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Nano-Structured Compaction Resistant Thin Film Composite Membranes

Asim K. Ghosh, Jodie Nygaard, and Eric M. V. Hoek. Civil & Environmental Engineering, UCLA Water Technology Research Center and California NanoSystems Institute, Henry Samueli School of Engineering and Applied Science, 5732G Boelter Hall, University of California, Los Angeles, Los Angeles, CA 90095

Thin film composite (TFC) polyamide membranes are commercially successful for desalination and water purification, but the application of hydrostatic pressures to these membranes causes a reduction of membrane permeability due to compaction. When a polymeric membrane is put under pressure, the polymers are slightly reorganized and the structure is changed, resulting in a lowered porosity, increased membrane resistance, and eventually lowered flux. As the applied pressure is increases, so does the extent of physical compaction. Generally the flux decline of TFC membranes in brackish water desalination is around 15-25% and in sea water desalination it is as high as 30-40% due to compaction. The compaction problem in polyamide thin film composite (TFC) reverse osmosis (RO) membranes arises mainly due to compaction of the thick porous polysulfone support layer rather than the thin compact polyamide layer.

In this paper, we discuss the mechanisms and relative extent of physical compaction of conventional TFC membrane as well as a new type of nano-structured TFC membrane. Attempts have been made to prepare nano-structured RO membranes by incorporating a wide array of organic and inorganic nanomaterials in conventional TFC membranes to provide resistance to physical compaction. Different nano-structured membranes were prepared by varying the types and loading of nanomaterials throughout the membrane cross-sections. These nano-structured membranes were characterized in terms of thermo-gravimetric analysis to understand the change in thermal and mechanical properties. In addition, surface hydrophilicity, charge, and morphology were determined from contact angle, streaming potential, atomic force microscope, and electron microscope analyses. Pure water flux was measured as a function of pressure and time for conventional and nano-structured TFC membranes. Time required and steady state flux for was compared for different nano-structured membranes, in addition to solute rejection. In all the membranes, compaction results in a notable membrane permeability loss. The relative resistance of conventional and nano-structured membranes to physical compaction will be presented at the conference.