Improving the separation performance for heavy metals by optimizing the structure of multilayered GO membrane

Abstract

Graphene oxide (GO) membranes have significant application potential for heavy metal separation. Most previous studies only investigated GO membranes composed of nanosheets with a fixed number of layers and a specific size. In this study, the effects of the size of nanosheets and the number of layers on the membrane performance were investigated to optimize the structure of multilayered GO membranes. Large and small GO nanosheets (LG and SG) were used to construct the membrane. As the numbers of layers increase, the water permeability decreases to some extent, while the retention rate increases significantly. The membranes constructed with a small size of nanosheets have better performance. For membranes with an interlayer spacing of 0.8 nm, a number of layers of three, and a gap width of 1.23 nm, the water permeability of the SG membrane was only 3 % lower than that of the LG membrane, while the retention rate increased by 1.22 times. The number-density maps showed that a large amount of Cd2+ gathered around the hydroxyl, epoxy, and especially carboxyl groups, which helped improve the retention rate. The umbrella sampling indicated that the energy barrier for Cd2+ migrating across the gap of the membrane was much higher than that of water molecules, hence reducing the size of GO nanosheets could effectively improve the water permeability without significantly reducing the retention rate. In summary, the preparation of GO membranes from nanosheets with the smallest possible size and an optimal number of layers may help improve the overall performance.