Scale up of Synthesis Seawater Reverse Osmosis Membranes with Surface Nano-Structured Hydrophilic Polymer Brush Layers

Tethered hydrophilic polymer layers have been shown to be effective in reducing the fouling propensity of reverse osmosis (RO) membranes as well as in improving membrane cleaning via solvent flushing. In the present work surface nano-structuring (SNS) of brackish water polyamide RO membranes with terminally anchored poly(acrylic acid) (PAA) chains resulted in RO membranes of salt rejection suitable for seawater RO (SWRO) desalination but with water permeability being about a factor of 1.5 greater than other commercial membranes. The tethered PAA layer was synthesized by a two-step process consisting of surface activation, to form active surface peroxide radicals, by an impinging atmospheric pressure plasma (APP) source, followed by graft polymerization in a monomer solution. The fouling propensity of the resulting SNS-polyamide (PA) RO membrane were shown to be more resistant to biofoulants, and moreover could be effectively cleaned with freshwater flushing to a greater extent relative to commercial SWRO membrane membranes. The improved SNS-PA membrane fouling resistance was attributed to the ionic strength-responsive swelling of the tethered polymer layer as ascertained via AFM force spectroscopy and analysis of contact angle measurements. Given the above success with small membrane coupons, scale up of the SNS-PA RO membrane manufacturing process was undertaken with the goal of producing membrane sheets suitable for manufacturing up to 21-inch spiral-wound RO elements. However, in order to achieve high level of membrane surface uniformity (with respect to salt rejection, flux and fouling resistance), several challenges had to be addressed: (a) uniformity of surface activation by a moving atmospheric pressure plasma source over an area of at least 30” x 30”, (b) accurate temperature control to avoid temporal and spatial variations of temperature within the graft polymerization reactor, and (c) adequate mixing of the monomer solution in the graft polymerization reactor to avoid differences in local surface graft polymerization rates (due to both variation in local concentration and solution temperature). In order to address the above, scale up of APP surface activation was achieved by utilizing a robotic system capable of translating the APP plasma source over the membrane surface area at a controlled speed and over repetitive cycles. Graft polymerization was achieved in a shallow film reactor with specially designed multiple fluid entry locations with recirculation of the reaction solution mixture, with multi-stage temperature control to minimize temperature variations in time and across the reactor. Evaluation of the above approach enabled the fabrication of SNS-PA RO membranes with high degree of uniformity with respect to surface structure, salt rejection and water permeability. The utilization of the present membranes in small spiral-wound RO elements for expeditionary systems will be discussed along with prospects for scale up to large commercial RO elements.