Investigation of hyperbranched polyethylenimine (PEI) as an osmotic agent in polymer enhanced forward osmosis

Abstract

Forward osmosis (FO) is an emerging technology and potentially useful in desalination process as an alternative to popular, yet very costly, reverse osmosis (RO) process and other desalting technologies. FO uses so-called “draw solution” to draw fresh water from saline water through osmosis while solutes are retained by semi-permeable membrane. Thus it can desalt saline water without applying external pressure. Up-to-date, most of studies on draw solution have been focusing on electrolyte draw solution; though some macromolecular osmotic agents and their innovative recovery technologies have been investigated, they pose a number of limitations to their application. Osmotic pressure of macromolecules is lower than that of salts, polymer solutions have high viscosity, and economics and recyclability of nanoparticles impede large scale application. In this study, potential use of hyperbranched polyethyleninime (PEI) as the draw solution for FO process followed by reconstitution via ultrafiltration (UF) has been investigated. Osmotic analysis of PEI by custom designed direct membrane osmometer has shown the polymer’s capacity to generate high osmotic pressure >35 bar at 25wt% and its globular structure enhances fluidity. Hyperbranched PEI’s flux performance in FO, though it was slightly lower than that of NaCl, is observed to be comparable to NaCl draw solution. This is likely due to higher degree of concentration polarization (CP) attributed to higher molecular weight of the polymer. However, it is expected that recovery of draw solution via UF will require relatively lower energy than distillation and the use of lower molecular weight PEI will reduce the effect of CP. In addition, implementation of the membrane with different chemistry has shown significant reduction of ICP which emphasizes the importance of further development of new FO membrane. UF reconstitution of PEI draw solution has been demonstrated from 5wt% to 10wt%. PEI rejection stayed above 99.8% d...