Journal of Membrane Science 484 (2015) 95–106
Contents lists available at ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Graphene oxide-assisted membranes: Fabrication and potential
applications in desalination and water purification
Hanaa M. Hegab a,b, Linda Zou a,c,n
a
Centre for Water Management and Reuse, University of South Australia, Adelaide, SA 5095, Australia
Institute of Advanced Technology and New Materials, City of Scientific Research and Technological Applications, Borg Elarab, Alexandria, Egypt
c
Department of Chemical and Environmental Engineering, Masdar Institute of science and Technology, Abu Dhabi, United Arab Emirates
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 26 May 2014
Received in revised form
18 February 2015
Accepted 5 March 2015
Available online 16 March 2015
Globally, the problem of fresh water scarcity has continued to escalate. One of the most powerful
techniques to fully secure the availability of fresh water is desalination. Searching for more efficient and
low-energy-consumption desalination processes is the highest priority on the research agenda. Recent
progress has been achieved using graphene oxide (GO)-assisted membranes in desalination applications.
GO's abundant functional groups, including epoxide, carboxyl and hydroxyl, provide functional reactive
sites and hydrophilic properties. Its freestanding membrane, with a thickness of a few nanometres, has
been applied recently in pressurised filtration, which is an ideal candidate for the application of
desalination membranes. The multilayer GO laminates have a unique architecture and superior
performance that enable the development of novel desalination membrane technology. With good
mechanical properties, they are easily fabricated and have the ability to be industrially scaled up in the
future. This review considers the different fabrication and modification strategies for various innovative
GO-assisted desalination membranes, including freestanding GO membranes, GO-surface modified
membranes and casted GO-incorporated membranes. Their desalination performance and mechanism
will be discussed, and their future opportunities and challenges will be highlighted.
& 2015 Elsevier B.V. All rights reserved.
Keywords:
Graphene oxide
Membrane
Water desalination
Reverse osmosis
Contents
1.
2.
3.
4.
5.
Global fresh water scarcity stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Challenges of water desalination membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Graphene oxide and its potential in membrane development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Recent developments on graphene oxide-assisted desalination membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.1.
Freestanding GO membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.2.
GO for membrane surface modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.3.
GO-incorporated composite membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Fabrication mechanism of GO-assisted water desalination membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.1.
Fabricating freestanding GO membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.2.
Structured GO-surface modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.2.1.
Covalent bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Abbreviations: 2D, two-dimensional; aGO, amino graphene oxide; aPES, amino poly ether sulphone; APTS, 3-amino propyl triethoxy silane; BSA, bovine serum albumin;
BW, brackish water; CA, cellulose acetate; CNT, carbon nanotubes; CR, Congo red; DMAc, di methyl acetamide; DNA, deoxyribonucleic acid; ED, electrodialysis; f-GO,
functionalised graphene oxide; FRR, Water flux recovery ratio; GO, graphene oxide; Gt, graphite; GtO, graphite oxide; HPEI, hyper-branched polyethylenimine; IPA, isopropyl
alcohol; JP, Permeation flux; JWI, Pure water flux; LbL-SA, layer-by-layer self-assembly; MB, methylene blue; MBR, membrane bioreactor; MD, membrane distillation; MDS,
molecular dynamics simulations; MF, microfiltration; MMAHPOEM, methyl methacrylate-hydroxy poly (oxyethylene) methacrylate; MO, methyl orange; NF, nanofiltration;
NMP, N-methyl pyrolidone; NPs, nanoparticles; OMWCNT, one dimension oxidised carbon nanotubes; PA, polyamide; PAA, polyacrylic acid; PAN, polyacrylonitrile; PEC,
polyelectrolyte complex; PEG, polyethylene glycol; PEI, polyethyleneimine; PES, polyethersulfone; PSF, polysulfone; PSVBP, poly (4-(2-sulfoethyl)-1-(4-vinylbenzyl)
pyridineiumbetaine); PVA, polyvinyl alcohol; PVDF, polyvinylidene fluoride; RB, rhodamine B; rGO, reduced graphene oxide; RO, reverse osmosis; TFC, thin film composite;
UF, ultrafiltration; UV, ultraviolet; WHO, World Health Organisation
n
Corresponding author. Tel.: +971 28109304.
E-mail address: lyuanzou@madar.ac.ae (L. Zou).
http://dx.doi.org/10.1016/j.memsci.2015.03.011
0376-7388/& 2015 Elsevier B.V. All rights reserved.
96
H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
5.2.2.
Non-covalent bonding via LbL-SA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Casting GO-incorporated composite membrane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.3.1.
Unfunctionalised GO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.3.2.
Functionalised GO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6. Summary and future prospects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
5.3.
1. Global fresh water scarcity stress
Nowadays global population growth has intensified, with an
estimated population increase from 7 to 10 billion by 2050. One
significant consequence of this trend is the increasing scarcity of fresh
water, which represents only 0.5% of Earth's overall water resources,
compared to seawater (97%) [1]. Recently, the universal need for fresh
water has increased more rapidly than in the past. The agriculture and
food-production sectors are the main consumers of water in most
countries, requiring 100 times more water than domestic users [2].
Desalination is one of the most important and promising
methods for fresh water augmentation [3]. The membrane based
desalination processes can be categorised according to membrane
pore size and rejection mechanism: membrane distillation (MD),
electrodialysis (ED), microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) (Fig. 1) [4].
RO membrane technology is presently considered the greatest
major membrane-assisted desalination technique and it is experiencing significant progress because of its effective salt rejection
and reasonable energy requirements. RO energy consumption has
reduced from approximately 5 kW h/m3 in the 1990s to 1.8 kW h/
m3 today, which is less than other techniques, including thermalbased desalination methods [5].
Membrane-assisted desalination technologies require improvement to enhance their performance in terms of water flux, salt
rejection and fouling resistance. The rapid progression of nanotechnology research has created a golden opportunity to combine
nanotechnology with polymeric RO membrane development.
Already, progress has been achieved at the laboratory scale in
incorporating nanomaterials such as carbon nanotubes (CNTs),
graphene and zeolites into RO membranes by modification or
fabrication processes [6]. However, there are challenges in using
these new materials practically and on a large scale. The greatest
challenges include the low salt rejection obtained from CNT-based
membranes [7] and the low water flux achieved by zeolite
membranes [8–10]. Moreover, most of these modified membranes
have been utilised in different applications, such as pervaporation,
rather than in RO applications.
In theory, graphene-assisted desalination membranes have
been considered promising [11]. Inspired by this theory, many
recent publications have confirmed the positive role of GOassisted membranes in water desalination [12–18]. This review
goes beyond previous work by providing not only a detailed
update on the current GO-assisted desalination membranes, but
also a discussion of how the GO enhances membrane efficiency.
Furthermore, the challenges facing GO-based membranes will be
outlined.
2. Challenges of water desalination membranes
Membrane fouling, high energy demand and trade-offs
between salt rejection and water flux are remained as challenges
of water desalination membranes (Fig. 2). In general, the operating
costs of the RO process remain high [19] because of the challenges
of limited water permeability, and high energy and chemicals
consumption [6]. Even though the required working pressure in
modern systems is close to the thermodynamic limit, further
reduction of the applied pressure will have no noticeable impact
on performance [20]. Cohen-Tanugi et al. [21] observed that a
tripling in permeability would decrease pressure by 44% and 63%
for RO seawater and BW plants respectively; this is equivalent to a
reduction of 15% and 46% in energy consumption. The drop in
energy consumption would be significant because of the high cost
of energy, which accounts for 50% of the total water desalination cost.
Membrane fouling is one of the primary concerns in desalination technology; the onset of fouling gives rise to a decline in the
RO membranes' performance [22,23]. Commonly, membrane fouling occurs via one of two mechanisms: the first is fouling in
membrane pores, the second is membrane surface fouling. The
occurrence of surface fouling is the result of a range of impurities
Fig. 1. Schematic diagram representing the different types of water desalination membranes according to pore size.
H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
97
Fig. 2. Key bottlenecks of water desalination membrane productivity.
Fig. 3. Schematic diagram indicating the separation mechanism of the freestanding graphene and GO membranes.
such as suspended organic or inorganic matter, biogenic materials
and dissolved solids or dissolved organic matter in the feed water
[24]. Moreover, impurities, particularly the biological ones, usually
build up on the membrane surface during the RO process to
develop a biofilm [25], which is believed to be the major limiting
factor in the seawater desalination process [26]. Bio-fouling is the
most critical factor affecting the membrane desalination process
[5]; it leads to lower membrane selectivity, permeate water flux,
higher energy consumption and shorter membrane durability [22].
Disinfectants such as chlorine, which are regularly used to remove
the growing membrane biofilm, tend to react with polyamide (PA)
on the membrane surface layer; this is evidenced by a number of
research studies on RO membranes [27–30]. The process of
chlorine disinfection can also degrade the top layer of polyamide,
resulting in major variations in the capacity of the thin film
composite (TFC) RO membrane [31], even if low concentrations
of chlorine are applied to the feed water [32].
3. Graphene oxide and its potential in membrane development
Since Geim and Novoselov were awarded the Nobel Prise in
Physics in 2010, graphene research has developed rapidly in both
academic and industrial applications [33]. Graphene is a layer of
strongly packed pure carbon atoms that are joined together in a
hexagonal honeycomb matrix. More comprehensively, it is a
carbon allotrope arranged in flat sp2 bonded atoms with a very
small molecule bond length (0.142 nm). Graphite is a threedimensional material formed by the arrangement of graphene
layers on top of each other, with tiny (0.335 nm) interplanar gaps
[11].
This novel material has progressed rapidly towards scaling up
production of 30-inch graphene membranes consisting of multiple
layers of graphene sheets [34]. The tiny thickness of graphene, i.e.
one atomic layer has a unique tensile strength [35]. Although this
single atomic layer is impermeable to all gases and liquids [36,37]
(Fig. 3A), many researchers are exploring the possibility of using
this material to develop new membranes for desalination applications. The currently available mass production of graphene makes
this development possible [38].
Moreover, nanopores could be created within the unsaturated
carbon atoms, which exist at the chemically passivised pore edge,
in the structure of graphene. Recently, experimental procedures
for introducing nanopores into graphene have been widely
explored and rapid developments achieved. Initial methods were
based on electron beam bombardment; however, the more modern approaches such as helium ion beam drilling, diblock copolymer templating and chemical etching were performed to achieve
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H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
Fig. 4. Schematic diagram illustrating the fabrication mechanism of GO nanosheets from graphite.
higher pore density and more precise pore size distribution [39–
42] (Fig. 3B). The most recent studies have focused on using the
prepared nanoporous graphene in gas separation and deoxyribonucleic acid (DNA) sequencing applications [43–47]. Further, the
efficiency of NaCl rejection by graphene membrane was simulated
using a molecular dynamics modelling approach. This research
concluded that one layer of nanoporous graphene can successfully
remove salt from water with rates ranging from 10 to 100 L/cm2/
day/MPa, which is two to three times greater than can be achieved
using the RO diffusion membrane [11].
Nanocarbon-based materials i.e., graphene and CNTs, also
possess the ability to inhibit bacterial growth upon direct contact
with cells [48–52]. This bactericidal ability is durable over time,
enabling a new kind of antimicrobial surface that is not depleting
or leaching [53]. Liu et al. [50] compared four derivatives of
graphene-based materials graphene oxide (GO), reduced graphene
oxide (rGO), graphite (Gt) and graphite oxide (GtO) in terms of
their antibacterial activity against the bacterial species of Escherichia coli. They observed that, under the same concentration,
incubation time and conditions, GO dispersion has the highest
antibacterial ability, followed consecutively by rGO, Gt and GtO.
In addition, it has been stated that graphene is an inert material
that hardly dissolves in classic organic solvents [54]. Therefore, it
was proposed that pristine graphene should be chemically modified in order to be practically utilised in several applications
[54,55].
The chemical exfoliation of GtO is the classic method of
obtaining GO. GO is produced via either the Brodie [56], Staudenmaier [57] or Hummers [58] method, or via various modifications
of these techniques (for example, the Hummers modified method)
[58–62], allowing significant amounts of epoxy, hydroxyl and
carboxylic groups in the resultant graphene [55,63]. The highly
oxidised graphene prepared by this method is usually called GO
[63]. The three techniques include graphite oxidation to serious
levels. Brodie and Staudenmaier oxidise the graphite using a
mixture of nitric acid (HNO3) and potassium chlorate (KClO3); in
contrast, the Hummers method oxidises the graphite using a
combination of potassium permanganate (KMnO4) and sulphuric
acid (H2SO4). The obtained graphite salts prepared via intercalating graphite with strong acids i.e., H2SO4, HNO3 or HClO4 are
created prior to the oxidation of graphite and, finally, the
exfoliation of GO in water, in order to obtain GO nanosheets [64]
as schematised in Fig. 4.
The GO could be reduced to graphene by hydrazine vapour [54];
however, the reduced graphene sheets tend to form heavy aggregates
and precipitate from the reaction medium because the recovered
graphite domain increases their hydrophobicity and corresponds to
stacking interactions. This problem can be overcome by introducing
sulphonic functional groups to the surface of partially reduced GO. The
negatively charged units can effectively prevent the graphene sheets
from aggregating [60,65]. GO could be covalently modified via the
amination coupling of carboxylic or epoxy groups [66,67], or noncovalently functionalised [68]. Also, a number of exceptional properties of modified GO have been examined i.e., optical characteristics
arbitrated by pH [69], biocompatibility for L-929 cells proliferation and
adhesion [70], water-insoluble drug delivery property and high antibacterial activity [48,50,71–73].
Although various applications involving GO have been triggered, its application as membranes for separation has not been
widely examined. The first serious investigation of GOincorporated membrane was reported by Nair and his group
[74], who fabricated a GO-incorporated membrane with the
capability of unhindered water vapour permeation and almost
the full retaining of other gas molecules. Further studies were
conducted by Cohen-Tanugi and Grossman [11], in which the
ability of nanoporous graphene to reject salt as an RO membrane
was simulated; this work was confirmed by Wang and Karnik [75].
It is believed that GO has the ability to enhance the membranes'
separation performance, although it is challenging to implement
this concept in reality, similar to another carbon nanomaterial
CNTs, which represent natural water channel [76,77].
Nevertheless, when GO is properly assimilated into a ceramic
[78] or polymer [79,80] matrices, the properties of the obtained
nanocomposites are significantly improved, and the GOincorporated membranes can be used in different applications i.
e., fuel cells [81,82], nanofiltration [83,84], ultrafiltration [85–87],
gas separation [88] and pervaporation [84]. In the same context,
GO may enhance the performance of the obtained hybrid polymer
membrane, such as its mechanical, antifouling and surface charge
properties. The estimated improvement of GO-incorporated membranes was attributed to the hydrophilic nature of some functional
groups in GO, which resulted in better GO dispersion in water and
H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
some organic solvents, and created laminates comprising GO
layers, in nanosize, that formed a mesh-like structure [89–91].
GO is a nanomaterial with an amphiphilic nature; water
molecules are adsorbed initially at the hydrophilic terminal
(hydroxides), then quickly diffused among the hydrophobic carbon
core, developing a water channel that improves permeation flux.
Once water molecules infiltrate the GO layers, they create a singlelayer configuration that drives the consecutive layers apart,
resulting in the increase of the d-spacing [92]. The unique properties of GO-incorporated water desalination membranes could open
the door to opportunities to overcome the challenges in making
clean water easily available around the globe.
4. Recent developments on graphene oxide-assisted
desalination membranes
In the fabrication of desalination membranes, GO could potentially
be applied to freestanding membranes, on the surface of membranes
or through incorporated casted membranes. The first technique
utilises GO as a separating layer directly, the second employs GO for
membrane surface modification and in the third technique, GO is
incorporated into a polymer matrix. In the following sections, the most
recent advances in the three techniques described above will be
overviewed along with their performance (Table 1).
4.1. Freestanding GO membrane
Various techniques were applied to fabricate freestanding GO
membranes with gaps among GO nanosheets by using the continuous vacuum filtration technique reported by Xu et al. [93]. This
group has fabricated GO/TiO2 composite NF membrane with an
average pore size of 3.5 nm by introducing TiO2 nanoparticles
between GO nanosheets to widen the gaps and form channels. The
NF GO/TiO2 membrane has achieved 100% rejection of Rhodamine
B (RB) and methyl orange (MO) from water. In a different study, by
Nair et al. [74], the intersheet gaps were minimised to 1 nm. The
researchers fabricated the freestanding Cu-assisted GO NF membranes via a spray- or spin-coating method, which utilised the
stable dispersed GO/water solution to create more laminates
among GO nanosheets. The obtained membranes were totally
impermeable to gases, vapours and liquids as well as helium;
however, these fabricated membranes permitted unrestricted
water permeation. Additionally, the water permeates over the
fabricated membranes at around 1010 times faster than helium.
Sun et al. [12] fabricated GO freestanding NF membranes via a
drop-casting technique, which diminished the intersheet spacing
to 0.82 nm. The obtained membranes were applied to efficiently
separate sodium salt from organic contaminants and copper salt.
Recently, Nicolaï's group [13] reported on their use of molecular
dynamics simulations (MDS), that freestanding GO membranes are
exceptional candidates for RO membrane applications to realise
proficient water desalination. Freestanding GO membranes have
the ability to reject salt by 100%, while simultaneously allowing
excellent water permeation, with double the permeation ability of
current RO techniques because of the GO membranes' ultrathin
thickness (approximately 10 nm).
4.2. GO for membrane surface modification
Membrane surface modification using GO could enhance several
membrane properties, including the antimicrobial effect, which is
greatly improved when GO on the membrane surface interacts directly
with the bacterial cells [50,51]. In addition, the GO-modified membranes are more chlorine resistant while maintaining the same RO
performance. Further, surface modification requires relatively small
99
quantities of nanomaterial (GO), making it cost-effective and minimising the environmental impacts caused by GO fabrication.
An early trial to prepare GO-surface modified membrane was
conducted by Kim et al. [94], who deposited GO nanosheets
followed by amino GO (aGO) nanosheets on the surface of amino
poly ether sulphone (aPES) membrane using the layer-by-layer
self-assembly (LbL-SA) method. The obtained aGO/GO/aPES RO
membrane had much higher chlorine resistance than the original
PA RO membrane, and good salt rejection (98%) and water flux
(28 L/m2 h) were also achieved. The same modification technique
was applied on a PA-TFC membrane; the LbL-SA technique was
used to deposit aGO and GO multilayers on the membrane surface.
A dual-functional protective layer was reported to improve both
membrane antifouling properties and chlorine resistance, while
maintaining the separation performance. The water flux was
improved by 10% and the salt rejection was slightly decreased by
0.7%, as compared to the unmodified membrane [95]. The LbL-SA
methodology was also employed by Gao et al. [96] to introduce
TiO2 nanoparticles and GO nanosheets consecutively to the surface
of a polysulfone membrane. A partial reduction of GO was
achieved by ethanol-UV pretreatment. The successive grafting of
TiO2 nano-particles and GO nanosheets onto the membrane surface was completed using a photocatalytic approach (i.e., UV and
sunlight radiations). The fabricated membranes can effectively
reject 90% of methylene blue (MB) and the water flux was
increased to 45 L/m2/h as a result of the enhanced hydrophilicity
by TiO2 on the membrane surface.
The effect of GO modification on membrane antibacterial properties was further explored by Perreault et al. [53]. This work
reported the initial provision to covalently attach GO nanosheets
to the surface of PA (TFC) membranes for antimicrobial purposes.
The GO nanosheets had a covalent bond with the PA thin layer. The
modified membrane demonstrated effective inactivation of the
bacterial cells by 65% after one hour, without sacrificing water flux
and salt rejection. In addition, it was observed that the modified
membranes became more hydrophilic, which was evidenced by
the decrease of the water contact angle from 811 for the control
membrane to 471 for the modified membranes, attributable to the
surface oxygen functionalities of GO.
In a study conducted by Hung et al. [92], the GO-surface modified
membranes were utilised efficiently as pervaporation membranes. The
researchers ordered flexible multilayers of GO on the surface of
modified polyacrylonitrile (PAN) membranes using the pressureassisted self-assembly method. The obtained composite membrane
demonstrated outstanding performance: approximately 99.5 wt%
water recovery and 4137 g/m2 h water permeation flux during the
pervaporation separation test, using 70 wt% isopropyl alcohol (IPA)/
water solution. The significant selectivity was attributed to the
deposited dense GO laminates, which were packed in perfect order,
permitting only water and avoiding permeation of IPA molecules.
4.3. GO-incorporated composite membrane
Recently, researchers have focused on adding GO to polymer
mixed solutions to be casted together to improve water permeability, antimicrobial properties and mechanical strength [85,97–
99]. GO can either be prefunctionalised prior to inclusion in the
polymer matrix, or applied without functionalisation.
The first strategy was employed by several researchers. GO was
functionalised by different modifiers: hyper-branched polyethylenimine (HPEI) [98], 3-amino propyl triethoxy silane (APTS) [100]
and isocyanate (i) [101]. Different ratios of functionalised GO (fGO) were then mixed with polymer solutions i.e., polyvinylidene
fluoride (PVDF) and polysulfone (PSF) respectively in order to be
casted by the common phase-inversion method. The obtained
fabricated ultrafiltration membranes (APTS f-GO/PVDF and i-f-GO/
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H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
Table 1
Overview regarding the recent research of GO-based water desalination membranes and its impact on the membrane performance.
Type of GO
Abbreviation of
modification membrane
composite
Insitu
Surface
Membrane
fabrication method
GO-based water desalination membranes performance
Flux
Rejection%
Contact
angle
Antifouling
(FRR %)z
BSAv 99%
BSA 79%
Na2SO4 72%
BSA 95%
PEGw 20,000 85% and
PVAx 30,000–70,000
90%
Mg2 þ 92.6% and Na þ
43.2%
82.5
60.5
53
74
63.1
N/A
BSA 88.6%
N/A
BSA 40.3%.
BSA 92.1%
[99]
[103]
[83]
[101]
[98]
N/A
N/A
[84]
N/Ay
52.5
BSA 98.3%
[97]
Dye 96%
BSA 57%
Yeast suspension 80%
20.4
51
60.5
[102]
[100]
[104]
NaCl 98%
55.4
Protein 90.5%
BSA 85%
Yeast
suspension
81.1%
N/A
Methylene Blue 90%
19
N/A
[96]
NaCl 96.4%
26
N/A
[95]
Isopropyl alcohol/
water 70 wt% 99.5%
N/A
N/A
[92]
NaCl 97.8%
47
N/A
[53]
Methyl orange 100%
N/A
N/A
[93]
UFp
UF
NFq
UF
UF
Phase inversion
Phase inversion
Phase inversion
Phase inversion
Phase inversion
100 kPa
100 kPa
400 kPa
100 kPa
100 kPa
450 L/m2/h
26.49 L/m2 h
50 L/m2 h
135 Kg/m2 h
153.5 L/m2 h
GO/PECsh
NF
500 kPa
GO/OMWCNTsi
/PVDF
GO/DMAc/PESj
APTSk f-GO/PVDF
GO/DAMc /PVDF
UF
Dynamic selfassembly LbLu
blending
Phase inversion
100 kPa
7.1 kg/m2 h MPa,
and 8.1 kg/
m2 h MPa
410 L/m2 h
NF
UF
MFr
Phase inversion
Phase inversion
Phase inversion
400 kPa
100 kPa
25 kPa
53 kg/m2 h
401.39 L/m2 h
324.5 L/m2 h
aGOl/GO/aPES
ROs
GO/aGO/PA
GO/PAN
o
GO/PA
GO/TiO2
n
Self-assembly LbL
deposition
Photocatalytic UV Self-assembly LbL
deposition
RO (TFC)t
Self-assembly LbL
deposition
NF
Pressure-assisted
Perevaporation self-assembly drop
casting
RO (TFC)
GO covalently
bonded membrane
surface
NF
Continuous vacuum
suction filtration
Reference
Applied
pressure
GOa/NMPb/PSFc
GO /DMAcd/PVDFe
GO/NMP/PSF
i-f-GOf/PSF
HPEIg-GO/PSF
GO-TiO2m/PSF
Freestanding
Membrane
classification
5500 kPa 28 L/m2 h
45 L/m2/h
69 kPa
2
1550 kPa 14 L/m h
5 kg/cm
2
2
4137 g/m h
2760 kPa 41.4L/m2 h
100 kPa
7 L/m2 h
[94]
a
GO: Graphene oxide.
NMP: N-methyl pyrolidone.
c
PSF: Polysulfone;
d
DMAc: Di methyl acetamide.
e
PVDF: polyvinylidine folride.
f
i-f-GO: Isocyante functionalized graphene oxide.
g
HPEI: Hyper branched polyethylenimine.
h
PECs: Polyelectrolyte complexes.
i
OMWCNTs: One dimension oxidised carbon nanotubes.
j
PES: Polyether sulphone.
k
APTS: 3-amino propyl triethoxy silane.
l
aGO: Amino graphene oxide.
m
TiO2: Titanium dioxide.
n
PA: Polyamide.
o
PAN: Polyacrylo nitrile.
p
NF: Nanofiltration.
q
UF: Ultrafiltration.
r
MF: Microfiltration.
s
RO: Reverse osmosis.
t
TFC: Thin film composite.
u
LbL: Layer by layer.
v
BSA: Bovine serum albumin.
w
PEG: Polyethylene glycol.
x
PVA: Polyvinyl alcohol.
y
N/A: Not Applicable.
z
FRR%: Water flux recovery ratio.
b
PSF) had the ability to reject bovine serum albumin (BSA) by 57%
and 95% respectively, whereas the HPEI-GO/PSF rejected PEG
20,000 by 85% and polyvinyl alcohol (PVA) 30,000–70,000 by
90%. Xu et al. [100] revealed that the mechanical properties
(elongation-at-break and tensile strength) of APTS f-GO/PVDF
membranes were improved by 48.38% and 69.01% respectively
compared to GO/PVDF membranes because of the strong interfacial reaction between APTS f-GO and the polymer matrix. The
same trend was found by Yu et al. [98], who reported that the HPEI
f-GO/PSF membrane showed significantly better Young's modulus
and tensile strength. Further, the antifouling properties of the
fabricated membranes were successfully improved by adding f-GO,
enhancing the hydrophilic properties of the membranes and
providing extra negative charge, which was evidenced by zeta
potential measurement and smooth membrane surface topography [98,101].
Another functionalised GO membrane was fabricated by introducing the GO with polyelectrolyte complexes (PECs) [84]. Polyethyleneimine (PEI)-modified GO and polyacrylic acid (PAA) were deposited
consecutively on the surface of a hydrolysed PAN ultrafiltration
H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
membrane substrate. The nanohybrid membrane was dipped in PVA
solutions, and then cross-linked via glutaraldehyde. Nanohybrid GO/
PECs membranes can remove dye molecules. The rejection of Congo
red (CR) achieved 99.5% and the water permeability was 8.4 kg/
m2 h MPa. For the rejection of monovalent and divalent ions, such
membranes exhibited good NF characteristics. The rejection of Na þ
and Mg2þ were 43.2% and 92.6% respectively.
Many researchers also employed the second strategy, in which
unfunctionalised GO nanosheets were incorporated into different
polymer matrices. They investigated the remarkable effect of
casting various GO-incorporated polymeric membranes such as
GO/N-methyl pyrolidone (NMP)/PSF [83], GO/di methyl acetamide
(DMAc)/polyethersulfone (PES) [102], NF membranes, GO/one
dimension oxidised carbon nanotubes (OMWCNTs)/PVDF [97]
and GO/DMAc/PVDF [103] UF membranes, GO/DMAc/PVDF [104]
MF membranes and GO/NMP/PSF [99] membrane bioreactors
(MBRs) by the phase-inversion method. The modified MBRs highly
suppressed the fouling; as a result, the time between chemical
cleanings was increased fivefold. Ganesh et al. [83] reported that
the membrane with 2000 ppm GO fillers showed a maximum
Na2SO4 rejection of 72%. Zinadini et al. [102] stated that the
asymmetric flat sheet (GO/DMAc/PES) membrane performed well,
rejecting dye molecules by 99% with water flux 65.2 kg/m2 h.
Zhang et al. [97] investigated the synergistic effect among GO and
OMWCNTs of (PVDF) composite membranes on the water flux and
antifouling performance. They reported that tortuous and lengthy
OMWCNTs can connect neighbouring GO nanosheets and prevent GO
aggregation, improving the anti-irreversible fouling and hydrophilicity
of the fabricated membranes [97]. Zhao et al. [103] verified that
introducing GO nanosheets enhanced the surface hydrophilicity of the
modified GO/DMAc/PVDF (UF) membrane. The modified membranes
had the ability to reject 44.3% of BSA and had a water flux of 26.49 L/
m2 h. Further, these membranes demonstrated much better antifouling properties than the pristine PVDF because of the changes of
membrane morphology and surface hydrophilicity.
5. Fabrication mechanism of GO-assisted water desalination
membranes
5.1. Fabricating freestanding GO membrane
The freestanding GO membrane is considered the most promising
candidate for water desalination application in the future because of
its unique structure. It has been suggested that GO nanosheets tend to
form laminates, as illustrated in Fig. 3C. It could be described as an
assembly of GO microcrystallites, which form interlocked layers that
are packed and stacked above each other; the gaps are created by the
existing interlayer space (d) [91,105,106]. These sub-mm thick membranes of GO laminates have a robust mechanical strength as well as
flexibility [38,91,105,106]. It was reported that GO sub-mm thick
membranes could be impermeable to vapours, gases and liquids as
well as helium; however, freestanding GO membranes allow water
molecules to permeate without any impediment. This phenomenon is
due to the presence of oxygen functional groups (i.e., hydroxyl, epoxy,
and so forth) attached to the GO nanosheets, which are responsible for
keeping these interlayer gaps (d) [107–110].
Importantly, these functional groups cluster by affinity and
create huge percolating areas of non-oxidised GO nanosheets
[106,111,112]. Consequently, it is expected that gaps have been
created among non-oxidised areas of GO laminates, because d for
reduced GO (rGO) is approximately 4 Å, while the gap size width
can be predicted as approximately 5 Å, which is suitable to
facilitate the passage of monolayer water [113,114]. It was estimated that these gaps form a network of graphene nanocapillaries
within GO laminates. Further, it was demonstrated that the low-
101
friction flow of monolayer water through two-dimensional (2D)
graphene nanocapillaries was achievable. However, the oxidised
areas were likely to react strongly with introduced water molecules, so did not participate in the water permeation. The
described freestanding GO membranes could be employed as
barrier membranes in order to filtrate and separate materials with
selectivity to permeate water [74].
GO/TiO2 sheets were simply assembled into well-packed alignments of GO/TiO2 membranes. These TiO2 nanoparticles could
support GO nanosheets and enlarge the interlayer space, which led
to the creation of suitable channels and pores in the fabricated
membranes and allowed them to be promising filtration membranes [93], as indicated in Fig. 3D.
5.2. Structured GO-surface modification
5.2.1. Covalent bonding
GO could be used to covalently modify the surface of desalination membranes. Covalent bonding could be achieved using amide
coupling to link the carboxylic groups attached to GO nanosheets
and the other carboxylic groups on the surface of the polyamide
thin layer PA (TFC) [53] (Fig. 5A).
The functionalisation step does not have any negative effect on
the membrane's performance. Also, when PA (TFC) is surface
functionalised, GO nanosheets can be assembled better on the
membrane surface, which has a positive and strong influence on
the antimicrobial and hydrophilic properties of the modified
membranes. It was reported that the surface hydrophilicity does
not enhance the membrane water flux as the flow of water was
governed via the solution diffusion mechanism by the active layer
of PA (TFC), independent of the surface modification [5].
5.2.2. Non-covalent bonding via LbL-SA
GO can modify the surface of desalination membranes noncovalently using a LbL-SA approach, as illustrated in Fig. 5B. GO
could be used directly or through bridging materials, depending
on the membrane surface net charge (i.e., aGO or TiO2) to noncovalently modify the membrane. The LbL-SA method of modification was reported to cause the preferential absorption of water
molecules into the oxygen functional groups attached to GO
nanosheets, which is then entered through the ‘gaps’ among the
GO laminates and were promptly spread over. The water molecules exhibited low-friction interaction with the hydrophobic
central carbon-rich region of GO, and created ‘channels’ that
facilitated their passage through the GO laminates membrane.
The proposed mechanism could possibly explain the membraneseparation ability and why the water permeate concentration by
the GO/PAN membrane reached approximately 99.5% through the
pervaporation process [92].
Many bridging nanomaterials were tested by researchers. One of
these materials was the aGO, which was used to achieve the dual
advantages of increasing the membrane surface resistance to chlorine and fouling by using the durable coating technique on the surface
of two different substrates i.e., PA(TFC) and aPES via LbL-SA deposition of GO and aGO nanosheets [94,95]. The depositions steps were
repeated as a layer of negatively charged GO nanoparticles was
applied to the surface of the aPES membrane, followed by a layer of
positively charged aGO [94]; alternatively, the process could be
reversed [95].
The PA active thin layer of the TFC membrane was described as
a tightly cross-linked ‘inner-core layer’ inserted in the middle of
two wider outer layers. Principally, the inner-core layer controls
salt rejection; however, the dense bulk of the PA(TFC) is a selective
layer that accommodates the water permeation [115–119].
102
H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
Can the GO multilayer alone be considered a salt-rejection
layer? Further, can it overcome the problem of reduced salt
rejection by a PA selective layer caused by chlorination? Choi
et al. [95] found that the rejection of NaCl by the GO multilayer
deposited on the surface of the PSF substrate (GO membrane) was
limited to 12.2 70.6%. They reported that GO multilayers alone do
not provide sufficient selectivity for NaCl salt rejection; this
finding supports the earlier report on the rejection behaviour of
the LbL-SA GO-based membranes [16].
Despite the conclusion that GO laminates are not sufficient to
be a thin separating layer, as mentioned in the previous work, Kim
et al. [94] have reported using a non-covalently (LbL-SA) coating
technique to fabricate aGO/GO/aPSF. Their membranes were
described as novel TFC RO membranes, with the characteristics
of high RO performance and robust chlorine resistance. GO/aGO
nanoparticles were introduced onto the aPES membrane surface
and the GO-surface modified membranes demonstrated enhanced
chlorine resistance. The modified aGO/GO/aPES membrane exhibited a salt (NaCl) rejection of 98.4%, which was higher than the
pristine aPES and PA membranes (94.3% and 98% respectively).
Recently, GO laminates have been investigated for pressurised
filtration, to clarify their potential promising properties [12,15–17].
The outcomes varied widely, according to the fabrication method
used. Moreover, it is inconsistent to permeate large molecules
through the GO laminates [91,106]. This could be explained by the
occurrence of pin holes or cracks in these laminates, which hinder
their inherent properties [14].
After the introduction of GO/aGO nanoparticles into the membrane, the amide bonds are more obscured. Also, the presence of
nanoparticles in membranes enhances hydrogen bonding via the
intermolecular reaction between the PA and the nanoparticles,
which is assumed to inhibit the substitute of hydrogen ions on the
amide terminals of PA membranes by chlorine. Additionally, amide
bonds in the active layer of the RO membrane are formed as a
result of the merge between the amino sites on the GO/aGO. These
bonds contribute together with the remaining nanoparticle amino
groups towards further protection of the membrane active layer
from chlorine [94].
TiO2 nanoparticles was used to bridge GO nanosheets to a PSFbased membrane surface via the LbL-SA approach [96]. First, TiO2
nanoparticles were adhered to the membrane through the Ti-O link
made between sulphonic groups and Ti4 þ and, to some extent, by the
hydrogen bonding between the sulphonic and hydroxyl groups of
TiO2 [120]. GO nanoparticles were then layered on the TiO2 deposit by
Ti-O and/or hydrogen bonding between the GO carboxyl groups
and Ti4 þ . Finally, GO underwent partial reduction and was
physiochemically bonded to TiO2 via the ethanol-UV method
[121,122]; hence, the colour of GO turned from light brown to
black [123].
5.3. Casting GO-incorporated composite membrane
5.3.1. Unfunctionalised GO
In general, it was found that the incorporation of carbon
nanomaterials into the membranes affected the pore configuration
[97]. Compared with the virgin membranes, the surfaces of the
modified membranes showed a dense porous structure resulting
from the precipitation of nanomaterials throughout the phaseinversion procedure [124]. After modification, the surface of the
membranes achieved favourable pore density and advantageous
cross-section pore structure. The considerable improvement in
membrane hydrophilicity resulted in enhanced permeability of the
modified membrane [125]. Consequently, the introduction of
carbon nanomaterials endowed the membranes with dry-loading
capability without affecting the permeability, which is advantageous for preserving the membrane's resistance to bacteria and
facilitating transportation.
The membrane's permeability was improved with the increasing GO concentration, until a certain critical concentration was
reached; after this, the flux began to decline. This decline was
ascribed to the blockage of pores and reduction in pore size by the
high concentration of the GO nanoplates. The decreased porosity
found in the membrane surfaces confirmed that the high GO
content increased the solution viscosity, slowing down the precipitation and leading to the development of a dense skin layer on
the surface and a wider finger-like pore arrangement, which was
further extended towards the central or bottom regions of the
membranes [104] (Fig. 6A).
Lee et al. [99] explain the progressive role of GO in the phase
inversion of the membrane-casting process. Normally, polymer solution solidifies quickly during separation at the edge between polymer
and non-solvent solutions because of the concentration gradient and
the rapid activity of all the components. At fragile sites on the
hardened polymer surface, cracks appeared as a result of the
pressures generated by the shrinkage occurring through continuous
desolvation. The incorporation of GO in the casting solution improved
hydrophilicity and affected the degree of substitution between nonsolvent and solvent during phase separation. As a result, the formation of cracks and macrovoids was minimised.
It has been stated that GO, with its acidic groups, can produce
negative charge on the membrane surface throughout the entire
pH range [126], leading to significant repulse between the
Fig. 5. Schematic diagram exemplifying the preparation mechanism of GO-based membrane surface modification.
H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
103
Fig. 6. Overview of the casted GO-incorporated membrane.
negatively charged surface and negatively charged ions. GO has
resulted in an increase in the hydrophilic properties of the
membrane, and led to enhanced permeation flux (JP) and pure
water flux (JW1); the more hydrophilic membrane becomes more
appealing to the water molecules within the membrane network,
enabling them to flow through the membrane [83,127].
5.3.2. Functionalised GO
It was also reported that functionalised hydrophilic nanoparticles could be mixed with polymer matrix to increase the amorphous feature of the membrane [128]. The porosity of f-GOincorporated membranes was enhanced by gaining more dense
pores through the rapid exchange between the solvent and nonsolvent during the phase-inversion procedure. The introduction of
f-GO increased the thermodynamic instability of the mixture in
the non-solvent gelation bath, which stimulated a quick demixing
stage, leading to outsized pore creation with a small quantity of
the additives on the surface of the membrane [100]. Generally,
pore size and porosity of membranes were improved by using
small amounts of f-GO, but were then decreased by the incorporation of more additives [100,101]. This may be a result of the
elevated viscosity level of the casting solution [129], which
normally retarded the exchange between non-solvent and solvent,
along with the inhibition of large-pore-size formation [130]
(Fig. 6B).This was confirmed by the improvement in water flux
upon the addition of a small amount of f-GO, when the flux
steadily decreased in line with the amount of added f-GO. The
improvement in the flux with a low f-GO amount could be
attributed to the improvement in hydrophilic property of the
membrane, as well as the creation of linear and more finger-like
macrovoids. When the f-GO amount surpassed 0.05%, the clotting
of f-GO throughout the phase-inversion method elevated the
viscosity of the casting solution and the porosity of the membrane
was greatly decreased [101].
Commonly, GO can attract water molecules into the membrane
matrix and facilitate them being diffused through the membranes,
improving the permeability. As the f-GO has greater attraction to
water than GO, the diffusion rate of water (non-solvent) into the
membranes was improved throughout the phase inversion. Moreover, the amplified thermodynamic instability of the casting
solution by the incorporation of f-GO resulted in an improved
diffusion rate of solvent from membranes to water [131]. These
phenomena resulted in the enlargement of the pore size of
modified membranes, which positively enhanced permeability
[132]. Nevertheless, with a high f-GO content, the static interference and electrostatic interactions between the f-GO
nanosheets or between the f-GO nanosheets and the membrane
polymer cause some f-GO nanosheets to be non-uniformly mixed
in the membrane matrix [100,101].
Further, it was observed that the increased number of f-GO layers
resulted in the delay of the rejection of some metal ions i.e., Na þ and
Mg2þ with a concurrent drop in water permeability. This was possibly
because the increased number of layers of f-GO escalated the effective
charge density and resulted in an increase of zeta potential on the
membrane surface. Consequently, the metal ions were retained
because of the Donnan and steric effects [84].
6. Summary and future prospects
GO has been used in different ways to improve membrane
properties for desalination application, including assembling freestanding GO membranes, GO-surface modified membranes and casted
GO-incorporated membranes. It has been demonstrated that various
membrane properties including mechanical strength, antimicrobial
and antifouling properties, selectivity, water flux and thermal properties are significantly improved after the incorporation of GO. Such GOmodified membranes (i.e., TFC RO, NF, UF, MF, photocatalytic, membrane bioreactors and pervaporation) performed significantly better
than the pristine membranes. The reported performances of GOassisted membranes in the removal of dyes, separation of monovalent
and divalent ions, and dehydration of solvent–water mixtures were
positive. In general, GO-assisted membranes do not suffer from any
practical restrictions and setbacks; it was found that the coupling of
GO and polymers is beneficial for the improvement of membrane
properties.
This interest has focused on developing efficient GO-assisted
desalination membranes through various fabrication strategies. More
work needs to be done to achieve a true understanding of the role and
mechanism of GO and membrane interaction, in order to enhance
membrane-separation performance, in particular for freestanding GO
membranes. The GO nanosheet is a promising building block for
fabricating GO-assisted desalination membranes, but more attention
needs to be paid to its potential drawbacks, such as mechanical
instability, unideal alignment and assembly, and surface defects.
Moreover, scaling up the commercial fabrication of an ultrathin
high-permeability GO membrane is one of the greatest challenges
104
H.M. Hegab, L. Zou / Journal of Membrane Science 484 (2015) 95–106
faced by researchers; a successful research outcome in this area will
lead to a reduction of the energy required for RO plants. The key to
success is to strike a balance between production costs and simplicity
in manufacturing operations. Overall, GO-assisted membranes could
provide one of the most promising tools to help solve the expected
global water crisis.
Acknowledgement
The authors would like to acknowledge the financial IPRS
scholarship (170747) in University of South Australia, funded by
the Australian government.
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