Fabrication and characterization of a polysulfone-graphene oxide nanocomposite membrane for arsenate rejection from water
© Rezaee et al. 2015
Received: 8 February 2015
Accepted: 9 August 2015
Published: 22 August 2015
Nowadays, study and application of modified membranes for water treatment have been considered significantly. The aim of this study was to prepare and characterize a polysulfone (PSF)/graphene oxide (GO) nanocomposite membrane and to evaluate for arsenate rejection from water.
Materials and methods
The nanocomposite PSF/GO membrane was fabricated using wet phase inversion method. The effect of GO on the synthesized membrane morphology and hydrophilicity was studied by using FE-SEM, AFM, contact angle, zeta potential, porosity and pore size tests. The membrane performance was also evaluated in terms of pure water flux and arsenate rejection.
ATR-FTIR confirmed the presence of hydrophilic functional groups on the surface of the prepared GO. FE-SEM micrographs showed that with increasing GO content in the casting solution, the sub-layer structure was enhanced and the drop like voids in the pure PSF membrane changed to macrovoids in PSF/GO membrane along with increase in porosity. AFM images indicated lower roughness of modified membrane compared to pure PSF membrane. Furthermore, contact angle measurement and permeation experiment showed that by increasing GO up to 1 wt%, membrane hydrophilicity and pure water flux were increased. For PSF/GO-1, pure water flux was calculated about 50 L/m2h at 4 bar. The maximum rejection was obtained by PSF/GO-2 about 83.65 % at 4 bar. Moreover, it was revealed that arsenate rejection depended on solution pH values. It was showed that with increasing pH, the rejection increased.
This study showed that application of GO as an additive to PSF casting solution could enhance the membrane hydrophilicity, porosity, flux and arsenate rejection.
In recent years, a growing public concern has arisen over release of toxic pollutants such as inorganic ions, metals and synthetic organic matters into the water due to increasingly industrial and agricultural activities. Among these toxicants, arsenic is a serious threat in water resources of some regions [1, 2]. Toxicological and epidemiological studies proven that inorganic arsenic could cause carcinogenic and non-carcinogenic effects in human . World health organization (WHO) and united state protection agency (USEPA) had classified arsenic as class A carcinogens list . International agency for research on cancer (IARC) also classified inorganic form of arsenic in class I carcinogens list . With regard to strict regulations for control and removal of arsenic in drinking water, and limitations of conventional water treatment processes (e.g. generation of toxic intermediates and low efficiencies) looking for new technologies is of great interest [3, 6]. Membrane process can be considered as a promising technology for arsenic removal due to its several advantages such as no need to add chemicals, no generation of sludge, ease of system capacity development, separation in continuous mode, ease of integration with other processes, minimum dependency to environmental conditions and capable of microorganisms and solutes removal [7, 8]. However, certain drawbacks associated with common membranes are low water recovery, fouling problem and high-energy consumption [9, 10]. In recent years to overcome these drawbacks, different studies have been conducted in order to modify polymeric membranes to enhance the permeability, rejection and decreasing fouling problem and reduce the investment and operational costs . Accordingly, various works such as physical blending, plasma treatment, polymer grafting and chemical reactions have been carried out to modify the membranes [12, 13]. Among these methods physical blending is preferred due to the simplicity procedure using phase inversion technique . Physical blending consist of mixing of polymeric materials with inorganic nanoparticles (e.g. TiO2 , ZnO , silica ) and recently carbon allotropes [11, 18, 19]. Adding inorganic nanoparticles to membrane matrix can enhance the membrane hydrophilicity, strength, permeability and antifouling characteristics [18, 20]. Graphene and its derivatives due to unique two-dimensional structure, one-atom-layer-thick, high theoretical surface area (2630 m2/g), good mechanical properties, non-harmful effects, low cost production have attracted interest for different application especially polymeric membrane modification [21, 22]. Graphene oxide (GO) is also highly hydrophilic due to presence of oxygen containing functional groups (e.g. hydroxyl, carboxyl, carbonyl and epoxy) [12, 23]. When thin sheets of carbon atoms (GO) are added to a polymer matrix at low content and proper procedure, it could significantly improve the physical properties of the base polymer [21, 24]. Among different synthetic polymers, polysulfone (PSF) is the one that is widely used for various membranes fabrication such as filtration, ultrafiltration, hemodialysis and bioreactor technologies [13, 25]. The reasons for wide use of this type of polymer are good characteristics such as desire mechanical and thermal properties, high chemical stability, high resistance in a wide range of pH and high solubility in a broad range of polar solvents (dimethylformamide, dimethylacetamide, dimethylsulfoxide) [13, 21, 25]. One of the main drawbacks of PSF membrane is fouling problem and consequently reduction of the membrane lifetime. Actually, this type of membrane is influenced by fouling problem more than other membrane materials because of the hydrophobic nature of the membrane and interactions between the membrane surface charges and the foulants [13, 21]. A few studies have used GO in casting solution to improve the water permeability, antifouling properties and mechanical strength characteristics of the mixed matrix membrane. Zhao et,al showed that synthesized PVDF/GO ultrafiltration membrane had higher pure water flux compared to PVDF due to improvement of the surface hydrophilicity . Wang et, al also reported that GO nanosheet as a hydrophilic modifier could enhance the water flux of the fabricated ultrafiltration membrane with an improvement in the antifouling property . In another study, Zinadini et.al showed that water permeability, hydrophilicity and antifouling properties of the PES/GO membrane were enhanced compared to pure PES membrane . Xia et, al also revealed that employment of a certain amount of GO in the matrix could improve the water flux, hydrophilicity and antifouling characteristics of a type of synthesized PVDF/GO membrane used for natural organic matter removal . The aim of this study is to synthesis and characterizes a PSF/GO nanocomposite membrane in order to reject arsenic from water. In this work, GO was applied to PSF matrix as a hydrophilic agent. The performance of the synthesized membranes was evaluated by pure water flux measurement and arsenate rejection.
Materials and methods
All chemicals used in the experiments were of reagent grade. Graphite fine powder extra pure (with a mean particle size of <50 μm) was purchased from Merck- Germany. PSF (with average Mw = 22,000 g/mol) was obtained from Sigma-Aldrich Co-Germany. N,N-Dimethylformamide >(DMF) was purchased from Sigma-Aldrich and used without purification as a solvent to prepare cast solution. Analytical grade H2SO4 (98 %-Merck), NaNO3 (99 %, Sigma–Aldrich), KMnO4 (99 %, Sigma–Aldrich) and H2O2 (30 % solution, stabilized-Merck) were used as received. Sodium arsenate dibasic heptahydrate (Na2HAsO4-7H2O) was obtained from Sigma-Aldrich. The deionized (DI) water was used in the sample preparation and for pure water flux measurements.
Preparation and characterization of graphene oxide (GO)
In this study GO was prepared using modified Hummer’s method . Firstly, 5 g graphite powder and 2.5 g sodium nitrate were added to a 500-ml neck flask containing 120 ml concentrated sulfuric acid in ice bath and thoroughly mixed for 30 min. Then under vigorous mixing, 15 g KMnO4 was slowly added to the suspension and mixing was continued for 30 min. The rate of adding was controlled to maintain temperature of the reaction below 20 °C. After that, ice bath was removed and the mixture was stirred overnight at room temperature. By elapsing the time, the mixture changed in to sticky and the color changed to brown. Then under mixing condition, 150 ml distilled water was slowly added to the mixture. The temperature was rapidly increased to 98 °C and the color turned to yellow. This aqueous suspension was stirred at 98 °C for 24 h. In order to remove KMnO4, 50 ml H2O2 (30 %) was added to the liquid mixture. For more purification, the liquid mixture of GO was washed by HCL (5 %) and DI water and centrifuged for several times to reach the pH to natural range. Finally, for exfoliating the product, sonication was conducted for 1 h. Then it was filtered and dried in a vacuum oven (at 40 °C for 24 h) to obtain a grey color GO nanoplate powder. Raman spectra of the GO was obtained in the spectral range of 100-4200 cm-1 and with 532 nm wavelength incident laser light (Almega Thermo Nicolet Dispersive Raman Spectrometer, Germany). The measurements of the attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) of the GO was performed using a ATR-FTIR spectroscopy in the range between 600 cm − 1 and 4000 cm−1 (Tensor 27, Bruker Inc., Germany).
Fabrication of PSF/GO nanocomposite membrane
In present work, PSF/GO nanocomposite membrane was fabricated via common phase inversion method [14, 29]. For this purpose, PSF was used as bulk material, DMF as solvent, GO nanoplate as the additive and hydrophilic modifier, DMF as the solvent and DI water as the nonsolvent in coagulation bath. The casting solution consist of PSF = 15 %wt, DMF = 85 wt% and GO = (0-0.5-1-2 wt% PSF). PSF and GO powder were dried in vacuum oven at 60 °C for 4 h. At first, four different amounts of GO were dispersed in DMF and was sonicated for 1 h to obtain a homogenous casting solution. Then, under continuous stirring condition, PSF was added to GO/DMF mixture and was allowed to stir for 24 h. Then the casting solution was maintained in room temperature for 24 h without stirring. Finally, casting solution was sonicated to remove remaining air bubbles. The prepared casting solution was casted uniformly onto a smooth and clean glass plate using a casting knife at a thickness of 200 μm. The casted film on the glass was left for air exposure (20 s) followed by immersing into the nonsolvent coagulation bath (DI water at 25 °C). The glass plate was kept in the coagulation bath for 10 min to guarantee complete phase inversion process. Finally the peeled off synthesized membrane was washed with DI water for several times until all the residual solvent removed. The membranes were kept in DI water for characterization and experiments. The synthesized membrane based on GO content named pure PSF, PSF/GO-0.5, PSF/GO-1 and PSF/GO-2.
Characterization of the prepared membranes
The structure and surface morphology of the membranes were evaluated using a field-emission scanning electron microscope (FE-SEM, S-4160, Hitachi, Japan). For sample preparation, membrane were cut into small pieces and washed with distilled water. For obtaining a good cross section image, the wet pieces were immersed in liquid nitrogen for 1 min to freeze. The frozen pieces of the membranes were fractured and kept in air to dry. The dried samples were coated with a thin layer of gold to increase the electric conductivity before FE-SEM imaging. Atomic force microscopy (AFM) was applied for top surface morphology and roughness analysis. Thermo microscopes Auto probe CP Research (Veeco Instruments, Sunnyvale, CA, USA) was used for AFM analysis. The samples were cut into small pieces (1 cm × 1 cm), washed with distilled water and dried in room temperature. In this study, surface hydrophilicity changes of different fabricated membranes were determined via the contact angle and Zeta potential. The contact angle was analyzed using a water contact angle measurement (OCA 15 Plus, Dataphsycs, Germany). Before contact angle measurement, the samples were dried in oven at 50 °C for 4 h. For more accuracy in the determination of contact angle, 5 different top surface points were measured and the average was reported. The zeta potentials of fabricated membrane were measured by streaming potential method using Electro kinetic Analyzer (EKA, Anton Paar GmbH, Austria) equipped with plated sample cell. For this purpose membrane were cut in 5 cm × 5 cm pieces and zeta potentials were measured at 26 °C and pH of 7. In this measurement method, 0.001 M KCl solution was applied as electrolyte and zeta potential were measured in triplicate for each membrane.
Membrane porosity and pore size
Permeation tests and arsenate rejection experiments
Where J w is the pure water flux (L/m2h), V is the volume of permeated pure water (L), A is the effective area of membrane (m2) and Δt is the sampling time (h).
Where R is the rejection of arsenate (%), and Cp and Cf are the concentrations of arsenate in the permeation and feed solution, respectively (μg/L). All pure water flux and rejection experiments were performed in triplicate.
Results and discussion
Characterization of graphene oxide
Characterization of the PSF/GO membrane
Effect of GO addition on membrane morphology
Surface roughness parameters of the prepared membranes obtained from analyzing six randomly chosen surface AFM images
Mean surface roughness (Ra-nm)
Root mean square roughness (Rq-nm)
2.9 ± 0.23
3.9 ± 0.47
2 ± 0.14
2.5 ± 0.15
2.5 ± 0.30
3.4 ± 0.36
4.4 ± 0.32
5.8 ± 0.50
Membrane hydrophobicity, Water permeation flux and pore structure parameters
Effect of GO content on water contact angle, pure water flux and pore structure parameters of the prepared membranes
Contact angle (deg)
Mean pore radius (nm)
Pure water flux (L/m2h)
73.5 ± 2.1
48.3 ± 2.6
6.9 ± 0.56
19.7 ± 3.2
66.7 ± 1.6
77.9 ± 2.2
8.3 ± 0.31
32.3 ± 3.5
51.3 ± 1.2
86.5 ± 1.8
9.1 ± 0.63
49.9 ± 2.6
54.8 ± 1.4
82.1 ± 1.3
8.7 ± 0.42
46.4 ± 2.0
Arsenate rejection performance evaluation
In present study, GO nanoplate were directly added to PSF casting solution to fabricate a mixed matrix membrane via phase inversion method. The results showed that presence of abundant containing hydrophilic functional groups on GO, strongly enhance the hydrophilicity and permeability of the synthesized membrane. Graphene oxide also could modify the morphology of the membrane so that the spongy structure and closed-end drop like pores of the pure PSF membrane could change to finger like pores and larger open-end channels in PSF/GO membrane. Adding GO up to 1 wt% in casting solution resulted in enhancement of membrane morphology so that the contact angle reduced and the porosity and pure water flux increased due to the improvement of the membrane surface hydrophilicity. The results also showed that the rejection of arsenic in the PSF/GO membranes has substantially increased compared to pure PSF membrane. In addition, with increase in GO weight in the casting solution the rejection of arsenate ions increased. The experiments also showed that the predominant mechanism of arsenate rejection is, Donnan repulsion due to the negative charges induced by GO on the membrane surface. The results of this study revealed that due to unique properties of GO especially hydrophilicity, it can be considered as a promising nanomaterial for membrane fabrication and modification.
This research was part of a PhD dissertation of the first author and has been financially supported by a grant (NO, 22716-46-02-92) from Center for Water Quality Research, Institute for Environmental Research, Tehran University of Medical Sciences, Tehran, Iran. The authors would like to express their thanks to the Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical Sciences for their collaboration.
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