ULTRATHIN GRAPHENE OXIDE COMPOSITE

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ULTRATHIN GRAPHENE OXIDE COMPOSITE MEMBRANE for WATER PURIFICATION Febri Baskoroab, Shingjiang Jessie Luea a Department

of Chemical and Materials Engineering, Chang Gung University, Kwei-shan, Taoyuan 333, Taiwan, ROC b Directorate General of Higher Education, Ministry of Education and Culture of Indonesia, Jakarta 10270, Indonesia ABSTRACT In this work graphene oxide (GO) was used as an ultrathin separation layer in thin film composite water selective membranes for water purification. The synthesized ultrathin GO separation layer was placed on the top surface of a porous polyvinylidene fluoride-co-polyacrylic acid (PVDF-PAA) intermediate layer, formed onto a non-woven support layer to improve the three-layer composite membrane mechanical properties. The resulting membrane was investigated using a cross-flow Nano filtration (NF) system to examine the water permeability and salt rejection efficiency. The operating parameter effects on the water purification performance were studied and the correlation between membrane structure and performance was elucidated. KEYWORDS Graphene Oxide, Water Purification, Nano filtration, Thin-film Composite Membranes 1. Introduction Nano filtration (NF) has recently become widely used in water treatment and desalination process applications. The crucial aspect of this technique relies on a highly selective top layer semipermeable NF membrane. It is well known that polymeric membranes have lower cost and easier processing compared to inorganic membranes. Among polymeric membrane materials, PVDF exhibits more advantages such as good crystalline properties, better thermal stability, peculiar antioxidant activity, good membrane forming properties and chemical resistance [1, 2]. PAA is one of the common polyanionic macro-molecules usually blended with polymers to increase membrane hydrophilic characteristics [3-5]. Graphene oxide (GO) is a two dimensional material derived from graphite that contains oxygen groups. Moreover, GO has been reported to have excellent water purification properties, including hydrophilic characteristics, flexibility, and mechanical properties [6, 7]. This research fabricated three-layer composite membrane nonwoven/PVDF-PAA/GO and evaluated its performance in an NF system for aqueous solutions containing monovalent salts (NaCl and Na2SO4).

2. Experimental and Methods 2.1 Materials Graphite powder, poly(vinylidene fluoride) (PVDF) Mw ~ 534,000 powder, poly(acrylic acid) (PAA), sodium chloride (NaCl) powder were purchased from Sigma-Aldrich, St. Louis, Missouri, USA. Potassium permanganate (KMnO4) powder was purchased from Nihon Shiyaku Industries .Ltd, Osaka, Japan. Sodium sulphate (Na2SO4) was purchased from Island Long Pharmaceutical Co. Ltd., Osaka, Japan. Sulfuric Acid (H2SO4) (95-98%) solution was purchased from Scharlab S.L., Barcelona, Spain. NMethyl-2-pyrrolidinone (Diquat) solution (NMP) was purchased from Mallinckrodt Baker Inc., Philipsburg, Pennsylvania, USA. Nonwoven grade 3706 made from Polyethylene terephtalate (PET) was purchased from Ahlstrom filtration Co., Helsinki, Finland. 2.2 Graphene oxide preparation GO was synthesized using a modified Hummer’s method [8]. An appropriate quantity of 2 g graphite flakes were dispersed in 98% H2SO4 (300 mL) at room temperature using a mechanical stirrer (200 rpm). After 10 min of stirring, 1 wt equiv of KMnO4 (2 g) was added. Additional portions of KMnO4 were added when the green MnO3- color diminished. A total of 5 wt equiv of KMnO4 was sequentially added. After the reaction completed, 400 g of ice was added into the mixture. This process was conducted in an ice bath, resulting in the mixture exhibiting a purple color. The mixture was left to stand overnight. The purple solution was then separated from the precipitate and 100 ml of DI water was added into the precipitate. The precipitate was then washed until the solution pH became neutral. The precipitate was dried at 60 °C under vacuum and suspended in DI water to form 200 ppm mixture. 2.3 Preparation of nonwoven/PVDF-PAA/GO Nonwoven was used as the supporting membrane for PVDF-PAA composite to increase the mechanical properties of the resulting membranes. A PVDF-PAA intermediate layer was prepared by dissolving PVDF and PAA into NMP solvent (6 g: 0.12 g: 24 g) at 60 °C for 12 hours. After 12 hours mixing the mixture was left to stand for 12 hours to release any bubbles in the mixture. The PVDF-PAA was cast onto the nonwoven using a casting knife with a clearance of 200 µm and 15 mm/sec as the casting speed. The cast film and the nonwoven was immersed in DI water to remove the solvent and dried at 60 °C. Ten milliliters of GO 200 ppm solution were deposited into the Nonwoven/PVDF-PAA membranes using vacuum filtration, with 2 ml of Isopropyl alcohol as the wetting agent. The resulting membranes were dried in a 50°C oven overnight before use for filtration performance testing. Water permeation and salt rejection were measured after the first drop of permeate solution and analyzed continuously after each 20 minute test run. The concentration and volume of the feed solutions were 2000 ppm and 1L, repectively. The permeation was calculated using Eq. (1) while the rejection performance was calculated using Eq (2).

𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 =

𝑚 𝐴.ℎ.𝑃

(1); 𝑅 (%) = (1 −

𝐶2 𝐶1

) × 100 % (2)

Where, Permeate is the water permeation through the filtration membrane (kg/m2hMpa), m is the permeate solution weight (kg), A is the filtration area (4.9 x 104 m2), h is the run time (0.33 h), P is pressure condition (0.49 MPa), R is the rejection (%), C1 is the feed solution concentration, C2 is the permeate concentration. 3. Result and Discussion As shown in Figure 1 the GO morphology in this experiment was seen as sheets with a wavy wrinkled shape. The GO size in this experiment was at micrometer scale around ~6 µm. This corresponds to a previous study that reported that wrinkled GO sheets appeared in different lateral sizes ranging from hundreds to thousands of nanometers [9]. Figure 2 describes the C1s core level XPS spectra of GO. It clearly shows four binding energy peaks characteristic of GO binding energy at 285, 287.1, 287.9 and 289 eV. Those four binding energy peaks are attributed to C-C/C=C (43%), C-O (45%), C=O (7%) and O=C-O (5%), respectively. The XPS spectra result also corresponds to previous studies [10-14]. Based on the C1s core level the XPS spectra of GO in this experiment, the total oxygen-containing groups was 57 %, and this number also supported that the GO oxidation degree in this work was higher compared to that in previous studies, which were 56.5 %[14], 51.8 % [12], 47.6 % [13] and 44.04 % [10]. The higher oxidation leads to higher GO hydrophilicity because it contains more oxygen-containing groups that can attract more water. Figure 3(a) shows the surface morphology of nonwoven/PVDF-PAA. It is clear that the PVDF-PAA composite is a porous membrane with an average pore size ~100 nm. Figure 3(b) shows a cross section image of the nonwoven/PVDF-PAA membrane. As shown in Figure 3(b) the morphology inside the composite is a finger like structure with small pores inside of its structure. Figure 3(b) clearly shows that the composite layer thickness is around 50 µm. After GO deposition onto the nonwoven/PVDF-PAA membrane surface, the composite membrane morphology changed significantly. As shown in Figure 3(c) the coated membrane morphology was like all membranes already covered by GO. This is confirmed by the wrinkled structure clearly show in Figure 3(c). It corresponds to the GO size in Figure 1. The resulting GO deposition rate on the composite membrane was 0.97 ± 0.013 mg/cm2. According to Figure 3(d) the GO layer thickness is in the nanometer scale around 378 nm.

Figure 1. FESEM image of GO

Figure 2. C1s core XPS spectra of GO.

Figure 3. FESEM image of the resulting membrane: (a) and (b) show nonwoven/PVDF-PAA before GO deposition with the surface and cross-section, respectively. While (c) and (d) show nonwoven/PVDF-PAA after GO deposition with the surface and cross-section, respectively.

According to Figure 4(a) it is clear that the pure water permeate of the resulting membrane is around 9.1 ± 2.3 kg/m2hMPa. Figure 4(a) shows that Na2SO4 and NaCl permeation were higher than that of pure water. This phenomenon is due to the GO layer hydration effect. Joshi et al. [15] reported that when GO membranes are soaked into an electrolyte solution the d-spacing of the interlayer GO sheets increase to ~0.9 nm, permitting higher electrolyte solutions permeation than that for pure water. Based on Figure 4(a) Na2SO4 has higher permeation than NaCl. This may due to the ionic strength of SO42- being higher, thereby enlarging the interlayer GO sheet d-spacing resulting is higher permeation compared to NaCl. As shown in Figure 4(b) the rejection of Na2SO4 was significantly higher than that for NaCl. This phenomenon can be explained by the Donnan exclusion theory. GO is negatively charged in a wide pH range due to it containing carboxylic and hydroxyl groups. It therefore tends to extrude co-ions, such as SO42− and Cl1− in order to maintain solution electro neutrality on each side of the GO composite membrane [16].

Figure 4. Membrane performance for NaCl and Na2SO4: (a) permeate comparison and (b) rejection comparison. 4. Conclusion Higher GO oxidation was synthetized in this research using a modified Hummer’s method. The FESEM image confirmed that a GO ultrathin separation layer was successfully deposited onto nonwoven/PVDF-PAA membrane. The performance of nonwoven/PVDF-PAA/GO for monovalent salts indicates a potential membrane for water purification with the rejection of Na2SO4 at almost 80%. 5. References [1] Z. Wang, H. Yu, J. Xia, F. Zhang, F. Li, Y. Xia, Y. Li, Novel GO-blended PVDF ultrafiltration membranes, Desalination, 299 (2012) 50-54. [2] F. Liu, N.A. Hashim, Y. Liu, M.R.M. Abed, K. Li, Progress in the production and modification of PVDF membranes, Journal of Membrane Science, 375 (2011) 127.

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