Electrospun Membrane for Textile wastewater treatment

Dyeing, printing and finishing in textile wet processing generates a lot of wastewater and these have to be properly treated before it is being discharged into the environment. The type of contaminants found in the textile effluent includes suspended solids, colourings, high water salinity and compounds with high chemical oxygen demand (COD). Electrospun membrane has been shown to be very effective in various water filtration applications and this has led to investigations in its use for treating textile wastewater.

One area of textile wastewater treatment is the removal of suspended solids and the reduction of compounds with high COD. Affandi et al (2017) compared the performance of electrospun Nylon 6 membrane against commercial membrane in the removal of contaminants in textile effluent. The textile wastewater was taken directly at the end of a pigment padding process and contains pigments, acrylic binder and waxes. Their result showed that electrospun Nylon 6 membrane was able to remove 99% of suspended solids compared to 34% in commercial membrane. However, the permeate flux in electrospun membrane was slower than commercial membrane due to greater accumulation of suspended solid particles. The amount of COD reduction using electrospun membrane was also significantly better at 64% reduction compared to just 16% for the commercial membrane. While electrospun membrane was found to perform favourably in terms of particulate removal and reducing COD, more investigation is needed to reduce fouling of the membrane in order to maintain a high flux for practical utilization.

Large surface area of electrospun nanofibers allows more dye to be removed from the wastewater through surface adsorption. Yu et al (2018) has shown that electrospun nylon-6 (PA-6) membrane was able to remove indigo dye solution without any additional modifications to its chemistry. However, for complete removal of the dye, 10 layers of the prepared membranes were hot-pressed together for filtering. A single layer of electrospun membrane had a thickness of about 0.03 mm and ten layers of hot-pressed membrane had a thickness of about 0.12 mm. Examination of the membrane after performing the dead-end filtration showed cake formation on the upper surface. This cake formation was the result of solute concentration of the membrane surface being greater than its saturation solubility resulting in precipitation of the dye. Although the pores of the membrane was not completely blocked, the flux dropped rapidly in the first 200 mins and stabilises at above 15 L m^-2 h^-1 after 500 mins.

Electrospun fibers can be easily functionalized for adsorption of 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%. Celebioglu et al (2017) prepared 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].

Beta-cyclodextrin-based polymer (PCD) are interesting as it changed the adsorption uptake due to the cavity molecular structure of Cyclodextrin which captures specific molecules. Guo et al (2019) constructed electrospun composite fibers made of polycaprolactone (PCL) and PCD. A high loading of beta-cyclodextrin up to 50% of PCL was possible for electrospinning into fibers. With increasing beta-cyclodextrin loading, the dye removal capacity also increases. With PCL/(50%)PCD, the dye removal efficiency could reach 24.1 mg/g towards 4-aminoazobenzene and 10.5 mg/g for methylene blue (MB). Adsorption reutilization test on PCL/(50%)PCD's removal of MB using ethanol showed 78% retention of maximum dye removal efficiency after repeated adsorption over eight cycles.

Chitosan is a natural polymer and its high contents of amino and hydroxyl functional groups makes it suitable for removal of contaminants from wastewater. Ghani et al (2014) used a blend of chitosan and polyamide-6 for adsorption of anionic dyes, Solophenyl Red 3BL and Polar Yellow GN. However, since most of chitosan amino group is protonated in acidic condition, the pH of the wastewater has significant influence on its adsorption performance. It was found that pH 5 was the optimum pH level for the dye adsorption with about 95% removal.

Inorganic material, commonly used for heavy metal ion removal may also be used for dye adsorption. Fard et al (2018) prepared a nano-cellulose/α-Fe₂O₃ with electrospun derived α-Fe₂O₃ nanofibers as the base layer. Cellulose nanoparticles were incorporated onto the surface of α-Fe₂O₃ nanofibers by first soaking the α-Fe₂O₃ nanofibers in silica gel citric acid before surface reaction with cellulose nanoparticles. The addition of cellulose nanoparticles on the surface of the nanofibers increases its surface area and potentially improve adsorption performance. The resultant hybrid nanofibers were tested for adsorption of BR46 and BB41 dye solutions. Their result showed that there was an optimum amount of cellulose nanoparticles that can be incorporated onto the nanofibers, beyond which aggregation occured and resulted in reduced surface area. The reduction in surface area also corresponds to a reduction in dye removal. A maximum dye removal efficiency of about 95% was achieved at maximum surface area of 178.74 m²/g as measured using BET. Maximum adsorption of basic dyes occured at pH 9. In acidic pH, protonation of the nanofiber created a respulsive force between the dye and positively charged nanofibers which reduces dye adsorption. The hybrid nanofibers point of zero charge(pH_pzc) was about 5.3. At pH > pH_pzc, the surface charge is negative and at pH < pH_pzc, the surface charge is positive. Inorganic particles may be added to adsorbent polymer matrix fibers to enhance its performance. Fendi et al (2018) demonstrated this in electrospun phenol-cresol formaldehyde/polystyrene membranes when zinc oxide nanoparticles were added to it for removal of methylene blue. With the addition of ZnO nanoparticles, the rate of methylene blue adsorption is faster and there is also an increase in the adsorption capacity with both reaching saturation point less than 15 minutes. Adsorption capacity of ZnO loaded electrospun phenol-cresol formaldehyde/polystyrene membranes continues to perform better than membrane without ZnO in the presence of NaCl and KCl in the dye solution and at elevated temperature.

Photocatalytic degradation of organic dyes is an attractive method to remove polluting dyes as it is theoretically possible to use the membrane for longer periods compared to dye adsorption membranes before replacing. 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. TiO₂ is a commonly used inorganic material for photocatalytic degradation of dye. AlAbduljabbar et al (2021) tested the efficacy of degrading methyl orange with TiO₂ nanoparticles either adhered to the surface of electrospun polyacrylonitrile (PAN) fibers or blended within the PAN fibers. The electrospun PAN membrane with surface coated TiO₂ nanoparticles was constructed by electrospraying a suspension of TiO₂ nanoparticles over the prefabricated PAN nanofibers membrane. Comparing their photocatalytic activity, it was shown that PAN nanofibers membrane with surface coated TiO₂ nanoparticles has a much higher activity at 92% against the TiO₂/PAN blended sample at 49.6%. The difference in photocatalytic activity can be attributed to the greater exposure of surface TiO₂ nanoparticles to methyl orange. Having TiO₂ nanoparticles on the surface also increases the surface roughness and contributes to increased surface area of the membrane.

When a carrier is used for the photocatalytic material, factors such as the location of the photocatalytic material and the bonding between the photocatalytic material and carrier may have a strong influence on the photocatalytic activity. AlAbduljabbar et al (2021) used electrospun chitosan nanofibers as the carrier for TiO₂ nanoparticles. Chitosan in particular has been used for adsorption of dye [Ghani et al 2014] and its combination with photocatalytic nanoparticles may enhance the removal of contaminants in the water. Two methods of loading TiO₂ nanoparticles were being investigated. The first method involves surface functionalization of chitosan nanofibers followed by electrospraying to deposit TiO₂ nanoparticles on the surface of the nanofibers. In the second method, TiO₂ nanoparticles were mixed into a chitosan solution and electrospun into fibers. Comparison of their photocatalytic performance was determined by the degradation of methylene blue dye. With the TiO₂ nanoparticles on the surface of functionalized nanofibers, the photodegradation rate under UV light was 89.30% and a reaction rate constant k at 0.0088 min^-1. For TiO₂ nanoparticles that were blended into the chitosan nanofibers, the degradation efficiency under UV was only 40.26% and the reaction rate constant k was 0.0016 min^-1. This significant difference in performance may be attributed to TiO₂ nanoparticles on the surface of the fibers being more exposed to the UV light and in direct contact with the methylene blue molecules as compared to the blended system where the TiO₂ nanoparticles were embedded within the carrier matrix material. The band gap of the TiO₂ nanoparticles on the surface of functionalized nanofibers was also found to be smaller than the TiO₂ nanoparticles/chitosan blended fibers. Surface roughness of chitosan nanofiber with TiO₂ nanoparticles on its surface is also higher and this increases the exposed reaction site with the dye molecules.

Layer-by-layer deposition technique has been used to immobilize metal oxide on the surface of electrospun fibers. Sandua et al (2022) demonstrated this method by first electrospinning poly(acrylic acid) (PAA) and β-Cyclodextrin (β-CD). PAA functions as a polyanion due to its property of negative charge hence the electrospun PAA/β-CD forms the first layer. Metal oxide particles with their positive charge were first sonicated in water before the electrospun fiber membrane was dipped into it. Photocatalytic TiO₂ particles were able to adhere to the electrospun membrane due to the negative charges on PAA. Photocatalytic enhanced metal oxide particles such as WO₃ and Fe₂O₃ were added to PAA anionic solutions to form the next layer on the TiO₂ coated membrane to produce a functional coating. When irradiated with both UV and visible light, photocatalytic degradation of methylene blue with membrane containing TiO₂ only, TiO₂ and WO₃ only, and all three TiO₂, WO₃ and Fe₂O₃ was 65%, 75% and 85% respectively. Photocatalytic activity of TiO₂ depends on irradiation in the UV spectrum. The addition of WO₃ decreases the band gap and facilities photocatalytic activity under visible light. The presence of Fe₂O₃ reduced the band gap of TiO₂ thus increasing the photocatalytic activity under visible and solar light for dye degradation.

Where a polymer is used as a carrier for catalytic nanoparticles, the hydrophobicity of the carrier polymer needs to be considered especially when the nanoparticles are blended into the polymer matrix. Medeiros et al (2023) electrospun poly (butylene adipate-co-terephthalate) (PBAT) and poly (lactic acid) (PLA) fibers loaded with titanium dioxide-rutile (TiO₂-R) and iron oxide-magnetite (Fe₃O₄) particles for photodegradation of Reactive Red 195 dye. Photodegradation of the dye at 1440 min was 64% for TiO₂-R/Fe₃O₄ blended in PBAT/PLA compared to only 24% for the polymer fibers containing only TiO₂. Further, the onset of degradation only occurs after 300 min for the electrospun polymer membrane. This is probably due to the hydrophobicity of PBAT/PLA which delayed the contact between the model dye and the encapsulated photocatalytic particles. To mitigate this, the electrospun membrane was immersed in distilled water for 1080 min prior to testing. The immersed electrospun membrane showed a 20% improvement in photocatalytic efficiency compared to the membrane without immersion after 900 min.

Bacteria may be used for the removal of polluting dye in textile wastewater. Two possible mechanisms of dye removal by bacteria are physical adsorption and enzymatic biodegradation. Zamel et al (2019) was able to demonstrate the biodegradation of methylene blue (MB) dye with Bacillus paramycoides encapsulated within electrospun cellulose acetate (CA)/poly(ethylene oxide) (PEO) nanofibrous membrane. To ensure viability of the bacteria, dimethyl sulfoxide (DMSO) was selected as the solvent which is known to be safe on bacterial cells. In water, CA/PEO nanofibers swell and this allows the dye to be adsorbed and the reductases enzymes which are produced and secreted by the bacteria to be released into the surrounding media for biodegradation of MB. MB removal by free bacteria and bacteria-immobilized CA/PEO nanofibrous membrane was similar after 48 h at 89.13 and 87.39% respectively. This showed that encapsulation of the bacteria in the electrospun CA/PEO fiber matrix does not reduce its MB removal efficiency. However, the percentage of MB removal decreases with each repeated usage. By the 4th usage, MB removal has been reduced to 44%. This may be due to detachment of cells during washing or cell death due to lack of nutrients.