Preparation of a novel PSf membrane containing rGO/PTh and its physical properties and membrane performance

Abstract

Recent advances in the fabrication of nanostructures such as graphene-related materials have received a lot of attention in membrane technology for the future of water supplies. Herein, we report the synthesis of a reduced graphene oxide/polythiophene (rGO/PTh) composite material using an in situ enzymatic polymerization reaction, which is an eco-friendly and a simple way to construct a nanocomposite material. Polysulfone (PSf) mixed matrix composite membranes containing rGO and rGO/PTh were prepared via a phase inversion method. The morphology of the membranes was evaluated by various characterization methods, including SEM, AFM, contact angle and porosity measurements. The performance and antifouling properties of the membranes were examined in detail. The PSf-rGO/PTh membrane showed a significant improvement in water flux permeability due to the enhancement of hydrophilicity and porosity. Moreover, the PSf-rGO/PTh membrane exhibited an approximately 10 times higher improved water flux than that of the rGO membrane as the pressure was increased. The fouling resistance ratio (FRR) and antifouling properties of the membranes were tested using two different protein solutions: bovine serum albumin (BSA) and cytochrome c (Ctc). The antifouling and FRR properties of the PSf-rGO/PTh membrane decreased due to not only the interactions between the functional groups on the membrane surface and fouling materials, but also the morphological properties of the membrane.

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1. Introduction

The development of ultrathin and nanoporous membranes with high mechanical strength has recently attracted significant interest for broad applications in water purification.^1,2 To date, the membranes produced using conventional polymeric materials (polyvinyl chloride,³ polyaniline⁴) or nanomaterial (ZrO₂,⁵ TiO₂,⁶ ZnO,⁷ AgNP,⁸ carbon nanotube⁹ and fullerene¹⁰) enhanced composite membranes have been prepared and they have exhibited good separation performance. In recent years, great effort has been focused on the incorporation of carbon-based nanomaterials due to their advantages such as ease of access, high mechanical properties and environmental friendliness.¹¹ As a rising star of carbon-based nanomaterials, graphene has been a novel and promising material for various applications, including electronics,¹² sensors,¹³ solar cells¹⁴ and super capacitors,¹⁵ as well as membrane research.^16,17 Usually, working with modified forms of graphene begins with chemically oxidized graphene (GO), which has recently emerged in membrane research as a fascinating material.¹⁸ However, Ying et al. have stated that due to low permeability, GO incorporated membranes exhibit relatively low separation efficiency, which limits its applications.¹⁹ For this reason, various graphene oxide–polymer hybrid materials have been recently used to arrange the nanoporosity of the membrane.²⁰ GO can form a stable aqueous suspension because of the presence of tunable oxygenated functional groups such as carboxyl groups and hydroxyl groups on the surface, side and edge of the GO sheets.²¹ Hence, when used as a nanofiller, GO possesses strong interactions with the polymer chain and can be easily dispersed into the polymer matrix, which is intimately related to its antifouling performance.²² In this regard, graphene-based polymer composites (G-PCs), which can be fabricated using a variety of simple chemical routes, such as non-covalent dispersion and in situ polymerization methods, have attracted considerable interest in modern membrane science and technology.²³ Despite the attention graphene has received, G-PCs have been barely used as membrane nanofillers in order to enhance flux and antifouling behavior, which are inversely proportional to the thickness and pore size of the membrane.¹⁶ Recently, Akin et al. proposed a reduced graphene oxide–polyaniline composite material incorporated polysulfone-based composite membrane. They have studied the resulting membrane's salt rejection and pure water flux performance, and reported that the membrane exhibits high salt rejection value and water flux.²⁰ Moreover, He et al. produced a composite membrane using polydopamine-modified graphene oxide sheets incorporated into a sulfonated polymer matrix.²⁴ They investigated its transfer properties and proton-conducting membrane fuel cell performance and stated that the results guarantee the nanocomposite membrane's promising prospects in high-performance fuel cells. Therefore, it is believed that G-PCs might improve the practical performance of the membrane such as the effects on the surface charge, antifouling and mechanical properties.

For in situ polymerization processes to produce G-PCs, enzyme-catalyzed reactions have received a lot of attention because the process is a simple, one-step, eco-friendly and does not require strong acidic media^25,26 (i.e. glucose oxidase, which is an oxido reductase enzyme, has been employed for the production G-PCs such as polypyrrole²⁷ and polyaniline²⁰).

The motivation of this study is to report on the synthesis of a novel G-PC and fabrication of the polysulfone membrane using it. In this context, rGO/PTh was synthesized via an in situ polymerization process and incorporated into a PSf matrix using a phase inversion technique. The performance of the obtained membranes was tested using pure water flux and protein rejection experiments. Moreover, the fouling resistance of the membranes was studied using different protein solutions such as BSA and Ctc.

2. Experimental

2.1. Materials and reagents

All chemicals used in the experiments were purchased from global suppliers and used without further purification. Graphite powder (99.99%), concentrated H₂SO₄ (98%), H₃PO₄, KMnO₄ (99%), H₂O₂ (30%), glucose oxidase (GOx), Aspergillus niger (E.C.1.1.3.4.) 295 U mg⁻¹, D-(+)-glucose, thiophene, PSf with M_w-35 000, DMF, bovine serum albumin (molecular weight: 66 kD) and cytochrome c (molecular weight: 12 kD) were purchased from Sigma-Aldrich Co., Germany. All aqueous solutions were freshly prepared using Milli-Q ultrapure water.

2.2. Synthesis of rGO/PTh composite

Graphene oxide (GO) and rGO/PTh composite were synthesized according to literature.^20,27 Briefly, the reduction of GO and polymerization of thiophene to form PTh on the reduced graphene sheets can be explained by the total reaction given in eqn (1).where thiophene (Th) is a monomer for polymerization, GOx is the enzyme generating hydrogen peroxide and glucose is the reducing agent of GO.

2.3. Preparation of PSf membranes

The composite membranes were prepared using a phase inversion technique with PSf as the bulk material, DMF as the solvent, rGO/PTh composite as the additive and distilled water as the non-solvent coagulation bath. The polymer casting solution was prepared by dissolving the polysulfone (15%, w/w) in DMF with vigorous stirring for 12 h to achieve a homogeneous polymer solution. The desired amount (0.1%, w/w) of rGO or rGO/PTh disbanded in DMF was added. Furthermore, the solution was stirred at 60 °C for 24 h to obtain a uniform dispersion of rGO or rGO/PTh in the casting solution. The casting polymer solutions were sonicated for 10 min to remove air bubbles. A spin coater (Laurell WS-400A-6NPP/LITE) was used for covering a polysulfone layer onto the non-woven fabric support (Hollytex 3329), which was pre-wetted using DMF.^28,29 Subsequently, the covered support was immersed into a DI water bath for 10 min at room temperature to induce the phase inversion polymerization. A schematic of this total procedure is given in Scheme 1.

Scheme 1 Schematic for the preparation of the PSf-rGO/PTh composite membrane.

2.4. Characterization

Fourier transform infrared and Raman spectroscopy were used to characterize the GO, rGO and rGO/PTh composite. Fourier transform infrared spectra of the samples were recorded between 550 and 4000 cm⁻¹ wavenumber range using an ATR FT-IR spectrometer (PerkinElmer 100 FT-IR). The Raman spectroscopy measurements were performed at room temperature with a Renishaw-inVia spectrometer combined with 532 nm laser. In order to characterize the composite membranes, AFM, SEM and CA measurements were used. AFM micrographs were taken using a Park XE7 instrument (scanning speed 1 Hz) and the mean roughness parameter (R_a) was analyzed at three different parts of each composite material. The structure of the composites was examined using a Zeiss EVO-LS10 scanning electron microscope. The membrane samples were cut into 0.5 cm² samples, attached with conductive double side tape to steel stabs, and scanned with gold prior to the SEM measurements. Contact angle measurements were monitored by a horizontal beam comparator (KSV CAM 200) equipped with video capture. The sessile drop method was used to measure the contact angle of the composite materials prepared.³⁰ A 4 μL droplet of distilled water was placed on the samples surface, and a magnified image of the droplet was recorded using a digital camera.

2.5. Pilot plant

A schematic of the pilot system (Prozesstechnick GmbH) is depicted in Fig. 1. The system consists of a feed tank, including heating/cooling jacket, a diaphragm pump controlled with a frequency converter (flow range: 1.8–12 L min⁻¹; pressure range: max 40 bar) and a flat-sheet membrane housing, which has an effective filtration area of 44 cm².

Fig. 1 Flow diagram of the pilot system: V₁ and V₂: emptying valve, V₃: pressure regulation valve, V₄: spring loaded valve, PI01 and PI02: pressure gauge, DP1: differential pressure indicator, TI01: temperature indicator.

2.6. Permeation and antifouling performance of the membranes

The permeation and antifouling performance of the composite membranes were tested by measuring pure water flux. The experiments were performed at 25 °C and 1 MPa. An aqueous solution of BSA and Ctc (0.2 g L⁻¹ and 0.05 g L⁻¹, respectively at pH = 7) was measured using a UV-Vis spectrophotometer (Shimadzu UV-1800). The pure water flux (L m⁻² h⁻¹) was calculated by the following equation:where M is the volume (L) of the gathered water, A (m²) and Δt (h) are the membrane area and the permeation time, respectively. The protein rejection R (%) was calculated by eqn (3):where C_p and C_f are the concentrations of the protein in the permeate and feed solutions, respectively.

In order to determine the fouling-resistant behavior of the membrane, the flux recovery ratio (FRR) was evaluated according to the following expression:³¹

where J_w,1 is the initial pure water flux, used as a reference for the membrane permeability and J_w,2 is the water flux of the cleaned membrane after the filtration process. After the pure water flux test, the flux for protein solution J_p (L m⁻² h⁻¹) was measured at 1 MPa for 6 h. Then, the fouled membranes were washed and immersed in distilled water for 30 minutes. Consequently, J_w,2 of the cleaned membranes was measured again.

In order to analyze the formation of a cake/gel layer and adsorption onto the membrane surface or within the membrane pores in detail, the fouling mechanism, the total fouling ratio (R_t), reversible fouling ratio (R_r) and irreversible fouling ratio (R_ir) were determined using the following equations:³²

Eventually, R_t is the sum of R_r and R_ir.

2.7. Porosity and pore size

The overall porosity (ε) of the obtained membranes was measured using the gravimetric method, as defined in the following equation:³¹where ω₁ and ω₂ are the weight of the wet and dry membrane, respectively, A is the membrane effective area (m²), l is the membrane thickness (m), and d_w is the density of water (0.998 g cm⁻³). The membrane mean pore radius (r_m) was calculated using the Guerout–Elford–Ferry³³ equation (eqn (9)) considering the porosity data and the pure water flux.where η is the water viscosity (8.9 × 10⁻⁴ Pa s), Q is the volume of the permeate of pure water per unit time (m³ s⁻¹), and ΔP is the operational pressure (1 MPa).

3. Results and discussion

3.1. Characterization of rGO/PTh composite

FT-IR spectral analysis was performed to characterize the formation of the rGO/PTh composite, which is the reduction of GO upon treatment with glucose. Fig. 2a shows the FT-IR spectra of GO and rGO/PTh composite. The spectrum of GO shows characteristic absorption bands for the stretching vibrations of O–H hydroxyl, C=O carbonyl, C–O carboxy, C–O epoxy and C–O alkoxy at around 3211 cm⁻¹, 1719 cm⁻¹, 1401 cm⁻¹, 1221 cm⁻¹ and 1044 cm⁻¹, respectively.³⁴ After the reduction process, the other peaks for oxygen-containing functional groups were significantly weakened, whereas the peak at 1719 cm⁻¹ belonging to carbonyl groups nearly disappeared. The results confirm that GO was successfully reduced to rGO.²⁰ Moreover, the presence of the peaks at 1526 and 1340 cm⁻¹ in the spectrum of rGO/PTh were related to the C=C and C–C stretching of the thiophene ring, whereas the peaks at 832 and 668 cm⁻¹ belong to the C–S bond in the thiophene ring.^35,36 The presence of these peaks reveals that PTh obtained by the in situ polymerization reaction was successfully settled on the rGO sheets.

Fig. 2 (a) FT-IR spectroscopic analysis of GO and rGO/PTh composite. (b) Raman spectra for GO and rGO/PTh composite. (c) Images of the rGO/PTh composite.

The successful reduction of the oxygen-containing groups in GO and the formation of the rGO/PTh composite was further confirmed using Raman spectroscopy. Fig. 2b shows the Raman spectra of GO and rGO/PTh. The spectrum of GO indicates two characteristic peaks at 1347 and 1597 cm⁻¹ corresponding to the D and G bands, which originate from the structural defects and the sp² graphitized segment in the structure, respectively.³⁷ After reduction with glucose, the D and G bands located at 1353 and 1593 cm⁻¹ show a decreased D/G intensity ratio compared to that found for GO (I_D/I_G ratio decreased from 0.85 to 0.78). Although the increased I_D/I_G ratio for rGO after reduction has been commonly reported in the literature,³⁸ this decrease indicates that the partial sp² domains have been restored³⁹ and the presence of a polyhydrocarbon template on the surface of the rGO.⁴⁰ Moreover, the spectrum of rGO/PTh exhibits a characteristic peak, related to the fully in-plane symmetric bending mode of the C–H bonds of the thiophenes at 1040 cm⁻¹, which can be attributed to thiophene structure in the composite.^41,42 In addition, Fig. 2c shows the images of the rGO/PTh composite thin film peeled from the petri dish. The occurring reduction of GO and polymerization of thiophene on the rGO sheets gives rise to a color change from brown to black indicating the successfully formation of rGO/PTh composite. The obtained composite shows good flexibility and unbroken properties as can be seen in Fig. 2c.

3.2. Characterization of the prepared membranes

The fabricated membranes containing PSf (blank), PSf-rGO and PSf-rGO/PTh were characterized using different methods. To evaluate the hydrophilicity and wettability of the membrane surfaces, contact angle measurements were obtained (Fig. 3). The PSf-rGO membrane showed the highest water contact angle of 102.07 ± 0.8°, whereas PSf and the PSf-rGO/PTh composite membranes showed water contact angles of 82.8 ± 0.5° and 78.2 ± 1°, respectively [n = 3]. The increase in contact angle on the rGO membrane surface can be explained by the hydrophobic character of rGO,^43,44 leading to a more hydrophobic membrane surface. In addition, the resulting PSf-rGO underwent a change from hydrophobic to hydrophilic due to the PTh molecules, which are coated onto the rGO sheets via π–π interactions.⁴⁵ The thiophene groups on the rGO played a key role for increasing the hydrophilicity. The improved hydrophilicity can also be explained by the fact that during the phase inversion process, the hydrophilic functional groups migrate spontaneously to the membrane surface and cause more adsorption of water, which improves the membrane water permeability.⁴⁶

Fig. 3 Drop images during the contact angle measurements of the (a) PSf membrane, (b) PSf-rGO and (c) PSf-rGO/PTh composite membrane.

The changes in the plane surface and cross-section view of the prepared membranes were examined by SEM before and after the modification process and are shown in Fig. 4. It can be seen that the blank PSf membrane surface is relatively smooth (Fig. 4a). The membrane prepared with rGO (Fig. 4b) showed morphological changes compared to the blank PSf membrane. The presence of rGO results in a porous structure on the membrane surface. The membrane prepared with rGO/PTh (Fig. 4c) exhibited smaller pore sizes and higher porosity on the membrane surface. This can be ascribed to the fast exchange of the solvent and non-solvent during the phase inversion polymerization reaction.⁴⁷

Fig. 4 SEM images of the surface (a–c) and cross sections (d–f) of PSf (bare), PSf-rGO, and PSf-rGO/PTh membranes.

The cross-sectional SEM images of the obtained membranes with different additives, such as PSf (blank), PSf-rGO and PSf-rGO/PTh, are presented in Fig. 4d–f, respectively. The structure of the rGO blended membrane has a dense skin layer and slightly irregular microvoids (Fig. 4b), which can be explained by the agglomeration behavior of rGO.⁴⁸ Yu et al. have reported a similar behavior for hyper-branched polyethylenimine graphene blended into PES membranes.²² The cross-section morphology of the PSf-rGO/PTh membrane is different with sponge-like micro-pores in the top layer of the membrane (Fig. 4f). This can be attributed to the processing conditions, which used pre-wetted hollytex fiber with DMF before the casting of the polysulfone layer.⁴⁹ When the amount of DMF increases, the non-solvent begins to move into the polymer solution film more slowly, while the vitrification front moves more quickly relative to the non-solvent front. As a result of this exchange, the membrane morphology shows a formation of sponge-like morphology.^50,51

Fig. 5 shows two and three dimensional surface AFM images for PSf (blank), PSf-rGO and PSf-rGO/PTh composite membranes, including roughness parameters. In these images, the membrane surfaces are not smooth and the dark regions indicate valleys or membrane pores. The mean roughness parameter R_a for PSf (blank), PSf-rGO and PSf-rGO/PTh membranes were obtained at 1.22, 3.92 and 1.70 nm, respectively. After the addition of rGO, the roughness value for the bare PSf membrane increased from 1.22 to 3.92, which is caused by rGO having hydrophobic characteristics.²⁰ Moreover, the addition of rGO-PTh (hydrophilic nature) leads to a reduction in the R_a value to 1.70 nm.

Fig. 5 2D and 3D AFM images of the PSf membranes: (a) PSf (blank), (b) PSf-rGO and (c) PSf-rGO/PTh composite membranes.

3.3. Porosity and pore size

The porosity of the PSf, PSf-rGO and PSf-rGO/PTh membranes are shown in Fig. 6. As can be seen in Fig. 6, the porosity of the PSf (blank), PSf-rGO and PSf-rGO/PTh containing membranes was calculated to be 45.4, 49.5 and 52.3%, respectively. The increase in porosity emerges from the rGO-based material's hydrophilic character that leads to a higher porosity in the membrane surface and thereby improves the water permeability. Moreover, the mean pore radius values were found to be 11.74, 9.44 and 1.95 nm for the PSf, PSf-rGO and PSf-rGO/PTh membranes, respectively. The results indicate that the rGO and rGO/PTh composite membranes have a smaller mean pore radius when compared to the blank PSf membrane because the mean pore radius decreases upon the addition of rGO and rGO/PTh. This is consistent with the study by Yang et al. who reported that adding appropriate TiO₂ nanoparticles to PSf bulk material can improve its porosity and increase the small pore numbers.⁵²

Fig. 6 Porosity of the composite membranes.

3.4. Membrane permeation and antifouling performance

In order to study the membrane performance in terms of the composite membranes containing rGO and rGO/PTh, the water flux and protein rejection were examined in detail. The obtained results for the pure water flux and protein rejection of the composite membranes are presented in Fig. 7a–c. The composite membrane including rGO/PTh showed a higher pure water flux than the PSf-rGO membrane (Fig. 7a). This improvement can be attributed to the increase in hydrophilicity of membrane surface and the boost in the water permeability, which attracts water molecules inside the membrane matrix and facilitates them to move through the membrane.⁵³ Moreover, the hydrophilicity effect of rGO/PTh can increase the solvent and non-solvent exchange during the phase inversion technique, therefore leading a membrane with a more porous surface and improves the water permeability.⁵⁴ When comparing the surface of rGO and rGO/PTh membranes, the rGO/PTh membrane has a smaller pore size than the rGO membrane, whereas it has higher porosity. Although the pore size of the rGO/PTh membrane is smaller, its higher porosity gives rise to a higher water flux. Eventually, it can be concluded that the porosity of the membrane seems to play a prominent role in good water flux. Moreover, the water flux permeabilities of the PSf-rGO and PSf-rGO/PTh membranes were evaluated under different pressures (Fig. 7b). An increase in pressure led to an increase in the water flux permeability for the PSf-rGO/PTh membrane, while the water flux permeability was slightly increased for the PSf-rGO membrane. This is related to the morphological differences, including membrane pore size, porosity and structure.⁵⁵ In particular, it may have been predicted that a higher pressure would lead to the deformation of the membrane pores, which can result in a decreasing/no change in the water flux. The less change in pore size through an increase in pressure can be ascribed to the interconnection of the pores with each other inside the sponge-like structure.^56,57

Fig. 7 (a) Effect of time on the water flux of PSf-rGO and PSf-rGO/PTh composite membranes at 1 MPa operation pressure (b) effect of pressure on the water flux of PSf-rGO and PSf-rGO/PTh composite membranes and (c) protein rejection of the PSf-rGO/PTh membrane.

Protein rejection measurements were carried out with BSA and Ctc protein solutions using a cross flow test system containing the rGO/PTh composite membrane. The rejections of the prepared membrane were about 89% for Ctc and 75% for BSA (Fig. 7c). This can be attributed to the interactions between the BSA and Ctc macromolecules and the functional groups on the blended membrane surface, as well as the extent of protein retention on membrane surface or pores.^58,59

The total fouling ratio (R_t), reversible fouling ratio (R_r), and irreversible fouling ratio (R_ir) values for the prepared composite membranes are presented in Fig. 8a and b. The total fouling resistance of the membranes prepared with the rGO, which is the sum of R_r and R_ir, was lower compared to the rGO/PTh membrane. For BSA, the R_ir value of the PSf-rGO membrane was 7.3% (more than 60% in total fouling), whereas R_ir value of the PSf-rGO/PTh membrane was 44.3% (93.1% in total fouling). For Ctc, the R_ir value of the PSf-rGO membrane was 16.6% (72.6% in total fouling). In addition, the R_ir value of the PSf-rGO/PTh membrane was 67.6% (94.9% in total fouling) (the values are an average of five hours). Contrary to our expectations, the rGO composite membrane exhibited higher antifouling properties than that of the rGO/PTh membrane for the BSA and Ctc protein solutions. As it is well known, the roughness parameters and hydrophilic properties of the membrane surface are important parameters to describe the fouling ability of a membrane.⁶⁰ In this point, the membrane fouling tendency can be increased with an increase in the roughness parameter (discussed in Section 3.2) because the protein tends to accumulate in the “valleys” of a rough membrane surface. Safarpour et al.⁶¹ reported similar behavior for a TiO₂ modified reduced graphene oxide embedding PVDF membrane. In addition, we can also speculate that the rGO/PTh membrane has further interactions with the protein chains by electrostatic attractions and π–π stacking, which can all contribute to the higher R_ir values. Due to the lower number of interactions between rGO and the protein chains, rGO can reduce the adsorption between the membrane and protein. When the BSA solution was changed to Ctc, which has a smaller size and molecular weight, the R_ir values of the membranes increased. These increases can be related to either well-adsorbed Ctc molecules on the membrane surface or plugged membrane pores.

Fig. 8 Fouling resistance ratio of the composite membranes for (a) BSA and (b) Ctc. (c) The water flux recovery for the rGO and rGO/PTh blended PSf membranes after fouling.

The water flux recovery ratio depicted in Fig. 8c can introduce the appropriate recycling properties of the modified membranes. The higher FRR implies a better antifouling property for the membrane. Two different studies were carried out with proteins having different molecular weights. For BSA, the FRR for the rGO blended PSf membrane (91.9%) is higher than the FRR for the membranes prepared with rGO/PTh (62.1%) (the average of 5 h). For Ctc, the FRR for the rGO blended PSf membrane (83.3%) is higher than the FRR for the membranes prepared with rGO/PTh (45.4%) (the average of 5 h). These results showed the high antifouling property of the PSf-rGO membranes.

4. Conclusions

In this study, a rGO/PTh composite was synthesized using an enzymatic method and characterized by FT-IR and Raman spectral analysis. This composite was used to obtain a novel PSf-based membrane using a phase inversion process. The SEM images showed that the prepared composite membrane (PSf-rGO/PTh) possessed a sponge-like micro pore structure on the top layer of the membrane. The addition of the rGO/PTh nanocomposite to the polymer matrix significantly improved the properties and played a favorable role on the characteristics of the membrane. The hydrophilicity and porosity of the blended membranes were enhanced by the addition of the rGO/PTh nanocomposite. The rGO/PTh blended PSf membrane also showed a significant improvement for promoting the water flux permeability with high pressure. The PSf-rGO/PTh membranes, which have a higher R_ir fouling resistance and lower FRR when compared to the PSf-rGO membrane, can be used for the filtration of water at high pressure in different applications.

Acknowledgements

This study was supported by the Scientific Research Projects of Necmettin Erbakan University (141210001).

Notes and references

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