(473c) Molecular Understanding of Cellulose Acetate Membrane Formation Processes for the Minimization of Internal Concentration Polarization in Forward Osmosis | AIChE

(473c) Molecular Understanding of Cellulose Acetate Membrane Formation Processes for the Minimization of Internal Concentration Polarization in Forward Osmosis

Authors 

Zhang, S. - Presenter, National University of Singapore
Wang, K. - Presenter, National University of Singapore
Amy, G. L. - Presenter, 4700 King Abdullah University of Science and Technology
Chung, T. - Presenter, National University of Singapore


Forward osmosis (FO) has received worldwide attention in recent years as an alternative desalination technology for the production of fresh water. As an osmotic-driven membrane process, FO possesses unique advantages of low energy consumption, high rejection towards ion contaminants and anti-fouling property. However, currently available membranes for FO application suffer from severe internal concentration polarization (ICP) due to their thick and low-porosity support layer, which leads to significant reduction in the efficiency of osmotic driving force. Therefore, innovations are urgently required in the design and fabrication of membranes that are structurally feasible for FO.

New science on phase inversion and membrane formation mechanisms has been revealed in this work, which demonstrates that the hydrophilic-hydrophilic interaction between the polymer and casting substrate can have deterministic effect on the morphology of the bottom layer of the membranes. An in-depth understanding of membrane structure and pore size distribution has been elucidated with Field Emission Electronic Microscopy (FESEM) and Positron Annihilation Lifetime Spectroscopy (PALS). A rather porous bottom surface is obtained when cellulose acetate (CA) is used as the polymeric material and hydrophobic Teflon plate works as the substrate. On the contrary, when glass plate is employed, bottom surfaces of distinct porosity are formed depending on the hydrophobicity of different CA derivatives. With relatively hydrophilic CA, a selective bottom layer much denser than the top which is traditionally considered as the barrier layer, is formed due to hydrophilic-hydrophilic interaction. The thickness of the ultra-thin bottom layer is identified to be around 95 nm. At the same time, a fully porous, open-cell structure is formed in the middle support layer as a result of spinodal decomposition. Consequently, the membrane shows low salt leakage with mitigated ICP in the FO process for seawater desalination. The structural parameter (St) of the membrane is analyzed by modeling water flux using the theory that considers both external concentration polarization (ECP) and ICP, and the St value of the double dense-layer membrane is much smaller than those reported in literatures. Furthermore, the effects of an intermediate immersion into a solvent/water mixed bath prior to complete immersion in water on membrane formation have been studied. The resultant membranes may have a single dense layer with an even lower St value. A comparison of fouling behavior in a simple FO-membrane bioreactor (MBR) system are evaluated for these two types of membranes. The double dense-layer membrane shows a less fouling propensity. This study may help pave the way to improve the membrane design for new-generation FO membranes.

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