Cross-linked GO membranes assembled with GO nanosheets of differently sized lateral dimensions for organic dye and chromium separation

Membrane microstructure and separation performance largely depend on the property of building units and its subsequent patterning. In this work, three differently sized graphene oxide (GO) nanosheets were prepared by a simple centrifugation method. Supported GO membranes were then fabricated based on these separated GO nanosheets. GO nanosheets of different lateral sizes show varied degree of oxidation and distribution of oxygenated groups at both edge and in-plane areas, which leads to different membrane microstructure and separation performance. The swelling and long-term stability of GO membrane were addressed by a pre-crosslinking process with ethylenediamine (EDA). The ultimate separation performance of these GO/EDA membranes was affected by several factors such as cross-linking density, d-spacing, and microstructure like wrinkles all relating to the lateral size of GO nanosheets. Specifically, compared to smaller GO nanosheets, larger GO sheets have more surface groups and thus higher cross-linking density. In terms of microstructure, larger sheets tend to form more surface wrinkles which could sacrifice the membrane separation performance. GO membranes were tested in different pH conditions for separating methylene blue (MB), methyl orange (MO) and chromium (VI). Overall, exceptional rejection rates of higher than 99.0% were achieved at optimal separation conditions. MB separation performance has been found closely related to GO lateral size at specific pH environments.