Electrospinning NF Membranes: Fabrication & Modification Review

Desalination 558 (2023) 116638
Contents lists available at ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal
Fabrication and modification of nanofiltration membranes by solution
electrospinning technique: A review of influential factors and applications
in water treatment
Amir Hossein Behroozi a, Muayad Al-Shaeli b, Vahid Vatanpour c, d, e, *
a
Department of Chemical Engineering, Queen's University, Kingston K7L 3N6, Ontario, Canada
Paul Wurth Chair, Faculty of Science, Technology and Medicine, University of Luxembourg, Avenue de l'Université, L-4365 Esch-sur-Alzette, Luxembourg
National Research Center on Membrane Technologies, Istanbul Technical University, 34469 Istanbul, Turkey
d
Department of Environmental Engineering, Istanbul Technical University, 34469 Istanbul, Turkey
e
Department of Applied Chemistry, Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran
b
c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
• This review presents recent de­
velopments in the electrospinning-based
nanofiltration (NF) membranes.
• Effects of electrospinning parameters on
electrospun
nanofiber
membranes
(ENMs) are summarized.
• Applications and performance of ENMs
in NF-based water treatment are
expressed.
• Post-electrospinning
modification
methods of NF membranes are
presented.
• Perspectives
and
challenges
on
electrospinning-based NF membranes
are discussed.
Abbreviations: ACE, acetone; BM, baking material; BOD, biological oxygen demand; BSA, bovine serum albumin; CA, cellulose acetate; CaAlg, calcium alginate;
CHI, chitosan; CNC, cellulose nanocrystal; CNT, carbon nanotube; COD, chemical oxygen demand; CR, Crystal Red; CV, Crystal Violet; DBP, 1,3-dibromo propane;
DCM, dichloromethane; DI, deionized water; DMAc, dimethylacetamide; DMC, dimethyl carbonate; DMF, dimethyl fluoride; DMFCs, direct methanol fuel cells;
DMSO, dimethyl sulfoxide; DW, distilled water; ECH, epichlorohydrin; ENMs, electrospun nanofibrous membranes; ES, electrospinning; GA, glutaraldehyde; GO,
graphene oxide; HFGR, anionic reactive black; IP, interfacial polymerization; LBL, layer by layer; MB, Methylene Blue; MF, microfiltration; MO, Methyl Orange; MPD,
m-phenylene diamine; MWCO, molecular weight cut-off; MWNTs, multiwalled carbon nanotubes; NF, nanofiltration; NMP, N-methyl pyrrolidone; PA, polyamide;
PAN, polyacrylonitrile; PANI, polyaniline; PBI, polybenzimidazole; PCL, polycaprolactone; PCU, polycarbonate urethane; PDLA, poly (d, l-lactic acid); Pebax 1074,
polyethylene oxide–block–polyamide 12 copolymer; PEG, polyethylene glycol; PEI, polyethyleneimine; PEO, polyethylene oxide; PES, polyethersulfone; PET,
polyethylene terephthalate; PHB, polyhydroxybutyrate; PI, phase inversion; PIP, piperazine; PLLA, poly (l-lactic acid); PMMA, polymethyl methacrylate; PPD, Pphenylene diamine; PS, polystyrene; PSf, polysulfone; PSS, polystyrene sulfonate sodium salt; PVA, polyvinyl alcohol; PVC, polyvinyl chloride; PVDF, polyvinylidene
fluoride; PVP, polyvinylpyrrolidone; Q-PVA, quaternized polyvinyl alcohol; RH, relative humidity; RhB, Rhodamine B; rGO, reduced graphene oxide; RO, reverse
osmosis; SA, sodium alginate; SF, silk fibroin; TBAB, tetrabutyl ammonium bromide; TBAC, tetrabutyl ammonium chloride; TEA, triethyl amine; TEOS, tetraethyl
orthosilicate; TFA, trifluoroacetic acid; TFNC, thin film nanofibrous composite; TFC, thin film composite; THF, tetrahydrofuran; TMC, trimesoyl chloride; TMP,
transmembrane pressure; UF, ultrafiltration.
* Corresponding author at: Department of Applied Chemistry, Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran.
E-mail addresses: [email protected], [email protected] (V. Vatanpour).
https://doi.org/10.1016/j.desal.2023.116638
Received 8 January 2023; Received in revised form 14 March 2023; Accepted 17 April 2023
Available online 20 April 2023
0011-9164/© 2023 Elsevier B.V. All rights reserved.
A.H. Behroozi et al.
Desalination 558 (2023) 116638
A R T I C L E I N F O
A B S T R A C T
Keywords:
Electrospinning
Nanofiltration membrane
Modification
Water treatment
Electrospun nanofibers
Nanofiltration (NF) is the most promising process for the efficient treatment of contaminated air and water
environments due to its flexible processing and excellent separation efficiency. Electrospinning is a membrane
fabrication and modification process that can generate electrospun nanofiber membranes (ENMs) with large
specific surface area, selective wettability, and reasonable interconnected pore structure. The modification of
ENMs with multifunctional nanomaterials has extended their applications to a wide range of separation pro­
cesses, mainly water treatment. Due to their superior advantages, this review summarizes the recent advance­
ments of the solution electrospinning-based NF membranes, preceded by a detailed discussion on its general
concepts and influential processing factors, polymer solution properties, and electrospinning variables. Another
appealing modification approach of ENMs is their post-treatment, which endows them with unique features to
improve the filtration performance of NF membranes. Moreover, potential challenges in the large-scale pro­
duction of ENMs for treating water via the NF process are represented. Overall, future electrospinning-based
developments could focus on designing a well-controlled system to produce more durable ENMs and devel­
oping multifunctional composite ENMs for improved practical NF-based water treatment.
1. Introduction
flux and solute rejection, they are vulnerable to membrane fouling and
concentration polarization, leading to a temporary or permanent flux
reduction, deteriorating membrane solute selectivity, and increasing
operational and capital costs [19,20]. Thus, modification of their ma­
terials and structures has been introduced to reduce membrane fouling
and enhance the water permeation flux and solute rejection for energy
savings [21].
One of the efficient modifications for improving membrane perfor­
mance is to apply nanofibrous structures to membranes. Nanofibers are
a new class of nanotechnology-based products, which have been
extensively exploited in biomedicine, sensors, energy, catalysis, textiles,
tissue engineering, air purifications, and water treatment because of
their high surface area to volume ratios, nanoscale diameters, and
interconnected nanoscale pore structures. They have excellent me­
chanical strength as well, and their surfaces can be easily altered
[22–24]. Considering these advantages, electrospun nanofibrous mem­
branes (ENMs) have been applied in various water purification appli­
cations, such as the pretreatment of feed solution prior to NF and RO,
separation of heavy metal ions, organic solvent, and membrane distil­
lation. These membranes can be produced by several approaches,
including drawing [25–28], template synthesis [29–31], phase separa­
tion [32,33], self-assembly [34], and electrospinning [35,36]. Among
them, electrospinning (ES), also named as electrohydrodynamic tech­
nique, is a versatile and desirable method to fabricate smooth ENMs
from a wide range of polymers (natural and synthetic) based on high
electric charge jet of the homogeneous solution (polymer/solvent) or
melt polymers [37,38]. It can generate nanofibrous materials with sizes
varying from micrometer to nanometer scales and diverse morphologies
by changing the electrospinning variables and polymer solution char­
acteristics [39].
ENMs can be used in different membrane applications, including
microfiltration (MF) [40] and ultrafiltration (UF) [41], due to their
propensity to lessen the resistance to water flow (i.e., high water
permeation flux). Also, they can be applied as porous supports in NF
membranes by modifying the middle supporting layer of the TFC
membranes to synthesize thin film nanofibrous composite (TFNC)
membranes [42]. All these ENMs-supported TFC NF membranes are
characterized by their higher water permeation flux, higher solute re­
jections, and less tendency to membrane fouling. Much research has
been conducted between 2010 and 2023 on implementing ENMs as the
TFC NF support layer, as reported in Fig. 1. To have an appropriate
search in this regard, the keywords “Electrospinning” and “Nano­
filtration membranes” were selected to find the number of publications
with these keywords in their abstract and title versus year (Fig. 1a) and
research category (Fig. 1b). The increasing trend in the published pa­
pers, particularly in engineering and chemical sciences, indicates that
the electrospinning technique has recently become more appealing
among scientists in fabricating NF membranes.
Due to the global population and urbanization, the demand for clean
freshwater has considerably increased in recent years. It is estimated
that >1.4 billion people have less access to clean freshwater, and >66 %
of the world population will be challenged to access the freshwater
supply by 2025 [1]. This demand has urged the scientific community to
seek alternative technologies to address this challenging issue.
Separation-based membranes have found their way as a promising
technology to meet the rising freshwater demand due to their superior
features, such as easy operation, higher separation performance, rela­
tively low cost, and being environmentally friendly [2,3]. As part of
separation-based membranes, nanofiltration (NF, pore size: 0.1–10 nm,
molecular weight cut-off (MWCO): 0.5–2 kDa), as a pressure-driven
process, represents an intermediate between ultrafiltration (UF) mem­
branes, due to the porous supporting structure, and reverse osmosis (RO)
membranes (pore size: 15–75 nm, MWCO: 0.2–10 kDa) [4–6].
NF has been extensively used in water treatment applications
because of its lower energy consumption, higher permeation flux, and
higher capability of eliminating diverse pollutants from water (e.g.,
pharmaceutical compounds, dissolved compounds, and organic pollut­
ants) based on the Donnan effect and size exclusion mechanism [7–9].
NF membranes can potentially provide high rejection of multivalent ions
(e.g., transition metals), but low rejection of monovalent ions (e.g.,
chloride). As compared with RO, NF operates at lower pressures (5–20
bar) to overcome the osmotic pressure barrier, making it possible to save
more energy (about 20 % of electric power) [10–12].
Most NF membranes are prepared by thin film composite membranes
(TFC) primarily according to asymmetric porous supports [13]. TFC
membranes consist of three fundamental layers: the top ultra-thin se­
lective layer, a middle porous support layer, and a bottom non-woven
fabric layer [14]. Such membranes can be prepared by diverse tech­
niques, such as plasma polymerization, interfacial polymerization (IP),
dip-costing, layer by layer (LbL), and phase inversion (PI) [15]. The
middle porous support layer is usually fabricated by the PI procedure
and has a dense surface skin, thereby adopting an asymmetric structure.
It commonly provides maximal compression resistance and mechanical
strength combined with minimal resistance to permeation [16]. The
porous support is of importance in forming the barrier layer, which
impacts the selectivity of the top thin layer. The top and middle support
layers can be modified meticulously to provide optimum separation
performance [17]. The bottom non-woven layer does not relate to the
separation performance and is exploited to present mechanical strength
during handling [18].
The advantages of TFC membranes are the capability of generating
diverse thicknesses of support and top thin layers and optimizing the top
ultrathin and support layers regarding structure, performance, and sta­
bility [12]. Although these membranes can improve water permeation
2
A.H. Behroozi et al.
Desalination 558 (2023) 116638
Although ENMs have some practical benefits, they still have some
problems for water treatment purposes, such as the requirement of
specific solvents and equipment and high applied voltage for producing
ENMs. Since ENMs are often thin layers, the fabrication time is relatively
long when the thickness is large [43]. Furthermore, it is difficult to
control their structure; therefore, modification of ENMs is needed to
extend their potential applications.
Different modification approaches have been considered to modify
ENMs, like modification of nanofiber surface and TFNC membranes. The
second approach is a typical process of building a selective layer on the
top surface of ENMs to make it suitable to reject pollutants with tiny
molecules [44]. The modification can be done either by crosslinking
polymerization or IP. Crosslinking polymerization can produce func­
tional polymer materials via various functional monomers, forming
network macromolecules on the nanofibrous membrane surface as a
selective layer. Since this method is conducted by spinning and precrosslinking, it can increase the manufacturing steps and create an
excessive gel. Thus, the in-situ crosslinking method has been suggested
to copolymerize the crosslinked material in the polymer directly. It is
interesting because in the existence of appropriate ultraviolet radiation,
the photo-initiator can prompt the generation of free radicals, and the
combination of such polymerized free radicals will generate a cross­
linked network. This occurrence has achieved excellent outcomes when
exploiting in-situ crosslinking [45]. On the other hand, IP is a common
procedure to modify TFNCs by creating an extremely high crosslinked
ultra-thin polymer film (thickness of approximately 300 nm) with a
network architecture at the interface of two immiscible solutions [46].
As a promising and versatile technique, electrospinning is welldeveloped for the fabrication of ENMs, and there are some decent re­
view papers on ENMs [47–50]. However, none of them has focused on
applications of the electrospinning method and the effect of its pro­
cessing variables on the performance of NF membranes for water
treatment. This review provides a comprehensive survey on the poten­
tial of electrospinning in preparing NF-based membranes, primarily for
water treatment applications. Table 1 reports the experimental condi­
tions, aim, and findings of recent works on NF-based membranes pre­
pared by the electrospinning technique to provide researchers with
comprehensive information on this topic. The impacts of influential
electrospinning factors on nanofiber-based NF membranes are dis­
cussed. Moreover, electrospun nanofiber supports and their coating for
TFC NF membranes are highlighted. The final section presents the out­
comes and perspectives for future research- and industrial-based
developments.
70
12
10
8
6
4
2
0
Year
50
40
30
20
10
0
Technology
14
Biological Sciences
Number of Publications
16
(b)
60
Chemical Sciences
(a)
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
Number of Publications
18
Nanofibers are a type of linear material with a specified lengthdiameter and nano-scale dimensions in the range of 1–100 nm. When
they are reduced from micron size to submicron or nanometer, they
exhibit superior characteristics, including high surface-to-volume ratio,
excellent pore interconnectivity, and numerous surface functional
groups, making them highly applicable for various practical fields, such
as filtration, protection, sensing, textile, energy, and biomedicine [95].
Several fabrication methods, like template synthesis, self-assembly,
physical stretching, electrospinning, and phase separation, have been
employed for preparing nanofibers, among which electrospinning is
more capable of generating nanofibers with diverse morphologies.
Electrospinning is a mechanism of producing smooth nanofibers with
tunable morphology with the help of a melt in a strong electric field or a
charged viscoelastic polymer solution. This process is highly complex
due to its association with the interaction of different physical instability
mechanisms [24]. It uses electrostatic forces to stretch the polymer so­
lution as it solidifies compared with conventional fiber spinning tech­
niques like melt and dry spinning [96]. However, similar to
conventional techniques, drawing the solution to generate the fiber will
resume until there is a sufficient amount of solution to feed the jet. As a
result, the fiber generation will be persistent without disrupting the jet.
Typically, electrospinning systems include four main parts: a highvoltage direct current (DC) power supply (usually 10–30 kV), spin­
neret (fiber generator), jet syringe, and metal collector. Based on the
spinneret type, electrospinning is generally classified into two needlebased and needleless methods [97]. Fig. 2 illustrates the schematic
setups of needle and needleless electrospinning techniques with their
classification in terms of the fiber fabrication method, spinneret motion,
and collection path of fibers.
In the case of needle-based electrospinning, a spinning liquid
thruster transfers the viscous polymer solution from the syringe into the
electric field during spinning, making the solution to be charged and
consequently producing a repulsion interaction to prevail the solution
surface tension. The droplet gradually turns to a cone shape, namely the
Taylor cone, and then transforms into a jet form. The ejected jet creates
an unstable bending phenomenon. When the charged jet moves toward
the low potential zone, the entanglement of the polymer chain impedes
the jet from breakage while evaporating the solvent, and the metal
collector receives the nanofiber at the other end [98].
Needleless electrospinning refers to the electrospinning of nanofibers
quickly and directly from an open fluid surface. In this method, various
planes are shaped simultaneously from the spinneret without the
Engineering
20
2. Effective parameters of electrospinning technique
Research Category
Fig. 1. Numerical reports on publications regarding applying electrospinning in fabrication and modification of NF membranes: number of papers published between
2010 and 2023 versus (a) year and (b) research category. Date of data extraction: February 2023.
3
4
Main
polymer &
conc.
Solvents (ratio)
Viscosity of
polymer
solution
Additives & conc.
Voltage (kV)
Chamber
humidity
(%)
Syringecollector
distance
(cm)
Post-treatment
Aim of study/main finding
Ref.
Q-PVA (10 %
w/v)
DI
–
KOH
25
99
15
–
[51]
PVA (10 %
w/v)
DW
–
Ramie CNC (1, 3, 5, 7.5 w%)
15
–
12
–
PVA (10 %
w/v)
DI
–
28
35–55
10
Crosslinking & coating
PVA (20 %
w/v) PSf
substrate
DMF
–
Triton X-100 (0.6 % v/w)
Crosslinker GA (0.05 mol/L in
acetone)
HCl catalyst (0.02 M)
GO coating (5 mg)
Sepiolite (0–0.1 w%)
TEOS (0.05–0.3 w%)
19
–
10
Crosslinking
PES (18–22
% w/w)
PET
substrate
PES (24–32
% w/v)
DMF:NMP
(60:40, 40:60,
50:50, 30:70 w/
w)
DMF:NMP (25–75
w% NMP)
0.31–1.84 Pa⋅s
PEO (0.5 w%)
30
<45–70
10.5
–
–
–
15
–
10
Oxidation
PAN (11 w
%)
DMF
–
–
12
45–48
10
–
PAN (7 % w/
v)
DMF
–
GO and rGO coating (0.01
mg/mL)
18
–
15
Hot-pressed coating
PAN (12 %
w/v)
DMF
–
SF (12 % w/v) in 98 % formic
acid PANI/TiO2 nanoparticles
(0, 2.5, 5, 7.5 w%)
22
–
14
–
PAN (4, 6, 8,
10 % w/w)
PIP:PPD:TEA
75:25:1 w/v%
–
–
30
70
15
–
PAN (10 %
w/v) PAN/
PET
PAN (8 % w/
v)
DMF
–
PIP (1 w%)
TEA (1 w%)
20
–
7
CN layer coating and IP
DMF
–
PIP (0.2–2 w%)
TEA (1 w%)
NaCl (0–25 w%)
20
–
12
Interfacial polymerization
DMF
–
–
12
<50
15
–
- Methanol permeability
- High-performance electrolyte fabrication in Direct
methanol fuel cells
- Increased power density of the fuel cell by 36.4 %
by adding electrospun Q-PVA composite.
- Separation of potential infective substances from
water
- Improved thermal resistance, but weaker
mechanical properties by adding nanocrystals
- Solute separation from water
- 91.01 % Na2SO4 (20 mmol/L)
- 98.12 % Eosin Y (10 mg/L)
- 76.92 % Methylene Blue (10 mg/L)
- 49.62 % for NaCl (20 mmol/L)
- Salt rejection for water purification
- Inhibited growth of Escherichia coli due to the
improved antibacterial property
- Rejection: 97.6 % MgSO4 and 96.95 % MgCl2
- Fabrication of high-flux and low-fouling membrane
- PES nanofiber–PET adhesion increased at relative
humidity (RH) of 50 % while decreased at higher
RHs
- Pure water permeability
- ~3000 L/m2 h water flux
- Improved mechanical properties
- Contaminant removal from water
- 100 % Indigo carmine dye (12 mg/L)
- 97.7 % NaCl (1700 mg/L)
- Salt separation
- NaCl: ~40 % by PAN-GO, ~20 % by PAN-rGO
- MgSO4: ~40 % by PAN-GO, ~90 % by PAN-rGO
- KCl: ~18 % by PAN-rGO
- Dye separation from water
- ~92 % HFGR removal by SF/PAN hybrid
nanofibrous membrane containing 7.5 w% PANI/
TiO2 nanoparticles
- Salt removal from water
- Investigation of polymer content on the surface
morphology of membranes
- Removal rate: ~83 % NaCl, ~96 % Na2SO4, ~94 %
MgSO4, ~91 % MgCl2
- Salt rejection for water purification
- >99 % MgSO4 rejection with 44.7 L/m2.s flux
- Improved flux by adding 20 w% bipiperidine
- Salt rejection for water purification
- 99.1 % Na2SO4 rejection with 129 L/m2 s flux
- Good stability and anti-fouling characteristics for
long-term applications
- Pre-filter membrane for PS particulate removal
from water
- 99 % of 10, 8 and 7 μm particles
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63]
(continued on next page)
Desalination 558 (2023) 116638
PSf (20 % w/
v)
A.H. Behroozi et al.
Table 1
Conditions, aim, and findings of recent literature on preparing NF membranes via electrospinning technique.
Solvents (ratio)
Viscosity of
polymer
solution
Additives & conc.
Voltage (kV)
Chamber
humidity
(%)
Syringecollector
distance
(cm)
Post-treatment
Aim of study/main finding
Ref.
PSf (20 % w/
v)
DMAC:acetone
(9:1 v/v)
1.13–1.88 Pa⋅s
GO and ZnO
30
–
15
–
[64]
PSf (20 % w/
v)
DMF
239–290 cP
GO and SiO2 (1 w%)
20
–
15
Hydrothermal
PSf (18 % w/
v)
DMF
–
Iron acetate (1.7 w%)
NaOH
18
–
15
PA coating
PSf (20 % w/
v)
DMF
0.37 Pa⋅s
Keratin (5 % w/v)
12
–
12
–
PSf (25 % w/
v)
DMF/NMP
(70:30)
5258–10,291 cP
PEI (0.5–2 w%)
Al2O3 (0.01–0.05 w%)
12.5
15–17
12
–
PVC (15 %
w/v)
PVC (15 %
w/w)
DMAC
–
PVP (0–5 w%)
20
–
30
–
DMAC
–
PVP (0–5 w%)
–
–
25
Vacuum filter
PET (20 %
w/v)
TFA:DCM (7:3 w/
w)
221.7 cP
–
20
–
10
–
PET (10–15
% w/v)
TFA:DCM (7:3 w/
w)
–
–
7–12
–
25
–
PET (8–12 %
w/v)
PVDF (15 %
w/v)
TFA/DCM (4:1 v/
v)
DMAC:acetone
(1:1 v/v)
0.8 Pa⋅s
–
19
–
20
–
–
15
60–70
15
PES/PVP coating by LBL
assembly
Heat treatment
PVDF (6–10
% w/v)
DMSO:acetone
6:4
49.6–199.3 cP
LiCl
12–18
–
10 & 20
Thermal
PVDF (18 %
w/v)
DMF/acetone
(1:3 v/v)
–
TBAC (0–0.04 mol)
30
–
15
PA coating
PS (8–16 w
%)
Chloroform
–
–
10
–
15
–
PVP (4–10 %
w/v)
Chloroform:DMF
(65:35 w/w)
47.9 cP
Lecithin (20–40 w%)
30
–
15–30
–
PSf (15 w%)
DMF (15 % w/v)
–
–
10, 15, 25
–
5–15
–
- Arsenic removal from water
- 71 % arsenite–As(III) rejection by composite
membrane, 43 % by PSf membrane
- 27 % arsenate–As(V) rejection by both membranes
- High flux oil/water separation
- 100 m3/m2 gasoline
- 115 m3/m2 kerosene
- 187 m3/m2 hexane
- Oil/water separation
- Improved mechanical properties
- Switched from hydrophobic (100◦ ) for PSf to
hydrophilic (37◦ ) for composite membrane
- Treatment of high toxic effluents radiating from
tannery industries
- Removal rate: 53 % COD, 66 % BOD, and 76 % azo
dye
- Pure water flux and BSA rejection
- Improved hydrophilicity and mechanical strength
- 48 % BSA rejection
- Removal of magnetic suspended particles from
water in terms of turbidity values
- Treating water containing Ca2+, Mg2+, hardness,
sulfates, nitrates, fluoride, chloride, alkalinity and
silica
- Treating air containing nanoparticles and
bioaerosols
- 98.4 % particle removal
- Smoke filtration
- Filtration capacity of higher than 43 times the filter
weight
- Salt rejection for water purification
- ~80 % MgSO4 rejection with ~75 L/m2.s flux
- Particulate removal from water, such as 1, 2, and
10 μm PS particles
- 90 % separation of microparticles
- Pure water permeability
- Produced electrospun nanofiber membranes with
less toxicity
- Salt rejection for water purification
- Achieved small pore size (130–140 nm)
- Rejection: 97 % MgSO4 and 76 % NaCl
- Ion and dye removal from water
- Removal: 98.6 % Cu2+ (5 mg/L), 98 % Cr6+ (5 mg/
L), 97.3 % MB (10 mg/L)
- Anti-biofouling membranes
- Increased hydrophobicity
- Zero protein adhesion for PVP membrane
containing 2 w% lecithin
- The increases surface charge of the jet reduced the
presence of beads.
- Longer spinning time resulted in more uniform PSF
nanofibers.
5
Main
polymer &
conc.
A.H. Behroozi et al.
Table 1 (continued )
[65]
[66]
[67]
[68]
[69]
[70]
[71]
[72]
[23]
[73]
[74]
[75]
[76]
[78]
(continued on next page)
Desalination 558 (2023) 116638
[77]
Main
polymer &
conc.
Solvents (ratio)
Viscosity of
polymer
solution
Additives & conc.
Voltage (kV)
Chamber
humidity
(%)
Syringecollector
distance
(cm)
Post-treatment
6
Distilled water
–
–
5–13
–
15
–
PAN (4–12 w
%)
DMF (100 cm3)
–
–
–
5, 10, 15
–
PVA (6 w%)
Water (100 mL)
–
SA
Charge
density =
1.5, 2, 2.5, 3
kV/cm
28–35
–
9
–
PAN (8 w%)
DMF (100 mL)
–
Hollytex backing material
15
–
–
Hot pressing at temp = 87 ◦ C
and 0.14, 0.28 or 0.41 MPa
pressure.
IP method
PVDF (18.5
w%)
DMF:NMP (1:1
w/w)
–
PEI (2.5 w%)
29
–
7.5
Crosslinking process using
TMC, DBP and ECH
PES (22 w%)
DMF:NMP (6:4
w/w)
–
PIP and TMC
30
–
10
IP by OMIC and BMIC
PAN (10 w
%)
DMF (100 mL)
–
PA
20
–
14
Hot-pressing at temp = 95 ◦ C
at 0.25 MPa for 40 min and IP
using PIP and TMC
PAN (8 w%)
DMF (100 mL)
–
PIP, biPIP and
dihydrochloride (0.125 %–1
% (w/v))
15
–
10
IP
PHB (2.5 w
%)
Chloroform: DMF
(9:1 w/w)
–
CNT (0, 0.5, 1, 1.5, and 2 w%)
16
–
20
CaAlg hydrogel coating
Ref.
- A short distance led to a shorter flight time and
solvent evaporation time, resulting in more beads.
- At a high voltage above 10 kV, the nanofiber
diameter increased.
- At lower voltages, the nanofiber diameter becomes
narrow.
- The increased charge density decreased the fiber
diameter.
- The optimal charge density was 2.5 kV/cm.,
achieving 116.8 nm mean fiber diameter.
- Broken nanofibers were observed at 28 kV.
- The fiber length became more continuous at 35 kV
or higher.
- The enhanced membrane rejection, lower
membrane fouling, better pressure tolerance, and
mechanical performance.
- After IP method, fluxes for TFNC-1 and TFNC-2 are
(291.58 and 261.70 LMH) and MgSO4 rejection was
88 % and 92 %
- Fluxes and rejection for commercial NF270 (239
LMH) and NF90 membranes (82 LMH), but they
have roughly lower salt rejection (8–12 %)
- The improved membrane performance.
- Flux: 30 LMH
- Rejection: 88 % MgCl2 and 65 % NaCl at pH =6 For
using TMC at 7 bar
- Rejection: 75.5 % MgCl2 and 22.9 % NaCl for using
DBP.
- Rejection: 76.4 % MgCl2 and 62.6 % NaCl for using
ECH.
- The improved membrane performance:
- TFNC flux and salt rejection: 50.0 LMH and 97.8 %.
- NF-90 flux and salt rejection: 27.2 LMH and 99 %
- NF-270 flux and salt rejection: 50 LMH and 97 %
when using OMIC
- 99.5 % rejection of the negatively charged dye
Sudan 4 (MW: 380 Da)
- Methanol flux: 9.87 LMH/bar
- Water flux: 22.40 LMH/bar
- Outstanding stability in aprotic solvents like DMSO
for up to 50 h.
- Flux: 3553 LMH Psi compared to unmodified TFC
51.2 LMH Psi with rejection of >96 %.
- Flux and rejection improvement: 21–42 % and
2–22 % for 0.25–1 % PIP.
- Flux for TFNC 38 % was higher than commercial
NF270 at 4.8 bar
- The contact angle was reduced from 83.6◦ to 17.8◦
- Flux: 150.72 LMH for the modified membrane and
68.61 LMH for the unmodified one.
- Rejection: above 90 % for dyes with the molecular
weight higher than 600 g/mol under 0.5 MPa.
[79]
[80]
[81]
[82]
[83]
[84]
[85]
[86]
[87]
(continued on next page)
Desalination 558 (2023) 116638
PVA (6–8 w
%)
Aim of study/main finding
A.H. Behroozi et al.
Table 1 (continued )
A.H. Behroozi et al.
Table 1 (continued )
Main
polymer &
conc.
Solvents (ratio)
Viscosity of
polymer
solution
Additives & conc.
Voltage (kV)
Chamber
humidity
(%)
Syringecollector
distance
(cm)
Post-treatment
Acetone:DMAC
(2:1 v/v)
–
–
12
–
15
Coating by using CHI SA, PSS
PVA (8–15 w
%)
Distilled water
–
Triton X-100 (0.02 and 1.2 %
v/w)
Pebax (1.0 w%)
MWNTs (0–20 w% based on
the polymer weight)
30
–
10
Coating polyetherb-polyamide
copolymer (Pebax 1074), or
crosslinked PVA hydrogel
incorporated with MWNTs
PAN (10 w
%)
DMSO
–
ETA (6 w% based on the total
solution)
0–16.5
36–40 %
12
Coating of PEG-600 (3 w%)
PAN (8, 10
and 13 w
%)
–
–
PDA/PEI and hydrolysis by Zr
(SO4)2
–
–
–
IP using PIP and TMC
PU (12 % w/
v)
DMF/THF (50/50
v/v)
–
LiCl (0.04 w%)
22
45 %
18
LbL and PEI and TMC
PVDF (8 g)
DMF/acetone
(92 g in 2:3 v/v)
–
–
–
–
IP by polyamide
PLA (5–15
w/v%)
DMC
–
Graphene oxide nano-sheets
(GOns) (1, 0.1, and 0.1 w%)
was added in the top, middle
and bottom
TBAB at concentrations of
1–15 w/w%
15
50–55 %
10
–
7
CA (16 w%)
Aim of study/main finding
- Acceptable antifouling performance toward protein
and oil.
- Flux: 60 and 40 LMH for 15 and 25 bilayered
membranes (CHI/SA and PSS/SA)
- NaCl rejection: 6 % and 15 % for 15 and 25
bilayered membranes.
- Improved membrane performance:
- Porosity: 82 %
- Flux for PVA/MWNTs (10 w%) was two times (330
LMH) that of MWNTs/Pebax (160 LMH) with the
same rejection (99.8 %), as compared to PAV and
Pebax (58 and 67 LMH)
- A uniform coating for PVA/Pebax and PVA/
MWNTs.
- Formed asymmetric micro/nano channels in
nanofiber membrane by phase inversion
- At 10 V and P = 0.1 bar: flux = 75 LMH, R% = for
MB = 98 % and for salt ≤ 5 %.
- At 10 V and P = 0.01 bar, flux = 7.4 LMH and R% =
99 % for saline solutions
- Higher flux of 38.2 LHM at 4 bar compared to
pristine NF membranes (21.5 LMH)
- Excellent rejection rate for divalent anions (97.6 %
2+ .
for SO2−
4 ) and cations (92.4 % for Mg )
- High filtration performance: flux of (70, 73, 72 and
74 LMH) and rejection of (99 %, 82 %, 81 % and 57
%) for different types of salts (MgCl2, MgSO4, NaCl,
and Na2SO4, respectively).
- Higher flux was achieved from 6.68 to 20.36 L/m2
h and the salt rejection remind on an incremental
basis as NaCl < MgCl2 < MgSO4 under 6 bar
pressure.
- Excellent acetone permeance up to 10 LMH⋅bar− 1
with a molecular weight cut-off in the range of
178–391 g/mol. Membrane demonstrated an oil
removal rate of 99.6 % in the produced water
treatment at a water permeance of 5.6 LMH⋅bar− 1.
Ref.
[88]
[89]
[90]
[91]
[92]
[93]
[94]
Desalination 558 (2023) 116638
A.H. Behroozi et al.
Desalination 558 (2023) 116638
Fig. 2. Schematic representations of electrospinning techniques: (a) needle-based and (b) needleless, along with their classifications based on nanofiber fabrication
approach, spinneret movement, and collection direction of nanofibers.
influence of the capillary impact, which is commonly attributed to
needle electrospinning [99]. However, in needleless electrospinning, the
spinning process is challenging to control because the fly start is a selfassembly phenomenon that occurs on the free surface of the fluid. In this
method, numerous spinneret shapes have been introduced with different
production levels.
Generally, three different regimes can be generally obtained in the
electrospinning process by changing the solution characteristics and
processing factors. The electrospinning starts with a Taylor cone
generated from competing between the surface tension and electric
stress on a droplet in the polymer solution [100]. When the electric
stress dominates the surface tension, a thin jet of the solution is evolved
and causes three major states of instabilities: two caused by electrostatic
repulsion (non-axisymmetric and axisymmetric) and one driven by the
Rayleigh instability [101]. The surface tension of the fluid primarily
drives the capillary breakup caused by the Rayleigh instability. Fluids
are reduced to droplets to decrease the total surface area and energy.
The Rayleigh instability can be fixated and even repressed for specific
length scales by viscoelastic stresses intrinsic in entangled polymer so­
lutions. This phenomenon creates three regimes of fiber morphology:
droplets, beads-on-string, and uniform fibers. When there is very low
viscoelasticity to hinder deformation caused by surface tension, droplets
are generated, called electrospraying. The second one has a form of
beads linked by a thin filament; this beads-on-string architecture is
associated with the partially stabilized fibers among droplets driven by
the entangled polymer structures that oppose capillary breakup from the
Rayleigh instability [102]. The last one demonstrates uniform fibers
when the Rayleigh instability is sufficiently suppressed for the length
scales attributed to the fiber generation. Hence, the electrospinnability
of polymer solutions is primarily measured by viscoelasticity and surface
tension [103].
Different parameters can affect the physical properties of nanofibers
generated by the electrospinning technique. Two influential factors,
including polymer solution properties and process operating parame­
ters, can control the nanofiber diameter. The first factor consists of
molecular weight, conductivity, surface tension, viscosity, and dielectric
property of solvents. The process operating parameters involve feeding
rate, collector's shape and/or motion, capillary tip to collector distance,
capillary diameter, applied voltage, humidity, pressure, and tempera­
ture. Among the mentioned factors, polymer solution characteristics can
greatly impact the structure and morphology of electrospun nanofibers
[104].
2.1. Polymer solution properties
2.1.1. Polymer type
Polymer solution properties directly affect electrospun nanofiber
morphology and characteristics. These properties are polymer type,
solution concentration, polymer relative molecular weight, conductiv­
ity, viscosity, and surface tension. Among them, polymer type is a crit­
ically influential factor because the entanglement of molecular chains of
each polymer in solution plays a pivotal role in the final properties of
synthesized fibers [105]. The content of polymer functional groups
placed on the nanofiber surface can be approximately 100 πd/D, in
8
A.H. Behroozi et al.
Desalination 558 (2023) 116638
which d indicates the polymer chain diameter and D presents the
diameter of fibers [106]. For improved properties of fibers produced,
efforts to reduce their diameters can be a feasible approach to increasing
their exposed polymer chain's ratio and functional groups.
Numerous techniques have been advanced as yet for generating fi­
bers from polymers, mainly wet, dry, melt, and gel spinning. In the case
of wet spinning, when a polymer solution is extruded from a spinneret
submerged within a chemical bath into the chemical bath, the polymer is
deposited caused by the chemical reaction or dilution impact, creating
nanofibers via solidification [107]. In the case of dry spinning, a poly­
mer solution is extruded into the air via a spinneret, and fibers are
generated through solvent evaporation from the jets assisted by a hot air
stream [108]. A polymer melt is extruded from a spinneret to produce
fibers upon cooling throughout the melt spinning [109]. Gel spinning is
used to generate fibers with high mechanical strength by spinning a
polymer in the gel form, followed by drying in the air and then cooling in
a liquid bath [110]. In these procedures, jets are primarily created under
mechanical drawing and/or external shearing forces while moving
through spinnerets, and fibers are generated upon jet solidification
because of drying or precipitation. The jets are just stretched to a limited
extent, associated with generating fibers with a diameter range of
10–100 μm. Even with additional mechanical drawing throughout the
solidification mechanism or after cooling of the jets, the resulting fibers
still cannot achieve the sub-micron scale [111]. However, the diameter
of nanofibers can be controlled by the electrospinning method.
Polymers generally have a low thermal conductivity of about 0.1 W/
m/K, which restrict their applications in various fields. This feature can
be because of their structural properties where many molecular chains
coiled up disorderly, limiting their heat transfer rate. Another reason is
that the molecular chains have an amorphous structure, decreasing the
average free pathway of heat-conducting phonons [112]. To encounter
this problem, one improving method is to draw polymer fibers that
enhance the chain alignment and crystallinity and therefore assist the
motion of phonons. In this way, electrospinning can generate wellaligned fiber arrays with ordered polymer molecular chains in each
fiber, thereby continually supplying the required conditions for phonon
Fig. 3. Polymer-based ENMs: (a) representation of ionic pathways in Q-PVA composite membranes containing electrospun nanofibers, reproduced with permission
from Ref. [51], Copyright Elsevier, 2016; (b) schematic representation of preparation process of the hollow fiber module. Copyright ACS Publication [57].
9
A.H. Behroozi et al.
Desalination 558 (2023) 116638
transfer. On the other hand, molecular orientation and crystallinity are
the essential parameters for improving thermal conductivity, which can
be provided by electrospinning effectively.
In the electrospinning technique, when the polymer solution jets are
created on the Taylor cone surface under the stretching of potent elec­
trical force, sufficient entangled chains are located along the jets' axial
direction to generate sequential fibers. The entanglement degree of
molecular chains in polymer solution grows by enhancing the relative
molecular weight. Jets are stretched to generate molecular chains
orientated well throughout the electrospinning process, which is ad­
vantageous for achieving beadless and uniform fibers. To date, diverse
polymers have been employed for preparing electrospinning nano­
fibrous membranes for nanofiltration applications [113,114], as its
general schematic for water purification is presented in Fig. 2a. These
polymers include polyvinyl alcohol (PVA), polyacrylonitrile (PAN),
polysulfone (PSf), polyvinyl chloride (PVC), polyethylene terephthalate
(PET), polyvinylidene fluoride (PVDF), polyethersulfone (PES), poly­
styrene (PS), etc. Some reports on employing such polymers in preparing
nanofiber membranes are summarized below.
agents, respectively) was more effective in eliminating turbidity
compared with the straightforward, direct filtration process operated
without any coagulants and filter media.
2.1.1.5. PET. Electrospun microfiber PET membranes were fabricated
by Bonfim et al. [71] to harness as an air filter media by electrospinning
in a solvent mixture comprising dichloromethane (DCM) and trifluoro­
acetic acid (TFA) with a ratio of 7:3. They recycled PET-type bottles as
plastic wastes, which are detrimental to the environment. The filter
membranes exhibited excellent mechanical resistance (4 MPa), good
permeability (4.4 × 10− 8 m2), and excellent porosity (96 %), along with
providing a perfect collection performance (approximately 100 %) and
low-pressure drop (0.212 kPa), where the face velocity was 4.8 cm/s for
the separation of aerosol particles, including fungi, bacteria, and viruses
like SARS-CoV-2.
In gas separation applications, Strain et al. [72] synthesized tough
mesomorphic PET fibrous membranes using solution electrospinning for
filtering cigarette smoke. By adjusting the spinning factors, they
controlled the fiber thicknesses in the range of 0.4–4.3 mm, achieving
the maximum fiber toughness and strength of 65.8 MJ/m3 and 62.5 MPa
with a mean average fiber diameter of 1 mm. The physical analyses
showed an inner morphology with a high molecular orientation degree
in the amorphous sections along the thinner fibers, compatible with a
limited mesomorphic phase created throughout their quick solidifica­
tion under the electrical force. The best smoke filtration performance
was related to the fibers with 0.4 mm thickness, showing a maximum
sorption capacity of 43 times its own weight in smoke residuals. In
contrast, the best-combined performance of mechanical robustness and
filtration capacity was attributed to the fibers with 1 mm thickness.
Therefore, the electrospinning method can prepare ENMs with
controlled thicknesses and diameters at the level applicable for different
purposes, even for treating gaseous molecules.
2.1.1.1. PVA. Liao et al. [51] generated electrospun quaternized PVA
(Q-PVA) nanofibers to incorporate into a composite membrane to
evaluate the methanol permeation rate. Fig. 3a depicts a schematic for
ionic paths in Q-PVA composite membranes containing electrospun
nanofibers. The composite nanofiber membrane containing about 6 w%
Q-PVA indicated excellent conductivity, high cell performance in direct
methanol fuel cells (DMFCs), and suppressed methanol permeation rate
associated with the quasi-coaxial architecture. Such nanofibers
demonstrated lower polymer crystallinity throughout the electro­
spinning process, particularly within the fibers' core, decreasing the
crystallinity in the core that created conducting channels in the mem­
brane while considerably enhancing the ionic conductivity.
2.1.1.2. PAN. Anka and Balkus [57] synthesized NF hollow fiber PAN
membranes using concentric electrospinning assembled within a filtra­
tion module for treating water contaminated with dyes. The schematic
view of hollow fiber membrane preparation is illustrated in Fig. 3b. They
introduced an inside-out mode in the module where a feed solution is
entered from the lumen. However, the filtrate is collected from outside
the fiber wall via a side arm. They prepared relatively smooth PAN fibers
with mean outer and inner diameters of 890 and 530 nm, respectively.
The filtration performance of the hollow fiber membrane was evaluated
by filtering NaCl and Indigo carmine dye solutions, exhibiting a com­
plete dye removal and a high salt rejection of 97.7 %.
2.1.1.6. PVDF. Gopal et al. [73] developed a fibrous membrane by
electrospinning PVDF nanofibers into membranes to separate poly­
styrene and remove other microparticles from the solution. The pre­
pared ENMs successfully separate polystyrene particles with diameters
of 1–10 μm and reject >90 % of microparticles. Under an appropriate
module design, such an ENM can be an excellent nominee to be
exploited as pre-filters to UF and NF processes to minimize the fouling
rate of micro-particles and micro-organisms in water.
Russo et al. [74] fabricated a PVDF ENM via dimethyl sulfoxide
(DMSO)/acetone mixtures as the solvent. They examined the influence
of operating electrospinning conditions, such as the needle-to-collector
plate distance and applied voltage, on the water permeability and
structural properties. By changing these parameters, the resultant
membrane depicted micron-size pores (0.2–0.8 μm), high water
permeation flux (11.000–38.000 L/m2⋅h⋅bar), and porosity (>80 %).
2.1.1.3. PSf. Gopal et al. [63] developed PSf nanofibers with a high
surface area through electrospinning to separate micro-particles from
aqueous solutions. The prepared membrane eliminated >99 % of par­
ticles with a diameter larger than 7 μm without precipitation. None­
theless, particles with diameters of 1 and 2 μm created an irreversible
cake formation on the membrane surface. The particles smaller than 1
μm adhered to the nanofiber surface, where the nanofiber membrane
behaved as a depth filter. Considering this range of diameter, the
developed membrane could be an efficient pre-filter for UF and NF
treatment of wastewater.
2.1.1.7. Polystyrene. Polystyrene nanofibers membranes were made by
Yang et al. [76] via electrospinning polystyrene liquor dissolved in
chloroform to treat water contaminated by Methylene Blue (10 mg/L),
Cr6+ (5 mg/L), and Cu2+ (5 mg/L). The fabricated membrane with 170
μm thickness demonstrated a nanofiber diameter and pore size in the
range of 250 nm–15 μm and 3 nm–0.5 μm, respectively. The filtration
tests in a plate membrane system showed removal rates higher than 91
% for all components, achieving a water permeation flux in the range of
5.8–15.4 mL/cm2⋅h.
Since each polymer has unique intrinsic properties, its selection in
electrospinning-based membrane fabrications relies on the target
application. In cases where the mechanical properties are important,
electrospinning can effectively manipulate molecular orientations in
fiber structures to achieve more strength. Moreover, the ionic conduc­
tivity of ENMs can be carefully controlled with the help of electro­
spinning parameters by adjusting the crystallinity within polymeric
2.1.1.4. PVC. Asmatulu et al. [69] prepared PVC solution mixed with
polyvinylpyrrolidone (PVP) to produce nanofiber membranes via the
electrospinning method for filtering various liquid suspensions con­
taining magnetite nanoparticles, abrasive particles from a water jet
cutter, and lake water. They indicated that PVC nanofiber membranes
are highly hydrophobic, interfering with their capability of being uti­
lized as a filtering agent. Moreover, in terms of prevailing the fouling/
blocking/biofouling phenomena, a hybrid filtration by coagulation
(chemical agents of Tanfloc and Alum as organic and inorganic chemical
10
A.H. Behroozi et al.
Desalination 558 (2023) 116638
fiber structures. Overall, an appropriate selection of polymer type could
produce high selective and permeable ENMs for practical applications
after optimizing electrospinning parameters.
concentration, thereby considerably affecting polymer solution spinn­
ability, electrospun fiber diameter and morphology [123]. Hence,
choosing a desirable solvent system is crucial for successful
electrospinning.
Different properties related to solvent nature play a significant role in
electrospinning. For example, its boiling point refers to the temperature
when the solvent vapor pressure equals the ambient pressure, which
indicates its evaporation rate. A lower boiling point means a faster
evaporation rate and vice-versa. For example, since DCM/ethanol
mixture solvent has a higher boiling point than pure DCM, it evaporated
slower from the ejected charged jet. This phenomenon changed the
viscoelastic jet characteristics and produced very smaller fiber diameters
with irregular and non-uniform shapes because of jet stretching [124].
Another property related to the solvent is the dielectric constant,
which is the amount of free charge induced into the polymer solution
throughout electrospinning. An increase in the solution conductivity or
charge density results in the initiation and growth of non-axisymmetric
perturbations in the electrospinning jet. In other words, the efficiency of
ion disassociation in solvents considerably decreases with a lowdielectric constant, lowering the amount of free charge characterized
by a lower solution conductivity. A decrease in the available charge on
the jet surface weakens the electrostatic repulsive force, possibly stabi­
lizing the jet for higher distances from the spinneret [125].
The solvent volatility is another critical solvent property in produc­
ing porous fibers, mainly in highly humid media. In a highly volatile
solvent, the evaporation rate is faster, favoring the rapid cooling of the
fiber surface to a minimal temperature, namely the wet bulb tempera­
ture (Twb). While cooling to complete saturation, the surrounding air
obtains the dew point (Tdp) and water condenses on the surface. How­
ever, in low volatile solvents, the condensed water vapor will have time
to migrate into bulk and make phase separation, producing wrinkled
fibers. Therefore, electrospinning solutions with higher solvent vola­
tility will generate fibers with more surface pores [126].
Moreover, the solubility of a solvent can measure the affinity be­
tween solvent and polymer, which indicates the possibility of preparing
a suitable electrospinning solution. This indicated is characterized by a
solubility parameter distance (Ra), in which a lower Ra value means a
better solution. For example, DMAC is a better solvent than acetone
(ACE) for preparing polymethyl methacrylate (PMMA) membranes
because of its lower Ra value (4.6) than acetone (6.2). The good solvent
demonstrates a low degree of macromolecule chain overlap and aggre­
gation in solutions, resulting in lower solution viscosity, and conse­
quently smaller fiber diameters [127].
To date, much research has been dedicated to scrutinizing the effect
of several single and hybrid solvents on the properties of ENMs. For
example, Song et al. [128] prepared polyethylene oxide (PEO) electro­
spun nanofibers by electrospinning PEO solution using DI water and
ethanol mixed solvent. The nanofibers with wrinkly surface morphol­
ogies appeared by increasing ethanol content because of a high ethanol
evaporation rate throughout the electrospinning process. Also, higher
ethanol concentrations lowered the solvent dipole moment, density,
dielectric constant, conductivity, solubility, and boiling point and
weakened the hydrogen-bonding interactions among solvents and PEO.
Therefore, with such a mixed ethanol-rich solvent, PEO nanofibers with
lower crystallinity degree and molecular chain orientation but with
larger nanofiber diameters.
Yoon et al. [56] modified the PES nanofiber membrane using a mixed
solvent (DMF: NMP) at different compositions to enhance the oxidation
mechanism and mechanical characteristics. The tensile strength and
Young's modulus significantly improved by 360 % and 570 % by adding
a high boiling mixed solvent of NMP with DMF. As a second treatment,
the nanofibers modified by 3w/v% of ammonium persulfate improved
the hydrophilicity of the prepared membranes where the water contact
angle decreased from 120◦ to 28◦ .
Yin et al. [129] investigated the influences of several mixed solvents
containing DMSO, DMF, tetrahydrofuran (THF) and ACE on the
2.1.2. Polymer concentration and viscosity
The electrical and rheological features of polymer solution are
directly affected by the relative molecular weight of polymers chosen for
nanofiber generation [115]. At a constant relative molecular weight of
the polymer, the solution concentration is the most influential param­
eter affecting the morphology and structure of electrospun fibers. When
the viscosity and concentration of the electrospinning polymer solution
are low, only beads can be produced under electrostatic pressure
because the polymer's molecular chains are not entangled at all or at
least not entangled sufficiently. Simultaneously, the molecular chains
are inclined to shrink because of the viscoelastic impact, leading to the
aggregation of chains and consequently the generation of beads [116].
When the viscosity and concentration of the polymer solution are
higher than a specific critical quantity when the jet is exposed to uniform
tensile force under the electric field force and surface charge, the
relaxation time of jets is higher because of the entanglement among
molecular chains and the improvement in stretching of the electrical
force [117]. The entangled chains are located along the axial path of jets,
effectively restricting the breakup of some chains in jets. Therefore,
continuous electrospun fibers are formed. During the drawing process of
jets, some beads would be generated due to the inconsistent alignment of
chains as well as uneven force, which produce electrospun fibers with
beads. The tensile force is much more uniform by increasing solution
concentration further. Jets move quickly under the influence of electric
field force and surface charge. Finally, the electrospinning solution will
be solidified into fibers after the evaporation of the solvent [118,119].
Under the same electrospinning conditions, the diameter of fibers
increases by enhancing the polymer concentration. Nevertheless, Deitzel
et al. [120] hypothesized a power law-based non-linear relationship
between the diameter of fibers usually generated and the solution con­
centration. This unexpected trend can be because of the non-linear
relation between the solution viscosity and polymer concentration.
Furthermore, they observed that solutions with high concentration
generated a bimodal distribution of electrospinning fiber sizes, similar to
the distributions seen in the droplet formation mechanism of
electrospray.
Nordin et al. [121] examined the effect of polybenzimidazole (PBI)
concentration (6.5 and 7.5 w/v%) in the polymer solution on the me­
chanical properties, morphology, and hydrophobicity of PBI nanofiber
membranes. They found that the mean nanofiber diameter was 97 and
75 nm for 7.5 and 6.5 w/v% PBI solution, respectively, along with an
increase in the contact angle from 111◦ to 125◦ depicting hydrophobic
membranes. Moreover, higher PBI content improved the mechanical
characteristics of nanofiber membranes, obtaining the maximum stress
quantity of 4.20 MPa for 7.5 w/v% PBI.
Three polymers, namely polycarbonate urethane (PCU), poly­
ethylene glycol (PEG), and polycaprolactone (PCL), have been electro­
spun by Nezarati et al. [122]. They investigated the impact of solution
viscosity by changing the solution concentration from 15 to 20 wt%.
Their results exhibited beaded fibers at low viscosity, i.e., 7.2 Pa⋅s,
uniform ones at an intermediate viscosity, i.e., 10.1 Pa⋅s, and larger ones
at high viscosity, i.e., 22.5 Pa⋅s, where the diameter of fibers enhanced
from 1.2 to 3.5 μm. Overall, the polymer solution viscosity and con­
centration mainly affect the type and diameter of fibers produced. Thus,
optimizing these factors can make generated fibers highly efficient for
practical membrane-based separation applications, for example,
removing pollutants with small sizes from water.
2.1.3. Solvents
The selection of a solvent system or an appropriate solvent is another
pivotal factor in producing nanofibers that should be considered. This is
because the solvent is essential in measuring the minimum solution
11
A.H. Behroozi et al.
Desalination 558 (2023) 116638
morphology of PVDF ENMs. With a low-toxicity solvent, e.g., DMSO, the
PVDF nanofiber membranes exhibited good physical properties, such as
bead-free, uniform nanofiber, and smooth surface. Moreover, the
fabricated membranes indicated a higher β-phase fraction, piezoelectric
output, and crystallinity when using DMSO/ACE (2:1) solvent with a
high dipole moment than that with DMSO/THF (1:2). Accordingly, the
introduction of novel, non-toxic, and inexpensive hybrid solvents could
be an effective way in manipulating the crystallinity and orientation of
molecular chains in ENMs.
salts (sodium chloride, potassium phosphate, and sodium phosphate) to
polycaprolactone (PCL) solutions lowered bead defects and diameter of
fibers in the fabricated semi-crystalline poly (l-lactic acid) (PLLA) and
poly (d, l-lactic acid) (PDLA) non-woven membranes. Additionally, the
density of the charges carried by jet and the size of ions play an indis­
pensable role in the resultant fiber diameter. Hence, ions with smaller
atomic radius have a higher charge density and consequently higher
mobility under an electric field [136]. Therefore, the lowest mean fiber
diameter is related to the smallest atomic radius (like NaCl).
2.1.4. Additives in polymer solutions
To date, various additives can be incorporated into the polymer so­
lution used in electrospinning to improve the structural characteristics
and filtration performance of ENMs. Fig. 4 illustrates the most applicable
additives mixed with the main polymer solution and their effects on the
membrane properties.
2.1.4.3. Nanoparticles. The incorporation of nanoparticles into polymer
solutions has recently received much attention due to their significant
impacts on the structural properties of ENMs, particularly fiber diam­
eter. Siddique et al. [64] featured PSf-based NF membranes containing
graphene oxide (GO) and zinc oxide (ZnO) via electrospinning for
arsenic removal from contaminated water. Adding these oxides into the
PSf matrix decreased the fiber diameter but enhanced the membrane
porosity, negative surface charge, and hydrophilicity, in addition to
improved water permeability and metal ion separation efficiency. Obaid
et al. [65] increased the removal rate of PSf electrospun nanofibers by
separately doping GO nanosheets and amorphous silica nanoparticles
extracted from agriculture waste to treat water polluted by petroleum
fractions. They found that the SiO2 addition improved Young's modules
of membranes, whereas GO negatively affected their mechanical char­
acteristics. Rochardjo et al. [52] fabricated NF membranes produced
from PVA nanofibers fortified with cellulose nanocrystal (CNC) via
electrospinning. The CNC content did not considerably influence the
fiber diameter in the prepared membranes. Nevertheless, adding CNCs
until a given amount enhanced the elongation and strength of mem­
branes in addition to their improved thermal resistance.
2.1.4.1. Surfactants. The addition of surfactants may reduce the surface
tension of the polymer solution. Lin et al. [134] expressed that adding
dodecyl trimethylammonium bromide (DTAB) as a cationic surfactant to
the polymer solution decreased the mean diameter of the electrospun
PAN nanofibers. Similarly, as reported in Jung et al. [131] work,
increasing the content of the amphoteric surfactant by 4 v/v% reduced
the PVA fiber diameter from 250 nm to 150 nm. However, a slight
reduction in the mean fiber diameter is observed when increasing the
concentration of both anionic and cationic surfactants in the polymer
solution.
2.1.4.2. Salts. Adding salts to the polymer solution enhances the sur­
face charge density and conductivity of the solution jet. Zong et al. [135]
asserted that when using a rotating drum as the collector, the addition of
Fig. 4. Additives in electrospinning polymer solutions for generating ENMs and their primary impacts on the membrane properties. SEM images extracted from the
literature with permission: salts [130], surfactant [131], polymer [68,132], NPs [65], and protein [67,133].
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2.1.4.4. Polymer. The addition of other polymers and their blends with
common polymers can improve the hydrophilicity and mechanical
strength of ENMs. The PSf solution was blended with Al2O3 nano­
particles and polyethyleneimine (PEI) at different concentrations by
Uzal et al. [68] to generate nanofibrous membranes for evaluating
bovine serum albumin (BSA) rejection and pure water permeation. Their
observations confirmed that blending PSf with 0.05 wt% Al2O3 and 2 wt
% PEI led to higher porosity and mechanical and tensile strength,
demonstrating a highly hydrophilic, permeable membrane because of its
excellent water flux (28,456 L/m2 h) and a low water contact angle
(23.5◦ ).
3. Electrospinning conditions
3.1. Voltage
The applied voltage is a critical factor in the electrospinning process
in producing smooth ENMs since their generation occurs when the
voltage exceeds the threshold voltage (approximately 1 kV/cm,
depending on the polymer solution). The voltage required for creating a
jet from a polymer/solvent solution must be sufficient to overwhelm the
surface tension on the droplet surface. As reported frequently, the pore
diameter of ENMs is not considerably influenced by the applied voltage.
The rise in the voltage will increase the electrostatic force of the poly­
meric solution, leading to the jet stretching (jet elongation) and the fiber
length reduction (formation of thinner fibers), as illustrated in Fig. 5a. It
turned out that varying the voltage will vary the shape of the initial
droplet, thus changing the structure and morphology of fibers
[136,137]. Zhang et al. [79] expressed that higher applied voltages led
to more polymer ejection, which produced nanofibers with higher di­
ameters. Another study by Ali and El-Hamid [80] supported the idea of
the previous work, depicting that the optimal charge density was 2.5
kV/cm, and the fiber diameter increased with the charge density.
Reneker et al. [138] reported that there is no impact on the PEO fiber
diameter when a higher voltage is applied. Also, the fiber diameter of
electrospun PAN did not significantly change when a wide range of
voltages was applied under different concentrations of polymer solu­
tions [139].
Other researchers have mentioned that the increased voltage reduces
fiber diameter [140–142]. For example, Wang and Kumar [143] varied
the charge density by enhancing the voltage from 13 to 27 kV while
maintaining the distance from the tip-to-collector constant at 10 cm.
Their results demonstrated that the fiber size decreased when using a
high voltage, or the fiber size did not show a noticeable reliance on the
2.1.4.5. Protein. Proteins can also change the hydrophobic nature of
polymers to hydrophilic when blending due to their abundant functional
groups. Karunanidhi et al. [67] added keratin as a bioinspired substance
to the PSf matrix for preparing MF-based ENMs for efficiently treating
highly toxic effluents released from tannery industries. The keratin-PSf
membranes with a ratio of 95:5 v/v% were partially hydrophilic
compared to hydrophobic PSf membranes, achieving a random fiber
arrangement with the interstitial spaces. Moreover, the modified
membranes illustrated suitable antifouling properties, improved me­
chanical characteristics, and excellent adsorption capacity.
Due to the importance of surface structural properties in membrane
separation applications, the selection of a suitable additive into an
electrospinning polymer solution can be determinative in the perfor­
mance of the final membrane. Since each additive can add distinct
features to membranes, the target separation application or contaminant
type in water should be carefully considered in addition to other elec­
trospinning parameters to achieve high-performant water purification.
Fig. 5. Effect of electrospinning conditions: (a) applied voltage on electrospinning process and nanofiber size (a); effect of relative humidity on the structure and
morphology of electrospun PS/THF fibers based on SEM images for RHs <25 % (b), 31–38 % (c), 40–45 % (d), 50–59 % (e), and 60–72 % (f).
Copyright American Chemical Society [146].
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Desalination 558 (2023) 116638
voltage for all given polymeric solution concentrations. In another
study, the as-spun fiber diameters first increased, reaching a maximum
quantity, and then lowered when applying higher voltages [144].
Moreover, beads can be formed on the fibers at higher voltages [145].
For example, Deitzel et al. [120] detected bead formation when a higher
voltage is applied to the PEO/water solution, with similar results to
those reported by Zong et al. [135]. However, Siddique et al. [64] re­
ported that the surface of the PSf fibrous membrane was smooth and
aligned randomly without any beads by increasing the voltage even after
adding GO, indicating the formation of stable nanofibers.
A higher voltage (>35 kV) causes faster acceleration of the emitted
jet; resulting in a shorter flight period for the jet to be stretched prior to
deposition and forming larger fiber diameters (Fig. 5a). An increase in
jet speed may also lead to incomplete solidification or lower fiber
thickening as the polymer/solvent system touch the collector in a
shorter time. The electrospinning process will happen only when the
voltage is higher than the surface energy of the polymer/solvent solu­
tion. When the applied voltage approaches the surface energy, the shape
of the droplet solution is distorted to form a Taylor cone. The critical
voltage at which this takes place relies on various factors, including
droplet diameter, solution surface tension, and the electric field
strength, where small variations in these parameters can compensate for
the voltage change impacts.
The contradictions in understanding the effect of higher applied
voltages on the fiber size of ENMs might be because of the difference in
processing conditions. Whether or not the fiber diameters decrease or
increase with the voltage, is considerably dependent on the conditions,
such as polymer solution viscosity. According to the literature related to
the voltage impacts in Table 1, the voltage mainly affects the fiber
diameter. However, the consequences differ from the distance between
the tip and collector as well as the concentration of the polymer solution.
To solve the concern related to the applied voltage, different DC voltages
can be used to provide a voltage in the range of 5–35 kV to maintain the
electric field strength constant during electrospinning.
In a membrane-based water purification study, Zhu et al. [90]
designed electrospun PAN NF membranes with narrowed pore sizes to
remove heavy metal ions and dyes from water and saline solutions. The
membranes were modified first with ethanolamine, and then the
asymmetric nanochannels were narrowed via the PI approach by con­
tacting the membrane bottom with a polymer solution and then
immersing the nearly dissolved membranes in water for solidification.
High voltages (0–16.5 kV) were applied under moderate relative hu­
midity (36–40 %). The intensification of the applied voltage from 5 to
10 V enhanced the rejections of negatives dyes, such as Methylene Blue
(MB), Crystal Red (CR), and Methyl Orange (MO), and positively
charged dyes, such as Crystal Violet (CV) and Rhodamine B (RhB).
Numerically, the MB rejection with the maximum valence (raised from
42 % at 0 kV to 96 % at 5 kV) was greater than CR (from 40 % at 0 kV to
80 % at 5 kV). Additionally, the membrane flux initially increased and
then decreased with voltage, in which the flux and MB rejection were 75
LMH and 98 % at 10 V and 0.1 bar. Furthermore, the rejection for Cu2+
and Ni2+ ions was above 99 % from a NaCl solution (0.5 %) at 10 kV
with a flux of about 7.4 LMH at 0.01 bar.
hydrophobic or hydrophilic) used.
Fiber diameter might decrease or increase with RH, relying on the
polymer nature. For hygroscopic and hydrophilic polymers dissolved in
hydrophilic or aqueous solvents, the rise in humidity would delay the
solidification time due to polymer plasticizing and water absorption,
which increases both fiber diameter and bead defects. Also, the fibers are
coarser at low humidity than at high ones. For example, Yang et al.
[148] demonstrated that the diameter of PEO fibers decreased by
increasing ambient humidity, which generate beaded fibers after elec­
trospinning PEO from water solutions.
In the case of hydrophobic polymers, the increased humidity typi­
cally lowers the solidification time. Thus, low humidity produces thin
fibers while, on the contrary, at high humidity, coarser fibers are ob­
tained, possibly forming porous fibers, cotton-like structures, and
nanonets. Higher humidity will delay the water evaporation, increasing
the drying time and producing thinner fibers. This would give water
sufficient time to evaporate before the jet completely hits the collector.
Tripatanasuwan et al. [149] studied the RH effect on the evaporation
and solidification of the charged jet after electrospinning PEO from an
aqueous solution. The results demonstrated that the RH increment
delayed the solidification mechanism, allowing the elongation of the
charged jet to keep going longer and forming thinner fibers. When the
RH increased from 5.1 % to 48.7 %, the fiber diameter decreased from
253 to 144 nm. However, the beads were created on the thinner fibers at
a higher RH than 50 %, indicating that the capillary instability occurred
prior to the jet solidification.
The non-aqueous solution can be influenced by humidity in different
ways. The first way, if the relative water vapor pressure is very high, the
evaporation will be postponed because of the saturation impact even
without any solvent in the atmosphere. There will be a limitation on how
much liquid the atmosphere can hold at a certain pressure before either
solvent or water start to condense back into liquid form. In such solu­
tions, the humidity can have several consequences: (1) thinner fibers can
be generated at longer solidification times, or (2) a congealed mat of
fibers may be formed due to incomplete solidification time. Another
influence of these solutions is that the solution can adsorb water during
the flight. The adsorption rate will rely on the material's affinity for
water and the solution's capability to absorb water. This could delay the
evaporation mechanism, which increased the fiber size, introduced
pores onto the nanofiber surface due to the differences in solvent con­
centration, and possibly produced congealed mats instead of unwoven
fibers. As stated by Jenus et al. [150], the increased RH from 30 to 70 %
made the PLLA fibers porous when electrospinning from a chloroform
solution. Casper et al. [146] considered various ranges of humidity
(<25 %, 31–38 %, 40–45 %, 50–59 %, and 60–72 %) to examine its
effects on the produced PS fibers from THF solution via electrospinning.
As shown in Fig. 3b–f, the humidity variation could generate PS/THF
electrospun fibers with different morphologies. At RH < 25 %, the fibers
were smooth in the absence of any pores on the surface (Fig. 3b). A
further RH of >30 % (31–38 %), uniform and circular pores started to
become visible on the fiber surface (Fig. 3c) with a random distribution
on the fiber surface. At RHs in the range of 40–45 %, the shape of pores
remained almost constant while they began to heavily populate the
surface (Fig. 3d). By bringing the RH in the range of 50–59 %, the
multitude of pores appeared on the surface, leaving little space between
adjacent pores. Due to non-uniform-shaped structures and the integra­
tion of smaller pores into larger ones, the pores were not circular. At
higher RHs (60–72 %), the pores are non-uniform in shape (Fig. 3f),
which are larger than the pores observed in the previous case (Fig. 3e).
In another study, Vrieze et al. [151] examined the influence of humidity
on the properties of the fibers generated by two different polymers,
namely poly(vinylpyrrolidone) (PVP) and cellulose acetate (CA). For CA
polymer, the mean fiber diameter is enhanced with the humidity, while
the mean PVP diameter is reduced. For PVP polymer, the absorption of
the surrounding water with higher humidity made the nanofibers so­
lidify slower. As well, the elongation of the charged jet could continue
3.2. Humidity
Humidity, or relative humidity (RH), is another influential factor in
characterizing ENMs produced via the electrospinning process. It can
control the structure and morphology of ENMs, affecting the solidifi­
cation time of fibers and the evaporation rate of solvents [147]. Whether
the solidification of the electrospun nanofibers occurs slowly or rapidly,
the structure of the mats and the fiber morphology may be changed,
depending on the processed formulation features. The humidity will
affect each polymeric solution or polymer/solvent solution differently
based on whether the hydrophobic or hydrophilic polymer solution
(hygroscopic, hydrophilic, or hydrophobic) and the solvent (aqueous,
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Desalination 558 (2023) 116638
longer, which resulted in thinner fibers. However, at high RH (60 %), the
nanofibers started to be fused, which enlarged fiber diameters. In the CA
polymer solution, more absorption rate of water with high humidity
could accelerate the precipitation process and consequently form larger
fiber sizes. Both polymers exhibited opposite trends caused by their
different molecular and chemical interactions with solvent and water
vapor. These differences caused the solidification process and solvent
evaporation of each polymer to respond differently.
Tang et al. [55] developed a PES electrospun nanofibrous scaffold for
TFNC NF applications prepared by electrospinning of PES solution on a
PET non-woven support. They investigated different relative humidity
(RH = 45 %, 50 %, 60 %, 70 %) under a voltage of 30 kV, a distance
between the spinneret and rotating drum of 10.5 cm, and a temperature
of 25 ◦ C, showing a significant humidity impact on nanofiber mem­
branes. A higher humidity led to extensive fibers where the fiber size
increased from 266 nm at 45 % RH to 492 nm at 70 % RH. In addition,
more water molecules formed between the collector and the spinneret at
higher RHs. Water molecules would improve the conductivity of this
area, thus varying the property of the electric field caused by the po­
larization of H2O molecules. Also, a higher RH lessened the intensity of
the electric field (dampening). Hence, the nanofiber membrane became
thicker at higher RHs since a thick-diameter fiber could proceed from a
weaker electric field strength and, thus, a lower draw-down force.
Another influence of RH on the adhesion between PET non-woven
support and PES nanofiber was also studied. At low RHs (<45 %), the
adhesion between PES and PET was relatively low (2.6 psi), whereas the
adhesion between PES and PET significantly increased to 40.2 psi when
enhancing the RH from 45 % to 50 %, probably because of the “skin”
creation, hindering the solvent evaporation. A further increment in the
RH to 70 % remarkably reduced the adhesion between PES and PET
because of the likely PES “precipitation” in the jet stream throughout the
electrospinning process.
Furthermore, the RH strongly impacts the fiber diameter or forma­
tion, considerably depending on the type of solvent and polymer uti­
lized. Increasing the humidity decreases the fiber diameter, producing
beaded fibers and vice versa. By the way, the humidity directly affects
the evaporation rate and solidification process, in which the solidifica­
tion process becomes slower as RH increases, allowing the charged jet to
elongate longer and resulting in thinner fibers [78]. Regarding the
evaporation rate, the increased RH would delay the water evaporation
rate (when using water as a solution), thereby enhancing the drying time
and forming thinner fibers.
for 12 h. The deposited rGO layer could remove approximately 90 %
MgSO4 with an excellent water flux because of the size exclusion phe­
nomenon, bearing an efficient separation of organic molecules and hy­
drated ions from water. The separation illustration of different
components by the PAN-rGO nanofiber membrane is depicted in Fig. 6b.
Kaur et al. [82] synthesized the electrospun PAN membranes with a
mean fiber diameter of 287 nm on the surface of a hollytex backing
material (BM) for NF applications. They studied the hot-pressing con­
ditions and pressure effects on the pore size distribution, filtration per­
formance, and the mechanical and thermal characteristics of the ENMs.
The TFNC based on the ENMs support layer was subjected to hot
pressing in a thermal transfer press at 87 ◦ C for 16.65 min and different
pressures (ENM-1: 0.14, ENM-2: 0.28, and ENM-3: 0.41 MPa). The hotpressed TFNC membranes represented improved rejection, more pres­
sure tolerance, and mechanical strength compared with the untreated
TFC membrane. Without hot pressing, the ENM-control exhibited a
larger pore size (5.6 μm) as compared with the treated ENM-1 (2.3 μm),
ENM-2 (1.1 μm), and ENM-3 (0.8 μm), as determined by the bubble
point procedure. The large pore size of ENMs was due to their loose
fibrous structure, highlighting the significance of hot-pressing impacts
on the pore size distribution of the membranes and, eventually, their
water flux. The thickness of the ENM layer decreased in the following
order: ENM-control (74 μm) > NENM-1 (48 μm) > NENM-2 (21 μm) >
NENM-3 (18 μm) since the ENMs were compacted by applying pressure.
Due to the increased fiber diameter and crystallization, the mechanical
characteristics also improved with hot-pressing pressure. For ENM-2,
tensile strength and yield stress were improved by 203 % and 313 %,
respectively, compared with the pristine samples, indicating that the
adhesion between the ENM and baking material (BM) significantly
improved. Furthermore, the interfacial polymerization process was then
implemented on the ENM-control, ENM-1, and ENM-2, and the resultant
TFNC membranes were labelled TFNC-control, TFNC-1, and TFNC-2,
respectively. NF experiments were conducted using a MgSO4 feed so­
lution (2000 mg/L). For TFNC control, the membrane was not stable at
higher fluxes. However, the TFNC-1 and TFNC-2 depicted higher capa­
bility to resist a TMP of 190 psig with a rejection of 88 % and 92 %,
respectively, reflecting the influence of hot pressing on separation per­
formance. Also, the developed TFNC-1 and TFNC-2 demonstrated higher
fluxes (291.58 LMH and 261.70 LMH) than commercial NF270 (239
LMH) and NF90 membranes (82 LMH), but with roughly lower salt re­
jections (8–12 %) due to the very high porous fibrous structure of ENM.
Therefore, their fouling tendency was expected to be lowered because of
the decreased roughness.
An excellent example of the other approach, Park et al. [53] prepared
composite NF membranes by chemically crosslinking GO nanosheets as
a selective layer on electrospun PVA fiber, as schematically shown in
Fig. 7. The GO layers became stable by crosslinking GO-to-GO nano­
sheets and by linking GO onto the PVA surface with the help of GA. The
modified nanofiber membranes exhibited a defect-free glutaraldehyde
(GA) crosslinked GO layer with ~67 nm thickness, verifying a successful
GA crosslinking of GO-PVA and GO interlayers using acetalization. The
best-modified nanofiber membrane could remove Na2SO4 (2 mmol/L),
NaCl (20 mmol/L), Eosin Y (10 mg/L), and MB (10 mg/L) from the
permeated water by about 91 %, 50 %, 98 %, and 77 %, respectively.
Due to improved filtration and separation performance, applying posttreatment modification methods on ENMs could be advantageous if
their processing conditions are optimized. However, the energy con­
sumption and complex modification process can be potential obstacles
to their practical applications.
3.3. Post-treatment
Post-treatment is the process of modifying ENMs to add more func­
tionalities to the existing membranes, enhance their pore size distribu­
tion, or/and improve their thermal and mechanical properties. Different
modification techniques have been exploited for these purposes, which
include hot-pressing, crosslinking, drawing/stretching, solvent welding,
heat treatment/annealing, hot stretching, and chemical crosslinking.
Among them, the two first methods have recently enticed much
consideration.
Hot-pressing is an approach that can improve the mechanical
integrity of membranes, allowing more control over thickness and pore
size and achieving higher permeation fluxes and separation efficiencies.
Pressure and temperature are two essential variables that require to be
worked with to optimize hot pressing treatment. This method generally
increases fiber diameter (size) because of heat-aided compaction of the
fibrous web, leading to inter-fiber welding and resulting structural
integrity [152].
Wang et al. [58] overlaid GO nanosheets on the surface of a hotpressed electrospun PAN nanofiber membrane to generate a crack-free
layer as an NF media for water treatment. They deposited GO layers
on the PAN membrane via vacuum-assisted filtration, then thermally
reduced to into PAN-rGO membranes within a vacuum oven at 150 ◦ C
4. Electrospun membrane as support of TFC NF membranes
TFC membranes have become the most common membranes used for
different membrane filtration technologies, including UF, RO, MF, and
NF. They offer the properties of decent water permeation flux, good
solute rejection, relatively low cost, less membrane fouling, and
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Desalination 558 (2023) 116638
Fig. 6. PAN nanofiber membranes coated by GO and rOG: (a) Scheme of paths of water molecules and various hydrated ions; Surface SEM images of (b) GO sheet on
as-electrospun PAN nanofiber membrane, and (c) GO layer edge area, (d) GO layer center area of the hot-pressed PAN ENM. Copyright IOP [58].
Fig. 7. GO-PVA composite NF membranes: Schematic preparation route without (a) and with (b) crosslinking by GA crosslinker with SEM images.
Copyright MDPI [53].
reasonable mechanical properties. As mentioned earlier, these mem­
branes include three different layers: the top ultra-thin selective layer, a
middle porous support layer, and a bottom non-woven fabric layer. The
thin top layer is responsible for the separation of impurities as the pore
size is very small on the nanoscale (i.e., RO and NF) [153]. The porous
support layer is asymmetric that can work as a filtration function or
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Desalination 558 (2023) 116638
support function (i.e., UF). The non-woven layer (substrate) provides
mechanical support for membranes. TFC NF membranes can separate
small molecules of undesirable components or pollutants from water.
Current research has indicated that the top thin and porous support
layers could be exchanged by ENMs with fiber sizes ranging from mi­
crometers to nanometers, called TFNC. A general schematic of TFNC
membranes is represented in Fig. 8a for treating water containing
monovalent and divalent ions.
Electrospun nanofibrous scaffolds are highly applicable in different
water treatments, including desalination and removal of heavy metal
ions, organic compounds, microparticles, and microbial. ENMs have
garnered tremendous interest in recent years as potential supports for
TFC NF membranes due to their exciting features, such as low tortuosity,
excellent surface area, good porosity up to 80 %, and high orientation or
alignment of nanofibers [55–58]. The pore size, nanofiber diameter, and
porosity of ENMs considerably impact the generation of the active layer,
which in turn, would affect their filtration performance and separation
efficiency. These membranes, as the support layer have higher internal
porosity, interconnected voids, and nano-composite architecture of the
selective layer, resulting in higher permeation flux and solute rejections
and lower energy consumption cost [48]. The higher permeation may be
ascribed to the higher porosity, lower transmembrane pressure (TMP),
and interconnected pores within ENMs. Despite all the advantages, they
are some drawbacks that limit the practical development of electrospunbased nanofiltration membranes, such as the requirement of specific
solvents and equipment and high applied voltage for producing ENMs.
Since ENMs are often thin layers, the fabrication time is relatively long
when the thickness is large [41]. Furthermore, it is hard to control their
structure; therefore, modification of ENMs is required to extend their
potential applications.
Numerous studies have been dedicated to using ENMs as a support
layer. For example, Subramanian and Seeram et al. [154] developed TFC
NF membranes by replacing the porous polymer layer with an ENM
layer to purify low salt-content water. As they claimed, TFNC mem­
branes had the capability of controlling the thickness of the ultrathin top
layer as well as the selection of suitable nanofibrous substrates. Thus, the
optimization of these two layers regarding stability and structure would
lead to excellent separation efficiency. Generally, the structure of TFNC
endows them with two remarkable features for NF applications: higher
permeation flux and energy-efficient composite membranes. These two
advantages could realize their practical applications in wastewater
treatment plants.
Using electrospun nanofibrous scaffolds as a porous support layer for
the TFC NF process is implemented by Yoon et al. via electrospinning
PAN nanofibers. The nanofibrous mid-layer was used as a support layer
in a high flux TFNC membrane. Then, the active top layer was produced
by IP of piperazine (PIP), trimesoyl chloride, and some other additives.
As a result, the PIP concentration critically participated in the IP process
for optimizing the membrane water flux and MgSO4 rejection. The TFNC
membranes improved the flux (51.2 LMH psi) and rejection (>96 %) by
21 % and 10 % compared to the unmodified membranes. However, the
flux of TFNC membranes became double (from 42 % to 84 %) when
using a mixture of amine solution (PIP:bipiperidine). Furthermore, the
TFNC membrane improved the water flux by 38 % while keeping the
Fig. 8. TFNC membranes for water treatment: (a) general schematic; SEM images of (b) PES TFNC membrane fabricated by IP using 1 % (w/v) PIP. Copyright
Elsevier [60]; (c) representation of the layers present in PAN TNFC membrane prepared by 4 % (w/w) PAN solution. Copyright Elsevier [82].
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Desalination 558 (2023) 116638
rejection similar to the commercial NF270 at 4.5 bar [86].
Yung et al. [84] developed ENMs for NF applications by dissolving
PES in a DMF solvent, then in a mixture of solvents (DMF:NMP) with
different ratios (25–75 %), and finally spun onto PET nonwoven support
layer. The mixed solvent could enhance the adhesion between the PET
nonwoven scaffold and the forming layer of PES nanofibrous. In the next
step, the active barrier layer formed on the surface of the nanofibrous
scaffold using IP and adding some surfactants to monomer solutions (e.
g., PIP and TMC). The prepared ENMs improved the performance of
TFNC membranes, in which their water permeation flux reached two
times (50 LMH) higher than that for commercial NF-90 (27.2 LMH) with
an approximately equal salt rejection rate (99 %).
The ENM applications as a support layer for TFC NF membranes were
recently by Kaur et al. in two distinct works. In the first research [60],
the effects of fiber size on the filtration performance and salt rejection of
TFC NF membranes were evaluated. The PAN thin film membrane
formed onto the surface of PAN nanofibers through the IP process was
confirmed by SEM images (Fig. 8b) for the TNFC membrane prepared by
4 % w/w PAN solution. According to the results, the pore size and the
flux decreased, but the salt rejection increased when the fiber diameter
decreased. The reduced cross-sectional thickness of the electrospun
layer, together with the decreased pore size, resulted in improved flux
and salt rejection. In the other research [82], as mentioned before, the
hot-pressing method was applied to evaluate the presence of the nano­
fiber layer on the PAN nanofibrous membrane performance (Fig. 8c).
The higher applied pressure membrane displayed lower water flux and
higher salt rejection than TFNC membranes. The important outcome
from both studies can be summarized into (1) the ENM layer must be
hydrophilic to boost the IP process successfully, (2) the ENM thickness
layer must be decreased to improve the permeation, (3) smaller pore size
is required to enhance the salt rejection, and (4) the BM and ENM layers
must be hot pressed prior to IP to increase the adhesion between these
layers and promote the mechanical features. Overall, the electrospun
nanofibrous layer can be considered a substrate or support layer for
advancing the filtration performance (flux and solute rejection) and
mechanical properties of TFC NF membranes.
5. Coated electrospun support as NF membranes
With the increasing demand for using ENMs as a support layer for
TFC NF membranes, the nanofibrous scaffold cannot function well in NF
processes because their selectivity is not quite efficient against ions,
and/or their mechanical properties cannot resist the high pressure
applied. Hence, it is necessary to modify their surfaces to solve this
crisis. Diverse modification methods have been used for the modifica­
tion of ENMs to enhance their functionality toward water purification.
Surface coating is a common approach to improve the selectivity of
ENMs toward different pollutants with tiny molecules. This method
generally involves introducing various substances or functional groups
on the top ENMs surface.
Zhijiang et al. [87] fabricated a novel type of ENMs as a support layer
for TFC NF membranes by coating a calcium alginate (CaAlg) hydrogel
Fig. 9. Coating of electrospun polymeric supports: (a) preparation for CaAlg-c-PHB/CNT composite nanofiber membrane. Copyright Springer [65]; (b) SEM images
of ENM (surface) and TFC NF membranes (surface and cross-sectional) from left to right. Copyright Walter de Gruyter GmbH [68]; SEM images of electrospun PVA
coated with (c) pure Pebax and (d) 10 wt% MWNT/PVA hydrogel nanocomposite.
18
A.H. Behroozi et al.
Desalination 558 (2023) 116638
on electrospun CNT/PHB nanofiber (Fig. 9a). As the coating of hydro­
philic and dense CaAlg film functions as a barrier layer, the resultant NF
membranes rejected 90 % of dyes with 600 g/mol as molecular weight,
demonstrating great antifouling properties for proteins and oils. In
another study, Ritcharoen et al. [88] electrospun CA fiber mat coated
with chitosan (CS)/sodium alginate (SA) and polystyrene sulfonate to
obtain electrostatic multilayers. As observed, the water flux was reduced
by increasing the number of bilayers. Numerically, this flux was 60 and
40 LMH for 15 and 25 bilayered membranes, respectively. Additionally,
the NaCl solution flux was lower than the water flux expectedly because
of osmotic pressure; it was reduced by increasing the NaCl content in the
solution. The filtration experiments exhibited that the NaCl rejection
was 15 % and 6 % for 25 and 15 bilayered membranes, respectively.
Mahdavi and Moslemi [155] prepared TFNC NF membranes based
on PET self-support nanofibrous mats, demonstrating a higher salt
rejection and four times higher pure water flux than the TFC NF mem­
brane. Na2SO4 rejection and water flux were 93 % and 34 LMH,
compared with 67 % and 8 LMH for the TFC NF membrane. The SEM
morphologies of the PET ENM (Fig. 9b left) depicted the bead-free and
large, open pore structure and higher porosity (~85 %) compared with
the PES UF membrane. The top surface morphology of nanofiber in
TFNC NF membrane illustrated a clear visible structure (Fig. 9b middle).
Also, the cross-section view of nanofiber in TFNC NF membranes
showed the formation of uniform thin barrier layers (Fig. 9b. right).
Wang et al. [89] developed electrospun PVA membranes after adding
a trace of Triton X-100 as a surfactant. The surfactant and PVA were
crosslinked chemically using glutaraldehyde in an acetone solution.
Pebax/MWNTs and PVA/MWNTs coating solutions were prepared in
advance. The PVA nanofiber was positioned on a nonwoven PET microfibrous substrate, washed with 1-butanol solution or water, and then
coated with PVA/MWNTs or Pebax/MWNTs solutions. A uniform
coating was obtained on the surface of PVA nanofibers, as proved by the
SEM images (Fig. 9c and d). The resultant membranes exhibited higher
porosity (82 %) than conventional membranes prepared by the PI
approach. The permeation flux for PVA/MWNTs (10 wt%) was two
times (330 LMH) higher than MWNTs/Pebax (160 LMH) with the same
rejection (99.8 %) compared with the PAV and Pebax (58 and 67 LMH),
which is due to higher PVA hydrophilicity. The PVA nanofibers mem­
branes displayed a substantially higher flux than the Pebax copolymer
composite membranes (50 LMH).
approach is the polymer grafting method, as a chemically posttreatment, that could improve the surfaces of ENMs, making them
more suitable for removing specific components from water by the NF
process. The third approach is the chemical crosslinking of TFNC
membrane to improve the hydrophilicity of ENMs and stabilize the
nanofibers with the promotion of coupling and bonding reactions be­
tween polymer chains. The fourth approach is surface coating with a
thin inorganic film to enhance surface conductivity. The introduction of
efficient coating materials and synthesis of composite ENMs can some­
how achieve a better performance considering the trade-off between
these factors.
ENs as support layers for TFC NF membranes could make membranes
more capable of removing bacteria, viruses, or toxic metal ions from
water, accompanied by higher water flux, higher solute rejections, and
less tendency to membrane fouling. A masterly manipulation of elec­
trospinning variables can produce ENMs with desired physical charac­
teristics and fiber size. These variables include polymer solution
properties (conductivity, surface tension, molecular weight, and vis­
cosity) and operating factors variables (feeding rate, electric field, col­
lector shape, capillary diameter, pressure, humidity, and temperature).
Thus, it is essential to optimize these influential variables consistently to
ensure the maximum performance of ENMs. For example, the properties
of polymer material directly affect surface hydrophilicity, roughness,
and their affinity to the coverage of the selective layer, which should be
considered before selection. In addition, their availability and cost are
other critical factors in choosing an appropriate polymer.
Despite several advantages, there are several limitations to applying
the electrospinning technique to a large industrial scale for commer­
cialization is still limited due to low production rate and timeconsuming synthesis procedure. The stability and corrosion resistance
of coated electrospun supports in the long-run treatment of water con­
taining corrosive pollutants are other challenging issues in scaling
coated ENMs up to bulk production. Additionally, the requirement of
specific solvents and equipment and high applied voltage has limited the
practical development of ENMs. Moreover, the electrospinning process
is still difficult to control because it tends to be disturbed by external
agents. In other words, its complicated controlling and vulnerability to
small changes in processing parameters have limited the bulk generation
of ENMs for industrial applications. To address these issues, special
consideration has been given to developing high durable ENMs,
designing a controllable continuous spinning system, and introducing
innovative functionalities to well-controlled ENMs.
Overall, due to the current great demand for more multifunctional
materials mitigating water pollution concerns, producing ENMs with
ever-improving performance criteria, mainly durability, would be ad­
vantageous in NF-based wastewater treatment plants. NF systems in the
modern era should not be limited to separating pollutants and particu­
lates but also increasingly discover new applications in non-traditional
fields such as membrane-catalytic systems, which are still an exquisite
field for development.
6. Conclusions and perspectives
Electrospinning is a membrane modification and fabrication process
to generate highly porous, versatile ENMs, in which operating condi­
tions directly characterize the membrane morphology and structure.
Such a well-controlled synthesis process endows membranes with su­
perior features, mainly high surface area, surface-to-volume ratio, pore
interconnectivity, and uniform pore size distribution. These features
enable ENMs to perform well in various water purification applications.
In this review, the detailed concept of electrospinning was first discussed
in detail, followed by its different types and influential factors. Then, the
applications of ENMs in NF-based water treatment processes were pre­
sented to elucidate the role of electrospinning in recent developments.
Generally, various membrane characteristics, such as pore size,
thickness, and diameter, can be tailored by varying processing variables
to manipulate the selectivity and permeability of ENMs. Additionally,
functional modifications of ENMs can be achieved through casting,
coating, crosslinking, grafting, IP, and LBL self-assembly for treating
water contaminated with a wide range of contaminants.
ENMs can also be post-treated either chemically or thermally to
modify the critical separation and filtration factors, such as hydrophi­
licity, pore size, mechanical features, and electrical conductivity. Hotpressing treatment is a modification approach to promote the mechan­
ical integrity of membranes, allowing more control over thickness and
pore size to achieve higher permeation and selectivity. The second
CRediT authorship contribution statement
Amir Hossein Behroozi: Data curation, Writing – original draft,
Visualization. Muayad Al-Shaeli: Data curation, Writing – original
draft. Vahid Vatanpour: Conceptualization, Supervision, Writing – re­
view & editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
19
A.H. Behroozi et al.
Desalination 558 (2023) 116638
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No data was used for the research described in the article.
Acknowledgments
The authors wish to acknowledge Kharazmi University (Iran) and
Istanbul Technical University (Turkey) for their financial support.
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