Using polyelectrolyte coatings to improve fouling resistance of a positively charged nanofiltration membrane

In this study, a coating technique was applied to prepare an antifouling nanofiltration membrane. We first designed a positively charged nanofiltration (NF) membrane by chemical modification of P84 copolyimide using branched polyethylenimine (PEI). Then, a layer of water-soluble polymers was adsorbed onto the membrane surface in a dynamic manner. With such coatings, membrane surface properties such as hydrophilicity, roughness and charge were modified to give improved resistance to fouling. Depending on the coating materials, the coating layer may be erasable or inerasable. For example, the neutral polymer polyvinyl alcohol (PVA) may be adsorbed onto the membrane surface by hydrogen bonding. Such interaction becomes weakened during acid cleaning so that the PVA layer can be detached. Thus, if membrane fouling occurs, the PVA layer and attached foulants can be removed by acid cleaning to refresh the membrane. Negatively charged polymers such as polyacrylic acid (PAA) and polyvinyl sulfate (PVS) can be adsorbed onto the membrane surface by electrostatic force. Such strong interactions made the coating layers stable during acid cleaning. However, these coating layers permit removal of the foulants by a simple treatment with acid.