Can Fibrous Mats Outperform Current Ultrafiltration and Microfiltration Membranes?

Abstract

Ultrafiltration (UF) and microfiltration (MF) membranes have important applications in separations related to proteins, pharmaceutical products, viruses, food and beverages, water treatment, and sterilization. Although phase inversion membranes have been used for MF and UF applications for decades, there has been, instead, an increase in interest in using electrospun fibrous mats as MF/UF membranes. Although the selectivity–permeability tradeoff for conventional phase-inversion UF membranes is now established, such an understanding for phase-inversion MF membranes and for fibrous mat membranes does not exist. Here, we report the first preliminary selectivity–permeability tradeoff for commercially available MF membranes. We also describe a theoretical framework that can be used to evaluate the performance of fibrous mats. Mats consisting of a random array of nanofibers were modeled with a gamma pore size distribution, based on previous work. The pore size distribution of the mat was related to the physical properties of the mat, such as porosity, fiber diameter, density, areal density, and mat thickness. This distribution was then used in conjunction with a procedure developed by Zydney and co-workers to conduct a priori predictions of the performance of fibrous mats, UF, and MF membranes in terms of their hydraulic permeability and selectivity to the model solutes, 3.65 nm radius bovine serum albumin (BSA) and 100 nm radius microspheres, respectively. We compared the performance of modeled mats of varying properties with the selectivity–permeability trade-off curve of current UF and MF membranes. A surprising finding was that, as modeled, the performance of fibrous mat membranes can only surpass that of current UF and MF membranes under very limited conditions. These conditions include very low fiber diameters of ∼2 nm and ∼70 nm and membrane thicknesses of <100 nm and <1000 nm for UF and MF membranes, respectively. These metrics are challenging to achieve under currently used manufacturing conditions.

Robust, Small Diameter Hydrophilic Nanofibers Improve the Flux of Ultrafiltration Membranes

“…Electrospinning can be used to manufacture textiles on the commercial scale that are comprised of randomly accumulated nanofibers with large surface-to-volume ratios, high specific surface areas, large interstitial spaces, and porosity values greater than 80%. , Nanofibers show promise in a wide range of separation applications, including membrane distillation, adsorption, pretreatment of feed, and reverse osmosis membranes. − Models have suggested that the performance of freestanding nanofiber mats with small fiber diameters of ∼2 and ∼70 nm with membrane thicknesses of <100 and <1000 nm, respectively, would surpass that of current ultrafiltration and microfiltration membranes . When Liu et al .…”

“…24−27 Models have suggested that the performance of freestanding nanofiber mats with small fiber diameters of ∼2 and ∼70 nm with membrane thicknesses of <100 and <1000 nm, respectively, would surpass that of current ultrafiltration and microfiltration membranes. 28 When Liu et al 19 deposited a ∼5 μm-thick layer of carbon nanofibers (diameter: 20−40 nm) onto ultrafiltration membranes, they demonstrated a minimal pure water flux decline and an improved removal of natural organic matter, including sodium alginate and bovine serum albumin. A layer of poly(acrylic acid) and poly(vinyl alcohol) nanofibers (<2 mg/cm 2 ) was electrospun onto ultrafiltration membranes, causing an increase in membrane hydrophilicity and a reduction in organic fouling without affecting permeability or protein rejection performance.…”

Solution Blown Nylon 6 Nanofibrous Membrane as Scaffold for Nanofiltration

“…Nanofibrous membranes, which are composed of electrospun nanofibers with random orientation, have several unique characteristics such as a large surface area to unit volume ratio, high porosity (up to >80%), nanosize pores and fully interconnected pores [ 23 , 24 , 25 ]. With these properties, nanofibrous membranes provide abundant water pathways with low hydraulic resistance and have drawn remarkable attention in microfiltration with high water flux and low-energy consumption [ 26 , 27 ]. Recently, polyacrylonitrile (PAN) nanofibrous membrane was applied as nanofibrous scaffold for a high flux thin-film nanofibrous composite (TFNC) nanofiltration membrane and it exhibited over 2.4 times more permeate flux than conventional thin film composite (TFC) membranes with the same chemical compositions and the same retention rate [ 28 ].…”

Fabrication of fibrous microfiltration membrane by pore filling of nanofibers into poly(ethylene terephthalate) nonwoven scaffold

“…Their performance relies on their geometrical structure of pores, corresponding pore size distribution, and undesirable macrovoid formation across the entire film thickness. Usually, porous polymeric filtration membranes have their other intrinsic limitations, e.g., poor permeability, high operating pressures, limited available materials, cost effectiveness, and environmental concerns [8][9][10].…”