1

Fouling and cleaning of polymer-entwined graphene oxide

nanocomposite membrane for organic and inorganic wastewater

treatment by forward osmosis

Seungju Kima, Ranwen Oua, Yaoxin Hua, Huacheng Zhanga, George P. Simonb,

Hongjuan Houc, Huanting Wang*a

aDepartment of Chemical Engineering, Monash University, VIC 3800, Australia.

bDepartment of Materials Science and Engineering, Monash University, VIC 3800,

Australia.

cEnergy and Environment Research Institute, Baosteel Group Corporation, Shanghai,

201999, China

2

Abstract

In this study, we have characterised membrane fouling of polymer-entwined graphene

oxide membranes with a simulated feed solution containing organic foulants for

wastewater treatment application, as well as with a simulated inorganic wastewater with

high iron and silicon concentrations relevant to steel industry wastewater reclamation.

Membrane cleaning processes by cross-flow surface flushing with water were then

applied to demonstrate water flux recovery for long term application. Salt rejection

property was retained constant during fouling process, whereas water flux was found

to be reduced continuously due to fouling, but was readily recoverable following

surface flushing.

Key words: membrane fouling; forward osmosis; graphene membrane; organic

wastewater; steel wastewater; desalination

3

Introduction

Membrane-based water purification technology has received extensive attention

in both academia and industry due to its advantages in easy processing and low energy

consumption as compared with other separation technologies. In particular, pressure-

driven membrane processes such as reverse osmosis (RO) and nanofiltration (NF) are

widely used in various water treatment applications.[1-3] However, in recent years,

osmotically-driven membrane processes such as forward osmosis (FO) and pressure

retarded osmosis (PRO) have been studied as a potential alternative to RO processes

since these processes could avoid using hydraulic pressure, but rather use osmotic

pressure as the driving force for water transport through a membrane which could be

beneficial to operational cost.[4] FO processes draw water through a semi-permeable

membrane from the feed solution using a concentrated draw solution for high osmotic

pressure, then produce clean water from the diluted draw solution by regenerating the

draw solute.[5] Osmotically-driven membrane processes can potentially be applied in

the field of wastewater treatment, saline water desalination, and even in energy

production as water flow by osmotic pressure gradient releases the free energy to

generate electricity.[6]

Since the environmental sustainability is a primary concern of environmental

technology, water footprint assessment in wastewater treatment has been introduced

and developed to evaluate the environmental effects of industrial processes.[7-9] This

has led to more regulation and investigation in wastewater treatment to lower

environmental impacts from domestic sewages and various industries such as steel

production.[9-11] The volume of organic sewages has increased with population growth,

and includes substances such as compounds with biocidal properties, food additives,

organic compounds or detergents. Steel manufacturing is also regarded as the major

4

source of industrial wastewater where a great deal of water was consumed mainly for

cooling facilities and processes such as wet gas cleaning and sanitary applications

during steel production departments in iron-making, steel-making, hot-rolling, cold-

rolling and steel pipe manufacture.[7-9, 12] Wastewater in steel industry includes a low

concentration of organic compounds and ammonia nitrogen, but mostly contains high

conductivity from iron and salts, which seriously effects on the environment, as well as

damaging production equipment. Various treatments such as electrodeposition,

adsorption, and membrane separation have been introduced to reclaim wastewater,[12,

13] and membrane processes have been regarded as suitable wastewater treatment

technologies as their energy demand is low. In the case of FO process, it uses osmotic

pressure difference as a driving force, which is advantageous in terms of high water

flux and low membrane fouling.[4]

Membrane fouling is a phenomenon in which the membrane performance

declines due to the continuous deposition of impurities on a membrane surface. This

can be a major drawback of membrane technology to be applied for industrial

applications.[14-17] Although membrane fouling in FO process is not as severe as in RO

process, membrane fouling remains an important consideration when introducing new

membranes into real industrial applications.[18-21] There are various types of fouling

such as colloidal and organic fouling, inorganic fouling, and biofouling, which have

different effects on membrane performance decline because of physical properties of

the foulants. Moreover, in the steel industry, the wastewater mainly contains acidic and

metallic compounds which can damage membranes and bring long-term instability.[9,

12] Most studies on membrane fouling in FO process have been focused on polyamide-

based membranes,[5, 19] but not many studies on newly-synthesised membranes have

been reported. However, it is necessary to characterise new FO membranes, especially

5

membranes with high water flux, because their different water transport and salt

rejection properties would be affected by membrane fouling.

In this study, we report on membrane fouling and water flux recovery

behaviours of our newly developed graphene oxide and polymer (GO-polymer)

composite membranes for wastewater treatment applications. GO-based membranes

have attracted increasing interest in membrane processes since they show remarkable

water transport properties based on hydrophilicity of GO, but membrane fabrication is

an important issue for practical membrane processes.[22] Incorporating GO nanosheets

with hydrophilic and crosslinked polymers is an effective strategy for e large-scale

membrane fabrication of GO-base membranes. The newly developed GO-polymer

membrane demonstrated remarkable water transport and salt rejection properties in FO

process which showed great potential to significantly reduce the operating cost of the

membrane process.[23] The use of GO in GO-polymer composite membranes will

improve their water transport and salt rejection as well as anti-fouling property which

will greatly benefit the water treatment process.[24-29] Two organic model foulants;

sodium alginate (SA) and bovine serum albumin (BSA) were used to understand

membrane fouling in general wastewater reclamation and the simulated steel

wastewater containing silicon dioxide (SiO2) and iron(III) ions was used to characterise

fouling behaviour in wastewater treatment in the steel industry.

Materials and methods

Fabrication of forward osmosis membrane

GO-polymer membranes in this study were fabricated using our previously

reported method.[23] GO-polymer membranes were prepared as a thin film composite

6

type, supported on a porous membrane substrate. Free radical polymerization of

monomers with an initiator was carried out to entwine GO with polymer chains and to

form a GO-polymer network. First, graphene oxide solution was prepared by the

modified Hummers method from graphite powder, and then the resulting aqueous GO

suspension with a concentration of 3.8 mg/ml was obtained.[22, 30] Monomers used to

fabricate GO-polymer membranes include N-isopropylacrylamide (NIPAM), N,N’-

methylenebisacrylamide (MBA), and ammonium persulfate (APS) purchased from

Sigma-Aldrich (St Louis, MO), and they were used without further purification. More

specifically, 38 mg of NIPAM, 38 mg of MBA, and 7 mg of APS were dissolved into

10 ml GO suspension diluted into 0.34 mg/ml. The solution composed of GO and

monomers was sonicated for 30 min for complete dissolution. Nylon membrane

substrates with a diameter of 47 mm and an average pore size of 450 nm was used as

the support, which was purchased from Sterlitech Corporation (product number

NY4547100, Kent, WA). Nylon substrate were floated on 10 ml of precursor solution

in a glass petri dish for 30 seconds for enough solution loading, set on a spin-coater

(WS-650-23B, Laurell Technologies Corporation, PA), and spun for 30 seconds at

1,000 rpm, then put in a convection oven (Thermal Fisher) at 70 °C for 2 h for complete

free-radical polymerisation and drying.[31-33] Coating and polymerising procedures

were repeated for 3 times to fabricate a defect-free GO-polymer composite layer on the

Nylon substrate.

The surface and cross-sectional morphology of GO-polymer membranes before

and after fouling experiments was observed by a field emission scanning electron

microscope (FE-SEM) with energy dispersive X-ray spectroscopy (EDX), (Nova

NanoSEM 450, FEI, Hillsboro, OR). Hydrophilicity of membranes was characterised

by measuring the contact angle of membrane surfaces using a contact angle goniometer

7

(OCA15, Dataphysics, Filderstadt, Germany) with five membrane samples. The

chemical structure was analysed by Attenuated total reflectance Fourier transform

infrared spectroscopy (ATR-FTIR, Spectrum 100, Perkin Elmer, Waltham, MA).

Determination of forward osmosis membrane structure parameters

The membrane structure parameter S represents the average distance a solute

molecule must transport through the supporting layer, which is defined from the support

layer resistance to solute diffusion, K, and the diffusion coefficient of the draw solute,

D,[20, 34, 35]:

𝑆 = 𝐾𝐷 = (𝐷

𝐽𝑤) ln

𝐵 + 𝐴𝜋𝐷,𝑏

𝐵 + 𝐽𝑤 (1)

where Jw is the measured water flux, A is the intrinsic water permeability (A= Jw /P)

measured at different osmotic pressures, B is the solute permeability coefficient, D,b is

the bulk osmotic pressure of the draw solution (47.3 bar for 1 M NaCl solution),[34] and

F,m is the osmotic pressure of the feed solution (0 atm for DI feed). The diffusion

coefficient of NaCl in water was assumed constant at 1.61 10-9 m2/s.[36] The salt

permeability coefficient, B is calculated from the measured salt flux, Js and the draw

solute concentration, cD[34]:

𝐵 =𝐽𝑠

𝑐𝐷𝑒−𝐽𝑤/𝑘 (2)

where k is the mass transfer coefficient calculated for a rectangular cell.[37] All

parameters were determined with the 1 M NaCl draw solution.

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Foulants

Sodium alginate (SA) extracted from brown algae with a molecular weight of

12 to 80 kDa and bovine serum albumin (BSA) with a molecular weight of 66 kDa were

obtained from Sigma-Aldrich (St Louis, MO) and used as model organic foulants

without further purification. SA and BSA were dissolved in deionised (DI) water under

magnetic stirring for 12 h to obtain homogeneous solutions with a concentration of 200

mg/L stored in a fridge before use.[5] Simulated steel wastewater consisted of FeCl3

(5.79 mg/L), SiO2 (14.56 mg/L), and NaCl (adjusted conductivity to be 1.26 mS/cm).[12]

These chemicals were purchased from Sigma-Aldrich and used without further

purification.

Membrane fouling and cleaning experiments

A commercial cellulose triacetate (CTA) FO membrane was purchased from

Hydration Technology Innovations (HTI) (Albany, OR) to compare membrane

properties with GO-polymer membranes. Water flux and reverse salt flux of

membranes were measured at room temperature by a laboratory scale FO testing system,

which was assembled with components (Natural acetal copolymer (Delrin) Cell

#CF042D-FO and FO Carboy Tank Pump #1220189) purchased from Sterlitech (Kent,

WA). For membrane fouling tests, a membrane was mounted into the cell with FO

mode where the membrane selective layer faces the feed side. When the FO mode is

applied on FO process, it can lead to less fouling problems than PRO mode where the

membrane selective layer faces draw solution side.[38] A membrane coupon with an

effective area of 4 cm2 was loaded into the cell, 3 L of feed and draw solutions were

employed, respectively, and solution cross-flow rates were both set at 500 ml/min.

9

2,000 ppm NaCl aqueous solution and simulated wastewater solutions were used as

feed solution, and a NaCl aqueous solution with a concentration of 1.0 M was used as

a draw solution. Note that SA and BSA solutions have similar, but slightly higher

osmotic pressure with high conductivity than 2,000 ppm NaCl solution, as summarised

in Table 1. Both weights of the feed and draw solutions were continuously measured

and recorded using two digital balances connected to a computer to calculate water flux

from the weight changes of the feed and draw solutions. The electrical conductivity

meter was used to determine salt concentrations by measuring the conductivity of the

feed and draw solutions and to calculate salt rejection and reverse salt flux. Three

samples of each membrane were measured to minimise experimental errors.

For the cleaning procedures, one side of the cell to the feed was connected by

three way valves to be switchable to the simulated wastewater solution and the cleaning

solution. After 3 days of FO process operation to ensure fouling, the foulant on the

membrane surface was then removed by circulating 2 L of DI water as a cleaning

solution at high cross-flow rate at 1,200 ml/min for 1 h, then FO process operation was

resumed with the same feed solution. A series of membrane fouling and cleaning

processes was repeated with the same procedure, but with different cross-flow rates at

1,000, 800, and 600 ml/min for cleaning to investigate the effect of the cleaning

procedure.

10

Table 1. Conductivity of solutions for fouling experiments.

Solutions Conductivity (mS/cm)

2,000 ppm NaCl solution 3.72

Simulated steel wastewater 1.26

Sodium alginate solution (200 mg/L) 5.14

Bovine serum albumin solution (200 mg/L) 5.33

1.0 M NaCl solution 81.9 2.2

Results and discussion

Membrane fouling with water flux decline

Water flux and salt rejection of GO-polymer membranes are presented in Fig.

1 and compared with commercial CTA membranes. The use of hydrophilicity

poly(NIPAM-MBA) accelerates water flux and controlled crosslinking density of the

polymer improves salt rejection of GO-polymer membranes. Although GO nanosheet

membranes are known to be impermeable for liquid water molecules, an ultrathin

effective layer of GO-polymer membranes with highly ordered GO nanosheets, enables

fast and selective water transport. As a result, GO-polymer membranes exhibit

remarkable water flux and salt rejection property.[23] The GO-polymer membranes

demonstrated a water flux of 25.8 L m-2 h-1, which is more than 3 times higher compared

to the commercial CTA membranes, and a reverse salt flux of 1.5 g m-2 h-1 when 1.0 M

NaCl solution was introduced as draw solute and pure water was used as feed. When

2,000 ppm NaCl solution was introduced as feed, both water flux and reverse salt flux

slightly decreased because of smaller osmotic pressure difference between a feed and a

draw solution, but the difference of reverse salt flux was relatively small. Water flux of

the GO-polymer membrane linearly increased as a function of salt concentration of the

feed solution because the asymmetric structure of the GO-polymer membrane with a

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thin effective layer on the highly porous supporting layer minimised internal

concentration polarisation.[23] Whereas the CTA membrane demonstrated relatively

low water flux when the high concentration of the feed solution (1.5 mol L-1) was used.

Fig. 2 shows long-term water flux and reverse salt flux of GO-polymer and CTA

membranes as a baseline experiment with a 2,000 ppm NaCl solution as feed. In the

case of GO-polymer membranes, the initial water flux was 25.6 L m-2 h-1, and after 5

days, the flux slightly changed to 25.2 L m-2 h-1, and reverse salt flux was remained

almost constant. Note that no significant flux changes observed over 5 days since

enough volume of feed and draw solutions as well as low reverse salt flux of the GO-

polymer membrane kept the water flux constant, although the salt concentration of a

draw solution was diluted by water transport. For CTA membranes, water flux also

slightly decreased from 8.3 L m-2 h-1 to 7.3 L m-2 h-1 because relatively fast reverse salt

flow increased salt concentration in a feed solution and it caused internal concentration

polarisation, decreasing osmotic pressure difference between a feed and a draw solution.

Structure parameter, S, has a unit of length and explains the average distance for a solute

particle to travel through the support layer, where membrane with greater S shows more

severe internal concentration polarisation. As summarised in Table 2, CTA membrane

has S values of 596 22 m, which corresponds with the previously reported value.[20]

On the other hand, the GO-polymer membrane in this study has S value of 64.1 5 m,

which is much smaller than other conventional RO and FO membranes.[20] Note that

the use of a highly porous Nylon substrate can effectively extend the path for molecular

transport through the membrane and reduce S value, which results in reduced internal

concentration polarisation.[23]

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Figure 1. (A) Water flux and (B) reverse salt flux of GO-polymer and CTA FO

membranes as a function of various feed and draw salt concentrations.

Figure 2. Long-term water flux and reverse salt flux of GO-polymer and CTA

membranes (Feed: 2,000 ppm NaCl solution, draw solute: 1.0 M NaCl solution).

13

Table 2. Structure parameters of the membranes.

Membrane Structure parameter, S (m)

CTA 596 22

GO-polymer 64.1 5

Organic fouling

Water flux decline is seen as the major influence of membrane fouling on

membrane performances. Fig. 3 and 4 shows water flux decline curves due to

membrane organic fouling in FO process. For both cases of GO-polymer and CTA

membranes with SA and BSA as the feed, water flux decline was clearly seen. When

SA solution was employed as feed to the GO-polymer membrane, the water flux decline

was pronounced, with initial water flux significantly decreased by 20 % after 24 h

operation and further declined as a function of time. Membrane fouling is generally

more significant when the high water flux membrane was introduced because foulants

in feed solution are more quickly accumulated on the membrane surface due to fast

water transport.[39] However, GO-polymer membrane exhibited relatively less fouling

tendency than CTA membrane that the water flux of CTA membrane was decreased

more than 50% after 72 h operation. Anti-fouling property of GO alleviated adhesion

of hydrophobic foulant on the membrane surface from its strong negative charges as

well as its hydrophilicity, resulting in less flux decrease even though water flux of GO-

polymer membranes was much faster than that of CTA membranes. Table 3

summarises the contact angle of GO-polymer and CTA membranes to characterise their

hydrophilicity. Both membranes exhibit hydrophilicity with contact angles less than

45,[40] but GO-polymer presents a lower contact angle value (22.4) than CTA (43.3)

meaning that more hydrophilic GO-polymer membranes have higher water flux as well

14

as anti-fouling properties for organic foulants. Normalised water flux decline curves

also explain that fouling behaviour at the initial stage for both GO-polymer and CTA

membranes is similar, but after a sudden decline of water flux of GO-polymer

membranes (after 20 h), fouling on CTA membranes is about 30% more severe than

GO-polymer membranes. Colloidal compounds, such as SA, tend to adsorb each other

and also on the membrane surface, leading to formation of a dense cake layer and water

flux decline. In addition, small-sized SA continuously accumulated on the membrane

surface, resulting in continuous flux decline.

BSA is an organic compound with a large molecular weight and size as

compared with SA, and is a model for proteins in wastewater. When BSA was

employed to the GO-polymer membrane, the water flux decline at the beginning was

similarly severe, but after 20 h water flux became relatively constant. However, the

water flux of CTA membrane continuously decreased with time. BSA is known to

attach on a polymer membrane surface with strong adhesion force, but the

intermolecular adhesion among BSA molecules is not strong enough to form a compact

or dense fouling layer as compared with that of SA. Organic molecules with weak

intermolecular adhesion forces, such as BSA, tend to result in less severe fouling.

Moreover, hydrodynamic interaction of BSA is relatively weak as compared with SA,

which is less favourable in foulant deposition and cake layer formation.[5] Large sized

BSA quickly blocks membrane surface and results in high initial flux decline rate, but

the fouling layer they produce was less compact than smaller sized SA and did not

effectively hinder the water transport, therefore, the water flux decline was less serious

as compared to that from SA fouling in both GO-polymer and CTA membranes. In

membrane water treatment performances, the water flux was significantly reduced by

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organic fouling, but the reverse salt flux was rarely changed during FO operation for

both GO-polymer and CTA membranes.

Figure 3. (A) Water flux and (B) normalised water flux decline of GO-polymer and

CTA membranes by SA fouling.

Table 3. Contact angle of the membranes.

Membrane Contact angle

CTA 43.3 0.3

GO-polymer 22.4 0.5

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Figure 4. (A) Water flux and (B) normalised water flux decline of GO-polymer and

CTA membranes by BSA fouling.

Inorganic fouling

Wastewater in the steel industry contains inorganic compounds in high

concentration and high conductivity from iron and salts and its fouling behaviour in

membrane operation is different from organic fouling. Simulated steel wastewater was

used as feed for GO-polymer and CTA membrane to compare organic and inorganic

fouling behaviour. As described in Fig 5, initial water flux decreased to about 80% for

both GO-polymer and CTA membranes, meaning that membrane inorganic fouling

from simulated steel wastewater was not severe as compared to organic fouling from

SA and BSA. Foulants in simulated steel wastewater consist of inorganic scales

including silica and ferric chloride, and these become saturated on the membrane

surface during FO process. The initial water-flux decline rate, mainly from saturation

of inorganic scales, was slower than that in SA and BSA cases. However, water flux

continuously reduced at longer operating times. For CTA membranes, fouling

17

behaviour was similar to that of GO-polymer membranes as inorganic fouling is related

to inorganic scaling, therefore, anti-fouling property from GO was less significant for

preventing inorganic fouling. During fouling experiments, there were no clear changes

in salt rejection properties for all cases, explaining that the membrane active layer was

not damaged by organic and inorganic foulants.

Figure 5. (A) Water flux and (B) normalised water flux decline of GO-polymer and

CTA membranes by inorganic fouling from simulated steel wastewater.

Water flux recovery by physical cleaning

Organic fouling

Physical cleaning procedures have been introduced for GO-polymer membranes

to remove foulants on the membrane surface and to restore water flux. Physical methods

usually correspond to membrane back flushing and surface flushing for foulant removal,

without the use of chemical agents.[5] Surface flushing methods in FO process are

effectively employed to clean organic foulants as they deposed on the surface by

electrostatic charges and the cleaning efficiency can be improved by increased cross-

18

flow rates and an extended cleaning duration, eliminating the interaction between

foulants and the membrane surface.[38] Fig. 6 shows the flux decline by organic fouling

and the flux recovery by surface cleaning with SA as the feed (the most effective foulant

in this study). After 3 days operation, the membranes were cleaned with DI water by

surface flushing at controlled cross-flow rate for 1 h and water flux and reverse salt flux

were monitored for 35 h. Initially, the membrane demonstrated a water flux of 28.5 L

m-2 h-1, but after 3 days, the water flux declined to 18.3 L m-2 h-1. However, after

membrane cleaning at 1,200 ml/min, the water flux was restored to 24.2 L m-2 h-1, about

85 % of initial water flux. Membrane cleaning at 1,000 ml/min was also effective as

water flux was recovered to 22.4 L m-2 h-1. The surface flushing rate should be carefully

determined to overcome adhesive force between foulants and the membrane surface for

sufficient cleaning, but not to damage membrane surface. Mechanically robust GO

nanosheets in GO-polymer membranes provide mechanical strength to stand high

surface resistance during surface flushing at high rates, and then more foulants on the

GO-polymer membranes are removed by surface flushing. Moreover, hydrophobic

organic foulants are easily detached from the hydrophilic membrane surface during

cleaning processes. It is observed that small-sized SA molecules can be easily removed

from the surface and re-dispersed to the solution even at the low rate, but the water flux

was restored enough when the high flushing rate over 1,000 ml/min was introduced.

However, the water flux started declining at high rates as for the decrease of initial

membrane fouling behaviour directly after the cleaning procedure, meaning that SA

particles from feed solution quickly form a cake layer again, although most fouled SA

particles on the membrane surface were removed after cleaning. The FO process was

applied with a cross-flow rate of 500 ml/min, which means the foulants were deposited

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on the surface at this rate, forming a cake layer. Therefore, 800 and 600 ml/min were

not high enough to effectively remove foulants and recover water flux.

0 20 40 60 80 100 120 14015

20

25

30

1,200 ml/min

1,000 ml/min

800 ml/min

600 ml/min

original

Wate

r F

lux (

L m

-2 h

-1)

Time (h)

Cleaning

Figure 6. Water flux decline of GO-polymer membranes by SA fouling and water flux

recovery after surface flushing at different cross-flow rates.

20

0 20 40 60 80 100 120 14015

20

25

30

1,200 ml/min

1,000 ml/min

800 ml/min

600 ml/min

original

Wate

r F

lux (

L m

-2 h

-1)

Time (h)

Cleaning

Figure 7. Water flux decline of GO-polymer membranes by BSA fouling, and water

flux recovery after surface flushing at different cross-flow rates.

The water-flux recovery rate was also dependent on the type of foulant used

when surface flushing was applied. When foulants with high molecular weight such as

BSA were used, surface flushing was not effective for removing foulants and restoring

water flux, as shown in Fig. 7. This is because large-sized organic foulants tend to

interact each other and aggregate, and then more energy is required to remove the

foulants from membrane surfaces. After membrane cleaning by surface flushing at

1,200 ml/min, the water flux was restored from 20.2 L m-2 h-1 to 23.2 L m-2 h-1, which

is about 58% recovery. However, the effect of cross-flow rate on water flux recovery

was weak as water flux changes after cleaning at 1,000, 800, and 600 ml/min were only

42%, 33%, and 25%, respectively. Since heavy organic foulants tend to stay on the

21

membrane surface and they are required strong flow force to be removed, cross-flow

surface flushing to BSA was a less effective method for the water flux recovery.

Inorganic fouling

Membrane fouling by deposition of foulants in simulated steel wastewater

showed different behaviour from fouling by simulated organic wastewater. Organic

foulants are continuously accumulated on the membrane surface whereas inorganic

foulants generally form and grow scales.[12, 29] As the growth rate of inorganic scales is

relatively slower than the accumulation rate of organic foulants, the water-flux decline

rate at the beginning was slow. Furthermore, inorganic scales were relatively easy to be

eliminated by surface flushing since inorganic scales do not make such strong

interactions with the membrane surface than organic foulants, resulting in significant

water flux recovery by surface flushing at high cross-flow rate as shown in Fig. 8. After

cleaning at 1,200 ml/min, water flux was restored to 25.5 L m-2 h-1, which is 97% of

initial water flux. Similarly, after cleaning at 1,000 ml/min, the water flux was

recovered to 25.2 L m-2 h-1, which is 95% of initial water flux. These results showed

that physical cleaning by surface flushing of GO-polymer membrane was more

effective for inorganic fouling than organic fouling. However, cleaning at the low rate

at 800 ml/min and 600 ml/min were not effective enough to recover water flux.

22

0 20 40 60 80 100 120 14015

20

25

30

1,200 ml/min

1,000 ml/min

800 ml/min

600 ml/min

original

Wate

r F

lux (

L m

-2 h

-1)

Time (h)

Cleaning

Figure 8. Water flux decline of GO-polymer membranes by inorganic fouling from

simulated steel wastewater and water flux recovery after surface flushing at different

cross-flow rate.

Effects of membrane fouling on the membrane morphology

The GO-polymer membrane in this study is a composite membrane, coated on

a porous Nylon substrate, which has the effective layer with a thickness of about 30 nm

as shown in Fig. 9. In the SEM image of the membrane surface (Fig. 9b), it is clear that

electron beam easily penetrated through the ultrathin GO-polymer layer, and Nylon

substrate was imaged underneath the GO-polymer layer. After 3 days fouling of

different foulants, a cake layer on the membrane surface was not clearly seen by SEM

as shown in Fig. 10a and b when organic foulants were introduced. Cake layers after

organic fouling are usually formed as a very thin and dense layer due to the well-defined

foulant accumulation, so the evidence of surface changes was hardly observed before

23

and after cleaning procedure. Existence of foulant layer on the GO-polymer membrane

was confirmed by FT-IR spectroscopy before and after organic fouling as shown in Fig.

11. The main peaks for the GO-polymer membrane are as follows: 3298 cm-1

(secondary amide N-H stretching), 2950 cm-1 (-CH3 asymmetric stretching), 1650 cm-

1 (amide C=O stretching, aka amide I bond), and 1210 and 1540 cm-1 (CN stretching)

for poly(NIPAM-MBA). 1630 cm-1 (skeletal vibrations from unoxidized graphitic

domains), 1420 cm-1 (C=C aromatic ring), and 1040 cm-1 (C-O stretching) for GO.[23]

After fouling of SA, representative peaks for alginate (3200 cm-1 for -OH stretching,

1595 and 1407 cm-1 for -COO asymmetric and symmetric stretching, and 1078 cm-1 for

C-O-C stretching) are observed.[41] For BSA fouling, chemical structure of BSA is

rather overlapped with that of poly(NIPAM-MBA), but intensity of peaks for secondary

amide N-H stretching at 3298 cm-1 and CN stretching at 1210 cm-1 is increased and a

peak for -COO side chain at 1400 cm-1 in BSA newly appears. When simulated steel

wastewater was employed as the feed, lumps of foulants on the membrane surface could

be seen as confirmed by EDX analysis. Inorganic foulants are usually aggregated and

formed irregular inorganic scales on the membrane surface whereas organic foulants

form a regular cake layer. However, after physical cleaning, most of the lumps were

eliminated. This also explains why water flux after physical cleaning could temporarily

be recovered, as foulants on the membrane surface hindered water transport until they

were removed.

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Figure 9. (A) Cross-sectional and (b) surface SEM images of a GO-polymer membrane

Figure 10. Surface morphology of GO-polymer membranes before and after surface

flushing when (A) SA, (B) BSA, and (C) simulated steel wastewater were used as a

feed solution, respectively.

25

Figure 11. Changes in FT-IR spectrum before and after fouling by (A) SA and (B) BSA.

Conclusions

Membrane fouling behaviour of a GO-polymer membrane has been studied to

evaluate performance of this new membrane material for practical applications and

compared with a commercial CTA membrane. SA and BSA were employed as model

organic foulants, simulating operating conditions of the wastewater treatment processes,

and simulated steel wastewater was also used for steel production applications. Water

flux decline as a result of membrane fouling and water flux recovery after cleaning was

dependent on the class of foulant. The rate of decrease of the initial water flux rate was

relatively high when organic foulants were used, with the type of organic foulants

effected on membrane fouling, as SA foulants with strong adhesion between each

foulant molecules were effectively covered the membrane surface and continuously

decreased water flux. However, when inorganic foulants were introduced from

simulated steel wastewater, they tended to develop scales as a result of membrane

26

fouling. In this case, water flux decline rate was initially slow beginning, but

continuously decreased by time.

Physical cleaning by surface flushing was introduced at controlled cross-flow

rates to restore water flux. Water flux recovery was strongly affected by cross-flow rate

of flushing water, but the nature of foulants was also an important factor. Surface

flushing of DI water at a rate of higher than 1,000 ml/min was effective in the most

cases, but organic foulants with high molecular weight were not sufficiently eliminated.

As water flux was decreased and restored by membrane fouling and cleaning, salt

rejection remained constant during these membrane fouling experiments.

Funding

This work is supported by the Baosteel-Australia Research and Development Center

(BA13005) and the Australian Research Council (Linkage Project No.: LP140100051).

The authors acknowledge the staff of Monash Centre for Electron Microscopy (MCEM)

for their technical assistance.

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