Preparation,Modification And Performance Study Of Graphene Nanofiltration Membrane

Graphene oxide (GO), an oxidized derivative of graphene, maintains most of the extraordinary properties of graphene. GO is easy to fabricate in large-scale and can be assembled into membranes simply by vacuum filtration or spin coating methods. The GO membrane (GM) contains abundant two dimensional (2-D) carbon nano-channels network which has promising application in gas and liquid separation. Most of the researchings focusing on the separation application of GMs are about gas separation or microfiltration, but the application in nanofiltration (NF) has not been reported yet. As a result, this dissertation proposes the concept of ultrathin graphene nanofiltration membrane (uGNM) and systematically investigates the influencing factors for the NF performance of uGNMs. The mass transfer and separating mechanisms of uGNMs are also discussed. Based on these mechanisms some structure modifications for the uGNMs are carried out to further improve the NF performance. In order to expand the application scope of uGNMs, the preparation route for positively charged uGNMs is also explored. Additionally, the anti-fouling performance and fouling mechanisms of uGNMs are systematically studied and accordingly the anti-fouling property of uGNM is further improved by some membrane modification.(1) Single-layer GO solution is prepared by modified Hummers method. The GO dispersion is properly reduced by refluxing under base or neutral condition. Applying this kind of lightly reduced GO dispersion, a series of uGNMs are fabricated on porous support with the thickness of graphene layer ranging from 20-50 nm. It is found that the reduction degree of GO, the loading of graphene and the type and pore radius of the porous support are the most important influencing factors for the uGNMs NF performance. The rGOm, which is fabricated by assembling the rGO (refluxing GO dispersion in neutral condition for 1 hour) on poly (vinylidene fluoride) (PVDF) ultrafiltration membrane with the pore size around 50 nm, shows the best NF performance. The rGOm shows a pure water flux of 4.76 L m-2 h-1 bar-1. The rejection ratios are 95% for Na2SO4 (the highest value among all the published works) and 98% for Methyl Orange. It is also proved that the water molecules show ultrafast permeation in the 2-D carbon nano-channels in uGNMs. The separating mechanism of uGNMs for charged solutes is the combination of both the size sieving effect and Donnan exclusion effect.(2) In order to further increase the water flux of uGNMs, the multiwalled carbon nanotubes (MWNTs) intercalated graphene NF membrane (G-CNTm) is fabricated by synergistic assembling of graphene and MWNTs. The SEM, TEM and AFM characterizations demonstrate that the MWNTs are uniformly dispersed in the 2-D carbon nano-channels and expand the interlayer space between neighbored graphene sheets. The NF performance of G-CNTm can be easily adjusted by the MWNTs loadings. The G-CNT(100)m with the mass ratio (rGO to MWNTs) of 2 shows a pure water flux up to 11.3 L m-2 h-1 bar-1, more than twice of the rGOm, while keeping high dye rejection (96.1% for Methyl Orange and 99.8% for Direct Yellow). MWNTs act as the space holder between the graphene layers and maintain high water flux under high pressure and high ion strength.(3) Positively charged uGNMs are prepared to expand the application scope of uGNMs. Catalyzed by 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC), the carboxylic groups on the rGO are modified by amine and quaternary ammonium reagents to obtain positively charged uGNMs. The Zeta potential tests showed that, after modification the positively charged uGNMs exhibite isoelectric points at approximately pH 4, indicating the alteration of the surface charge. Additionally, the XRD characterization demonstrates that EDC also catalyzes the cross-linking reaction between rGO sheets leading to smaller interlayer space between graphene layers in water. As a result, the positively charged uGNMs show much higher rejection ratios for MgCl2 compared with unmodified samples and the rejection sequence for different salts is R (Na2SO4)> R (MgSO4)> R (MgCl2)> R (NaCl).(4) The antifouling performance and fouling mechanism of uGNM are studied with bovine serum albumin (BSA), sodium alginate (SA) and humic acid (HA). Due to the hydrophilic and smooth surface of rGO sheets, the uGNM shows excellent anti-fouling performance for SA and HA but inferior performance for BSA because of the strong interaction between protein and GO sheets. Accordingly, cyclodextrin (CD) is applied to modify the uGNMs, and the resulting rGO/CD membranes show better antifouling performance for BSA. EDC reaction is applied to further modify rGO/CD membrane by reducing the electrostatic attraction between protein and rGO sheets, which is named as rGO-c-β-CD membrane. The rGO-c-β-CD membrane shows a high flux recover ratio (FRR) of 97.71%, and the total fouling ratio (Rt) is only 20.73%. Additionally, the p-rGO membranes prepared by the pressurized ultrafiltration method also show good antifouling property (Rt=16.41%) because the pressurized ultrafiltration method leads to smoother membrane surface with less defects.For the first time, graphene NF membranes are proposed and prepared in this dissertation. The graphene NF membranes are facile to fabricate and modified, and exhibit high NF performance and good anti-fouling property. It is believed that after more dedicated design and modification, graphene NF membranes are very likely to become the next generation NF membranes and have great potential application in water treatment field.