Water pollution control by adsorption using electrospun fibers

Water pollution is a major concern in many region with industrial and agricultural presence. Release of pollutants such as dyes, chemicals and heavy metals into rivers and streams further diminishes the availability of usable fresh water. 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.

A major class of pollutant in the river system is the heavy metals which mainly come from industrial discharge. 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⁺. Yang et al (2017) loaded imino-acetic acid (IDA) type polymer synthesized from polyethyleneimine (PEI) into polyvinyl alcohol (PVA) solution for electrospinning. The resultant composite fibers were cross-linked using glutaraldehyde (GA) vapor to reduce its dissolution in water. The chelating nanofibers were found to adsorb over 80% of lead and copper at low pH of 2 to 3. Optimum adsorption of Cu(II) was at pH 3.0 while Pb(II) was at pH 5.5. More than 70% of the total metal ions adsorption occurred within the 10 minutes. Metal ions adsorption may be achieved using bacteria immobilized on electrospun fiber membranes. Páez-Vélez et al (2020) constructed an electrospun polycaprolactone (PCL) membrane (fiber diameter 3.5 µm) with Lysinibacillus sphaericus immobilized on its surface for the purpose of capturing gold from industrial wastewaters. Immobilisation of L. sphaericus is by dipping the electrospun PCL membrane into the bacteria suspension. Due to the fibrous architecture, L. sphaericus was able to adhere well on the membrane. Sonication and repeated washing was used to remove any non-Immobilized bacteria. The resultant biocomposite was shown to be able to remove 93% of gold ions in the aqueous medium after 120 h while neat PCL was only able to remove 57%. This showed that the presence of L. sphaericus on the electrospun fibers contributed significantly to the removal of gold from the water. To increase the metal ions capturing efficiency, Páez-Vélez et al (2020) set up a filtration system where the contaminated water cycles through the biocomposite membrane. After 15 cycles of filtration which took about 3 hours, 51% of the gold was removed from the water sample. The pH of the polluted water may have a significant influence on the adsorption ability of the filter towards ionic pollutants. For Cr (VI) ion in solution, it may exist as chromate (CrO₄^2-), dichromate (Cr₂O₇^2-), and hydrogen chromate (HCrO₄^-) depending on the pH of the solution. At pH lower than 6.8, HCrO₄^- is stable while Cr₂O₇^2- is stable above that. Protonated amine uses electrostatic attraction to bond Cr (VI) thus there will be greater affinity at lower pH in filters with amine functional groups. At higher pH, there is negative charge on the adsorbent surface which reduces its attraction towards negatively charged Cr (VI) anions [Parlayici et al 2019]. Parlayici et al (2019) constructed nylon-6,6 (N6,6)/graphene oxide (GO) Nanofibers through electrospinning for removal of Cr (VI) in polluted water. Maximum adsorption capacity is recorded at pH 2 and the Langmuir model equation identified the maximum capacity as 47.17 mg/g. For metal ions, the pH of the water is an important parameter for its adsorption as it affects both the adsorbent and absorbate. In the case of using electrospun Fe₃O₄ fibers as adsorbent for Pb(II), low pH causes its surface functional groups to protonate [Shin et al 2020]]. This causes coulombic repulsion between the positively charged surface and Pb(II) ions. As the pH value increases, the reduction in H+ ions encourages greater adsorption of Pb(II) ions on Fe₃O₄ fibers. At pH above 6, the bivalent Pb(II) ions gradually transformed to Pb(OH)+ and this too got adsorbed onto Fe₃O₄ fibers. At pH 5, adsorption of Pb(II) ions on both Fe₃O₄ fibers and powders reached equilibrium in about 60 min with maximum adsorption capacity of 16.78 mg/g and 15.80 mg/g respectively. The poorer adsorption by the powders may be due to agglomeration in water while electrospun fibers maintain its porous network. The Fe₃O₄ absorbents may be regenerated by washing in NaOH solution. Fe₃O₄ fibers and powders retained 95.3% and 89.5% of the original adsorption capacity respectively after six adsorption-desorption cycles. The reduction in Fe₃O₄ powders performance may be due to powder loss during the washing process therefore demonstrating the better practical usability of Fe₃O₄ fibers.

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
PVA/imino-acetic acid	Pb2+	111.15 mg/g	Yang et al 2017
PVA/imino-acetic acid	Cu2+	101.28 mg/g	Yang et al 2017
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

Guo et al (2021) constructed an electrospun metal-organic frameworks (MOF) hybrid nanofiber membrane for the removal of As(III) and As(V) from water. Synthesized Zr-based MOF (UiO-66) was blended into a polyacrylonitrile (PAN) solution for electrospinning into nanofiber membranes. The optimum concentration of UiO-66 loaded into the PAN nanofibers was found to be 10 wt% and the MOF was evenly distributed throughout the fibers. Above this concentration, irregular knots were found on the fibers and these are probably aggregates of the MOF. The maximum adsorption efficiency for arsenic was found to be 93% at 10 wt% loading which was close to that of pure MOFs and the maximum adsorption capacities for As(V) and As(III) were 42.17 mg/g and 32.90 mg/g, respectively. With the nanofiber diameter at 324 nm, the MOF within the PAN matrix and the As ions may be sufficiently close for efficient adsorption. Maximum adsorption occurs at neutral pH with a reduction of up to 20% at lower and higher pH. The UiO-66/PAN nanofibers membrane was found to be stable with no detectable MOF leakage after five cycles of recycling. However, there was a 28.5% reduction in the adsorption capacity for As(III) relative to its initial efficiency. The membrane is still good enough for reuse with the reduced capacity.

Where adsorbents were encapsulated within the electrospun fiber matrix, the matrix may hinder adsorption of targeted environmental pollutants. One way of increasing exposure of the pollutants to the embedded adsorbents is to stimulate swelling of the matrix. This method causes the polymer macromolecular chains to open up which potentially create more paths for the pollutants to enter the matrix and expose more active sites on the MOF for adsorption. Guo et al (2022) tested the effectiveness of this method using Zr-based MOF (UiO-66) blended into polyacrylonitrile (PAN) solution. Earlier report of UiO-66/PAN adsorption of arsenate was 93% [Guo et al 2021] but in this study, the untreated UiO-66/PAN adsorption of arsenate was just 50.8%. To improve adsorption of arsenate, electrospun UiO-66/PAN membrane was soaked in swelling solvents. There were no observable changes in the fiber morphology after treatment in the swelling solvents. Using pyridine as the swelling solvent, the adsorption efficiency of UiO-66/PAN reached 75.7% after soaking the membrane for 1h in the solvent. No Zr was detected in the swelling solvent or the test solution which demonstrates the stability of MOF in the PAN nanofiber membrane.

The impact of the pH of the water on the adsorption of metal ions by the adsorbent may be reduced by deliberate and careful material selection. Niu et al (2022) constructed a nano zero-valent iron (nZVI) doped electrospun carbon nanofiber (CNF) composite for aqueous hexavalent chromium removal. Preparation of this material was by electrospinning a blend of FeCl3 and polyacrylonitrile (PAN) solution into nanofibers. Carbonization process was carried out at higher temperature of 800 °C for the reduction of PAN into carbon nanofibers while FeCl3 was reduced by carbon to nano zero valent iron nanoparticles in the absence of oxygen. The presence of nZVI in the CNF significantly increases the adsorption of Cr(VI). Pure CNF had a low removal efficiency of about 10%. At a high concentration of 60% nZVI-CNFs, the removal efficiency increased to 91.5%. In more acidic conditions, removal efficiency was higher due to the reduction of Cr(VI) and reduction in the surface passivation of iron. In alkaline conditions, the removal efficiency drops but still at a high efficiency of 81% which may be due to the stability afforded by carbon coating. The ability of the nZVI/CNF composite to adsorb Cr(VI) over a wide pH range makes it suitable for use in complex environments where pH may fluctuate from acidic to alkaline.

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. Jang et al (2020) tested the adsorption performance of electrospun polyacrylonitrile (PAN) fibers loaded with cetyltrimethylammonium chloride (CTAC) modified exfoliated graphene oxide (GO) for methylene blue (MB) and methyl red (MR) dyes in the aqueous system. GO with its various oxygenated functional groups and phenyl backbones is able to induce attractive forces to MB and MR dye molecules hence a higher loading of GO was shown to increase adsorption efficiency for both dyes. Surface modification of GO using CTAC was able to increase the loading capacity of PAN solution from less than 10 wt% to 30 wt% of GO without any detrimental effect to electrospinning and fiber formation. However, electrospun PAN membrane loaded with 30 wt% cGO was too brittle hence 20 wt% cGO was used instead. The electrospun PAN/cGO membrane demonstrated good adsorption efficiencies for both MB and MR dyes with a higher adsorption efficiency for MR molecules. This is attributed to better compatible polarity between the MR dye and membranes and its smaller molecule size which aids in the diffusion of the solution into the depth of the electrospun membrane. 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%. Ebrahimi et al (2022) demonstrated the use of electrospun polyacrylonitrile (PAN) decorated with silica nanoparticles (SiNPs) for adsorption of cationic dyes (CDs) of Malachite Green (MG) and Methylene Blue (MB) from aqueous solutions. PAN nanofiber membrane was first fabricated using electrospinning. Next, SiNPs mixed with diluted PAN solution were electrosprayed onto the prepared PAN nanofiber membrane to form SiNPS coated PAN nanofiber membrane. Nanofiber membranes with surface SiNPs offered several advantages over SiNPs blended into the PAN nanofiber matrix. Surface roughness of SiNPs coated PAN nanofibers is higher than the smooth surface of SiNPs blended PAN nanofibers. Hydrophilicity of SiNPs coated PAN nanofibers is also better as SiNPs are more hydrophilic and are exposed on the surface. The SiNPs coated PAN nanofibers have a dye removal efficiency of 98.3 and 83.9% at 25 mg/L solution flow rate within 7 min for MG and MB respectively. Maximum adsorption of MG is at pH 6 and the adsorption is significantly lower at low pH. This is due to the adsorbents acquiring a positive charge in the presence of excess H+ at low pH which creates an electrostatic repulsion with the dye molecules. The membrane may be recovered by washing in acid and the flux recovery ratio (FRR) of SiNPs coated PAN nanofibers was about 70 to 80%. Camiré et al (2019) tested the adsorption performance of electrospun polyvinyl alcohol (PVA) loaded with alkali lignin (AL) on pharmaceutical residues (fluoxetine (FLX), venlafaxine (VEN) , ibuprofen (IBU), and carbamazepine(CAR)) found on surface water. The electrospun PVA/AL membrane was able to achieve adsorption levels of 78 mg/g for fluoxetine compared to 5-10, 49 and 75-80 for unfunctionalized silica, zeolites and ion-exchange resins respectively. Recovery yield is good with more than 90% recovered through desorption of the drug. However, the adsorption affinity differs across the different contaminants with the highest affinity for FLX and in decreasing order, VEN, CAR and IBU. This is due to the contaminants molecular structure and chemical properties. Both FLX and VEN are alkaline pharmaceuticals that protonate at neutral pH. CAR and IBU are neutral and anionic at neutral pH which prevents any ionic bonding with lignin.

With organic pollutants, adsorbents may be incorporated with catalysts for photodegradation of organic compounds. Maafa et al (2023) constructed a carbon nanofiber membrane incorporated with NiTiO₃ and TiO₂ by electrospinning of their precursors followed by sintering. The base material used in the electrospinning was poly(vinylpyrrolidone) (PVP) which formed the carbon base and titanium isopropoxide and nickel acetate tetrahydrate was used as the precursors for TiO₂ and NiTiO₃ respectively. Activated carbon is known to be a good organic compound adsorbent and TiO₂ is the photocatalyst. By having both carbon and photocatalyst in the same nanofiber, dye molecules adsorbed onto the nanofiber can be degraded by the catalyst. The addition of NiTiO₃ formed a heterojunction with TiO₂ and this increased photodegradation of Methylene blue (MB) in visible light by 20% to 82% after 120 min compared to carbon nanofiber with TiO₂ only.

The adsorption kinetics of the material may also be influenced by the environment. The presence of impurities such as salt may affect the targeted molecule. Fendi et al (2018) showed that increasing NaCl and KCl in the solution containing methylene blue increases the adsorption of the dye by electrospun phenol-cresol formaldehyde/polystyrene membranes. Increasing ionic content in the solution may have encouraged aggregation of the dye molecules and facilitate adsorption to the membrane surface. Similarly, increasing the solution temperature also increases the adsorption capacity of the dye. This has been attributed to permeation of the dye molecules into the fiber material.

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). For maximum adsorption performance of the fiber, the material which made up the fiber is by itself, capable of adsorbing the targeted pollutant. Celebioglu et al (2017) was able to prepare a poly-cyclodextrin (poly-CD) solution using hydroxypropyl-β-cyclodextrin (HPβCD), 1,2,3,4-butanetetracarboxylic acid (BTCA), sodium hypophosphite hydrate (SHP) initiator for electrospinning. The resultant free standing poly-CD nanofibrous membrane demonstrated above 90% removal efficiency of concentrated methylene blue (MB) (40 mg/L) dye under high flux (3840 Lm^-2h^-1.

Filtration performance of poly-CD nanofibrous membrane. (A) The photographs of membrane cell part of HP4750 dead-end system and the cropped poly-CD nanofibrous membrane with a definite active filtration area (14.6 cm2). The schematic view of HP4750 filtration system. For each test, 50 mL solution is passed through the poly-CD nanofibrous membranes with a definite N2 pressure. Then, the permeated solution is collected in a clear beaker. (B) The visual illustration of the MB solutions prepared at two different MB concentrations (40 and 80 mg/L) before and after filtration test. The photographs and SEM images (scale bar-10 µm) of the poly-CD nanowebs exposed to these two concentrated MB solutions during the experiments. As clearly seen, both the macroscopic visual appearance and the fibrous morphology of poly-CD nanofibers were protected under such applied pressure [Celebioglu et al 2017].

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%.