This study was performed to synthesize membranes of polyethersulfone (PES) blended with graphene oxide (GO) and PES blended with GO functionalized with photoactive semiconductor catalyst (TiO2 and ZnO). The antifouling and self-cleaning properties of composite membranes were also investigated. The GO was prepared from natural graphite powder by oxidation method at low temperature. TiO2 and ZnO nanopowders were synthesized by anhydrous sol–gel method. The surface of TiO2 and ZnO nanopowders was modified by a surfactant (myristic acid) to obtain a homogeneously dispersed mixture in a solvent, and then GO was functionalized by loading with these metal oxide nanopowders. The PES membranes blended with GO and functionalized GO into the casting solution were prepared via phase inversion method and tested for their antifouling as well as self-cleaning properties. The composite membranes were synthesized as 14%wt. of PES polymer with three different concentrations (0.5, 1.0, and 2.0%wt.) of GO, GO-TiO2, and GO-ZnO. The functionalization of membranes improved hydrophilicity property of membranes as compared to neat PES membrane. However, the lowest flux was obtained by functionalized membranes with GO-TiO2. The results showed that functionalized membranes demonstrated better self-cleaning property than neat PES membrane. Moreover, the flux recovery rate of functionalized membranes over five cycles was higher than that of neat membrane.
Textile processing industry is one of the common sectors in developing countries. The sector uses excessive amounts of water and produces wastewater resulting in pollution loading being too much. The wastewater contains high amount of suspended and dissolved solids, non-reacted dyestuffs (color) and other chemicals that are used in the different stages of dyeing and other processing. The presence of even small amounts of dye in water (e.g. 10–20 mg/L) is highly visible and affects the water transparency (Rajkumar & Kim 2006).
So far, many wastewater treatment technologies have been developed for color removal, including physical, chemical and biological processes. All of them have some advantages and drawbacks (Alver & Metin 2012; Kurt et al. 2012; De Jager et al. 2014; Hayat et al. 2015; Dehghani et al. 2016). The conventional chemical coagulation process generates a large volume of hazardous sludge and has a problem of sludge disposal. However, the biological treatment of textile wastewater shows low degradation efficiency due to presence of biologically inert high molecular weight dyestuffs (Rajkumar et al. 2007). Additionally, a combination of two or more treatment methods for the further treatment was investigated by different researchers in order to satisfy the legal requirements (Doumic et al. 2015; Jung et al. 2015; Punzi et al. 2015).
Recently, membrane technologies have gained significance because of important advantages such as easy operation, small footprint, high quality of effluent water, and cost-effectiveness (Lee et al. 2016a). Thus, membranes have been used intensively for color removal, wastewater treatment, and water recovery in the past decade (Kurt et al. 2012). However, wastewater has organic and inorganic pollutants. These pollutants can cause serious membrane fouling, which raises operating costs, decreases membrane life time, and increases maintenance (Dizge et al. 2011; Jegatheesan et al. 2016). Complicated interactions between membrane surface and dye molecules as well as microbes existing in textile wastewater can cause membrane fouling. This type of fouling can be considered reversible or irreversible and it is very difficult to remove from the membrane surface due to the interactions between membrane polymer chains and dye molecules or microbes (Drews 2010; Chidambaram et al. 2015; Chidambaram & Noel 2015). Therefore, many efforts have been made to prevent membrane fouling, such as modification of membrane materials, improvement of biomass characteristics, and optimization of operating conditions (Chang et al. 2002; Torretta et al. 2013; Zinadini et al. 2014; Zinadini et al. 2015). Among these methods, modification of membrane materials especially blending modification can significantly prevent the membrane fouling (Ozay et al. 2016). Moreover, self-cleaning membrane production with inorganic nanoparticles such as TiO2 and ZnO has been widely studied. Additionally, ZrO2, Al2O3, and SiO2 nanoparticles, among others, were used successfully and decreased the biofouling (Richards et al. 2012; Safarpour et al. 2014; Xu et al. 2014a, 2014b; Xu et al. 2016).
Application of graphene-based materials has increased considerably in the last decade because of their extraordinary features such as large specific surface area, thermal conductivity, and good optical transparency (Geim 2009; Dreyer et al. 2010; Zhu et al. 2010). Moreover, graphene oxide (GO) is an ideal nanomaterial for attaching a semiconductor catalyst such as TiO2 and ZnO. Its large surface area and high charge carrier mobility enhance further photocatalytic efficiency of semiconductor catalysts (Jiang et al. 2011; Wang et al. 2013). Indeed, the functionalized structures which are constructed by modifying GO with different materials have led to speeding up these trends in this field (Liu et al. 2010a, 2010b, 2010c; Guo & Dong 2011; Tang et al. 2013; Huang et al. 2014; Lee et al. 2016b). Among these materials, semiconductor catalysts which show photocatalytic properties attract much more attention. A photocatalyst can be defined in a simple manner as a semiconductor which generates free radicals after photon absorption of radiation. TiO2 and ZnO are the most commonly used semiconductors for photocatalytic studies due to their non-toxic behaviors, stability, inexpensiveness, high photosensitivity and photocatalytic degradation activity against organic pollutants (Sakthivel et al. 2003; Chakrabarti & Dutta 2004; Gaya & Abdullah 2007; Ocakoglu et al. 2012; Ocakoglu et al. 2013; Huang et al. 2014; Doruk et al. 2016). These remarkable properties are taken into account for the selection of these materials in this study. The fabrication of functionalized structures combining graphene-based materials and photocatalytic nanostructures could expand the application fields such as environmental areas.
The aim of this study was to create an antifouling and self-cleaning membrane surface. For this purpose, the polyethersulfone (PES) membranes blended with GO and with GO functionalized by photoactive semiconductor catalyst (TiO2 and ZnO) were synthesized via the phase inversion method and successfully used in methylene blue dye filtration. GO and semiconductor catalysts were prepared according to Hummers' and sol–gel methods, respectively. The characterization of the prepared GO and functionalized GO with TiO2 and ZnO catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray powder diffraction (XRD). Also, the structures of membranes and properties were investigated using energy-dispersive X-ray spectroscopy (SEM-EDX), atomic force microscopy (AFM), and static contact angle measurements. Additionally, antifouling properties of the prepared composite membranes were tested by filtration of Escherichia coli as a model bacteria solution. Finally, the performance of self-cleaning properties of the composite membranes was examined by filtrated methylene blue dye as a model solution. After that, the dye-contaminated membranes were subjected to visible light photocatalytic irradiation and the membrane surfaces were cleaned without using any chemical cleaning agents.
Graphite powder (325 mesh, 99%) was purchased from Alfa Aesar. Titanium(IV) chloride (TiCl4) (99.9%), benzyl alcohol (99.8%), zinc nitrate hexahydrate (Zn(NO3)2.6H2O), polyethylene glycol (PEG300), and N-methyl-2-pyrrolidione (NMP) were purchased from Sigma-Aldrich. Sulfuric acid (98%, H2SO4), hydrochloric acid (38%, HCl), hydrogen peroxide (30%, H2O2), potassium permanganate (KMnO4), ammonia (NH3) and tetrahydrofuran were purchased from Merck. Polyethersulfone (Ultrason E 6020 P) as a polymer was kindly provided by BASF Company from Turkey. Endo agar used in preparing plates for the antimicrobial membrane filtration test was purchased from Merck. Endo agar plates were prepared according to Merck Microbiology Manual (Merck 2005) E. coli (ATCC 25922) bacteria was obtained from our Biotechnology Research Organization for Science and Technology (Mersin, Turkey). All chemicals were analytical grade and a two-stage Millipore Direct-Q3UV purification system was used to obtain distilled water.
Characterization of GO, GO–TiO2 and GO–ZnO nanopowders
A schematic diagram of GO–TiO2 and GO–ZnO nanocomposite formation is shown in Figure S1 (Supporting Information, available with the online version of this paper). However, the synthesis of GO, GO–TiO2 and GO–ZnO nanopowders is described in detail in the Supporting Information. XRD analysis was undertaken using a Rigaku Smartlab model X-ray diffractometer at Cu-Kα radiation (λ = 1.54 Å). The XRD analysis of dried TiO2 nanoparticles and ZnO nanorods samples were carried out continuous scans from 10 to 90° at 2° scan rate at 2θ min−1 in ambient air. SEM images of GO–TiO2 and GO–ZnO nanopowders were taken using a Zeiss Supra 55. The presence of alkyl chains on the metal oxide surfaces was confirmed by FTIR-attenuated total reflectance spectroscopy. The characterization of synthesized nanopowders (GO, GO–TiO2 and GO–ZnO) is given in the Supporting Information (Figures S2–S6, available with the online version of this paper).
Preparation of neat PES membrane
Neat PES membranes were synthesized by phase inversion method. The PES beads were dried at 80 °C in an oven for 2 h before use. The casting solution was prepared by dissolving PES beads (14% w/w) in NMP solvent at room temperature. The composition of casting solution is given in Table 1. The prepared solution was stirred vigorously at 60 °C for 6 h and then agitated at room temperature overnight to obtain a clear homogeneous solution. Afterwards, the polymer solution was ultrasonicated for 20 min to remove air bubbles from the solutions. The bubble-free solution was cast onto a glass plate with a casting knife of about 200 µm gap at 100 mm/s. The glass plate was immediately immersed into a coagulation bath including distilled water. After that, the formed membranes were stored in distilled water for 1 day for complete removal of the residual solvent. Thickness of the prepared membranes was measured by a digital micrometer and it was calculated as 163 ± 25 µm.
|Membrane sample .||PES (%wt.) .||NMP (%wt.) .||Sample (%wt.) .|
|Membrane sample .||PES (%wt.) .||NMP (%wt.) .||Sample (%wt.) .|
Preparation of functionalized PES membrane
The neat and functionalized GO with photocatalytic semiconductor catalyst (TiO2 and ZnO) composite membranes were prepared according to the phase inversion method. In this method, the GO, GO–TiO2 and GO–ZnO nanopowders were added to the casting solution in order to prepare functionalized membranes. Different ratios of GO, GO–TiO2 and GO–ZnO nanopowders (0.5, 1.0, 2.0%wt.) were dispersed in NMP solvent using an ultrasonication bath for 15 min, and PES beads were then added to this solution. The PES membranes with different GO content were prepared, and the casting solution composition of various membranes is presented in Table 1. The casting solutions were ultrasonicated for 20 min to remove air bubbles from the solutions. Then, the casting solutions of PES/GO, PES/GO–TiO2, and PES/GO–ZnO were spread with a casting knife gap setting of 200 μm at 100 mm/s casting shear on a glass plate. The glass plates were immediately dipped in a deionized water bath to obtain polymer precipitation. After membrane formation, the membranes were taken from the water bath and stored in fresh deionized water for at least 1 day to guarantee the complete phase inversion.
Characterization methods for synthesized membranes
The surface and cross-section morphologies of the prepared membranes were characterized by SEM-EDX using a Zeiss/Supra 55 FE-SEM model operating at 15 kV. All samples were dried and coated under vacuum with a thin layer of platinum–palladium by a sputtering system.
AFM was used to characterize the surface roughness of prepared membranes. The average of the surface roughness (Ra) values was calculated as the standard deviation of all the height values within the given area. The AFM analyses were performed on a Park System XE-100 SPM AFM microscope. Small squares (0.5 × 0.5 cm) of dried samples were cut and 5 μm × 5 μm areas were scanned by contact mode in the air. Three different regions for each sample were scanned, and the average values of Ra are presented.
The contact angle of the neat and composite membranes was measured by a KSV CAM 200 goniometer (KSV Instruments). The analyses were performed for at least five different locations on the membrane surfaces.
The performance of the prepared membranes was measured by a dead-end flat sheet membrane module with a filtration area of 14.6 cm2 at a temperature of 25 ± 1 °C and an operating pressure of 5 bar.
where ΔP refers to transmembrane pressure.
In order to calculate the membrane porosity, the dry membranes were cut to a definite size (14.6 cm2) and then immersed in distilled water for 1 day. Water on the surface of the membranes was removed carefully with a clean cloth and the membranes were weighed. Then, the membranes were dried for 24 h in a desiccator and weighed again to measure the membranes' weight in dry state. The membrane porosity (ε) was determined by gravimetric method, as defined in the following equation (Zhao et al. 2014):
Microbial experimentation and antifouling property of the membranes
In this study, antibacterial activity of the prepared membranes containing GO, GO–TiO2 and GO–ZnO nanopowders against E. coli bacteria was investigated using the agar diffusion method. All types of membrane were filtered with 100 mL of E. coli suspension using a dead-end filtration system set-up. The bacterial load of the influent was about 1.2 × 106 colony-forming units (CFU) per millilitre. The CFU is a measure of viable bacterial cells. Bacteria were grown aerobically on agar plates at 37 °C overnight. Endo agar cup plate method was used to determine antibacterial activity of the GO, GO–TiO2 and GO–ZnO nanopowders against the test pathogen, E. coli. However, in order to test antibacterial activity of prepared membranes, they were cut into circular disks and put on the agar plate. The neat PES membrane was considered as the control sample (Ozay et al. 2016).
Self-cleaning activity of the membranes
In order to analyze the effect of semiconductor nanopowders for membrane cleaning, PES-GO composite membrane functionalized with TiO2 and ZnO nano powders was used to remove methylene blue dye from the membrane surface. Methylene blue dye (25 mg/L) was filtered for 2 h. After that, the membranes were exposed to UV light for 2 h. Then, the flux of membranes exposed to UV light was measured and compared with distilled water flux. The quartz photoreactor was surrounded by six UVA (Philips TL 8W Actinic BL) type lamps emitting waves at 365 nm predominantly. Lamps were placed in a hexagonal position inside an aluminum-foil-coated tube for equal reflection. The light intensity of the lamps was measured for UVA as 3.5 mW/cm2, respectively, using a UV-light meter (Lutron UVA-365 sensors) inside the quartz reactor (Doruk et al. 2016).
RESULTS AND DISCUSSION
Characterization of neat PES and PES/GO, PES/GO–TiO2, and PES/GO–ZnO blended membranes
Pure water permeation flux and hydrophilicity properties of membranes
It is well known that surface hydrophilicity is a significant factor in determining the flux and antifouling performance of membranes (Xu et al. 2016). The contact angle results of various membranes are compared in Figure 1. As shown in this figure, static contact angle decreased with addition of the GO and for functionalized GO-ZnO membranes. Neat PES membrane showed the highest water contact angle of 66.2 ± 1.3°. Blending of 0.5, 1.0, and 2.0%wt. GO nanopowders reduced the water contact angles to 56.4 ± 0.9°, 53.1 ± 1.1°, and 51.5 ± 0.8°, respectively. However, blending of 0.5, 1.0, and 2.0%wt. GO-ZnO nanopowders enhanced the water contact angles to 54.8 ± 1.2°, 50.6 ± 0.7°, and 49.2 ± 1.0°, respectively. Finally, blending of 0.5, 1.0, and 2.0%wt. GO-TiO2 nanopowders had 55.1 ± 1.2°, 51.7 ± 1.5°, and 50.5 ± 0.9°, respectively. The decreasing of contact angle can be due to the presence of carboxylic acid and hydroxyl groups on GO nanosheets, which increase hydrophilicity of GO. Composite membranes of GO functionalized with TiO2 and ZnO had an almost similar trend as the GO blended membranes. An increasing of nanopowder amount from 0.5 to 2.0%wt. caused a little decreasing of water contact angle of the composite membranes. Additionally, the ZnO nanorods can have a needle-spike structure and this structure decrease water contact angle.
The increased hydrophilicity together with the bigger pore and higher porosity enabled the GO and GO-ZnO nanopowder blended membranes to have a good pure water permeation flux compared with the other GO-TiO2 nanopowders blended membranes.
Morphology of the GO and functionalized GO blended PES membranes
|Membrane sample .||Rq (nm) .||Ra (nm) .||Rz (nm) .|
|Membrane sample .||Rq (nm) .||Ra (nm) .||Rz (nm) .|
Antimicrobial performance of neat PES and PES/GO, PES/GO–TiO2 and PES/GO–ZnO blended membranes
Self-cleaning properties of PES/GO–TiO2 and PES/GO–ZnO nanopowder blended membranes
The PES membranes blended with GO and with GO functionalized with photocatalytic semiconductor catalyst (TiO2 and ZnO) were prepared by direct addition of the nanopowders in the casting solution. The effects of blended GO and functionalized GO composite membranes on the morphology were examined by water hydraulic permeability, antibacterial resistance and self-cleaning properties of the functionalized membrane surface. The results showed that the carboxylic acid, hydroxyl, and other functional groups formed on the graphene supplied hydrophilicity and increased water flux when compared with neat membrane. Moreover, GO functionalized with ZnO composite membrane brought more hydrophilicity to the PES membrane. But the water flux of GO membrane functionalized with TiO2 decreased due to blocking of the membrane pores by round-shape TiO2 nanoparticles. A significant improvement was observed in biofouling prevention in the GO and functionalized GO composite membrane. The cross-section SEM images showed that all of the synthesized membranes had finger-like structure. The self-cleaning properties of the functionalized GO with TiO2 and ZnO semiconductor catalyst were also investigated. The results indicated that methylene blue dye was successfully degraded by TiO2 and ZnO when exposed to UV light on the membrane surface. Semiconductor catalysts protected the membrane surface from binding dye molecules into the polymer chain. It was concluded from this study that the GO is excellent antifouling material. Moreover, functionalized GO with ZnO nanopowders showed unique self-cleaning properties, which is promising for new applications for this type of membrane.
This research was supported by the Department of Scientific Research Projects of Mersin University (project number: 2017-1-TP2-2010).