development of antifouling reverse osmosis membranes for .... development of...
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Development of antifouling reverse osmosis membranes forwater treatment: A review
Guo-dong Kang, Yi-ming Cao*
Dalian National Laboratory for Clean Energy (DNL), Dalian Institute of Chemical Physics (DICP), Chinese Academy of Science (CAS),
457 Zhongshan Road, Dalian 116023, PR China
a r t i c l e i n f o
Article history:
Received 5 August 2011
Received in revised form
7 November 2011
Accepted 14 November 2011
Available online 23 November 2011
Keywords:
Reverse osmosis
Membrane fouling
Antifouling property
Surface modification
Abbreviations: AA, acrylic acid; ADMH, 3-aldihydrochloride; AMPS, 2-acrylamido-2- metspectroscopy; ATRP, atom transfer radical po3,40,5-biphenyl triacyl chloride; DTAB, dodecychloride; iCVD, initiated chemical vapor decomposite; MA, methacrylic acid; MDI, 4,4microfiltration; MPD, m-phenylenediamine;poly(ethylene glycol) diacrylate; PEGDE, poethyleneimine; PIP, piperazine; P(NIPAm-coosmosis; SEM, scanning electron microscopemission electron microscopy; TFC, thin-filmvinylsulfonic acid; XPS, X-ray photoelectron* Corresponding author. Tel.: þ86 411 843790E-mail address: [email protected] (Y.-m.
0043-1354/$ e see front matter ª 2011 Elsevdoi:10.1016/j.watres.2011.11.041
a b s t r a c t
With the rapidly increasing demands on water resources, fresh water shortage has become
an important issue affecting the economic and social development in many countries. As
one of the main technologies for producing fresh water from saline water and other
wastewater sources, reverse osmosis (RO) has been widely used so far. However, a major
challenge facing widespread application of RO technology is membrane fouling, which
results in reduced production capacity and increased operation costs. Therefore, many
researches have been focused on enhancing the RO membrane resistance to fouling. This
paper presents a review of developing antifouling ROmembranes in recent years, including
the selection of new starting monomers, improvement of interfacial polymerization
process, surface modification of conventional RO membrane by physical and chemical
methods as well as the hybrid organic/inorganic RO membrane. The review of research
progress in this article may provide an insight for the development of antifouling RO
membranes and extend the applications of RO technology in water treatment in the future.
ª 2011 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5852. RO membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5863. Development of new RO material or improvement of interfacial polymerization process . . . . . . . . . . . . . . . . . . . . . . . . . 587
3.1. Selection of new interfacial polymerization monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5873.2. Improvement of interfacial polymerization process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587
lyl-5,5-dimethylhydantoin; AFM, atomic force microscope; AIBA, 2,20-azobis(isobutyramidine)hylpropane-sulfonic acid; ATR-FTIR, attenuated total reflectance Fourier transform infraredlymerization; BSA, bovine serum albumin; BTEC, 3,30,5,50-biphenyl tetraacyl chloride; BTRC,ltrimethylammoniumbromide; HEA, 2-hydroxyethyl acrylate; ICIC, 5-isocyanato-isophthaloylposition; iLSMM, in situ hydrophilic surface modifying macromolecules; LFC, low fouling0-methylene bis(phenyl isocyanate); MDMH, 3-monomethylol-5,5-dimethylhydantoin; MF,NF, nanofiltration; PEG, poly(ethylene glycol); PEGA, poly(ethylene glycol) acrylate; PEGDA,ly(ethylene glycol) diglycidyl ether; PEGMA, polyethyleneglycolmethacrylate; PEI, poly--AAc), poly(N-isopropylacrylamide-co-acrylic acid); PVA, polyvinyl alcohol; RO, reverse; SPEEK, sulfonated poly(ether ether ketone); SPM, 3-sulfopropyl methacrylate; TEM, trans-composite; TFN, thin-film nanocomposite; TMC, trimesoyl chloride; UF, ultrafiltration; VSA,spectroscopy.53; fax: þ86 411 84379329.Cao).ier Ltd. All rights reserved.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0 585
4. Surface modification of conventional RO membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5914.1. Physical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591
4.1.1. Surface adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5914.1.2. Surface coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592
4.2. Chemical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5934.2.1. Hydrophilization treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5934.2.2. Radical grafting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5934.2.3. Chemical coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5934.2.4. Plasma polymerization or plasma-induced polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5954.2.5. Initiated chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596
5. Preparation of hybrid RO membranes with inorganic particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5965.1. Directly coating or depositing inorganic particles onto RO membrane surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5965.2. Incorporating inorganic particles via interfacial polymerization process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596
6. Conclusions and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597
1. Introduction consumption and operation costs, making RO technology
The world’s population tripled in the 20th century, and it will
increase by another 40e50% within the next fifty years. This
population growth e coupled with industrialization and
urbanization e will result in a rapid increasing demand for
fresh water. Furthermore, some existing freshwater resources
are gradually polluted and unavailable due to human or
industrial activities. Problemswithwater are expected to grow
worse in the coming decades, with water scarcity occurring
globally, even in regions currently considered water-rich
(Shannon et al., 2008). Therefore, many researchers have
focused on suitablemethods to obtain freshwater by saltwater
desalination and water reuse to sustain future generations
(Van der Bruggen and Vandecasteele, 2002; Khawaji et al.,
2008; Kim et al., 2009). The reverse osmosis (RO) technology
therein, which has been developed more than half a century
of industrial operation, is considered as a promising way and
gaining worldwide acceptance at present (Schiffler, 2004;
Fritzmann et al., 2007; Greenlee et al., 2009).
RO is a pressure-driven process whereby a semi-permeable
membrane (i.e., RO membrane) rejects dissolved constituents
in the feeding water but allows water to pass through (Malaeb
and Ayoub, 2011). Although the concept of RO has been known
for many years, the use of RO as a feasible separation process
is a relatively young technology (Williams, 2006). The progress
in RO technology is greatly depended on the development of
RO membranes because the membrane plays a key role and
determines the technological and economical efficiency of RO
process. In fact, only since Loeb and Sourirajan developed
a method for making asymmetric cellulose acetate
membranes with relatively high water flux and separation
factor in the early 1960s (Loeb and Sourirajan, 1962), especially
the subsequent invention of thin-film composite (TFC)
aromatic polyamide membrane prepared via interfacial
polymerization (Cadotte et al., 1980), RO process became both
possible and practical. In particular, the research and use of
energy recovery systems in recent years, such as the Pelton
wheel, turbocharger, pressure exchanger and Grundfos Pelton
wheel (Avlonitis et al., 2003), have greatly reduced energy
more competitive.
So far, most commercially available RO membranes are
still asymmetric cellulose type (cellulose acetate, triacetate,
cellulose diacetate or their blend) and TFC type. The asym-
metric cellulose ROmembrane is prepared by phase inversion
method, while the TFC ROmembrane is fabricated by forming
a dense aromatic polyamide barrier layer on a microporous
support such as polysulfone via an interfacial polymerization
process (Petersen, 1993). Compared with cellulose membrane,
the TFC aromatic polyamide membrane exhibits superior
water flux and salt rejection, resistance to pressure compac-
tion, wider operating temperature range and pH range, and
higher stability to biological attack (Li and Wang, 2010).
Therefore, it dominates RO membrane field nowadays.
Despite its many advantages, one of obstacles to the
widespread use of TFC polyamide RO membrane is the
proneness to fouling (Subramani and Hoek, 2010). Fouling is
a process where solute or particles in feeding water deposit
onto RO membrane surface in a way that causes flux decline
and affects the quality of the water produced. Although the
performance of fouled RO membranes can be partially
restored by appropriate cleaning method (Ang et al., 2006;
Creber et al., 2010), it will inevitably increase operation diffi-
culty and decrease membrane’s life time, which will be
translated into higher costs.
As a result, many efforts have been made to mitigate this
problem, including the combination with pretreatment
processes (Shon et al., 2004; Pontie et al., 2005), the design of
new membrane modules (van Boxtel et al., 1991) and the
development of antifouling RO membranes. Among these
efforts, the last one is a fundamental route and has been paid
much attention by many researchers and membrane manu-
facturers. So far, numerous papers concerning the improve-
ment of inherent antifouling properties of RO membranes
have been published in last few decades. Nevertheless, there
is no up-to-date review on this topic although many review
papers have been devoted to RO membrane materials or RO
technology (Petersen, 1993; Greenlee et al., 2009; Li andWang,
2010; Lee et al., 2011). In this article, the ROmembrane fouling
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0586
was discussed followed by the review of development
methods for antifouling RO membranes, including the selec-
tion of new starting monomers, improvement of interfacial
polymerization process, surface modification of conventional
RO membrane and the incorporation of inorganic particles.
This paper may provide a reference to the researchers and-
manufactures who are developing fouling resistant RO
membranes.
2. RO membrane fouling
There are mainly four types of foulants in RO membrane
fouling: inorganic (salt precipitations such as metal hydrox-
ides and carbonates), organic (natural organicmatters such as
humic acid), colloidal (suspended particles such as silica) and
biological (such as bacteria and fungi). Because RO
membranes are nonporous, the formation of a fouling layer on
the membrane surface is referred to as the dominant fouling
mechanism (Greenlee et al., 2009). RO membrane fouling is
closely related to the interaction between the membrane
surface and the foulants. Previous studies indicated that the
physicochemical properties of RO membrane surface, such as
hydrophilicity, roughness and electrostatic charge, are major
factors influencing membrane fouling (Louie et al., 2006).
Moreover, if RO membrane has surface-bounded long-chain
molecules (i.e., polymer brush), the steric repulsion effect is
also a factor that should be considered (Kang et al., 2007).
Firstly, it is generally accepted that an increase in hydro-
philicity offers better fouling resistance because many fou-
lants such as protein are hydrophobic in nature (Rana and
Matsuura, 2010). A pure water layer is easily formed on
highly hydrophilic surface, which can prevent the adsorption
and deposition of hydrophobic foulants onto membrane
surface, thus reducing fouling (Fig. 1a). In fact, numerous
studies have been conducted to enhance surface hydrophi-
licity of membranes aiming at the improvement of antifouling
performance. However, it should be noted that membrane
surface hydrophilicity may have a negative effect on fouling
resistance for the hydrophilic components as major foulants
(Kwon et al., 2005).
Secondly, a smoother surface is commonly expected to
experience less fouling, presumably because foulant particles
are more likely to be entrained by rougher topologies than by
smoother membrane surfaces (Sagle et al., 2009). Elimelech
et al. investigated the role ofmembrane surfacemorphology in
colloidal fouling of cellulose acetate and TFC aromatic
Fig. 1 e Schematic diagrams of antifouling mechanisms: (a) pur
polyamide RO membranes, and the results indicated a signifi-
cantly higher fouling rate for the TFCmembranes compared to
that for the cellulose acetate membranes (Elimelech et al.,
1997). The higher fouling rate for the TFC aromatic poly-
amide RO membranes was attributed to larger surface rough-
ness. Another study revealed that the surface roughness was
positively correlated with colloidal fouling of RO membranes
(Vrijenhock et al., 2001). Consequently, the decrease of surface
roughness can improve antifouling property of ROmembranes
(However, low surface roughness may be disadvantageous to
membrane flux (Jeshi and Neville, 2006.)).
Thirdly, the surface charge is also an important factor
influencing membrane fouling. It is easy for us to understand
that the electrostatic repulsive force but not the attraction
force between the charged membrane surface and foulant in
feeding solution is advantageous to reducing membrane
fouling (Fig. 1b). In other words, the antifouling RO
membranes should be developed according to the electro-
static character of foulants in practical situation. For instance,
for negatively charged cellulose acetate membrane and
aromatic polyamide TFC RO membrane, they both exhibited
distinct tolerance to feeding waters containing cation surfac-
tant and anion surfactant. Thesemembranes are easily fouled
by matters with opposite charges. On the basis of this,
Hydranautics Corporation designed a series of low fouling
composite (LFC) RO membranes with different surface
charges, such as LFC-1, LFC-2 and LFC-3. Compared to
conventional negatively charged RO membrane, LFC-1 and
LFC-3 are neutrally charged, while LFC-2 is positively charged.
However, asmentioned above, the application of these LFC RO
membranes should consider the charge properties of targeted
foulants in feeding water. That is to say, none of them can be
used on all occasions.
Finally, some previous research results showed that the
surface-bound long-chain hydrophilic molecules (e.g. poly-
ethylene glycol, PEG) were very effective in preventing
adsorption of macromolecules such as protein onto
membrane surface due to the steric repulsion mechanism
(McPherson et al., 1998; Wang et al., 2002; Nie et al., 2004).
When hydrophilic polymer chains are grafted or created on
membrane surface, this diffused hydrophilic layer will exert
steric repulsion to hydrophobic proteins that reach the
surface (Fig. 1c). Steric repulsion is due to the loss of config-
urational entropy resulting from volume restriction and/or
osmotic repulsion between the overlapping polymer layers
(Wang et al., 2005). The application of polymer brush to
reducing membrane fouling is relatively common in
e water layer; (b) electrostatic repulsion; (c) steric repulsion.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0 587
microfiltration (MF) and ultrafiltration (UF), but rare in nano-
filtration (NF) and RO. Moreover, its effectiveness was affected
by the density, length and regularity of grafted chains. Thus,
a lot of work should be further carried out on this topic.
The understanding of membrane fouling mechanism can
assist in the development of antifouling RO membranes. In
the following sections, the progress in development methods
is reviewed. Most researches are based on the discussions
above, for example, the introduction of hydrophilic layer, the
reduction of surface roughness, the improvement of charge
property and the utilization of steric repulsion effect.
3. Development of new RO material orimprovement of interfacial polymerizationprocess
3.1. Selection of new interfacial polymerizationmonomers
Among the used active monomers to form functional poly-
amide layer in RO membrane, m-phenylenediamine (MPD)
and trimesoyl chloride (TMC) are most common in the past
and present. In fact, many commercial RO membranes are
produced from MPD and TMC by adjusting interfacial poly-
merization conditions. Fig. 2 shows the polyamide RO
membrane dense layer based on TMC and MPD via interfacial
polymerization.
However, the researchers never stop to find new interfacial
polymerization monomers to improve membrane fouling
resistant performance. The new starting monomers usually
contain more functional or polar groups, so the prepared RO
membrane exhibits smoother surface or better hydrophilicity,
which is advantageous to the improvement of antifouling
property. For example, Li et al. synthesized two novel tri- and
tetra-functional biphenyl acid chloride: 3,40,5-biphenyl triacylchloride (BTRC) and 3,30,5,50-biphenyl tetraacyl chloride
(BTEC), which were then used to prepare TFC RO membranes
with MPD (Li et al., 2007). The structures of BTRC, BTEC
and other monomers or modifiers in this article are listed
in Table 1. The atomic force microscope (AFM) images
showed that the BTECeMPD membrane exhibited a smoother
surface and similar hydrophilicity compared to TMCeMPD
HN NH C
O
COCl
ClOC COCl
+
H2N NH2
TMC
MPD
Fig. 2 e The polyamide RO barrier layer derived from
membrane. Based on the analysis above and the research
results of Louie et al. (2006), the BTECeMPD membrane prob-
ably owned better resistance to fouling than those of
TMCeMPD membrane. Nevertheless, it should be noted that
the authors did not conduct fouling experiments, so firm
conclusion cannot be provided.
Similarly, Liu et al. presented a novel RO composite
membrane prepared from 5-isocyanato-isophthaloyl chloride
(ICIC) and MPD (Liu et al., 2006a,b, 2008). ICIC was a func-
tional monomer with trifunctional groups containing both
eCOCl and eN]C]O. The antifouling performance of
resultant polyamide-urea ICICeMPD membrane was tested
with lake water and four simulated aqueous solutions, and
compared with the TMCeMPD membrane and ESPA
membrane (a commercial polyamide RO membrane from
Hydranautics Corporation). Since ICICeMPD membrane had
favorable hydrophilicity and smoother surface (the static
contact angle was 28.5�, 44.3� and 35.0�, and the average
roughness was 43.89 nm, 54.36 nm and 160.2 nm for
ICICeMPD, TMCeMPD and ESPA membranes, respectively), it
showed better resistance to fouling in all five fouling tests.
The results probably proved that the antifouling properties of
RO membranes were closely correlated with their hydrophi-
licity and surface roughness. In addition, Jenkins and Tanner
compared the fouling resistance of two types of TFC RO
membrane with different barrier layers: polyamide and
polyamide-urea (Jenkins and Tanner, 1998). The operational
data also indicated that the latter one exhibited better anti-
fouling property.
3.2. Improvement of interfacial polymerization process
Besides of the exploration of new starting monomers, some
researches have been also focused on the improvement of
interfacial polymerization process. Similarly, the aim of this
concept is also to improve the membrane surface character-
istics, such as increasing the hydrophilicity, reducing the
roughness and introducing polymer brushes, hence, to
enhance the antifouling property of prepared ROmembranes.
The first method is to add active organic modifiers into
TMC or MPD solution. The modifiers can participate in the
reaction and are introduced into functional barrier layer
during the interfacial polymerization process, thus improving
C
O
C O
HN NH C
O
C
O
C O
OH n-1nNH
NH
MPD and TMC via interfacial polymerization.
Table 1 e Structure summary of monomers or modifiers in this article.
Monomer or modifier Structure Reference
3,40,5-Biphenyl triacyl chloride (BTRC)COCl
ClOC
ClOC
Li et al., 2007
3,30,5,50-Biphenyl tetraacyl chloride (BTEC)
COClClOC
ClOC COCl
Li et al., 2007
5-Isocyanato-isophthaloyl chloride (ICIC)
N=C=O
ClOC COCl
Liu et al., 2006a,b, 2008
4,40-Methylene bis(phenyl isocyanate) (MDI)OCN NCO Tarboush et al., 2008;
Rana et al., 2011
Polyethylene glycol (PEG)HO CH2CH2O H
n Tarboush et al., 2008;
Rana et al., 2011
Aminopolyethylene glycol monomethylether
(MPEGeNH2)
CH3O CH2CH2O CH2CH2NH2n Kang et al., 2007
3-Monomethylol-5,5-dimethylhydantoin (MDMH)
N
N
CH3
CH3
H
O
O
HOH2C
Wei et al., 2010a,b
T-X series polyethylene-oxide surfactantC
CH3
CH3
H3C CH2 C
CH3
CH3
O CH2 CH2 OH
n Wilbert et al., 1998
P series polyethylene-oxide surfactantH O CH2 CH2 CH2 O CH2 CH2 O H
m n Wilbert et al., 1998
Polyethyleneimine (PEI)N
NH2
N
H
N
N
H
NH2
N
H
N
NNH
2H2N n
Zhou et la., 2009
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0588
Table 1 e (continued )
Monomer or modifier Structure Reference
Sulfonated poly(ether ether ketone)
(SPEEK)
C
O
O O
SO3Hn
- +Ba and Economy, 2010
Polyvinyl alcohol (PVA)
CH2 CH
OH nHachisuka and Ikeda,
2001; An et al., 2011
PEBAX� 1657C
O
OH CH2C
O
NHC
O
OCH2 CH2
OH
5 X yLouie et al., 2006
Poly(N-isopropylacrylamide-co-acrylic acid)
(P(NIPAm-co-AAc))
H2C
HC
H2C
C
HC
NH
CH
O
H3C CH3
C
OH
Oa b
Yu et al., 2011
Poly(ethylene glycol) diacrylate (PEGDA) H2C CH
C
O
OCH2CH2 O C CH
O
CH2 Sagle et al., 2009
Poly(ethylene glycol) acrylate (PEGA) H2C CH
C
O
OCH2CH2 OH7
Sagle et al., 2009
2-Hydroxyethyl acrylate (HEA) H2C CH
C
O
OCH2CH2 OH Sagle et al., 2009
Acrylic acid (AA) H2C CH
C
O
OH Sagle et al., 2009
Methacrylic acid (MA)H2C C C
O
OH
CH3Belfer et al., 1998a,b
Polyethyleneglycolmethacrylate (PEGMA)H2C C C
O
O
CH3
CH2CH2O OHn Belfer et al., 1998a,b
(continued on next page)
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0 589
Table 1 e (continued )
Monomer or modifier Structure Reference
3-Sulfopropyl methacrylate (SPM)H2C C C
O
O
CH3
CH2 SO3 K3 Belfer et al., 1998a,b
Vinylsulfonic acid (VSA)
H2C CH
SO3 NaBelfer et al., 1998a,b
2-Acrylamido-2-methylpropane-sulfonic acid
(AMPS)
H2C CH
C
O
NH
C
CH3
CH2SO3HH3C Belfer et al., 1998a,b
3-Allyl-5,5-dimethylhydantoin (ADMH)
N
N
CH3
CH3
H
O
O
H2CHCH2C
Wei et al., 2010a,b
Poly(ethylene glycol) diglycidyl ether (PEGDE)
CH2 CH2CH2O OCH2n
HCH2CO
CH CH2O Van Wagner et al., 2010
Poly(ethylene glycol) derivative
H2N
CH3
O
CH3
OO
CH3
NH2
x y zKang et al., 2011
Trimethylene glycol dimethyl ether (triglyme)O
H3CH2C C
H2O C
H2
H2C O
H2C C
H2O CH3
Zou et al., 2011
Zwitterionic modifierN S
O
O O
Yang et al., 2011
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0590
the surface property and fouling resistance of resultant RO
membranes. For example, Rana and coworkers added 4,40-methylene bis(phenyl isocyanate) (MDI) and PEG (average
molecular weight 200 and 1000 Da) into organic phase con-
taining TMC in interfacial polymerization to incorporate in
situ hydrophilic surface modifying macromolecules (iLSMM)
into the TFC membranes (Tarboush et al., 2008; Rana et al.,
2011). The prepared membranes, which exhibited signifi-
cantly more hydrophilic surface, were then subjected to long-
term fouling studies using model foulants including sodium
humate, silica particles and chloroform spiked in the feeding
NaCl solution. The results showed that the flux decline was
reduced significantly after incorporating iLSMM into the TFC
membranes, indicating better antifouling performance. A
similar study was conducted by An et al. who added polyvinyl
alcohol (PVA) into piperazine (PIP) solution during the inter-
facial polymerization to prepare antifouling NF membrane
(An et al., 2011).
Different from the method above, the author and
coworkers proposed another idea (Kang et al., 2007). As we
know, the TFC polyamide RO membrane prepared from TMC
and MPD via interfacial polymerization usually contains
carboxylic acid groups on the surface, which are from the
hydrolysis of unreacted acyl chloride groups (Petersen, 1993).
In other words, the nascent polyamide RO membrane surface
has numerous acyl halide groups. Based on these active acyl
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0 591
chloride groups, a novel surface modification method of
polyamide RO membrane by chemical coupling was devel-
oped, which is summarized in Fig. 3. A kind of hydrophilic
polymer (aminopolyethylene glycol monomethylether,
MPEGeNH2) as modifier was grafted onto membrane surface
to improve the antifouling property. The prepared RO
membrane exhibited relative better resistance to fouling
owing to the enhanced hydrophilicity and steric repulsion
effect. However, since the modifier was macromolecular
having comparative lower activity, the resultant membrane
surface was not completely covered and had larger roughness
which was undesired.
After that, Wei et al. adopted the same method to graft
a hydantoin derivative with smaller molecule, 3-
monomethylol-5,5-dimethylhydantoin (MDMH), onto the
nascent RO membrane surface (Wei et al., 2010a,b). Through
modification, the membrane surface hydrophilicity was
enhanced obviously as contact angles decreased from 57.7� to50.4e31.5� without obvious change in surface roughness. The
test results using Escherichia coli (E. coli) as a model microor-
ganism foulant verified a substantial prevention of modified
membranes against biofouling. It was worth notice that the
MDMH-modified RO membrane also possessed high chlorine
resistances, offering a potential use as a new type of chlorine
resistant and anti-biofouling RO membrane.
Recently, Zou et al. further developed this idea (Zou et al.,
2010). In their research, the nascent RO membrane fabri-
cated from TMC and MPD via interfacial polymerization was
then placed in MPD solution again to react with residual acyl
halide groups onmembrane surface. The results of attenuated
total reflectance Fourier transform infrared spectroscopy
(ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)
revealed that the active skin layer of resultant RO membrane
contained a large amount of hydrophilic eNH2 groups. More-
over, the scanning electron microscope (SEM) images indi-
cated that this approach offered smoothermembrane surface.
Consequently, the prepared RO membrane showed a rela-
tively better antifouling property than traditional membrane
when dodecyltrimethylammoniumbromide (DTAB) and
humic acid were used as model foulants. Most importantly,
the reactivity of amino groups is very high, thus it is possible
to further develop multifunctional RO membranes on the
basis of them.
Fig. 3 e Surface modification of nascent polyamide RO membra
4. Surface modification of conventional ROmembranes
Surface modification of existing membranes is also consid-
ered as a potential and effective route to develop antifouling
membranes. So far, there are many articles related to the
surface modification of conventional RO membranes to
improve the surface morphology and properties, thus
enhancing the antifouling ability. The surface modification
method ranges from physical to chemical treatments.
4.1. Physical method
4.1.1. Surface adsorptionPhysical adsorption is a simple tool for modification and
structuring of polymer surfaces. Some researchers adopted
this method to modify the surface properties of water filtra-
tion membranes (Xie et al., 2007). For example, Wilbert et al.
used a homologous series of polyethylene-oxide surfactants
(T-X series and P series) to modify the surface of commercial
cellulose acetate blend and polyamide RO membranes
(Wilbert et al., 1998). The surface adsorption, where the
hydrophobic portion of the surfactant had a favorable free
energy of attraction for the polymeric surface, made a change
in membrane surface character. The tests showed that the
roughness of polyamide RO membrane after treatment was
reduced, and it exhibited improved antifouling property in a
vegetable broth solution compared to unmodified membrane.
However, the results of cellulose acetate RO membrane were
inconclusive.
Besides surfactants, the charged polyelectrolytes are also
used for surface modification of RO membrane. Zhou and
coworkers modified the polyamide RO membrane by electro-
static self-assembly of polyethyleneimine (PEI) on the
membrane surface (Zhou et al., 2009). The charge reversal on
the membrane surface due to the application of the PEI layer
was shown to increase the fouling resistance to cationic fou-
lants because of the enhanced electrostatic repulsion as well
as increased surface hydrophilicity.
Similarly, Ba and Economy developed a nearly neutrally
charged NF membrane by adsorption of a layer of negatively
charged sulfonated poly(ether ether ketone) (SPEEK) onto the
ne based on the unreacted acyl chloride groups on surface.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0592
surface of a positively charged NF membrane (Ba and
Economy, 2010). When using bovine serum albumin (BSA),
humic acid and sodium alginate as the model foulants, the
modified membrane exhibited much better fouling resistance
than both the positively and the negatively charged
membranes. The foulants would less likely deposit onto the
membrane due to the elimination of the charge interaction
between the membrane and the foulants. Although this study
was focused on NF membrane, it could also provide a refer-
ence to modify RO membranes.
4.1.2. Surface coatingSurface coating is a convenient and efficient technique for
membrane surface modification, and it has been widely
adopted to tailor the surface properties of conventional RO
membranes. In this method, the RO membranes can be not
only directly coated using proper water-insoluble polymers
(commercial or artificially synthesized), but also coated with
water-soluble molecules followed by cross-linking to make
them water-insoluble. Here, the coating acts as a protective
layer to reduce or eliminate the adsorption and deposition of
foulants onto membrane surface. Surface coating is a simple
way and easily operated, so it has been paidmuch attention by
many researchers and membrane manufacturers so far.
Hachisuka and Ikeda coated hydrophilic and electric
neutral PVA onto polyamide RO membrane to improve the
antifouling properties (Hachisuka and Ikeda, 2001). After
coating, the hydrophilicity of membrane surface was
enhanced. Moreover, the surface zeta potential (x) at pH 6
changed from �25 mV to 0 mV. Therefore, the coated RO
membrane exhibited a better antifouling property in indus-
trial wastewater and cationic surfactant feeding solution.
Similarly, when the PVA coated RO and NF membranes with
decreased surface charge and surface roughness were used to
treat dyeing process wastewater, the results showed that the
coating reduced fouling significantly (Kim and Lee, 2006). In
addition, some researchers further crosslinked the coated
PVA on membrane surface with glutaraldehyde and hydro-
chloric acid to increase the stability of coating layer (Wu et al.,
2006), which was an important factor in long-term operation.
Louie et al. performed another physical coating study of
commercial polyamide RO membranes with PEBAX� 1657,
which was a very hydrophilic block copolymer of nylon-6 and
poly(ethylene glycol) (Louie et al., 2006). The coating greatly
reduced surface roughness without significant change in
contact angle. During a long-term (106-day) fouling test with
an oil/surfactant/water emulsion, the rate of flux decline was
slower for coated than for uncoated membranes. However,
the coating resulted in large water flux reduction, especially
for high-flux RO membranes (ESPA1 and ESPA3). Most
recently, the authors systematically investigated the effects of
surface coating process conditions on the water permeation
and salt rejection properties to increase or restore the water
flux of coated RO membrane (Louie et al., 2011).
Yu and coworkers synthesized a thermo-responsive
copolymer, poly(N-isopropylacrylamide-co-acrylic acid)
(P(NIPAm-co-AAc)), for the surface modification of TFC poly-
amide RO membranes (Yu et al., 2011). The coating layer was
shown to increase membrane surface hydrophilicity and
surface charge at neutral pH. The results of the fouling
experiments with BSA aqueous solution and cleaning exper-
iments with de-ionized water revealed that the P(NIPAm-co-
AAc) coating layer improved the membrane fouling resis-
tance to BSA and the cleaning efficiency.
In addition, Sarkar et al. prepared two types of dendrimer-
based coatings for polyamide RO membranes aimed at elim-
ination of fouling by organic contaminants and biological
species (Sarkar et al., 2010). Dendrimers are highly branched,
globular, nanoscopic macromolecules composed of two or
more tree-like dendrons emanating from a central core which
can be either a single atom or an atomic group. The coating in
their study was a crosslinked honeycomb-like network of
dendritic cells prepared from highly hydrophilic polyamido-
amine and polyethylene glycol. After coating, the contact
angle decreased from 60� to 35�, and the coating generally
smoothed RO membrane surface. Since the improvement of
surface hydrophilicity, the decrease of roughness and the
dynamic brush-like topology are all advantageous to
enhancing antifouling properties, the dendrimer-based coat-
ings in their study may provide a novel approach to develop
fouling resistant RO membranes.
Moreover, the researchers from Freeman group in The
University of Texas at Austin developed a series of fouling
resistant coatingmaterials by lightly cross-linking, whichwere
used for thesurfacemodificationofwaterfiltrationmembranes
including commercial ROmembrane (Ju et al., 2008; Sagle et al.,
2009; Hatakeyama et al., 2009; La et al., 2011). Many promising
results were obtained. In this method, the liquid prepolymer
mixture (monomer, crosslinker and photoinitiator) was firstly
coatedonsurfaceofROmembraneand thenphotopolymerized
to form a water-insoluble coating. For example, Sagle et al.
modified commercial RO membranes with crosslinked PEG-
based hydrogels using poly(ethylene glycol) diacrylate
(PEGDA) as the crosslinker and poly(ethylene glycol) acrylate
(PEGA), 2-hydroxyethyl acrylate (HEA), or acrylic acid (AA) as
comonomers (Sagle et al., 2009). Model oil/water emulsions
were used to probe membrane fouling. The testing results
indicated that the surface-coated membranes exhibited
improved fouling resistance and an improved ability to be
cleaned after fouling compared to the unmodifiedmembranes.
In other references, Hatakeyama and La et al. introduced new
protein-resistant coatings based on quaternary ammonium
and phosphonium polymers or ammonium salt, which had
potential applicationperspective indevelopmentof antifouling
RO membranes (Hatakeyama et al., 2009; La et al., 2011).
It can be seen that, the materials for surface coating to
improve membrane antifouling property are usually hydro-
philic polymers containing hydroxyl, carboxyl or ethylene
oxide groups. This is consistent with the research results by
Tang et al. (2007, 2009a,b). They fully characterized several
widely used commercial RO and NF polyamidemembranes by
AFM, transmission electron microscopy (TEM), contact angle
measurement and streaming potential analysis, and found
that some commercial RO membranes were coated with
aliphatic polymeric alcohol (which seemed to be PVA). The
presence of coating layer could significantly enhance hydro-
philicity and reduce surface charge and roughness of
membrane, rendering a better antifouling property.
It should be noted that here, however, the coatedmaterials
may penetrate into the ridge-and-valley structure of
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0 593
polyamide RO membrane and increase the permeation resis-
tance, resulting in the decline of water flux after modification.
Therefore, for practical purposes, the coating layer should
have an inherently high water permeability and be made
sufficiently thin to maintain the water flux as possible. On the
other hand, the modifiers in physical modification are only
connected with membrane surface by van der Waals attrac-
tions, hydrogen bonding or electrostatic interaction, so the
antifouling property of modified RO membranes may be
gradually deteriorated due to the loss or leaching of coating
layer during long-term operation.
4.2. Chemical method
4.2.1. Hydrophilization treatmentAs mentioned above, surface hydrophilization treatment of
membrane is advantageous to enhancing fouling resistance
because many foulants are hydrophobic in nature. Kulkarni
et al. used some hydrophilizing agents, including hydro-
fluoric, hydrochloric, sulfuric, phosphoric and nitric acids to
modify the surfaces of TFC RO membranes (Kulkarni et al.,
1996). At solvatable sites along the polyamide chain, the
reactions caused the partial hydrolysis to more hydrophilic
eNH2 and eCOOH, which was consistent with the contact
angle measurement. The surface characterization indicated
an increase in hydrophilicity of the membrane surface after
treatment. The proposed method was very simple and easily
carried out. However, the concentration of acid and the time
of exposuremust be well controlled to avoid the breakdown of
polymeric structures, resulting in the decrease of salt rejec-
tion. Moreover, the antifouling property of treated RO
membranes was not investigated in their study.
4.2.2. Radical graftingRadical grafting is an effective way for polymer modification.
In this process, the free radicals are produced from the initi-
ators and transferred to the polymer to react with monomer,
realizing the modification of membrane material. In general,
the proposed grating site for polyamide chain is the hydrogen
in amide bond.
One successful system is developed by Belfer group in
Israel (Belfer et al., 1998a,b, 2001, 2004; Gilron et al., 2001;
Freger et al., 2002). This method is based on a redox-initiated
radical grafting of vinyl monomers onto polyamide RO or NF
membranes surface. A redox system, composed of potassium
persulfate and potassium metabisulfite, was used to generate
radicals. They attacked the polymer backbone (abstracting the
hydrogen atom in amide bonding), thus initiating the grafting
of monomers to the membrane surface. Polymerization then
occurred via propagation. Various hydrophilic monomers
were used such as acrylic acid (AA), methacrylic acid (MA),
polyethyleneglycolmethacrylate (PEGMA), 3-sulfopropyl
methacrylate (SPM), vinylsulfonic acid (VSA) and 2-
acrylamido-2-methylpropane-sulfonic acid (AMPS). The MA
modified membrane had a higher negative zeta potential over
the whole pH range because of the higher degree of dissocia-
tion of carboxylic groups. The membranes modified with
PEGMA and SPM showed lower receding contact angle or
advancing contact angle, implying more hydrophilic than the
unmodified membranes. On the other hand, the modified RO
membranes showed some reduction of roughness compared
to the virgin membrane irrespective of the type of monomer
used. In general, the membrane after grafting with hydro-
philicmonomers showed less adsorption of foulants andwere
more easily cleaned than the unmodified membranes.
Wei et al. performed a similar radical grafting study.
Nevertheless, the initiator in their report was 2,20-azobis(iso-butyramidine) dihydrochloride (AIBA), which can be ther-
mally decomposed to generate free radicals (Wei et al., 2010b).
In their study, 3-allyl-5,5-dimethylhydantoin (ADMH) was
used as grating monomer. Similarly, the ADMH-grafted RO
membranes had lower contact angles than those of the raw
membranes, indicating the increase of surface hydrophilicity.
After exposures to microbial cell suspension, the modified
membranes showed slighter decrease in pure water flux and
less adsorption in microbial colonies on surface, which veri-
fied the improvement of anti-biofouling properties.
The schematic diagramof radical grafting can be presented
in Fig. 4.
4.2.3. Chemical couplingThe conventional polyamide RO membrane surface has free
carboxylic acid and primary amine groups (on chain ends)
(Petersen, 1993). These relatively active groups provide the
possibility of surface modification via chemical reaction or
coupling. Some researches have been conducted on basis of
this to improve membrane surface properties and other
performances.
Van Wagner et al. modified commercial polyamide RO
membranes based on the reaction of primary amine groups
with the epoxy end groups of poly(ethylene glycol) diglycidyl
ether (PEGDE) (Van Wagner et al., 2010). Although
membranes after modification experienced minimal changes
in surface properties (e.g., surface charge, hydrophilicity and
roughness), they generally demonstrated improved fouling
resistance to charged surfactants and emulsions containing
n-decane and a charged surfactant. Moreover, they found
that PEGDE molecular weight had a stronger influence on
fouling resistance than did PEGDE treatment concentration.
The modification of RO membrane with lower concentrations
(i.e., less than 1% (w/w)) of higher molecular weight (i.e.,
greater than 1000) PEGDE may be a means of optimizing the
balance between water flux and fouling resistance. Similarly,
Mickols and Koo et al. independently adopted the same idea
to modify polyamide RO membranes using glycidyl ether-
type materials, and the resultant membranes showed better
fouling resistance (Mickols, 2001; Koo et al., 2005). Fig. 5
presents the schematic diagram showing surface modifica-
tion of polyamide RO membrane based on the chemical
reaction between primary amine groups and the epoxy-end
modifiers.
In addition, the author and coworkers developed
a different surface modification method of polyamide RO
membrane based on the existing carboxylic acid groups on
surface with the help of carbodiimide (Kang et al., 2011). The
carbodiimide is a coupling reagent for the activation of
carboxylic acid groups, promoting the modification reaction.
The grafting process of PEG derivatives onto polyamide RO
membrane is shown in Fig. 6. Similar results were obtained in
fouling test. Compared to the original membrane, the
HN NH CO
CO
C O
HN NH CO
CO
C OOHn 1-n
HN NH2
HN NH CO
CO
C O
HN NH CO
CO
C OOHn 1-n
HN NH CH2 CH OH
R
HC CH2RO
Epoxy-end modifier
Fig. 5 e Surface modification of polyamide RO membrane based on the chemical reaction between primary amine groups
and the epoxy-end modifiers.
HN NH CO
CO
C O
HN NH CO
CO
C OOHn 1-n
Redox system
N N CO
CO
C O
N N CO
CO
C OOH n-1n
H2C CH
R
N N CO
CO
C O
N N CO
CO
C OOH
n 1-n
CH2
HC R
CH2
CH2
R
n
H2C
HC R
CH2
CH2
R
n
CH2
HC R
CH2
CH2
R
nH2C
HC R
CH2
CH2
R
n
or initiator
H2C C
R
(Modifier)
Radical
Fig. 4 e Surface modification of polyamide RO membrane via radical grafting.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0594
Fig. 6 e Surface modification of polyamide RO membrane by carbodiimide-induced grafting with PEG derivates.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0 595
modified RO membranes were more resistant to fouling in
protein and cationic surfactant feeding solutions.
4.2.4. Plasma polymerization or plasma-inducedpolymerizationPlasma treatment is a technique for the surface modification
of polymer materials to improve the surface properties. This
method includes plasma polymerization and plasma-induced
polymerization. Plasma polymerization is a one-step process
as the plasma is used to deposit the polymer onto membrane
surfaces, while the plasma-induced polymerization utilizes
plasma to activate the surface to generate oxide or hydroxide
groups, which can then be used in conventional polymeriza-
tion methods (two-step process) (Zou et al., 2011). So far,
plasma treatment has been utilized on a variety of materials
including the surfacemodification of TFC ROmembranes (Wu
et al., 1997; Yu et al., 2007).
For example, Zou grafted a PEG-like hydrophilic polymer
(trimethylene glycol dimethyl ether) onto aromatic polyamide
RO membrane by plasma polymerization to reduce organic
fouling tendency (Zou et al., 2011). After modification,
a reduction in contact angles from 32� to 7� was achieved for
the treated membranes, indicating the enhanced surface
hydrophilicity. The fouling experiments revealed that the
modified membranes achieved an excellent maintenance of
flux compared to the untreatedmembranes. Specifically, after
210-min of filtration using BSA and alginate asmodel foulants,
Fig. 7 e Modification of polyamide RO membrane via plasma-in
polymerization.
no flux decline was found for the modified membranes, while
a 27% reduction of the initial flux was observed for the
untreated membrane. Moreover, the modified membranes
were easily cleaned. The flux recovery after cleaning by water
only was up to 99.5% for the modifiedmembrane, while it was
only 91.0% for the untreated one. The plasma polymerization
showed a clear improvement in membrane antifouling
performance. However, the plasma polymerization in their
study caused a increased roughness, from 61.9 nm for the
untreated sample to 66.2 nm, 86.9 nm and 89.3 nm after 15 s,
30 s and 60 s of treatment, respectively, which is disadvanta-
geous to the colloidal fouling resistance according to the
research results by Elimelech et al. (1997).
In addition, Lin and Kim and coworkers presented a study
on surface nano-structuring of RO membranes via atmo-
spheric pressure plasma-induced graft polymerization for
fouling resistance and improved flux performance (Lin et al.,
2010; Kim et al., 2010). The surface modification process is
summarized in Fig. 7. The polyamide RO membranes were
activated with impinging atmospheric plasma, followed by
a solution free-radical graft polymerization of water-soluble
monomers, including methacrylic acid (MAA) and acryl-
amide (AA), onto the surface of membrane. The results
showed that, PMAA and PAA brush layers on the polyamide
surface resulted in RO membranes of significantly lower
mineral scaling propensity, compared to the commercial RO
membrane (LFC-1) with same salt rejection and surface
duced surface activation followed by surface grafting
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0596
roughness. Moreover, an additional advantage of the treated
membranes was their high permeability, relative to
commercial ROmembranes of the same rejection and organic
fouling resistance.
4.2.5. Initiated chemical vapor depositionInitiated chemical vapor deposition (iCVD) is an all-dry free-
radical polymerization technique performed at low tempera-
tures and low operating pressures, which has shown great
promise as a surface modification technique (Yang et al.,
2011). By using this technique, Yang et al. synthesized
a copolymer containing poly-(sulfobetaine) zwitterionic
groups, which was covalently grafted on to RO membrane for
surface modification. The cell adhesion tests using E. coli
showed that the modified RO membranes exhibited superior
antifouling performance compared to the bare ROmembrane.
Unlike the physical method, the modifiers are covalently
connected with membrane surface in chemical method,
belonging to permanent modification. Therefore, it is better
for long-term operation. Nevertheless, the chemical modifi-
cation of RO membranes may require special equipments,
reagents or complicated operation processes, limiting its
practical application.
5. Preparation of hybrid RO membranes withinorganic particles
Apart from the organic modifiers, another important develop-
ment in antifouling RO membrane is the incorporation of
nanoscale inorganic particles into membrane. This process
combines important properties of conventional membrane
polymers (highdesalinationperformance, flexibility andease of
manufacture)with the unique functionality ofmolecular sieves
(tunable hydrophilicity, charge density, pore structure and
antimicrobial capability along with better chemical, thermal
andmechanical stability) (Jeongetal., 2007). Thehybridorganic/
inorganic ROmembranes can be prepared by directly coating or
depositing inorganic particles onto membrane surface and
incorporating inorganic particles into membrane structure via
interfacial polymerization process. The commonly used inor-
ganic particles include TiO2, SiO2, Zeolite A and silver nano-
particles and as well as mesoporous materials.
5.1. Directly coating or depositing inorganic particlesonto RO membrane surface
Kwak and Kim and coworkers synthesized positively charged
particles of the colloidal TiO2 by the controlled hydrolysis of
Fig. 8 e Conceptual illustration of (a) TFC
titanium tetraisopropoxide (Kwak and Kim, 2001; Kim et al.,
2003; Kwak et al., 2003). The resulting nanosized particles in
acidic aqueous solution were about 10 nm or less. Then, the
hybrid organic/inorganic aromatic polyamide RO membrane
was prepared by dipping virgin membrane into TiO2 colloidal
solution. The self-assembly of TiO2 nanoparticles onto
membrane surface was realized through the coordination and
H-bonding interaction with eCOOH functional groups in
aromatic polyamide layer. The antibacterial fouling potential
of TiO2 hybrid RO membrane was verified by determining the
survival ratios of E. coli as a model bacterium. The less loss of
RO permeability was observed, suggesting a potential use as
a new type of anti-biofouling TFC membrane.
TiO2 is a photocatalytic material and has been widely used
for disinfection and decomposition of organic compounds.
These properties make it interesting as a self-cleaning
coating. For example, Madaeni and Ghaemi created a kind of
self-cleaning RO membrane using TiO2 as coating. In their
study, the TiO2 particles were coated on RO membrane by
dipping method. The TFC-SR composite membranes used
were not polyamide-type, and the top layer was made of PVA
(Madaeni and Ghaemi, 2007). By optimizing the coating
conditions, the modified membranes exhibited better anti-
fouling and self-cleaning properties. Similarly, Yang et al.
used nanosilver particles as coating to modify commercial
polyamide RO membrane for biofouling control (Yang et al.,
2009). The results showed that this approach is beneficial to
biofouling prevention.
Similar to surface coating with organic modifiers, the
coated or deposited inorganic particles onto RO membrane
surface also face a problem of loss or leaching. Therefore,
further studies should be performed to strengthen the
combination between RO membrane surface and coated
inorganic particles, hence, to maintain the long-term
improvement of antifouling property.
5.2. Incorporating inorganic particles via interfacialpolymerization process
Another method to prepare hybrid organic/inorganic RO
membranes is adding nanosized inorganic particles in TMC
phase or MPD phase to realize their incorporation into
membrane structure via interfacial polymerization process.
This idea is similar to the study on organic modifiers
mentioned in Section 3.2 above.
Jeong et al. reported a method to prepare thin-film nano-
composite (TFN) polyamide RO membrane (Fig. 8) by
dispersing 0.004e0.4% (w/v) of synthesized zeolite A nano-
particles (particle sizes range from 50 nm to 150 nm) in TMC
and (b) TFN membrane structures.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 5 8 4e6 0 0 597
solution (Jeong et al., 2007). Nanoparticle dispersion was ob-
tained by ultrasonication for 1 h at room temperature imme-
diately prior to interfacial polymerization. Other fabrication
processes were same to the traditional method of TFC
membrane. The prepared zeoliteepolyamide membrane
surface showed enhanced hydrophilicity, more negative
charge and lower roughness, implying a strong potential use
as an antifouling membrane. Most recently, Fathizadeh et al.
performed a similar study (Fathizadeh et al., 2011). Never-
theless, they did not investigate the fouling resistance of
prepared hybrid RO membranes.
On the other hand, Rana et al. added 0.25 wt% of silver
salt (silver nitrate, or silver citrate hydrate or silver lactate)
into aqueous MPD phase instead of organic TMC phase to
prepare hybrid organic/inorganic RO membrane (Rana et al.,
2011). Meanwhile, the hydrophilic surface modifying
macromolecules (polyurethane end-capped with PEG) were
also added into aqueous phase containing MPD. In other
words, the authors combined the organic and inorganic
modifiers into RO membranes, optimizing the fouling resis-
tance. The results showed that silver salts incorporated in
the TFC membranes indeed improve the anti-biofouling
property.
At present, this method was also used in the development
of antifouling nanofiltration membranes (Lee et al., 2007;
Jadav and Singh, 2009). For example, Lee et al. prepared
polyamide/Ag nanocomposite membranes from in situ inter-
facial polymerization between aqueous MPD and organic TMC
together with 10 wt% of silver nanoparticles. The hybrid
membranes were shown to possess the dramatic anti-
biofouling effect on Pseudomonas. Moreover, most of the Ag
particles remained on the surface even after the performance
test, confirmed with SEM, XPS and AFM. It should be noted
that, however, besides of on membrane surface, some nano-
particles were also encapsulated within polyamide thin films,
reducing the antifouling or antimicrobial activities.
6. Conclusions and future perspectives
The development of antifouling is an important research
direction in RO technology for water treatment and has
attracted wide attention in recent years. In this paper, the
progress in this area is reviewed. The development methods
are related to the surface modification of conventional RO
membranes, improvement of interfacial polymerization
process and exploitation of new RO membranes.
Surface modification is an effective way to tailor
membrane surface properties, thus improving the fouling
resistant performance. Apart from the approaches and
hydrophilic modifiers mentioned above, some other methods
such as atom transfer radical polymerization (ATRP) tech-
nique and other modifiers such as zwitterionic charged
materials are also potential to develop antifouling RO
membranes. However, surface modification, either physical
method or chemical method, usually leads to the decline of
water flux. The trade-off of flux reduction and antifouling
property should be optimized and balanced. Moreover,
surface modification is conducted after the formation of RO
membrane, increasing the production difficulty and/or
operation cost. The method whereby the membrane fouling
resistance can be enhanced in situ (i.e., in preparation
process) is of particular interest from a practical point of view.
The addition of inorganic particles into polymeric
membranes is a new development direction in RO technology.
The hybrid organic/inorganic RO membranes show attractive
permeability characteristics, antifouling and self-cleaning
properties, and they are very promising in commercial use.
In fact, the nanocomposite RO membranes have been indus-
trialized in market at present (for example, http://www.
nanoh2o.com/) and may be extensively used in future.
Despite the achievements, there are still some issues or
challenges facing antifouling RO membranes. Firstly, many
development methods are confined to scientific research
currently due to high cost, complicated operation procedure
or difficulty in scaling up, and only few methods are ready for
commercial use. Secondly, the studies on long-term fouling
test should be paid further attention. The stability ofmodifiers
should be verified in actual application. In fact, the improve-
ment of antifouling property through some physical modifi-
cations, such as surface adsorption or even surface coating,
may be easily deteriorated in long-term operation due to the
loss of modifiers. Generally, the chemically covalent linkage
between membrane and modifiers is superior to physical
combination and has better practical utility. However, special
equipments or chemical reagents are usually needed in
chemical modification method. These will increase the
production cost or cause environmental pollution. Thirdly,
few studies are focused on the stability of surface modifiers in
cleaning operation. In fact, the cleaning is a necessary process
in RO membrane use. The acid, alkaline or other cleaning
environments may cause the degradation of modifiers, which
should be also considered in practical application.
Last, but not least, the fouling cannot be thoroughly pre-
vented even for antifouling membranes. There are no
membranes that are free from fouling under any circum-
stances (Rana and Matsuura, 2010). The selection and use of
RO membrane should be based on the foulants character in
feeding solution. Moreover, some other measures such as
module design optimization, proper pretreatment and effec-
tive membrane cleaning are also necessary.
Acknowledgments
Financial support from National Natural Science Foundation
of China (Grant No. 20906086) and Major State Basic Research
Development Program of China (Grant No. 2009CB623405) are
gratefully acknowledged.
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