Desalination Performance Of Lameller Graphene Oxide Reverse Osmosis Membranes

Graphene-based nanomaterials are widely studied due to its large specific surface area, excellent electrical, thermal, optical and mechanical properties, especially the application in the field of water treatment. Graphene oxide(GO)as the derivative of graphene, containing a lot of oxygen functional groups, its unique structure shows good potential application in water purification. According to the latest study, graphene-based membranes have showed excellent desalination performances to remove organic molecules and large radius ions, but a low rejection rates for sodium chloride of brackish water.In this paper, we have fabricated a series of sub-micrometer thick lameller GO based membranes by vacuum-filtrating with porous anodic alumina(AAO) substrate, and conducting a series of experiments through a home-made test apparatusunder the condition of reverse osmosis(RO). The desalination mechanism of the GO films indicates that the rejection rates are mainly affected by the size of the hydrated salt ion radius and ionic screening, which is that the larger ion radius, the stronger intensity, the higher rejection rates. Furthermore, protonation and deprotonation of the GO films could increase the desalination performance.We have made modification on GO thin films in order to improve the desalination performances, such as reduced by HI, the insertion of titania(Ti0.87O2, TO) nanosheets and cross-linked by chitosan(CTS). The deslanation performance of the reduced graphene oxide(RGO) inrease with the time, and the highest value up to 30.7%.The analysis indicates that the HI make ruduce of the functional groups.The cross-linging effects between GO and Al3+ releasing from the substract are weakened.The RGO film peeled off from the substrates when immersed into the salt solutions. The low rejection rates are caused by the large peeled off areas and defects of the membranes.To adjust the GO layer spacing, the GO nanosheets are not only inserted by titania(TO) nanosheets but also cross-linked by chitosan(CTS) chains.Three sub-micrometer-thick GO-based lamellar membranes: GO-only, reduced GO(RGO)/TO nanosheets and GO(RGO)/TO/ CTS are prepared on AAO substrate. The membranes were characterizated by optical microscope, scanning electron microscope(SEM) and X-ray diffraction(XRD). The results show that the insertion of TO and crossed-linked by CTS can adjust the layer spacing of GO nanosheets pecisely. And the rejection rates increase slowly with the membrane thickness. The rejection rates of RGO/TO hybrid membranes is 1.5-fold, RGO/TO/CTS 3-fold improvement comparing with GO-only membrane.The highest rejection rate of the membranes in the experiments is only about 30%. The water flux can remain between 71.6 and 143.2 L/m2 h MPa, which is higher compared to other RO membranes.Even the rejection rates in our experiment based on the GO thin films are not optimistic. We have concluded that intercalating and crosslinking methods can reduce the layer spacing.Therefore, GO-based membranes have great potential applications in water desalination. The rejection rates have much room to be improved, such as optimizing the experiment conditions and parameters, inserting another functional nanoflakes or molecular chains to shrink GO d-spacing, so as to achieve the ideal desalination performances. With the advantages of simple, low-cost-preparation, high permeability and selectivity of GO-based lamellar membranes, which may be promising to act as high performance separation membranes for water desalination applications.