![MXenes obtained by bottom-up methods. Atomic layer deposition: (a) preparation of Ti 3 AlC 2 MAX thin film by sputtering of Ti, Al and C on a sapphire substrate, (b) diagram of Ti 3 C 2 T x after selective etching of Al, Ti 3 C 2 T x . Chemical vapor deposition approach: (c) synthesis process of Mo 2 C [32]. (Figure reproduced by permission of respective publishers).](https://www.researchgate.net/profile/Gim-Lim-2/publication/359422421/figure/fig4/AS:1138509956034561@1648453334105/MXenes-obtained-by-bottom-up-methods-Atomic-layer-deposition-a-preparation-of-Ti-3_Q320.jpg)
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Ceramics International xxx (xxxx) xxx
Please cite this article as: Gim Pao Lim, Ceramics International, https://doi.org/10.1016/j.ceramint.2022.03.165
Available online 22 March 2022
0272-8842/© 2022 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Recent progress and new perspective of MXene-based membranes for water
purication: A review
Gim Pao Lim
a
, Chin Fhong Soon
a
,
*
, A.A. Al-Gheethi
b
, Marlia Morsin
a
, Kian Sek Tee
c
a
Biosensor and Bioengineering Lab, Microelectronics and Nanotechnology-Shamsuddin Research Center, Institute for Integrated Engineering, Universiti Tun Hussein Onn
Malaysia, 86400, Parit Raja, Batu Pahat, Johor, Malaysia
b
Micro-Pollutant Research Centre (MPRC), Faculty of Civil Engineering and Built Environment, Universiti Tun Hussein Onn Malaysia, Parit Raja, 86400, Johor,
Malaysia
c
Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, 86400, Parit Raja, Batu Pahat, Johor, Malaysia
ARTICLE INFO
Keywords:
MXene membrane
Water ltration
Emerging organic pollutants
Oil-water separation
ABSTRACT
MXene is a new emerging 2D material which has been extensively studied for its applications in biosensors,
energy storage, cancer theranostics, microbiology and electromagnetic interference shielding. In spite of the
growing interest in MXenes, there are relatively few studies on MXene-based membranes especially for treatment
of water and emerging organic pollutants. Thus, this review aims to provide an up-to-date account on recent
progress in MXene-based membranes. This includes MXene synthesis, unique properties, membrane preparation,
characterization, and their applications in treating various types of dyes and salts, oil/water emulsion, and
emerging organic pollutants. The results demonstrated that antifouling multifunctional MXene based membranes
are a promising alternative to graphene-based membranes for water purication applications. Moreover, we also
discuss the current research gaps and further perspectives regarding MXene based membranes.
1. Introduction
Water pollution has become an increasingly serious problem owing
to anthropogenic activities and deprived management of water re-
sources [1]. Various organic and emerging pollutants found in the
aquatic environments can cause undesirable effects to humans and an-
imals due to their persistence, mobility, and toxicity. To combat this
issue, traditional and advanced treatment processes that involve liquid
separation have been used to treat water and among these, membrane
separation has turned out to be the most promising technology to tackle
this challenge [2]. The main benets of membrane separation are
providing low carbon footprint and great manufacturing consistency
[3]. Unfortunately, this membrane separation process suffers from water
permeance or selectivity. The commercially used membrane materials
such as polypropylene and polysulfone are hydrophobic, which has an
inuence on separation performance and water permeance. To over-
come this problem, numerous research have described the use of inno-
vative membranes integrating with nanomaterials, such as metal
organic frameworks carbon nanotubes, graphene oxides, and MXenes
[4–9]. Graphene oxides have been extensively used in membrane tech-
nology [10]. However, poor stability and minor interlayer distance
between the graphene nanosheets, limiting their industrial application
in large scale. It is desired to nd the idyllic 2D nanomaterials for sep-
aration membrane, the investigation of novel materials with good pos-
sessions can be one of the alternative strategies.
Ti
3
C
2
T
x
MXenes is a novel 2D material that was rst investigated by
Naguib M. et al. [11] in 2012. MXenes were characterised by high
electrical and thermal conductivity, large surface area, hydrophilicity
and high stability [12,13]. More recently, the use of MXenes as
adsorption materials and membrane processes have been investigated
[14–17]. In [18,19], the researchers established the fabrication of
MXene/polymer composites by mixing Ti
3
C
2
T
x
with either poly(vinyl
alcohol) (PVA) or polydiallyldimethy lammonium chloride (PDDA) [18,
19], respectively. As reported in [18,19], the interaction between
polymer chains and the MXene akes not only improved the tensile
strength but also enhanced the intercalation of cations, leading to a
remarkably high volumetric capacitance of the composite lm. The
intercalation of polymer chains reduces the restacking of MXene akes
and improving MXene stability in oxygen atmosphere [20]. Till date,
different MXene/polymer composites have been well fabricated with
several types of polymers. Due to the synergistic effect between the
MXene and polymer precursors, the MXene/polymer composite shows
* Corresponding author.
E-mail address: soon@uthm.edu.my (C.F. Soon).
Contents lists available at ScienceDirect
Ceramics International
journal homepage: www.elsevier.com/locate/ceramint
https://doi.org/10.1016/j.ceramint.2022.03.165
Received 16 December 2021; Received in revised form 16 March 2022; Accepted 17 March 2022
Ceramics International xxx (xxxx) xxx
2
outstanding properties, such as improved conductivity, enhanced me-
chanical properties, and thermal stabilities. The properties of MXene/-
polymer composite can be modied by different types of polymer and
MXene, as well as their composition [21,22]. MXene/polymer mem-
branes are generally made-up of vacuum-assisted ltration (VAF), drop
casting (DC) and hot press (HP) [23–25]. MXene-based membranes have
great potential to provide efcient separation and sustainable
manufacturing processes.
The global distribution of the research based on the data analysis is
as presented in Fig. 1. It was noted that 44.9% of the research on the
MXenes is currently conducted in China followed by 14.35% in USA and
5.5% in South Korea. The high research activity in the eld of MXenes in
China might be related to the high industrial activities and applications
of MXenes in the environment and medicine.
The analysis for the keywords used in the studies conducted using
MXene is as depicted in Fig. 2. It was noted that 5% of the studies have
applied MXene for the preparation of adsorbents for removing heavy
metals and different pollutants from the water and wastewater, 2% of
the studies investigated the application of MXene in the biomedical
applications such as antimicrobial activities and animal experiments,
4% of the studies focused on the use of MXene in the energy applica-
tions. In contrast, 13% of the studies have been using MXene in the
nanotechnology applications, while 11% of the studies have explored on
the utilization of MXene in the photocatalysis and 19% of the studies
have involved MXene in the water and wastewater treatment. However,
only 2% of the studies have applied MXene in the membrane technology
such as a composite membrane and nanoltration membranes. Only two
reported studies have demonstrated the application of MXene for
removing emerging pollutants [26,27]. However, the analysis results
reected that more studies are required for studying the efciency of
MXene membrane.
There is a total of 266 research papers that contain the keywords
“MXene”, “membrane”, “water purication”, “water contaminant”, and
“pharmaceutical MXenes” that have been found from 2014 to 2022 in
lens.org. (Fig. 3a). The total number of research articles signicantly
increases year after year, especially from 2016 to 2019 (Fig. 3a), indi-
cating that MXenes are becoming a popular potential application in
membrane separation and water purication. However, the use of
MXene to treat water contaminants and emerging pollutants of phar-
maceutical presented only in 30 publications indicating that the po-
tential of MXene in this area has not been fully unveiled. Hence, there
are many opportunities for exploration especially relating to the use of
membranes incorporated with MXenes to remove various environmental
pollutants. As summarized in a timeline of Fig. 3b, MXenes were rst
used as a desalination membrane in 2014. MXene Ti
3
C
2
T
x
membranes
were then translated as ion sieves in 2015 and 2016. MXene-graphene
oxide membranes were also applied as selective molecular separators
in 2017. In the same year, functionalized MoS
2
were successfully
synthesized for nanoltration and desolation. In 2018, 2D MXene
membranes were applied in seawater desalination and distillation.
Then, the MXene membranes were found with potential in oil/water
emulsion separation and forward osmosis in 2019. In the same year,
MXene-biopolymer membranes were explored for solvent resistance and
solar driven water purication. In 2020, MXene membranes were
applied in organic solvent recovery, wastewater treatment and anti-
biotic separation. In 2021, pristine MXene and MXene nanocomposites
were demonstrated with success to remove micropollutants from
wastewater containing model dye and 4- nitrophenol. In 2022, the
MXene membranes were applied in oil/water separation, dyes, molec-
ular separation, and toxic metallic pollutant.
The aim of this review is to evaluate the present results on the
research method for MXene based membranes and the application of
MXene membranes to solving environmental and industrial issues
especially in the treatment of wastewater and emerging organic pol-
lutants. In addition, the upcoming prospective through nding gaps
would help the reader to identify the future potential of MXene in
solving water pollution problems.
2. MAX phase and MXene
2.1. MAX phase and MXene structures
The general formula of the MAX phases is Mn+1AXn. M is an early
transition metal, the n is in the range of 1–3, A is a group element such as
Al, Cd, Gl, In, Sn, Ge, Si, P, Pb, Ti, S, Bi, As), and X is either nitrogen or
carbon. MXenes are commonly formed through selectively removing
aluminum from the MAXphase. The layered structure of the MXenes
comprises transition metal carbides and carbonitrides or nitrides. The
chemical formula of the MXenes is Mn+1XnTz, where T is represent the
surface functional groups (F, O, H or OH). The thickness of the indi-
vidual MXene akes is approximately less than 1 nm. The lateral size of
MXenes ranges from a 1 nm
2
to 5.1
μ
m
2
[28].
2.2. Preparation methods and modication approaches
The synthesis MAX phases which are the precursor to MXenes can be
produced using different approachs, including chemical vapor deposi-
tion (CVD), combustion synthesis, hot isostatic pressing (HIP), arc
melting, spark plasma sintering (SPS), self-propagating high-tempera-
ture synthesis (SHS), mechanical alloying, physical vapor deposition,
and reactive sintering. These approaches generally can be divided into
the bottom-up or top-down approach [29]. The surface of a MAX phase
can also be altered to improve the separation performance of
MXene-based membranes. The overall synthesis path of MXenes and
their surface alterations are systematically studied in this sub-division.
2.3. Top-down synthesis methods
The top-down synthesis method is a conventional technique to pro-
duce MXene nanosheets. The approach is established on the cleavage of
comparatively large MAX-phase precursor materials using intercalants.
The top-down synthesis methods for MXene are as follow;
2.3.1. Exfoliation based on MAX-phase
The chemical bonding between M and A atom is typically metallic
and thus it is challenging to detach M
n+1
X
n
layers through conventional
mechanical shearing methods. However, the chemical bonds between M
and X atoms are less chemically active and stronger than the M–A bonds,
and therefore engraving of the A-atom layers can be carried out. The
etching process of the MAX phase is usually accomplished by using
robust etching reagents containing uoride ions (F
−
) such as HF, NH
4
F
[30], ammonium biuoride (NH
4
HF
2
), a mixture of HF and HCl, and a
combination of LiF and HCl solution [31] (Fig. 4a and b). MAX-phases
containing aluminum are commonly used for the synthesis of MXenes.
Fig. 1. Global distribution of the studies conducted on MXene.
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
3
In a typical exfoliation process, the MAX phase precursor (Ti
3
AlC
2
or
Ti
2
AlC) is immersed in hydrouoric acid or acid-uoride solution at
25 ◦C and then undergoes sonication process (Fig. 4b). The MXene stacks
(Ti
3
C
2
or Ti
2
C) are permitted to adhere to each other. This etching
process eliminates the aluminum species and terminates the surface
layer with –OH, –O and –F groups (Fig. 4a). Single- or few-layer MXene
is attained by the etching of the glue-like ionic bonding layers (Fig. 4b).
2.3.2. Exfoliation based on etchants
The exfoliation method based on the use of etchants can be catego-
rized as hydrouoric acid -etching or non-hydrouoric acid etching. The
use of highly concentrated hydrouoric acid causes the MXene stacks to
delaminate and to form single or few layers MXene [33]. Free F
−
ions are
generated in-situ by reacting hydrouoric acid with sodium uoride or
Lithium uoride, then using elevated temperatures to obtain molten
uorides. In the latter case, the ions F
−
may corrode the attaching
metals. Hydrouoric acid is a precarious solvent, numerous substitute
reagents have been introduced to substitute hydrouoric acid.
Fluorine-free methods include using mechanical forces such as ultra-
sonication in combination with liquid exfoliation using a non-uoride
intercalant [34]. Recently, Ti
3
C
2
MXene was produced by ultra-
sonication in the tetrabutylammonium hydroxide (TBAOH) as an
intercalant [93]. The surface termination groups on the MXene are
regulated through the type of etchant used. For instance, etching with a
uoride-based etchant produces an abundance of uoride groups on the
surface of the MXene. The exfoliation method based on the use of
intercalants can be categorized as either metal-ion or
organic-intercalant.
Functionalized titanium carbide MAX phase have a binding energy of
up to about 2–6 times higher than that of analogous materials such as
bulk MoS
2
or graphite. In fact, a binding energy in the range of 1.0–3.3
J/m
2
must be overcome to achieve complete exfoliation of MXene [35].
It is inadequate to synthesize single-layer MXenes using mechanical
exfoliation methods. Thus, various intercalants were used to intercalate
the MXene precursors before the mechanical stressing process. These
intercalants include metal hydroxides, halide salts, polar organic mol-
ecules (e.g., isopropylamine and dimethyl sulfoxide (DMSO)), or large
organic base molecules (e.g., n-butylamine, TBAOH, choline hydroxide,
tetrapropylammonium hydroxide (TPAOH)).
2.4. Bottom up methods
Bottom-up methods, such as atomic layer deposition and chemical
vapor deposition can be applied for the synthesis of MXenes. Halim and
co-workers [94] fabricated Ti
3
ALC
2
MAX thin lms by direct current
magnetron sputtering of Ti, Al, and C onto an insulating sapphire sub-
strate (Fig. 4 a and b). Then, after the selective etching of aluminum
using aqueous HF or NH
4
HF
2
, the authors managed to obtain 1 ×1 cm
2
Ti
3
C
2
thin lms with 90% light transmittance in the visible-to-infrared
range. MAX phase (Mo
2
GaC) and non-MAX phase (Mo
2
Ga
2
C) thin
lms were fabricated using a similar method, and Mo
2
C thin lms were
produced after undergoing an etching process using hydrochloric acid.
The other method of interest is chemical vapor deposition and it is a new
method to fabricate ultrathin MXene materials [32,36]. Xu et al. [36]
produced ultrathin
α
-Mo
2
C (~3 nm) by vapor deposition of carbon from
methane onto a Cu/Mo alloyed surface at temperatures above 1085 ◦C
(Fig. 4c). This method was further improved by Ref. [37] which pro-
duced β-Mo
2
C nanosheets using a MoO
2
nanosheet as the template and
Mo source. The authors managed to obtain β-Mo
2
C nanosheets with
fewer defects and no terminations compared to 2D materials that were
obtained using other methods. However, for large scale fabrication of
MXenes, a uoride-based etching protocol is preferred because the
MXene sheets can be simply delaminated by mechanical vibration or
sonication in polar solvents to obtain single or few layer MXenes [38].
3. Preparation methods for MXene-based membranes
There are three approaches to produce membranes that involve the
incorporation of MXenes: (a) MXenes are applied as a base for the pro-
duction of a lamellar-structure membrane; (b) MXenes are mixed with
dissimilar additives or different nanomaterials to produce a mixed ma-
trix membrane; and (c) MXenes are used as a coating material to alter a
membrane. Fig. 6 displays the methods for producing MXene-based
membranes supported on different materials, including polyvinylidene
Fig. 2. Most common applications of MXene in water purication.
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
4
uoride, polycarbonate and anodic aluminum oxide. Table 1 summa-
rizes the methods, separation performance, and ltration capability of
numerous MXene-based membranes for the removal of organic, inor-
ganic species and main ndings. A lm with ultra-thin thickness is
important to achieve high separation performance. This is due to sepa-
ration performance being highly reliant on the lateral size of the nano-
sheets and stacking conformation. The lateral size of 1–2
μ
m exfoliated
sheets is required for making a well-ordered 2D-stacking structure with
abundant in-plane gaps for high-efciency separation performance.
MXenes are suitable for embedment in membranes because they are
resilient to oxidation, thermal shock, chemical attack, durable and
destruction-tolerant.
3.1. Vacuum assisted ltration (VAF)
There are several methods to produce MXene/polymer membranes
such as vacuum assisted ltration (VAF), drop casting (DC), hot pressing
(HP), layer-by layer (LbL) assembly, cold pressing (CP), electrospinning,
and electrochemical deposition [65]. Vacuum-Assisted Filtration (VAF)
is a widely used method to fabricate MXene/polymer membranes due to
its low cost and simple operation. Typically, appropriate amounts of
polymers and MXenes are mixed together in a polar solvent and then
subjected to vigorous stirring or sonication to obtain the composite. The
MXene/polymer composite can then be recovered and formed on a
substrate as a lm using VAF (Fig. 5 a). An example is shown in Fig. 5 b
where poly(vinyl alcohol) (PVA) and charged poly-
diallyldimethylammonium chloride (PDDA) lms are obtained using the
VAF method(Lin et al., 2014). The Ti
3
C
2
/PVA lms demonstrated better
electrical conductivity when the MXene percentage weight was
increased from 40% to 90%. The addition of the polymer between the
MXene akes increases exibility and encourages cationic intercalation
of the composite membrane. The thickness of the MXene/polymer
membrane can be modied by adjusting the solution concentration and
volume before VAF treatment. Vahid Mohammadi et al., [97] modied
Ti
3
C
2
with polyaniline (PANI) by in situ polymerization of aniline (Fig. 5
c). The authors obtained Ti
3
C
2
/PANI hybrid lms with various thick-
nesses (4–90
μ
m) on Celgard membrane lters using the VAF method.
The electrochemical performance of their Ti
3
C
2
/PANI membranes was
found to exhibit much less dependence on thickness compared to the
pristine Ti
3
C
2
lms. In recent years, biopolymers have been widely
investigated due to its abundance and environment friendly nature.
These polymers are also good potential candidates for use in synthe-
sizing MXene/polymer hybrid membranes. Shahzad et al., 2016 rst
produced Ti
3
C
2
/sodium alginate composite membranes using the VAF
method on the mixed colloidal solution (Shahzad et al., 2016). The
Ti
3
C
2
/sodium alginate composite membranes were found to exhibit very
high electromagnetic interference (EMI) shielding efciency. Further-
more, calcium ions can be used to cross-link the Ti
3
C
2
/sodium alginate
Fig. 3. (a) Quantity of published articles on MXenes have been used for environmental applications, searched with keywords “MXene”, “membrane”, “water”, “water
purication”, “water contaminant”, and “pharmaceutical” (Source: lens.org, 2014 to 2021).(b) Timeline of environmental application of MXenes per year from 2014
to 2022.
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
5
membranes to form sponge-like Ti
3
C
2
/calcium alginate (CA) aerogel
lms after a freeze-drying process (Fig. 5 d) [66]. showed that the EMI
shielding performance of Ti
3
C
2
/CA aerogel membrane was further
improved due to its lower density and spongelike structure [66].
Vacuum-assisted ltration is an accessible method to arrange MXene
akes onto a support to form ultrathin MXene membranes under vac-
uum condition [39]. The MXenes then can be stacked to construct a
robust layered MXene membrane. It can be easily detached from the
support as a free-standing membrane. This relatively simple method is
suitable for large -scale production.
MXene membrane was prepared for water purication using
vacuum-assisted ltration to pass a colloidal Ti
3
C
2
T
x
solution through a
polyvinylidene diuoride (PVDF) substrate [39]. The nal product is a
binder-free, exible Ti
3
C
2
T
x
lm. The MXene sheets restacks to produce
a membrane with slit-like channels and well-dened interlayer spacing.
This permits the membrane to serve as a size-selective ion sieving sep-
aration and charge mechanism. The water ux of this membrane was
determined to be 37.4 Lbar
-1
h
−1
m
2
with exceptional ion separation.
The charge of the ions and hydration radius play a role in the ion sep-
aration process. The results showed that Na
+
, K
+
, and Li
+
cations were
easily diffused through Ti
3
C
2
T
x
lms faster than Al
3+
and Mg
2+
during a
concentration gradient-driven diffusion process [39]. It is also noticed
that ions with a higher charge, larger hydration radius(Å), and a smaller
than the interlayer spacing of Ti
3
C
2
T
x
have established an order of
slower permeation compared to monovalent cations.
A lamellar MXene membrane was produced using vacuum-assisted
ltration with controlled porosity [39]. They intercalated their nega-
tively charged MXene nanosheets with Fe(OH)
3
NPs followed by
vacuum-assisted ltration (VAF) on anodic aluminum oxide substrate.
Then, Fe(OH)
3
NPs was dissolved in hydrochloric acid to attain a porous
MXene membrane. The MXene-Fe(OH)
3
NPs(M3) membrane was found
to have approximately 5–10 times higher water permeance at 1084 L
m
−2
h
−1
bar
−1
than the MXene membrane with intercalated Fe(OH)
3
(M2) and original MXene membrane (M1), respectively. The MXene-Fe
(OH)
3
NPs(M3) membrane also exhibited a 90% rejection rate for Evans
blue (EB) dye. The results indicated that membranes with a thickness
greater than 0.8
μ
m were found to reject 100% of EB, gold NPs, and
Cytochrome (Cyt. c) molecules. The durability test results found that the
water permeance and rejection efciency retained at constant level. In
spite of the high ux accomplished, the low rejection of salts is the main
disadvantage of this type of membrane. However, MXene membranes
still exhibited a better separation performance under comparable
experimental conditions compared to other 2D membranes [10,67].
The percentage weight of polymer in the MXene/polymer membrane
is difcult to control when using a VAF approach due to part of the
water-soluble polymer being lost when using vacuum ltration. There-
fore, the nal composition of MXene/polymer membrane will be un-
controllably less than intended, making the elucidation of the specic
relationship between the MXene/polymer composition and its property
challenging.
3.2. Casting method
The casting method is a widely used method to prepare MXene/
polymer membranes with well-dened composition. Typically, exact
amounts of MXene and polymer are dispersed in a solvent to form a
homogeneous solution under vigorous stirring or sonication. Then the
solution is cast onto a substrate and the solvent is allowed to evaporate.
Finally, the free-standing MXene/polymer membrane can be peeled off
from the substrate. Hydrophilic polymers that are commonly applied to
fabricate MXene/polymer lms include poly(ethylene oxide) (PEO), PEI,
polyurethane (PU),polyacrylamide (PAM), and PVA [68–73]. As shown
in Fig. 7 a, MXene can be mixed into a PAM matrix to produce a exible
membrane using the casting method [68]. Pan et al., 20 fabricated
Ti
3
C
2
/PEO membranes with differing MXene content by casting mix-
tures of high molecular weight PEO and Ti
3
C
2
[72]. The resultant
Fig. 4. MXenes obtained by bottom-up methods. Atomic layer deposition: (a) preparation of Ti
3
AlC
2
MAX thin lm by sputtering of Ti, Al and C on a sapphire
substrate, (b) diagram of Ti
3
C
2
T
x
after selective etching of Al, Ti
3
C
2
T
x
. Chemical vapor deposition approach: (c) synthesis process of Mo
2
C [32]. (Figure reproduced
by permission of respective publishers).
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
6
Ti
3
C
2
/PEO lm exhibited improved ionic conductivity and stability at
room temperature, indicating possible application for lithium metal
batteries [74]. established an effective method to produce polymer
electrolyte membrane by casting a mixture of chitosan and Ti
3
C
2
MXene
onto a glass plate [74]. The authors found that the agglomeration of the
two-dimensional akes can be prevented by a synergistic effect between
the functionalized boron nitride nanosheets (fBNNS) and Ti
3
C
2
(Fig. 7b)
[74]. Interestingly, an inter space lling and bridging phenomena were
observed in the f-BNNSTi
3
C
2
/PBI lm, which caused the lm to have
higher thermal conductivity and enhanced mechanical property
compared to well-ordered PBI lms. Casting is a simple and low-cost
method to assemble MXene/polymer composite lms. To date, many
kinds of polymers (both hydrophilic and hydrophobic) have been used
in the fabrication of MXene/polymer membranes using the casting
method. However, aggregation of MXene akes may occur during the
evaporation process, especially when using the solvents with high
boiling point [74]. To fabricate high-performance MXene/polymer
membranes, it is desired that the interaction between MXene akes and
polymer matrices is enhanced to improve the dispersion of MXene akes
in the matrix.
3.3. Hot press method
Melt blending is the preferred method in industrial production as it
has the advantage of being solvent-free, environmental-friendly and can
be tailored to suit the application. MXene akes can be uniformly
dispersed in the polymer matrix by melt blending with controlled stir-
ring intensity and temperature. The composite membranes obtained can
then be cut into suitable shapes and sizes for use [75]. fabricated
Ti
3
C
2
/UHMWPE membranes with varying MXene content using the hot
press method [75]. The surface of Ti
3
C
2
was functionalized with iso-
propyl (dioctylphosphate)titanate before blending with UHMEPW to
enhance the dispersion of MXenes in the matrix. As a result, the lms
demonstrated enhanced hardness, creep resistance, and yield strength
[76]. introduced a supercial and common technique to fabricate
Ti
3
C
2
/TPU membrane with superior mechanical and thermal properties
through a combination of melt blending and hot-pressing [76]. Ti
3
C
2
was pretreated with polyethylene glycol (PEG) before melt blending to
avoid agglomeration. A series of homogeneously dispersed Ti
3
C
2
/TPU
membranes were obtained due to the successful intercalation of PEG
into the MXene layers. Ti
3
C
2
akes can also be modied using cetyl-
trimethylammonium bromide (CTAB) and tetrabutyl phosphine chlo-
ride (TBPC) to improve interaction with hydrophobic polymers. The hot
press method can be easily scaled-up for the fabrication of
Fig. 5. Fabrication of MXene/polymer membrane by VAF method: (a) Filtration procedure and the obtained free-standing MXene/ANF membrane. (b) Schematic
diagram of MXene/PDDA and MXene/PVA membranes. (c) Schematic picture of the synthesis of Ti
3
C
2
T
x
/PANI lm. (d) Schematic diagram of the fabrication process
of Ti
3
C
2
T
x
/CA aerogel lm. (Figure reproduced with permission from respective publishers).
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
7
MXene/polymer membranes in the industrial setting. While hot press is
not suitable for those polymers with low degradation temperatures or
high melting points, there are a lot of thermoplastic engineering mate-
rials that make ideal candidates for the large-scale fabrication of MXe-
ne/polymer membranes. Furthermore, functionalization of MXene
and/or polymers is required to avoid the aggregations of MXene akes
and assuring good dispersion of the MXene akes in the membranes.
3.4. Layer-by layer (LbL) assembly
Other established methods to fabricate MXene/polymer membranes
include layer-by layer (LbL) assembly, cold press (CP), electrospinning,
and electrochemical deposition. The LbL assembly approach is used to
control the thickness of the membranes [77]. Multilayer lms with high
uniformity can be attained using the LbL method through depositing
layers functionalized with oppositely charged species. Multilayer
MXene/polyelectrolyte membranes have been prepared using a dip
coating LbL approach to assemble alternate layers of negatively charged
Ti
3
C
2
akes and positively charged poly(diallyldimethylammonium
chloride) (PDAC) [78,79]. The thickness of obtained Ti3C2/PDAC
membrane can be controlled to the nanometer, enabling their applica-
tion for ultrafast humidity sensing [79]. In addition, spin-spray LbL
method have also been applied to fabricate Ti
3
C
2
/PDAC membranes and
it resulted in improved mechanical properties and electrical conduc-
tivity (Fig. 8a) [78]. The work in [80] fabricated MXene/carbon nano-
tube lms using the spin-spray LbL method with PVA and poly(sodium
4-styrenesulfonate) (PSS) as the polymer matrices [80]. Their lms
exhibited good stability in water and have high electrical conductivity.
MXene/polymer membrane with controlled thickness can be coated
onto various mechanically deformable objects using the spin-spray LbL
method.
Fig. 6. Fabrication diagram of (a) MXene membrane supported on anodic aluminum oxide [39], (b) polyvinylidene uoride [40], and (c) polycarbonate [16].
(Figure reproduced by permission of respective publishers).
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
8
Table 1
Summary of methods, ltration capacity, and separation performance of organic and inorganic compounds by MXene-based membranes.
MXene Types Experimental
condition
Water permeance/
percentage removal
Key nding References
Ti
3
C
2
T
x
NaCl
BSA
C
o
=10000 mg L
−1
NaCl
C
o
-2000 mg L
−1
Total ux =10 Lm-
2
h
−1
MXene-coated PVDF membrane obtained a 56–64%
reduction in ux decline.
Tan et al.
[41]
Ti
3
C
2
T
x
GV
CR
NaCl
MgCl
2
Na
2
SO
4
C
o
=100–1000 mgL
-1
405 Lm
−2
h
−1
bar
−1
/80%
(GV), 92% (CR), 13.8%
(NaCl)
2.3%(MgCl
2
), 13.2%
(Na
2
SO
4
)
Effective permeability and selectivity in the separation of
dyes from salts.
[6]
Ti
3
C
2
T
x
EB
Cytochrome
RhB
C
o
=10–20 mg L
−1
1084 Lm
−2
h
−1
bar-
1
/90%
(EB), 97% (Cytochrome),
85% (RhB)
An excellent water permeance, a 90% removal rate. [39]
Ti
3
C
2
T
x
Oil 1% v/v oil in water
emulsion
472 L Lm
−2
h
−1
bar-
1
with
oil/>99%(TOC)
The print paper was used as supported layer to improve
mechanical strength and exibility. MXene layer as a
selective layer to separate emulsied oil from water.
[42]
Ti
3
C
2
T
x
NaCl
KCl
LiCl
MgCl
2
C
a
Cl
2
0.2 M 1.1–8.5 L m
−2
h
−1
89.5–99.6%
High rejection of NaCl in the range of 89.5%–99.6% with
fast water ux.
[43,44]
Ti
3
C
2
T
x
NaCl
KCl
LiCl
MgCl
2
AlCl
3
0.2 mol/L <1 ×10
−2
mol m
−2
h
−1
99.5%(Na
+
rejection)
High ions rejection ability of up to 99.5% with
permeation rates down to 10
−3
mol m
−2
h
−1
.
[45]
Ti
3
C
2
T
x
NaCl
KCl
LiCl
0.2 M 0.05–0.06
L m
−2
h
−1
bar-
1
>97%(ion rejection)
An excellent anti swelling property with high metal ion
rejection.
[46]
Ti
3
C
2
T
x
NaCl
River water
0.5 M
0.01 M NaCl
94.1% selectivity MXene membrane showed the high efcient charge
separation.
[43,44]
Ti
3
C
2
T
x
Urea 30 mg urea 99% removal
94% removal from
dialysate
MXene membrane showed the maximum urea absorption
capacity of 10.4 mg/g
[47]
Ti
3
C
2
T
x
NaCl, MgCl
2
Na
2
SO
4,
MgSO
4
VOSO
4
C
o
=10 mM 5–25 L m
−2
h
−1
bar-
1
/
50–99%, Rejection order
VOSO
4
>Na
2
SO
4
>MgSO
4
>NaCl >MgCl
2
MXene membrane showed the maximum ion removal
99%.
[48]
Ti
3
C
2
T
x
E.coli
B.Subtilis
C
o
=2700 CFUmL
−1
37.4 L m
−2
h
−1
bar-
1
/>99%
E. coli
>99% B. subtilis
Surface oxidation of aged membrane showed above 99%
growth inhibition for bacteria’s as compared with the
pristine membranes.
[40]
Ti
3
C
2
T
x
-Ag Rh B, MG, BSA C
o
=50–100 mg L
−1
~420 m
−2
h
−1
bar-
1
/79.9%
(Rh B), 92.3% (MG), >99%
(BSA)
Superior bactericidal performance was recorded [49]
Ti
3
C
2
T
x
-graphene
oxide
BB, RB, MB, MR,
MgSO
4
NaCl
C
o
=10 mg L
−1
~25 L m
−2
h
−1
bar-
1
/95.4
(BB) 94.6 (RB),
40 (MB), 5 (MgSO
4
)
<1 NaCl
High removal of organic dyes in the range of 40%–95.6%
with hydrated radii greater than 0.5 nm.
[16]
Ti
3
C
2
T
x
-graphene
oxide
RhB, MB, CV, NR,
Na
2
SO
4
NaCl
C
o
=10 mg L
−1
(dyes);
5 mmol L
−1,
89.6 Lm
-2
h
−1
bar
−1
/>97%
(dyes), 61% (Na
2
SO
4
), 23%
(NaCl)
High enhanced water permeance, 7.5 times of pristine
GO membrane and 2.5 times of GO/TiO
2
membrane.
[50]
Ti
3
C
2
T
x
graphene
oxide
Chrysoidine G NR,
MB, CV, BB, Humic
acid, BSA
C
o
=10 mg L
−1
71.9 L m
−2
h
−1
bar
1
/>99%
(dyes), 61% (Na
2
SO
4
), 23%
(NaCl)
High removal of organic dyes [17]
Ti
3
C
2
T
x
graphene
oxide
MO,MB,
AY 14,
Indigo carmine,
Eeosin
C
o
=10 mg L
−1
8.5–11 L m
−2
h
−1
bar
1
/
>95%
High ux for pure solvents and outstanding dyes
molecular separation performance (>90%).
[7]
Ti
3
C
2
T
x
-TiO
2
Dextran C
o
=3000 mg L
−1
Molecular weight =
10–500 kDa
90 L m
−2
h
−1
bar
1
/>95%
(molecular weight, >30
kDa)
High permeance of 90 L m
−2
h
−1
bar
−1
and a molecular
weight cut off of 22 kDa.
[51]
Ti
3
C
2
T
x
-TiO
2
Dextran C
o
=6500 mg L
−1
Molecular weight =
10–70 kDa
100–140 L m
−2
h
−1
bar
−1
/
>90% (molecular weight,
>20 kDa
High permeance of 140 L m
−2
h
−1
bar
−1
and a molecular
weight cut off of 20 kDa.
[52]
Ti
3
C
2
T
x
-TiO
2
BSA C
o
=1 g/L 756.8 L m
−2
h
−1
bar
−1
/
95% BSA
Excellent self-cleaning function under UV radiation and
improved antifouling performance.
[53]
Ti
3
C
2
T
x
- PSS NaCl
KCl
LiCl
MgCl
2
0.2 M 0.08 mol m
−2
h
−1
Highly selective transport of Li
+
. [54]
Hal@MXene-PDA Lubricating oil 1.5% v/v oil in water
emulsion
5636.2 L m
−2
h
−1
bar
−1
/
Ethanol (99.8%)
Lubricating oil (99.8%)
Higher hydrophilicity and good separation performance
of lubricating oil (99.8%).
[55]
MXene @CS/TA-
FeOOH
Oil in water emulsion Volume ratio of oil in
water emulsion 1:99
500.38–1022.7 L m
−2
h
−1
bar
−1
/96.32–99.72%
High permeation ux and good separation performance
for oil in water emulsion.
[35]
(continued on next page)
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
9
3.5. Electrospinning
Electrospinning is a typical technique for the production of nano-
bers and brous lms [81]. [82] fabricated exible Ti
3
C
2
/PVA mem-
branes using an electrospinning approach (Fig. 8b) [82]. With addition
of PVA, the Ti
3
C
2
/PVA lms can be twisted, and compressed randomly.
Utilizing the Ti
3
C
2
/PVA membrane, a exible triboelectric nano-
generator was successfully synthesized, which can be used to screen
different types of body motion.
3.6. Electrochemical deposition
Electrochemical deposition is another facile method to fabricate
conductive MXene/polymer membranes [83,85,95]. The work in [83]
described a two-step method to fabricate Ti
3
C
2
/PPy hybrid lm, in
which uniform Ti
3
C
2
particles lms were rst formed by electrophoretic
deposition (Fig. 8 c) and then the intercalated pyrrole was subjected to in
situ polymerization (Fig. 8 d) [83]. The membranes were then peeled off
from the electrode and used to fabricate an ultrathin all-solid-state
supercapacitor. The Ti
3
C
2
/PPy hybrid lm was found to have
excellent capacitance and cycling stability [83]. Qin et al. [85] devel-
oped a novel electrochemical polymerization approach to fabricate
MXene/conjugated polymer composite lms without the use of an
electrolyte. The thicknesses of Ti
3
C
2
/PPy and Ti
3
C
2
/PEDOT membranes
could be controlled by adjusting the amount of applied current on the
electrode, and different microstructures of the membranes can be ach-
ieved by photolithography of the conductive substrate. This technique is
a new approach in designing and fabricating free-standing MXene/-
conductive polymer membranes with unique electrochemical proper-
ties, increasing their potential applications in the eld of energy storage.
4. Liquid separation performance of MXene-based membranes
MXenes have a negatively charged surface, hydrophilic behaviour,
surface tunability, and high surface area [13]. These properties are
benecial for the removal and catalytic degradation of organic dyes
[50]. The research in [50] synthesized a graphene-oxide-Xene-TiO
2
membrane for use in water ltration. When compared to unmodied
membranes, graphene-oxide-MXene-TiO
2
membrane had a water
permeability of 89.6 L m
−2
h
−1
bar
−1
and a 7.5- and 2.5-fold greater
Table 1 (continued )
MXene Types Experimental
condition
Water permeance/
percentage removal
Key nding References
PDA@MXene/CA DR28
DB38
C
o
=100 mg L
−1
271.2 L m
−2
h
−1
bar
−1
/
88.9%(DR 28), 88.6% (DB
38)
Good hydrophilicity and the pure water ux more than
the virgin membrane about 277%.
[56]
MXene
nanocomposite
membrane
Na
2
SO
4
MgSO
4
MgCl
2
NaCl
C
o
=2000 ppm 4.7 Lm
−2
h
−1
bar
−1
/97.6%
(Na
2
SO
4
)
The water ux of the MXene nanocomposite membrane
was 1.7 times higher than the TFC membrane. MXenes
nanosheets showed good antifouling properties and high
salt rejection.
[57]
MXene@Cals/SA 90% ethanol 2 L/min, Cals (5 %wt) 938 g m
−2
h
−1
Separation factor =4612
Separation performance is improved due to synergistic
effect of MXene and CaLS.
[58]
Ti
3
C
2
T
x
-Al
2
O
3
RhB, MB, OG
Na
2
SO
4
MgSO
4
NaCl
15 dye ppm
1000 ppm
Commercial
membrane
88.8 LMH/bar (MB)
84. (OG), 86.5 (RhB)
75.0 (OF)/99.8% (RhB),
99.5% (MB), 97.2%(OG),
87.2%(OF)
The permeability of the Ti
3
C
2
T
x
-Al
2
O
3
was 6 times
higher than the pristine MXene membrane.
[59]
MXene-O-MWCNT Lubricating oil
MB, CV, RB, JG
11 mg/mL oil-water
emulsion
10 mg L
−1
dye
PAN UF membrane
Pure water ux 293
Lm
−2
h
−1
bar
−1
Permeate ux oil 110
Lm
−2
h
−1
bar
−1
/98% MB,
98% RB,
100% CV, 100% JB,
The synergy effect of MXene and O-MWCNT improves
the ux and rejection of oil and dyes. The composite
membrane exhibited good antifouling, recyclable
properties, and chemical stability in harsh condition.
[60]
MXene-CNT CR, Rh B, MO,
Na
2
SO
4,
MgSO
4
, NaCl
MgCl
2
10 ppm dye solution
10 mM salt solution
Anodic aluminum
oxide (200 nm)
76.5 Lm
−2
h
−1
bar
−1
(pure
water)
10.8 (CR), 11.1
(Rh B), 13.2 (MO)
17.4 (Na
2
SO
4
), 20.9
(MgSO
4
), 23.5 (NaCl)
25.9 (MgCl
2
)/Salt Rejection
R (Na
2
SO
4
)>R (MgSO
4
)
>R(NaCl)>R(MgCl
2
)
The MXene-CNT membranes showed excellent
separation performance, water permeance, and anti-
swelling behavior.
[61]
PEI/MXene MgCl
2
NaCl
Na
2
SO
4
MgSO
4
C
o
=50 ppm salt
solution
9 Lm
−2
h
−1
bar
−1
(MgCl
2
), 8
(NaCl)
7.5 (MgSO
4
), 8 (Na
2
SO
4
)/
82% MgCl
2
50%NaCl, 40% MgSO
4,
40% Na
2
SO
4
Salt Rejection R ((MgCl
2
)>
R(NaCl)
R>(MgSO
4
)>R((Na
2
SO
4
)
High salt rejection and water permeance during the
nanoltration process.
[62]
PA/MXene NaCl
Na
2
SO
4
C
o
=1000 ppm salts 27.8 L m
−2
h
−1
bar
−1
/99%
(Na
2
SO
4
)
PA/MXene Nano ltration membrane showed high
separation for salt solution up to 480
α
NaCl/Na
2
SO
4
than commercial NF membrane and reported NF
membranes.
[63]
PICL/MXene BSA
CV
C
o
=50 ppm crystal
violet
1 g/L BSA
1.64 L/m
−2
h
−1
bar
−1
/
98.92% crystal violet
The membrane loaded with 4% wt MXene showed four
time higher water permeance than pristine membrane.
[64]
PES-Ni@MXene
nanoparticles
BSA
CV
Humid acid(HA)
Oil-water emulsion
C
o
=50 mg L
−1
crystal
violet
1 g/L BSA
20 mg L
−1
HA
1181 L m
−2
h
−1
bar
−1
/
64.6% BSA 99.35 %HA
PES-Ni@MXene nanoparticles membrane was shown 2.5
times higher than that of PES membrane. This membrane
was shown excellent de-coloration ability for dyes and
oil-water emulsion.
[53]
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
10
water permeance, respectively [50]. TiO
2
nanocrystals with a
well-dispersed laminar structure were utilized as intercalators to create
homogeneous nano-channels for water transport within the
graphene-oxide membrane. Additional nano-channels were provided by
the use of MXene. All four dyes such as methyl blue, rhodamine B,
neutral red and crystal violet were removed with greater than 97% of
efciency using this membrane [50].
Inorganic salts, on the other hand, were only removed at low rates by
graphene-oxide-MXene-TiO
2
membrane (61% for Na
2
SO
4
and 23% for
MgCl
2
). The adsorption process was found to only account for about
10% of dye removal, suggesting that electrostatic interaction or size
exclusion may be more important mechanisms of the process [50]. The
separation ability of Ti
3
C
2
T
x
graphene-oxide membranes was tested for
different dyes as well as inorganic ions (MgSO
4
and NaCl) [16]. The data
obtained showed that the dyes have low uxes, but higher removal rates
compared to monovalent or divalent anions or cations. The low uxes
are presumably, due to the organic dyes causing signicant fouling
during the ltration process [86]. The order of removal rate of dyes was
Brilliant blue (100%) =methylene blue (99.5%) >rose Bengal (93.5%)
>methylene red (61%) using this membrane. The removal rates of dyes
with various hydrated radii and charges are dependent on electrostatic
interaction between the membrane, the dyes, and size exclusion [87].
Although electrostatic repulsion occurs between a negatively charged
membrane and the salt ions, the poor removal rate of ions is caused by
the interlayer spacing (0.5–0.96 nm depending on Ti
3
C
2
T
x
swelling)
being too big to lter the ions properly.
The work in [43] established non-swelling Al
3+
intercalated MXene
membranes for seawater desalination. The MXene membranes demon-
strate high rejection of NaCl in the range of 89.5%–99.6% and possess
water ux (~1.1–8.5 lm−2 h
−1
) [43,44]. Because of interactions be-
tween Al
3+
and oxygen functional groups on the MXene nanosheets,
MXene membranes exhibit outstanding stability up to 400 h in aqueous
solution. It is noticed that MXene membranes only required to prepare
by simple ltration and ion-intercalating methods, which holds promise
for large scale production [46]. The study in [46] proposed
self-crosslinked MXene membrane for ion rejection to alleviate swelling
problem often suffered by 2D membrane when immersed in aqueous
solution. The self crosslinked MXene membrane is prepared by a simple
thermal self-crosslinking treatment. The FTIR and XPS characterizations
conrm the formation of Ti–O–Ti and release of –OH groups [46]. The
permeation rate of the monovalent ions through the self-crosslinked
MXene membrane is reduced from the order of 10
−1
to 10
−3
mol h
−1
m
−2
. The membranes show superb long-term stability for monovalent
ions for more than 70 h due to formation of the strong between two
nearby MXene nanosheets. The MXene (Ti
3
C
2
T
x
)/PSS composite mem-
brane was developed for lithium-ion separation [54]. The composite
membrane shows highly Li+selectivity from ionic mixture solutions
with permeation rate of 0.08 molm
−2
h
−1
[54]. The MXene
(Ti
3
C
2
T
x
)/PSS composite membranes exhibit good stability in aqueous
solution. As a result, the MXene (Ti
3
C
2
T
x
)/PSS membranes can be
further expanded with the application in the eld of Li recovery and
nanouidic device [43,44]. Previous works [43,44] developed opposite
charged Ti
3
C
2
T
x
MXene membranes (MXMs) with 2D nanouidic
channels to harvest osmotic energy. The 2D MXene nanochannels show
surface-charge-governed ion transport and exhibit excellent ions selec-
tivity. The MXene membranes are usually fabricated by vacuum ltra-
tion. 2D MXene membranes with a large area are prepared by
electrophoretic deposition (EPD) within 10 min [45]. The EPD-MXene
membranes display excellent ions rejection ability of up to 99.5% for
the light metal ions. The EPD of MXene nanosheets makes it more easily,
rapid to prepare uniform and large area lamellar membranes with
controllable thickness than for a VF membrane. This membrane prepa-
ration method is expected as a good candidate for fabrication of a
different of 2D membranes, targeting for seawater desalination and
Fig. 7. Fabrication of MXene/polymer membrane by casting approach: (a) synthesis of MXene/PAM lm. (b) Preparation process of binary Ti
3
C
2
T
x
/PBI, f-BNNS/PBI
and ternary f-BNNS-Ti
3
C
2
T
x
/PBI membranes. (Figure reproduced with permission from respective publishers).
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
11
Nano ltration.
Organic dyes such as crystal violet, chrysoidine G, methylene blue,
neutral red, and brilliant blue, as well as humic acid and bovine serum
albumin were used to assess the removal performance of Ti
3
C
2
T
x
-gra-
phene membrane [7,17,50]. Even though graphene-oxide-based mem-
branes are unstable in water due to electrostatic repulsion between the
nanosheet layers, it’s have been shown to be stable in water, likely
because MXene tends to increase
π
-
π
attraction and reduce electrostatic
repulsion.
Generally, the pristine graphene-oxide membrane was found to
exhibit a good water permeance and membrane permeability was found
to increase with increasing MXene concentration [17]. The existence of
MXene nanoparticles reduces the H-bonding between H
2
O molecules
and the O-containing functional groups of graphene oxide nanosheets,
resulting in increased water ux due to a lower interaction between the
water molecule and the membrane. Both electrostatic interaction and
size exclusion mechanism govern the transport of organic dyes in the
MXene membrane though it also depends on the types of the target dye.
5. MXene based membrane in water treatment applications
Functionalization of the MXene surface can be carried out using
electrostatic attraction, adsorption, and the creation of covalent bonds.
The –F and –OH surface groups cause MXenes to usually have a negative
surface charge. As a result, a positively charged substance is electro-
statically attached to the MXene surface. The MXenes surface with OH-
groups have signicant acidic behavior with a pKa of 2.7–3, (Lin et al.,
2017) and can potentially adsorb “basic” liquids.
The development of ammonia selective membranes for the Haber-
Bosch process should benet from this unique adsorption feature.
When compared to dissolution in water followed by drying in a
hydrogen-nitrogen stream or ammonia condensation, the use of mem-
branes for selective extraction of NH
3
from a complex mixture can
potentially result in lower operational costs. A positively charged
polymer, such as poly (vinylpyrrolidone) (PVP) or polyethyleneimine
(PEI) [88], can be composited with MXene akes and then adsorbed
onto a substrate surface to produce the membrane. The amino groups in
PEI can create robust hydrogen bonds with the oxygen moieties in
MXenes and so the MXenes can be properly dispersed in PEI [23].
Furthermore, crosslinking PEI with MXene nanosheets powerfully links
the two, creating a robust lm. The surface charge of Ti
3
C
2
T
z
MXene can
also be altered from positive to negative at neutral pH using a
silane-coupling agent that is attached to the membrane [89]. The
functionalization permits for in situ production of the self-assembled
membranes through the layer-by-layer assembly.
Nanomaterials with antimicrobial properties could be used for water
treatment, wastewater treatment, fumigation, environmental remedia-
tion processes and anti-biofouling coating. The attractive characteristics
of MXene such as nanometer sharp edges, hydrophilicity, and generation
of oxidative stress render MXenes as potential candidates to have anti-
microbial effects. Current studies have revealed outstanding antimi-
crobial activity of MXene against bacteria [40,90]. The high
Fig. 8. (a) Synthesis of MXene/PDAC membrane by LbL assembly process. (b) Fabrication process of MXene/PVA lm by electrospinning approach. (c) Electro-
phoretic deposition of Ti
3
C
2
akes. (d) Synthesis of Ti
3
C
2
/PPy membrane by electrochemical polymerization of PPy on Ti
3
C
2
lm (Figure reproduced with permission
from respective publishers).
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
12
hydrophilicity of MXene nanosheets can easily bind to bacteria, causing
inactivation. The high conductivity of MXene aids the production of a
conductive connection over the shielding lipid bilayer, transporting
electrons away from intracellular components under potential causing
in component breakdown and eventual bacteria death. Besides,
hydrogen bonds between the MXene surface groups and lipopolysac-
charide strings of the cell membrane increases MXene and bacteria
contact and lead to bacteria destruction. MXenes can be employed as
antimicrobial coating by coating with PVDF membranes. MXenes in a
thin lm form also showed effective antibacterial activity. The fresh
MXene lms have been shown to inhibit E. coli and B. subtilis when
compared to pristine PVDF [90]. Aged MXene membranes have also
been found to inhibit the growth of both bacteria [40]. This is due to a
synergetic effect between MXene, defective 2-D carbon structure, and
free radicals from TiO
2
. The results also found that MXenes coating
surface membranes have good anti-biofouling properties. MXene
membranes have shown higher conductivity than graphene oxide even
though both have a similar hydrophilicity. Therefore, MXene lm have
better antibacterial potential compared to GO [88]. The silver nano-
particles can be adhered onto MXene nanosheets to increase the anti-
bacterial activity of MXene lm. In a study reported in [39] MXene
composite membranes showed a 21% higher growth inhibition of
Escherichia coli compared to the control. The excellent antimicrobial
activity of this membrane was credited to the synergetic effect of TiO
2
nanoparticles and Ag nanoparticles imposing oxidative stress on the cell
membrane leading to membrane breakdown, and nally, microbial cell
death.
Oil spills arise repeatedly leading to a great impact to the ecological
environment due to urbanization and industrialization. The membrane
technology has prominent advantages to treat complex oil-water
pollution [84]. The work reported in [84] developed an ultra-thin
MXene membrane supported on polyethersulfone substrate using vac-
uum ltration method. Their results showed that MXene based mem-
brane established high antifouling performance, good reusability, and
high oil rejection ratio for oil/water emulsion [84]. A previous study
prepared ultra-thin Ti
3
C
2
T
x
MXene membrane, which effectively sepa-
rates diverse stabilized emulsions with obtained 99.4% of oil removal
efciency and excellent recyclability [91]. He et al. [92] developed
MXene@UIO-66-(COOH)
2
composite membrane by using nylon 66
micro porous substrate. This membrane exhibited good chemical sta-
bility and good separation ability for multicomponent pollutant oil/-
water emulsion. In order to produce ultra-high oil–water separation,
halloysite nanotubes(Hal) and polydopamine (PDA) are utilized to
modify the MXene based membrane [55]. Zeng et al. [55] prepared a
new type of Hal-MXene-PDA composite membrane using vacuum
ltration. The composite membrane exhibited an ultra-high pure water
ux of 5036.2 Lm
-2
h
−1
bar
−1
, and the rejections of lubricating oil and
petroleum ether were shown 99.8%. The composite membranes also
demonstrates good antifouling and anti-swelling properties. Therefore,
it is possible for MXene-based composite membrane applied for
oil-water separation. While MXene based membrane can be a favorable
candidates for oil wastewater treatment, there are still some weaknesses
that need to be tackled. Based on the previous studies, the current uxes
membrane are still relatively low, which could limit their practical ap-
plications. It is needed to develop high ux and antifouling multifunc-
tional MXene based membrane to deal with complex oil wastewater.
The emerging organic pollutants, including pharmaceutical com-
pounds and personal care products have been detected in aquatic en-
vironments, causing adverse effects to aquatic animals and human
health. The uncontrolled release such as antibiotics and pharmaceuticals
in water from the conventional sewage discharge and medical waste
became a worldwide problem. Therefore, highly efcient treatment
technologies for separation of antibiotics and pharmaceuticals are
desired. The MXene based membranes with regular interlayer space
nanosheets can be one of the methods to obtain good separation per-
formance in antibiotics-contaminated wastewater treatment [26]. The
study in [26] successfully prepared Ti
3
C
2
T
x
MXene membranes with
regular slit-shaped nanochannels with large Ti
3
C
2
T
x
sheets. The Ti
3
C
2
T
x
MXene membrane was applied in antibiotic separation from water and
organic solvents. The stacked structure of the Ti
3
C
2
T
x
MXene membrane
was characterized using TEM and FESEM. The membrane thickness and
interlayer spacing of Ti
3
C
2
T
x
MXene membrane were 500 nm and 1.35
nm, respectively. The highly regular laminated Ti
3
C
2
T
x
MXene mem-
brane was demonstrated good antibiotics separation performance in
aqueous solution and ethanol solution. The mechanism of the Ti
3
C
2
T
x
MXene membrane in antibiotic separation is mainly size-selective mo-
lecular sieving and electrostatic interaction. The water permeance
through the MXene membrane is 1 times to 2 times higher than other
nanoltration membrane with comparable antibiotics rejection [26].
Ti
3
C
2
T
x
MXene membrane can extend the applications in drug puri-
cation and medical wastewater.
The investigation in [27] has explored the removal of selected
pharmaceuticals by multilayer MXene. They evaluated the adsorption of
selected cationic and anionic pharmaceutical compounds on MXene.
They found that the absorption capacity for cationic pharmaceutical
compounds to be as high as 58.7 mg/g at pH 7. The removal of cationic
pharmaceutical compounds was also enhanced by the use of sonicated
MXene, which had the highest adsorption capacity of 214 mg/g at 28
Khz frequency. This is due to the stronger bubble cavitation effect
generated by the high frequency, causing MXene to be well dispersed
and also the formation of oxygen containing functional groups on the
surface. The sonicated MXene also showed a high percentage removal,
selectivity, and reusability, indicating its potential as an alternative
MXene base membrane for pharmaceutical compounds. In future
studies, the synthesis of amine –functionalized MXene is suggested to
improve adsorption of anionic pharmaceutical compounds.
6. Conclusion and areas of future study
Recent studies have demonstrated the use of several MXene-based
membranes for water purication and waste treatment. However, only
30 publications have been found to study MXene-based membranes for
the treatment of water contaminants and emerging pollutants of phar-
maceuticals. This indicates that the potential of MXenes has not been
fully unveiled. Therefore, there is plenty of room for exploration per-
taining to the use of membranes incorporated with MXenes to remove
various environmental pollutants. Generally, MXene-based membranes
exhibit better performance compared to pristine membranes based on
their total ux and percentage removal for various dyes, salts, organic
solutions (Dextran and bovine serum albumin), and oil in water emul-
sion. These ndings can be rationalized by considering three main ef-
fects. MXene incorporated with TiO
2
nanoparticles or graphene oxide
offer extra water transport pathways than pristine membranes thereby
improving water permeance and antifouling performance. MXenes
based nanoparticles tend to distinct the pores on the MXene nanoakes,
resulting in an abundance of nanochannels. Finally, the attractive
feature of MXenes such as hydrophilicity and inherent negative charge
can easily incorporate specic species or functionalities that improve
membrane fouling resistance and selectivity, owing to adsorption, steric
exclusion and repulsion and attraction interaction.
The current MXene-based membrane studies only focus on Ti
3
C
2
T
x
MXenes. It is worthwhile to consider other different types of MXene in
the fabrication of novel membranes that may deliver high efcient
separation and safe operation. The current approach to assess the sep-
aration process of the MXene-based membranes is still in the laboratory
scale with limited water quality, selected solutes (dyes and inorganic
salts), and xed operation condition. As a result, systematic investiga-
tion of different MXene-based membranes for different water contami-
nates and emerging pollutants of pharmaceuticals under different water
chemistry and operation condition is lacking. More studies should be
carried out to investigate effective MXene–based membranes using a
variety of crosslinkers, 2D nanomaterials, and substrates for efcient
G.P. Lim et al.
Ceramics International xxx (xxxx) xxx
13
separation performance. The current conventional fabrication tech-
niques prepare the MXene-based membrane at the laboratory scale.
Comprehensive studies are desired to mass produce MXene-based
membrane for commercial application in industrial wastewater treat-
ment. When taking into account its cost-effectiveness, application of
MXenes based membranes should be applied to treat water contami-
nants that are difcult to treat using current conventional treatment
processes. The degradability study of the MXene-based membrane is also
required to be investigated for environmental safety. This review pro-
vides important information on the effects of preparation techniques on
MXene-based membrane properties, compounds properties, and opera-
tive condition which in turn affects the liquid separation performance in
water purication applications.
CRediT authorship contribution statement
Gim Pao Lim: Writing-Original Draft, Formal Analysis, Resources,
Visualization. Chin Fhong Soon: Conceptualization, Validation,
Writing – Review & Editing, Supervision. A.A. Al-Gheethi: Writing-
Original Draft, Writing – Review & Editing. Marlia Morsin: Conceptu-
alization, Validation. Kian Sek Tee: Writing – Review & Editing,
Supervision.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgment
This research is funded by the Malaysia Ministry of Higher Education
under the Research Excellence Consortium Grant Scheme (KKP) or KPM-
Special Grant RMK-10 (JPT(BPKI)1000/016/018/25(54) or KKP Vot.
No. K343).
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15
List of abbreviations
MB: Methylene Blue
AB80: Acid blue 80
MO: Methyl Orange
MR: Methyl Red
CR: Congo Red
EB: Evans Blue
CR: Congo red
GV: Gentian violet
RhB: Rhodamine B
DR 28: Direct red 28
DB 38: Direct black 38
Cyt c: Cytochrome c
NaCl: Sodium chloride
MgCl
2
: Magnesium chloride
VOSO
4
: Vanadyl(IV) sulfate
Na
2
SO
4
: Sodium sulfate
MgSO
4
: Magnesium sulfate
BSA: Bovine serum albumin
G.P. Lim et al.
