBr pellets. The absorption spectra of the samples were measured by using a Lambda35 spectrophotometer (Perkins-Elmer, USA).

3. Results and discussion

3.1. Morphology of the PWT/PVDF composite

The SEM image of the PWT/PVDF membrane with 25% of PWT is shown in Fig. 1a, and the corresponding X-ray emission map obtained using the X-ray energy dispersive spectrometry (EDS) detector is presented in Fig. 1c, in which white points of tungsten can be observed. Fig. 1b shows the membrane with 30% of PWT. As seen from Fig. 1a and b, the surface morphology of the membrane with 25% PWT is much more even than that of the membrane with 30%. Besides, there are some holes and agglomerates present, as observed in Fig. 1b. This demonstrates that a suitable amount of PWT can be well dispersed into the polymer PVDF. Fig. 1c exhibits a uniform dispersion of tungsten, indicating that PWT was intermingled homogeneously in the polymer PVDF. Thus, we selected a PWT/PVDF membrane of 25% PWT to examine the photocatalytic experiments.

Fig. 1 SEM image of the PWT/PVDF composite with 25% PWT (a) and the corresponding X-ray emission dot map (c), obtained with an X-ray fluorescence microprobe showing tungsten as white points, and a SEM image of PWT/PVDF with 30% PWT (b).

3.2. FTIR spectra

As shown in the PWT spectrum (Fig. 2a), the absorption peaks at 1081, 1043, 955, 904, 508 cm⁻¹ are fundamentals for PW₁₂O₄₀³⁻, which indicate that the Keggin units were not destroyed during the ion-exchange. Fig. 2b exhibits the FTIR spectrum of the PWT/PVDF composite membrane. In the spectra of the PWT/PVDF composite samples, the characteristic bands of POM appear slightly shifted, indicating that PWT and PVDF interact with each other and the structure of PW₁₂O₄₀³⁻ is still preserved inside the PWT/PVDF composite. It is concluded that the POM structure remains stable after immobilization in the PVDF matrix. Fig. 2c is the FTIR spectrum of the PWT/PVDF composite after 8 cycles of photocatalytic reaction. Its absorption bands are very similar to those shown in Fig. 2b, confirming that the original framework of PW₁₂O₄₀³⁻ was not destroyed after several cycles of photocatalytic reaction in the composite membrane.

Fig. 2 Infrared spectra of PWT/TBA (a), PWT/PVDF (b), and PWT/PVDF after photodegradation reaction (c).

3.3. Photocatalytic property

The variation in decolorization efficiencies of the MO solution under various conditions is shown in Fig. 3. Curve a in Fig. 3 is the initial time (0.5 hour for all samples) during which the whole system was stirred in the dark, and curve b (from 0.5 hour to 8.5 hours) denotes the time for the mixture to be exposed to UV light irradiation. In the dark, there is negligible degradation of the MO solution, even in the presence of a catalyst and H₂O₂. Only after the UV light irradiation started, the degradation of MO was initiated.³⁵ After 8.5 hours, the decolorization efficiency of the MO solution can achieve 98.35% (curve b). Only limited photodegradation of MO in the absence of the H₂O₂ system (curve e) was observed, indicating that H₂O₂ is an efficient electron acceptor in the heterogeneous PWT/PVDF system. Meanwhile, the decolorization efficiency in the presence of only H₂O₂ (curve f) was very low, implying that PWT/PVDF is an effective photocatalyst. To understand the role of PWT and PVDF in the composite, we also compared the decolorization efficiencies of PWT, PVDF, and PWT/PVDF. As shown in Fig. 3a and g, the decolorization efficiency of PWT is a little higher than that of PWT/PVDF, using the same effective catalyst dosage, which indicates that the PVDF support doesn’t hinder the catalytic reaction activity. Whereas the decolorization efficiency of polymer PVDF is almost zero, which shows that the PVDF support has no photocatalytic activity under UV irradiation. However, the central atom in the Keggin unit of POM has a significant effect on the properties. We compared the photocatalytic activities of three composite membranes of different POM under the same conditions. The results are also presented in Fig. 3c and d. It is obvious that the photocatalytic activities among the Keggin unit of POM for MO follows the order of PWT/PVDF > SiWT/PVDF > GeWT/PVDF, which is consistent with the reactivity order in other photocatalytic reactions.^36,37

Fig. 3 Photocatalytic decolorization efficiency for methyl orange (MO) with different POM/PVDF composite membranes. MO concentration: 15 mg l⁻¹; pH: 2.5; POM/PVDF dosage: 2 g l⁻¹ (effective dosage, 0.5 g l⁻¹); PWT, 0.5 g l⁻¹ (a) PWT; (b) PWT/PVDF/H₂O₂; (c) SiWT/PVDF/H₂O₂; (d) GeWT/PVDF/H₂O₂; (e) PWT/PVDF; (f) H₂O₂; (g) PVDF.

3.4. Photocatalytic kinetic

The PWT/PVDF composite membrane was selected as a photocatalyst to investigate the kinetic of the MO degradation in an aqueous solution, and the results are depicted in Fig. 4.

Fig. 4 First-order linear ln(A₀/A) = f(t). MO concentration: 15 mg l⁻¹; pH: 2.5; PWT/PVDF dosage: 2 g l⁻¹ (PWT, 0.5 g l⁻¹); H₂O₂, 1.5 mmol l⁻¹.

It follows an apparent first-order reaction, which is in good agreement with a generally observed Langmuir–Hinshelwood kinetics model:²⁶

where r is the degradation efficiency of the reactant (mg l⁻¹ h⁻¹), C is the concentration of the reactant (mg l⁻¹), t is the illumination time (h), k is the kinetic constant (mg l⁻¹ h⁻¹), and K is the adsorption coefficient of the reactant (l mg⁻¹). When the initial concentration, C₀, is a micromolar solution, the eqn (2) can be simplified to an apparent first-order equation:

The plot of ln(A₀/A) versus time represents a straight line, the slope of which gives a rate constant, k_app, of 0.607 h⁻¹ upon linear regression.

3.5. Effect of H₂O₂

Fig. 5 shows the initial decolorization efficiency of methyl orange (15 mg l⁻¹, pH = 2.5), containing different H₂O₂ concentrations, after two hour irradiation. The decolorization efficiency increases with an increase in H₂O₂ concentration from 0.4 mmol l⁻¹ to 1.5 mmol l⁻¹, and then decreases with an increase in H₂O₂ concentration from 1.5 mmol l⁻¹ to 2.0 mmol l⁻¹. The optimum concentration of added H₂O₂ is about 1.5 mmol l⁻¹. In the photocatalytic degradation process, the addition of adequate H₂O₂ is beneficial to the formation of hydroxyl radicals.^38–40 H₂O₂ will facilitate the generation of ˙OH and promote the decolorization efficiency. However, under the conditions of an excessive H₂O₂ concentration, H₂O₂ acts as a scavenger of ˙OH and exhaust ˙OH in the solution, and the decolorization reaction will be retarded. Therefore, only moderately added H₂O₂ is beneficial to achieve a higher degradation efficiency.^38–40

Fig. 5 Effect of H₂O₂ on the decolorization efficiency of the methyl orange (MO) solution with the PWT/PVDF composite. MO concentration: 15 mg l⁻¹; pH: 2.5; dosage: 2 g l⁻¹ (PWT, 0.5 g l⁻¹); irradiation time: 2 hours.

3.6. Proposed mechanism

It is well known that POM can decompose a variety of organic pollutants via the formation of a common powerful oxidizing reagent from the reaction of the excited state or water or other electron acceptors, being very similar to TiO₂.^10–12 In this work, the polymer PVDF support makes it easier to recycle and reuse the catalyst, and has no contribution to the photocatalytic activities. Meanwhile, the added H₂O₂ plays an important role in the photodegradation process. Thus, the photocatalytic mechanism of the PWT/PVDF composite membrane may be depicted as follows:²

PVDF-in-POM* + H₂O → PVDF-in-POM⁻ + H⁺ + ˙OH (5)

PVDF-in-POM⁻ + O₂ → PVDF-in-POM + ˙O₂⁻ (6)

H₂O₂ + ˙O₂⁻ → O₂ + OH⁻ + ˙OH (7)

˙OH + MO (dye) → degradation product (9)

UV excitation of POM induces oxygen-to-metal charge transfer (eqn (4)), and the charge-transferred excited state POM* has a highly oxidizing power that is strong enough to oxidize H₂O to generate hydroxyl radicals (eqn (5)).² The POM core is like a reservoir of electrons, which can undergo electron reduction processes.⁴ In the presence of dioxygen, the reduced catalyst (PVDF-in-POM⁻) undergoes an easy reoxidation to the parent catalyst, through the transfer of an electron from the reduced species to dioxygen (eqn (6)), keeping the photocatalytic cycle persisting. H₂O₂ will facilitate the generation of ˙OH and promote the photodegradation efficiency (eqn (7) and (8)). Then, the radicals attack the organic substrates and degrade the dye (eqn (9)).^2,38

3.7. Reproducibility of the photocatalyst

The reproducibility of the photocatalytic activity of the photocatalyst is a very important parameter to assess the practicability of the photocatalyst.³⁹ Fig. 6 exhibits the decolorization efficiency of MO by the PWT/PVDF composite membrane after different recycling times. It is observed that the decolorization efficiency decreases from 69.05% to 67.23% after eight cycles, which maintains 97.36% of the initial decolorization efficiency, indicating that the photocatalytic activity has a better repeatability. To further investigate the reason of the little decrease in decolorization efficiency with an increase in recycling times, the weight of PWT/PVDF before and after the photocatalytic degradation reaction was measured (Table S1†), and the PWT concentration in the PWT/PVDF was analyzed before the 1^st and after the 8^th cycle (Table S1†). For comparison, the weight of PVDF photodecomposed by PWT, was also measured (Table S2†). As can be seen from Tables S1 and S2,† the weight of PWT/PVDF, the PWT concentration in PWT/PVDF, and the weight of PVDF are all a little decreased with increased recycling times. Thus, it can be deduced that the reason of the very little decrease in decolorization efficiency with an increase in recycling times may be the degradation of PVDF and the losing of POM. In conclusion, the PWT/PVDF composite membrane exhibits a simple recycling procedure and a better reproducibility of photocatalytic degradation, which is of great significance for the practical use of the POM photocatalyst.

Fig. 6 Photocatalytic decolorization efficiency for methyl orange using the PWT/PVDF composite after different recycling times. MO concentration: 15 mg l⁻¹; pH: 2.5; dosage: 2 g l⁻¹ (PWT, 0.5 g l⁻¹); irradiation time: 2 hours.

4. Summary

We have successfully developed a PWT/PVDF composite membrane by combining a doctor-blade method, immersion precipitation and colloid interface techniques. The PWT/PVDF composite membrane exhibits good UV light photocatalytic activity to decompose an MO solution and excellent photocatalytic repeatability. Moreover, the composite membrane can be easily handled and recycled, thus indicating great practicability. This study also shows that the water-soluble POM can be immobilized into a polymer matrix, which may provide a new way to prepare an insoluble and readily separable solid for the photocatalytic degradation of organic pollutants in solution.

Acknowledgements

This work was financially supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (14KJB150007), and High-level Scientific Research Funds for the overseas talent of JUST (635211301).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11409d

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