1. Introduction
Micropollutants found in water systems continue to pose a threat to humans, aquatic biota, other living organisms as well as the environment. These micropollutants are classified as organic (e.g., herbicides, pesticides, dyes, pharmaceuticals, phenols, polyaromatic hydrocarbons, endocrine-disrupting chemicals and natural organic matter)
[1,2[1][2][3],
3], inorganic (such as heavy metals, mineral acids, metal compounds, and cyanides)
[4[4][5][6],
5,6], and biological pollutants (such as parasites, bacteria, pathogens, and viruses)
[7,8][7][8]. Pollution of water by micropollutants can occur naturally and/or through the release of contaminants either intentionally or accidentally due to human activities such as mining, manufacturing, and agriculture. Recently, research has given much attention to the treatment and removal of strong recalcitrant pollutants such as phenols, alcohols, nitrogenous compounds, sulphur compounds and dyes that are mostly hydrophobic and resistant to biodegradation other methods of water treatment
[9,10][9][10].
The fabrication and application of electrospun nanofiber membranes embedded with photocatalytic and antimicrobial nanomaterials have been at the forefront of research in recent times
[11]. Polymers such as polystyrene, polysulfone, polyethersulfone, polyester and polyacrylonitrile (
Figure 1) have often been used in the production of nanofiber membranes with desired properties for various applications via an electrospinning process or other desired methods such as polymer blending and sea/island cross-section conjugation
[12,13,14][12][13][14]. These polymers can be electrospun on their own or co-polymerised with other polymers depending on the required application. Polymers are often coupled with other materials to produce polymer products with superior properties compared to their mono-polymer counterparts
[14].
Figure 1. Chemical structures of different types of polymers that are often used in the fabrication of nanofiber membranes. The asterisk (*) indicates that the structure is continuous.
Polystyrene (PS) is one of the widely used polymers in nanofiber production. It exudes high electrical resistance and low dielectric loss. It is stiff and brittle
[12], cheap, easy to handle, and displays a good balance of electrical, mechanical and chemical properties
[15]. Polystyrene also finds application in heavy-duty polymer materials such as in containers and packaging of electronic goods, ion-exchange materials, membranes, sensors and filtration due to its ease of fabrication, dimensional stability and contact efficiency
[16]. However, it is hydrophobic and this limits its full use in water treatment applications
[17]. On the other hand, polyester (PET) is used to synthesise nanofibers, membranes and nanotubes for various applications
[13]. Natural PETs have advantages such as low cost, ease of separation, low density, CO
2 sequestration, biodegradability and enhanced energy recovery compared to synthetic PET
[18,19,20][18][19][20]. Beside nanofibers for water treatment, PET resins have been reinforced with natural fiber to make materials such as engine covers
[20]. Commercially available bio-PET include poly(lactic acid) (PLA), polycaprolactone (PCL) and poly(ester amide) (PEA), among others
[18].
In the class of thermoplastics, polysulfone (PSf) has been extensively studied in membrane technology and nanofiber fabrication. PSf materials have good heat-ageing resistance, high mechanical property, thermal and chemical stability
[14,21][14][21]. PSf based materials have been widely applied in food processing, biotechnology, and water treatment
[14]. Polyethersulfone (PES) is another thermoplastic used in various material preparation processes as a modifier or as the main polymer. PES is a synthetic polymer that is non-degradable and biocompatible, oxidative, thermally stable, and exhibits hydrolytic stability, good film-forming and excellent mechanical properties
[22]. It has found tremendous application in the fields of filtration, tissue engineering, bioreactors and haemodialysis
[22,23,24][22][23][24].
Polyacrylonitrile (PAN) is one of the most widely used polymers for fabricating different types of membranes due to its excellent properties, which include ease of electrospinning, high solvent resistance, high mechanical strength, enhanced thermal and chemical stability, good membrane forming ability, biocompatibility and ease of modification
[25,26,27,28,29][25][26][27][28][29]. PAN is also the predominant precursor to produce nano- to microscale carbon fibers due to its high fiber yield, high mechanical strength and elastic modulus tailoring
[30,31][30][31]. The PAN polymer fibers are subjected to thermal treatment where they undergo carbonisation and graphitisation at the desired temperature, and are subsequently transformed into carbon fibers
[31,32][31][32]. Other polymeric materials that are used for the fabrication of different types of membranes, including nanofiber membranes include chitosan, polyaniline, polyvinylpyrrolidone, and polyvinylidene fluoride
[33,34,35,36][33][34][35][36] as shown in
Figure 1.
Polymeric membranes are however susceptible to drawbacks such as fouling, poor flux, poor rejection, and short lifespan. As a result, efforts have been made to eliminate or reduce the occurrence of these setbacks and produce composites with superior properties. Methods of modification include additive blending, chemical treatment and surface grafting
[37,38][37][38]. The commonly practiced methods include the blending of two or more polymers, incorporation of nanoparticles and blending with photocatalysts, depending on the desired application and properties. Blending polymers and/or incorporation of nanoparticles may enhance or suppress the intrinsic properties or even add new/novel properties to the bare polymer material
[39,40,41][39][40][41].
Figure 2 shows an example of a nanofiber membrane produced via electrospinning using fine and coarse polyacrylonitrile polymer coated with chitosan
[42,43][42][43]. The nanocomposite membranes were fabricated with three layers: (I) nanofiber polyacrylonitrile coarse layer which was coated with (II) fine nanofiber polyacrylonitrile and finally with (III) chitosan
[42,43][42][43]. It is demonstrated that traditional flat-sheet membranes can be coupled with nanofiber membranes to produce composite membranes with enhanced adsorption capacity, increased surface area to volume ratio, and ease of modification properties.
Figure 2. An example of a composite nanofiber membrane consisting of an electrospun polyacrylonitrile (PAN) layer coated with a chitosan layer. Reprinted from
[42] with permission from Elsevier.
On the other hand,
Figure 3 demonstrates the electrospinning of PES nanofiber membranes infused with TiO
2 nanoparticles for simultaneous adsorption and photodegradation of water pollutants (organic dyes) as reported by Xu et al.
[44]. The TiO
2-PES nanofiber composite membrane was prepared via a combination of blending modification and electrospinning technology. Adsorption activity was reported to be via electrostatic attraction. Photodegradation studies resulted in the elimination of residual toxins completely and adsorption active sites were regenerated by continuous UV irradiation without any other treatments. Recyclability enhancement of over 95% after 5 cycles was obtained
[44]. The incorporation of TiO
2 nanoparticles to the adsorption membrane introduced photocatalytic and self-cleaning properties, rendering the membrane more efficient and highly recyclable. In latter sections of this review, various other types of polymer-photocatalyst nanofiber membranes with specific examples are discussed in comprehensive detail.
Figure 3. Fabrication of Polyethersulfone (PES)-TiO
2 nanofiber composite membrane via electrospinning as well as simultaneous adsorption and photodegradation of micropollutants. Reprinted from
[44] with permission from Elsevier.
While this review is focused on nanofiber membranes infused with photocatalytic and antimicrobial nanoparticles, Nasreen et al. previously reviewed the general advancement of modification and application of electrospun nanofiber membranes in water treatment. The review emphasizes the importance of nanofiber membrane modification for enhanced efficiencies. Modifications discussed include surface modification (improved selectivity and hydrophilicity) and interfacial polymerization (improved strength, chemical/thermal stability and introduction of selective barrier layer, porous support and/or maintaining strength and configuration). The specific application of these nanofiber membranes covers the removal of heavy metals and microbes as well as desalination application
[45].