Overcoming the trade-off between water flux and salt rejection of an RO membrane via simultaneous optimization using highly porous microstructured support and sulfonated porous organic polymer-based polyamide active layer

Many researchers have devoted much effort to addressing the trade-off between water flux and salt rejection of reverse osmosis (RO) membranes till now, but it still needs to be solved. In this work, as a strategy to overcome the trade-off, simultaneous optimization was carried out with porous organic polymers (POPs) and highly porous microstructure support (HPμS) layers featuring regularly arranged columnar macrovoids formed by carefully adjusting the thermodynamic instability of a polymer solution. POPs were introduced into the active layer of an RO membrane to impart the water channel and hydrophilicity. POP fillers were functionalized with amine and sulfonic groups, and the synthesized POPs were introduced into different phases depending on sulfonation. Sulfonated POPs with high hydrophilicity provided additional water pathways and hydrophilic properties while maintaining the active layer's thickness, notably improving water permeability. Surprisingly, the simultaneous optimization not only enhanced the permselectivity further but also made thin-film nanocomposite (TFN) membranes overcome the trade-off because the shortened diffusion pathway by the HPμS was more beneficial for improving water permeability than salt permeability. As a result, the simultaneously optimized TFN exhibited 2.2 to 2.7 times higher water permeability and 2.6%p to 3.4%p higher salt rejection than a control TFC membrane.