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. © 2014 Elsevier B.V. All rights reserved.

This study explored the impact on FO properties resulting from the application of a thin film of PDA on the PSu support structure of a commercial TFC RO membrane. A four and six fold enhancement in the FO mode osmotic flux of the BW30 and SW30-XLE membranes, respectively, was observed after modification with PDA. Overall, these membranes were shown to have modest flux under desalination conditions with a 2.0 M NH3–CO2 draw solution and a 0.25 M sodium chloride feed; however, low sodium rejections were observed due to cation exchange between the draw and feed solutions. Evidence for this ion exchange is provided by the unequal fluxes between sodium and chloride. A tuning of process conditions or membrane chemistry may enable higher rejections for both ions within the feed solution. Acknowledgments The authors acknowledge funding from the NSF CBET Chemical and Biological Separations Program #1160098 and #1160069, and USEPA (Project No. R834872). The authors also acknowledge the NWRI-AMTA Fellowship for Membrane Technology and National Science Foundation GK-12 Program, which provided support for Jason T. Arena. The authors also wish to thank Dow Water & Process Solutions for providing membranes for this study. Additionally the authors would like to thank Dr. James V. Arena at Central Connecticut State University and Dr. Abhay Vaze at the University of Connecticut for providing technical assistance and feedback for some of the analytical techniques used.