Introduction to Water Filtration

Fig 1. General schematic of electrospun liquid filtration membrane.

High porosity of electrospun membrane per unit area makes it an appropriate candidate for filtration applications. While electrospun membrane has a long history of utilization as air filter media, it is only in the 2000s that it is been investigated for use as water filtration membrane. This may be due to the higher pressure that a membrane needs to withstand as water filtration media which may lead to some perception that nanofiber is too fragile for this purpose. Nevertheless, numerous studies have demonstrated the feasibility of electrospun membrane as stand-alone filters. Various materials (eg. polyethersulfone, cellulose, polyvinylidenefluoride) commonly used for liquid filtration membrane has been electrospun to form non-woven nanofibrous membrane. Thus electrospun membrane has since been investigated for use in various water filtration applications [Nasreen et al 2013]. The requirements of the electrospun membrane depend on the targeted liquid filtration application. Liu et al (2022) investigated the hydraulic permeability of membranes with nonwoven and quasi-parallel fiber distributions using electrospun polyacrylonitrile (PAN) fibers with different fiber diameters. Their studies showed that smaller fiber diameters (200 nm) exhibited greater pressure drop with increasing flow rate compared to larger diameter fibers. With increasing flow rate, the porosity of the electrospun membrane decreases which shows that the membranes are compressible. The permeability constant of larger diameter fibers membrane is greater than that of smaller diameter fibers although the porosity of larger diameter fibers (700 nm) membrane is less than smaller diameter (200 nm) fibers membrane. In other studies, the pore size of a membrane is greater when fibers diameters are larger and this may have a greater influence in permeability compared to porosity. With quasi-parallel fiber distributed membranes, the drop in porosity with increasing flow rate is more pronounced than nonwoven fibers. This is probably due to greater compaction along the length of the fibers. While porosity of quasi-parallel fiber with fiber diameter of 200 nm and 300 nm are similar at the beginning, the drop in porosity at increasing flow rate is more pronounced with the smaller fiber diameter membrane. Pressure drop is also much faster with quasi-parallel fiber arrangement compared to nonwoven membrane. Due to the change in membrane properties with flow rate, such membranes may be useful as sensors of the flow rate or pressure.

Microfiltration is usually used for pre-treatment to other separation processes such as ultrafiltration or reverse osmosis. The main objective is to filter out microparticles ranging from the size of 0.1 microns to 10 microns. Pore size of typical commercial membrane ranges from 0.1 to 5 microns. Operating pressure is about 0.2 to 2 bar with typical flux of about 100 to 1000 l/m2/h. As microfiltration membrane, electrospun nonwoven membrane can be used without further modifications. However, it is recommended to perform heat treatment on the electrospun membrane so that its dimensional stability is maintained when pressure is applied on it [Homaeigohar et al 2010]. Several characteristics of electrospun microfiltration membrane has been investigated such as separation factor for particle sizes ranging from 0.5 microns to 1 microns, flux recovery and filtration mechanism.

Smaller size particle or molecular level separation requires ultrafiltration (0.1 to 0.01 um) and nanofiltration (0.01 to 0.001 um). For these applications, the pore size of the electrospun membrane is too large to use without any modifications. Instead, the electrospun membrane now functions as a supporting substrate to hold the separation layer. Due to the decrease in pore size, the pressure that the membrane needs to withstand is higher at about 1 to 5 bars and 20 to 50 bars for ultrafiltration and nanofiltration respectively. Whether the composite membrane is used for ultrafiltration or nanofiltration is dependent on the coated separation layer. Once again, high porosity and small fiber diameter of electrospun membrane makes it an excellent supporting substrate as it gives a larger effective separation area (separation surface without underlying obstruction). Nano and ultrafiltration membrane using electrospun membrane will theoretically give it a higher flux compared to other conventional membranes. Shen et al (2021) investigated the potential of electrospinning poly(acrylonitrile-co-vinyl acetate) copolymer (PAN-co-VA) and polyvinylidene fluoride (PVDF) as supporting substrate for graphene membrane for organic solvent nanofiltration. For the production, PAN or PVDF was directly electrospun onto commercially available CVD-grown monolayer graphene on copper foil. Hot pressing was used to fuse the graphene layer to the electrospun fibers. Etching using ammonium persulfate (APS) was used to remove the copper foil before defect sealing by interfacial polymerization (IP), and selective nanopore creation by oxygen plasma to complete the process. Between PAN-co-VA and PVDF, the former showed a better coverage by graphene with areal coverage of 69% while PVDF have a low coverage of 30%. Polyhedral oligomeric silsesquioxane (POSS) was added to the electrospinning solution to improve binding between graphene and the electrospun fibers and the graphene coverage increases to 98% for PAN-co-VA and 85% for PVDF nanofibers. The constructed nanofiber-supported monolayer graphene membranes showed effective molecular separations in both diffusion- and pressure-driven processes with 94.5% rejection to Rose Bengal (RB) at a high ethanol permeance of 156.8 liters m^-2 hour^-2 bar^-1.

Other than use as size exclusion membrane, electrospun membrane has also been explored for use as affinity membrane, water-in oil coalescence membrane and distillation membrane. Affinity membranes use functional groups on its surface to remove unwanted substance from the water. Large surface area per volume ratio of electrospun nanofibrous membrane maximizes the capturing efficiency of the membrane. Such membranes are usually either made out of inorganic material or are functionalized for the targeted contaminants. Water-in oil coalescence membranes are used in petroleum production to separate water from the oil. This works by filtering out the water droplets from the oil and allowing the droplets to coalesce to form larger water droplets that flow down the filter to a settling tank. In this case, it works like a microfiltration membrane except that the "particles" are water droplets. In membrane distillation, the membrane is used as a porous barrier to allow liquid vapour (eg. water) from the feed solution to pass through the membrane and condensate at the other side. There are several advantages using electrospun membrane for this application such as high hydrophobicity, high porosity and low thickness. While electrospun membrane generally performs better than flat-sheet membrane, hollow fiber membrane still has an edge over electrospun nanofiber membrane in terms of the flux rate [Tijing et al 2014]. Nevertheless, investigation of electrospun nanofiber membrane for distillation is still at its infancy and further optimization will see better improvement in the future.