Preparation And Characterization Of Free-standing Forward Osmosis Membrane

Forward osmosis (FO) was an osmotically driven membrane process that simulated the osmosis phenomenon in nature, making use of the osmotic pressure difference across a semi-permeable membrane as driving force to drive water transport from (low osmotic pressure) side to draw solution (high osmotic pressure) side. Due to nearly no hydraulic pressure required, FO delivered many potential advantages such as less energy input and lower fouling tendency. As the most important factor in FO separation technology, FO membrane acquired the similar structure with reverse osmosis (RO) membrane which composed of an active layer and a porous support layer. Researchers found that concentration polarization (CP), especially internal concentration polarization (ICP), and membrane fouling restricted FO membrane performance in actual research and application. Taking above into account, we focused on the preparation of FO membranes in this study to overcome the effect of ICP and membrane fouling using hydrophilic substance, e.g. polyvinyl alcohol (PVA) and graphene oxide (GO), and common membrane materials in FO membranes, e.g. m-phenylenediamine (MPD) and trimesoyl chloride (TMC), by solution casting method.First of all, PVA/GO membranes were synthesized by solution casting method using PVA and GO. The SEM showed that PVA/GO membranes possessed no porous layers. The obtained results indicated that water flux of PVA/GO membranes was3.7LMH with salt reverse flux of39.8gMH, and also implied that rejection of PVA/GO membranes should be further increased.Secondly, MPD and TMC were selected as membrane materials to prepare free-standing membranes. The free-standing membranes possessed better performance with water flux and salt reverse flux of3.5LMH and9.8gMH, respectively. The results of SEM and water flux manifested that the FO membranes were synthesized with symmetric structures in which no distinct porous layer was distinguished. When tested with deionized (DI) water as feed solution (FS), the changes of water flux of free-standing membranes were more stable than HTI membranes, indicating that free-standing membranes could overcome the adverse effects of ICP on water flux. Moreover, the free-standing membranes were as thick as38μm with mechanical strength of50MPa which was qualified for strength requirements in FO process. Further the free-standing membranes exhibited substantial hydrophilic property with the contact angle as low as7degree. Finally, water flux of free-standing membranes was also more stable than HTI membranes when running in the osmotic membrane bioreactor (OMBR) system. The adsorbed extracellular polymeric substance (EPS) mass on free-standing membranes was much lower than that on HTI membranes. Additionally, the EPS on free-standing membranes could be easily eliminated by simple washing with running tap water because water flux recovery of free-standing membranes after cleaning was also higher than HTI membranes. Thus it could be deduced that anti-fouling property of free-standing membranes was much better than HTI membranes...