Enabling graphene oxide nanosheets as water separation membranes

We report a novel procedure to synthesize a new type of water separation membrane using graphene oxide (GO) nanosheets such that water can flow through the nanochannels between GO layers while unwanted solutes are rejected by size exclusion and charge effects. The GO membrane was made via layer-by-layer deposition of GO nanosheets, which were cross-linked by 1,3,5-benzenetricarbonyl trichloride, on a polydopamine-coated polysulfone support. The cross-linking not only provided the stacked GO nanosheets with the necessary stability to overcome their inherent dispensability in water environment but also fine-tuned the charges, functionality, and spacing of the GO nanosheets. We then tested the membranes synthesized with different numbers of GO layers to demonstrate their interesting water separation performance. It was found that the GO membrane flux ranged between 80 and 276 LMH/MPa, roughly 4-10 times higher than that of most commercial nanofiltration membranes. Although the GO membrane in the present development stage had a relatively low rejection (6-46%) of monovalent and divalent salts, it exhibited a moderate rejection (46-66%) of Methylene blue and a high rejection (93-95%) of Rhodamine-WT. We conclude the paper by emphasizing that the facile synthesis of a GO membrane exploiting the ideal properties of inexpensive GO materials offers a myriad of opportunities to modify its physicochemical properties, potentially making the GO membrane a next-generation, cost-effective, and sustainable alternative to the long-existing thin-film composite polyamide membranes for water separation applications.