Conventional flat-sheet and hollow fibers membranes are widely being used in pressure-driven osmosis (Reverse Osmosis – RO) applications due to their superior permselectivity. Owing to high operating/maintenance costs required for RO process, alternative processes with less-energy intensive nature have been under investigation by leading membrane scientists. One of such processes, called Forward Osmosis (FO), uses an osmotic pressure difference instead.
When two solutions of differing concentration are placed on two sides of a semi-permeable membrane, osmotic pressure is developed which in turn drives the separation of salt-free water across the membrane. FO processes have not yet been fully commercialized on a large scale for various reasons. One of the key reasons is the lack of an efficient membrane specifically designed for FO.
Scientisits Liwei Huang, and Jeffrey R. McCutcheon from University of Connecticut, USA attempted to address the challenge. In general, FO membrane structure consists of two key layers; an organic selective layer on top of a support layer. Conventional thin film composite (TFC) membrane possesses the thick nonwoven fabric layer as support (baking) layer that causes severe mass transfer resistance near the interface of the selective thin film layer, results in poor water flux. Prof Jeffrey R. McCutcheon’s team decided to replace the conventional thick support layer with a thinner, more porous and less tortuous nylon 6,6 structure as they speculated that structural parameter played key role in FO separation process. To produce the porous support layer, they adopted electrospinning as the technique and nylon 6,6 as material. Electrospinning was chosen because its popularity in producing thin and a highly porous and interconnected (low tortuosity) structure. Nylon 6,6 polymer was the choice of material for support layer, mainly due to its hydrophilicity, good mechanical strength, and excellent compatibility with a polyamide selective film. The team successfully fabricated the support structure using nylon 6,6 nanofibers via electrospinning for the first time. They found that intrinsic hydrophilicity and mechanical strength of nylon 6,6 nanofibers were superior compared to other nanofiber materials. The polyamide selective (top) layer was prepared in situ on the nanofiber support using interfacial polymerization (IP) chemistry, and using acetone as co-solvent.
The nanofiberous support layer allowed for complete saturation (i.e. wetting) throughout its structure, thanks to superior hydrophilicity. High osmotic flux was achieved as a result of the intrinsically high porosity with an interconnected pore structure and low tortuosity of the nanofiber mats. As a result, the developed novel polyamide TFC membrane based on a nylon 6,6 nanofibrous mat outperformed the standard commercial FO membrane by 1.5 – 2 folds increase in water flux. Higher flux and selectivity were contributed by the structural parameter, which is 3 times higher than that of commercial FO membrane (about 2000 μm).
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