Water Pollution control using electrospun fibers

Potential application of electrospun fibers along water treatment process.

Easy functionalization, high surface area to volume ratio and interconnected fibrous nonwoven membrane has given electrospun nanofibers the potential to be used on various aspects of pollution control. As a non-functionalized membrane, electrospun fibers can be used as a pre-filter for removing microparticles. Electrospun fiber membrane may also be used as a supporting substrate for casting of a thin film such that the assembly may be used for ultra and nano-filtration. Functionalized fibers can be used for detection of pollutants, removal of pollutants by adsorption and even neutralization of toxic chemicals. Given the multi-faceted treatment capability of electrospun membrane, it has been investigated for use in industrial water treatment.

Detection

High surface area of electrospun nanofibers make it highly sensitive for detection of selected industrial pollutants. Given that its cross-sectional diameter is in the hundreds of nanometer, the targeted pollutant will easily gain access to the reactive component on or within the nanofibers. Pule et al (2015) constructed an electrospun fiber composite with polystyrene as the matrix for encapsulation of gold nanoparticles to be used in the detection of 17β-estradiol in dairy effluents. In the presence of 17β-estradiol, the gold nanoparticles within the matrix start to cluster which changes the color of the nanofibrous strip from white to pink. At higher concentration, the surface plasmon resonance band shift to a longer wavelength turning the color blue from pink. Color change was observable at a concentration of 100 ng/ml and the color change to pink is specific to 17β-estradiol with other compounds such as cholesterol, p,p'-DDE, deltamethrin, 4-tert-octylphenol and nonylphenol turning the probe brown.

Electrospinning may also be used to coat a probe with nanofibers for solid phase microextraction (SPME) which works by adsorption of analytes from the water and subsequently transferring the analytes into a gas chromatography for analysis. Bagheri and Roostaie (2014) coated a stainless steel wire with a modified silica-polyamide nanocomposite for this purpose. The team used selected chlorobenzenes as model compounds for extraction and determination from water sample. The limits of detection of the electrospun SPME was 10 ng/L and a relative standard deviation of 7%. Extraction efficiency is comparable to other commercial fibers although the exposure time for electrospun fibers was only 15 minutes while the exposure times by other researchers was 60 minutes. Roostaie et al (2022) used ionic liquid (IL) doped electrospun polybutylene terephthalate (PBT) fiber membrane for solid phase microextraction (SPME) of preconcentration and isolation of selected analytes. 1-butyl-3- methylimidazolium chloride was mixed into PBT solution and electrospun into fibrous membrane for capturing of chlorophenols from aqueous samples. At optimum IL loading of 12 wt% into the PBT membrane, the lower limit of detection for the chlorophenols was 0.75-5 ng L^-1 and the limit of quantification of 5-15 ng L^-1 with equilibrium time of 15 min. The membrane was able to undergo repeated extraction and desorption cycles of more than 115 times without significant change in extraction efficiency of the chlorophenols. Hence, the IL/PBT electrospun membrane has a potential use as SPME for chlorophenols.

Detection of oil contaminants may be carried out using electrically conductive polymer composites (ECPC). This works by measuring an increment in the ECPC resistivity due to diffusion of oil into its matrix which causes it to swell. As there is a finite amount of conductive fillers in the matrix, the swelling increases the space between the conductive fillers and therefore increases the resistivity of the material. Moghaddasi et al (2019) used this method for the detection of vegetable oil in water. In their study, electrically conductive nanocomposites based on styrene-isoprene-styrene copolymer (SIS) and multiwall carbon nanotubes (MWCNT) fabricated by electrospinning were used. Their electrospun nanocomposite fibers were able to detect a steep increase in relative resistivity after 7 min for 50 ppm of oil in water. Increasing level of oil reduces the time taken for detection and 1000 ppm oil in water was detected after 4 min. Higher concentration of oil causes the SIS fibers to swell at a faster rate hence the faster detection. Note that at oil levels lower than 50 ppm, it was not possible to obtain a stable reading due to low changes in resistivity.

Filtration

The typical pore size of electrospun membrane is less than 5 µm. Filtration by electrospun membrane is combination of screen filtration where particles are trapped on the surface or depth filtration where the particles are being trapped within the network of fibers. Shirazi et al (2013) tested the performance of electrospun polystyrene membrane for treatment of biodiesel water-washing effluent. The polystyrene membrane was only given heat treatment to improve its stability and dimensional stability thus removal of pollutants is purely through physical means. Using a membrane with fiber diameter of 1.35 µm and mean pore size of 0.79 µm, chemical oxygen demand (COD), Biological oxygen demand (BOD), Total solid (TS), Total dissolved solid (TDS) and Total suspended solid (TSS) were reduced by 75%, 55%, 92%, 96% and 30% respectively. The effluent is a complex emulsion system of water, glycerol, biodiesel, un-reacted oil, soap and catalyst with the water, soap and pollutants forming agglomers which are larger than the pore size of the membrane. Thus the membrane is able to remove the agglomers through size exclusion.

Pollutant adsorption

A major class of pollutant in the river system is the heavy metals which mainly come from industrial discharge. Large surface area of nanofibers enables more functional groups to be exposed to the pollutants for their removal. Where the base material does not contain appropriate functional group, functionalization method such as blending and chemical treatment may be used. A wide range of functional molecules has been incorporated into electrospun nanofibers for pollutant adsorption. Lee et al (2013) loaded polymethylmethacrylate (PMMA) with Rhodanine (Rhd) through blending and tested its performance for the removal of Ag (I) and Pb (II) ions through dead-end filtration. The highest adsorptivity value was found to be 65.1% and 60.4% of the initial silver ion and lead ion concentration respectively at 10 s. Comparing the metal ions adsorption capability with Rhodanine powder, it was found that Rhodanine powder performed better in Ag (I) ion adsorption but poorer in Pb (II) ion. The reason for the poorer performance of Rhodanine powder in the adsorption of Pb (II) ion compared to Rhodanine loaded nanofibers is not apparent although it has been attributed to the greater surface area of nanofibers. The membrane can be recovered by washing in nitric acid solution. Rad et al (2014) used NaX nanozeolite for blending with polyvinyl alcohol (PVA) solution to form electrospun PVA/zeolite nanofibers for the adsorption of Ni²⁺ and Cd²⁺ ions. The time of adsorption equilibrium was found to be 60 minutes with greater adsorption capacity for Cd²⁺.

Table 1: Electrospun materials for adsorption of pollutants.

Material	Chemical	Adsorption Capacity	Reference
PVA/zeolite nanofibers	Ni2+	342.8 mg/g	Rad et al 2014
PVA/zeolite nanofibers	Cd2+	838.7 mg/g	Rad et al 2014
Rhodanine/PMMA	Ag+	125.7 mg/m2	Lee et al 2013
Rhodanine/PMMA	Pb2+	140.2 mg/m2	Lee et al 2013
Mesoporous polyvinyl pyrrolidone (PVP)/SiO2	Cr3+	97 mg/g	Taha et al 2012
Vinyl-modified mesoporous poly(acrylic acid)/SiO2	malachite green	240.49mg/g	Xu et al 2012
Polyvinylidene fluoride (PVDF) blend (m-PEI/PVDF)	methyl orange	633.3 mg/g	Ma et al 2016

To further increase the surface area of the nanofibers, surface pores or mesopores can be introduced to improve its adsorption capacity. Taha et al (2012) fabricated electrospun amino functionalized mesoporous polyvinyl pyrrolidone (PVP)/SiO₂ composite nanofiber membranes for the removal of Cr³⁺ ions. They were able to remove more than 97% of the Cr³⁺ ions from the spiked solution after 20 minutes.

Apart from the adsorption of metal ions, electrospun fibers have also been used for adsorption of organic molecules such as dyes. Xu et al (2012) tested the adsorption capacity of vinyl-modified mesoporous poly(acrylic acid)/SiO₂ composite nanofiber membranes for the adsorption of malachite green, a triarylmethane dye used for materials such as silk, leather and paper. Creation of the pores is by removal of cetyltrimethyl ammonium bromide (CTAB) from the electrospun fibers. The fibrous membrane has an adsorption capacity of 240.49mg/g and retained good removal rate for the first three cycles of regeneration. However the removal rate was reduced to about 44% after six cycles of regeneration. Ma et al (2016) tested the adsorption performance of polyethylenimine (m-PEI) and polyvinylidene fluoride (PVDF) blend (m-PEI/PVDF) for anionic dyes using methyl orange (MO) as the model. In neutral pH, a maximum adsorption capacity of 633.3 mg/g was recorded for a nanofibrous blend containing 49.5% m-PEI which is much higher than previously reported adsorbents. The high adsorption capacity was attributed to the swelling of hydrophilic m-PEI in water which increases the diffusion rate of the dye and adsorption capacity. The pH has a significant influence on the adsorption capacity of m-PEI/PVDF fibrous mat with higher pH leading to reduced adsorption capacity. PEI is a cationic active polymer at low pH which attracts anionic dyes. At high pH, the captured anionic dyes are released which regenerates the membrane. Desorption efficiency of the membrane using NaOH solution was about 87%.

A disadvantage of using blending to functionalize the nanofibers is that the functional molecule may be shielded from the polluting molecules. Greater exposure of the functional group on the surface of the nanofibers will allow greater utilization of the functional groups and corresponding removal of the pollutants. Li et al (2014) used a three steps process to introduce Fe_x O_y onto the surface of electrospun polyamides 6 (PA6)/Chitosan. First electrospinning was carried out to produce polyamides 6 (PA6)/Chitosan nanofibers. The membrane is then dipped in FeCl₂ solution to chelate Fe(II). Finally, pyrolysis was carried out to grow iron oxide nanoparticles on the surface of the nanofibers. The resultant PA6/Chitosan/ Fe_x O_y nanofibers were successfully used for the removal of Cr(VI).

In field application of pollutant adsorption by electrospun membrane, it is important to note that the water to be treated would also contain other contaminants which may affect the actual efficiency of pollutant removal. Xu et al (2015) tested the efficiency of triclosan removal using three hydrophobic electrospun membranes in controlled condition and simulated polluted water condition. Triclosan is a commonly used antimicrobial agent found in personal care products such as shampoo, shower gel and toothpastes which are washed away into the sewage system. Under controlled condition, methoxy polyethylene glycol-poly(lactide-co-glycolide) (MPEG-PLGA), poly(D,L-lactide-co-glycolide) (PLGA) and poly(D,L-lactide) (PDLLA) were able to achieve about 90% triclosan removal. The presence of dissolved organic matter (DOM) commonly found in sewage and polluted water will reduce the removal efficiency of triclosan due to competitive adsorption. Despite this, the membranes were still able to achieve removal efficiencies of more than 60%.

Heavy metal ions are often removed from the water through adsorption following by a regeneration process to recover the membrane for subsequent usage. For other polluting compounds, it is possible to remove them through neutralization or catalytic degradation to render the compound harmless.

Chemical Neutralization/ Catalytic degradation

An advantage of treating polluted water through catalytic degradation of harmful compounds is that there is no need for the membrane to undergo a recovery process. However, the presence of ultra violet light may be needed to activate the catalytic activity of the inorganic material. TiO₂ is well known for its photocatalytic property. Reduction of TiO₂ or other inorganic material to the nanoscale will significantly improve its photocatalytic performance but it is a source of pollutant itself if it is introduced into the water. In contrast, nanofibers are structurally sound and it can be used as a carrier for inorganic nanoparticles. Rajak et al (2015) investigated the photocatalytic property of nanofiber composite with Styrofoam as the matrix nanofiber and TiO₂ nanoparticles as the active compound. TiO₂ nanoparticles blended into the nanofiber matrix was found to reduce the blue textile dye by about 60% after 30 hours of irradiation. An et al (2014) constructed a composite nanofibrous membrane by simultaneously electrospinning nylon and electrospraying titania nanoparticles. The electrospun nylon fibers act as a carrier for the titania nanoparticles and this process ensures that the nanoparticles are on the surface of the fibers for maximum exposure to the pollutant. The composite membrane was shown to be capable of significantly reducing the toxicity of 2,4-dichlorophenol and 2,4,6-trichloro- phenol when tested against D. magna. To improve photocatalytic rate, Lu et al (2019) used a hydrothermal process to grow ZnO nanorod projections from electrospun polyimide (PI) and polyimide/Ag fibers. Due to the presence of the surface projections, this significantly increase the surface area of the membrane in contact with the dye. A 98% photocatalytic degradation of methylene blue (MB) solution was achieved for a UV irradiation duration of 120 min using a hybrid, ZnO and Ag projections on PI/Ag nanofibers membrane. The presence of Ag on ZnO has been shown to significantly increase the membrane photocatalytic activity. ZnO on PI and ZnO on PI/Ag (Ag in PI fibers) was able to achieve a photocatalytic degradation rates of 84% and 87% respectively. The synergistic effect of Ag on ZnO has been attributed to the Ag nanoparticles functioning as an electron sink that reduce the recombination of the photoinduced electrons and holes.

There are growing interests in metal organic frameworks (MOF) as photocatalysts due to its high surface area and high photocatalytic activity under visible light irradiation. However, MOF is seldom used as it is due to difficulty in removal from the aquatic system and aggregation which would limit its activity. Electrospun fibers with its high surface area is an attractive carrier for MOF. Koushkbaghi et al (2023) used polyacrylonitrile (PAN) as the carrier for UiO-66-NH₂/TiO₂ composites MOF. To construct a layered membrane for the removal of Cr(VI) and phenol from water under visible light, polyethersulfone (PES) was electrospun to form a base layer. The PAN/MOF blend was then electrospun onto the PES base layer. The optimum pH for removal of Cr(VI) and phenol was found to be pH 2 and pH 3 respectively. At lower pH, reduction of the Cr anions improves its electrostatic attraction to MOFs. In the absence of visible light, the removal efficiencies of phenol and Cr(VI) using PES/PAN/UiO-66-NH₂/TiO₂ 5% were 84.9 and 77.3% at 120 mins. Under visible light, the maximum removal efficiencies of phenol and Cr(VI) increased to 92.7 and 96.3% respectively. The nanofibrous membrane may be regenerated using HCl without significant loss in removal efficiencies after five cycles.

Pure inorganic nanofibers may also be fabricated by electrospinning of the precursor solution. Singh et al (2013) fabricated ZnO nanofiber by electrospinning of polyacrylonitrile and zinc acetate solution followed by sintering to generate pure ZnO nanofiber. The photocatalytic activity of ZnO nanofibers was studied by the degradation of naphthalene and anthracene dyes. It was found that photocatalytic activity of ZnO requires the presence of light especially towards ultraviolet range. There was no drop in the photocatalyic activity of the membrane and its physical structure with 6 cycles of testing and washing. MXene is a transition metal nitride/carbon nano-2D layered material that is popular after graphene due to its extraordinary physical and chemical properties and layered structure. Huang et al (2019) investigated the photocatalytic property of a novel poly(vinyl alcohol)/poly(acrylic acid)/Fe₃O₄/MXene@Ag nanoparticle composite nanofiber membrane with electrospinning for wastewater treatment. This composite membrane was tested against nitro compounds, 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA) which are harmful to humans and environment. Under UV-vis spectroscopy at room temperature, 4-NP and NaBH4 mixture was unchanged for 24 h without catalyst indicating that the reaction did not occur. In the presence of poly(vinyl alcohol)/poly(acrylic acid)/Fe₃O₄/MXene@Ag composite nanofiber membrane, 4-NP was completely reduced and catalyzing 4-NP could be regarded as the pseudo-first-order reaction. The catalytic reaction of the composite nanofibers to 2-NA could also be considered as a pseudo-first-order reaction. Wang et al (2021) demonstrated the photocatalytic performance of graphitic carbon nitride (g-C₃N₄)/niobium pentoxide nanofibers (Nb₂O₅ nanofibers on photodegradation of rhodamine B and phenol under visible light irradiation. The structure of g-C₃N₄ is known to offer rapid photoinduced charge separation but its photocatalytic activity is reduced due to fast recombination of the photogenerated electron/hole pairs. The well matched band gap edges between Nb₂O₅ and g-C₃N₄ encourages charge carrier separation when used together. This significantly improves the photocatalytic performance over individual material. Nb₂Cl₅/ g-C₃N₄/PVP precursor solution was prepared and electrospun into nanofibers. Calcination of the precursor fibers gave nanofibers. UV-Vis absorption spectra results showed that g-C₃N₄ absorbs light in the visible region but pure Nb₂O₅ absorbs light in the UV spectra. The combination of Nb₂O₅/ g-C₃N₄ shifts the absorption edge into the visible light range. The band gap of the Nb₂O₅/ g-C₃N₄ is also narrower than pure Nb₂O₅ which favors photocatalytic activity in visible light. When the electrospun Nb₂O₅/ g-C₃N₄ nanofibers were tested with Rhodamine B (RhB), a photodegradation efficiency of 98.1% was recorded after 120 min of visible light irradiation and complete photodegradation of phenol over the same period . There was no apparent reduction in photocatalytic activity after four cycles of photodegradation and washing, demonstrating the stability and reusability of the composite membrane.

Apart from man-made material, natural organisms may also be introduced with the help of electrospun nanofibers to remove water contaminants. To demonstrate the use of bacteria immobilized on electrospun membrane in wastewater treatment, San et al (2014) immobilized three types of bacteria (Aeromonas eucrenophila, Clavibacter michiganensis and Pseudomonas aeruginosa) on cellulose acetate electrospun membrane. Using methylene blue as the contaminant, they showed that all three bacteria were able to achieve more than 95% removal efficiency which is comparable to removal from free bacteria. However, more works need to be done to reduce the amount of bacteria lost after subsequent washing step as the efficiency drops to below 50% after the forth cycle of washing. To avoid loss of bacteria, another method is to encapsulate the bacteria within the fiber matrix.

For practical utilization of inorganic nanofibers, a supporting base substrate is preferred to ensure the integrity of the material remains sound. Ramasundaram et al (2013) used a combination of stainless steel filter, poly(vinylidene fluoride) nanofibers and TiO₂ nanofibers to form a photocatalytic filter media for degradation of pharmaceutical compound, cimetidine. In this setup, the poly(vinylidene fluoride) nanofibers serve as a binder to adhere TiO₂ nanofibers to the stainless steel filter. A hot press was used to combine the three layers together for the test. The filter media was tested by passing the spiked solution through the filter. At a flow rate of 10 L/m²h, the degradation of cimetidine was 90% in the presence of ultraviolet light and the photocatalytic efficacy was maintained for 72h. Thus this setup has the potential for use in a waste water treatment for removal of organic compounds.

Pathogen removal

Small pore size of electrospun fibrous membrane may be effective as a barrier against bacteria entry. Lev et al (2012) tested the performance of electrospun polyurethane nanofibers on polypropylene substrate for removal of E. coli from water. At higher nanofibrous layer weight of 3.8 g/m2, exclusion of the bacteria was found to be comparable with commercial water treatment membranes. With insufficient membrane thickness, the nanofibers may shift position resulting in expanding pore size and bacteria leakage. Bjorge et al (2010) found that the removal of pathogens (culturable micro-organisms and coliform bacteria) by electrospun polyamide unsupported membrane is just 1.5 log₁₀ while commercial microfiltration membranes are in the range of 2 log₁₀ to 4 log₁₀. With a supporting layer, the electrospun membrane was able to reach a removal efficiency of 2 log₁₀. To reach higher removal efficiency, silver nanoparticles and commercial biocide are added to the nanofiber membrane. This improves the removal efficiency to 4 log₁₀ and 6 log₁₀ respectively.

Biofilm formation prevention

A constant challenge faced by wastewater treatment technology is the formation of biofilm on the surface of the membrane. Biofilm will significantly reduce the useful life of the membrane as it prevents water from flowing through the membrane. One way of reducing biofilm formation is to inhibit microbial growth on the membrane. Wang et al (2016) constructed cellulose acetate nanofibrous membrane loaded with silver nanoparticles for dye wastewater treatment and exhibiting antibacterial property. Their study showed that the presence of silver nanoparticles does not affect the adsorption of rhodamine B aqueous solution. The membrane also demonstrates bacteria inhibition compared to cellulose acetate nanofibrous membrane without the nanoparticles.

Electrospinning of materials that are naturally nonfouling due to its resistance to nonspecific protein adsorption may also prevent bacteria adhesion. Zwitterionic polymers which contain both negatively and positively charge moiety are known to exhibit nonfouling properties. Poly (sulfobetaine methacrylate) (polySBMA), which is a zwitterionic polymer has been successfully electrospun by Emerick et al (2013). Their study showed that a maximum molecular weight for obtaining smooth polySBMA fibers was 420 kDa. Beyond this, fibers were melted at the intersections between them. However, further studies are required to determine the resistance of the membrane to fouling.

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