Solute and water transport in forward osmosis using polydopamine modified thin film composite membranes

Forward osmosis is a rapidly emerging technology that has potential to enable low cost water treatment and desalination. Previous investigations have found that reverse osmosis (RO) membranes were unsuitable for forward osmosis in part due to their hydrophobic support layers, which inhibit wetting. Poor wetting hinders water and solute transport in the support layer, dramatically increasing the severity of internal concentration polarization. In this study, RO membrane support layers were modified with polydopamine (PDA) to increase their hydrophilicity and promote wetting. The results indicate that the modified RO membranes exhibited a four to six fold increase in forward osmosis (FO) water flux under test conditions relative to unmodified membranes. Additional tests were performed under model desalination conditions using an ammonia-carbon dioxide draw solution with a sodium chloride feed. The sodium and chloride rejections were measured independently and in some instances substantial differences were observed. Additionally sodium and chloride rejections were lower than anticipated with a peak rejection of 90%. The substantial difference between sodium and chloride rejections was attributed to a cationic exchange effect between the draw and feed solutions.