Cavitating substrates to boost water permeance of reverse osmosis membranes

Abstract

Reverse osmosis membranes have been shown to achieve reliable freshwater supply by means of water desalination. Nevertheless, breaking the long-standing trade-off is a major challenge for realizing high-efficient desalination. Herein, we show that selective swelling of substrates made by a block copolymer, polysulfone-block-poly(ethylene glycol) (PSF-b-PEG), is very promising in the fabrication of high-flux reverse osmosis membranes. Polyamide layers are first formed on relatively dense PSF-b-PEG substrates by interfacial polymerization. Then, the substrates are cavitated into highly porous substrates by selective swelling, which follows the mechanism of selective swelling-induced pore generation. The substrate porosity can be well tuned over a wide range via adjusting swelling duration. Selective swelling does not compromise the structural integrity and surface properties of polyamide layers atop the substrates. Importantly, the boosted porosity reduces the water transport resistance in substrates markedly, which in turn enables fast water permeation in reverse osmosis membranes. Thus, the optimal membrane shows exceptional water permeance of 50.4 L m−2 h−1 MPa−1 and a high NaCl rejection of 99.2%. Our work not only offers a novel strategy to enhance the water permeance of reverse osmosis membranes, but also demonstrates that the transport resistance of substrates also significantly influences water permeance of reverse osmosis membranes.