The electrospinning method goes back a long way that the first investigations were based on the generation of aerosols from liquid drops under high electrical potential in the 1700s. Then, the developments continued until the 1900s, and the first patented device was developed in 1929 by fabricating artificial silk by Kiyohiko.¹⁴ The first application of polymers in electrospinning was performed by Simm et al. in 1934, and after that date, growing attention has been paid to this method.¹⁵ The fabrication of continuous fibers with ultrathin diameters becomes possible with electrospinning, and solutions, suspensions, or melts from a variety of materials can be applied as precursors. The method allows the control of the surface morphology and topography of resulting materials, and their properties are strongly variable depending on the process parameters.¹⁶ Moreover, not only the polymer precursors but also the ceramic and metal fibers can be fabricated, and the post-modification of fibers with various functionalizing agents is possible.¹⁷ The setup comprises three main components, which are a syringe pump with a small diameter conductive needle to feed the polymer solution, a high voltage supply to apply a direct current potential and create an electrical field, and a grounded collector to gather fibers on itself (Figure 2). Through the process, the polymer solution is pumped, and the droplet that comes to the capillary nozzle acts as an electrode. When the high voltage is applied to the nozzle, the jet is created, and the polymer is transferred through the electrical field. The electrical field depends on the processing parameters, but it varies between 1.0–5.0 kV cm⁻¹. While the polymer solution is transferred through the electrical field, the solvent evaporates, the polymer solidifies, and the solidified electrospun fibers are collected on the grounded collector.¹⁸

The morphology of collected fibers depends on various parameters, which are the polymer solution properties (concentration, viscosity, surface tension, and vapor pressure of solvent), flow rate of syringe pump, applied electrical field, and environmental conditions (temperature and humidity). Thus, the biggest advantage of electrospinning is its tailorable properties by changing the processing parameters. All these parameters have a strong influence on the fiber morphology. For example, using highly volatile solvents results in porous structures with enhanced hydrophobicity or high concentrations of polymer solutions that have larger fiber diameters with lower surface area at the end, while the low concentrations lead to the formation of beads instead of fibers. Moreover, the viscosity of the solution affects the fiber morphology because the low viscosity results in a lack of chain entanglement and bead formation. The conductivity of the solution promotes the fiber formation, but higher conductivity than the required levels results in bead formation.¹⁹ Another advantage of this fiber fabrication technique is the ability to use a wide variety of polymer types. In the literature, there are various types of polymers have been used for the fabrication of nanometer to micron scale fibers by using synthetic polymers (polystyrene, polymethylmethacrylate, polyvinyl alcohol, etc.), protein blends (silk, fibrinogen, collagen, etc.) and polysaccharides (cellulose derivatives).²⁰ Alongside its advantages, electrospinning also has some challenges. The major problem faced during the process is the clogging of the nozzle due to the very fast evaporation of solvent on the needle or high viscosity. Also, the polymer and solvent selection is an important step in that there are a limited number of conductive polymers, and it is not possible to process neat, non-conductive polymers. The use of corrosive or harmful solvents might be an environmental issue during the process for synthetic polymers, and sustainable selection of the solvents is required. However, the continuous and fine nanofiber fabrication and bulk production possibilities of the method make electrospinning suitable for various applications.

Due to urbanization and industrialization all over the world, there are various water pollutants such as oil spills, heavy metals, pathogens, and dyes. The use of electrospun fibers is an emerging trend for water remediation due to their high surface-to-volume ratio, and various studies have been conducted in the literature. Xu et al. utilized polysulfone fibrous mats to remove the suspended particles from bio-polluted water sources²¹ In another study, Gopal et al. fabricated polyvinylidene fluoride electrospun fibers for the pretreatment of wastewater treatment plants before ultrafiltration, and these fibers removed 90% of the microparticles.²² Shi et al. modified the polyacrylonitrile (PAN) nanofibers with silver nanoparticles and investigated their antimicrobial activities against Escherichia coli and Bacillus cereus bacteria.²³ Zhao et al. fabricated PAN electrospun nanofibers with microbial carriers by immobilizing aerobic denitrifying bacteria, and different carbon-to-nitrogen (C/N) ratios were obtained. The optimized parameters were found to be 12% PAN concentration with C/N > 7, and these fibers showed 90% degradation with 200 ppm nitrate-polluted wastewater.²⁴ Proteins have also been used to modify electrospun fibers for wastewater remediation. Xie et al. developed polyvinyl alcohol (PVA) and zein composite nanofibers via electrospinning for the adsorption of copper ions from wastewater. The results showed that a 50% protein addition to the polymer composite improved the Cu(II) ion adsorption capacity up to 26.62 mg/g.²⁵ Chitosan and cellulose composite fibers were fabricated by Phan et al. by using a co-solvent system followed by sodium carbonate treatment for the neutralization of chitosan and deacetylation of cellulose acetate. The mechanical properties reached up to 17 MPa tensile strength for composite fibers with 5.5% elongation in break. The resulting fibers were utilized for the adsorption of heavy metal ions from aqueous solutions, and the results were recorded as 39, 57, and 112 mg/g for arsenic, lead, and copper ions, respectively.²⁶ Janus electrospun fibers have also been used in wastewater treatment applications, and Yan et al. fabricated PAN/carbon nanotube composite electrospun fibers to remove oil from aqueous solutions efficiently. The micro and nanopores through the fibers create channels for the adsorption of oil, and the results showed that 99.8% separation efficiency was recorded with 2188 L/m².h flux.²⁷ As the latest development, Ahmadijokani et al. reported the use of metal-organic frameworks (MOFs) combined with electrospun fibers by taking advantage of both methods that easy-design, controllable structure and flexibility of electrospun fibers and high permeability, water treatment applicability, and tailored structures of MOFs.²⁸

In recent years, recycling plastic waste has become a crucial issue worldwide. The long degradation time and microplastic formation increase the negative impact on the environment, and precautions are required to eliminate the plastic waste problem. At this point, there are various traditional methods in the literature for the recycling of plastics. Electrospinning of waste polymers is one of the latest approaches that can be used for both recycling and reusing waste products for water remediation purposes. Polyethylene terephthalate (PET) is one of the most frequently generated wastes. Disposable water bottles are the main source of PET waste. After their first use, PET bottles run to waste in a quite clean and undeformed way. Thus, recycling and reusing them for further applications is a proper approach to waste management. Siskova et al. recycled PET bottles in binary organic solvents (hexafluoro-2-propanol/dichloromethane), and nano-sized electrospun fibers were fabricated for filtration application of 120 nm particles with 98% efficiency.²⁹ Likewise, Strain et al. processed PET bottles via solution electrospinning for the fabrication of micron-sized electrospun fibers with 62.5 MPa fiber strength. The resulting fibers were utilized for smoke filtration and absorbed smoke 43 times their weight.³⁰ Moreover, polystyrene (PS) is another frequently generated plastic waste, and the main source of this waste is Styrofoam. Esmaeili et al. recycled the PS wastes by varying the processing parameters and investigated their effect on fiber diameter.³¹ Isik et al. recycled Styrofoam wastes via solution electrospinning and investigated the effect of solvent composition and polymer concentration on the surface morphology. By tailoring the solvent composition, interior and exterior porosity were generated, and high adsorption capacity and hydrophobicity were achieved, respectively. Then, the recycled electrospun fibers were used for oil adsorption from wastewaters with up to 124 g/g adsorption capacity and 99% separation efficiency³² (Figure 3). Alongside synthetic polymers, bio-based polymers can be recycled via electrospinning. Keratin extracted from chicken feathers or cellulose has been recycled via electrospinning, and their unique chemical structure makes them available for various filtration applications.^33,34

One of the purification methods frequently used in water remediation applications is flocculants. Flocculants are made of polymers, and their working principle is based on the aggregation of suspended solid particles in solution to form large agglomerates, which are called as “flocs.” The formed flocs further aggregate into larger flocs through Van der Waals attractions and they settle out of the solution by separating the clean water and suspended solids (contaminants in water).³⁵ Depending on their chemical structure, flocculants can be synthesized with different surface charges, such as nonionic, anionic, or cationic, and each leads to various applications. Polyacrylamides (PAMs) are among the most commonly used polymers in flocculation applications and can be synthesized with different surface charges through various modifications. Nonionic PAMs stand out due to their hydrophilic nature and can be synthesized through the polymerization of the acrylamide monomer via free radical polymerization with the help of an initiator (Figure 4a). Since the resulting long-chain continuous polymer structure does not contain any functional groups on its backbone, it is referred to as nonionic PAM, and it is widely used in various water remediation applications.³⁶ Anionic polyacrylamide carries negative charges on its backbone and can be synthesized in different ways. One such method involves the alkaline hydrolysis of amide groups on nonionic polyacrylamide using sodium hydroxide (NaOH). In the presence of heat and NaOH, a portion of the amide groups on the polymer chain undergoes hydrolysis by forming negatively charged functional groups³⁷ (Figure 4b). However, the flocculants synthesized via alkaline hydrolysis result in non-uniform charge distribution along the polymer chain and may limit the separation efficiency during flocculation. Some studies in the literature indicate the effect of negative charge distribution along the polymer chain on flocculation performance. Charge distribution is an important parameter for flocculant synthesis, as well as the molecular weight of the polymer and degree of hydrolysis. As another option, Feng et al. synthesized block copolymers using acrylamide and sodium allyl sulfonate and used them as anionic flocculants for industrial wastewater treatment. They observed that the synthesized polymer formed compact and large flocs while exhibiting a highly efficient flocculation performance.³⁸ For this reason, the use of copolymers synthesized from acrylamide and acrylic acid monomers as anionic flocculants is also frequently reported in the literature (Figure 4c).

Moreover, cationic polyacrylamides carry positive charges along their backbone, and they are synthesized through the reaction of amide groups on formaldehyde with the secondary amine groups on monomers. As a result, a polymer structure with tertiary amine is formed and becomes positively charged under acidic conditions. The presence of hydrophobic groups on the cationic flocculants makes them favorable for the removal of organic substances from the wastewater, such as oils and dyes³⁹ (Figure 4d).

The working principle of flocculants is based on three main mechanisms: charge neutralization, particle adsorption onto the polymer chain, and bridge formation between target particles to create large flocs. In an aqueous system, target particles become destabilized through charge neutralization and interact with the opposite charges on the polymer chain via adsorption. Subsequently, they become trapped within the long polymer chains, agglomerating to form large flocs.³⁸ Both anionic and cationic flocculants work in similar ways during the flocculation process. After the interaction of opposite charges, the target particles are adsorbed on the chain, and the polymer forms a bridge between target particles by forming large flocs and facilitating separation. The only difference between cationic flocculants is that the hydrophobic groups are on their chain because there is minimal interaction with the aqueous solution, and they interact with organic structures present in the system. This results in the formation of larger flocs, making the flocculation process more efficient and faster.⁴⁰ Figure 5 schematically represents the working mechanisms of flocculants in the presence of target molecules.

The use of flocculation in water remediation occurs in three main steps: the selection of flocculant, its reaction with target contaminants, and the separation of flocs from water through sedimentation. Among the many natural and synthetic polymers available today, polyacrylamide-based flocculants are the most widely used ones in water remediation applications. They function as “thickeners” in municipal wastewater treatment plants by aiding in the removal of suspended solids. Once the polymer is separated from the system, clear and purified water is obtained.

Nonionic flocculants are widely utilized due to their high hydrophilicity and ease of synthesis. Vanotti et al. used synthesized nonionic flocculants in combination with sand drying beds in municipal wastewater treatment and significantly improved treatment performance. This method removed 97% of the total solids from the wastewater while reducing the post-process discharge time of drying beds to just 1 hour.⁴¹ Similarly, Yang et al. achieved wastewater treatment in high-phosphorus hematite flotation effluents using nonionic acrylamide flocculants. These wastewater types are known to be difficult to treat due to their high solid and organic content, but in this study, the turbidity value was successfully reduced to 0.8 NTU using nonionic flocculants.⁴² Subramanian et al. synthesized various copolymer microgels using diallyldimethylammonium chloride (DADMAC) and acrylamide monomers. These copolymers were applied for the treatment of colloidal suspensions containing titanium dioxide, and the copolymers with 30% DADMAC content achieved the highest turbidity removal efficiency.⁴³ Additionally, Amuda and Amoo utilized nonionic polyacrylamides in industrial wastewater treatment, recording a 97% reduction in total solids.⁴⁴

On the other hand, anionic polyacrylamides have been used in wastewater remediation due to the advantage of the interaction between negative charges on the polymer and positive charges on the target particles. In a study by Michaels, anionic polyacrylamide was used to treat clay-containing suspension solutions. It was reported that as the molecular weight of the flocculant increased, the number of hydrolyzed groups also increased, leading to improved flocculation performance in high-molecular-weight polymers.⁴⁵ Kurenkov et al. synthesized anionic flocculants by hydrolyzing high-molecular-weight polyacrylamides at low temperatures and applied them in wastewater treatment. They reported that when the degree of hydrolysis reached 19%, the turbidity values of the solution were minimized.⁴⁶ Amuda et al. used anionic polyacrylamide for the treatment of Abattoir wastewater in Nigeria and reported a 94% reduction in total solids.⁴⁴ Martin et al. applied anionic polyacrylamides in industrial wastewater treatment and achieved a 72% reduction in turbidity.⁴⁷ In another study, Hamza et al. synthesized an anionic polyacrylamide-based copolymer flocculant and reported a 99.42% reduction in turbidity.⁴⁸

Similar to anionic polyacrylamides, cationic polyacrylamides synthesized with positive charges are also used in wastewater treatment, particularly for the separation of organic content and carbon in wastewater. Liu et al. synthesized chitosan-grafted polyacrylamide-itaconic acid flocculants in cationic forms via UV-initiated graft copolymerization and investigated their dye removal performance from wastewater. They reported 81.6% dye removal efficiency with 0.7 mg/mL use of polymer at alkaline pH.⁴⁹ In another application, Irfan et al. used cationic polyacrylamides for the treatment of wastewater from paper mills and reported a 95% reduction in turbidity.⁵⁰ Cheng et al. used cationic polyacrylamide synthesized via inverse emulsion polymerization as a flocculant in kaolin suspensions.⁵¹ Sun et al. compared the performance of a cationic and branched flocculant synthesized through vinyl polymerization using acrylamide and acryloxyethyltrimethylammonium chloride with a nonionic branched polyacrylamide flocculant. They reported that the cationic flocculant performed better.³⁵ In another study, Yang et al. synthesized a cationic flocculant using acrylamide, dimethyl diallyl ammonium chloride, and butyl acrylate monomers through free radical polymerization. The presence of hydrophobic groups on the polymer chain reduced its interaction with the solution, increasing the polymer’s hydrodynamic size and, consequently, the solution’s viscosity. In a study on the separation of oil molecules from aqueous solutions using the synthesized cationic polymer, 93.4% oil separation was achieved with 50 ppm polymer.⁴⁰