PVDF membranes containing reduced graphene oxide: effect of degree of

reduction on membrane distillation performance

Ahmed Abdel-Karim1,2, Jose Miguel Luque-Alled1, Sebastian Leaper1, Monica Alberto1,

Xiaolei Fan1, Aravind Vijayaraghavan3, Tarek A. Gad-Allah2, Amer S. El-Kalliny2, Gyorgy

Szekely1, Sayed I.A. Ahmed4, Stuart M. Holmes1, Patricia Gorgojo1*

1 School of Chemical Engineering and Analytical Science, The University of Manchester

2 Water Pollution Research Department, National Research Centre, Giza, Egypt

3 School of Materials, The University of Manchester

4 Faculty of Engineering, Ain Shams University, Abbassia, Cairo, Egypt

*Corresponding author: p.gorgojo@manchester.ac.uk

Abstract

Hydrophobic polyvinylidene fluoride (PVDF) membranes have been successfully used in

membrane distillation (MD) for desalination applications; however, there is still room for

performance enhancements both regarding water flux and salt rejection. In this work, reduced

graphene oxide (rGO) nanoplatelets with different degrees of reduction (36%, 58%, 65 and

69% removal of oxygen, as characterized by XPS) were incorporated as fillers in PVDF

matrices in order to evaluate the effect of the oxygen content of the fillers in the MD

performance. UV-Vis and raman spectroscopies were also used to characterize the fabricated

rGO. Changes in morphology of the prepared mixed matrix membranes (MMMs) were

assessed via scanning electron microscopy (SEM) and were related to the increased

hydrophilicity and viscosity of the casting solutions when fillers were added. MMMs

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containing 0.5 wt% rGO with an optimum degree of reduction of 58% exhibited an improved

MD performance, with fluxes of ~7.0 L m-2 h-1 (LMH), representing an enhancement of

~169% in comparison with the plain PVDF membrane, without compromising salt rejection

(greater than 99.99%). Continuous testing for up to 96 h showed a stable performance of the

developed MMMs, without compromising the quality of the permeate.

Keywords: Air gap membrane distillation (AGMD); Desalination; Hydrophobic

polyvinylidene fluoride (PVDF); Mixed matrix membranes (MMMs); Reduced graphene

oxide (rGO).

1. Introduction

Today, seawater desalination is mainly achieved by one of the following three

technologies: multistage flash distillation (MSF), multi-effect distillation (MED), or reverse

osmosis (RO). However, significant amounts of thermal energy in MSF and MED, and

electrical energy in RO are required to carry out the separation on a large scale and typically

rely on non-renewable energy sources. In addition, the capital investment and maintenance

costs for these technologies can be very high. Consequently, alternative technologies that are

capable of performing effectively but in a more sustainable way are currently being

investigated. Membrane distillation (MD) seems to be a suitable option. In this process, only

water vapor is able to permeate through a non-wetted porous hydrophobic membrane as a

consequence of the difference in water vapor pressure between the two sides of the

membrane. The pressure difference results from the thermal gradient between the hot feed

and the cold permeate stream [1, 2]. MD requires lower feed temperatures than those in

thermal-based processes, and offers a variety of advantages when compared to conventional

pressure-driven membrane separation processes; it can achieve higher rejection of ions at

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much lower operating pressures than for instance RO and it shows greater simplicity and

higher resistance to fouling [2-5].

Recently, MD has begun to be implemented at pilot plant scale, as described in the

work by Duong et al., where a pilot-scale spiral-wound air gap MD system was successfully

used to treat coal steam gas (CSG) RO brine with 80% water recovery [6]. The work by

Woldemariam et al. also reports the use of MD at pilot scale, where the purification of

effluent from a municipal wastewater treatment plant was carried out in an air gap membrane

distillation (AGMD) system [7]. Yet, this technology has still not been fully implemented at

the industrial level, with the lack of specially-tailored low-cost membranes being one of the

major reasons for this [8]. Although commercial hydrophobic microfiltration (MF)

membranes can be utilized for MD systems, they can still suffer from wetting issues and sub-

optimal flux performance [4, 8]. Thus, robust membranes specific for MD processes with

high hydrophobicity, high porosity, narrow pore size distribution and good chemical stability

need to be developed [3].

Polyvinylidenefluoride (PVDF) is a semicrystalline polymer characterized by having

good chemical resistance and thermal stability, high mechanical strength and excellent aging

resistance, which are very important requirements for practical applications [9]. Moreover,

PVDF is soluble in some common solvents as dimethylacetamide (DMAc), N,N-

dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). Consequently, PVDF flat

sheet and hollow fiber membranes can be produced at scale from solution by the widely used

non-solvent induced phase separation (NIPS) process [8-11]. Attempts have been carried out

using PVDF as a base polymer due to its relatively high hydrophobicity, low thermal

conductivity, and good processablility. For instance, Tomaszewska used PVDF in

combination with LiCl to prepare membranes for direct contact membrane distillation

(DCMD) with 98% of salt rejection and a permeate flux of 9.7 LMH (L m-2 h-1) [12].

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Furthermore, advances in nanotechnology have been used in water desalination technologies

to improve salt rejection and flux [13]. Prince et al., prepared electrospun nanofiber

membranes comprising PVDF blended with clay nanocomposites, which were tested for

DCMD and gave a flux of 5.7 LMH and a rejection of 99.5% [14]. TiO2 was used by Meng et

al. as a surface coating for PVDF membranes, leading to improvements in salt rejection and

membrane stability [15]. In addition, Roy et al. immobilized functionalized carbon nanotubes

(CNTs) on porous polypropylene supports, achieving higher fluxes in DCMD [16].

A considerable amount of literature has been published on using graphene oxide (GO)

nanosheets in MD [17-19] and ultrafiltration (UF) applications [20, 21]. GO can be prepared

via several cost-effective mass production methods using inexpensive graphite as raw

material with high yields [22]. Leaper et al. reported an 86% flux enhancement of neat PVDF

membrane by adding 3 (aminopropyl)triethoxysilane (APTS)-functionalized GO [19].

Zahirifar et al., fabricated and tested octadecylamine functionalized graphene oxide/PVDF

dual-layer membranes for desalination via AGMD, showing a decrease in water flux but

boosted salt rejection as compared to the unmodified PVDF membrane [18]. Bahdra et al.,

immobilized GO on the surface of polytetrafluoroethylene (PTFE) membranes for

desalination via DCMD, significantly improving the overall permeate flux with complete salt

rejection [17].

GO can be partly reduced to graphene-like sheets (i.e., reduced graphene oxide, rGO)

by removing the oxygen-containing groups, thus, recovering the conjugated structure in a

large extent [23]. Although the electronic properties of rGO are far from those shown for

pristine graphene, other features such as mechanical strength, high ratio area/volume and

thermal/chemical stability are still presents in rGO nanosheets. The relatively high

hydrophobicity of rGO compared to GO makes it a promising material for improving the

hydrophobic membranes in MD systems.

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In the present study, PVDF flat-sheet membranes incorporated with reduced graphene

oxide (rGO) have been employed for water purification of aqueous solutions containing

35,000 mg L-1 of sodium chloride via AGMD. To the best of the authors’ knowledge, this is

the first time that rGO has been used for the preparation of MD membranes, and the first time

that the amount of oxygen-containing functionalities has been linked to the morphology,

hydrophobicity and desalination performance of MD membranes. GO and four different rGO

fillers with increasing degrees of reduction have been prepared, characterized and

incorporated into PVDF. The concentration of graphene-based nanoadditives in the

membranes has been optimized for membranes prepared with the most reduced rGO sample,

and that optimum concentration has been used in the preparation of membranes containing

GO and the other three rGO fillers with lower reduction degrees.

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2. Experimental

2.1. Materials

For the preparation of graphene oxide (GO) graphite powder

(Naturegraphit GmbH, Germany) was used. Potassium permanganate

(KMnO4, 99%), hydrogen peroxide (H2O2, 30%), sulphuric acid (H2SO4,

98%), sodium hydroxide (NaOH), nitric acid (HNO3, 68%) and hydrochloric

acid (HCl, 37%) were purchased from Sigma Aldrich, UK and were all used

as received. For the preparation of membranes, PVDF powder with a

molecular weight of 534,000 g mol-1 and N,N dimethylformamide (DMF)

were purchased from Sigma-Aldrich. Nonwoven support (NWS) fabric

(Viledon Novatexx-2471) was acquired from Freudenberg Filtration

Technologies (UK). The NWS fabric has a thickness of ~0.16 mm as

measured in the lab using a digital Mitutoyo IP65 micrometer screw gauge (accuracy

of ±0.5 µm) and it has an air permeability value of 150 L m-2 s-1 at 200 Pa (as

reported by the supplier). Its maximum tensile force along/across is

270/170 N for 5 cm pieces with elongations of 25% and 30 %,

respectively, as also specified by the supplier. Millipore deionized (DI)

water (18 MΩ·cm resistivity) was used as non-solvent in the coagulation

bath. NaCl was provided by Acros, Belgium. All reagents and materials

were used as received.

2.2. Preparation and characterization of reduced graphene oxide

GO was prepared by following a modified Hummer’s method as

reported elsewhere [24]. GO was reduced by dispersing 100 mg of the as-

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synthesized GO in 100 mL of DI water and placed in a one-neck round

bottom flask. The dispersion was heated up to 90 °C, at which point 700

mg of ascorbic acid (AA) (Sigma Aldrich, UK) dissolved in 5 mL of DI water

were added. The mixture was stirred for 4 h at 300 rpm, and afterwards,

the solid was removed by filtration using qualitative filter paper

(Fisherbrand, 110 mm size, France) and washed with DI water several

times until the filtrate reached a pH of 7. Finally, the material was

thoroughly washed and dispersed in DMF. This rGO sample is labelled as

12.0-rGO, with 12.0 indicating the oxygen content in %. The same

procedure but shorter reaction times (90 and 45 min) were followed to

prepare rGO samples with lower degrees of reduction (13.1-rGO and 15.5-

rGO, respectively). In addition, an even lower degree of reduction was

obtained using an equal weight ratio of GO and AA (sample 21.8-rGO). For

simplification, Table 1 shows the reduction conditions of the rGO samples.

Table 1. Reaction conditions for the reduction of GO. Samples have been labelled based on oxygen content (%).

Sample

Reaction conditions

Temp. (°C) GO (mg) AA (mg) Reaction time (min)

12.0-rGO 90 100 700 240

13.1-rGO 90 100 700 90

15.5-rGO 90 100 700 45

21.8-rGO 90 100 100 45

2.3. Material characterization

The surface composition of the GO and rGO samples was

determined by X-ray photoelectron spectroscopy (XPS) using an Axis Ultra

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spectrometer (Kratos Analytical Limited, Manchester, UK)) with a

monochromatic Al Kα source (1486.7 eV). All the XPS spectra were

analyzed using CasaXPS software. GO and the reduced samples were also

characterized through ultraviolet–visible (UV–vis) spectroscopy using a UV

2700 spectrometer from 500 to 220 nm by dispersing the materials in

water at concentrations of 0.1 mg mL-1. Raman spectroscopy was carried

out in a WITec Apyron Raman Spectroscope using a

laser excitation wavelength of 532 nm in the range 700 - 3500 cm-1.

2.4. Preparation and casting of the graphene/polymer solutions

Casting solutions containing 16 wt% of PVDF in DMF with varying concentrations of

rGO in the range 0.1 - 0.7 wt% (relative to PVDF) were prepared with the most reduced

graphene-like material (i.e. 12.1-rGO). This range of concentrations was selected based on

previous work by Leaper. et al. [19]. The optimum concentration of filler for MD

performance was 0.5 wt% in the prepared membranes, and others containing less reduced

rGO materials (13.1-rGO, 15.5-rGO and 21.8-rGO) and GO were prepared at that optimum

concentration. The rGO/GO was first dispersed in DMF via sonication. Casting solutions of

pure PVDF without graphene-like materials were also prepared to fabricate control

membranes. The detailed compositions of the casting solutions are

presented at Table 2. The casting solutions were mechanically stirred at 40 °C for at

least 5 h and subsequently left overnight without agitation to allow for degassing. The

viscosities of the PVDF and PVDF/rGO dope solutions were measured using an Elcometer

2300 Rotational Viscometer, using a TL7 spindle with a spindle speed of 20 rpm.

The prepared dope solutions were cast into films on atop pieces of non-

woven fabric, using an automatic film applicator (Sheen) with a doctor

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blade set to a gap-height of 200 µm at a coating velocity of 0.05 m s-1.

After casting, the films were left for 10 s before immersing it in deionized

water at room temperature to produce the solidified membrane. They

were left in the coagulation bath for 5 min and were then transferred to

another DI water bath and left for 24 h. PVDF membranes were air dried in

a fume cupboard for 48 h to ensure complete drying and used for

characterization. Membranes were labelled as M_rGO type/yy where yy

represents the wt% of the filler in the prepared membrane, as shown in

Table 2.

2.5. Characterization of PVDF/rGO mixed matrix membranes (MMMs)

The membrane porosity was obtained via a gravimetric method reported elsewhere

[2]. In short, square membrane samples (2 cm × 2 cm) were cut and immersed in ethanol for

30 min, weighing the samples before and after immersion. The membrane porosity value (ε)

was determined by Eq.(1):

ε=(W ¿¿1−W 2)/ ρe

[(W ¿¿1−W 2)/ ρe]+W 2/ ρp¿¿ Eq. (1)

whereW 1 ¿W 2 are the weights (g) of the saturated and the dry membrane, respectively, ρe is

the density (g m-3) of ethanol and ρp is the density (g m-3) of PVDF.

The morphology of the membrane surfaces was examined under SEM (QUANTA FEI

250, USA) at an accelerating voltage of 20 kV. Membrane samples were sputtered with

platinum (5 - 6 nm Pt layer) using an MTM 10 Thickness Monitor (Cressington, USA) prior

to the analysis. For the preparation of cross sections membranes were immersed and fractured

in liquid N2.

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Atomic force microscopy (AFM) was carried out to determine the roughness and

surface topography of the fabricated membranes using a Fastscan microscope (Bruker, USA).

The measurements were conducted using tapping mode at room temperature over an area of

32 μm × 32 μm. The average roughness (Ra) and the root mean square roughness (Rq) were

all calculated using Nanoscope analysis software version 1.5.

The contact angle of the membrane surface, θ, was analyzed using an optical

tensiometer (Attension Theta, Biolin Scientific, UK) via the sessile drop method. The

experiments were conducted at room temperature with the reported values being an average

of three separate measurements.

The bubble point, mean pore size and pore size distribution (PSD) of

the flat sheet membranes were investigated using a Capillary Flow

Porometer (Porolux 1000, Porolux, Belgium) by the wet/dry flow method.

The membrane samples were completely wetted with Porefil 125 (Porolux,

Belgium, surface tension of 15.88 ±0.03 dyne/cm) prior to the analysis.

Pure nitrogen gas was then permitted to gradually flow into the sample

chamber until the N2 pressure became higher than the capillary force of

the fluid within the largest pore. This was identified as the bubble point.

Afterwards, the pressure was further increased measuring the N2

permeation rate until all the pores were emptied and the sample was

considered dry. Then a repeated flow test was carried out with the

membrane in the dry state. The intersection of this curve with the wet

curve allows for the calculation of the minimum pore size whilst the

intersection of the half-dry curve and the wet curve allows for the

calculation of the mean flow pore size (MFP). All reported data were

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recorded, calculated and plotted automatically by the POROLUX TM 1000

software.

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Table 2. Prepared PVDF MMMs containing rGO fillers. This table contains some relevant information about the preparation and characterization of the membranes: amount of solvent and fillers in the casting solutions (all the solutions contained 1 g of PVDF which represents 16 wt% of the whole solution), measured viscosities of the casting solutions, contact angle, bubble point pore size, smallest pore size and roughness parameters of the prepared membranes. Surface roughness parameters of membranes resulted from analyzing AFM images for pure PVDF and membranes containing different loadings of rGO.

Membrane code

Fillertype

DMF (g)

Filler (mg) / wt%*

Viscosity of

casting solution (mPa

s)

Membrane thickness

(µm)

Contact angle, θ

(0)

Bubble point

pore size (µm)

Smallest pore size

(nm)

Roughness

parameters

Ra(nm)

Rq(nm)

M_0 - 5.250 0 1550 18 84 (± 5) 0.18 76 52 67M_12.0-rGO/0.1

12.0-rGO

5.249 1 / 0.1 1650 22 - - - - -M_12.0-rGO/0.3 5.247 3 / 0.3 1710 24 70 (± 6) 0.50 77 40 56M_12.0-rGO/0.5 5.245 5 / 0.5 1840 35 70 (± 3) 0.88 83 48 60M_12.0-rGO/0.7 5.243 7 / 0.7 2250 43 66 (± 2) 0.17 119 59 75M_13.1-rGO/0.5 13.1-rGO 5.245 5 / 0.5 - 24 73 (± 1) - - - -M_15.5-rGO/0.5 15.5-rGO 5.245 5 / 0.5 1930 26 59 (± 2) - 83 - -M_21.8-rGO/0.5 21.8-rGO 5.245 5 / 0.5 1890 22 57 (± 2) 0.34 83 - -

M_GO/0.5 ** GO 5.245 5 / 0.5 2654 24 70 (± 1) 1.13 155 84 106M_APTS-GO/0.5 (APTS-GO) 5.245 5 / 0.5 2482 20 77 (± 1) 0.26 134 50 65

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**

* wt% of filler in the prepared membranes after casting and phase inversion.** Membranes prepared as reported by Leaper et al. [19] for comparison purposes against the original membrane in this work.

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2.6. Membrane distillation (MD)

The air gap membrane distillation (AGMD) setup used for the

evaluation of the membranes performance is presented in and is

described in more detail elsewhere [19]. Membranes were inserted

into the module along with gaskets, a spacer, a perforated support

disk and a condenser plate. The air-gap width was 3 mm and the

membrane area was 28.27 cm2 whereas the effective area, due to the

perforated membrane support was 7.16 cm2. However, to give more realistic values,

the full membrane area was used in the flux calculations. The feed solution,

aqueous NaCl solutions of 35 g L-1, was heated to 80 °C and

recirculated using a pump at a flow rate of 385 ± 5 mL min-1. The

condenser plate was maintained at 20 °C using tap water circulated

by a Julabo F12-ED chiller. To assess reproducibility, three repeats

were carried out for each membrane, with the permeate flux being

recorded each hour up to either 2 or 8 h. In addition, a 96-hour test

was carried out for selected membranes to assess long-term

stability. The permeate volume (J) was calculated using equation (2):

J=V p

A t Eq. (2)

Where, V pis the volume of the permeate collected during a running time t, and Ais the

membrane area. Salt rejection (SR) was calculated using equation Eq.

(3):

SR=(1−Cp

C f)×100 Eq. (3)

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Where, C p and C f are the conductivities of permeate and feed solutions (µS cm-1),

respectively.

Fig. 1. Schematic diagram of the experimental set up for AGMD.

3. Results and discussion

3.1. Characterization of fillers

Raman spectroscopy is an effective way to identify the

structural fingerprint of carbon materials (such as GO and rGO)

since Raman scattering can observe vibrational modes these

systems. In Fig. 2, the Raman spectra of the prepared GO and rGO

with different degrees of reduction are shown. The characteristic G

and D peaks are present for all the samples. It is well-known that

the G peak at ~1600 cm-1 corresponds to the sp2 hybrid structure,

which reflects the symmetry and crystallinity of carbon materials;

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the D peak at 1350 cm-1 accounts for defects in the structure. The

ratio of the intensities for both peaks (ID/IG) can be used as a tool to

evaluate the degree of oxidation/ reduction of the graphene-based nanofillers. The

obtained ratio for GO (ID/IG = 0.96) is similar to that reported in the

literature [23]. For all the reduced samples higher ratios are

obtained, which indicates an increase in the number of sp2 domains

in the GO structure as a consequence of the removal of oxygenated

groups and the restoration of the graphenic lattice. A clear trend is

observed with higher ID/IG values for stronger reduction conditions:

1.23 (12.0-rGO) > 1.21 (13.1-rGO) > 1.20 (15.5-rGO) > 1.10 (21.8-

rGO). In addition, a shift in the G peak from 1,598 cm-1 (for GO, 21.8-

rGO and 15.5-rGO to 1,593 cm-1 in 12.0-rGO and 13.1-rGO reveals

the highest reduction degree present in the latter [23].

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Fig. 2. Raman spectra of GO and rGO fillers.

The variation in the oxygen functionalities of the fillers was

further confirmed by XPS. The obtained carbon to oxygen ratios

(C/O) and oxygen content (%) of GO and the reduced GO samples

are shown in the inset table in Fig. 3 and reveal that, as expected,

the atomic percentage of oxygen decreases upon the reduction of

GO. The C/O ratio increases with the amount of reducing agent AA

and the time of reaction; 2.3 for GO, 3.58 for 21.8-rGO, 5.45 for

15.5-rGO, 6.65 for 13.1-rGO and 7.36 for 12.0-rGO. A decrease in

the oxygen content of 36% is obtained for the reduced sample using

mild conditions (21.8-rGO: 100 mg AA, 45 min reaction time). For

samples 15.5-rGO, 13.1-rGO and 12.0-rGO the decrease in oxygen

rises up to 58, 65 and 69%, respectively, which confirms that most

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of the oxygen functionalities have been removed. The conformal

changes of oxygen functionalities for different reduction conditions

can be assessed by the deconvolution of the C1s core level spectra

of GO and rGO samples (Fig. 3) [25]. The spectrum of GO shows four

peaks located at 284.5, 284.8, 286.9 and 288.6 eV corresponding to

sp2/sp3 carbon (C=C and C-C), C-O bond in epoxy/alkoxy/hydroxyl

groups, carbonyl (C=O) and carboxyl (O-C=O) groups, respectively

[26, 27]. There is a clear decrease in C-O and C=O peaks as the

amount of AA and the time of reaction increases, in good agreement

with what is reported in the literature [26, 28]. It is worth noting that

a peak for C-C and another one for C=C are proposed for GO and

21.8-rGO. However, when a higher degree of reduction is present, it

was considered for simplicity that most of the groups are C=C.

Therefore, only one peak (asymmetric due to the presence of

delocalized electrons) was proposed, although rGO is always

defective and some C-C domains are still present. A tiny peak

assigned to π→π* satellite is also included for 12.0-rGO and 15.5-

rGO due to the presence of delocalized electrons. C/O ratios were

obtained from C 1s and O 1s (not included) in the high resolution

spectra.

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GO 12.0-rGO 13.1-rGO 15.5-rGO 21.8-rGO

C/O ratio: 2.30 7.36 6.65 5.45 3.58

Oxygen content (%): 30.3 12.0 13.1 15.5 21.8

Fig. 3. XPS spectra of GO and rGO fillers. The C/O ratios and oxygen content in each of the fillers are shown in the table at the top of the figure.

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The reduction degree was also analyzed by UV–vis

spectroscopy as depicted in Fig. 4. At 230 nm a maximum peak

corresponding to π→π* transitions from C=C double bonds is

observed. In addition, at 300 nm a shoulder peak related to n→π*

transitions from C=O [23] is also present. With the increase of

reduction time as well as AA loading, the peak at 230 nm is shifted

to higher wavelengths (i.e. 261 nm for 21.8-rGO, 269 nm for 15.5-

rGO and 270 nm for 13.1-rGO and 12.1-rGO) due to the partial

recovery of the aromatic structure. Moreover, the shoulder peak at

300 nm disappears, which confirms effective reduction of the

oxygen-functionalities, respectively [24].

Fig. 4. UV-Vis absorption spectra of GO and rGO samples.

3.2. Characterization of the fabricated mixed matrix

membranes(MMMs)

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Rheological measurements of the casting solutions were

performed in order to get better insights of the kinetics of the phase

inversion process and therefore how membrane were formed [29].

The results are shown in Table 2. As expected, the viscosity increases with

increasing loadings of rGO; from 1550 mPa s for the bare PVDF

solution (without fillers) to 2250 mPa s when 0.7 wt% of rGO is

added (both solutions containing 16 wt% of polymer). This

significant viscosity increase may be attributed to greater steric

interactions between the nanofiller and the polymer chains. The

implications of the increased viscosity on the membrane

morphology are discussed in the coming sections.

The surface morphology was analyzed under SEM and sample

micrographs are shown in Fig. 5. It can be seen that the addition of

12.1-rGO in the PVDF matrices leads to an increase in the number of

surface pores of varied sizes, being indeed obvious for membranes

containing 0.3 and 0.5 wt%. This can be explained by the increased

affinity towards the non-solvent (water) when the graphene-like

nanosheets are added on the casting dope solutions, thus increasing

the rate of de-mixing. As the percent of 12.1-rGO raises up to 0.7 wt

%, the number of pores decreases dramatically. This is likely due to

the higher viscosity of the resultant casting solution (see Table 2).

For 0.7 wt%, the increase in rGO loading produces much more

viscous casting solutions, which hinders the solution de-mixing

process due to increased kinetic hindrance, resulting in lower pore

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size and membrane porosity. This observation is in agreement with

other reports for similar systems [30].

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Fig. 5. SEM surface images of pure PVDF: (a) M_0, and PVDF membranes containing

increasing amounts of 12.1-rGO: (b) M_12.1-rGO/0.3, (c) M_12.1-rGO/0.5 and (d) M_12.1-

rGO/0.7. Scale bars in all the micrographs represent 2 µm.

During the non-solvent induced phase separation (NIPS) method,

different structures can be formed depending on the system

composition (i.e. polymer, solvent, non-solvent and additives):

closed cell foam, an open cell foam, a lacy or net structure, a

granular or packed sphere structure and an unconnected powder

[31]. As shown in Fig. 6, the addition of rGO nanosheets alters the

cross-section microstructure of the fabricated PVDF/rGO

membranes. The macrovoid structures seem to become wider and

longer after adding a loading of 0.5 wt% of 12.1-rGO into the PVDF

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a) b)

c) d)

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membranes. The reason for this might be the presence of residual

hydrophilic groups on the rGO nanosheets that cause a faster rate of

in-diffusion by the non-solvent (water) during the phase separation

process. This is further evidenced by the presence of more

elongated voids in the structure of membranes containing more

oxidized rGO (15.5-rGO and 21.8-rGO) specimens. Consequently,

rapid solidification of the membrane occurred and led to formation

of larger pore channels [32]. However, a further increase in the

loading of 12.1-rGO of 0.7 wt% increased the solution viscosity and

this, as explained earlier, resulted in a delay in the exchange rate

between the solvent and non-solvent during the NIPS process and a

more sponge-like structure formed.

Fig. 6. SEM images of cross sections of PVDF and PVDF/rGO membranes, (a) M_0 (pure PVDF), (b) M_12.1-rGO/0.5, (c)

M_12.1-rGO/0.7, (d) M_15.5-rGO/0.5 and (e) M_15.5-rGO/0.7. The scale bars in the micrographs correspond to 20 µm.

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In general, membrane porosity refers to the void volume

fraction of the membrane; the higher the porosity, the higher the

permeate flux and lower the conductive heat loss [33]. However, the

porosity is limited by the mechanical strength requirement of the

membranes and there must be a compromise between high porosity

and mechanical stability. The membrane porosities for MD typically

lie in the range 30 - 85% as reported by El-Bourawi et al. [1] and

indeed, the values shown in this work lie in this range. Porosity

values for the fabricated MMMs (plotted in Fig. 7) show an increase along with

the increase in 12.1-rGO loading from 0.1 up to 0.5 wt% (M_12.1-

rGO/0.1, M_12.1-rGO/0.3, and M_12.1-rGO/0.5). The highest porosity value

of 70.1% was obtained for the membrane with a 0.5 wt% rGO

loading, which corresponds to the best pore interconnectivity

among the fabricated membranes, as also suggested by the SEM

images shown in Fig. 6. It is worth noting that the PVDF membrane

without filler, M_0, possesses a porosity of just 54 %. The increase in

porosity for the MMMs may be attributed to the increased

thermodynamic instability as a result of the introduction of rGO in

the casting solution. As previously explained, rGO has terminal -OH

and -COOH groups that provide some affinity for the non-solvent

molecules (i.e. water) and therefore, the rate of de-mixing is

enhanced leading to greater porosity, being this effect also reported

in the literature for other hydrophilic additives [34, 35]. However, at

a higher loading of 0.7 wt% (membrane M_12.1-rGO/0.7) the porosity

dropped down to 56.6 %, which may be attributed to the increased

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viscosity of the casting solution as discussed earlier, and is

consistent with the measured flux values that are presented in the

coming sections.

Fig. 7. Porosity data of the fabricated membranes (SD <5%). Average of three membrane pieces for each membrane type.

For MD applications, in addition to high porosities, membranes

with narrow pore size distributions (PSD) are preferred, and should

be centered on an optimum pore size. This has to be small enough

to prevent any liquid penetration inside the membrane, but large

enough to allow for efficient vapor transport [36]. To measure this,

capillary flow porometry was conducted on each membrane type

using the wet/dry flow method. Fig. 8a depicts the pore size

distribution of all the fabricated mixed matrix flat-sheet membranes

with distinct peaks at the mean pore size values). The peak

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percentage flow value for the pure PVDF sample was 67%, which

suggests a highly homogeneous membrane with a narrow PSD. After

adding rGO to the PVDF, the pore size distribution (PSD) %

increased till 85% for the membrane containing 0.3 wt% 12.1-rGO

(M_12.1-rGO/0.3). It is worth noting that slight heterogeneities in the

membranes are likely created by restacking or partial

agglomeration of the rGO flakes in the casting solutions. However,

the high viscosity in case of 0.7 wt% rGO may be the reason of

restored narrow PSD. Nonetheless, the fact that single peaks at the

mean pore size value dominate all curves is a promising

characteristic of these membranes and is reflected in the reported

increased flux and high rejections.

Fig. 8. a) Pores size distribution (PSD) data of fabricated membranes containing 0.1,

0.3, 0.5 and 0.7 wt% 12.1-rGO in the casting solution and a pure PVDF membrane.b)

Mean pore size vs. concentration of 12.1-rGO in the PVDF membranes.

Capillary flow porometry was also used to measure the mean

flow pore size (MFP) values for membranes containing different

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concentrations of 12.1-rGO in the range 0 - 0.7 wt% (values of MPD

vs rGO loadings are plotted in Fig. 8b). As the rGO loading

increased, so too did the MFP from a value of 0.09 µm for pure PVDF

to 0.11 µm, 0.10 µm and 0.17 µm for the 0.1, 0.3 and 0.5 wt%

loadings, respectively, being again the hydrophilic nature of rGO

responsible for this increase. However, at a loading of 0.7 wt% the

MFP fell slightly down to 0.13 µm, which suggests again that the de-

mixing rate is partially lowered due to the increased viscosity and

henceforth a slightly lower MFP value is obtained.

For AGMD applications, the membrane is a contactor between

two phases (i.e. liquid and vapor) formed by at least one

hydrophobic layer that is not wetted by the feed liquid. This will

ensure that salts (which are non-volatile solutes) are nearly 100%

retained. To assess the degree of hydrophobicity, water contact

angle (θ) values for the membranes were measured and are

presented in Table 2. As can be seen, the addition of rGO reduced θ from 84 to 66° in the range 0 – 0.7 wt% loading. This can be

attributed to a combination of the reduced surface roughness

(values also shown in Table 2 and AFM images presented in Figure

S2) and the interactions between the water and the remaining

oxygenated groups on the rGO flakes. However the potentiality of a

membrane to be wetted, does not only depend on its hydrophobic

nature; other properties such as the maximum pore size and shape

geometry, and the surface tension of the wetting liquid contribute to

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this phenomenon. In order to account for this, a parameter called

liquid entry pressure (LEP), defined as the minimum pressure

required for liquid water to penetrate the pores of the membrane

[10], can be estimated using the Cantor-Laplace equation However,

given the difficulty in obtaining accurate values of the physical

parameters involved in the equation (i.e. maximum pore size), LEP

calculations do not often match experimentally obtained pressure

values. In addition LEP only gives information on the first event of

total liquid intrusion in the biggest pores and neglects other known

wetting forms.

Membrane thickness data is shown in Table 2. The membrane

thickness increased significantly with the addition of rGO

nanosheets. The pure PVDF membrane (M_0) had a thickness of 18

µm, whereas that of the membrane containing 0.7 wt% 12.1-rGO

was 43 µm (~2.4 times thicker). There are possibly two factors at

play. The first is the increase in the viscosity of the casting solution,

which inhibits the rapid penetration of the polymer solution into the

pores of the non-woven support, and thus, results in thicker films

prior to coagulation. The second factor is to do with the coagulation

process itself. As described previously, the higher rGO loadings

result in faster de-mixing of the solvent and non-solvent, giving rise

to higher porosity and larger pores. The growth of these pores

reduces the densification of the polymer as it solidifies and so the

membranes with higher porosity, cast from the same amount of

polymer solution, should have slightly higher thickness. The

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dominant factor here is the reduced penetration of the casting

solution containing 0.7 wt% 12.1-rGO into the NWS. The obtained

membrane thicknesses are in line with the values that are reported

in the literature (i.e. 10-400 µm) [37-39] although the relationship

between thickness and performance for AGMD is different than for

DCMD. For DCMD it is generally stated that the flux decreases with

increasing membrane thickness as a consequence of the increase

mass transfer resistance [37, 38, 40]. However, at lower

thicknesses, more conductive heat loss occurs, reducing the driving

force and resulting in lower fluxes. Therefore, there is a trade-off

where the optimum thickness will depend on process conditions,

membrane properties and feed salinity. Conversely, El-Bourawi et al.

and Lagana et al, reported that the effect of membrane thickness in

AGMD can be neglected, since the stagnant air gap, which

represents the predominant resistance to mass transfer, can be 2-3

orders of magnitude thicker than the membrane [1, 41]. In order to

confirm this hypothesis, two more pure PVDF membranes (without

fillers) were prepared at different casting knife heights (50 µm and

100 µm). The obtained flux values for these two membranes are

shown in Table S1 in the SI. In addition, a graph showing flux and

membrane thickness is provided in the SI (Figure S1), suggesting

that flux does not depend significantly on the membrane thickness

in AGMD configuration.

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3.3. Membrane distillation performance

Membrane distillation (MD) utilizes a hydrophobic microporous

membrane where vapors are able to pass through while the liquid

feed, containing dissolved salts, is retained. This makes MD a

suitable technique for separation of salts from saline water such as

seawater [3]. The desalination performance of the fabricated PVDF

membranes was assessed using the AGMD configuration with a feed

concentration and temperature of 35,000 mg L-1 and 80 °C,

respectively. The water flux and salt rejection values obtained over

a period of 2 h are shown in Fig. 9, where values correspond to the

pure PVDF membrane (M_0) and rGO-modified PVDF membranes. As

shown, membrane M_0 exhibits the lowest flux of 2.6 LMH but

higher fluxes of 2.8 and 3.9 LMH are obtained for the composite

PVDF membranes with 0.1 and 0.5 wt% 12.1-rGO, respectively.

However, at a loading of 0.7 wt%, this drops down again to 2.6 LMH,

which correlates well to the changes in porosity of these

membranes, as described earlier (see Fig. 7).

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Fig. 9. Flux and rejection data for the fabricated membranes. Membranes containing GO and

APTS-functionalized GO (M_GO0/0.5 and M_APTS-GO/0.5) were prepared as reported by Leaper et al. elsewhere [19]. Three membranes of each type

were tested, and the error bars correspond to the standard deviation.

Besides this, the rejection of NaCl is almost 100% for all

prepared membranes. In order to make a better comparison

between the membranes’s permeate quality, conductivity values

were tabulated, as seen in Table 3. The addition of 0.1 wt% 12.1-

rGO resulted in a lower permeate conductivity than the pure PVDF

(38 ± 9 instead of 57 ± 8 µS cm-1) but this slightly increased with

the rGO content to a value of 79 ± 13 µS cm-1 for the maximum

loading of 0.7 wt%. This is a good indication that rGO in small

concentrations imparts anti-wetting properties to the membranes,

despite the increased mean pore size, bubble point size and lower

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contact angle. For comparison, PVDF/GO and PVDF/APTS-GO

membranes with 0.5 wt% loading of these nanofillers have been

prepared and tested. It was found that the same loading of GO gave

a permeate conductivity value of up to 643 µS cm-1 while APTS-GO

gave comparable value to rGO. These data are very similar to the

work done previously by Leaper et al. [19]. This can be attributed to

the larger amount of oxygen functionalities of GO rather than rGO or

GO-APTS, allowing for stranger interactions with water molecules,

and therefore crating a greater propensity for wetting.

Table 3. Permeate conductivity of all tested membranes. Feed conductivity was (59 ± 4) ×103 µS cm-1.

Membrane Permeate conductivity (µS cm-1)

M_0 57 (± 8)

M_12.0-rGO/0.1 38 (± 9)

M_12.0-rGO/0.3 51 (± 8)

M_12.0-rGO/0.5 72 (± 9)

M_12.0-rGO/0.7 79 (± 13)

M_13.1-rGO/0.5 52 (± 12)

M_15.5-rGO/0.5 41 (± 4)

M_21.8-rGO/0.5 26 (± 5)

M_GO/0.5 643 (± 179)

M_APTS-GO/0.5 65 (± 15)

From this work, it is clear that the degree of graphene oxide reduction has a

profound effect on the membrane properties. To show this, flux and permeate

conductivity values were plotted as a function of the carbon to oxygen (C/O) ratios of

the nanofillers, as depicted in Fig. 10. As the C/O ratio increases from 2.3 (for GO) to

5.45 (for 15.5-rGO), the flux increases from 6.4 to 7.0 LMH. In addition, the

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permeate conductivity drops dramatically from 643 to 41 µS cm-1, implying less

pore wetting occurred for the membranes prepared with more reduced GO. The best

performing membrane in terms of flux and salt rejection was prepared by using 0.5

wt.% 15.5-rGO, which had an intermediate degree of reduction between GO and 12.1-

rGO. However, as the C/O ratio increased to a value of 7.36, the flux decreased

dramatically to 3.9 LMH. This suggests that there is an optimum degree of reduction

with a C/O ratio of about 5.45.

As discussed earlier, the addition of these 2D nanofillers simultaneously

affects both the thermodynamic and kinetic characteristics of the phase separation

process. Furthermore, the mass of the flakes will be lower for the more reduced

materials, due to the removal of oxygen groups. This means that for the same wt%,

there will be a higher number of flakes for the more reduced rGO materials. This will

have an additional effect on the thermodynamic/kinetic trade-off. In fact, each

nanomaterial variant will have its own subtle effects on the phase inversion process,

resulting in morphological differences in the final polymer.

Fig. 10. Flux (a) and permeate conductivity (b) values of membranes containing

nanofiller loadings of 0.5 wt% as a function of nanofiller carbon to oxygen ratios.

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Whilst morphological differences (such as porosity, pore size etc.)

play a large role in affecting the performance of the membranes,

certain results reported here suggest that other factors may be

having an effect also. For example, the best performing membrane,

M-15.5-rGO/0.5, achieved nearly double the flux of the

M-12.0-rGO/0.5 membrane, despite having 10% lower porosity.

Various suggestions have been made in the literature to account for

the improvement in flux for membranes containing GO-like

materials [17]. Graphene consists of a 2D sheet of sp2-bonded

carbon atoms which provide effective sorption sites for water vapor

but repels liquid water molecules [42]. In addition, the presence of

remaining polar functional groups on rGO (i.e. hydroxyl and

carboxyl groups) can favorably alter the membrane-water vapor

interaction, thus increasing the overall vapor permeation rate [42].

The MMMs prepared with other rGO samples of different degrees of

reduction (15.5-rGO and 21.8-rGO) presented enhanced flux,

reaching up to 7.0 and 6.7 LMH for membranes M_15.5-rGO/0.5 and

M_21.8-rGO/0.5, respectively. It is worth noting that the same wt%

of either GO or GO-APTS have lower flux than PVDF/rGO

membranes, which suggests that there is an optimum degree of

reduction for GO nanosheets at about a C/O ratio of 5.45.

Furthermore, the addition of such fillers has not compromised the

high salt rejection (SR) obtained for pure PVDF membranes.

These results are in line with those in previous studies with

oxidized carbon nanotubes and nanodiamonds [16, 17]. The 35

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creation of nanopores in PVDF/rGO binary system and the presence

of the hydroxylated and carboxylated carbon atoms adjacent to the

pores [43] also aid in increasing the flux through selective sieving of

pure water with respect to salt water clusters [44]. In addition,

nanoscale wrinkles and structural defects are known to form due to

the distribution of functional groups on the rGO sheets, providing

additional passages for water transport and reducing the

temperature polarization without augmenting conductive heat loss

[44].

Finally, to assess the long-term performance stability, plain

PVDF and selected membranes with the optimum 0.5 wt% of rGO

concentration (M_12.0-rGO/0.5, M_15.5-rGO/0.5 and M_21.8

-rGO/0.5) were tested for 8 h. In addition, the best-performing

membranes containing 0.5 wt% of an rGO filler with a reduction

degree of 58% (M_15.5-rGO/0.5) and the pure PVDF membrane

(M_0) were tested for a longer period of 96 h. Results are shown in

Figs. 11a and 11b for the 8 and the 96 h tests, respectively, and

suggest a good stability of the membranes under the testing

conditions, without reduction in salt rejection.

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Fig. 11. Flux data as a function of time for a) PVDF (squares) and selected

PVDF/rGO membrane containing 0.5 wt% of rGO: 12.0-rGO (circles), 15.5-rGO (up

triangles) and 21.8-rGO (down triangles) for 8-hour testing, and b) PVDF and best

performing membrane M_15.5-rGO/0.5 for 96-hour testing.

3.4. Performance comparison with other reported MD membranes

The performance of commercially available MD membranes (PTFE)

and other reported PVDF membranes, plain or containing graphene-

based nanofillers (e.g. graphene, graphene-oxide (GO), APTS-GO

and CNTs), is presented in Table 4. Best permeate fluxes in this work for

the MMMs lie in the range of 4-7 LMH as seen in previous sections, and are

generally higher than the best fluxes shown in Table 4 for other

works [19, 45-51], which suggests the potential of the prepared

PVDF membranes incorporated with optimum reduction degree rGO.

Another conclusion that can be drawn from the literature survey is that in

general, the addition of graphene-like nanofillers improves the

separation properties of the bare polymer membranes. It is worth

noting that there are also a few values of flux that are higher than the ones found in

this work. For instance, membranes prepared by Woo et al. [10], Silva et al.

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[52] and Bhadra et al. [17], possess extraordinarily high values of

water flux of about 20.5, 34.2 and 83 LMH, respectively. Our lower

values (along with most of the other values in Table 4) can be attributed to the use of

the total membrane area (rather than effective membrane area) to calculate flux, as

described in the experimental section, and other different experimental parameters

such as differences in feed compositions or feed flow rates.

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Table 4. Literature survey of some commercially available MD membranes and others developed by research groups that have been reported in the literature.

Material MD config.

Module config.

Feed solution Flux (LMH)

Ref

PVDF AGMD Hollow fibre

7 g L-1 NaCl 3.5-6.5 [45]

PVDF coated with nanofibers

VMD Flat sheet 35 g L-1 NaCl 3-3.2 [46]

PVDF-HFP AGMD Flat sheet 35 g L-1 NaCl ~0.5-1.8 [47]

PVDF nanofibers AGMD Flat sheet 3.5 wt % NaCl 4.2 [48]

PVDF (0.22 µm) AGMD Flat sheet, 4-12 wt % NaCl 4.2-4.8 [49]

PVDF (0.45 µm) AGMD Flat sheet 1.5-4.5 wt % NaCl 1-3.5 [50]

PVDF DCMD Flat sheet 3 wt% NaCl 1 [51]

PVDF-GO AGMD Flat sheet 3.5 wt% NaCl 5 [19]

PVDF-APTS/GO AGMD Flat sheet 3.5 wt% NaCl 6.2 [19]

PVDF/Graphene MMMs AGMD Flat sheet Produced water 20.5 [10]

PVDF/MWCNT MMMs DCMD Flat sheet 35 g L-1 NaCl 34.2 [52]

PVDF-clay nanocomposite electrospun membranes

DCMD Flat sheet 3.5 wt% NaCl 5.7 [14]

PVDF-Cloisite 15 A® composite membranes

DCMD Hollow fibre

Dye solutions 7 [53]

GO-immobilised PTFE membrane

DCMD Flat sheet 3.4-34 g L-1 NaCl 83 [17]

Commercial PTFE AGMD Flat sheet Produced water 1-7 [54]

Commercial PTFE TF200 AGMD Flat sheet 46.4 g L-1 NaCl ~ 10.8 [55]

PVDF/0.5wt.% 15.5-rGO AGMD Flat sheet 3.5 wt% NaCl 7 This work

PVDF/0.5 wt% 21.8 rGO AGMD Flat sheet 3.5 wt% NaCl 6.7 This work

PVDF- HFP: poly(vinylidene fluoride-co-hexafluropropylene)VMD: Vacuum membrane distillationMWCNT: Multiwalled carbon nanotube

39

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622623624

4. Conclusions

Returning to the hypothesis posed at the beginning of this

study, it is now possible to state that the addition of reduced GO,

prepared using environmentally friendly ascorbic acid, into PVDF

mixed matrix membranes significantly enhances the membrane

distillation performance. This enhancement can be explained by

differences in morphology (as evidenced by SEM, AFM, and pore-size

data analyses) as well as water vapor interactions with the rGO

nanosheets. The degree of GO reduction, as defined by the carbon-

oxygen atomic ratio, was also found to be a significant factor in

determining membrane performance. The intermediate carbon-

oxygen atomic ratio of 5.45 (oxygen content of 15.5%) produced the

best performing membrane. One of the more significant findings

from this study is that membranes containing just 0.5 wt% rGO with

such an oxygen content (corresponding to a degree of reduction of

58%) show an increase in water flux by a factor of 2.4 compared to

the pure PVDF membrane, reaching a value of 7.0 LMH. This was

achieved without reductions in the product water conductivity value.

Another major finding in this work is the good performance stability

of this best-performing membrane under a longer testing time of 96

hours. The current findings add to a growing body of literature on

blending nanofillers like rGO in PVDF matrix for MD applications. If

the debate is to be moved forward, a better understanding of

fouling issues in MD needs to be developed.

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Acknowledgements

The authors would like to acknowledge EPSRC for funding this work (grant number

EP/K016946/1). A. Abdel-Karim would like to thank the Ministry of Higher Education of

Egypt and the Newton-Mosharafa Fund.

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