Functional graphene nanosheets: The next generation membranes for water desalination

Desalination xxx (2014) xxx–xxx

DES-12309; No of Pages 18

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Desalination

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Functional graphene nanosheets: The next generation membranes forwater desalination

Khaled A. Mahmoud a,⁎, Bilal Mansoor b, Ali Mansour b, Marwan Khraisheh a,⁎a Qatar Environment and Energy Research Institute (QEERI), Qatar Foundation, P.O. Box 5825, Doha, Qatarb Mechanical Engineering Program, Texas A&M University at Qatar, Doha, Qatar

H I G H L I G H T S

• Graphenes are exceptional materials for the next generation water separation membranes.• Graphenes prove ultrafast permeance, excellent mechanical strength and precise ionic sieving.• Modified NPG and GO membranes showed exceptional antifouling properties.• Need full understanding of the transport mechanism of NPG and GO membranes• Mechanical performance of fully wetted NPG and GO membranes must be addressed.

⁎ Corresponding authors.E-mail addresses: [email protected] (K.A. Mahmo

http://dx.doi.org/10.1016/j.desal.2014.10.0220011-9164/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: K.A. Mahmoud, etlination (2014), http://dx.doi.org/10.1016/j.d

a b s t r a c t
a r t i c l e i n f o
Article history:Received 8 October 2014Received in revised form 16 October 2014Accepted 17 October 2014Available online xxxx

Keywords:GrapheneGraphene oxideDesalinationMembranesFabricationAntifoulingThin filmsFiltrationMechanical propertiesWater flux

Membrane desalination and water purification technologies have become important energy-efficient means tosecure freshwater resources around the globe. Among the significant recent advancements in the design and de-velopment of newmembrane systems is the use of graphenes. Graphenes have offered a novel class of mechan-ically robust, ultrathin, high-flux, high selectivity, and fouling resistant separation membranes that provideopportunities to advance water desalination technologies. The facile synthesis of nanoporous graphene (NPG)and graphene oxide (GO) membranes opens the door for ideal next-generation membranes as cost effectiveand sustainable alternative to the long-existing thin-filmcomposite polyamidemembranes forwater purificationapplications. In this review, we highlight the structure and preparation of NPG and GOmembranes. We also dis-cuss the recent experiments, computer simulations and theoretical models, addressing the unique mechanicalproperties, ion selectivity, and possible transport mechanisms through NPG and GO membranes. We will focuson the fabrication and functionalization schemes of graphene oxide membranes. Particular emphasis is on theantifouling properties of the NPG and GO modified membranes. We believe this review will open new avenuesfor new innovations and applications of NPG and GO in water desalination and treatment.

© 2014 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02. Graphene structure and synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03. Mechanical properties of nanoporous graphenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

3.1. Effect of temperature and strain on nanopore formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03.2. Effect of nanopore size, shape and density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

4. Transport mechanisms through graphene membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 04.1. Nanoporous graphenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 04.2. Graphene oxide nanosheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

5. Multifunctional graphene oxide membranes for water separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05.1. Graphene oxide membrane fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

ud), [email protected] (M. Khraisheh).

al., Functional graphene nanosheets: The next generation membranes for water desalination, Desa-esal.2014.10.022

http://dx.doi.org/10.1016/j.desal.2014.10.022

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2 K.A. Mahmoud et al. / Desalination xxx (2014) xxx–xxx

5.2. Functionalization of graphene oxide membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05.3. Antifouling properties of the GO modified membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

6. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

1. Introduction

The increasing scarcity of freshwater sources across the globe hasurged the need to develop alternative water supplies, including seawa-ter desalination, reuse and recycling of wastewater and stormwater [1,2]. Membrane-based desalination techniques, mainly reverse osmosis(RO), are currently considered as more environmentally friendly andenergy-efficient than that of thermal desalination methods such asmultistage flash and multiple-effect distillation. However, these tech-nologies suffer from low desalination capacity and high capital costs.For example, RO consumes ~2 kWhm−3 for a 50% recovery with a the-oretical minimum energy of ~1 kWh m−3. Moreover, conventionalpolymeric membranes currently used in RO plants are prone to fouling,suffer from flux decline under high pressure, undergo rapid degrada-tion, and have low tolerance to high temperature, acids/alkaline, chlo-rine, and organic solvents [3]. This has urged the need for developingnovel membranes which can reduce the energy consumption of theRO process by showing high water permeability coupled with highsalt rejection capacity [4,5].

The ideal membrane should provide higher flux, higher selectivity,improved stability, and resistance to chlorine and fouling. Also it shouldbe as thin as possible and mechanically robust to maximize permeabil-ity, chemically inert and must retain a high salt rejection rate through-out its service life [6]. Recently, nanostructures such as zeolites, metalorganic frameworks, ceramics and carbon based materials haveattracted considerable attention as alternative membrane materials toreplace polymeric membranes due to good chemical resistance, highflux, and high rejection rates [4].

On the other hand, zeolite membranes have failed to realize eco-nomical fabrication on a large scale due to manufacturing cost, repro-ducibility and defect formation [7]. Also, ceramic membranes arecostly and very brittle under high pressure which limits their practicalapplications in membrane technologies. Although it is possible to fabri-cate high-flux and high selectivity membranes from carbon nanotubes(CNTs), it is currently difficult to synthesize highly aligned and highdensity CNTs with large lengths. CNTs remain an active area of researchformembrane technologies but costs and operational issues have great-ly hindered the development and integration of CNTs into large areamembranes [8].

Recently, graphene based materials have attracted great interest fortheir potential exploitation inwater desalination and purificationmem-branes. This can be attributed to their unique properties including dis-tinctive structural characteristics [9], high mechanical strength [10]and negligible thickness [11]. The advancement inmolecular simulationof graphene family opens the door for their potential contribution indeveloping novel membrane desalination technologies. Graphene'sunique electronic properties, high tensile strength and impermeabilityto small molecules is now a well determined fact [12–16] and thesehave been utilized to construct extremely thinmembranewith size tun-able pores (for molecular sieving) allowing for high flux. Graphenenanosheets display ideal chemical and physical properties in the desali-nation process. Despite its negligible thickness, membranes made ofgraphene exhibit adequate mechanical strength, capability of function-ing under higher pressures that is superior to conventional polymericRO membranes currently in circulation [11,17].

Several simulation studies have identified nanoporous graphene(NPG) structures among the most promising membrane materials thatcan provide high water flow rates and high salt rejection as a function

Please cite this article as: K.A. Mahmoud, et al., Functional graphene nanolination (2014), http://dx.doi.org/10.1016/j.desal.2014.10.022

of nanopore morphology [17]. On the other hand, these hypothesesare based on a single layer of graphene sheet which is difficult to assem-ble in the real world [18].

Thus, despite the great theoretical promise, there remains a majorchallenge to achieve cost effective and scalable manufacturing of largeNPG membranes displaying the required subnanometer pores and nar-row size distributionwhile preserving graphene's intrinsic structural in-tegrity. To this end, continuous efforts are in quest to identify affordableand scalable NPG frameworks while maintaining the desired molecularand ion sieving performance. The evaluation of NPG performance andfeasibility needs to be carried out through a detailed study of its saltrejection and water flux. In addition, its mechanical durability andchemical stability need to be investigated [8]. Alternatively, the wideavailability of reactive surface sites and layered structure of GO allowsit to be a better candidate for developing free standing and GO/polymerhybrid membranes for water separation applications. GO nanosheetsexhibit excellent antifouling capacity, a property greatly desired in thefield of water desalination processes [19]. GO-based thin membranesexhibit promising qualities in the fields of a readily accessible, waterpermeable membrane to be incorporated in the desalination process[17]. GO films have also shown to be effective in allowing the flow ofwater while subsequently blocking penetration of other vapors, liquids,or gases [20].

This review highlights the preparation, characteristics and applica-tions of functionalized NPG and graphene oxide (GO) membraneswith the focus on their potential engineering into promisingmembranematerials for water desalination technologies. In the text, we refer tomicropores and mesopores as “nanopores” for convenience because oftheir nanoscale pore widths. We will also cover the structural aspectsof nano-channels across the graphene-based membranes and ion/mol-ecule interaction with their sheets, the permeation and rejection mech-anisms, and themode of water transport in graphene nano-channels, aswell as the latest advancement in graphene membrane fabrication andenhanced separation strategies. The emphasis will be on NPG and GOmaterials that participate directly in the desalination process or indi-rectly by providing selective properties such as anti-fouling, whichcould lead to next-generation desalination systems with increased effi-ciency and capacity.

2. Graphene structure and synthesis

Graphene can be defined as one-atom-thick 2D sheets, consisting ofsp2 bonded carbon atoms arranged in a hexagonal, honeycomb lattice.Due to their large theoretical specific surface area (2630 m2 g−1), highthermal conductivity (~5000Wm−1 K−1), and excellent electrical con-ductivity, graphene has attracted a great deal of interest for over40 years [21]. Themost critical characteristic of graphene is its extreme-ly versatile and tunable carbon backbone, leading to facile function-alization, and incorporation in a variety of applications [21]. Theperfect one atom thick graphene sheets have been prominent for theirimpermeability to all standard gases [10,22,23]. Also, it can be fabricatedon large scale as there is recent evidence of 30 inchmultilayer graphenesheets being produced and transferred on roll-to-roll fabrication [24].Stable defects in a graphene sheets, such as adatomvacancies, and topo-logical defects were experimentally proven to be numerous and stableunder electron irradiation. The electrical properties of graphene sheetscan be altered by inducing defects through ion irradiation under ultra-high vacuum [25]. Defects induced in graphene trigger different charge

sheets: The next generation membranes for water desalination, Desa-


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transport patterns that are proportional to charge carrier density andmobility. Other experimental and simulation outcomes suggest alsothat sub-nanometer pores can be generated and controlled by differentmethods such as electron ion beam, oxidation, ion/cluster bombard-ment, or by doping [26–29]. This has paved the way to explore thetransport of molecules like gases and ions through the pores ingraphene membranes [30].

GO exhibits similar properties as graphene, except the sheets areconsidered asymmetrical due to oxygen-containing functional groupson the edges and basal planes as shown in Fig. 1 [20,31–33]. GO mayhave their basal plane modified with various groups, such as epoxide,hydroxyl, carbonyl, and carboxyl groups, presumably found at theedges of the sheet according to the Lerf–Klinowski model [34]. Theseoxygen functional groups on GO provide facile dispersion in aqueousmediums without the need for surfactants or stabilizing agents, whichfacilitates thin film assembly of GO from aqueous solutions. They alsoallow for a variety of surface-modifications, which can be used to devel-op a series of functionalized GO-based membranes with superior sepa-ration performance. In addition, nanopores can be introduced into GOsheets to allow for water permeation while rejecting other unwantedionic substances [19]. These functional nanopores are expected to en-able GO sheets with the capacity for selective sieving, improve waterflux, as well as enhance properties such as antifouling [19]. Another im-portant aspect is the stacking pattern of GO nanosheets. The amorphousdistribution of epoxy, hydroxyl and carboxyl groups tends to formnanoscalewrinkles and structural defects in the basal plane of GO sheets[33], which provide primary passages for water transport across thestacked GO nanosheets. Also, the hydrophobic nature of the stacks cancreate an almost frictionless surface, resulting in an unexpected fastflow of water across the membrane.

Graphene was first mechanically exfoliated from graphite by the sim-ple scotch tape method [35,36]. This was followed by other techniquessuch as chemical vapor deposition (CVD) from carbon-containing gaseson a catalytic metal surface, or by surface segregation of carbon dissolvedin the bulk of suchmetals. In a typical process, a mixture of CH4 and H2 isheated at above 1000 °C and subsequently deposited on a Ni surface, cre-ating a concentration gradient between thebulk and the surface and caus-ing the carbon atoms to diffuse into the Ni surface which after saturationforms graphite [36–38]. Ambient-pressure CVD has been used to synthe-size 1–12 layer graphene films on polycrystalline Ni films, while ethylenedecomposition on pre-annealed Pt (111) surfaces resulted in the forma-tion of a single layer of epitaxial graphite [39,40]. Copper surfaces on theother hand are ideal for forming one monolayer in CVD processes, asthe solubility of carbon in copper is minimal (0.001 atom% at 1000 °C)when comparedwith nickel (1.3 at.% at 1000 °C) [36]. Afinal etching pro-cess is required after the deposition process to remove metal catalystlayers [41]. Li et al. [42] demonstrated a CVD method that utilized acentimeter scale copper substrate, thus opening a new potential forlarge-scale production of high-quality graphene films, primarily due tothe flexible copper foils. The previously utilized Ni was far too rigid, andtherefore limited scalability of production [41]. The flexibility of Cu foilsubstrate provided essential properties required for a roll-to-roll transfermethod offering the potential to fabricate graphene sheets up to 30 in.long [43–45].

GO can be mass synthesized by the chemical exfoliation and chemi-cal oxidation of graphite [19,20,34]. The capacity to mass-produce GOfrom graphene significantly lowers its material cost, providing excellent

Fig. 1. Representative graphene oxide structure [33].


integration opportunities of this material in industrial processes [19].Brodie [46], Hummers [47], and Staudenmaier [48] are the three mainmethods being used to produce GO. All three methods involve the oxi-dation of graphite using strong acids and oxidants to exfoliate surfaceproperties. Brodie's approach consisted of mixing graphite with nitricacid and potassium, which proved time consuming and unsafe, andwas soon overcome by the Hummers method [34]. Hummers' methodconsisted of fewer steps, and mixed graphite with mixtures of sodiumnitrite, sulfuric acid, and potassium permanganate [47].

In all cases, the degree of oxidation of the basal planes of GO is direct-ly dependent on reaction conditions, such as temperature and pressure.Substantial variation in the structure and properties of the produced GOis likely caused by variations in the degree of oxidation due to the differ-ences in starting materials or oxidation protocol [49]. Electrochemicaltechniques can provide simple, economic, non-destructive, and envi-ronmentally friendly alternative for producing graphene and GO sheets,which can be easily scaled up for large-scale production [50]. Few recentstudies have reported surfactant-assisted electrochemical method forthe production of GO and graphene nanosheets on carbon paper orglassy carbon (GC) substrates from graphite powder and anionic surfac-tants and by using the electric current as oxidizing or reducing agent[51,52]. The process of sonication coupled with chemical exfoliationprocesses, can prove effective in fully exfoliating graphene layers, withminor defects that do not compromise mechanical integrity [53].

3. Mechanical properties of nanoporous graphenes

An ideal RO desalination membrane should be permeable and asthin as possible to maximize flux, mechanically robust to prevent frac-ture and fatigue, and havewell-defined pore sizes to increasemolecularselectivity at a low energy cost. Graphene nanosheets represent the bestpossible combination of the thinnest cross-sections achievable andexcellent mechanical properties [35]. In fact, pristine graphene is speci-fied by extremely high in-plane tensile stiffness (Youngmodulus on theorder of 1 TPa) — and the highest ever measured tensile strength (onthe order of 100 GPa) [15]. Despite of having these two ideal attributes,defect-free graphene nanosheets, even when they are one atom thick,are impermeable to the smallest of molecules. Therefore, to improvepermeability and tune selectivity, nanopores of various diameters,geometry, edge quality and density must be realized in graphene nano-sheets [27,30].

It is expected that NPG sheets with improved permeability may en-able fast water transport under low pressures and theymaywork undera wider range of operating conditions than previously possible [54]. Infact, several researchers have been evaluating graphene as a membranematerial since 2009 with the main focus on fabricating nanosheets anddeveloping variousmethods of creating controlled pores (0.4–10 nm), arange suitable for desalination but without compromising essentialmechanical properties [27,30]. A number of studies have shown thatgraphene membranes with artificial nanopores have great promise fornano-filtration and RO based seawater desalination [17,54], gas separa-tion [55,56] and selective ion passage [20,23,57–59]. However, severalkey technological issues still remain when creating nanopores for im-provedflux and selectivefiltering including (i) the ability to reliably cre-ate pores that have the appropriate attributes such as size, shape anddensity, (ii) the ability to create pores with minimum plastic deforma-tion around the pores in order to preserve mechanical properties, and(iii) the ability to create nanopores on large area nanosheets.

3.1. Effect of temperature and strain on nanopore formation

Despite the predictions of their huge potential as high-performancemembranes for RO desalination, it is expected that the presence of pro-cessing induced imperfectionsmay impact themechanical properties ofNPGs [60–66]. To this end, many factors may affect the creation andquality of the nanopores but the first step is obviously selecting the



Fig. 2. TEM images show a suspended graphene sheet with (a) a nanoporemade by electron beam ablation and (b) multiple nanopores made in close proximity to each other. Scale barsare 2 and 10 nm [73].


method of nanopore creation [67]. Over the past 5 years, several tech-niques have been investigated, that generally involve high-energyimpact with the intent of dislocating tens to thousands of adjacentcarbon atoms from the graphene lattice. Forming nanopores of varyingsizes in a monolayer graphene can be achieved by electron beamexposure, ultraviolet-induced oxidative etching, diblock copolymertemplating, and helium ion beam drilling [17,23,57,58,68–72]. Electronbombardment, although precise, is less efficient than ion/cluster bom-bardment in terms of pore size control. On the other hand, chemicaltechniques such as oxidative etching need careful control and havethe potential to leave unexpected residues on the membrane.

Fischbein and Drndić [73] have attempted to drill nanoporesthrough suspended graphene sheets by a focused electron beam usinga transmission electron microscope as shown in Fig. 2. Normally theminimum pore diameter drilled by a focused electron beam tends tobe between 2 and 5 nm [68,73,74]. In another study, Lu et al. [75]used a 200–300 keV electron beam at room temperature for drilling

Fig. 3. TEMmicrographs show shrinkage of nanopores in a graphene sheet at 400 °C. The sequenclearly be seen that the refilled area in both cases (c, f) has relatively poor crystallinity [76].


nanopores. The beam voltage was much higher than the 140 keVknock-on voltage for carbon atoms in graphene [76]. Therefore, thepore sizes were easily enlarged and substantial damage was inducedto the graphene around the nanopores as shown in Fig. 3. This undesir-able amorphization of the crystalline lattice aswell as carbon depositionon the surface,may adversely impact the over-allmechanical propertiesof theNPG. This studydemonstrated an approach for tailoring the size ofthe graphene nanopore by in-situ TEM studies by using a combinationof electron beam irradiation and controlled heat treatment [75].Nanopores (thosewith a diameter of up to 10 nm) could even be closedcompletely by choosing the appropriate heat treatment temperatureand exposure time. They also concluded that the nanopore shrinkingprocess can be stopped by blocking the electron beam and that thisapproach may be applied for successfully tailoring the size of thegraphene nanopore through a combination of electron beam irradiationand controlled heat exposure [75]. Although, Lu et al. were successful inreducing the size of nanopores, however the amorphization around

tial images of nanopores with an initial diameter of ∼2.3 nm (a–c) and∼9 nm (d–f). It can




nanopores is a more challenging problem. Therefore, an important re-quirement in producing NPGs is to develop nanopore drilling methodsthat cause negligible damage to the surrounding graphene.

In order to create an efficient nanoporous structure as a semiperme-able membrane, there is a critical need to create nanopore arrays withconsistent size, density and quality. Cluster bombardment is anothermethod for nanopore creation that has received some attention fromthe graphene research community. It tends to give the highest efficiencyin damaging graphene surface because of the non-linear cluster/targetcollision process [67]. Becton et al. in their molecular dynamic (MD)studies expand upon previous works on the creation of nanopores ingraphene, to improve the understanding of howparameters, such as en-ergy of impacting cluster, the temperature and strain of the targetgraphene, affect the creation of nanopores in graphenewhen using car-bon clusters. In their study, they choose C180 fullerene molecule as theincident cluster because of its hollow structure and with the aim to cre-ate nanopores of approximately 1 nm2. As shown in Fig. 4a [67], MDsimulations by Becton et al. provide suggestions that incident energyof fullerene is the best way for global control of nanopore size. The tem-perature and pre-strain of the target graphene allow finer control of sizeand determine edge quality of the nanopores. Temperature increase al-lows the fullerene cluster to stretch and deform the graphene sheetmuchmore before fully penetrating, and a nanoporewithmore danglingcarbon chains is consistently created (Fig. 4b). The configuration ofgraphene with a plethora of dangling carbon chain is considered to beenergetically favorable and it possesses unsaturated bonds to offer afeasible avenue for the achievement of adsorption of foreign atoms ormolecules in graphene. On the other hand, increased pre-strain in targetgraphene produces nanopores with cleaner edges, with few to no dan-gling carbons when compared with unstrained graphene (Fig. 4c).

This provides compelling predictions that the energetic cluster im-pact may be a feasible and promising way to fabricate nanometer-sizepores in graphene. Therefore, we believe that there is a strong needfor unambiguous experimental identification, computer modeling andtheoretical description of the fundamental effects involved duringnanopore drilling processes including both e-beam sculpting and cluster

Fig. 4. (a) MD simulation of nanopore formation with the impact of fullerene cluster. The plastshows the impact of (b) temperature and (c) biaxial strain on nanopore size with the impact oAdopted from Ref. [67].


bombardment in combination with the chemical techniques with theaim to improve NPG membranes.

3.2. Effect of nanopore size, shape and density

A number of research articles have addressed the influence of chiral-ity and defects on the mechanical properties of graphene sheet [15,60]but on the other hand, however, there is no systematic experimentalstudy to date aimed at revealing the effects of size, shape, and densityof subnanometer pore on the mechanical properties of NPG mem-branes. There is some experimental literature available on mechanicalresponse of GOmembrane [77–79], but till datewhatever littlework ex-ists on NPG mechanical response has been carried out using moleculardynamic simulations [80]. Therefore, we have highlighted the possiblemechanical property design considerations that are vital for the fabrica-tion of NPGs with ordered nanopore structures to enable its use as amembrane in desalination processes. It iswell known that in RO seawaterdesalination processes, the working pressures can reach up-to approxi-mately 10 MPa [80,81]. Recent MD studies on NPG that considered pres-sures typically observed in real RO systems (1–10MPa) have shown thatwhile higher pressures may lead to appreciable deformation at thenanopore edge, the flow of water across the nanopores remains largelyunaffected. However, the MD literature available so far on NPGmechani-cal properties researchers has not investigated macro-level strain fieldsunder RO pressure conditions and how they may impact transport prop-erties. In a recent molecular dynamic study conducted by Liu and Chen[80], the effects of size, porosity, and shape of nanopores on mechanicalproperties of NPG sheets were observed. They considered configurationsfor five types of nanopore as shown in Fig. 5. The smoother edges onnanopores correlated to higher strengths than those with sharp edges. Itcan be seen in Fig. 5(a) that the nanopore has a sharp tip normal to thearmchair direction, which induces stress concentrations on the structure,while in the zigzag direction smaller stress concentrations are noticed dueto the smooth edges. Although, this is a typical result with stress concen-tration seen at sharp edges, nanopore size increase may lead to a reduc-tion in strength. It is well known that in pristine condition, zigzag

ic deformation and disorder in the lattice around the nanopore are visible. MD simulationf fullerene. The initial energy of fullerene cluster is 900 eV.



Fig. 5. (a) The configurations for five types of nanopores, with the average diameter ranging from 4 to 13 Å. x is the armchair direction and y is the zigzag direction. (b) Important me-chanical properties of NPG as predicted by MD studies: (1) normalized modulus, (2) normalized strength, (3 and 4) Fracture strain w.r.t to porosity and chirality [80].


graphene shows a higher elastic modulus and tensile strength comparedto the armchair graphene [15]. However as shown in Fig. 5(b) adaptedfromLiu et al., it was shown that elasticmodulus has a strongdependenceon effective porosity and both armchair and zigzag direction largely show


similar trends. Also, that the classical Griffiths fracture criterion breaksdown and the tensile strength and fracture tensile strain in the zigzag di-rection were much higher than those in armchair tension, this is consis-tent with results reported for pristine graphene [15,80].




It was concluded that the strengths of nanopores are largely depen-dent on their shape and the impact of chirality is rather weak. Sincelarge pressures are often required to push water molecules throughnanopores, mechanical instability arising from pore shape and densitymay limit the use of NPG sheets for separation and filtration applica-tions. TheMD results reported by Liu et al. need extensive experimentalvalidation and could lead to form a basis for understanding the na-ture of dislocation based plasticity in graphene membranes and mo-tivate design of high performance NPG filtrationmembranes. Furtherinvestigations aimed at understanding the effect of hydraulic pres-sure on NPG and GO based membrane's load bearing capacity, fractureand fatigue characteristics and their implications on the permeabilityand salt rejectionwould help determine their desalination performance[54,80].

We believe that developing detailed experimental understanding ofdeformation and fracture micro-mechanisms under typical RO condi-tions is of crucial significance to promote graphene as a candidatemem-branematerial. The key aspects that must be studied for their impact onmechanical behavior include: (i) impact of size, shape and density ofnanopores; (ii) influence of chirality and processing induced defects;(iii) stability under large operating pressures with and without expo-sure to elevated temperatures; (iv) possibility to plastically form curvedgraphene specimens for complex geometry desalination systems;(v) impact of functionalization on over-all mechanical properties;and (vi) understand and control durability and service life of mem-branes. In order to realize these challenges, novel micro- and nano-mechanical characterization methods including electron and atomicforce analytical microscopy tools must be applied to accuratelymeasureimportant mechanical properties of graphene membranes under ROconditions [82].

4. Transport mechanisms through graphene membranes

4.1. Nanoporous graphenes

Water transport through NPGmembrane has received relatively lessattention in spite of its potential application inwater purification. In thissection we present a brief overview of recent experiments, computersimulations and theoretical models developed to investigate watertransport through different types of porous graphene membranes.This will help identify the potential applications of nanoporous andfunctionalized graphene membranes and the advantage they mayoffer over polymer based commercial filtration membranes.

It is well-known that defect-free graphene sheets, even when theyare one atom thick, are impermeable primarily due to the repulsivefield created by the dense and delocalized π-orbital cloud [83]. The π-orbital cloud fills the gap within its aromatic rings and can block even

Fig. 6. (a) Functionalized graphene nanopores, (left) the F–N terminated nanopore, (right) the HNa+ and Cl-ions through the F–N-pore and H-pore, respectively, on the applied electric field [5


the smallest ofmolecules, such as hydrogen and helium, to pass througheven under higher pressure conditions [10,22,23]. The ability to with-stand such pressure differences (6 atm approximately) in graphene isa result of its superior mechanical properties as discussed above.Imparting controlled pores to graphene structure while retaining thestructural integrity can render graphene an ultimate thin membranefor gas and liquid purification. Pores howevermust remain large enoughto allow for the passage of water molecules, while not allowing ions orunwanted substances to pass through themembrane. A number of the-oretical studies have suggested that artificial pores in graphene can in-crease its permeability and permeation selectivity [83]. Therefore,introducing nanopores into graphene's structure creates potential ap-plications in membrane separation processes, as it allows for thefunctionalization and monitored alteration of surface properties fortargeted filtration. Water flux through NPG is greatly dependent onpore size and the chemistry of the pores, as well as how the pores arefunctionalized [17]. Konatham et al. [18] found that in NPG, pore diam-eter had the largest effect on free-energy profiles of the graphenesheets. It was also realized that the permeation of molecules throughNPG membranes can be related to molecular adsorption on thegraphene sheet, as well as chemical functionalization of the graphenesheet and pores. This could make them serve as ionic sieves of high se-lectivity and transparency. For example, the NH3 functionalized porefailed to reject Cl− ions at a moderate ionic strength due to ion accumu-lation at the pore entrance. On the other hand, OH− functionalizedpores gave promising results for the removal of Cl− ions at both lowand moderate ion concentrations indicating their strong potential forwater desalination [18]. As shown in Fig. 6, pores terminated by nega-tively charged fluorine or nitrogen highly favor the passage of cations[58]. Nanopores terminated by positively charged hydrogens, respec-tively, favor the passing of anions [58,84,85].

Molecular dynamic simulations helped in studying the transport ofions through 0.5 nm pores in graphene [58]. The graphene used in thisstudy was terminated by either hydrogen or nitrogen. It was observedthat pore in a graphene layer was terminated by nitrogen allowed lith-ium, sodium, and potassium ions, whereas those terminated by hydro-gen allowed chloride and bromide ions but did not let fluoride ion topass. Unexpectedly, smaller ions showed lower passage rates whencompared to larger ions. This might be due to strongly bound hydrationshells of smaller ions. Also, like protein ion channels, charged terminalgroups of pores helped water molecules to move out of hydrationlayer [58]. In another research, a comparison is made between watermolecule transport through CNTs having 0.75–2.75 nm diameter and2–10 nm length against a graphene membrane of similar diameterpores [30]. The observed flux through graphene was almost double tothat of CNTs. The entrance regions of pores were the major contributortoward resistance. These studies show that graphene membranes could

-terminated nanopore; (b) dependence of the atomic radii and flow rates of the hydrated8].




outperformpolymeric ROmembranes forwater desalination in terms ofallowable flux [68].

Flow rate of water through nanopores in graphene increases, as ex-pected, with increasing pore diameter and applied pressure. Porousgraphene exhibits opposing characteristics to conventional diffusiveROmembranes, as salt rejection decreases with higher applied pressurein a range of ~90–225 MPa [17]. This result is rather peculiar and it isopposite to what is typically observed in diffusive RO membranes.They attributed this observation to the large effective volume of saltions in solution, which may respond more sensitively to pressure in-creases than water molecules. They concluded that this is in contrastwith the kinetics of ion passage across diffusive RO membranes, inwhich osmotic pressure is the driving force for the passage of salt ionsand it leads to faster increase in water flux than salt flux with increasein pressure. Introducing charged species to graphene nanopores byoxidation and other chemical groups can prove beneficial in desalina-tion [86]. Evidently, graphene nanopores subjected to negativelycharged functional groups exhibit excellent rejection of salt ions [87].When dealing with seawater, charged nanopores in graphene provedadvantageous in the desalination process using electro-dialysis, as thenanopores had a much lower energy requirement compared to currentRO ormulti-stage flash distillation [88]. Graphenewith induced charged

Fig. 7. (a) High pressure molecular and ionic sieving across a one atom thick graphene sheet. Cfunctionalization with hydroxyl groups improved the flux [106]. (b) Performance of functiona


nanopores has the capacity to address problems in membrane perfor-mance including high permi-selectivity, low electrical resistance, aswell as strong chemical and mechanical stability [88,89]. Cohen-Tanugi and Grossman [17] investigated pores passivated with hydroxylgroups and hydrogen atoms to study the effect of pore chemistry ondesalination dynamics. The selection of pore size and chemical function-al groups was based on previous studies conducted by Suk and Aluru[30] and Sint et al. [58]. Hydrogen terminated nanopores showed betterwater selectivity while functionalization with hydroxyl groups en-hanced the rate of water transport. Furthermore, hydroxyl groupswere found to replace water molecules, thus allowing some salt ionsto pass, decreasing salt rejection [17]. Simulation also showed that thetransport of water through these nanoporous membranes could reachup to 66 L/cm2/day/MPawith greater than 99% salt rejection. In contrast,water transport through a conventional ROmembrane approximate-ly reaches 0.01–0.05 L/cm2/day/MPa with similar salt rejection. Thenanopores investigated showed salt rejection and water permeabili-ty at 2–3 orders of magnitude greater than commercialized mem-branes. These values revealed the great potential for the utilizationof functionalized NPG as a high-permeability desalination membrane(Fig. 7). The ability of GO blended polysulfone GO/PSF membranes toachieve high salt rejection has been shown by Ganesh and co-workers

hemical functionalization of the pores with hydrogen increases water selectivity, whereaslized nanoporous graphene versus existing desalination technologies [12].




[90]. That the resultant compositemembrane dopedwith 2000 ppmGOgave an optimum salt rejection of 72% for a 1000 ppm sodium sulfatesolution at an applied pressure of 4 bar. This observationwas attributedto the surface charge of various functional groups (mainly carboxylicand phenolic groups) attached to theGO surface, producing a negativelycharged membrane surface at a higher pH [90].

4.2. Graphene oxide nanosheets

Despite the huge potential suggested byMD simulation for usingNPGmembranes in water desalination [17,30] as well as selective ion separa-tion ion selectivity [58], scale production of large-area graphene films re-mains a major hurdle [42]. In addition, introducing nanopores into thegraphene basal plane is technology intensive as described above. GOmembranes, on the other hand, overcome these shortcomings and ren-der GO amore suitable candidate as an ion sievingmembrane. AlthoughGO nanopores pose high hydrophobicity, the GO nanosheets themselvesare extremely hydrophilic [19,91]. Ideally, these GO nanosheets shouldhave approximately 3 Å of interspacing between the sheets [92]. Howev-er, when immersed in ionic solutions, GO nanosheets experience hydra-tion that subsequently increases the spacing of the nanosheets toapproximately 9 Å. In this case, any ionic species with a hydrated radiuslarger than 4.5 Å is blocked, while ionic species with hydrated radius lessthan 4.5Å passes through the hydrophobic channels [92]. As indicated byBoehmet al. [93], the interlayer distance betweennanosheets can further

Fig. 8. Proposed schematic illustration of GOmembrane and the interaction with different ionsCu2+) are significantly lower than those of the alkali salts,which are due to the tight coordinatio


increase if immersed in a polar liquid such as sodium hydroxide. Apossible way of counteracting the spacing expansion between GOnanosheets lies in the partial reduction of film thickness. This wouldthereby enable the nanosheets to be in closer proximity with one an-other, allowing for the filtration of ions with hydration radius as smallas K+ (0.53 nm) and Na+ (0.79 nm). A second approach in reducingthe spacing between nanosheets lies in covalent bonding GO nano-sheets via small molecules, allowing these covalent bonds to overcomethe unavoidable hydration forces [92]. According to Joshi et al., filtrationis exclusively related to hydration radii of particles and not dependenton the charge of ions passing through the system [91]. For example, de-spite triply charged nature of AsO4

3− the ion permeates across themem-brane at the same rate of single charged Na+ or Cl−. On the other hand,Hu and Mi [19] indicates that permeation through GO nanosheets isgreatly dependent on the particle charge Debye lengths, or the electro-static repulsion between ions and the membrane charge, therefore,charges of the ions contribute to the separationmechanismof GOmem-branes [20]. This difference in salt rejection trends with respect to ioncharge by these two studies requires further investigation [91]. Sunet al. [20] has demonstrated the selective ion penetration andwater pu-rification properties of freestanding GO membranes. Sodium saltsquickly permeate through GO membranes, whereas heavy-metal saltsinfiltrate much slower. CuSO4 and organic molecules, such as rhoda-mine B, were blocked entirely because of their strong interactionswith the GO membranes (Fig. 8). The nano-capillaries that formed

. The penetration abilities of aqueous solutions of heavy-metal salts (e.g., Mn2+ Cd2+, andn between theheavy-metal ions and the functional groups decorated on theGOsheets [21].




within the membranes were responsible for the permeation of metalions, whereas the coordination between heavy-metal ions with theGO membranes restricted the passage of the ions. In addition, alkaliand alkaline earth cations that interact with π sites of GO membranesalso derive the selective permeation of cations. Finally, the penetrationprocesses of hybrid aqueous solutions were investigated; the results re-vealed that sodium salts can be separated effectively from copper saltsand organic contaminants. The presented results demonstrate the po-tential applications of GO in areas such as barrier separation andwater purification [20]. The key aspects of water transport mechanismsand salt rejection processes in NPG and GO membranes as reported incomputer modeling literature require unambiguous experimental veri-fications and theoretical descriptions. The authors believe that develop-ing better understanding of transport and rejection kinetics underactual RO conditions can lead to systematic design and optimizationof graphene based desalination membranes that can offer significantimprovement and energy saving.

5. Multifunctional graphene oxide membranes for water separation

As described above, substantial efforts have been devoted to devel-op highly efficient GO membranes. Several approaches have been usedto incorporate GO into polymer casting solutions, either during mem-brane fabrication or post-coating of the prefabricated membrane, inorder to improve antimicrobial properties, increase permeability, andenhancemechanical strength [94–98]. This section highlights the fabri-cation and physicochemical properties of standalone and supported GOmembranes and discusses the latest progress in GO membranes interms of enhanced separation performance and unique antifoulingproperties.

5.1. Graphene oxide membrane fabrication

GO membranes can be fabricated from GO nanosheets by differentmethods including vacuum filtration, layer-by-layer (LbL) deposition,drop casting, and spin coating [92]. Membranes fabricated by vacuumfiltration often have weak bonding between nanosheets depending onthe spacers including nanoparticles or polyelectrolytes [19]. LbL anddrop-casting methods are cost effective and scalable and have the

Fig. 9. (A) Photograph of a GO membrane covering a 1-cm opening in a copper foil. (B) Schemmembrane into two compartments referred to as feed and permeate. (C) Permeation through(inset) Permeation rates as a function of C in the feed solution. Chloride rates were found the s


capacity to stabilize covalent bonding between GO interlayers as wellas electrostatic interactions [92]. Asymmetrically located oxygen func-tional groups in GO membranes maintain a distance during the drop-casting process, creating void spaces between nonoxidized regionswhich form a series of nanocapillaries within the film, increasing thewater fluxwhen potentially utilized in desalination processes [31]. A re-cent study by Joshi et al. [91] investigated the permeation throughmicrometer-thick laminates prepared by vacuum filtration of GO sus-pensions. The laminates were vacuum-tight in the dry state but whenimmersed in water they act as molecular sieves, blocking all soluteswith hydrated radii larger than 4.5 Å. Smaller ions permeate throughthemembranes at rates thousands of times faster thanwhat is expectedfor simple diffusion. This behavior is caused by a network ofnanocapillaries that open up in the hydrated state and accept only spe-cies with the right size (Fig. 9).

Free-standingmembranes were also fabricated by vacuum filtrationof GO laminates on Anodisc filters (0.2 μm pore size) [99]. Althoughkeeping the support filter did not add to the permeation barrier, itwas important for providing good mechanical support for the mem-brane. The sub-micrometer-thick membranes made from GO werefound to be completely impermeable to liquids, vapors, and gases, in-cluding helium, and allowed for unimpeded permeation of water atleast 1010 times faster than He. This behavior is attributed to a low-friction flow of the water monolayer through two-dimensional capil-laries formed by closely spaced graphene sheets (Fig. 10).

It should be noted that the mechanical strength of pristine GOmembrane made via solution filtration demonstrates outstandingmechanical durability in dry conditions [23,99], however the mem-branes are very week in a fully wetted state due to the extreme hy-drophilic nature of GO nanosheets [49]. The potential interchelationbetween the laminar GO sheets and molecules or ions can enforcethe interspace expansion between the adjacent GO sheets, causingdamage to the membrane integrity. Moreover, such free standingmembranes made by simple filtration methods are not likely tosurvive the cross-flow testing conditions, typical real-world membraneoperations. One solution to encounter this issue is forming stablebonding between GO nanosheets to stabilize the membrane integri-ty. These crosslinking spacers could also act as barriers to form a newchannel which would greatly enhance the water permeability. To

atic of the experimental setup. A U-shaped tube 2.5 cm in diameter is divided by the GOa 5-mm-thick GO membrane from the feed compartment with a 0.2 M solution of MgCl2.ame for MgCl2, KCl, and CuCl2 [91].



Fig. 10.He-leak–tight GOmembranes. (A) Photo of a 1-mm-thick GO film peeled off of a Cu foil. (B) TEM of the filmcross section. (C) Schematic view for possible permeation through thelaminates. (L/d is ~1000). (D) Examples of He-leak measurements for a freestanding micrometer-thick GO membrane and a reference (polyethylene terephthalate) PET film [99].


this end, few studies have focused on fabricating cross-linked GOmembrane with an effort to improve the mechanical integrity of GOmembranes.

A promising approach in applying graphenes in the desalinationmembranes is by incorporating GO nanosheets in the polymer matrixrather than surface coating. These hybrid materials present significantperformance enhancements not usually attained by typical nanocompos-ites or pristine polymers [85, 100–102]. Hu andMi [19] prepared a waterfiltration membrane with layered GO nanosheets on polydopamine-

Fig. 11. Schematic illustration of (a) a step-by-step procedure to synthesize the crosslinkedGOm(c) the proposed mechanism of crosslinking between GO and TMC [19].


coated polysulfone support with 1,3,5-benzenetricarbonyl trichloride ascrosslinkers. Interestingly, they did not observe any decline of the waterfluxwith increasing GO layers which presume no direct relation betweenwater flux and number of GO layers (Fig. 11) [19]. The ionic flux ofsulfonated polyethersulfone (SPES) membrane can be improved by 19%by the incorporation of 10% GO as compared to unmodified SPES mem-brane. The strong interfacial interactions due to GO nanofillers into theSPESmatrix improve the thermal andmechanical properties of the nano-composite membranes [103].

embrane, (b) the proposedmechanismof reactions betweenpolydopamine and TMC, and




Inspired by cell wall assembly of higher-order plants, ultra-stiff GOthin films can be formed by crosslinking GO with borate [104]. Thestudy has reported a remarkable enhancement by over 255% in thestiffness of the graphene film by adding 0.94 wt.% boron to the finalcomposite GO film. The strong crosslinking between the adjacent nano-sheets has been attributed to the formation of covalent bonds betweenborate ions and the hydroxyl groups on the GO nanosheets (Fig. 12)[104].

5.2. Functionalization of graphene oxide membranes

Surface functionalization of graphene is not readily achieved at thebasal panels, and therefore tends to occur at the edges of graphenesheets [105]. Molecular channels consisting of zeolites, carbon, silica,and other materials often assist in the filtration process of rejecting un-wanted ions in an aqueous solution. Graphene pores can be functional-ized by either hydroxyl groups or hydrogen atoms, hydrophilic andhydrophobic by nature respectively, both producing different effectson the desalination process [58]. However, it was noted that water per-meability experienced a favorable increase when pores were hydroxyl-ated rather than hydrogenated. This interesting behavior is caused by

Fig. 12. (Left): Schematic illustration of the formation of the borate‐crosslinked network acrosnanosheets in an unmodified graphene oxide through hydrogen bonding with epoxide and hyannealing to improve the mechanical properties by formation of more covalent bonds withultra‐stiff films are only obtained after annealing borate‐modified films under dry N2 [104].


the tendency for hydrophilic functional groups to increase the waterflux, directly related to the increase in hydrogen-bonding. Although hy-droxylated pores tended to increase water flux, hydrogenated pores ap-peared to bemore effective in rejecting unwanted salts from passing bythe filtration system.

The higher water flux through the hydroxylated pores as comparedwith hydrogenated pores can be attributed to larger cross-sectional areaavailable for the water molecules to pass. Analysis to identify which ofthe above forces was conducive to the increased rate of water flux bycalculating density maps of oxygen atoms of water molecules withinthe hydroxylated and hydrogenated pores. The hydrogenated poresonly experienced an approximate 25% reduction in surface area whencompared to the hydroxylated pores. This data cannot explain the 69–113% drop in water permeability found in the hydrogenated pores. Hy-droxylated pores, in a study conducted by Konatham et al. [18], alsoportrayed favorable trends in filtrating unwanted Cl− ions in the desali-nation process as well. The study attributes this massive difference inwater permeability to entropic effects, primarily the higher level order-ing of water molecules due to the fact that hydrogen passivation is con-sidered to be hydrophobic, thereby restricting the volume of watermolecules through the membrane. Hydroxylated groups, in contrast,

s two adjacent graphene oxide nanosheets in a thin film. (a) Water molecules bridge thedroxyl groups. (b) Borate anions bond with surface‐bound hydroxyl groups. (c) Thermalin the intersheets. Right: Mechanical properties of respective films, demonstrating that




provide a surface with less friction, leading to an overall faster flow ofwater molecules [106].

Water flux and antifouling properties of UF membranes modifiedwith pristine graphene can be restricted by the graphene's strong affin-ity to aggregation [107,108]. This would lead eventually to a significantmechanical deterioration of the membrane due to the incompatibilitybetween the pristine graphene and polymers [109,110]. It is wellknown that GO has several functional groups (epoxy, hydroxyl, and car-boxyl) which present reactive sites for covalent functionalization [109].Chemical functionalizationwould greatly alter Van derWaals forces be-tween GO nanofiller, increasing their dispersal in the polymer matrix.Furthermore, due to their extensive polymer chains, functionalizedGO can entangle with polymer matrix, facilitating a stronger interfacialinteraction between GO and the matrix [109,111]. In an effort to im-prove the performance of graphene-based nanocomposite UF mem-branes, Crock et al. [112] integrated metallic nanoparticle (NP)-basedderivatives as nanofillers into the membrane matrix. In their study,graphene nanoplatelets decorated with AuNPs were used as hierarchi-cal nanofillers for the preparation of innovative PSF nanocompostitemembranes as described in Fig. 13. The Au loading was found to specif-ically controlmembrane reactivitywhile themembrane structure solelyrelied on the graphene loading. The flux increased linearly with an in-creased catalyst loading of the nanocomposite membranes which wasfound to be controlled by the loading of catalytic Au NPs andwas ratherindependent of xGnP loading [112]. This design concept proposed byCrock et al. can be further developed to include different supports, alter-native NPs, or nanofillers with more hierarchy levels. Zhao et al. [113]showed that the addition of graphitic carbon materials including GOto PVDF membranes greatly enhanced the membrane microstructurewhichwas attributed to the surface characteristics of GO, such as surfacecharge, functional groups, and active site availability. This significantimprovement was attributed to the high surface roughness as well asthe enhanced membrane porosity and hydrophilicity.

5.3. Antifouling properties of the GO modified membranes

Biofouling is major challenge in the membrane separation industry.Bacteria and other microorganisms adhere to the membrane surface

Fig. 13. Schematic diagram of hierarchical nanofillers as eleme


and form a viscous gel-like biofilm causing a severe decline in the flux.Communal membranes used for pressure driven desalination processesare mostly hydrophobic polymers with high mechanical, thermal, andchemical stability. Usually these materials are prone to fouling. Thepresence of a hydrophilic layer on the membrane surface tends toform hydrogen bonds with water molecules and reconstruct a thinwater boundary between the membrane and bulk solutions. This layeris able to prevent or reduce undesirable adhesion or adsorption of hy-drophobic foulants on themembrane surface [114]. Also, themembranecharge is an important factor for the reduction of membrane foulingwhen foulants are charged. This would create electrostatic repulsionforces between the foulants and membrane surfaces of similar chargepreventing the foulants' deposition on themembrane, thereby reducingthe fouling [115].

One of the main approaches to reduce the organic fouling and bio-fouling of polymeric desalination and water treatment membranes isthrough surface modification with antimicrobial coating. The formationof hybrid organic/inorganic membranes with biocidal properties wouldlimit the frequent flux decline. Several studies have compared the anti-bacterial activity of graphene-based materials (graphite (Gt), graphiteoxide (GtO), GO, and reduced GO (rGO)) against Gram-negative andGram-positivemicrobes through direct contact and comparedwith syn-thetic carbon nanomaterials, such as fullerenes and CNTs [116–122].This bactericidal effect is stable over time, allowing for a new type ofnonleaching, nondepleting antimicrobial surface.

In some cases, the antibacterial activity of GO and rGO has been at-tributed tomembrane stress induced by sharp edges of graphene nano-sheets, which may result in physical damages on cell membranes,leading a loss of bacterial membrane integrity [118,122]. In this case,the destruction effect due to graphene nanosheet edges is expected todecline significantly when graphene nanosheets fuse in a large-area ofthe matrix. Hu et al. [117] have reported on the antibacterial activityof two water dispersible graphene films (GO, rGO) fabricated fromtheir suspension via simple vacuum filtration. The free standing andflexible films efficiently inhibited the bacterial growth and Escherichiacoli (E. coli) cells lost their membrane integrity on the films (Fig. 14).This indicates the superior antibacterial efficiency of such graphene-based films against E. coli bacteria while showing minimal cytotoxicity

nts for multifunctional nanocomposite membranes [112].



Fig. 14. Antibacterial activity of GO and rGO films. Photographs of E. coli growth on GO (a) and rGO (b) paper (overnight incubation at 37 °C). SEM images of E. coli attached to GO (c) andrGO (d) paper (12 h incubation at 37 °C) [117].


[117]. Other studies reported that GO can act as a terminal electron ac-ceptor frommicrobes [119]. A recent work by Salas et al. has confirmedthat GO and rGO nanosheets can extract phospholipids from E. colimembranes and destroy the membrane integrality, thus inhibitingE. coli, the model bacteria used in this study [120]. High hydrophilicityinduced by the functional groups at the basal plane and terminals ofGO sheets should also have contribution to high water permeationand antifouling owing to the low interfacial energy between a surfaceand water [123].

It was found that post-coating of the membrane will enable the GOnanosheets to be in direct contact with the bacterial cells and therebysignificantly enhance the antimicrobial effect of the GO nanosheets[120–122]. Modified ultrafiltration membranes prepared by blendinghyperbranched polyethylenimine (HPEI) GO with polyethersulfone(PES) via phase inversionmethod showed effective antibacterial perfor-mance against E. coli. However, the hybrid membranes exhibited alower water flux as compared with the unmodified PES membranes

Fig. 15. Surface AFM images of (a) pure PV


[96]. The higher recovery ratio indicated that the adsorbed foulingfilm is comparatively loose and can be easily detached. This was consis-tentwith a previous studywhere theflux recoveries ofmembraneswithhydrophilic properties were found to be relatively more effective uponbackwashing [124]. A 0.2 wt.% GO modified PVDF ultrafiltration mem-brane can improve the permeability by 96.4% which implies an im-provement in the anti-fouling ability of the GO modified membranecompared to the pure PVDF [95].

Choi et al. [125] prepared GO coated polyamide membranes for ROdesalination application. The membrane showed an inherent improve-ment in the membrane's resistance to foulants and chlorine ions.Integrating GO sheets with polymers was found to increase the hydro-philicity of the sheets, a much desired property, while reducing theroughness of surface properties on the sheets. This, in turn, leads to im-proved antifouling properties The strong chemical nature of GO allowedit to act as a protective layer for themembrane during chlorination, pre-serving the high degree of salt rejection [125].

DF (b) GO/PVDF UF membranes [126].




Membrane surface characteristics were found to play a role in theantifouling property [126,127]. As shown in Fig. 15 [126], pure PVDFmembranes exhibited high roughness while GO/PVDF membranesdisplayed a more smooth surface. Higher membrane surface roughnessleads to contaminants accumulating in the dents, thereby increasingthe fouling potential [128,129]. Optimized GO/PVDF membraneswere prepared by the Taguchi method and similarly applied in mem-brane bioreactor system [130,131]. When compared to the purePVDF membranes, the composite required less frequent cleaning andlonger filtration period. GO in the composite membrane was found toincrease the hydrophilicity due to the vast presence of hydroxyl groups.Consequently, less accumulation of extracellular polymeric substances,specifically polysaccharide was observed on the membrane surface.This decreases the development of biofilms on the membrane surface[132,133].

In addition, the improved surface hydrophilicity enhanced the com-posite membrane's resistance to reversible and irreversible fouling[131]. Based on the previous findings [95,127], Lee et al. prepared GO/PSFmembranes for the treatment of wastewater [98]. It was anticipatedthat the presence of GO in membrane material would enhance the rateof water transport and, thus, hinder biofouling. Furthermore, its func-tional groups would provide a large negative zeta potential, whichmay further inhibit the attachment of biofoulants on the membrane

Fig. 16. (A) Schematic diagram of states of additives in (a) P/GO and (b) P/f-GO membranes anrecovery % of nascent membranes and (b) mixed PVDF membranes containing 1 wt.% graph[139].


surface [134,135]. The biofilm thickness decreased with increasingavailability of GO on the PSFmembranes. It is worth noting that thema-jority of microbial substances in aquatic environments possess nega-tively charged surfaces [136,137]. The electrostatic repulsion betweenthe microorganisms and membrane surface caused by large negativezeta potential of themembranewas able to inhibit surface accumulationof the microorganisms.

Zinadini et al. [138] also prepared a novel polyethersulfonenanofiltration membrane blended with GO aiming for better perfor-mance and antifouling properties. Higher dye rejection was achievedby the GO/PES membrane as compared to pure PES membrane due toelectrostatic repulsive forces on the membrane surface. Similar towhat was observed by Yu et al. [96], the compositemembranes showeda significant improvement of antifouling capability demonstrated bymore than 55% decrease in irreversible resistance of the membranes.

Xu et al. [139] sought to modify GO through chemical function-alization with 3-aminopropyltriethoxysilane (APTS) [113]. They blendedAPTS-functionalized GO (f-GO) and GO with various proportions in aPVDF matrix. APTS was chosen due to its extensive polymer chainswhich can tightly intertwine with PVDF matrix (Fig. 16A) [139]. Higherflux recovery rate and better antifouling performance were obtainedfromGOmodifiedmembranes as compared to the neat PVDFmembranes(Fig. 16B) due to thewell-dispersedGO inmembranepore channelwhich

d the relationship between surface morphology and fouling behavior. (B) (a) Water fluxenes after surface and inner fouling and the fouling resistance of different membranes




caused the attached contaminants to be easily washed away from thepores [139]. Liu et al. [140] showed that (graphene/polypyrol) G/PPyand GO/PPy modified composite materials acted as cathode membraneswith enhanced conductivity and antifouling properties for wastewatertreatment. The G/PPy modified membranes showed 3 times more en-hanced conductivity than pure PPy membranes. The antifouling proper-ties of the modified membranes were tested by filtering yeast solutionthrough the membranes with and without an applied electric field.Results showed that the G/PPy modified membranes inhibited foulingmore efficiently than the PPy modified membranes, demonstrated by a10% higher permeation rate.

Indeed, graphene-based membranes showed very promising anti-fouling properties. The antimicrobial activity of G andGOwas attributedto the synergy of both “chemical” and “physical” effects. Most of theabove studies have attributed the antibacterial activity of GO and rGOto cellular membrane stress induced by sharp edges of graphene nano-sheets, whichmay result in physical damages on cell membranes, lead-ing a loss of bacterial membrane integrity. On the other hand, highhydrophilicity induced by the functional groups at the basal plane andterminals of GO sheets should also have contribution to high waterpermeation and antifouling owing to the low interfacial energy be-tween a surface and water. Nevertheless, physicochemical propertiesof graphene-based materials, such as the density of functional groups,size, and conductivity, can be better tailored to increase their antimicro-bial potentials.

6. Conclusions

This review highlighted the evident promise of NPG and GO nano-sheets as emerging membrane materials for next generation desalina-tion and water separation technologies; thanks to their ultrafastpermeance, excellent mechanical strength and precise ionic andmolec-ular sieving capabilities in aqueous media. Indeed, NPG and GO nano-sheets offer great opportunities to assemble novel class of ultrathin,high flux, and energy-efficient water separation membranes. Freestanding and supported GO membranes showed superior antifoulingproperties against commonwaterborne bacteria. Thus would eventual-ly improve the membranes' life time and energy consumption of thewater purification processes.

However, several challenges are yet to be fully addressed such ascreating controlled nanopores to improve ion permeability withoutcompromising integrity and improving the mechanical performance offully wetted NPG and GO membranes in order to achieve the desirablewater separation performance. More efforts are yet to be seen in closingthe gap between theoretical and experimental findings on water/iontransport mechanisms across NPG and GO nanosheets in aqueousmedia; whether it is charge or size selective ion exclusion. The futurefocus should be also devoted to the large scale production of mechani-cally stable NPG and GO sheets. New reduction methods that minimizeresidual oxygen functionality will be of great value for large-scale pro-duction of graphenes. Furthermore, the utilization of graphite as an in-expensive raw material should significantly lower the manufacturingcosts of GO nanosheet. To further improve the outstanding antifoulingcapability of GO based membranes, it is crucial to balance the compro-mise between the fouling rejection and flux decline. This can be doneby improving the antimicrobial efficiency and mechanical strength ofthe freestanding membranes.

Acknowledgments

The authors would like to thank the Qatar Foundation Research &Development for the financial support. The authors would also like tothank Dema El-Masri and Adnan Ali from Qatar Environment and Ener-gy Research Institute at Qatar Foundation for their valuable contributionto the manuscript.


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