Polydopamine Modified Thin Film Composite Membranes for Forward and Pressure Retarded Osmosis

Engineered Osmosis (EO) is an emerging technology platform that can purify water, dewater solutions, or generate power by  using concentration gradients to drive water through a semi-permeable membrane.  Thin film composite membranes offer a breadth of permselectivities readily available from commercial vendors.  These membranes are based off a three tiered structure consisting of a selective layer, a phase inverted porous polymer mid-layer, and a non-woven fabric support layer.  The selective layer is typically formed on a hydrophobic polymer such as polysulfone (PSu) or polyethersulfone.  TFC membranes have been previously found to exhibit poor flux performance in osmosis due to the characteristic hydrophobicity of the polymer mid-layer.  This inhibits osmotic transport because of a lack of wetting throughout the pore structure.    [1].  If the pore structure does not completely wet, the effective porosity of the support is reduced and internal concentration polarization is enhanced through reduced solute transport through the PSu.

Our previous study revealed that dramatic improvement in osmotic flux performance in conventional reverse osmosis membranes is achievable through  chemical modification of the support layer with polydopamine [2].  Polydopamine is a hydrophilic polymer with similar characteristics to the adhesive secretions of mussels [3].  It self-polymerizes from dopamine in an oxygenated solution, typically between pH 8.5-9.0, on surfaces exposed to the polymerizing aqueous bath [2, 3, 4].  A thin film of polydopamine forms on the internal structure of the membrane, increasing its hydrophobicity and overall wettability of the membrane’s support structure.  The polydopamine modification coating presents a stable and permanent chemical modification, increasing the hydrophilicity of the porous polymer support layer without altering conventional TFC membrane fabrication process or selectivity.

These modified membranes were tested in osmosis and reverse osmosis systems to examine the role of polydopamine deposition on permselectivity and osmotic flux performance. Contact angle and mercury porosimetry were used to characterize hydrophilicity and porosity after the application of the polydopamine. Variations in dopamine monomer loading and pH allow for some degree of optimization. 

The modified membranes exhibit 5-20 fold increases in osmotic water flux relative to the native membranes while retaining the high selectivity that is expected of these membranes.  The high selectivity resulted in dramatically lower salt fluxes than those seen in commercial FO membranes..

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]     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.

[3]     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.

[4]     McCloskey BD, Park HB, Ju H, Rowe BW, Miller DJ, et al.. Influence of polydopamine deposition conditions on pure water flux and foulant adhesion resistance of reverse osmosis, ultrafiltration, and microfiltration membranes. Polymer (2010) 51: pp. 3472-3485.