Synthesis and physicochemical properties of reduced graphene oxide membrane for humic acid rejection

Abstract

Polyethersulfone (PES) has received tremendous attention in water treatment because it has excellent membrane-forming capability. However, PES is naturally hydrophobic which tends to increase the potential of fouling. Therefore, in this study, a modification was carried out by adding reduced graphene oxide (rGO) into the polymer matrix to improve the hydrophilicity of the membrane, thus reducing the fouling problem. The effect of reduction time on the properties of rGO was investigated. Graphene oxide (GO) was prepared via modified Hummer’s method and it was reduced using 0.5 M sodium borohydride (NaBH4). The effects of different reduction times (8, 16, 24 and 32 hours) were studied. The prepared rGO was then added into PES to produce membrane for ultrafiltration process. Polyvinylpyrrolidone (PVP) was added into the polymer matrix to study the effect of the material on the pores formation. The membrane was prepared using phase inversion method. X-ray diffraction (XRD), attenuated total reflectance Fourier-transform infrared spectroscopy (ATR FTIR), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDX) were used to characterize the material. It was found that the interlayer spacing of GO was reduced from 7.87 to 3.68 Å after reduction process due to the removal of some of the functional groups from the material. The FTIR peak at 1718 cm-1, attributed to the carbonyl (C=O) group, was absent after GO was reduced. The morphology of GO showed that the surface was smooth and had wrinkles while rGO was rough and agglomerated. EDX results suggested that the percentage of oxygen was lower due to effective reduction process. All the membranes were characterized using scanning electron microscopy (SEM), water contact angle, water flux and humic acid (HA) rejection. Based on the SEM of membrane cross section, it was found that the addition of PVP greatly improved the formation of pores. When rGO was added, the finger-like porous sub-layer increased. Besides that, the hydrophilicity showed a significant improvement after rGO was introduced into the polymer matrix. PES/rGO 24 hour showed the greatest contact angle (32.99o). Meanwhile, PES/GO showed the highest water flux (174.29 L/m2h). Nevertheless, in terms of HA rejection, PES/rGO 24 hour showed the best performance where the rejection reached up to 46.88% as compared to that of PES/GO which was only 24.68%.