Journal of Membrane Science 389 (2012) 155 161 [621916]

Journal of Membrane Science 389 (2012) 155– 161
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science
jo u rn al hom epa ge: www.elsevier.com/locate/memsci
A new outlook on membrane enhancement with nanoparticles: The alternative
of
ZnO
Stefan Baltaa,b, Arcadio Sottoa,c, Patricia Luisa, Lidia Beneab, Bart Van der Bruggena, Jeonghwan Kimd,∗
aDepartment of Chemical Engineering, K.U. Leuven, Belgium
bDepartment of Environmental and Material Engineering, University Dunarea de Jos, Galati, Romania
cDepartment of Chemical and Energetic Technology, Rey Juan Carlos University, Madrid, Spain
dDepartment of Environmental Engineering, Inha University, Incheon, Republic of Korea
a r t i c l e i n f o
Article history:
Received
16 April 2011
Received
in revised form 18 October 2011
Accepted
21 October 2011
Available online 25 October 2011
Keywords:MembranesNanofiltrationZnOMembrane synthesis
Nanoparticlesa b s t r a c t
Although several studies explored the use of nanoparticles as additives in membrane structures, mixed
matrix membranes still suffer from difficulties in synthesis and applications. In this paper, a new outlook
on enhancement of membranes with nanoparticles is proposed by using ZnO as an alternative to TiO 2.
Although ZnO has attractive features that potentially could fill the objectives of mixed matrix membranes
with lower cost and better performance, challenges in development remain. This paper investigates the
synthesis of ZnO enhanced membranes and evaluates the performance of mixed matrix membranes with
ZnO nanoparticles. Polyethersulfone (PES) membranes manufactured by diffusion induced phase inver-
sion in N-methyl-pyrrolidone (NMP) using a range of procedures were blended with ZnO nanoparticles
in a wide range of concentrations from ultralow to high (0.035–4 wt%). It was shown that the new mem-
brane materials embedded with ZnO nanoparticles have significantly improved membrane features. The
influence of the ZnO nanoparticles on the characteristics of PES/ZnO membranes was investigated with
microscopic observations, contact angle measurement, filtration experiments, fouling resistance deter-
mination and observation of the rejection of selected dyes. The results showed an overall improvement
compared to the neat membranes in terms of permeability as well as dye rejection and fouling resistance
by adding ZnO nanoparticles even in small and ultralow concentrations.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Nanoparticles, having unique physico-chemical properties that
differ from bulk materials, are of high interest in the manufacturing
of membranes to achieve a high degree of control over membrane
fouling and the ability to produce desired structures as well as
functionalities [1–5] . Membrane fouling is the main problem that
limits the use of membranes in a wide range of applications from
an economic or even technical point of view. Nanoparticles may
offer a key to resolve this problem. The fouling phenomenon is
often attributed to adsorption of organic compounds on the mem-
brane surface, although other types of fouling such as biofouling
and scaling may also occur. The main effect of membrane foul-
ing is a dramatic decrease of the flow through the membrane. A
diminished membrane performance occurs together with exces-
sive operating costs [6,7] and a decrease in rejection of the target
compounds [8–10] and permeation properties [11–13] caused by
the higher hydrophobicity of the membrane surface. It depends on
∗Corresponding author. Tel.: +82 32 860 7502.
E-mail address: jeonghwankim@inha.ac.kr (J. Kim).the membrane characteristics [14–17] and on the filtration mode
(cross-flow or dead-end filtration) [18] but it can be said that the
application of membranes is threatened by the critical formation
of fouling and the reality shows that their use in industrial applica-
tions is extremely restricted if this problem is not solved.
Strong efforts are being done in several membrane applications
using nanoparticles, advancing towards a good performance and
trying to understand the fouling phenomenon. Two methods are
reported in general to prepare composite membranes, one by dis-
persing the nanoparticles in the casting solution and prepare the
membrane via phase inversion [19–25] and a second by dipping the
prepared membrane in a suspension with nanoparticles [26,27] . In
addition, many types of nanoparticles have been studied to improve
the membranes properties, silica (SiO 2), carbon nanotubes, alumina
(Al2O3), silver (Ag), zirconia (ZrO 2), gold (Au), zerovalent iron (Fe0),
palladium (Pd) and, as most studies focus on, TiO 2nanoparticles [5].
TiO 2is a functional nanomaterial, but also has disadvantages in the
use, such as the simultaneous filtration and photocatalytic activity
that is aimed at, the dispersion in the bulk of the polymer or in the
top layer, and the overall cost of the TiO 2enhanced membranes,
which is not only related to the intrinsic cost of the nanoparticles,
but also to the synthesis procedure.
0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2011.10.025

156 S. Balta et al. / Journal of Membrane Science 389 (2012) 155– 161
In this work, a new outlook on the use of nanoparticles in mem-
brane structures is explored, through the use of zinc oxide (ZnO)
nanoparticles. ZnO is one of the most important multifunctional
semiconductor materials and exceptionally important for appli-
cation in photo-catalysis and anti-bacterial materials, due to its
excellent optical, electrical, mechanical and chemical properties
[28]. The lower cost of ZnO and the increase of the surface-to-
volume ratio obtained when ZnO is used as particles in a nano-sized
scale make this alternative a potential system that can meet the
demand of an efficient and lower-cost device.
Furthermore, another issue that can limit the application of
nanoparticles is their toxicity since it is thought that nanoparti-
cles may persist as small particles in aquatic systems and that their
bioavailability could be significantly greater than that of larger
particles. For ZnO nanoparticles, positive conclusions have been
obtained [29–33] . The use of nanoparticulate ZnO does not pro-
duce an increase in toxicity since the size distribution and surface
area are not related to toxicity. Already in 2007 Franklin et al. [34]
compared the toxicity in algae of nanoparticulate ZnO, bulk ZnO
and ZnCl 2and observed that toxicity is attributable solely to dis-
solved zinc, this is, to simple solubility of the compounds since
ZnO nanoparticles aggregate in freshwater systems forming flocs
of even several microns with a saturation solubility similar to that
of bulk ZnO. Also, since ZnO nanoparticles will be embedded in a
solid matrix (the membrane), a stable system can be developed,
keeping their physical properties associated to their size and the
chemical activity related to their availability in the membrane.
Thus, the use of ZnO in the nano-size scale incorporated in mem-
branes is a promising and novel system that may be the solution
for the development of low-cost and fouling-prevention membrane
technology.
The novelty of this paper is in the synthesis of ZnO enhanced
membranes in view to significantly enhance the performance of
nanofiltration technology in terms of permeation, rejection and
fouling resistance. The eventual membrane structure was also stud-
ied in this work.
2. Experimental
2.1. Materials
Polyethersulfone (PES, type Radel) supplied by Solvay (Belgium)
was employed as the base polymer. 1-methyl-2-pyrrolidone (NMP,
99.5%) was used as the polymer solvent. The support layer (Viledon
FO2471) used for the PES membrane manufacturing was obtained
from Freudenberg (Weinheim, Germany). ZnO nanoparticle and
humic acid (HA) were purchased from Sigma–Aldrich (St. Louis,
MO). HA is known as a foulant in natural waters being fraction of
humic substances obtained from chemical and biological degrada-
tion products from plant and animal residues [35].
Six different dyes were used to explore the size interaction
in the interface solute-membrane pore. Organic compounds pur-
chased from Acros Organics (Belgium) were selected in order to
cover a large range of molecular mass. The selected dyes were
methyl red (269.21 Da), neutral red (288.77 Da), methylene blue
(319.85 Da), Sudan black (456.54 Da), Victoria blue (506.10 Da),
Congo red (696.67 Da).
2.2. Preparation of membrane
Neat PES membranes and ZnO-entrapped PES membranes
were prepared using phase inversion induced by immersion
precipitation. PES cast from four different concentrations in N-
methyl-pyrrolidone (NMP) (25, 27, 30 and 32 wt%) was used as the
polymer matrix. The ZnO-entrapped membranes were prepared bydissolving
different amounts of nanoparticles in the corresponding
volume of NMP for 3 h by mechanical stirring at 200 rpm and room
temperature. Eleven different concentrations of ZnO nanoparticles
were used: 0.035, 0.07, 0.085, 0.125, 0.250, 0375, 0.500, 0.750, 1, 2
and 4 wt%. Subsequently, the polymer was added to the solution,
which was stirred for 24 h at 500 rpm and 40◦C. After formation of
a homogenous solution, the films were cast with 250 /H9262m thickness
using a filmograph (K4340 Automatic Film Applicator, Elcometer)
in an atmosphere with controlled relative humidity on nonwoven
polyester as support layer. Prior to the casting, the support layer
was wetted with NMP to prevent the polymer solution of intruding
in the pores of the support layer. The prepared films were immersed
in a non-solvent bath (distilled water at 20◦C) for precipitation. The
membrane was afterwards repeatedly washed with distilled water
to remove the remaining solvent, and stored wet. For each polymer
solution composition, five identical membrane sheets were made
and tested to obtain an average value of flux and solute rejection.
2.3. Characterization of the membrane surface
A contact angle measuring system DSA 10 Mk2 (Krüss, Germany)
was used to measure the water contact angle of the synthesized
membranes. A water droplet was placed on a dry flat homogeneous
membrane surface and the contact angle between the water and
membrane was measured until no further change was observed.
The average contact angle for distilled water was determined in
a series of 8 measurements for each of the different membrane
surfaces. To visualize membrane surface characteristics, scanning
electron microscopy (SEM) measurements were performed. SEM
images were made with a Philips XL30 FEG instrument with an
accelerating voltage of 20 keV. Cross-sections were prepared by
fracturing the membranes in liquid nitrogen.
2.4. Filtration experiments
The prepared membranes were characterized for water flux,
pure water permeability (membrane hydraulic resistance) and dye
rejection studies using dead end filtration experiments. Compar-
ison of the fouling-resistant ability of the manufactured neat and
blended membranes was explored by cross-flow experiments.
Pure water permeability and dye rejection were determined for
a wide range of membranes. Four membrane coupons of the same
membrane sheet for eight membranes of each type were tested.
Therefore, the obtained results are the average of 32 experimental
values. The maximum experimental errors were less than 5% and
8% for PES and PES/ZnO membranes, respectively.
The pure water flux was determined from a compaction exper-
iment at a transmembrane pressure of 10 bar and a constant
temperature of 25◦C in dead-end mode with a Sterlitech HP4750
Stirred Cell. A nitrogen cylinder coupled with the pressure regulator
was connected to the top of vessel to pressurize the cell. The active
membrane area was 14.6 cm2. The thoroughly washed membrane
was cut into the desired shape and fitted in the dead end device. The
volume of the appropriate solution was 250 mL. The initial water
flux was measured 30 s after the pressurization. Permeate was col-
lected in a graduated cylinder for a time interval until steady state
[36].
After compaction, the pure water permeability (PWP) was
determined by measuring the pure water flux (Jw) at different trans-
membrane pressures (/Delta1P) from 2 to 14 bar; the slope of the linear
regression of the water flux as a function of transmembrane pres-
sure was determined as the permeability. The PWP was calculated
by the following equation:
PWP =Jw
/Delta1P(1)

S. Balta et al. / Journal of Membrane Science 389 (2012) 155– 161 157
Rejections were measured at a transmembrane pressure of
10 bar. Concentration polarization at the membrane surface is min-
imized by driven a Teflon coated magnetic stirring bar on top of the
membrane. Four samples of permeate, 5 mL each, were taken.
In order to study the effect of membrane fouling, the membranes
were tested in a cross-flow filtration set-up [37] fed with 5 ppm of
HA. To compare flux decline between different membranes, relative
fluxes were defined as the relation of the permeate flux to the pure
water flux of the respective membrane as follows:
RF =Jv
Jw(2)
Concentration polarization is minimized by using a cross-flow
velocity of 4.5 m s−1. This feed velocity corresponds to a Reynolds
number of 30,000, which is situated far in the turbulent region.
2.5.
Analytical methods
A Shimadzu UV-1601 double beam spectrophotometer was
used to determine the concentration of dyes. Regression factors
(R2) obtained for calibrations within the experimental concentra-
tion range were above 0.99. The rejection R of the dissolved dyes
was calculated as follows:
R(%)
=/parenleftbigg
1 −Cp
Cf/parenrightbigg
× 100 (3)
where
Cpand Cfare the permeate and feed concentrations of dyes,
respectively.3.
Results and discussion
3.1. Membrane characterization with SEM observations
Membrane surfaces synthesized were observed with scanning
electron microscopy (SEM). The influence of the addition of ZnO
nanoparticles to the membrane surface is shown in Fig. 1. The
SEM images of the PES membrane surface before addition of ZnO
nanoparticles was similar to these of the same membranes after
embedding ZnO. However, further increase of added nanoparti-
cles to 0.7 wt% promoted the formation of clusters or aggregates of
ZnO on the membrane surface significantly. The SEM images show
that pore-like structures on the membrane surface appeared, which
was improved by the addition of ZnO nanoparticles. However, con-
clusions cannot be drawn about the relative pore sizes between
the control PES membrane and the PES/ZnO membrane from the
SEM images, and more quantitative analysis is needed. Assuming
that the pores of UF or NF membranes are in the same range as
molecular weight cut-off (MWCO) measurements using synthe-
sized PES membrane embedded with TiO 2nanoparticles from our
previous study [38], the MWCOs between control PES membranes
and PES/ZnO membranes were compared using model dye com-
pounds (results are discussed later).
Cross-sectional observations of synthesized membranes were
also made with SEM. Fig. 2 shows that the number of
macrovoids decreases with increasing polymer concentration, and
the sponge-like structure is more pronounced at higher poly-
mer concentrations. The membranes with ZnO nanoparticles in
Fig. 1. SEM images of the surface of the synthesized membranes at different concentrations of ZnO (PES 27%) (A) Neat PES membrane, (B) 0.125 wt%, (C) 0.5 wt%, and (D)
0.7
wt%.

158 S. Balta et al. / Journal of Membrane Science 389 (2012) 155– 161
Fig. 2. Cross-sectional SEM images of synthesized PES (without ZnO) and PES/ZnO (0.125 wt% of ZnO) at different PES concentrations (A) 27% PES, (B) 30% PES, (C) 32% PES,
(D)
27% PES/ZnO, (E) 30% PES/ZnO, and (F) 32% PES/ZnO.
the casting solutions showed that the separation layer (upper
layer) became thinner than that of the control PES membrane.
Furthermore, macrovoid formation of PES/ZnO membranes was
more improved for higher polymer concentrations by adding
ZnO nanoparticles. The exchange between solvent (NMP) and
non-solvent (water) is slower because of a hindrance effect of
nanoparticles during the phase-inversion process, thus promot-
ing macrovoid formation [39,40] . Similar results were observed at
higher concentrations of nanoparticles than proposed in this work
(0.125% ZnO) [41].
3.2. Hydrophilicity of membranes
Figs. 3 and 4 show contact angle measurements for both dif-
ferent polymer and ZnO nanoparticle concentrations. As shown in
Fig. 3, an increasing polymer concentration increases the contact
angle of the membrane surface, indicating that these membranes
are more hydrophobic. As a result of the high affinity of nanopar-
ticles to water, the addition of ZnO can increase the hydrophilicity
of membrane. This fact can be explained by the presence of
hydrophilic ZnO nanoparticles in membrane structures, and the
contact angle should be lower because a larger fraction of water
diffuses through the membrane structure.32 30 27 254050607080Contact angle (ș)
PES concentra/g415on (%) PES
PES/ZnO 0.125 wt.%
Fig. 3. Contact angles measured for neat membranes (no ZnO used) and PES/ZnO
membranes
(0.125 wt% ZnO) at different PES concentrations.

S. Balta et al. / Journal of Membrane Science 389 (2012) 155– 161 159
41 0,75 0,5 0,375 0,25 0,125 0,07 0,0350505560657075
PES concentra/g415on 27 wt.%Contact angle (ș)
ZnO Concentra/g415on (wt.%)
Fig. 4. Contact angles measured for PES/ZnO membranes at different nanoparticle
concentrations.
In contrast to the increase of the contact angle with polymer
concentration as observed in Fig. 3, hydrophilicity did not change
significantly, in spite of the increasing content of ZnO nanoparti-
cles (Fig. 4). The remarkable conclusion from Fig. 4 is that even
at ultra-low concentration (0.035 wt%), however, embedding ZnO
nanoparticles into PES membrane can drop contact angle signifi-
cantly from 70◦(control PES membrane) to about 56◦.
3.3. Comparison of permeability between PES and PES/ZnO
membrane
Fig. 5 shows the combined effect of the polymer and ZnO
concentration on the membrane permeability. The polymer con-
centration has a negative effect on the water permeate flux as a
result of the increasing hydrophobicity of membrane. In addition,
if the polymer fraction becomes higher, the membrane porosity
could be smaller during phase-inversion because the solvent
inter-diffusion rate can be decreased. However, the systematic
addition of ZnO nanoparticles has a positive effect on the water
permeability for all of polymer concentrations tested. This result
Fig. 5. Permeability of the newly synthesized membranes (L h−1m−2bar) as a func-
tion
of ZnO concentration and PES concentration.421 0,75 0,5 0,375 0,25 0,0850,125 0,07 0,03501020304050607027 wt.% PESPermeability ( L h-1 m2 bar-1)
ZnO content (wt.%)
Fig. 6. Permeability of the newly synthesized membranes (L h−1m−2bar) in the
ultralow
to high concentration range for ZnO at 27 wt% of PES.
is in agreement with the improvement of macrovoids formation
described by SEM images (Fig. 2) and also with the increase of
hydrophilicity observed by contact angle measurements (Fig. 3).
For ZnO nanoparticle concentrations below 0.4%, the permeabil-
ity of the membranes was two times higher than the initial value
in the absence of ZnO nanoparticles. In the literature [40], a sim-
ilar increase of water fluxes (almost double) has been observed
with addition of TiO 2nanoparticles, but the TiO 2concentration
was much higher (about 1 wt%) than the proposed in this work.
In general, as the ZnO concentration increases, the water perme-
ability increases and this effectiveness becomes more pronounced
at lower polymer concentrations.
At nanoparticle concentrations below 1 wt%, the maximum per-
meability was found in the concentration interval between 0.3
and 0.5 wt%, suggesting that an optimum concentration of ZnO
should exist. Fig. 6 also shows a sudden decrease of permeability at
nanoparticle concentrations of 0.75 and 1%. This fact could be asso-
ciated with the reduction of the dispersion rate of nanoparticles.,
which is evidenced by the increase of contact angle measurement
observed in these concentration values. The permeability increased
again, however, as the ZnO concentration was higher than 1 wt%
and it was recovered to the permeability values obtained under the
ZnO concentrations between 0.3 and 0.5 wt% interval eventually. As
shown in Fig. 4, the contact angle of a PES/ZnO membrane becomes
smaller as the ZnO concentration is higher than 1 wt%. Thus, the
observation of higher permeabilities for the higher ZnO concentra-
tion can be explained by the more hydrophilic characteristic of the
PES membrane due to the presence of ZnO nanoparticles.
3.4. Reduction of membrane fouling
In order to explore the fouling resistance of PES/ZnO mem-
branes, filtration experiments were performed using humic acid
as a model organic foulant in aqueous solution. A concentration
of 5 mg/L of humic acid was selected in this test. As shown in
Fig. 7, showing the relative fluxes at different nanoparticle concen-
trations, the addition of ZnO nanoparticles improves the fouling
resistance considerably. Due to the increase of the hydrophilic-
ity of the membrane by the addition of ZnO nanoparticles, the
adsorption of organic pollutants within membrane structure can
be reduced. The results also indicate again that an optimum con-
centration of ZnO nanoparticle exists. Increasing the nanoparticles
concentration to 0.125 wt% increased the relative flux, but at higher
concentrations, no further increase in the flux was observed.
Using ZnO nanoparticles in membrane structures, a 23% increase
in permeate flux was observed (0.5 wt% ZnO). A similar effect on
the permeate flux, i.e., an enhancement of 25%, was reported for a

160 S. Balta et al. / Journal of Membrane Science 389 (2012) 155– 161
25 20 15 10 5 00.50.60.70.80.91.0
23 %
PES
PES/ZnO 0.035 ZnO wt.%
PES/ZnO 0.125 ZnO wt.%
PES/ZnO 0.5 ZnO wt.% Rela/g415ve flux
Time (h)Humic acids 5mgL-1
Fig. 7. Relative flux of PES membranes with four concentrations of ZnO nanoparti-
cles
(0, 0.035, 0.125, and 0.5 wt%) in transient regime.
PES/TiO 2composite membrane [42]. However, the concentration
of TiO 2was about five times higher than proposed in this study. In
some cases, the extent of flux improvement was even lower than
observed with PES/ZnO membranes here [43].
3.5.
MWCO measurement and rejection performance
In order to estimate the MWCO of prepared PES/ZnO membranes
(0.125 wt% of ZnO), different commercial dyes with increasing
molecular mass were used. Fig. 8A shows a typical tendency for the
rejection of organic compounds by a nanofiltration (NF) membrane,
which can be quantified as about 500 Da of MWCO. However, the
difference of MWCO between the PES/ZnO membrane and the con-
trol PES membrane was not significant, suggesting that the addition
of ZnO nanoparticles did not yield a large improvement in the pore
size of PES membranes.
It is interesting to note that the rejection of PES/ZnO membranes
was higher when the molecular weight of dye compounds was
smaller than 400 Da while no difference in the rejection pattern
between these two membranes was observed when the molecu-
lar mass of dye was higher than 400 Da. Perhaps, in the interval
between 250 and 400 Da, increasing hydrophilicity and charge
effects by the addition of ZnO nanoparticles may play an important
role in the rejection of dye compounds in the PES/ZnO membrane
although the sieving mechanism becomes more dominant for the
rejection potential as the size of dye compound increases.
In order to better understand the rejection of organic dye
compounds smaller than 400 Da with PES/ZnO membranes, the
rejection of 5 mg/L methylene blue (MW = 319.8 Da) was measured
at different PES concentrations, which is shown in Fig. 8B. For the
control PES membrane, the polymer concentration has a signifi-
cant effect on the rejection potential of the selected dye compound.
Increased rejection of the methylene blue with the control PES
membrane is assumed to be caused by the macrovoids decrease in
the membrane because higher polymer content can decrease the
inter-diffusion rate during phase inversion as discussed with Fig. 2.
As shown in Fig. 8B, the rejection of selected dye compound
with PES/ZnO membrane is higher than the one with control PES
membrane under all polymer contents tested in this study. To con-
firm if the ZnO nanoparticles embedded in PES membrane have any
adsorption capacity of methylene blue compound or not, adsorp-
tion batch tests were performed with the same ZnO content as
tested (0.125 wt%) and 5 mg/L of methylene blue. After 1 h contact
time, we observed that the adsorption effect of ZnO nanoparticles
for the methylene blue was almost negligible. Since methylene
blue is a cationic dye and the zero point of charge (ZPC) of ZnO700 600 500 400 300020406080100
ΔP = 10 bar
T = 25 șC
PES
PES/ZnO 0.125 ZnO wt.% Rejec/g415on (%)
Molecular weight (Da)
32 30 28 26 24020406080100
PES
PES/ZnO 0.125 ZnO wt.% Methylene blue 5 mgL-1ΔP = 10 bar
T = 25 șCRejec/g415on (%)
PES content (wt.%)A
B
Fig. 8. MWCO measurement of control PES and PES/ZnO membranes with 0.125 wt%
ZnO
(A) and rejection performance at different polymer concentrations (B).
nanoparticles is at pH of ca. 9.8, electrostatic repulsion between the
dye compound and ZnO nanoparticles should be involved under
the neutral pH tested in this study. This suggests that the charge
effect between methylene blue and ZnO nanoparticles is an impor-
tant factor that determines the rejection potential of the organic
compound with the PES/ZnO membrane.
Considering the hydrophobic character of the selected dye
compound, the increasing hydrophilicity of the membrane with
addition of ZnO can also decrease solute adsorption in the mem-
brane structure, and therefore increase the rejection potential. The
dye solute may diffuse through the PES/ZnO membrane at a slower
rate than the control PES membrane and this leads to lower con-
centrations in the membrane permeate. Usually for hydrophobic
compounds, solute adsorption into the membrane can be promoted
with hydrophobic membranes due to hydrophobic–hydrophobic
interactions [13]. Avoiding hydrophobic interaction in the inter-
face solute-membrane surface is thought to be an effective way to
improve the rejection of hydrophobic organic compounds.
4.
Conclusions
The results from this study show that ZnO nanoparticles are
an excellent competitor to TiO 2as anti-fouling material, which
may lead to new applications of membranes in the forthcoming
years due to the attractive properties of ZnO. It was shown that
ZnO blended membrane showed lower flux decline and a bet-
ter permeability compared to neat polymeric membrane due to
a higher hydrophilicity of the ZnO membranes. This has not been

S. Balta et al. / Journal of Membrane Science 389 (2012) 155– 161 161
evidenced before. In addition, ZnO nanoparticles provide a remark-
able improvement in the dye rejection potential. The rejection of
methylene blue increased from 47.5% for neat membranes to 82.3%
for blended membranes. This was observed even at unusually low
concentration (0.035 wt%) of ZnO nanoparticles.
Acknowledgements
This research was supported by Basic Science Research Pro-
gram through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2010-0024286). Stefan Balta would like to acknowledge the sup-
port provided by the European Union, Romanian Government and
Dunarea de Jos University of Galati, through the project POSDRU-
6/1.5/S/15. Arcadio Sotto would like to acknowledge the support
provided by the Regional Government of Madrid through project
REMTAVARES (S2009/AMB-1588).
References
[1] J.F. Li, Z.L. Xu, H. Yang, L.Y. Yu, M. Liu, Effect of TiO 2nanoparticles on the surface
morphology and performance of microporous PES membrane, Appl. Surf. Sci.
255 (2009) 4725–4732.
[2] M.M. Cortalezzi, J. Rose, A.R. Barron, M.R. Wiesner, Characteristics of ultrafil-
tration ceramic membranes derived from alumoxane nanoparticles, J. Membr.
Sci. 205 (2002) 33–43.
[3] M.M. Cortalezzi, J. Rose, G.F. Wells, J.Y. Bottero, A.R. Barron, M.R. Wiesner,
Ceramic membranes derived from ferroxane nanoparticles: a new route for the
fabrication of iron oxide ultrafiltration membranes, J. Membr. Sci. 227 (2003)
207–217.
[4] I. Soroko, A. Livingston, Impact of TiO 2nanoparticles on morphology and
performance of crosslinked polyimide organic solvent nanofiltration (OSN)
membranes, J. Membr. Sci. 343 (2009) 189–198.
[5] J. Kim, B. Van der Bruggen, The use of nanoparticles in polymeric and ceramic
membrane structures: review of manufacturing procedures and performance
improvement for water treatment, Environ. Pollut. 158 (2010) 2335–2349.
[6] E. Matthiasson, B. Sivik, Concentration polarization and fouling, Desalination
35 (1980) 59–103.
[7] D.E. Potts, R.C. Ahlert, S.S. Wang, A critical review of fouling of reverse osmosis
membranes, Desalination 36 (1981) 235–264.
[8] B. Van der Bruggen, M. Manttari, M. Nystrom, Drawbacks of applying nanofil-
tration and how to avoid them: a review, Sep. Purif. Techol. 63 (2008) 251–263.
[9] B. Van der Bruggen, L. Braeken, C. Vandecasteele, Flux decline in nanofiltration
due to adsorption of organic compounds, Sep. Purif. Techol. 29 (2002) 23–31.
[10] L.D. Nghiem, P.J. Coleman, C. Espendiller, Mechanisms underlying the effects of
membrane fouling on the nanofiltration of trace organic contaminants, Desali-
nation 250 (2010) 682–687.
[11] L. Braeken, R. Ramaekers, Y. Zhang, G. Maes, B. Van der Bruggen, C. Vande-
casteele, Influence of hydrophobicity on retention in nanofiltration of aqueous
solutions containing organic compounds, J. Membr. Sci. 252 (2005) 195–203.
[12] J.M. Arsuaga, M.J. López-Mu ˜noz, A. Sotto, Correlation between retention and
adsorption of phenolic compounds in nanofiltration membranes, Desalination
250 (2010) 829–832.
[13] L. Braeken, K. Boussu, B. Van der Bruggen, C. Vandecasteele, Modeling of the
adsorption of organic compounds on polymeric nanofiltration membranes in
solutions containing two compound, Chemphyschem 6 (2005) 1606–1612.
[14] J.M. Laine, J.P. Hagstrom, M. Clark, M. Mallevialle, Effects of ultrafiltration mem-
brane composition, J. Am. Water Works Assoc. 81 (1989) 61–67.
[15] M. Elimelech, X. Zhu, A.E. Childress, S. Hong, Role of membrane surface
morphology in colloidal fouling of cellulose acetate and composite aromatic
polyamide reverse osmosis membranes, J. Membr. Sci. 127 (1997) 101–109.
[16] C. Jucker, M.M. Clark, Adsorption of aquatic humic substances on hydrophobic
ultrafiltration membranes, J. Membr. Sci. 97 (1994) 37–52.
[17] J. Cho, G. Amy, Interactions between natural organic matter and membranes:
rejection and fouling, Water Sci. Technol. 40 (1999) 131–139.
[18] D.B. Mosqueda-Jimenez, R.M. Narbaitz, T. Matsuura, Membrane fouling test:
apparatus evaluation, J. Environ. Eng. – ASCE 130 (2004) 90–98.[19] Y.N. Yang, H.X. Zhang, P. Wang, Q.Z. Zheng, J. Li, Effect of TiO 2nanoparticles
on the surface morphology and performance of microporous PES membrane, J.
Membr. Sci. 288 (2007) 231–238.
[20] L. Yan, Y.S. Li, C.B. Xiang, Preparation of poly(vinylidene fluoride) (PVDF)
ultrafiltration membrane modified by nano-sized alumina (Al2O3) and its
antifouling research, Polymer 46 (2005) 7701–7707.
[21] X.C. Cao, J. Ma, X.H. Shi, Z.J. Ren, Effect of TiO 2nanoparticle size on the perfor-
mance of PVDF membrane, Appl. Surf. Sci. 253 (2006) 2003–2010.
[22] L. Yan, Y.S. Li, C.B. Xiang, X.D. Shun, Effect of nano-sized Al2O3-particle addi-
tion on PVDF ultrafiltration membrane performance, J. Membr. Sci. 276 (2006)
162.
[23] P. Jian, H. Yahui, W. Yang, L. Linlin, Preparation of polysulfone–Fe 3O4composite
ultrafiltration membrane and its behavior in magnetic field, J. Membr. Sci. 284
(2006) 9–16.
[24] Y.N. Yang, P. Wang, Preparation and characterizations of a new PS/TiO 2hybrid
membranes by sol–gel process, Polymer 47 (2006) 2683–2688.
[25] C.M. Wu, T.W. Xu, W.H. Yang, Fundamental studies of a new hybrid
(inorganic–organic) positively charged membrane: membrane preparation and
characterizations, J. Membr. Sci. 216 (2003) 269–278.
[26] T.H. Bae, I.C. Kim, T.M. Tak, Preparation and characterization of fouling-resistant
TiO 2self-assembled nanocomposite membranes, J. Membr. Sci. 275 (2006)
1–5.
[27] M.L. Luo, J.Q. Zhao, W. Tang, C.S. Pu, Hydrophilic modification of poly(ether sul-
fone) ultrafiltration membrane surface by self-assembly of TiO 2nanoparticles,
Appl. Surf. Sci. 249 (2005) 76–84.
[28] W. Hu, C. Shiyan, B. Zhou, H. Wang, Facile synthesis of ZnO nanoparticles based
on bacterial cellulose, Mater. Sci. Eng. B 170 (2010) 88–92.
[29] W. Lin, Y. Xu, C.C. Huang, Y. Ma, K.B. Shannon, D.R. Chen, Y.W. Huang, Toxicity of
nano- and micro-sized ZnO particles in human lung epithelial cells, J. Nanopart.
Res. 11 (2009) 25–39.
[30] X. Deng, Q. Luan, W. Chen, Y. Wang, M. Wu, H. Zhang, Z. Jiao, Nanosized zinc
oxide particles induce neural stem cell apoptosis, Nanotechnology 20 (2009)
7, 115101.
[31] J.H. Yuan, Y. Chen, H.X. Zha, L.J. Song, C.Y. Li, J.Q. Li, X.H. Xia, Determination,
characterization and cytotoxicity on HELF cells of ZnO nanoparticles, Colloids
Surf. B 76 (2010) 145–150.
[32] B. Berardis, G. Civitelli, M. Condello, P. Lista, R. Pozzi, G. Arancia, S. Meschini,
Exposure to ZnO nanoparticles induces oxidative stress and cytotoxicity in
human colon carcinoma cells, Toxicol. Appl. Pharm. 246 (2010) 116–127.
[33] W. Song, J. Zhang, J. Guo, J. Zhang, F. Ding, L. Li, Z. Sun, Role of the dissolved zinc
ion and reactive oxygen species in cytotoxicity of ZnO nanoparticles, Toxicol.
Lett. 199 (2010) 389–397.
[34] N. Franklin, N.J. Rogers, S.C. Apte, G.E. Batley, G.E. Gadd, P.S. Casey, Comparative
toxicity of nanoparticulate ZnO, Bulk ZnO, and ZnCl 2to a freshwater microalga
(Pseudokirchneriella subcapitata): the importance of particle solubility, Envi-
ron. Sci. Technol. 41 (2007) 8484–8490.
[35] A.R. Costa, M.N. de Pinho, M. Elimelech, Mechanisms of colloidal natural organic
matter fouling in ultrafiltration, J. Membr. Sci. 281 (2006) 716–725.
[36] K. Boussu, C. Vandecasteele, B. Van der Bruggen, Study of the characteristics
and the performance of self-made nanoporous polyethersulfone membranes,
Polymer 47 (10) (2006) 3464–3476.
[37] B. Van der Bruggen, K. Everaert, D. Wilms, C. Vandecasteele, Application of
nanofiltration for the removal of pesticides and hardness from ground water:
retention properties and economic evaluation, J. Membr. Sci. 193 (2001)
239–248.
[38] A. Sotto, A. Boromand, S. Balta, J.H. Kim, B. Van der Bruggen, Doping of
polyethersulfone nanofiltration membranes: antifouling effect observed at
ultralow concentrations of TiO 2nanoparticles, J. Mater. Chem. 21 (2011)
10311–10320.
[39] A. Rahimpour, S.S. Madaeni, A.H. Taheri, Y. Mansourpanah, Coupling TiO 2
nanoparticles with UV irradiation for modification of polyethersulfone ultra-
filtration membranes, J. Membr. Sci. 313 (2008) 158–169.
[40] M.L. Luo, J.Q. Zhao, W. Tang, C.S. Pu, Hydrophilic modification of poly(ether sul-
fone) ultrafiltration membrane surface by self-assembly of TiO 2nanoparticle,
Appl. Surf. Sci. 249 (2005) 76–84.
[41] R.A. Damodar, S.J. You, H.H. Chou, Study the self cleaning, antibacterial and
photocatalytic properties of TiO 2entrapped PVDF membranes, J. Hazard. Mater.
172 (2009) 1321–1328.
[42] G. Wu, S. Gan, L. Cui, Y. Xu, Preparation and characterization of PES/TiO 2com-
posite membranes, Appl. Surf. Sci. 254 (2008) 7080–7086.
[43] T.H. Bae, T.M. Tak, Effect of TiO 2nanoparticles on fouling mitigation of ultra-
filtration membranes for activated sludge filtration, J. Membr. Sci. 249 (2005)
1–8.