The role of interlayers in enlarging the flux of GO membranes.

A graphene oxide (GO) membrane can be easily made by filtering a GO solution onto a supporting layer, and such a membrane is effective at adsorbing ions. But low flux and a high work pressure become an obstacle for its application in wastewater treatment. In this study, a positively charged mixture of carbon nanotubes and chitosan (CNTS) served as an interlayer to improve the GO membrane's flux. The three-layer membrane is known as MCG, while one without an interlayer is known as MG. For MCG and MG with the same GO load, the water flux of MCG reaches 2-8 times larger than that of MG. A better water permeability is consistently detected for MCG, with a contact angle descent speed of 3.3°/s, which is significantly faster than that of MG (0.5°/s). The ion rejections of MCG and MG are mostly attributed to GO adsorption, which stay at the same level. The flux varies with GO load, CNTS load and membrane dryness, while the ion rejection is correlated with the GO load. Optimized membrane fabrication conditions are suggested as being a CNTS load of 0.72 g m and a GO load of 0.4 g m. A 'gap' mechanism is suggested to explain the interlayer effects. The rougher interlayer surface produces gaps between the GO and CNTS layers, which results in the faster water permeation and higher flux of MCG. These results demonstrate that it is possible to fabricate high flux GO membranes by adding a controlled-roughness interlayer.