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Thin film composite membranes currently represent the best technology platform for nanofiltration and reverse osmosis due to their high permselectivity and developed commercialized fabrication. The thin film composite structure consists of three layers: a selective layer formed through an interfacial polymerization reaction; a porous phase inverted polymer mid-layer support; a non-woven fabric support layer. The porous polymer layer is typically made of polysulfone or polyethersulfone. These polymers have an inherent hydrophobic character preventing these structures from completely wetting which, under osmotic flow, leads to low performance as a result of severe internal concentration polarization [1].
As a means of mitigating this poor membrane wetting performance, our recent study has discovered that thin layers of the hydrophilic polymer polydopamine can improve the mid-layer hydrophilicity. Polydopamine is a polymer similar to the adhesive secretions of mussels [2]. It polymerizes in a buffered aqueous solution at pHs typically between 8.5 and 9.0. The application of polydopamine to a conventional thin film composite membrane yields significant improvements to the rate of osmotic flow [3]. This polydopamine modification is the final step of a process which initially wets out the structure to facilitate complete coating of the membrane support layer.
Modified membranes were tested under both the pressure retarded osmosis and forward osmosis orientations. This study examines the roll of membrane orientation and seeks to optimize the polymerization by variation the solution pH and dopamine monomer concentration.
Works Cited
[1] McCutcheon JR & Elimelech M. Influence of membrane support layer hydrophobicity on water flux in osmotically driven membrane processes. Journal of Membrane Science (2008) 318: pp. 458-466.
[2] Lee H, Dellatore SM, Miller WM & Messersmith PB. Mussel-inspired surface chemistry for multifunctional coatings. Science (New York, N.Y.) (2007) 318: pp. 426-430.
[3] Arena JT, McCloskey B, Freeman BD & McCutcheon JR. Surface modification of thin film composite membrane support layers with polydopamine: enabling use of reverse osmosis membranes in pressure retarded osmosis. Journal of Membrane Science (2011) 375: pp. 55-62.