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Thin lm composite membranes embedded with graphene oxide for

water desalination

Mohamed E.A. Ali a,c, Leyi Wang b, Xinyan Wang b, Xianshe Feng a,a Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canadab Zhaojin Motian Co., Ltd., Zhaoyuan, Shandong, Chinac Water Desalination & Treatment unit, Hydrogeochemistry Dept., Desert Research Center, Cairo, Egypt

H I G H L I G H T S

Thin lm composite membranes embedded with graphene oxide were prepared. Membranes exhibited improved water ux, mechanical strength.

Membranes were stable in acidic and alkaline solutions.

The presence of graphene oxide increased chlorine and fouling resistances.

a b s t r a c ta r t i c l e i n f o

Article history:

Received 29 September 2015

Received in revised form 24 February 2016

Accepted 25 February 2016

Available online xxxx

This work deals with thin lm composite membranes prepared from m-phenylenediamine and 1,3,5-

benzenetricarbonylchloride by interfacialpolymerization on the surface of a polysulfonesubstrate,and graphene

oxide was embedded into the membrane during membrane formation to improve the membrane performance.

The desalination performance of the membranes was evaluated in terms of water ux and salt rejection, along

with a baseline membrane containing no graphene oxide. The membrane morphology and surface properties

were also studied using contact angle measurements, FT-IR, XRD and SEM. Incorporating a small amount of

graphene oxide into the membrane was shown to improve the water ux, mechanical stability, and chlorineand fouling resistances of the membrane. At 15 bar, a water ux of 29.6 L/m2h and a salt rejection of97%

were obtained for a salinesolution(2000 ppmof NaCl) when theamine reactant contained100 ppmof graphene

oxide during membrane fabrication. The membranes were found to be stable in acidic and alkaline solutions.

2016 Elsevier B.V. All rights reserved.

Keywords:

MembranesThinlm composite

Graphene oxide

Chlorine tolerance

Fouling resistance

1. Introduction

The shortage of clean and fresh water has become one of the critical

problems for sustainable development to meet the growing social, eco-

nomic and environmental needs of the society[1]. The desalination of

brackish and sea water based on thermal processes and membrane

technologies holds great promises to address these issues [2,3]. Espe-

cially, the development of thin-lm-composite (TFC) membranes com-

prising of a substrate and an interfacially polymerized polyamide (PA)

skin layer has signicantly advanced the membrane technology for

water desalination[4]. In the thin lm composite membranes, the sub-

strate provides mechanical strength to the membrane against the oper-

ating pressure applied across the membrane, whereas the PA active

layer is responsible for rejecting salt while allowing water to pass. The

PA skin layer also determines the membrane resistances to fouling

and chlorine[5]. Ideally, the membranes should be chemically and me-

chanically stableover a long period of operation at high pressures, while

maintaining their desired waterux and salt rejection characteristics.

Unfortunately, the current generation of membranes faces two main

challenges: chlorine sensitivity and fouling propensity[6]. The amide

groups in PA skin layer are vulnerable to chlorine attack even at a low

chlorine dosage in the feed water [7], and membrane chlorination nor-

mally leads to reduced salt rejection that compromises the quality of

the permeated water. In addition, the surface fouling of the thinlm

composite membranes is often a serious problem because frequent

cleaning will not only increase the operating cost but the service

life of the membrane will also be shortened if harsh cleaning agents

are needed[8].

Therefore, many efforts have been made to modify the membrane

surface in order to improve water ux, salt rejection, and fouling and

Desalination 386 (2016) 6776

Corresponding author. Tel.: +1 519 888 4567.

E-mail address:[email protected](X. Feng).

http://dx.doi.org/10.1016/j.desal.2016.02.034

0011-9164/ 2016 Elsevier B.V. All rights reserved.

Contents lists available atScienceDirect

Desalination

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chlorine resistances of the membrane. One common approach is the

development of hybrid organicinorganic membranes with tailored

surface properties. Over the last two decades, various hydrophilic

nanomaterials (including alumina, silica, titanium dioxide, zeolites,

and carbon nanotubes) have been used to improve the performance of

TFC membranes[9]. In general, a small amount of such nanollers is

often insufcient to alter the membrane permselectivity signicantly,

while the uniform dispersion of nanollers in themembrane will be af-

fected because of nanoparticle aggregation if too much nano

llers areused. Nanoparticles with unique structures and functional groups

(e.g., hydrophilicized graphene oxide nanosheets) that enhance their

compatibilities with the polymer matrix of the membrane are of partic-

ular interest. Because of the oxygen-containing functional groups in

graphene oxide (GO) nanosheets (e.g., hydroxyl and epoxy groups on

the basal plane and carboxyl groups at the edge), they generally have a

better dispersibility in water or polar solvents than other nanoparticles

[10,11]. It may be mentioned that a great deal of work has been done

on the synthesis of various polymer-carbon based hybrid materials

through different approaches to enhance mechanical, electrical, catalytic

or other properties (see, for example[1221]). GO nanosheets have re-

cently attracted signicantattention in membrane development because

of their unique nanostructures and physical and mechanical properties

[2224]. Because of the different functional groups (e.g., hydroxyl, car-

boxyl, and epoxide) in graphene oxide, there is a good compatibility be-

tween the nanosheets with the host (polymer) materials through

covalent or non-covalent attachments [25]. When graphene oxide

nanosheets are embedded into a membrane matrix, the surface hy-

drophilicity of the membrane will be enhanced, which are helpful

to enhance water permeability and fouling resistance. In addition,

because of the intermolecular hydrogen bonding between the

amide groups of PA and the functional groups of graphene oxide,

the active amide groups in PA vulnerable to chlorine attack are

shielded by the nanosheets, resulting in improved membrane resis-

tance to chlorine[26]. Moreover, the embedded GO nanosheets in

the membrane matrix also increases the mechanical strength and

enhances the membrane stability against high transmembrane pres-

sures[27].

The main objective of the present work was to prepare hybridorganicinorganic thin lm composite membranes by incorporating

graphene oxide nanosheets into the interfacially polymerized polyamide

skin layer. The effects of the addition of GO nanollers in the membrane

on the desalination performance of the resulting membrane were investi-

gated. It wasshown that by properly controlling themembrane formation

conditions, thinlm composite membranes with signicantly improved

performance (i.e., mechanical stability against high pressure, tolerance

for high chlorine dosages, resistance to strong acidic and alkaline solu-

tions) were produced. These membranes compared very favorably

with those GO-modied membranes recently reported in the literature

[26,2830]. It may be mentioned that unlike GO-modied membranes

where GO was incorporated into the membrane by coating or layer-by-

layer deposition[26,30], the composite membranes developed in this

work comprised of GO that was embedded in the polyamide skin layerduring the interfacial polymerization.

It may be noted that prior work on modication of TFC mem-

branes with GO involved surface coating or layer-by-layer assembly.

In one approach, GO wasdepositedon topof a polyamide active layer

by electrostatic self assembly, and the GO coating layer led to an

additional resistance to water permeability[14,16]. In another ap-

proach, oppositely charged GOs were deposited on a porous sub-

strate by electrostatic self assembly, and the polyamide layer was

formed on top of the GO layer by conventional interfacial polymeri-

zation [30]. In both cases, the GO layer and the polyamide layer

were not interpenetrated, and the stability of the electrostatically as-

sembledGO layermay be a potential concern when used in water de-

salination because of the ionic solutes involved. Therefore, it is

desirable to incorporate GO into the polyamide layer during the

course of interfacial polymerization so as to form a nanostructured

hybrid membrane. In the present work, GO nanollers were dis-

persed in the aqueous amine reactant phase, which was allowed to

react with the organic phase of acyl chloride reactant on a substrate

surface to achieve interfacial polymerization, thereby incorporating

the GO in situ into the thin polyamide layer during the course of in-

terfacial polymerization.

2. Experimental

2.1. Materials

Graphene oxide was prepared from graphite powder, potassium

permanganate, sulfuric acid, nitric acid and hydrogen peroxide; all the

chemicals were supplied by Fisher Scientic, except for graphite pow-

der which was supplied by Acros Organics. Microporous polysulfone

ultraltration membranes supplied by Sepro Membranes were used as

the substrate. They had a molecular weight cut-off of 10,000, and a

pure water permeability of approximately 90 L/m2hbar. 1,3,5-

benzenetricarbonyl chloride (i.e., trimesoyl chloride, TMC) (N98%), m-

phenylenediamine (MPD) (N99%), and camphor sulphonic acid (CSA)

were supplied by Fischer Scientic. n-Hexane was purchased from Cal-

edon Laboratories. Sodium lauryl sulfate (SLS) was purchased from

Matheson Coleman & Bell Chemical. NaCl (EMD Chemical) was used

to characterize the salt rejection of the TFCmembranes. Thechlorineso-

lutionwas prepared from a commercially available sodium hypochlorite

solution (NaClO,14.5% availablechlorine, Alfa Aesar). When needed, the

feed solution pH was adjusted to desired values using hydrochloric acid

(37 wt%, Sigma-Aldrich) or sodium hydroxide (Caledon Laboratories).

Bovine serum albumin (BSA) supplied by Sigma-Aldrich was used as a

model foulant in membrane fouling experiments.

2.2. Preparation of graphene oxide

GO was prepared following a modied Hummers' method [31].

Briey, in an ice bath, 1 g of graphite powder was dispersed ina mixtureof 25 ml of cold concentrated sulfuric acid and 1 g of sodium nitrate.

Then, 3 g of potassium permanganate was slowly added under vigorous

stirring and cooling conditions to keep the temperature below 20 C.

Thereafter, the reaction mixture was placed in a water bath at 35 C

with continuous stirring for 1 h. Then, 50 ml of deionized water was

slowly added, and the reaction mixture was allowed to stay at 98 C

for 12 h, producing a bright-yellow suspension. Finally, the reaction

was terminated by sequentially adding 140 ml of deionized water and

3 ml of hydrogen peroxidesolution (30%). Thesolidproduct wasltered

out of the solution, rinsed with a dilute hydrochloric acid (3.4 wt.%)

until a pH of 7, and vacuum-dried at room temperature.

Fig. 1.Schematic diagram of the cross-ow membrane testing unit.

68 M.E.A. Ali et al. / Desalination 386 (2016) 6776


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2.3. Preparation of thinlm composite membranes

The thin lm composite membranes with and without graphene

oxide were prepared on the top surface of the polysulfone substrate

using the interfacial polymerization technique. In brief, the substrate

membrane was rinsed with deionized water for at least 2 h to remove

any preservatives. After that, 10 ml of the aqueous solution of MPD

(2 wt.%) containing additives (camphor sulphonic acid 1 wt.%, and

Fig. 2. Schematic representation of chemical interactionsbetween MPD and TMC with and without GO relevantto membrane formation by interfacial polymerization. (For interpretation

of the references to color in this gure legend, the reader is referred to the web version of this article.)

69M.E.A. Ali et al. / Desalination 386 (2016) 6776


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sodium lauryl sulfate 0.2 wt.%) was allowed to sit on the top surface of

the substrate for 1 min. These additives were used to enhance MPD

sorption onto the substrate. After draining off any droplets of the

amine solution from the substrate surface, 10 ml of the TMC in hex-

ane solution (0.1 wt.%) was brought into contact with substrate sur-

face for 50 s to induce interfacial polymerization between TMC and

the MPD deposited. The excess organic solution on the membrane

surface was removed, and the resulting thin lm composite mem-

brane was subjected to heat treatment at 65 C for 5 min, therebycompleting the interfacial polymerization to attain the desired sta-

bility of the TFC membrane against high pressure in desalination

processes. The membranes were stored in water after thorough

washing with water. To prepare TFC membranes incorporated

with graphene oxide, the same membrane fabrication procedure

was followed, except that the aqueous reactant solution of MPD

contained different amounts of graphene oxide (0300 ppm). The

MPD/GO solutions were sonicated in an ultrasonication bath for

30 min before being deposited on the substrate surface for interfa-

cial polymerization with TMC. For convenience, thin lm composite

membranes with and without GO are designated as TFC and TFC/GO

membranes, respectively.

2.4. Membrane characterization

The membranes were characterized with infrared spectroscopy

using a Bruker Vector 22 FT-IR spectrophotometer. The X- ray diffrac-

tion patterns of the membranes were recorded using a Bruker-AXS D8

Discover diffractometer (Co-K source). The surface morphology of

the membranes was characterized using a scanning electron microsco-

py (SEM, Hitachi S-4800). The roughness and thickness values were

estimated by using the image-J software program. The surface hydro-

philicityof themembranes wasevaluated using a contact angle analyzer

(Cam-plusMicro, TantecInc.). Themembranesamples were air-dried at

ambient temperature for measuring contact angles of deionized water

on the membrane surface using the sessile droplet technique, the con-

tact angle reported was an average of at leastve measurements on dif-

ferent locations of a single membrane sample.

2.5. Membrane performance for water desalination

The performance of the membranes for water desalination was

measured in term of water ux and salt rejection using a cross-

ow permeation apparatus (Fig. 1). It consisted of six membrane

cells, and each cell had an effective membrane area of 14.75 cm2.

This allowed six membranes to be measured under identical condi-

tions. Unless stated otherwise, the operating conditions were: pres-

sure 15 bar gauge, NaCl concentration in feed water 2000 ppm,

temperature 22 C, and pH 7. During the reverse osmosis runs, the

retentate solution was recirculated to the feed tank, and the perme-

ate samples from the membrane cells were also recycled back to the

system upon measurements of their solute concentrations individu-

ally. This way, an essentially constant saline concentration in thefeed was maintained during the experiment. The salt rejection (R)

and permeation ux (J) were determined using the following two

equations,

R 1CPCF

100% 1

J V

At 2

where CFand CPare the salt concentrations in the feed and perme-

ate, respectively. Vis the volume of permeate collected over a time

intervalt for a membrane area of A. The salt concentrations in the

permeate and feed solutions were determined using an Orion

conductivity meter. The desalination experiments were conducted

in triplicates for every membrane sample, and the average values

were reported.

Fig. 3.FT-IR spectra of graphene oxide (GO), and the TFC and TFC/GO membranes.

Fig. 4.XRD patterns of GO, the TFC and TFC/GO membranes. The TFC/GO membrane was

prepared using 100 ppm of GO in the MPD solution.



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2.6. Evaluation of membranes resistance to chlorine

To evaluate the chlorine resistance of the membranes, the waterux

and salt rejection were measured witha feed saline solution (2000 ppm

NaCl) containing 500 ppmof NaOClat a pressure of 7 bar gauge for 10 h.

The variations in water ux and salt rejection were monitored as per-

meation proceeded with time.

2.7. Evaluation of membrane resistance to fouling

Membrane fouling was evaluated withltration-cleaningcycles. The

feed saline solution contained 2000 ppmof NaCl, and BSA was used as a

model protein foulant. First, the membraneswere preconditioned in the

membrane unit with a saline solution (2000 ppm NaCl) at a pressure of

15 bar gauge, and theinitial water ux was determined when the water

ux reached a steady state. Then, the membranes were subjected to a

ltration test with a saline solution of 2000 ppmof NaCl (in theabsence

of BSA) for 6 h, followed by a ltration test with a saline solution

(2000 ppm of NaCl) containing 100 ppm of BSA for another 6 h. After

that, the membranes were washed thoroughly with de-ionized water

for 2 h, followed by ltration tests with the two saline solutions again.

Theltration-cleaning cycles were repeated to determine the signi-

cance of membrane fouling as reected by the water ux decline.

3. Results and discussion

3.1. Characterization of TFC and TFC/GO membranes

The TFC and TFC/GO composite membranes were prepared by theinterfacial polymerization technique.Fig. 2is a schematic of interfacial

polymerization between MPD and TMC; the interactions between GO

and PA are also illustrated in the gure. GO can form a hydrogen bond

with terminal primary and secondary amines, as well as covalent

bond through condensation reactions with terminal carboxyl groups

of TMC in the linear portion of PA. During the experiment, we noticed

Fig. 5. SEM images ofthetopsurfacesandcrosssectionsof TFC (aandd),TFC/GOwith 100 ppm ofGOin MPD solution(b and e),andTFC/GO with300ppmof GOin MPDsolution (candf).

Table 1

Estimated skin layer thicknesses of the membranes.

Membrane Thickness (m)

TFC 4.56 1.39

TFC/GO 100 ppm 1.93 0.78

TFC/GO 300 ppm 4.06 0.75



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a color change of the MPD solution from transparent to dark green then

to black in a few hours at ambient conditions, presumably dueto oxida-

tion. On the other hand, when GO (yellowish brown) was added to the

amine solution, there was little change in color of the solution over a

long period of time. This may be due to blocking the reactive amine

sites in MPD through formation of hydrogen bonds with the functional

groups of GO.

Fig. 3shows the FT-IR spectra of GO nanosheets, polyamide active

layer in the thin

lm composite membranes with and without GO.For the GO nanosheets, the bands at 3429 cm1, 1734 cm1 and

1052 cm1 correspond to hydroxyl, carboxyl and epoxide functional

groups, respectively, and these results conrm the preparation of GO

by the oxidation process of graphite [29,32]. For thethin lm composite

membranes, the characteristic band at 1694 cm1 was attributed to C=

O stretching vibration(amideII) of thepolyamideactive layer,while the

band at 1589cm1 was due toNHandCN stretching vibrationsof the

amide group (amide II). The latter band was shown to have a lower in-

tensity and broader width for the TFC/GO membrane, accompanied

with a band shifting to 1660 cm1, suggesting the interaction between

PA and GO nanosheets. The characteristic band at 1747 cm1 in the FT-

IR spectrum of the TFC/GO membrane corresponded to the stretching

vibration of C=O ester groups formed between carboxylic or hydroxyl

groups of GO and the carboxylic groups of the PA active layer. The

band at around 1113 cm1 corresponding to C-O stretching in the TFC

membrane also showed a reduction in band intensity and a shifting to

1106 cm1 for the TFC/GO membrane. The band shifting and change

in band intensity, especially from 400 to 1600 cm1, were apparent in

the FT-IR spectra when GO was incorporated into the membrane due

to the interactions between GO with the polyamide active layer in the

membrane.

Fig. 4shows the XRD diffractogram of GO and the TFC and TFC/GO

membranes. GO showed a sharp peak at a 2 of 10.09, with a d-

spacing of 0.88 nm. This indicates the presence of functional groups in

the GO basal structure, which is in agreement with prior work on GO-

containing composite materials [23,33]. The PAactivelayer inTFC mem-

brane was characterized by the presence of three semi-crystalline peaks

at 2 of 18.14, 23.25 and 26.55, corresponding to d-spacing of 0.48, 0.38

and 0.34 nm, respectively. A similar trend in the XRD pattern was ob-served for the TFC/GO composite membrane, but with a decreased

peak intensity and d-spacing. It has been reported that a small amount

of GO in the membrane can change the d-spacing between thepolymer

chains[33].

Fig. 5 shows SEM imagesof thesurfacesand the cross-sectionsof the

thin lm composite membranes. A comparison of the three membranes

(that is, TFC,TFC/GO with 100 ppm of GO, and TFC/GO with 300 ppm of

GO) shows that all the membrane surfaces showed a ridge and valley

morphology. Similar observations can also be found in other studies

on thin lm composite membranes containing nanollers[29,34]. As

expected, the incorporation of different amounts of GO in the mem-

brane resulted in different surface morphologies of the membranes.

Table 1shows the thicknesses of the active skin layers of the thin lm

composite membranes estimated from the SEM images using ImageJprocessing software. The TFC/GO membranes tended to have rougher

surface than the TFC membrane containing no graphene oxide. As far

as the effective skin layer thickness is concerned, the TFC/GO (with

100 ppm of GO) was thinnest among the three membranes considered.

Compared to the GO-free TFC membrane, there is a hindrance effect of

GO nanosheets to the diffusivity of MPD from the aqueous phase to

reach the interface for reaction with TMC in the organic phase [24],

resulting in a thinner interfacially polymerized skin layer. However, ex-

cessive loading of GO will result in aggregation of the GO nanosheets,

which will cause an increase in the active layer thickness.

Fig. 6 shows the effects of incorporating GO in the thin lm compos-

ite membranes on the surface hydrophilicity of the membranes as

measured by the water contact angle on the membrane surface. Gener-

ally speaking, a high surface hydrophilicity is advantageous to the

membrane performance with respect to water permeability and fouling

resistance [35]. The pure water permeability is also shown in the gure.

With an increase in the concentration of GO in the reactive amine solu-

tion from 0 to 300 ppm, the contact angle decreased from 64 for the

GO-free TFC membrane to 48 for the TFC/GO membrane. Thus, there

was a considerable improvement in the membrane surface hydrophilic-

ity, and this was expected because of the hydrophilic functional groups

(carboxyl, hydroxyl and epoxy) of GO. A hydrophilic membrane surface

facilitates the uptake of water molecules onto the membrane surface,which enhances the water permeability. The data in Fig. 6shows that

thepure water permeability(PWP) wasincreased when a small amount

of GO was present in the membrane. However, when the GO concentra-

tion in theamine reactant was sufciently high, a further increase in the

Fig. 6. Water contact angle and pure water permeability of the TFC and TFC/GO

membranes as a function of GO concentration in the MPD solution used in membrane

preparation. Operating conditions for pure water permeability tests: pressure, 15 bar;

temperature 20 C; feed, deionized water.

Fig. 7. Effects of graphene oxide concentration in MPD solution used in membrane

preparation on the RO performance of resulting composite membranes. Operating

conditions: pressure 15 bar, temperature 20 C, feed NaCl concentration 2000 ppm, and

pH 7.



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GO content led to a decrease in the water permeability probably be-

cause aggregated GO nanosheets would increase the length and tortu-

osity of the passageways for the penetration of water molecules

through the membrane. Nonetheless, it was shown that over the GO

concentration range (0300 ppm) studied, all the TFC/GO membranes

had a pure water permeability higher than that of the GO-free TFC

membrane.

3.2. Desalination performance

The effects of GO concentrations in the reactive amine solution dur-

ing membrane formation on the water ux and salt rejection of the

resulting membranes are shown inFig. 7. At low GO concentrations,

the water ux of the membrane increased with an increase in the GO

concentration, whereas the salt rejection decreased. These trends

began to change at relatively high GO concentrations. There was a de-

crease in water ux and a slight increase in the salt rejection when the

GO concentration was above 150 ppm. Compared to the GO-free TFC

membrane which had a water ux of 21.4 L/m2.h and a salt rejection

of 98.5%, the TFC/GO membrane showed a 39% increase in water ux

and a slight reduction (1%) in salt rejection at a GO concentration of

100 ppm. The presence of GO in the membrane enhances water trans-

port through the membrane matrix[36]. On the one hand, water mole-cules can transport through the channels of GO nanosheets, and on the

other hand, the hydrophilic functional groups (hydroxyl, carboxyl and

epoxy) in GO facilitate the adsorption of water molecules on the mem-

brane surface. Both effects favor water transport. However, water uxis

also inuenced by the thickness of the skin layer[37]. On the one hand,

the presence of GO in the diamine reactant tends to result in a thinner

interfacially formedpolyamide layer. On theother hand, theGO embed-

ded in the skin layer also contributes to the skin layer thickness, which

increases the membrane resistance to water transport[38]. Because of

these two opposing effects, a maximum water ux was observed for

TFC/GO membrane at a GO concentration of 100 ppm among the mem-

branesstudied here. This membrane was used for further tests in subse-

quent studies of mechanical stability, chlorine resistanceand antifouling

properties. In general, there was a slight decrease in the salt rejectionwhen GO was incorporated into the membrane, presumably due to

the nanosheet structure of the GO randomly positioned in the mem-

brane skin layer. The GO nanosheets positioned perpendicularly to the

membrane surface are not effective to block the passage of salt while

allowing water to permeate. In addition, too much GO in the membrane

will aggregate, which will also affect the formation of the polyamide

skin layer by interfacial reaction between MPD and TMC because the

GO aggregates dispersed in the polyamide layer can lead to localized

defectsin the aggregates, which tends to lower salt rejection.

It may be pointed out that GO-modication of MPD/TMC-based

polyamide TFC membranes has been done by coating or layer-by-layer

deposition of GO onto a substrate prior to polyamide layer formation

by interfacial polymerization[26], or simply onto the surface of a TFC

membrane after skin layer formation[30]. Not until earlier this year

was the concept of embedding GO into the polyamide layer during in-

terfacial polymerization attempted[24,29]. Chae et al.[24]dispersed

GO in the aqueous solution of MPD for interracial polymerization,

while Bano et al.[29]used several additives (including triethyl amine,

dimethyl sulfoxide, 2-ethyl-1,3-hexane diol, and camphor sulfonic

acid) in theMPD solution. In this work, we chose to use camphor sulfon-

ic acid and sodium lauryl sulfate as additives in the aqueous MPD solu-

tion in order to help adjust the solution pH and dispersity, respectively,

thereby facilitating deposition of MPD and GO onto the substrate sur-

face for subsequent interfacial polymerization with TMC. The mem-

branes so formed compared well with other GO-modied membranes

for water desalination, as illustrated by the water ux and salt rejection

shown inTable 2.

The operating pressure is an important parameter for water desali-

nation by membranes.Fig. 8shows the water ux and salt rejection of

TFC/GO membrane at different transmembrane pressures. To assess

the effect of addition of GO on mechanical stability of resulting mem-

branes, the performance of the GO-free TFC membrane was plotted in

Fig. 8 as well. As expected,the wateruxfor bothTFC and TFC/GO mem-

branes increased as the operating pressure increased from 7 to 35 bar.

Unlike the TFC/GO membrane, which showed an almost constant salt

rejection, the TFC membrane experienced a sharp decline in the salt

Table 2

MPD/TMC based thin lm composite membranes containing graphene oxide llers.

Monomers of TFC

membrane

Additives in MPD

solution GO conc. and addition procedure

Pressure

(bar)

NaCl conc.

(ppm)

Waterux

(L/m2h)

Salt

rejection (%)

Water contact

angle () Ref.

MPD, TMC 76 ppm in MPD 15 2000 16.6 99 47 [24]

MPD, TMC

1000 ppm, deposited onto polysulfone substrate 55 32,000 28 98 55.4 [26]

MPD, TMC TEA, DMSO, EHD, and CSA 2000 ppm in MPD 15 2000 22 88 65 [29]

MPD, TMC TEA 20,000 ppm, deposited onto TFC membrane 15.5 2000 14 96 26 [30]

MPD, TMC CSA, and SLS 100 ppm in MPD 15 2000 29.6 98 56 This work

TEA: triethyl amine.

DMSO: dimethyl sulfoxide.

EHD: 2-ethyl-1,3-hexane diol.

CSA: camphor sulfonic acid.

SLS: sodium lauryl sulfate.

Fig. 8.Salt rejection and waterux of TFC and TFC/GO membranes at different feed

pressures. The TFC/GO membrane was prepared using 100 ppm of GO in the MPD

solution. Other RO operating conditions were the same as in Fig. 7.



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rejection when the operating pressure was above 25 bar. This was pre-

sumably caused by membrane deformation at high pressures. It has

been reported that the mechanical stress on the membrane may lead

to membrane deformation as a result of compaction, changes in the

pore size and shape, or rupture of the skin layer under extreme condi-

tions. The data inFig. 8show that incorporating GO into the membrane

enhanced the mechanical stability of the membrane, which is under-

standable because of reinforcement of skin layer by the presence of in-

tercalation GO nanosheets[39,40].

It is preferable to operate thin lm composite membranes with

aqueous solutions at pH ranges between 2 and 10[41]. The effects ofGO on the membrane performance over a wide range of pH (2 to 12)

were tested, with the pH of the feed solution being adjusted with HCl

or NaOH accordingly. The results are shown inFig. 9. Under alkaline so-

lution conditions, an increase in the solution pH increased the water

ux and decreased the salt rejection for both TFC and TFC/GO mem-

branes. This can be explained in light of de-protonation of carboxyl

groups of the linear portion of the polyamide in the membrane

skin layer, as well as the carboxylic groups of GO in case of TFC/GO

membranes[42,43], making the membrane surfaces more negatively

charged. Consequently, water sorption into the membrane skin was

enhanced, leading to an increased water ux and reduced salt rejection

because of the increased membrane swelling. On the other hand, com-

pared to neutral feed solutions at pH of 7, the salt rejection was lower

for both membranes under acidic conditions, which was accompaniedwith an increase in the water ux of the TFC membrane and a slight

decrease in the water ux of the TFC/GO membrane. Under acidic con-

ditions, the terminal acidic and amine groups in the interfacially poly-

merized PA skin layer will be protonated, and this will cause the

membrane surface to be more positively charged. As a result, water

sorption in the membrane will the enhanced, leading to increased

membrane swelling, which tends to improve water permeability in

the membrane while lowering the salt rejection. When GO is embedded

in the membrane, the membrane swelling is constrained, and the

hydrophlicGO nanosheets also function as passageway to water perme-

ation. Consequently, the water permeability did not change signicant-

ly. On the other hand, GO is known to have a strong adsorption

properties with respect to salts in aqueous solutions [44], and this favors

salt rejection of the TFC/GO membrane. However, the ion sponge

effect of GO appears to be inuenced by the solution pH. For instance,

Wang et al.[45]used a GO/amidoxime hydrogel to capture uranium

by adsorption, and the sorption capacity was shown to decrease signif-

icantly when the pH decreased in acidic solutions. If the salt sponge ef-

fect of GO to NaCl is affected by pH in a similar fashion, then the salt

rejection of TFC/GO membrane will decrease when the feed solution

pH changes from 7 to 2, as observed inFig. 9. The study of Ganesh

et al.[46]on GO/polysulfone mixed matrix membrane also exhibited a

reduced salt rejection under acidic conditions when the solution pH de-creased. A further study of NaCl sorption uptake in GO at different solu-

tion pH, which is beyond the scope of the present work, will be needed

to have a better understanding about the pH effect on salt rejection of

the TFC/GO membrane.

Fig. 10shows the changes in water ux and salt rejection for both

TFC and TFC/GO membranes after exposure to 500 ppm of chlorine so-

lution over different periods of time. For the sake of illustration, the

water ux and salt rejection were normalized by the performance

data of the membrane without chlorine exposure. As expected, there

was a signicant increase in water ux and a slight decrease in salt re-

jection after exposure of the membrane to chlorine. The TFC/GO mem-

brane was found to be more stable against chlorine attack than the

GO-free TFC membrane. Chlorine is considered to degrade polyamide

macromolecules via a nucleophilic substitution reaction between chlo-rine and thehydrogenof the secondary amide group (NH) in polyam-

ide[47]. The improved chlorine resistance of the TFC/GO membrane is

believed to result from hydrogen bonding between the secondary

amide groups in polyamide and the functional groups of GO nanosheets

[28]that functioned as a protection against chlorine.

3.3. Anti-fouling property of the membrane

Surface fouling is an important issue that affects the separation per-

formance of membranes. Using BSA as a model foulant, the water uxes

of the TFC and TFC/GO membranes were investigated. Fig. 11 shows the

normalized water uxes (Jr) through the membranes relative to their

initial wateruxes. A commercial membrane was also tested as a refer-

ence under the same conditions for comparison (the supplier of this

Fig. 10.Effects of chlorine exposure time on salt rejection and water ux of the TFC and

TFC/GO membranes (The TFC/GO membrane was prepared using 100 ppm of GO in the

MPD solution). Chlorine dosage, 500 ppm. RO operating conditions: pressure 7 bar,

temperature 20 C, feed NaCl concentration 2000 ppm, and pH 7.

Fig. 9.Salt rejection and waterux of TFC and TFC/GO membranes at different feed

solution pH. The TFC/GO membrane was prepared using 100 ppm of GO in the MPD

solution. Other RO operating conditions same as in Fig. 7.



9/10

membrane is not disclosed pernondisclosure agreement). It was shown

that all membranes were fouled by BSA (100 ppm) when present in the

saline feed solution (2000 ppm of NaCl), as shown by the decreased

waterux over time. Although membrane cleaning withwater could re-

mediate the membranes to some extent, the membrane performance

could not be fully recovered by periodically cleaning the membranes

with deionized water. Compared to the TFC membrane, which was

fouled by BSA to a similar extent as the commercial TFC membrane,

the TFC/GO membrane was shown to be much more resistant to BSA

fouling. In addition, a waterux recovery as high as 85% was achieved

with the TFC/GO membrane after membrane cleaning, while the other

two membranes showed a water ux recovery of around 50%. The im-provement on the anti-fouling property of the TFC/GO membrane is

considered to derive from the increased surface hydrophilicity of the

membrane and the more negatively charged surface when GO was em-

bedded in the active membrane layer. As BSA is negatively charged at

pH 7, themembrane fouling is less signicant due to thefavorable elec-

trostatic interactions between the negatively charged membrane and

the foulant.

4. Conclusions

Thinlm composite membranes were prepared via the interfacial

polymerization technique, and graphene oxide was used as a modier

to improve the membrane properties. It was shown that incorporatingGO into the membrane improved the hydrophilicity, water permeabili-

ty, chlorine resistance and antifouling properties of the membrane. The

membrane surface and morphology were also studied using contact

angle measurements, FT-IR, XRD and SEM. Compared to a GO-free TFC

membrane, a 39% increase in water ux was achieved when the

amine reactant contained 100 ppm of GO during membrane formation.

Thethin lm composite membranesembedded with GO were shown to

be more resistant to chlorine attack and surface fouling than the mem-

braneswithout GO. A higherwaterux recovery wasalso achieved with

the TFC/GO membrane after membrane cleaning. At 15 bar, a water ux

of 29.6 L/m2h and a salt rejection of97% wereobtainedfor a saline so-

lution (2000 ppm of NaCl) when the amine reactant contained 100ppm

of graphene oxide during membrane fabrication. The membranes were

found to be stable in acidic and alkaline solutions.

Acknowledgments

Research support from the Natural Sciences and Engineering

Research Council (NSERC)of Canadaand ZhaojinMotian Co. is acknowl-

edged. We are also grateful to theScience andTechnology Development

Fund (STDF), Ministry of Scientic Research of Egypt, for awarding a fel-

lowship to oneof theauthors (M.E.A.A.) to carry out theresearch at Uni-

versity of Waterloo.

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Fig. 11.A comparison ofux recoveries for the TFC and TFC/GO membranes prepared in

this study (The GO content in MPD for preparing the TFC/GO membrane was 100 ppm).

A commercial TFC membrane was also tested as a reference. The membrane fouling

tests were carried out with feed solutions containing 2000 ppm of NaCl and 2000 ppm

of NaCl + 100 ppm of BSA, respectively, in the ltration cycle.

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