Large specific surface area, high porosity, good permeability, adjustable pore shape, and outstanding functional abilities are some of the advantages of electrospun nanofiber membranes (ENMs) [69][70][71]. ENMs have overcome the drawbacks of high energy consumption and limited separation efficiency, particularly in wastewater treatment [72][73]. In recent years, ENMs has received a lot of attention to act as adsorbents because of its interpenetrating porous structure (70% porosity), adequate specific surface area (approximately 10–20 m2g−1), and easy scale-up feature (even several meters) [74][75]. Furthermore, unlike activated carbon, which has a high internal specific surface, the specific surface of ENMs is is almost exclusively generated from the exterior surface, which aids the regeneration process. ENMs were found to be a suitable membrane for obtaining an adsorbent that does not require further shaping and is easy to recycle and replace [76][77].
2.1. Inorganic membrane
Recently, Inorganic membranes have been rapidly developing and innovating in recent years. Chemical resistance, high temperature and wear resistance, high chemical stability, longer lifetime, and autoclavability are all advantages of inorganic membranes [78]. Inorganic membranes were an excellent contender for water treatment and desalination applications because of all of these outstanding features. Because of its capacity to have both better permeability and selectivity, inorganic membranes, which are classed as metal membranes, ceramic membranes, and carbon membranes, have gotten a lot of interest.
2.1.1. Metallic Membrane
Metallic membrane is a type of porous material having a thin layered, smooth surface with pore sizes as small as 0.01 micron. They appear to be suitable to clarify rainwater because of their high treatment efficiency of microorganisms and particulates [79].
2.1.1.1 Silver
Chou et. al demonstrated that a silver-loaded asymmetric cellulose acetate (CA) hollow fiber membrane may inhibit the growth of Escherichia coli and Staphylococcus aureus for water treatment [80]. The sponge-like nature of these hollow fibres, as well as their dense inner and outer surfaces, make them a good option for water treatment. Alt et al. also found that polymethylmethacrylate bone loaded with 5–50 nm metallic silver particles had no in vitro cytotoxicity and was highly effective against multi-resistant bacteria [81]. Ag-PA/PES membranes have good antibacterial and antifouling properties, according to Zhu et al., and it can be employed to kill bacteria in ballast water and saltwater [82]. Electrostatic forces arise between the Ag+ ions in the bacterial solution and the negatively charged cytoplasm. The cytoplasm is assumed to have enough electrostatic attraction to rip the cell membrane apart, allowing the cytoplasm to squeeze out of the phospholipid bilayer and into the bacterial solution, killing the bacteria. The zwitterionic of silver nanoparticles combined with surface modification of poly(carboxybetaine acrylate-co-dopamine methacryamide) (PCBDA) copolymers significantly inhibited biofilm growth on polyamide membrane surfaces, indicating a possible pathway to achieve long-term biofouling resistance while maintaining water flux for conventional MF membranes [68]. Not only that, the water disinfection performance of PCBDA@AgNPs membrane demonstrated that hazardous bacteria in water could be effectively inactivated in contact with the membrane surface during the filtration process, resulting in pure drinking water.
2.1.1.2 Zinc
ZnO membranes have received a lot of interest in recent years because of their unique properties [83]. They are preferred over freely suspended nanoparticles because they are easier to remove from cleaned water. Hong et. al revealed that the performance of PVDF microfiltration membranes was increased by nanosized ZnO [84]. PVDF-ZnO0.005 had the highest pure water flux (452.1 L m− 2 h− 1), maximum porosity (75.16%), largest pore size (0.08 μm) and lowest surface roughness. Clearly, the improved hydrophilicity and reduced roughness of the composite membrane improved anti-fouling performance during recovered water treatment. The composite membrane surpassed the pure PVDF membrane in terms of breaking strength (2.92 MPa) and elongation at break value during mechanical testing (210.6%). Purushothaman et. al demonstrated that adding ZnO to PEES membrane improves hydrophilicity of the membrane [85]. The fouling-resistant capability of the membranes was tested using a model foulant, humic acid (HA), and the resulting membrane demonstrated an enhanced anti-fouling irreversible feature with a corresponding flux recovery rate of 92.43 %. The rejection rate and flux permeability of HA were 98.03 % and 166.73 L m−2 h−1, respectively, contributed by the hydrophilic properties of ZnO particles. According to the findings of Taherizadeh, ZnO nanoparticles incorporated with ferric chloride are strong suggestions for enhancing municipal wastewater treatment quality, and the treated wastewater is of extremely high quality and may be used for a number of reasons [86].
2.1.2. Ceramic Membrane
Ceramic membranes have cemented their place in wastewater treatment systems when the environment is hostile due to their durability and chemical stability. Recent studies have demonstrated that wastewater treatment using membrane bioreactor (MBR) systems with ceramic membranes is very effective and produces high yields [78]. The MBR is a revolutionary wastewater treatment method that combines an effective membrane separation process with a traditional activated sludge process in which the filtering membrane replaces the secondary clarifier [87]. As a result, MBR has solved many of the shortcomings of the activated sludge process, such as low solids separation efficiency, minimal mixed liquor suspended solids (MLSS), and a delayed biological response rate. Silica, alumina, titania, and zirconia are commonly used in the development of ceramic membrane materials.
2.1.2.1 Silica
Silica membranes for water treatment were studied by Yang et. al by recovering ammonia from sewage sludge using a molecular sieve silica membrane in pervaporation (PV) [88]. The silica membrane has a flux up to 3kgm-2h-1. Elma et. al measured the water flux and salt rejection using a silica-based ceramic membrane at various salt concentrations. measured the water flux and salt rejection using a silica-based ceramic membrane at various salt concentrations [89]. The water flux and salt rejection were directly proportional with the increasing salt concentration, producing a maximum water of 9.5 kgm-2h-1 with a rejection rate up to 99.6% for 0.3% NaCl solution, and 1.55 kgm-2h-1 with a rejection rate of 89.2% for 15 wt% NaCl solutions. Under the experimental conditions used, the eco-nanomagnets silica coated dithiocarbamate showed high efficiency for Hg2+ uptake (74%) even at contamination levels as low as 50 μg L-1 [90]. As a result, these materials have a lot of promise for magnetic separation to remove heavy metal ions from polluted water. By functionalizing chitosan–silica hybrid materials with (ethylenediaminetetraacetic acid) EDTA ligands, Repo et al successfully synthesised a new adsorbent [91]. The synthesised adsorbents were found to benefit from the advantages of both silica gel (high surface area, porosity, rigid structure) and chitosan (surface functionality). The maximal adsorption capacities of the combined materials for metal ions rejections ranged from 0.25 to 0.63 mmol/g under the analysed experimental conditions.
2.1.2.2 Alumina
Alumina membrane is one of the most prevalent used ceramic membranes for water filtration. Alumina can be used as a substrate, intermediate layer, and active layer in the structure of a ceramic membrane because of its intrinsic properties of high strength, chemical and thermal stability, and ease of production [92]. Das et. al showed that the maximum flux obtained for clay-alumina membrane for desalination of brine was 98.66 L/m2 day at a temperature difference of 60 °C. This result came together with water impurities rejection rate up to 99.96% [93]. Another study revealed that the high separation efficient can be attained with oil rejection at 98% and water flux of 21.62 L/m2.h by using alumina membrane composite contained surfactant sodium perfluorooctanoate[94]. The fluorosurfactant significantly plays a role in simultaneously acting as interfacial surface material and improving the dispersion of alumina particles on the alumina composite membrane. He et. al developed an alumina double-layered membranes with increased flux, which have a lot of promise in water UF [95]. Sol-gel, dip coating, and sintering procedures were used to tighten the pore size of alumina MF membranes, resulting in alumina UF membranes. The resultant UF membranes had higher hydrophilicity and better particle size retention performance in comparison to the MF membrane. The double-layered UF membrane had superior anti-fouling properties in comparison with the single-layered UF membrane. It could be due to the gradient pore sized structure of the bi-layered membrane, which showed 1.7 times higher flux than the single-layered membrane. This suggests that the profile of a double-layered active layer with gradient pore sizes could improve flux compared to a coating single active layer directly over the MF membrane.
2.1.2.3 Titania
The efficiency of the fabricated electrospun Nano-Palm Frond Titania Fiber (Nano-PFTF) membrane was tested with methylene blue (MB) dye and hexavalent chromium (Cr (VI)) under UV-C and visible light irradiation [96]. Within 120 minutes, 97.82 % rejection percentage of 10 ppm MB was achieved by Nano-PFTF membrane (CA/N-TiO2) while 99 % rejection percentage of 10 ppm Cr (VI) was achieved by Nano-PFTF membrane under visible and UV light irradiation respectively. Based on the results, the Nano-PFTF membrane showed remarkable potential in industrial wastewater treatment and increase the potential usefulness of oil palm frond. Chang et. al showed that nano-titania/polyethersulfone composite membrane demonstrated high rejections (≥ 92.3%) on filtration against BSA aqueous solutions [97]. This membrane also possessed the highest water flux (181 LMH/bar) and antifouling capability among all prepared membranes; specifically, the flux recovery ratio value remained as high as 94% even after three cycles of filtration-cleaning tests.
2.1.2.4 Zirconia
Because zirconia has been found to be used in ceramic membranes alongside other ceramic membranes made of silica, alumina, and titania, zirconia has been identified as a favoured option in microfiltration for wastewater compared to polymeric membranes due to its chemical stability and ability to endure high temperatures and pressures [98]. Yang et al. stated that zirconia membranes are one of the most well-known ceramic membranes because of their high chemical resistance, that promotes steam sterilisation and cleaning procedures at extremely high and low pH, outstanding pure water permeability, and excellent permeation flux due to their unique surface properties, as well as their high thermal stability [98]. Nishiyama et al. added zirconia to the silica-based membrane to increase the dissolution of the silica-based membrane in alkaline condition [99]. Similar procedure used by Kumar et al. which also added zirconia-based materials to the surface of kaolin-based membrane in order to use it in alkaline condition [100]. Furthermore, Pauzan et. al conducted another study to attach zirconia to kaolin suspension to overcome the dissolution of kaolin in high alkali solution [101]. Zirconia was used in these studies because zirconia is reported to be resistant in high alkali condition and the results showed that zirconia-kaolin hollow fiber membrane (ZKHFM) had the best mechanical strength (21 MPa) and outstanding membrane flux (~1,600 Lm2/h), indicating that ZKHFM can be used in alkaline solution. Separation of whey components was also done using zirconia-based ceramic composite membranes [102]. The obtained membrane improved remarkably high protein content (80%) and low lactose retention (7%), with a permeate flux value of 40 L/m2h.
2.1.3. Carbon Membranes
2.1.3.1 Carbon Nanotube (CNT)
CNT has been identified as a potentially transformative technology for addressing existing water scarcity and pollution issues [103]. CNTs, as members of the fullerene family, are made up of cylindrical graphite sheets that are rolled up into a seamless tube-like structure with a nanometer-scale diameter and a lattice-like appearance. CNTs are categorised as single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), or multiwalled carbon nanotubes (MWCNs) based on the layers of graphene shells (MWCNTs). The atomic arrangement (chirality), morphology (defect development), and nanotube diameter and length all play a role in the properties of nanotubes. CNT has the potential to be a useful absorbent in the removal of heavy metal ions from aqueous solutions [104]. It was reported that oxidation of CNTs with HNO3, NaOCl, and KMnO4 can considerably improve the sorption capacity of metal ions. The sorption mechanism appears mainly attributable to chemical interaction between the metal ions and the surface functional groups. Researchers have investigated the adsorption of aqueous cadmium (II) onto customised MWCNT. Yu and colleagues have also investigated about lead sorption and discovered that smaller and rich oxygen content enhance adsorption performance [105]. Pb(II), Cd(II) and Cr(VI) exhibited superior sorption capacity on oxidized CNTs from water [106]. Wei et. al have developed another study to improve the antifouling and separation performance of CNT by coupling CNTs/ceramic flat sheet ultrafiltration membrane with electro-assistance via crosslinking technique [107]. The resulting membrane features a good permeability 1.8 times higher than that of the membrane without electro-assistance. Furthermore, the electro-assisted membrane filtration process showed 70% reduction in energy consumption compared with the filtration process of the commercial membrane. CNTs have also been shown to have successful desorption of divalent metal ions while simultaneously having better sorption ability, which helps to reduce pollution in the environment [36][108].
2.1.3.2 Graphene
Graphene is a two-dimensional material that contains carbon atoms in the sp2 hybrid orbitals, has high heat conductivity and stiffness, and can reconcile the brittleness and ductility qualities [109]. Due to its extraordinarily large specific surface area and ease of broad functionalization, graphene has been incorporated into a number of composite materials, providing ample ‘anchoring’ sites for various functional nanoparticles [110]. Membrane strength and desalination performance can be improved with graphene-containing polymer composites. GO membranes with weak and stable interactions (hydrophobic or π–π interactions) may not be sturdy enough to endure actual water filtration process [111]. As a result, changing the membrane spacing and surface functional groups can improve the performance of GO-based membranes. In addition, Liu et al. developed a novel composite chitosan-graphene oxide (CS-GO) membrane with tunable characteristics with a simple cross-linking process at ambient temperature, which shows great promise for water treatment applications [112]. The permeability and separation performance of the CS-GO composite membranes were found to be influenced by increased interlayer spacing. This is due to the fact that CS molecules increased the binding force between GO nanosheets by providing hydrogen bonds, electrostatic interactions, and chemical bonds such as C-OH and C-N. The results show that CS-GO membranes have excellent separation performance for bovine serum albumin (BSA), sodium alginate (SA), and humic acid (HA) was very good (>95% rejection). Because porous reduced graphene oxide (PRGO), which is generated by perforating graphene following reduction, may overcome the problem of GO aggregation while preserving flow and retention, PRGO has been applied to dye removal, desalination, and other disciplines [113][114]. To develop a composite membrane with significantly enhanced flow, Zhu et al. employed poly (sodium-p-styrenesulfonate) (PSS) and modified halloysite nanotubes (HNTs) intercalated PRGO [115]. The interlayer spacing of PRGO rises after intercalation, and the dye removal rate exceeds 97%. Monovalent and divalent ions are removed at a rate of less than 10%. As a result, the composite membranes can be used to ease the colour separation from salt in mixed solutions.
2.1.4. Zeolite Membranes
Zeolites are aluminosilicates that are porous and have a well-defined molecular channel structure. They can tolerate a variety of monovalent and divalent cations (Na+, K+, Ca2+, Mg2+) and so interchange easily in a contact solution [103]. Both gas and liquid processes have shown interest in zeolites as potential membrane materials. Because of their unique molecular sieving properties, which allow them to precisely segregate molecules based on the size exclusion ability offered by controlled pore channel sizes. Zeolite membranes have various distinguishing characteristics that typical polymeric membranes lack. Thermal and chemical stability, adjustable pore sizes, and reduced fouling are just a few of the benefits [14]. The addition of inorganic materials to the active layer has been shown to be capable of breaking the trade-off phenomenon during the separation process based on their excellent hydrophilicity and the sieving effect [116]. As a result, zeolite is the optimum membrane material for using the FO method to remove heavy metals. This inorganic substance has the ability to adsorb heavy metals and can be shaped into a thin layer membrane. The inclusion of porous and hydrophilic zeolite might significantly improve the support qualities, resulting in increased water permeability [117]. By adding NaY zeolite nanoparticles in the polyamide rejection layer, Ma and colleagues investigated the effect of zeolite on FO performance. The addition of zeolite to the polyamide layer enhanced the water permeability of the membrane [118].
2.2. Organic membrane
2.2.1 Chitosan
Chitosan is an example of natural and biodegradable polysaccharides. Physical and chemical properties of chitosan include hydrophilic, bioadhesive, high crystallinity, antimicrobial, biocompatibility, good chelating and complexing agent, and good ionic conductivity[119]. However, the use of chitosan directly (as raw materials itself) to prepare electrospun membranes is still a challenge as chitosan particles have small surface area and weak mechanical strength thus limited chitosan application especially in metal adsorption [71], [120]. Shi et al. reported the fabrication of antibacterial hydrogel coating from alkynyl chitosan using electrophoretic co-deposition method. The alkynyl chitosan was prepared by reacting chitosan with 3-bromopropyne. The hydrogel was shown to have better antibacterial activity against E. coli and S. aureus than pure chitosan assessed [121]. Deng et al. reported the fabrication of chitosan-rectorite nanospheres immobilized on polystyrene (PS) fibrous mats for copper ions adsorption. The incorporation of rectorite increased the surface area of the composite mats thus increased the uptake capacity of the PS mats for copper ions up to 134 mg/g.[120] Bumgardner et al. added elastin to chitosan electrospun membranes to improve the mechanical strength and bioactivity of the membranes. The fiber diameters increased as the amount of elastin increased, while the water contact angle decreased upon addition of more elastin showing greater hydrophilicity.[122] Meanwhile, Li et al. prepared chitosan stacking membranes for adsorption of copper ions. The stacking membranes were stabilised by sodium carbonate and the proposed adsorption mechanisms of copper ions is due to the presence of large number of amino and hydroxyl groups which can interact with the metal through chelation [123].
2.2.2 Polyacrylonitrile (PAN)
It has been shown that adjusting the pH of the solution can easily influence the adsorption/desorption process based on non-specific electrostatic interaction [124][125]. As a result, adding protonable groups to the ENFM scaffold surface, such as nitrogen-containing groups, is thought to be a smart strategy to improve the membrane's adsorption ability for negatively charged Cr(VI) ions. Wang et al. developed a polyacrylonitirile/hyperbranched polyethylenimine (PAN/HPEI) aminated electrospun nanofiber membrane that was used as a permeable reactive barrier material for in-situ Cr(VI) polluted soil remediation [126]. Because of its amine group-rich membrane surface and interpenetrating porous structure, the as-prepared PAN/HPEI electrospun nanofiber membrane has a remarkable Cr(VI) adsorption capacity (206 mg g-1) and outstanding reusability (>9 cycles).
2.2.3 Poly(vinyl alcohol) (PVA)
Poly(vinyl alcohol) (PVA) is widely reported as heavy metals adsorbent due to the presence of large number of hydroxyl groups in the polymer backbone. The chemical composition also gives the hydrophilic properties to PVA. Tian et al. fabricated crosslinked PVA nanofibers by treating electrospun PVA nanofibers with glutaraldehyde solution to adsorb Cu2+ and Pb2+ ions. The crosslinked PVA nanofibers show enhanced water resistance, mechanical properties and fibers morphology compared to the non-crosslinked nanofibers with decreased adsorption equilibrium time for both metals.[127] Tian et al. later prepared grafted PVA by incorporating octaamino-silicon sesquisiloxane (octaamino-POSS) to further improved the adsorption efficiency of PVA nanofibers towards metals. The presence of large number of amino groups increased the adsorption capacity of the fibers with significantly improved for adsorption of Cu2+ ions.[128] Karim et al reported the fabrication of composites nanofibers membranes consisting of PVA and chitosan for Pb2+ and Cd2+ ion removal in wastewater. At the optimum conditions, the reported maximum adsorption capacity was 266.12 mg/g (Pb2+ ions) and148.79mg/g (Cd2+ ions).[129] Other application of PVA-based nanofibers reported include methylene blue dyes removal [130], [131] and oily wastewater treatment (for antimicrobial activity).[132]
2.2.4 Polyethersulfone (PES)
The properties of polyethersulfone (PES) include hydrophobic and robust in term of mechanical and thermal properties, thus PES is among the raw material to fabricate ultrafiltration membranes.[133], [134] Several functionalised PES fibers have been reported for removal of common pollutants in wastewater. These include porphyrins functionalised PES mats for removal of toxic compound, para-nitroaniline [133]; positively-charged PES nanofibrous membranes prepared through electrospinning and in-situ cross-linked polymerization of poly ([2-(methacryloyloxy)-ethyl] trimethyl ammonium chloride) in PES solution for removal of bacteria and cationic dyes [135]; and PES nanofibrous membrane prepared by electrospinning of blended mixture of poly(acrylic acid-co-methyl methacrylate) and PES for removal of methylene blue.[136]
2.2.5 Poly (vinylidene fluoride) (PVDF)
Poly (vinylidene fluoride) (PVDF) has been reported to produce superhydrophobic membranes due to excellent properties. This include the low surface energy, sound chemical inertness, and robust in term of thermal stability and mechanical strength. Thus, Zhou and Wu reported the fabrication of ultrathin fibrous PVDF-based superhydrophobic membrane by electrospinning for low-cost, high-efficiency oil/water separation. Herein, the properties of the membranes (superhydrophobicity and superoleophility) could be controlled by adjusting the PVDF added in the electrospinning solution [137]. Alvarez et al. reported the fabrication of porous fibers consisting of polyvinylpyrrolidone (PVP) and poly(vinylidene fluoride) (PVDF). TiO2 was embedded and immobilised in the fibers to produce photocatalytic mat and have shown to degrade common water pollutants [138]. Due to the membrane's high surface hydrophobicity and adequate pore diameters, Feng et al. explored chloroform, a VOC that could be eliminated by membrane gas stripping utilising electro-spun PVDF nanofiber membrane [139]. Chloroform mass transfer coefficient over the nanofiber membrane was reported to be 2.40 × 10− 5 m/s. This value is higher than that found for a hollow fibre module-based membrane air-stripping system. It could be because the nanofiber membrane performs better than the hollow fibre membrane in terms of chloroform mass transfer. It could possibly be attributable to the flat sheet module's lower boundary layer resistances than the hollow fibre module employed in this study.
2.2.6 Polyaniline (PANI)
Polyaniline (PANI) is well known as material for conducting polymer, thus only few has been reported for PANI to be used in wastewater treatment. Advantages of PANI include easy to synthesise, inexpensive starting materials (monomers), properties that are tunable, and robust in term of environmental and thermal stability.[140], [141] In most fabrication of nanofibrous membranes, PANI was used as coating materials. Dognani et al. reported the fabrication of PANI-coated polyvinylidenefluoride-co-hexafluoropropilene (PVDF-HFP) nanofibrous membranes for removal of toxic chromium (VI) ions.[142], [143] PANI coating enhanced the adsorption intake of the membranes up to 15.08 mg/g at pH 4.5 with good reusability (efficiency >70% even after five cycles).[142] Ali et al. fabricated electrospun membranes combining PANI-b-cyclodextrin and cellulose acetate for removal of cationic dyes in water. The PANI-modified cellulose acetate membranes showed good methylene blue removal which was reusable up to 3 cycles with adsorption efficiency >80%.[144] In another study of methylene blue removal, Mohammad and Atassi reported the fabrication of PANI-coated electrospun nanofibrous membranes of polylactic acid (PLLA) and polyacrylonitrile (PAN). Herein, the PANI membranes (PLLA-PANI and PAN-PANI) were prepared and their performance were compared with the pure PLLA and PAN membranes. The PANI-coated membranes were found to have higher methylene blue adsorption capacity compared to the pure polymer membranes.[145] For removal of heavy metals, Mohammad and Atassi fabricated PANI-coated nonofibrous polyacrylonitrile membranes to adsorb lead and chromium (VI) ions. The membranes showed higher lead ions removal (99%) compared to chromium (VI) ions (90%) at 5 mg/L.[146]
2.2.7 Polyvinylpyrrolidone (PVP)
Polyvinylpyrrolidone (PVP) is the most common hydrophilic additive used to improve membrane hydrophilicity among the hydrophilic additives available. It can serve as a pore-forming agent and an anti-biofouling agent [147]. According to multiple studies, any increase in PVP molecular weight tends to increase membrane pore diameter, resulting in increased water permeability [148][149].