Elsevier

Desalination

Volume 313, 15 March 2013, Pages 199-207
Desalination

Enhanced hydrophilicity and salt rejection study of graphene oxide-polysulfone mixed matrix membrane

https://doi.org/10.1016/j.desal.2012.11.037Get rights and content

Abstract

Graphene oxide (GO) dispersed polysulfone (PSf) mixed matrix membranes were prepared by wet phase inversion method. The morphology of membranes was studied using scanning electron microscope (SEM) images. The variation in hydrophilicity was studied by measuring surface wettability and water swelling experiments. The performance of membranes in terms of pure water flux and salt rejection was studied. SEM images depict enhanced macrovoids, while the contact angle data reveals that, GO incorporated membrane surface is moderately hydrophilic. Membranes exhibited improved salt rejection after GO doping. Membrane with 2000 ppm GO loading has exhibited maximum of 72% Na2SO4 rejection at 4 bar applied pressure. The salt rejection seems to depend on pH of the feed solution and it has been witnessed that the salt rejection showed an increasing trend with increase in the pH.

Graphical abstract

Graphene oxide (GO) dispersed polysulfone (PSf) mixed matrix membranes were prepared by the wet phase inversion method. Membranes exhibited improved salt rejection after GO doping. Membrane with 2000 ppm GO loading has exhibited a maximum of 72% Na2SO4 rejection.

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Highlights

► Graphene oxide dispersed polysulfone mixed matrix membranes were prepared. ► Membranes were characterized. ► Membranes exhibited improved salt rejection after graphene oxide doping. ► Membranes showed increased hydrophilicity after doping.

Introduction

In recent times, considerable efforts have been put towards obtaining fresh water from abundantly available saline sea water. In this direction the pressure driven membrane filtration has gained significant ground [1]. This is majorly due to two factors, one is because of its ease of operation and more importantly these filtration processes are cost effective compared to all other conventional filtration techniques. For membrane filtration, polysulfone (PSf) based membranes are used most extensively because of its excellent heat resistance, chemical compatibility and resistance over wide range of pH [2]. The major drawback of these PSf membranes is that, they are hydrophobic in nature, whereas the desalination membranes demand appreciable hydrophilicity in order to cope up with productivity issues. Also in general membranes suffer from fouling and hence lead to low membrane life time. The low permeability and fouling are mainly due to low surface hydrophilicity and low porosity. So, all over the globe, desalination researchers are looking for a membrane with finely-balanced characteristics.

The most widely employed method for increasing the surface hydrophilicity is to blend the PSf with some surface modifier. Mahesh et al. showed technique of increasing hydrophilicity by blending PSf with hydrolyzed poly isobutylene-alt-maleic anhydride and witnessed enhanced performance for the same [3]. Enhanced hydrophilicity as well as improved salt rejection was achieved through PSf blend membrane with modified poly isobutylene-alt-maleic anhydride [4]. In this direction, Vatanpour et al. and Evrim et al. were successful in increasing hydrophilicity and antifouling property of the membrane by preparing polyethersulfone (PES) composite membrane with functionalized multi walled carbon nanotubes (MWCNT) [5], [6]. Choi et al. prepared MWCNT mixed matrix membrane and witnessed enhanced hydrophilicity as well as the dependence of pore size on the content of MWCNT [7]. Also chitosan modified carbon nanotubes (CNTs) were used as a performance modifier in mixed matrix polyimide composite membranes [8]. From these works it can be understood that, further modification of existing membranes is required in order to improve their performance. Keeping this in view, in the present work graphene oxide (GO) is being utilized as a performance modifier for the preparation of PSf based mixed matrix membrane.

GO is gaining much more interest in the field of material research due to its high surface area, outstanding electron transport and mechanical properties. These atomically thin carbon sheets when incorporated appropriately into polymer matrix, can significantly improve physical properties of the host polymers at extremely low dope concentration. The objective behind choosing GO as an additive lies in its hydrophilic and pH sensitive behavior. Also it has been studied and proved that GO can induce surface negative charge. It is also demonstrated that GO exhibits this property of negative surface charge throughout the entire pH range [9]. The polymer composite of GO is found to enhance the mechanical strength of the host polymer. It has also been studied that, incorporation of GO or graphene into polymer mixed matrix membranes, can appreciably increase the rejection of gas like nitrogen and oxygen [10], [11], [12], [13]. Due to different types of hydrophilic functional groups present on the surface of GO, it can take up water very easily and it has also been well understood that the water uptake increases as the degree of oxidation increases [14]. Keeping these characteristic features of GO in view, present investigation has been designed.

Section snippets

Materials

Natural graphite was procured from Graphite India. PSf with Mw-35000 was obtained from Sigma-Aldrich Co Germany. N-methyl-2-pyrrolidone (NMP) was obtained from Merck India and was used without purification. Analytical grade H2SO4 (purity 98%), KMnO4 (purity 99%) and H2O2 (30% aqueous solution) were used as received.

Preparation of GO

There are many methods in practice for the preparation of GO [15], [16], [17], [18]. Initially it has been prepared by oxidation of natural graphite using NaClO3 as an oxidizing

Characterization of GO

In order to understand the structure of GO (Fig. 1S) (see supporting information for representative chemical structure of GO), it has been characterized by infrared spectrum, solid state 13C NMR, FESEM, TEM, XRD and TGA. Fig. 1 displays the stacked IR spectra of graphite and GO. IR spectrum of GO demonstrates the presence of many functionalities on the graphite sheet after oxidation. IR assignments are tabulated in Table 1S (see supporting information). Here one can observe the broad IR band

Conclusions

Graphite was oxidized to graphite oxide (GO) using KMnO4 in order to introduce hydrophilic functional groups onto its surface. Thus prepared GO was impregnated into polysulfone (PSf) matrix to prepare PSf/GO mixed matrix membranes. The GO doping into polymer matrix has resulted in enhanced hydrophilicity, water flux, and salt rejection property of the membrane. It has been observed that GO plays major role in modification of membrane morphology. As it is evident from SEM cross sectional images

Acknowledgment

AMI thanks the Department of Atomic Energy, Board for Research in Nuclear Sciences—Government of India for the financial support and ‘Young Scientist’ award. AMI also thanks, Vision Group on Science & Technology, Government of Karnataka, India for the ‘Best research paper award’.

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