Fabrication Of High Flux Thin Film Nanofibrous Composite Ultrafiltration Membrane For Oil Water Treatment

Electrospinning nanofibrous membranes offer an effective solution for a wide range of environmental issues and have been widely used in different levels of water purification because of their light weight, high surface area and interconnected porous structure. For water filtration, composite membranes with multi-layer composite structures have been developed with the purposes of reducing the membrane resistance and enhancing the flux. It's usually consists of a porous low resistant substrate offering mechanical support and a thin top coating layer offering filtration function.In this study, we attempted to manufacture a hydrophilic ultrathin barrier layer on porous nanofibrous nonwoven supporting layer by electrospinning technique combined with vertical melting method. High flux thin film nanofibrous composite (TFNC) membrane based on polyacrylonitrile (PAN) electrospun scaffold coupled with a thin hydrophilic barrier layer of cross-linked poly(vinyl alcohol) (PVA) was fabricated by electrospinning technique combined with vertical melting method, and used as an ultrafiltration media to separate an oil/water emulsion. The morphology, structure and filtration performance of the composite membrane was investigated in detail. It is believed that the strategy for fabricating TFNC membranes described here can be extended easily to fabricate TFNC membranes from many other polymeric membrane materials simply by choosing the suitable solution system for vertical melting treatment.To fabricating the TFNC membrane, firstly the double-layer nanofibrous mat containing very thin hydrophilic PVA top layer and PAN supporting layer was manufactured via electrospinning technique. Secondly, the hydrophilic PVA nanofibrous top layer was vertically melted to merge imperceptibly into an integrated barrier film on the supporting layer by immersing PVA/PAN double-layer mats into suitable water/acetone solution, and then chemically crosslinked by glutaraldehyde in water/acetone solution. The water content of water/acetone solution and the immersion time were optimized to achieve the integrated and nonporous PVA barrier layer. Filtration performance of the resulting PVA/PAN TFNC membranes was evaluated by the oil/water emulsions separation system. Results showed that the optimized TFNC membrane possessed high flux (218.3 L/m2h) with high rejection rate (99.5%) at very low feeding pressure (0.2 MPa).The aligned electrospun PVA nanofiber top layer was also prepared for the composite membrane. It was found that the resulting PVA/PAN TFNC membrane can possess high flux (283.0 L/m2h) with high rejection rate (99.5%) at very low pressure (0.2 MPa) for oil/water emulsion filtration. The PVA/PAN TFNC membrane also had good anti-fouling characteristic.The PVA nanofibers containing multi-walled carbon nanotubes (MWNTs) were prepared for the composite membrane fabrication. The free volume characteristics of MWNTs/PVA layer were revealed by positron annihilation lifetime spectroscopy (PALS) and the apparent fractional free volume (fapp) were calculated according to free volume cavity size and intensity of o-Ps. Filtration performance of the resulting MWNTs-PVA/PAN TFNC membranes was evaluated by the oil/water emulsions separation system. Results showed that the fapp of MWNTs-PVA/PAN TFNC membranes increased significantly with MWNTs content in the PVA layer rising from 0 wt.% to 10wt.%. The flux (320.5 L/m2h,0.2 MPa) and the degree of swelling of 15wt.% MWNTs-PVA/PAN TFNC membranes were much higher than the pure PVA/PAN TFNC membrane.