Water and humic acid transport in graphene-derived membrane: Mechanisms and implications to functional membrane design

Graphene-derived membranes (GDM) have drawn broad attentions due to their superb water permeation and contaminant retention capabilities. In this paper, the transport of model humic acid (HA) and water through GDM is studied using the molecular dynamics simulation approach. By systematically varying the positions of functional groups and membrane pore dimensions, we find that the introduction of functional groups at the slit edge, the basal plane or the interlayer of bilayer graphene nanosheets reduces water permeation through the membranes to significantly different extents. Comparatively, -COO- at the slit edge of GE nanosheet produces the strongest electrostatic interaction for enhanced HA retention and greater water permeability than graphene oxide membranes. Furthermore, HA adsorbed inside GDM may cause membrane fouling due to pore blocking at the slit entrance or pore constriction inside the slit or the interlayer. Meanwhile, the reversibility of membrane fouling depends upon HA desorption, which is easier for HA anchored via functional groups than stacked via pi-pi interaction onto graphene surfaces. Our findings emphasize that the performance of functional GDM for water treatment is highly dependent upon the specific position of chemical modification, which has been commonly overlooked in previous studies.