Electrospun Ultrafiltration Membrane

Fig 1. Electrospun ultrafiltration membrane comprising of a separation layer, an electrospun layer and a nonwoven microfiber membrane

Relatively large pore size of electrospun membrane makes it unsuitable for filtration below the microfiltration level. For use in ultra- and nanofiltration, electrospun membrane is often used as a supporting substrate for the separation film layer. This supporting substrate is vital to prevent rupturing of the film under high water pressure while allowing water to flow through. An advantage of electrospun membrane is that the large pore size between the small diameter fibers reduces the barrier to liquid flowing through the membrane. This potentially improves the flux while the thin film provides the separation function. Several studies have been conducted to examine the potential use of electrospun membrane in ultra and nanofiltration.

Ultrafiltration membrane may be constructed by cast-coating with the separation material on the electrospun membrane. Wang et al (2006) used electrospinning to construct a polyvinyl alcohol (PVA) base substrate followed by cross-linking to improve the structural stability of the membrane. To further improve the mechanical integrity of the composite membrane, the electrospun fiber may be transferred to a non-woven microfiber substrate to form a three-tier composite membrane. The surface of the electrospun PVA was coated with a layer of PVA solution to form the separating layer.

Another way to create the three-tier composite membrane structure is to carry out electrospinning over the non-woven microfiber substrate followed by cast-coating. Using this technique Yoon et al (2006) first coated the surface of a poly(ethylene terephathalate) (PET) microfilter substrate with chitosan solution. Polyacrylonitrile (PAN) solution was directly electrospun onto the substrate before cast-coating to form a chitosan film over polyacrylonitrile (PAN) electrospun membrane. Pre-treating of the electrospun PAN membrane with 1 N NaOH was used to reduce penetration of the chitosan solution into the supporting membrane. Comparing the constructed membrane with commercial nanofiltration membrane (NF 270), the electrospun PAN composite membrane exhibited greater rejection (>99.95%) of oily waste water with much higher flux. The study also found that thicker chitosan membrane reduces the flux although it was still much higher than the commercial nanofiltration membrane.

To reduce the film thickness, modification of the application material or technique is needed. Solution with higher viscosity would form a thicker layer but insufficient concentration will result in non-uniform separation layer. Thus, there is a limit which the viscosity can be reduced through lowering of the solution concentration. Tang et al (2009) found that UV-cured poly(vinyl alcohol) (PVA) can be pretreated with potassium persulfate (K2S2O8) to increase the viscosity of low PVA concentration. This allows them to cast film layer thickness down to 0.55 µm using 5 wt% PVA solution compared to 2.2 µm thick layer cast from 15 wt% PVA solution without pretreatment.

Beyond rejection and flux, another important criterion for its application is its longer term performance. Yoon et al (2009) tested the rejection performance of cross-linked PVA film cast on PAN composite membrane. At a high pressure of under 130 psig, the rejection of oil/water emulsion permeates was over 99%. Over 24 h operation, the permeate flux for the electrospun based film remains stable while commercial ultrafiltration membrane (PAN 10) experience severe flux decline. Operating for 190 h, the electrospun-based membrane experiences slow reduction in flux while the commercial membrane (PAN 10) showed a stable but very low flux after the initial decline. However, the flux for the electrospun-based membrane is still 12 times higher than commercial membrane after 190 h. Rejection rate for both electrospun based film and commercial ultrafiltration membrane remains the same at about 99% throughout the operation.