Solvent activation before heat-treatment for improving reverse osmosis membrane performance

Abstract

In this work, a novel and simple strategy, i.e., solvent activation before heat-treatment was proposed to improve reverse osmosis (RO) membrane performance. This activation strategy is different from that adopted in previous reports. With this strategy, solvent activation was conducted before heat-treatment. While in previous reports, solvent activation was conducted after heat-treatment. By systematic characterization, solvent activation before and after heat-treatment were compared for the first time. The results showed that exchanging the order of solvent activation and heat-treatment created completely different effects on the RO membrane structures and performance. Specifically, solvent activation before heat-treatment affected the RO membrane structures more obviously and was superior in terms of improving RO membrane performance. Besides, the solvent activation was discussed deeply. The results showed that it had better choose an activation solvent, which is non-polar and low viscous as well as molecular size is small. By using an appropriate activation solvent, such as hexane, solvent activation before heat-treatment improved the RO membrane performance markedly: the flux was improved from 46 ± 1 L m−2· h−1 to 75 ± 5 L m−2 h−1 (by 63%) and meanwhile a high rejection of 98.97 ± 0.08% was retained (the testing condition: 15.5 bar pressure, 2000 ppm NaCl feed and 25 °C). Additionally, an unexpected phenomenon was found: increasing the activation time led to the decrease of flux and increase of rejection, which is because more activation solvent (hexane) molecules remained in the polyamide layer and then impeded the transport of water and salt.