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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: ymcao@dicp.ac.cn (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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