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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
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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
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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
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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
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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
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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
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(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.
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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.
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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).
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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
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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
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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-
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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.
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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
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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.
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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
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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.
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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
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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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