Enhancement of surface properties and performance of reverse osmosis membranes after surface modification: A review

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Enhancement of surface properties and performance of reverse osmosis membranes after surface modification: A review Mahdieh Asadollahi⁎, Dariush Bastani, Seyyed Abbas Musavi Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Reverse osmosis Membrane Surface modification Fouling Water purification

Reverse osmosis (RO) membrane process has become the most promising technology for desalination to produce purified water. Among numerous polymeric materials used to fabricate RO membranes, aromatic polyamide thin film composite (TFC) membranes are dominant in commercial RO membrane processes because of their high salt rejection and water permeability as well as their excellent chemical, thermal, and mechanical stability. However, the major hindrance to the effective application of polyamide TFC RO membranes is membrane fouling. Furthermore, polyamide TFC RO membranes have limited stability to chlorine, which is commonly used as disinfect to control membrane biofouling. These two factors deteriorate membrane performance and shorten membrane life span. Membrane fouling depends strongly on membrane surface morphology and properties. Up to now, many physical or chemical surface modifications have been reported to alter the surface properties so as to improve the fouling resistance of the RO membranes. In this paper, different kinds of RO membranes fouling, chlorine effects, factors that influence RO membranes fouling, and the ways for reduction of fouling in RO membranes were discussed. Moreover, as a main part of this paper, all physical and chemical surface modification methods for fabricated polyamide TFC RO membranes were completely reviewed.

1. Introduction

and the poor water quality of the local feed water, RO membrane processes have rapidly developed and now surpass thermal processes in new plant installations [3,14–16]. Fig. 1 shows the global desalination capacity by process, highlighting the high capacities shares of RO and MSF [17]. There are many significant advantages of using membranes for industrial processes which make membranes as a popular technology in various fields such as water treatment. Membrane technology are modular which is easy to scale up, simple in operation, relatively low energy consumption, no chemical additives, etc. [18–21]. Separation with membranes occurs because of the existence of a gradient across the membrane, and membrane processes may be divided into groups based on the specific type of gradient. The most common way of categorizing membranes is to divide them into two groups: non-pressure driven and pressure driven. In non-pressure driven membrane processes, the driving forces are concentration gradient (such as gas separation and pervaporation), temperature gradient (membrane distillation), and electrical potential gradient (electrodialysis). The pressure driven process includes four main groups depending on the size of particle they retain: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) (Fig. 2) [22–27].

Today water scarcity is one of the most serious global challenges and needs to be urgently solved, as it is exacerbated by population growth, industrialization, and climate change [1,2]. The fact that only around 0.8% of the total earth's water is fresh water conduct numerous researches in an effort to develop more sustainable technological solutions that would meet increasing water consumption [3–6]. Desalination is the process of removing salts or other minerals and contaminants from seawater, brackish water, and wastewater effluent and it is an increasingly common solution to obtain fresh water for human consumption and for domestic/industrial utilization. Desalination technologies can be classified by their separation mechanism into thermal and membrane based processes. Thermal desalination separates salt from water by evaporation and condensation and includes multistage flash (MSF), multiple effect distillation (MED), and vapor compression (VC). In membrane desalination, water diffuses through a membrane, while salts are almost completely retained and includes reverse osmosis (RO) and electrodialysis (ED) [7–13]. While thermal desalination has remained the primary technology of choice in the Middle East due to easily accessible fossil fuel resources

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Corresponding author. E-mail address: [email protected] (M. Asadollahi).

http://dx.doi.org/10.1016/j.desal.2017.05.027 Received 18 December 2016; Received in revised form 5 April 2017; Accepted 23 May 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Asadollahi, M., Desalination (2017), http://dx.doi.org/10.1016/j.desal.2017.05.027

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membrane (g/(cm2 s bar)), ΔP is the transmembrane pressure difference (bar) and Δπ is the difference in osmotic pressure between the feed and permeate (bar). In many cases it is more appropriate to refer to salt rejection, R, which is a measure of the ability of the membrane to separate solute from the feed solution. The definition of salt rejection is either the observed rejection (Ro) or real rejection (Rr). Observed rejection is calculated from bulk (feed) and the permeate concentrations according to:

Ro = 1 −

CP Cb

(2)

While the real rejection is calculated from membrane surface and the permeate concentrations as:

Rr = 1 −

CP Cm

Here, the difference between Cm and Cb is due to rejected solute concentration polarization (Section 5) [31–35]. Polymeric RO membranes have dominated commercial applications. Due to their technological maturity they offer low-cost fabrication, ease of handling and improved performance in selectivity and permeability [29]. Aromatic polyamides and cellulose acetate (CA) are two of the main polymeric materials used in the fabrication of commercially available RO membranes [36]. Cellulosic derivatives and their fabricated membranes have shown a number of good properties, including hydrophilicity, mechanical strength, wide availability, chlorine tolerance, fouling resistance, and low cost. However, CA membranes have some drawbacks [37,38]. These membranes are known to undergo hydrolysis in both alkaline and acidic conditions. The operational pH range for these membranes is hence very narrow (pH 4–6). Slow hydrolysis increased the flux, but reduced the rejection drastically [28]. The rate of hydrolysis also increases with temperature, and therefore CA is limited to an operating temperature below 30 °C. Furthermore, CA membranes are compacted under high operating pressures causing a period of flux decline [39]. Since the concept of interfacial polymerization (IP) of creating polyamide thin film composite membrane was introduced by Cadotte [40], the polyamide TFC membranes form the most important and widely used class of RO membranes [23,28]. Polyamide TFC RO membranes are composed of three layers: an ultra-thin skin polyamide polymer barrier layer on the upper surface, a microporous interlayer support, and a non-woven polyester fabric base acting as structural support as shown in Fig. 3. The polyester support layer cannot provide direct support for the barrier layer because it is too irregular and porous. Therefore, between the barrier layer and the support layer, a microporous interlayer is added to enable the ultra-thin barrier layer to withstand high pressure compression [29].

Fig. 1. Global desalination capacity by process [17].

Fig. 2. Particle size retention for pressure driven membranes [26].

Polyamide TFC RO membranes are the dominant technology for fresh water supply, but the major obstacles for application of these membranes is membranes fouling. Also, those RO membranes are vulnerable to chlorine. These two factors cause a decrease of membranes permeability, deterioration of permeate quality, increases in energy consumption, and shortening membranes life. In this paper, the basic principles of polymeric RO membranes and their drawbacks are completely reviewed. Also, the surface properties of RO membranes are explained. In addition, the methods for reduction of RO membranes fouling are introduced. Moreover, as a main part of this article surface modification of polyamide TFC RO membranes for improving the membrane surface properties and performance of those RO membranes are thoroughly reviewed. 2. Reverse osmosis membranes Today reverse osmosis membrane is the most widely used desalination process. Over the past few decades remarkable advances have been made in the preparation of RO membranes from different materials. Commercial interest in RO technology is increasing globally due to improvement in RO technology in terms of membrane material and energy consumption, which has enabled a reduction in cost of pure water production [28–30]. There are two parameters used to indicate RO membrane performance: water flux and salt rejection. Water flux, Jw, is the superficial velocity of water through the membrane, and is defined as the amount of water transported across membrane per unit time per unit area. The water flux is described by: (1)

Jw = A ( ΔP − Δπ )

(3)

where, Jw is water flux (g/(cm s)) (which can be converted to (L/ (m2 h))), A is the intrinsic water permeability coefficient of the 2

Fig. 3. Schematic illustration of a polyamide TFC RO membrane [41].

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and pH range (from 1 to 11), and higher stability to biological attack [36]. However, the polyamide TFC RO membrane has two drawbacks, limiting its wide application and long term performance. The first and the most important drawback is its proneness to fouling from all kinds of matters. Therefore critical pretreatment conditions and frequent membrane cleaning are required, which lead to significant capital and energy cost. The second drawback is its vulnerability to chlorine, which is the most widely used disinfectant in water treatment for biofouling control. The membrane dramatically loses its salt rejection characteristics when exposed to even a few parts per million of chlorine [23,43]. Also, it is well known these RO membranes suffer from of trade-off between permeability and salt rejections. It means that a higher permeation flux is always combined with a lower salt rejection, and vice versa. Thus it is a key problem and obstacle and exploration of polyamide TFC RO membranes with increased permeations and constant high salt rejections are very important [43].

3. RO membranes fouling Membrane fouling is the accumulation of unwanted materials on exterior surfaces of membranes, which diminish membrane performance and its lifecycle. It is the most common and significant problem associated with the RO membranes processes. Accumulation of foulants on the surface of RO membrane results in the formation of a cake/gel/ scaling layer along the membrane [44,45]. According to the characteristics of foulants, fouling in RO membranes can be classified into four major groups as shown in Fig. 5 [5,46,47]:

Fig. 4. Synthesis of polyamide layer on the supporting layer using TMC and MPD as monomers by interfacial polymerization [43].

Polysulfone (PSF) has been widely applied in lab and industrial scale fabrication of polyamide TFC membranes because of its chemical tolerance to a wide range of pH, resistance to compaction, ease of availability, low cost, and relatively high hydrophilicity [28,36]. Polyamide TFC RO membrane preparation technique is based on interfacial polymerization reaction between two monomers, a polyfunctional amine and a polyfunctional acid chloride, dissolved in water and hydrocarbon solvent, respectively [42]. The most commonly used monomers in interfacial polymerization process for synthesizing the polyamide RO membranes are m-phenylene diamine (MPD) and trimesoyl chloride (TMC) and the process is shown in Fig. 4 [43]. As compared with CA membranes, the interfacially polymerized polyamide TFC RO membranes exhibit excellent permeability to water and high salt rejections as a result of ultra-thin but high cross-linked polyamide layer. Also these membranes are characterized by pressure compaction resistance, wider operating temperature range (0 °C–45 °C)

• Colloidal fouling-deposition of colloidal particles on the membrane • Organic fouling-deposition and adsorption of macromolecular organic compounds on the membrane • Inorganic scaling-precipitation or crystallization of sparingly dissolved inorganic compounds on the membrane • Biofouling-adhesion and accumulation of microorganisms, and development of biofilm on the membrane

Fouling formation mechanism can be understood by examining the forces of interaction between the particles (foulants) and the membrane surface, and is best described by the classical Derjaguin-LandauVerwey-Overbeek (DLVO) theory. The DLVO theory states that the net Fig. 5. Schematic illustration of different types of fouling in the polymeric RO membrane.

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common inorganic salts with low solubility responsible for scaling on the membrane surface. The occurrence and severity of scaling is strongly dependent on the concentration and solubility of potential scalants (or their precursors) in the feed solution. From the thermodynamic perspective, a mineral may start to deposit when its concentration is greater than the saturation limit [60]. Therefore, inorganic particles deposit on the membrane surface and subsequently form a scale layer. Unlike the colloidal and organic fouling that can occur with a low foulant concentration, the scaling only occurs if the super saturation exceeds a critical value. The hydrolysis and oxidation can accelerate the deposition of inorganic salts during membrane filtration. The diminution of membrane scaling due to the inorganics may be accomplished by addition of antiscalants or adjustment of pH [61–63]. Scale formation was reported to occur by two crystallization pathways, surface (heterogeneous) crystallization and bulk (homogeneous) crystallization. Scaling in membrane systems is a combination of these two extreme mechanisms and is affected by membrane morphology and process conditions. Surface crystallization occurs due to the lateral growth of the scale deposit on the membrane surface, resulting in flux decline and surface blockage. Bulk crystallization arises when crystal particles are formed in the bulk phase through homogeneous crystallization and may deposit on membrane surfaces as sediments/particles due to convective transportation to form a scale layer that leads to flux decline. Simultaneous bulk and surface crystallization may also occur for high recovery operating conditions. A schematic representation of these crystallization processes is illustrated in Fig. 6 [48].

particle-surface interaction (or particle-particle) is a summation of the van der Waals and the electrostatic interaction force (also known as the electrical double layer forces). If the particle and surface have different charges, they will have attractive interaction, while if the particle and surface have similar charges, they will be repulsive of each other. In order to minimize fouling, the surface and the particle should be kept repulsive of each other or reduce the interaction between them [48–50]. In the next sections (Sections 3.1–3.4) different types of RO membranes fouling and their formation conditions were completely explained for comparison of them. 3.1. Colloidal fouling Colloids/particulates can be defined loosely as fine particles roughly in the size range of 1–1000 nm (0.001–1 μm). Colloids are inherently prone to cause RO membrane fouling as a result of their size range. In contrast, smaller particles (in the molecular size range) can easily diffuse away from a membrane surface via molecular diffusion, and larger particulate matter can be removed from a surface via shear-induced diffusion or lateral migration [51–53]. Colloids in the feed water are accumulated on the RO membrane surface, which results in the formation of cake layer and creates a hydraulic resistance to permeate flow through the membrane. Colloids can be categorized into the following general groups [54,55]:

• Inorganic colloids: Common inorganic colloids include silica, iron •

(oxy) hydroxide, and various aluminum silicate minerals. Other less frequently found inorganic colloids include aluminum oxide, manganese oxide, elemental sulfur, and metal sulfides. They are usually compact and rigid particles. Organic macromolecules: Macromolecules can be further classified into rigid biopolymers (mainly large molecular weight polysaccharides with a long persistence length), fulvic compounds, and flexible biopolymers (including proteins and some low molecular weight organic molecules (molecular weight < 1000).

3.4. Biofouling Biofouling is one of the most serious problems in RO membrane process and is defined as undesirable accumulation of microorganisms at the membrane surface. Biofoulant includes bacteria, fungi, algae, viruses, and higher organisms such as protozoa, living or dead, or biotic debris such as bacterial cell wall fragments [64–66]. Biofouling is inherently more complicated than other membrane fouling phenomena because microorganisms can grow, multiply, and relocate on the membrane surface. Usually, membrane biofouling is initiated by irreversible adhesion of one or more microorganisms to the membrane surface followed by fast growth and multiplication of the sessile cells in the presence of feed water nutrients [67,68]. When the microorganisms adhere to the membrane surface, they start building up their aggregates “biofilm” which is a gel-like diffusion barrier layer. Biofilm has a strong structure that results from the capacity of microorganisms on developing layers of polymer-like materials called Extra-cellar Polymeric Substances (EPS), which can protect the microorganisms from biocides and toxins. Biocides are disinfecting agents which are capable of inactivating microorganisms (such as

The most problematic feeds are those containing colloidal particles not easily removed by granular beds either because of their minute size or because of electrostatic repulsion effects of the media. In such cases it is necessary to add a coagulant or flocculating agent [56]. 3.2. Organic fouling Dissolved organic matter (DOM) is abundant in surface water and sewage, which can be classified into three different categories according to their origins: (1) refractory natural organic matter (NOM) derived from drinking water sources, (2) synthetic organic compounds (SOC) added by consumers and disinfection byproducts (DBPs) generated during disinfection processes of water and wastewater treatment, and (3) soluble microbial products (SMP) formed during the biological treatment processes due to decomposition of organic compounds. NOM is a complex heterogeneous mixture of compound formed as a result of decomposition of animal and plant materials in the environment. Most NOM comprise of a range of compounds, from small hydrophobic acids, proteins and amino acids to larger humic and fulvic acids [57]. The mechanisms include adsorption on the membrane surface and formation of a gel or cake layer [58]. 3.3. Inorganic fouling The term “mineral scale” is used to differentiate fouling due to inorganic salt deposits from organic fouling and biofouling. In NF and RO systems, the dissolved salts are normally concentrated by 4–10 times, causing high concentrations exceeding the solubility at the membrane surface [57,59]. Calcium carbonate (CaCO3), barium sulfate (BaSO4), silica (SiO2), and calcium sulphate (CaSO4) are some of the most

Fig. 6. Schematic representation of the surface and bulk crystallization mechanisms during inorganic fouling of RO membranes [48].

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chlorination because the polarization of the amide group due to the chlorine substitution at the nitrogen makes the carbon more susceptible to nucleophilic attack by hydroxide [81]. The formation of N-chlorination and ring-chlorination reactions disrupt the intermolecular hydrogen bonds and destroy the symmetry of polyamide network, resulting in conformational changes of the polymer chains, and thus causes the failure of the polyamide RO membrane resulting in decreased salt rejection and increased water flux after chlorination. Therefore, chlorine can affect the polyamide TFC RO membranes both on the structures and the separation properties [78,79,82]. Various approaches have been devised to develop chlorine resistant membrane by eliminating chlorine sensitive sites of the polyamide RO membrane [83–86] or protecting the sensitive sites using chlorine resistant coating materials which will be mentioned in Surface treatment section (Section 7.3).

chlorine). The microorganisms presented in the matrix of biofilm can obtain nutrients for maintaining their life functions by secluding organic or inorganic substances from the surrounding environment. As a result, severe biofilm formation decreases membrane performance, which is referred to as membrane biofouling [69–72]. While the first three types of fouling can be reduced greatly by simply reducing the foulant concentration in the feed water by pretreatment, biofouling cannot be reduced by pretreatment alone. Pretreatment of feed water enables to decrease the content of bacteria and nutrients consumed by the bacteria during their activity in water. However, such a pretreatment is rather labor consuming and expensive. Additionally, even if 99.99% of all bacteria are eliminated by pretreatment, a few surviving cells will enter the system, adhere to surfaces, and multiply at the expense of biodegradable substances dissolved in the bulk aqueous phase, especially at high feed temperature and in wastewater treatment or desalination where high contents of nutrients are available in the feed for bacterial growth [68,73–75].

5. Concentration polarization in RO membranes 4. Chlorine effect on RO membranes

Concentration polarization (CP) is one of the most important factors influencing the performance of RO membrane separation processes and it is a natural consequence of the selectivity of a membrane [87]. In the RO process, solute and solvent (water) molecules are transported to the membrane surface by convective flow. Then, the solvent can cross the membrane while a major part of the solutes is retained by the membrane. Consequently, the concentration of solutes in the vicinity of the membrane surface is higher than in the bulk solution. In order to equalize the solute concentration, the retained solutes should diffuse back to the bulk solution due to the concentration gradient. Whereas the diffusion is far slower than the convection, the concentration of solutes gradually enhances near the membrane by the passage of time, making a boundary layer near the membrane surface. This phenomenon is known as concentration polarization as shown in Fig. 8 [88–91]. Concentration polarization affects RO membranes performance via several ways [90–92]:

One limitation of polyamide TFC RO membranes is that the polyamide active layer deteriorates when exposed to free chlorine. Chlorine type disinfectants are often dosed into the RO feed stream to prevent biological membrane fouling (biofouling), as well as cleaning agents to enhance biofoulants removal from RO membrane surfaces [76–79]. Disinfectants that provide HOCl and OCl− (which are known as free chlorine) are aggressive oxidants that can adversely affect membrane performance. When chlorine gas (Cl2) or sodium hypochlorite (NaOCl) are present in the RO feed water, they undergo hydrolysis to form hypochlorous acid (HOCl), which is further deprotonated to form hypochlorite ion (OCl−) [80]. Amide nitrogen and aromatic rings of the polyamide RO membrane are the sites that are sensitive to free chlorine and can be easily attacked by it [77]. Chlorine mainly attacks the polyamide layer in four routes as shown in Fig. 7: (1) the hydrogen of amide group can be substituted by chlorine atom when polyamide is attacked by active chlorine species such as hypochlorous acid, leading to the formation of N-chlorination (2) direct ring chlorination mechanism in which the aromatic ring of mphenylene diamine (MPD) can be directly replaced by chlorine atom when they are attacked by active (electrophilic) chlorine species and the substitution mainly occurs at the para position (3) indirect ring chlorination in that a rapid N-chlorination first occur, then the chlorine migrates to the ring by intramolecular rearrangement called Orton rearrangement and (4) hydrolysis of amide group which is promoted by chlorine, leading to the formation of amido and carboxylic groups. Furthermore, the hydrolysis of CeN bond can be facilitated by

• Reduction of water flux due to higher osmotic pressure associated with higher salt concentration at the feed side membrane surface. • Scaling or fouling in the case of accumulation of solutes on the membrane surface with a higher concentration than bulk. • Increasing concentration gradient of the solutes across the mem•

Fig. 7. Four approaches for the attack of polyamide by active chlorine species [81].

brane, leading to an enhanced solute flux and, therefore, declined in solute rejection. Increase in solution viscosity near the surface, offering more resistance against the solvent flux.

Fig. 8. Schematic of concentration polarization in RO membranes [44].

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stability, and [103–105].

As a result of CP, the solute concentrations at the membrane surface, Cm, is greater than in the feed Cb and the ratio of Cm is known as the Cb

degree of CP (f). The factors that enhance the back diffusion reduce CP during the filtration process, such as increasing the cross flow velocity to sweep solute molecules flowing parallel to the membrane surface, a higher temperature, and a greater diffusion coefficient of the solute/ particle. In contrast, an increased filtration pressure or permeate flux increase CP [61,93]. CP is different from fouling in that fouling involves the deposition of foulants as an immobile solid phase on the membrane surface [94]. Also, once the filtration process is stopped, the phenomenon of CP disappears, therefore it is reversible [61].

foulant-foulant/foulant-membrane

interactions)

6.2. Feed water characteristics The extent and rate of fouling can be strongly dependent on the type, properties, and concentration of foulants in the feed water. Also, the fouling behavior can be extremely dependent on the feed water chemistry such as pH, ionic strength, and divalent ion concentration, since the feed water chemistry can significantly influence the physicochemical properties (surface, shape, and conformation) of foulants and the their interactions with themselves and membranes surface. These interactions in the feed water may change the morphology and hydrophobicity of a membrane, hinder solute back diffusion, affect van der Waals forces between solutes, and accelerate the growth of microorganisms. It is known that the increase of foulant concentration leads to the increase of rate and extent of initial fouling in the membranes. The increased foulant concentration can increase the frequency of foulant collision with membrane and thus increase the rate of foulant attachment on the membrane at the initial stage of membrane fouling where increased rate of flux decline is observed [46,95]. Feed water pH can affect the morphology and charge of solutes and has an impact on charge pattern and hydrophilicity of a membrane, thus affecting CP and membrane fouling. Some of the membranes are unable to withstand extreme acidic or alkaline conditions. Over exposure to these extreme conditions may cause membrane failure such as membrane structure damage, irreversible fouling, and alteration of membrane characteristic which will significantly retard the membrane system performance. Under alkaline conditions, membrane charge and hydrophilicity are improved, and accordingly, electrostatic repulsion between membrane and solutes reduces the CP and membrane fouling. At low pH, the effective radius of solute molecules may decrease, making it easier to be adsorbed onto the membrane surface. Moreover, a low pH (close to the isoelectrical points) reduces solute charge and induces aggregation of molecules, increasing CP and membrane fouling. Thus, study related to the effect of feed water pH is essential in order to optimize the RO membrane performance particularly on its rejection, permeability, and service life span [96,106–108]. Ionic strength in feed has a significant impact on CP and membrane fouling. Fouling is more severe when feed water with higher ionic strength is performed. When ionic strength in feed solution is very high, it can scrape electrical charges of membrane, decrease hydration radius of solutes, enhancing the effect between foulants and membrane, and causing formation of a more compact cake layer. At high ionic strength and low pH value, the charge between solute molecules is reduced and its surface hydration layer becomes thin, therefore charge repulsion and hydrophilic effect are weakened and solutes more easily aggregate and deposit on the membrane surface [96,109]. Also, it is showed that fouling was enhanced by high divalent ion concentration (Ca2 + and Mg2 +). The bridging between carboxyl groups on the surface of polyamide TFC RO membrane and foulant by divalent cations greatly increases fouling rate. Selective removal of calcium ions via pretreatment can significantly reduce the fouling capacity of carboxylic rich macromolecules. However, calcium and alum had synergistic effects in lowing the resistance and making the cake layer more compressible [110–113]. Therefore, it can conclude that by low pH, high ionic strength, and high divalent ion concentration the fouling is increased.

6. Factors affecting RO membranes fouling Fouling is a complex phenomenon, which is affected by different physical and chemical factors. Understanding the fouling phenomena is a requisite to have a better approach in minimizing, mitigating, and cleaning the fouling formation. In general, these factors can be classified into three groups [95]: 1. Hydrodynamic conditions such as water flux, cross flow velocity, and temperature. 2. Feed water characteristics such as foulant type, foulant physicochemical properties (shape, size, charge, and functional group), foulant concentration, solution pH, ionic strength, and ionic composition (divalent cation). 3. Membrane surface properties such as hydrophobicity, surface roughness, and surface charge. 6.1. Hydrodynamic conditions Hydrodynamic conditions such as water flux, cross flow velocity, and temperature strongly affected the membrane fouling. Generally, more severe fouling occurs at higher water flux (or pressure). At higher water flux larger volume of water permeated through the membrane and thus greater amount of foulants brought towards the membrane surface due to the water convection. As a result, the concentration of foulants on the membrane surface increases and more severe CP and adsorption of foulants happened on the membrane. In addition, greater hydrodynamic drag force towards the membrane surface created and made foulant layer denser [96–98]. Cross flow velocity over the membrane surface is another hydrodynamic factor affecting the membrane fouling through the influence of CP and mass transfer near the membrane surface. In the cross flow membrane filtration system, foulants transport towards the membrane due to the permeate water flux (the hydrodynamic drag perpendicular to the membrane surface) and meanwhile they are moved away from the membrane surface by the cross flow (the shear rate tangential to the membrane surface) and back transport towards the bulk solution. Therefore, the CP of foulants is mitigated by the back transport of those particles to avoid severe membrane fouling. As a result, a higher cross flow velocity permits a decrease of the CP by elevating the mass transfer rate and mitigating membrane fouling by enhancing back transport of foulants, thus improving flux behavior [99–102]. Temperature is regarded as another important physical parameter which can strongly affect membrane fouling and filtration performance of RO membrane. With an increase of temperature, the viscosity of liquid decreases which reduces the membrane resistance and thus increases the water permeability. Also, higher temperature increases solute diffusion coefficient leading to an improvement of back diffusion of solutes and reduction of CP. The effect of temperature on membrane fouling is virtually through the influence of hydrodynamic conditions (initial water flux level and mass transfer of foulant) and thermodynamic conditions (solution osmotic pressure, foulant solubility/

6.3. RO membranes surface properties In RO membrane desalination, fouling is caused by the interaction between the membrane surface and foulants particles and depends strongly on membrane surface morphology and properties. In water treatment applications foulants can adsorb to the membrane surface through hydrophobic interaction, hydrogen bonding, van der Waals 6

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boundary layer or flow distribution over the surface. Furthermore, a greater roughness increases the total surface area to which foulants can be attached, and the ridge-valley structure of polyamide TFC RO membranes favors accumulation of foulants at the surface. As a result, membranes with rougher surfaces are observed to be more favorable for attachment of foulants resulting in faster fouling rates. However, low surface roughness may be disadvantageous to membrane flux [97,126,127]. As a result, it can be concluded that the polyamide TFC RO membranes with smooth and hydrophilic surface with similar surface charge to the foulants possess good antifouling property.

attraction, electrostatic interaction, and several important membrane surface properties such as hydrophilicity, charge, and roughness are known to be strongly related to fouling [114–116]. 6.3.1. Surface hydrophilicity Surface hydrophilicity is considered as one of the major reasons of membrane fouling and it is related to the affinity of a surface for water over oil. In general, hydrophilicity is analogous to water attracting and these membrane materials prone to water adsorption. On the other hand, hydrophobicity is analogous to water repelling and such membrane materials have a limited ability to adsorb water [117–119]. It is accepted that an increase in hydrophilicity of membranes offers better fouling resistance because protein, microorganisms, and many other foulants are hydrophobic in nature. However, membrane surface hydrophilicity has a negative effect on membrane fouling by natural organic matter (NOM). The hydrophobicity behavior of membrane can be determined by the contact angle measurement. The contact angle, θ is an angle between water and surface of membrane. Hydrophilic membrane surfaces have a contact angle in the range of 0° < θ < 90°, while hydrophobic membrane surfaces have contact angle in the range of 90° < θ < 180° [120,121]. The major reason for hydrophobic membrane fouling with hydrophobic foulants is that there are almost no hydrogen bonding interactions in the boundary layer between the membrane interface and water. The repulsion of water molecules away from the hydrophobic membrane surface is a spontaneous process with increasing entropy and foulants molecules have a tendency to adsorb onto membrane surface and dominate the boundary layer. In contrast, the membrane with the hydrophilic layer possesses a high surface tension and is able to form the hydrogen bonds with surrounding water molecules to reconstruct a thin water boundary between the membrane and bulk solutions. It is difficult therefore for hydrophobic solutes to approach the water boundary and break the orderly structure because an increase of energy would be required to remove the water boundary and expose the membrane surface [122].

7. Reduction of fouling in RO membranes Membrane fouling is an inevitable challenge in all membrane processes. Lower RO membrane fouling allows higher water productivity, less cleaning, longer membrane life, and reduced capital and operational costs [128]. Membrane fouling is an extremely complex physicochemical phenomenon and usually several mechanisms are involved simultaneously. The complexity of membrane fouling predetermines the exploiting of a variety of approaches to control this adverse process. Here these approaches, which are used to minimize the RO membrane fouling, are categorized under three main topics: (1) pretreatment of feed, (2) membranes cleaning, and (3) membrane surface modification [129].

7.1. Pretreatment of feed Generally, for RO desalination, feed water with constant high quality is the key factor to a successful operation. The purpose of feed water pretreatment is to minimize membrane fouling by removing foulants precursors and adapting the physicochemical and biological characteristics of water [130]. Comprehensive understanding of the feed water quality and characteristics and type of water resource (surface water, brackish water, seawater, and industrial water) is essential to select the appropriate pretreatment technology for RO system. For instance, surface waters have high turbidities, Silt Density Index (SDI) and NOM as compared to water from the well source due to adsorption and filtration effect on underground water reserves. Similarly, well waters contain high silica content than surface waters [131]. Also, pretreatment design determines the costs of maintenance and operation of the plants. Moreover, an adequate design of pretreatment would avoid the corrosion, scaling, and the premature damage of equipment in addition to membrane fouling [130,132]. Pretreatments can be divided into two groups: conventional pretreatments and unconventional pretreatments [130]. Conventional pretreatments consist of a combination of chemical treatment with a media filtration to achieve a conditioned feed suitable as RO feed [130]. The typical conventional pretreatment process includes of all or some of the following treatment steps [131,133]:

6.3.2. Surface charge The electrostatic charge of membranes is a particularly important consideration for the reduction of membrane fouling where foulants are charged, which is often the case. When the surface and the foulant have similar charge, electrostatic repulsion forces between the foulants and the membrane prevent the foulant deposition on the membrane thereby reducing the fouling [58,123]. Interactions between charged foulants and the membrane can be reduced by enhancing electrostatic repulsion through altering the membrane surface charge. The polyamide TFC layer in RO membrane is formed via a polycondensation reaction of polyfunctional amine and acid chloride monomers at the interface of 2 immiscible solvents. The eventual hydrolysis of the acid chloride groups that did not react with the amine leaves the carboxylic acid groups on the membrane surface. Since the pKa value of the carboxylic acid group is < 4, the surface charge of the polyamide TFC RO membrane is negative under typical operating pH conditions (> pH 4 to 5). Enhancing the negative charge of a membrane surface would increase electrostatic repulsion of anionic foulants and, hence, lead to a decrease of fouling by these compounds. However, not all foulants are negatively charged. If metal cations and negatively charged natural organic matter are present in the feed water, they can combine to form insoluble complexes that deposit onto the membrane surface [124,125].

• Coarse/fine screening • Disinfection • Dissolved air floatation (DAF) • Coagulation/flocculation • Filtration with granular media • Acid addition • Antiscalant addition • Adsorption

6.3.3. Surface roughness Another important factor that affects the performance of the RO membranes is the surface roughness. A strong correlation has been shown between the fouling and surface roughness for RO and NF membranes and membrane surface roughness may be influenced by the manufacturing process. Increased roughness may lead to uneven

Unconventional pretreatments has been a movement towards the use of larger pore size membranes such as MF, UF, and NF membranes to pretreat RO feed water. 7

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Table 1 Comparison chart for disinfectants used for biofouling control of seawater RO membranes [75]. Disinfection Physical

UV

Chemical

Membrane Sand filtration HOCl, OCl−

NH2Cl ClO2 Ozone

Advantage

Disadvantage

installation and maintenance • Easy inactivation • Effective of organic matter • Oxidation with membrane pretreatment • Combined installation and operating cost • Low inactivation efficiency • High matter removal • Organic low cost • Relatively harmful on membrane than HOCl • Less inactivation • Residual damage on membrane • No inactivation • Effective • High oxidation potential for organic matter

formation • Scale • No residual effect capital and operating cost • High bacterial removal efficiency • Low corrosion of RO membrane • Chemical • THMs, HAAs formation • Relatively low efficiency toxicity • Chlorite formation • Bromate small half life • Very • Damage by residual ozone

skimmed off for disposal, while the low-turbidity seawater is collected near the bottom of the tank [142,143]. The time (and therefore, the size of the flocculation tank) needed for the light fine particulates contained in the seawater to form large flocs is usually 2 to 3 times shorter than that in conventional flocculation tanks because the flocculation process is accelerated by the air bubbles released in the flocculation chamber of the DAF tanks. Another benefit of DAF as compared to conventional sedimentation is the higher density of the formed residuals [142].

7.1.1. Disinfection Disinfection (biocide) is necessary to control the growth of microorganisms such as bacteria, algae, fungi, and viruses, which can cause serious biological fouling and to prevent the growth of microorganisms on the walls of pipes and tanks inside the system. Disinfection is achieved through addition of a strong oxidant, such as chlorine (Cl2), sodium hypochlorite (NaClO), monochloramine (NH2Cl), chlorine dioxide (ClO2), ozone (O3), UV irradiation or potassium permanganate [134,135]. Comparison of disinfection materials for the RO process to inhibit biofouling have summarized in Table 1 [75]. Pre-chlorination has become much less common in some countries such as United States, and it is only carried out in plants where there are no problems with the formation of carcinogenic substances known as trihalomethanes. The formation of trihalomethanes occurs when chlorine reacts with the organic compounds in water during a longer contact time [98]. Moreover, if chlorine is used as the disinfectant, activated carbon (typically contained in a filter) or sodium bisulfite is used at the end of the pretreatment system to remove the residual chlorine due to sensitivity of aromatic polyamide RO membranes to chlorine. Sodium bisulfite is less expensive and more commonly used for large desalination facilities [136]. Ozone may be a good alternative to chlorination, mainly in the prechlorination of plants with risk of formation of trihalomethanes, which can be destroyed by ozone. As disinfectant, ozone is more powerful and faster than chlorine and it can produce less flavor and smell of treated water. However, the use of ozone in this field is not more wide spread because of its higher cost compared to other commonly used disinfectants [137,138]. However, ozone will cause the formation of bromate, a well-known and regulated carcinogen, in waters containing bromide [139]. Monochloramine and chlorine dioxide show relatively low antibacterial efficiency and high cost despite less damage on RO membranes [140,141]. On the other hand, UV irradiation and ozone perform effective inactivation, but they lack residual effect and the damage to membrane surface structure. Therefore, it is concluded that all disinfection processes cause the formation of disinfection by-products, which are potentially toxic oxidation products formed by reactions between the disinfectant and organic and inorganic water compounds.

7.1.3. Coagulation/flocculation Coagulation/flocculation are effective physicochemical treatment methods and have been widely used to remove particulate and colloidal matters such as silica, iron, NOM, etc., both prior to media filtration and low pressure membrane filtration (MF/UF) [46]. However, several issues still remain to be resolved before coagulation pretreatment can be applied optimally especially in the water treatment membrane field, such as the selection of a proper type of coagulant for water characteristics, the optimal dosage, coagulation strategies (coagulationsettlement process and in-line coagulation), and overall cost and benefits of chemical pretreatment to MF and UF membrane systems [133]. Aqueous particulate and colloidal matters are typically negatively charged and thus stay separated because same charges repulse each other. The role of coagulants is to effectively neutralize same charges and allow the suspended solids to group together in flocs (large groups of loosely bound suspended particles) [143]. Therefore, coagulants are typically positively charged molecules. Two types of coagulants are available in commercial market: inorganic salt and organic macromolecules. Inorganic coagulants are commonly iron or aluminum salts such as ferric chloride or aluminum salts (aluminum sulfate (alum) or poly-aluminum chloride), while organic coagulants are typically cationic, low molecular weight (< 500,000 Da) polymers (dimethyldiallylammonium chloride or polyamines) [144–146]. Ferric chloride has been shown to be the most successful coagulant in NF/RO pretreatment applications [147]. Aluminum is not as commonly used in pretreatment coagulation prior to membrane filtration due to potential damage to the membrane system, because it is difficult to maintain aluminum concentrations at low levels in dissolved form since aluminum solubility is very pH dependent [142]. When a feed water has a high SDI (> 10), flocculation is often used with coagulation before media filtration. The process of flocculation and sedimentation is a well-known method of particle removal in water treatment. Flocculants are often high molecular weight (> 1 × 107 Da), anionic polymers [148,149]. In summary, evaluating each chemical used in pretreatment to determine its effects must be based on the successive steps performance and how much it can reduce RO fouling potential and increase the

7.1.2. Dissolved air floatation (DAF) Dissolved air flotation (DAF) is a physical separation process suitable for removal of floating particulate foulants such as light particles (suspended plankton, algae, fine silt, and debris) and organic substances (oil and grease) or other contaminants that cannot be effectively removed by sedimentation or filtration. DAF process uses very small air bubbles to float light particles and other compound contained in the seawater. The floated solids are collected at the top of the DAF tank and 8

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Polyphosphates, especially sodium hexametaphosphate (SHMP), ((NaPO3)6), was the first antiscalant commercially available to the membrane industry [156,157]. Adding antiscalant is an attractive selection because it has many kinds of advantages such as operation cost reduction, environmental acceptability, and harmlessness compared to the alternative methods [158]. Although the scale formation in the RO desalination systems can be reduced using the antiscalants, they can enhance the formation of biofilm on the surface of the RO membranes due to an alteration of membrane surface properties such as hydrophobicity and surface charge. As a consequence, it is recommended that antiscalants should be screened for their biofouling contribution prior to their application [159].

permeate water quality. Usually, sufficient jar tests and bench scale experiments or pilot tests with different types and dosage concentrations of coagulants are necessary to determine the optimal coagulation condition [133]. 7.1.4. Filtration with granular media Direct filtration, using mono, dual media or mixed media filtration, is the most common technology used for the filtration of seawater prior to the RO system. Filtration depends primarily on a combination of complex physical and chemical mechanisms, the most important being adsorption. As water passes through the filter bed, the suspended particles contact and adsorb (stick) onto the surface of the individual media grains or onto previously deposited material [150]. Granular media filtration includes materials such as sand, anthracite, glass fiber, gravel, and garnet, and often, a combination of materials is used in layers in the filtration bed to take advantage of materials' different effective sizes. For the well intake of seawater for RO desalination, sand filtration is extensively used as a pretreatment method to remove large-size particulate matters [151,152]. Use of coagulant is critical for the effective and consistent performance of granular media filtration systems. Without coagulation of the particulates in seawater, conventional granular media filters are likely to remove completely only particles which are larger than 50 μm. Coagulation allows granular filtration process to remove finer particles and microplankton from the source seawater [142]. Cartridge filtration is used as a last pretreatment step in conventional RO pretreatment. The filter cartridges are usually 1–10 μm and act as a final polishing step to remove larger particles that passed through media filtration [148,153].

7.1.7. Adsorption Adsorption is another promising method prior to RO membranes to remove free chlorine and soluble nonpolar organics in feed, such as NOM and endocrine disrupting compounds, which are the foulants responsible for flux decline [160]. There are two main types of adsorbents: suspended powders and fixed adsorbent contactors. Powdered activated carbon (PAC) is the most intensive adsorbent and has relatively large specific surface areas to uptake certain substances (humic substances and micro-pollutants), and is suitable to eliminate foulants and flux decline for membranes [161,162]. 7.1.8. Membrane-based filtration pretreatments A new trend in pretreatment has been a movement towards the use of larger pore size membranes (MF, UF, and NF) to pretreat the feed water RO system and those membranes can remove a variety of foulants, such as biomass flocs, bacterial cells, colloidal particles, and macromolecular organics. Depending on the specific contaminants to be eliminated, it will be advisable to use a type of membrane or another. While MF membranes are appropriate for removal of larger particulate matter at higher permeate fluxes, NF membranes are used to remove dissolved contaminants, particulate and colloidal material. On the other hand, UF membranes offer better balance than MF and NF membranes, because they have greater fluxes than NF membranes and smaller pore sizes than MF membranes [163–165]. Although conventional pretreatment has been widely used for seawater and brackish water RO plants, unconventional membrane pretreatments have significant benefits versus conventional pretreatments because they produce significantly higher RO flux and need lower space. Also, the replacement of RO membranes occurs rarely, as well as the frequency of chemical (acid or base) cleaning. In addition, it is possible to treat feed water with variable quality. Moreover, as membrane costs decrease and raw water qualities decline, the use of membranes based filtration prior to RO plant has become a more practicable alternative to conventional pretreatment [166–168]. The keystone of membrane pretreatment technology could be the combination of conventional and unconventional pretreatments (hybrid technologies). In this way, it could be possible to unify the advantages from different types of pretreatments in order to produce better water quality and minimize the overall treatment cost. For instance, coagulation has been successfully used in line with MF, UF, and NF membranes to prevent fouling during RO feed water pretreatment [163]. However, the important disadvantages of membrane pretreatment are that MF, UF, and NF membranes are fouled in the process, and fouling causes membrane damage and flux decline. Previous research has shown that hydrophobic materials (hydrocarbons or oil) and cellular or extracellular polymer substances are dominant foulants for UF and MF membranes [169–171]. NF membranes can also be subject to salt precipitation and membrane scaling, due to much smaller pore sizes [172]. Although membrane technologies are used as the pretreatment of RO systems, fouling control of membrane themselves is a regenerative research issue.

7.1.5. Acid addition Acid addition involves reduction of pH of the feed water to 5–7 and increasing the solubility of alkaline scale, especially CaCO3 which is a potential scalant in all feed water types [133]. The solubility of CaCO3 depends on pH as:

Ca 2 + + HCO3− ↔ H+ + CaCO3

(4)

Addition of H+ in the form of acid shifts the equilibrium to the left and maintains the calcium carbonate in the dissolved form. Generally, sulfuric acid or hydrochloric acid (HCl) is used for pH adjustment, however hydrochloric acid is used when sulfuric acid addition has the potential to cause sulfate precipitates such as CaSO4, BaSO4, and SrSO4. Acid addition is also effective for controlling calcium phosphate scale. Thus, acid addition will lead to other problems such as corrosion due to lower pH value of the feed water, transport and storage problems, and safety problems [148,154]. 7.1.6. Antiscalants addition Antiscalants (scale inhibitors) addition are useful conventional pretreatment methods that can be used to control scaling (such as carbonate scaling, magnesium hydroxide scaling, sulfate scaling, and calcium fluoride scaling), thus, improve the performance of RO membrane [131]. One of the major advantages of antiscalants is the low dosage levels required (substoichiometric amounts), which has minimal impact on the feed water quality. The inhibition of scale formation does not involve bond formation or breaking between the antiscalant and the scale forming constituent. With antiscalant addition, scale inhibition occurs by disrupting one or more aspects of the crystallization process. Generally, antiscalants do not eliminate the scaling constituents or its tendency; instead they delay the onset crystallization (nucleation phase of crystallization) or retard the growth of mineral salt crystals (growth phase of crystallization). Addition of antiscalants increases the effective solubility limits of scaling salts and hence the economic benefit of achieving higher product recovery [155]. Commercially available antiscalants can be classified into three major categories: phosphates, phosphonates, and polycarboxylates. 9

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precipitated salts such as CaCO3, while alkaline cleaning is used to remove adsorbed organic foulants [178,180]. The cleaning agents must be able to dissolve the majority of the fouling materials on the surface and removed them from the surface but not damaging membrane surface, thus maintaining membrane properties. Also, the cleaning agent should be low cost and safe and show chemical stability and the ability to be removed with water [181]. The cleaning agents may react with the foulant to weaken the cohesion forces between the foulants and the adhesion between the foulants and the membrane surface. The possible reactions between foulant and cleaning agent are: hydrolysis, peptization, saponication, solubilisation, dispersion (suspension), and chelation [176,178]. In detail, a cleaning agent can affect fouling material present on a membrane surface in three ways: removal of the foulants, changing morphology of the foulants (swelling or compaction), and/or alteration of surface chemistry of the deposit so that the hydrophobicity or charge is modified [182,183].

7.2. RO membranes cleaning Membrane fouling is a phenomenon that will always occur at some extent. Therefore, membrane cleaning to remove the fouling layer on the membrane surface is a necessary process to ensure stable operation of membrane systems and restore permeate flux and thus decrease salt passage. Cleaning cycles are recommended to be conducted when transmembrane pressure, permeate flow and/or permeate quality vary between 10 and 15% with respect to the initial values [173]. However, the cleaning frequency depends on concentration of existing foulant and it varies from weekly to yearly cleaning [174]. Cleaning is classified as physical and chemical cleaning. In practice, physical cleaning followed by chemical cleaning is widely applied in membrane applications to confine the extent of membrane fouling. However, in RO desalination, chemical cleaning is the most applied method [175]. 7.2.1. Physical cleaning Physical cleaning methods use mechanical forces to dislodge and remove foulants from the membrane surface [176]. Physical cleaning methods used for RO membranes include forward and reverse flushing and backwashing [175]. Forward flushing consists in pumping permeate water at high crossflow velocity through the feed side (from feed to retentate) in order to remove foulants from the membrane surface. Because of the more rapid flow and the resulting turbulence, particles absorbed to the membrane are released and discharged. In reverse flushing the feed is first flushed for a few seconds in the forward direction and afterwards, for few seconds in the reverse direction (from retentate to feed). In backwashing, permeate water direction is reversed and permeate is flushed from the permeate side to the feed side [106,175]. In reverse osmosis membranes, backwashing is conducted by direct osmosis and is based on negative driving pressure between the operating pressure and the osmotic pressure of the water solution in the feed side. This is achieved by (i) reducing the operation pressure below the osmotic pressure of feed solution or (ii) by increasing the permeate pressure [177]. Backflow from permeate to the feed side of the membrane expands the thickness of the fouling layer and fluidizes it. After this, a forward flush is usually used to wash out the detached layer or dilute the fouling layer. Some significant factors affecting physical cleaning when combining forward and backward flushing are production interval between cleans, duration of backwash and pressure during forward flush [176].

7.3. RO membranes surface modification Separation of polyamide TFC RO membrane is a surface phenomenon in which the top selective polyamide layer plays a major role. As aforementioned, in these membranes, fouling is widely correlated with their surface properties such as hydrophilicity, surface charge, and roughness and the membranes with smooth and hydrophilic surface with similar charge to the foulant possess better antifouling property. Therefore, a great deal of research efforts has been devoted to improve antifouling properties of polyamide TFC RO membranes by changing surface properties of polyamide selective layer. Surface modification of polyamide TFC RO membranes can be done through physical or chemical surface modification [158,174]. In physical modification, the materials interact with polyamide layer of RO membranes and attach it by van der Waals attraction, electrostatic interaction or hydrogen bonding, which may not be stable in long-term operation. In contrast, in chemical modification, the materials connected to the surface of RO membranes by covalent bonds and have better chemical and structural stabilities [23]. The whole discussion in this section is a complete review of physical or chemical surface modification of polyamide TFC RO membranes after synthesis of them (post modification) and the modification of membrane surface in the synthesis step of polyamide layer is not included (using new monomers or addition of hydrophilic monomers). Although incorporation of nano particles in the polyamide TFC RO membranes is a method for surface modification, that method is not discussed here and it can be found in other review articles [184–189]. Since most of researchers used commercial polyamide TFC RO membranes in their studies to investigate the effect of surface modification on the performance of membranes, some process were required prior to modification to remove any extractable components and preservatives (glycerin) from the membrane materials, for example by soaking polyamide TFC RO membranes in deionized water or isopropanol and then immersion in deionized water for some hours and replacing the water every hour [190,191]. Recently, based on investigation of the surface properties, Tang et al. concluded that at least 50% of commercial polyamide TFC RO membranes are modified as well, primarily via polymer coating, as part of manufacturing [192]. In Table 2, the performance of commercial polyamide TFC RO membranes, which were used by researcher for surface modification in the following sections, are listed.

7.2.2. Chemical cleaning Although physical cleaning exhibits great promise for mitigation of membrane fouling, it cannot perform well if the foulant has strong interaction with the membrane [46]. Chemical cleaning is the most common membrane cleaning method, especially in RO membranes and it means removing impurities by means of chemical agents. Generally there are five common categories of chemical cleaning agents:

• Acids (nitric, phosphoric, hydrochloric, sulfuric, citric acids) • Alkalis (sodium hydroxide (NaOH), sodium hypochlorite (NaOCl)) • Metal chelating agents • Surfactants (surface-active agents, including anionic, cationic, nonionic, and amphoteric electrolytes) • Enzymes In addition to the five main categories, disinfectants (O3), oxidants (H2O2, KMnO4), or sequestration agents (sodium ethylenediaminetetraacetic acid (EDTA)) are often used for cleaning chemicals of membranes [178]. In chemical cleaning, finding appropriate materials as cleaning agents is critical. This depends mainly on membrane material and type of foulant and in most cases is performed using a trial and error method [179]. For example, acid cleaning is suitable for the removal of

7.3.1. Physical surface modification Physical surface modification of polyamide TFC RO membranes were investigated by surface adsorption or surface coating. In Table 3, different materials which were used in physical surface modification of polyamide TFC RO membrane, operating conditions, and performance of membranes are summarized. 10

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Table 2 Performance of commercial polyamide TFC RO membranes according to the information provided by manufacturers. Membrane name

Manufacturer

Pressure (psi)

NaCl aqueous solution (ppm)

Water flux (L/m2 h)

Rejection (%)

XLE SW30HR SW30XLE BW30LE SW30 LCLE BW30FR BW30 BW30-2540 XLE-2540 LE SWC1 SWC2 SWC3+ (SWC3) SWC4+ (SWC4) ESPA1 ESPA3 CPA-2 LFC3 AD AG SE 80 B SUL-H SHN Hangzhou RO ES20 Tianchuang RO HR95PP, HR98PP Beidouxing RO TFC-HR SHN RE4021-TL RE2521-TL RE4021-TE NanoH2O DF30 TFC-ULP

Dow Dow Dow Dow Dow Dow Dow Dow Dow Dow Dow Hydranautics Hydranautics Hydranautics Hydranautics Hydranautics Hydranautics Hydranautics Hydranautics GE Osmonics GE Osmonics GE Osmonics Toray Toray Toray Chemical Korea Hangzhou Water Treatment Technology Development Center Nitto Denko Hangzhou Tianchuang Environmental Technology Niro-Hudson Hangzhou Beidouxing Membrane Koch Membrane System Woongjin Chemical Woongjin Chemical Woongjin Chemical Woongjin Chemical LG Chem Beijing Originwater Technology Koch Membrane Systems (Fluid Systems)

125 800 800 150 800 125 225 225 225 100 150 800 800 800 800 150 150 225 225 800 225 425 800 110 800 150

2000 32,000 32,000 2000 32,000 2000 2000 2000 2000 500 2000 32,000 32,000 32,000 32,000 1500 1500 1500 1500 32,000 2000 2000 32,000 500 32,000 1500 500 500 5000 1500 2000 32,000 2000 2000 2000 2000 – 500

53.85 29.3 38.3 44.71 41.67

99 99.7 99.8 99 99.4 99.2 99.5 99.5 99.5 99 99.3 99.5 99 99.8 99.8 99.3 98.5 99.7 99.7 99.7–99.75 99.5 98.5 99.8 99.5 99.75 99.2 99.7 98 95.5 96.7 99.55 99.75 99.2 99.5 99.5 99.5–99.6 – 98.5

a b

5a 350 1.05b 225 800 225 225 225 225 – 100

45 45 51.28 51.28 49.55 28.85 36.54 29.7 27.58 51 60.13 46.58 46.72 27.5–31.7 43.77–44.57 37.3–39.2 27 35.7–38.7 26 41.7–46.7 30 36 50 46.2–46.6 27.5 50.50 41.67 50.50 44.4–50.1 – 46.5

Pressure is bar. Pressure is MPa.

and non-fouled membranes. In surface coating process only a very thin coating (of the order of hundreds of nanometers) would form on the surface of membranes. It is done by relatively simple procedure which added at the end of the existing membrane fabrication process and creates a surface functional layer on membrane surface [195,196]. However, weak interaction between the coated layer and membrane is leading shortcoming of surface coating since the coating layer may wash away in the chemical cleaning process or long-term operation process, which means that antifouling property of the membrane may be gradually deteriorated [158,174]. The surface coating process of polyamide TFC RO membranes involve two general steps: (1) immersion of the selective polyamide surface in a polymer/solvent coating solution; and (2) evaporation of residual solvent at moderate temperature. Both of these process steps can affect water-polymer and polymer-polymer interactions, which could influence transport of compounds through the selective polyamide layer. For example, solvents can swell dense polymers to various degrees, which can influence membrane diffusivity and hence permeability. With respect to water flux, solvents can have a positive effect. If short-chain aliphatic alcohols used as solvents, it would have facilitated increases in water flux. These increases were attributed to enhanced swelling, removal of residual monomers or additives, or morphological changes that effectively increase membrane hydrophilicity. In contrast, solvent evaporation (drying) from a swollen membrane may lower permeability of the polymer matrix by reducing water-polymer interactions, or by collapsing voids via capillary forces. The extent of flux

7.3.1.1. Surface adsorption. Adsorption is a simple tool for modification and structuring of polymer surfaces. Researchers have carried out surface modification of RO membranes by adsorption of compounds such as surfactants [193] and charged polyelectrolytes [194]. Wilbert et al. [193] applied a homologous series of polyethyleneoxide (PEO) surfactants with either octylphenol or polypropylene oxide head groups (T-X series and P series) to modify the surface of commercial polyamide TFC RO membranes. The strategy in that study combined the benefits of increased hydrophilicity with steric hindrance by adsorbing surfactants. The results showed that surfactants decreased the roughness of membranes without a large change in zeta potential and the membranes exhibited improved antifouling property in a vegetable broth solution compared to unmodified membrane. Charged polyelectrolytes adsorption also carried out for surface modification of RO membrane. The modification was done by electrostatic self deposition of polyethyleneimine (PEI) (a branched hydrophilic cationic polyelectrolyte) on the membrane surface, and the modified membrane showed significantly improved antifouling properties to cationic foulants because of the enhanced electrostatic repulsion as well as increased surface hydrophilicity [194]. 7.3.1.2. Surface coating. Surface coating is a convenient way used to modify the polyamide TFC RO membranes surface and it has been widely used in membrane industry. Clearly, the key requirement of such modification is to eliminate the fouling while retaining rejection characteristics and permeate fluxes comparable to those of unmodified 11

PEBAX® 1657: 1 wt% in n-butanol or 1:1 ethanol: water (mass ratio)

PEI

P(AMPS-co-Am-co-VI)

PEBAX® 1657

MMA-HPOEM

Triglyme

Hangzhou RO

DF30

SWC3, SWC4

SHN (Woongjin Chemical Co.)

SW30HR

12 3 min

25

80–90

MMA-HPOEM: 0.2 wt% in a mixture of ethanol and water (1:1 vol. ratio)

–

Plasma polymerization time: 10, 15, 30, 60, 120 s

30 min

60

1, 5, 20, 35 h

24 h

Ambient condition

24 h

Time of coating

25

P(AMPS-co-AM-co-VI): 0.5, 1, 2, 5 g/L

PEI: 50–2000 mg/L

298 K

PEO: 0.1, 0.001% (w/w)

PEO with octylphenol or polypropylene oxide head group

Commercial

Temp. (°C) of coating

Concentration of coating agentb

Coating agenta

Membrane

Mixed protein solution containing 1 g/L BSA & 1 g/L AAS

Seawater: 32,000 ppm & BSA: 30 ppm, seawater & E. coli: 7.44 × 106 cells/mL

–

30 g/L

11.4 bar

–

1600 ppm

55 bar

900, 800, 370 psi

0.4 MPa

MgSO4: 2000 ppm, NaClO: 2000 ppm for 5 to 40 h exposure time

–

0–0.8 MPa

1750 kPa

Pressure

MgCl2: 75 mg/L DTAB: 50 ppm

Vegetable protein broth: 1% (w/w)

Other foulants testsc

90 mg/L

2.2 g/L

NaCl aqueous solution test

Table 3 Physical surface modification of polyamide TFC RO membranes: materials, operating conditions, and performance of membranesd,e.

–

25

25

25

Room temp.

25

Temp. (°C)

Stirred cell

Cross flow

Cross flow

–

–

[193] Decline in flux relative to untreated membrane, only T-X 100 improved rejection significantly, P-F87 was the only treatment that resulted decline in rejection after fouling, surfactant cause decrease in roughness without a large change in zeta potential after fouling. Coating reduced pure water [194] permeability by 37%, by increase in the PEI concentration, flux decreased (from 23.5 L/ m2 h to 15 L/m2 h), salt rejection improved from 77.9% to 82.2% for NaCl and from 72.2% to 91.2% for MgCl2, improved fouling resistance and the increased surface hydrophilicity. [195] Coated membrane became more hydrophilic (decreased CA to 15), neutral, and smoother, with increasing the coating concentration, the water flux increased and then decreased, the salt rejection increased monotonically, strong chlorine tolerance. Ethanol re-soaking of [197] PEBAX-coated membranes increased the water flux of membranes by almost 70%, from 2.6 L/(m2 h) to 4.5 L/ (m2 h) and decrease salt rejection from 96.8% to 95.4%. [198] Slower flux decline of modified membrane than the virgin membrane for BSA (37% and 44%) and seawater, antifouling property against reversible and irreversible fouling of E. coli, durability of coating layer to chemical cleaning. By increasing the time of [201] plasma polymerization, CA reduced and surface (continued on next page)

Cross flow

Ref.

Performance

Permeation test unit

M. Asadollahi et al.

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13

Chlorination treatment followed by chitosan deposition

Hangzhou RO

PEG based coating

AG

PEBAX® 1657

PEGA followed by GA crosslinking

RE8040 BE

ESPA1, ESPA3, SWC4

Coating agenta

Membrane

Table 3 (continued)

–

NaClO: 0–20 min, chitosan: 30 min

19

NaClO: 200–1000 mg/L, chitosan in acetic acid: 0.1 to 0.5 wt%

Exposure to UV light for 90 s at an intensity of 3000 μW/cm2

–

60

–

–

PEGA: 1 min GA: 30 s

1500 mg/L

1500 ppm

2000 mg/L

NaCl aqueous solution test

Time of coating

Temp. (°C) of coating

PEBAX® 1657: 0.06, 0.25, 1 wt% in nbutanol

Coating solution: 40 wt % of mixture that contains: PEGDA: 50 mol%, monomer (PEGA, HEA, AA): 50 mol%, HPK: 1.0 wt%

PEGA: 0.005–3% (w/w), GA: 0.005–1 wt%

Concentration of coating agentb

MgCl2: 1500 mg/L, MgSO4: 1500 mg/L

Oil/surfactant/water emulsion: 10,000 ppm motor oil and 1000 ppm silicone glycol

Oil-in-water emulsions containing 135 ppm ndecane & 15 ppm of SDS or DTAB, SDS: 200 ppm, DTAB: 200 ppm

BSA: 100 ppm, HA: 30 ppm & CaCl2 30 ppm solution of HA, E. coli: 0.34 optical density

Other foulants testsc

0.8 to 3 MPa

11.3 bar

15.5 bar

225 psi

Pressure

19

21.5

25

25

Temp. (°C)

–

Cross flow

Cross flow

Cross flow

Permeation test unit

Ref.

roughness increased, lower organic fouling, a little reduction of salt rejection (97.9–98.1%), more reversibility of the fouling Reduction in CA (from 49.2 [202] for bare membrane to 45.6) and surface roughness by coating, The pure water flux decreased with increasing the PEGA concentration and increasing GA concentration, exhibited better antifouling properties, recovered 100% of their initial water flux after physical cleaning. [203] Coating reduced the water flux of membrane, salt rejection of both coated and uncoated membranes is 99.0% or higher, coating caused small reduction in the negative charge of membranes, negatively charged membranes fouled extensively in the presence of positively foulant and minimal fouling in the presence of negatively foulant, more resistant of coated membranes to fouling in surfactant feed solutions and in an oil/ water emulsion made with DTAB [204] Coating reduced surface roughness without significant change in CA, enhanced fouling resistance against a model oil/ surfactant/water emulsion feed, the coating reduced water flux whiteout change salt rejection, coating remained on the surface of the membranes after 106 days of filtration. [206] Appropriate conditions for modification were: NaClO concentration 200 mg/L, chlorination time 2–5 min, chitosan concentration (continued on next page)

Performance

M. Asadollahi et al.

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14

PVA, PHMG

LE-400

P(MDBAC-r-Am-r-HEMA) coating followed by GA crosslinking

LCLE, BW30

P(NIPAM-co-Am)

Coating with sericin followed by GA crosslinking

Hangzhou RO

Hangzhou RO

Coating agenta

Membrane

Table 3 (continued)

PVA, PHMG and PVAPHMG solutions in ratios 95:5 and 99:1 (PVA:PHMG) with a total polymer content of 2 wt%.

Room temp.

Ambient condition

5h

2 Min soaking followed by heat curing at 50 °C for 2h

–

Terpolymer 200–10,000 mg/L, GA: 0.3 wt%

P(NIPAM-co-Am): 20 to 3000 ppm

Sericin: 5, min GA: 5 min followed by heat cured at 40 °C for 5 min

25

Sericin: 50–200 mg/L, GA: 0.2% (w/w)

Time of coating

Temp. (°C) of coating

Concentration of coating agentb

10 mM

2000 ppm

Bacteria adhesion of P. aeruginosa, antimicrobial activity of E. coli and B. subtilis

HCl: 0.5 mol/L, NaClO: 500, 1000, 2000, 3000 ppm for 1 h exposure time

NaClO: 500 mg/L for 1 to 50 exposure time, BSA: 100 mg/L

BSA: 100 mg/L

500 mg/L

2000 mg/L

Other foulants testsc

NaCl aqueous solution test

25

23

27.6 bar

25

25

Temp. (°C)

1.5 MPa

1.5 MPa

5 bar

Pressure

Cross flow

Cross flow

Cross flow

Cross flow

Permeation test unit

Ref.

1000 mg/L, coated membrane performed better than the original PA membrane (modified: flux of 57.7 L/m2 h and salt rejection of 95.4%, original: flux of 50.3 L/ m2 h and salt rejection of 91%), good performance for rejection of divalent salts (99.8% for MgCl2 and 98.5% for Na2SO4) [207] Sericin coating improved surface hydrophilicity, smoothed surface morphology, enhanced negative charge, lower pure water permeability, enhanced salt rejection, improved fouling resistance [208] Coating decreased flux, increased salt rejection, more tolerance chlorine exposure (7–10 times the pristine membrane), better wettability, fouling resistance, antimicrobial properties. [209] By increasing copolymer from 0 to 200 ppm, the CA decreased from 62.5° to 36.6°, and then leveled off, both the water flux and salt rejection of membrane slightly increased at lower coating concentration and then gradually decreased, both of the acid stability and chlorine resistance of the modified membrane increased with increasing the coating concentration. [210] Smoothening the membrane surface with all coatings (67.8 nm for uncoated and lowest RMS for PVA by the value 42.5 nm), reduction in CA (from 53.2 for uncoated to 15.9 for PHMG), Lower number of adhered bacteria on all coated membranes, membranes with higher PHMG percentage had (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

15

P(MPC-co-AEMA)

ES20

P(NIPAM-co-Am)

Hangzhou RO

P(NIPAM-co-Am)

P(NIPAm-co-AAc)

Hangzhou RO

Hangzhou RO

Coating agenta

Membrane

Table 3 (continued)

P(MPC-co-AEMA): 0.1 wt%

P(NIPAM-co-Am): 20 to 2000 mg/L with weight ratio of NIPAM/ Am = 87/13

P(NIPAM-co-Am): 5 to 5000 ppm with ratios NIPAM/Am = 90/10, 85/15, 80/20, 75/25, 70/30, 65/35, 60/40

P(NIPAm-co-AAc): 25, 50, 75, 100, 200, 500, 1000, 2000 ppm with 1, 2, 3, 4, 5 mol% of AAc

Concentration of coating agentb

24 h

25

In a refrigerator

3h

1 to 25 h

4h

Ambient conditions

Ambient condition

Time of coating

Temp. (°C) of coating

Bacterial adhesion of NBRC 13935

–

0.75 MPa

145 psi

100 psi, 0.65 MPa

–

BSA: 30, 60, 100 mg/L

0.5, 1.55 MPa

Pressure

MgCl2: 500 ppm, Na2SO4: 500 ppm, BSA: 100 ppm

Other foulants testsc

2000 mg/L

500 ppm

500, 2000 ppm

NaCl aqueous solution test

–

25

25

25

Temp. (°C)

Cross flow

Cross flow

Cross flow

Cross flow

Permeation test unit

Ref.

higher antimicrobial performance, reduced water permeability and salt rejection after modification. [211] Coating increased membrane surface hydrophilicity and surface charge at neutral pH, decreased the salt permeability of NaCl and Na2SO4, improved the fouling resistance to BSA, enhanced cleaning efficiency. [212] Increase in surface roughness by coating (from 87 nm to 106), decrease in CA from about > 55 for the unmodified membrane, increase in CA by increasing environmental temperature, little influence of coating on salt rejection (between 97% and 98% as that of the original membranes), decreases of water flux by increasing coating concentration, good durability and high performance stability of coating layer. Reduction of CA from about [213] 58.6° to 40.5°with increasing the coating concentration, the pure water permeability slightly increased and then gradually decreased by increasing deposition time, increase in water permeability by 12% and the salt rejection slightly (from 98.5% to 98.9%), improved fouling resistance of membrane to BSA, enhanced cleaning efficiency. [214] The coating reduced CA (from 36 of bare membrane to 18.6) and change the surface charge from a negative to a neutral value, (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

16

PDA coating followed by covalently immobilization (MPC-coAEMA) as the PZ

PDA

ES-20

XLE

Dopamine 0.1, 0.5, 2, 4, 8 mg/mL

Dopamine: 0.5 kg/m3, MPC-co-AEMA: 1 kg/m3

Room temperature

PDA: 28, PZ: –

30, 60, 120 min

PDA: 3 h, PZ: 24 h

1.5, 3, 6, 15, 24 h

PDA

ES20 28

5 min

60

SPGE: 0.01, 0.05, 0.1, 0.15, 0.2, 0.25 wt% DMAP: 0.01, 0.05, 0.1, 0.15, 0.2, 0.25 wt% glycerol: 1, 1.5, 2, 2.5, 3 wt%

Ring-opening by DMAP followed by SPGE and glycerol coating

IP of 2.25 wt% MPD and 0.06 wt% TMC

Dopamine: 0.1, 0.5, 1, 1.5, 2 kg/m3

Time of coating

Temp. (°C) of coating

Concentration of coating agentb

Coating agenta

Membrane

Table 3 (continued)

2000 ppm

0.05 wt%

0.05 wt%

2000 ppm

NaCl aqueous solution test

Oil/water emulsion containing 1500 of soybean oil/DC193C surfactant in water (9:1 ratio of oil to surfactant)

25

25

150 psig

–

25

Temp. (°C)

0.75 MPa

0.75 MPa

–

Bacterial suspension of P. putida: 850 g/m3

1.5 MPa

Pressure

NaClO: containing100 ppm free Cl2 for 540, 1620, and 3780 ppm h Cl2exposure time

Other foulants testsc

Cross flow

Cross flow

Cross flow

Cross flow

Permeation test unit

Ref.

decline in water permeability about 20% an increase in salt rejection (from 94.7 to 96.9), high resistance to bacterial adhesion [215] SPGE membranes were more hydrophilic and smoother, improved chlorine stability, with increasing DMAP concentration, water flux increased and salt rejection decreased, by increasing SPGE concentration water flux decreased and salt rejection decreased (above 0.05 wt%). [216] PDA did not affect the hydrophilicity, surface charge and salt rejection, PDA enhanced the biofouling resistance of membranes, improvement in the bacterial adhesion resistance No change of hydrophilicity [217] with PDA and/or PZ modification, changing surface charge from negative to neutral by PZ immobilization, permeability and rejection were not affected by modification, enhanced biofouling resistance and lower bacterial attachment of with PZ modification. [220] No change in CA with modification of membranes (about 19), decreasing pure water flux and salt rejection with increasing dopamine concentration and deposition time, more resistant of modified membranes to in oil/water emulsion fouling (as judged by higher permeate flux), little effect of variations in dopamine concentration, deposition times, and alkaline pH on the fouling resistance of coated (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

17

HPOEM or PEI coating followed by GA crosslinking, then dipping GA-treated PEI membranes in aqueous bromoacetic acid to obtain carboxylated PEI coating

PDA

SWC3 +

SHN (Toray)

PDA deposition followed by PEG-NH2 grafting

XLE

P(4-VP-co-EGDA) deposition followed by reaction with 3-BPA to obtain zwitterionic pCBAA

PDA deposition followed by PEG-NH2 grafting

XLE

TFC-HR

Coating agenta

Membrane

Table 3 (continued)

Reaction: 24 h

HPOEM or PEI: 1 min, GA: 30 s, bromoacetic acid: 10 h

Bromoacetic acid: 30 °C

HPOEM or PEI: 0.1 wt%, GA: 0.01 wt%, bromoacetic acid: 0.1 wt %

60 min

–

Filament temp.: 200, stage temp.: 20, reaction: 60

Dopamine: 30, 60, 90 min, PEG-NH2: 15, 30, 60, 90 min

PDA: 30 min, PEGNH2: 1 h

PDA: –, PEGNH2: 60

PDA: ambient conditions, PEG-NH2: 60

Time of coating

Temp. (°C) of coating

–

Dopamine: 2 g/L

Dopamine: 2 mg/mL, PEG-NH2: 2 × 10− 4 mol/L (0.2 g/ L of 1 kDa PEG, 1 g/L of 5 kDa PEG-NH2, or 4 g/ L of 20 kDa PEG-NH2)

Dopamine: 2 mg/mL, PEG-NH2: 1 mg/mL

Concentration of coating agentb

E. coli bacterial adhesion

Alginate: 30 ppm, BSA: 10 ppm

2000 mg/L

2000 or 32,000 ppm with 70 ppm CaCl2

Saline hydraulic fracturing flowback water

BSA adhesion

Pure water flux was measured

–

Oil/water emulsion (organic concentration: 1500 pm)

Other foulants testsc

2000 ppm

NaCl aqueous solution test

400 or 800 psi

300 psi

–

150 psi

10.2 atm

Pressure

25

21

–

Cross flow

Cross flow

Cross flow

Dead end cell

Cross flow

–

–

Permeation test unit

Temp. (°C)

Ref.

membranes. [221] PDA increased 30–50% water flux and improve the oil/water fouling resistance, the PD-g-PEG coating exhibited no flux decline and no fouling, modified membranes exhibited an increase in irreversible fouling resistance. [222] coating exhibited a small decrease in flux with increasing PDA deposition, PEG grafting reduced RO flux, PDA deposition reduced BSA adhesion and more BSA adhesion reduction was observed when PEG grafting. [223] The PDA-coated RO membranes did not enhance water flux or depress transmembrane pressure difference relative to the unmodified module due to the cleanliness of the RO feed after UF pretreatment, salt rejection was higher and more stable in the modified membrane. [230] 85% 4-VP is the optimal chemistry for copolymer, zwitterionic coatings reduce CA (from 60.4° for bare membrane to 29°), enhanced resistance against bacterial adhesion, decline in permeate flux (about 17%) and increase in salt rejection (about 2%), stability of copolymer in DI water [232] By surface coating, roughness decreased slightly, HPOEM coating made the membrane less negative and PEI coating made it more neutral, for HPOEM coating, the CA increase from 30.6 to 40.2 and for PEI coating the CA decrease from 57.8 to 43, PEI coating improved (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

p(4-VP-co-EGDA) deposition followed by reaction with 3-BPA to obtain zwitterionic pCBAA P(4-VP-co-DVB) deposition followed by reaction with PS to obtain pyridine-based sulfobetaine

TFC-HR

18

L-DOPA

HEMA-co-PFDA

HEMA-co-PFDA

SW30XLE

TFC-HR

TFC-HR

TFC-HR

Coating agenta

Membrane

Table 3 (continued)

Filament temp.: 250, stage temp.: 20

DVB content: 0, 4, 17, 100%

PFDA content: 0, 20, 40,

Film compositions of 17 to 100% PFDA

Filament temp.:

Filament temp.: 220, stage temp.: 30

–

Filament temp.: 200, stage temp.: 20

–

DOPA: 2.0 g/L

Temp. (°C) of coating

Concentration of coating agentb

–

0.2 M

–

–

35,000 ppm

–

–

–

–

4 to 24 h

NaCl aqueous solution test

Time of coating

Bacteria adhesion E. coli

E. coli bacterial adhesion, BSA adsorption

BSA adhesion, BSA: 100 mg/L, AAS: 100 mg/L, DTAB: 50 mg/L

NaClO: 1000 ppm for 2, 10, 24 h exposure time, bacterial adhesion of V. cyclitrophicus

Adsorption of BSA and HA, bacterial adhesion of P. aeruginosa and B. licheniformis.

Other foulants testsc

4825 kPa

–

18, 50 bar

25

–

20

Cross flow

–

Stirred dead end cell, cross flow

Dead-end filtration unit

–

700 psi

–

–

–

Permeation test unit

Temp. (°C)

Pressure

Ref.

fouling resistance in SW feed and HPOEM coating have better fouling resistance under BW feed. Significant enhancement in [233] resisting biopolymers (BSA and HA) adsorption, reduced attachment of seawater bacterial species [234] Coating decreased surface roughness (from 1.3 nm to 0.8 nm), with the 30-nm coating thickness, the water flux is reduced only by ~ 14% compared to untreated membranes whiteout changing the salt rejection, the addition of 4% DVB produces a major increase in the resistance to chlorine, whereas additions beyond 4% result in minor additional resistance, good fouling resistance against BSA and bacterial attachment. No change in zeta potential [237] after modification, reduction of CA from 55 to 15, intact salt rejection intact (about 97%), increased water flux with increasing the coating duration (1.27 times of original membrane), improved the BSA adhesion resistance, enhanced the organic and surfactant fouling resistance, high water flux recovery ratio (98%). [238] By increasing PFDA content, the roughness of surface (< 5 nm) and static contact angle increased, coatings was very smooth and conformal, the intermediate chemistry (40% PFDA) showed less alginate adsorption and highest resistance to bacterial adhesion (2 orders of magnitude). Increasing the PFDA [239] (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

19

HEMA-co-PFDA

HEMA-co-PFDA

TFC-HR

HEMA-co-PFDA

80 B, AD, SW30HR

TFC-HR, AD

Coating agenta

Membrane

Table 3 (continued)

Compositions: 0, 10, 40, 75, 100% PFDA on the copolymer

Composition: 40% PFDA on the copolymer

20 g/L

0.2 M

–

Filament temp.: 210, stage temp.: 30

2000 ppm

NaCl aqueous solution test

–

–

Time of coating

–

Filament temp.: 220, stage temp.: 30

210, stage temp.: 30

50, 100%

40% PFA on the copolymer

Temp. (°C) of coating

Concentration of coating agentb

–

SA: 100 mg/L

–

Other foulants testsc

700 psi

800 psi

50 bar

Pressure

25

28

25

Temp. (°C)

Cross flow

Cross flow

Cross flow

Permeation test unit

Ref.

content in the films leaded to increase the CA (from 38 for bare membrane to 64) and surface roughness, increasing the thickness of the coatings increased the surface roughness and decreased the permeation rates (from 170 L/m2 h for bare membrane to 80 L/ m2 h for 100 nm coating thickness), by increase the PDFA content the permeation rates of the coated membranes decrease, reduction in cell attachment on the coated surface. Deposited film were smooth [240] and conformal, modified surface were initially very hydrophobic but quickly assumed a hydrophilic character within few minutes, modification caused flux decline about 10% of the original value and negligible change in rejection. [241] Continuous and dense foulant layer on uncoated membrane while porous and discontinuous one on coated membrane, lower flux decline for coated membrane (10.51% vs. 19.82% for uncoated membrane) without change the salt rejection, better resistance to foulant adsorption, reduction in CA of coated membrane after the deposition of SA (~20) significant decrease (~20°) in the CA of the coated membrane. [242] Slight increase in the surface roughness and increase in CA with increasing PFDA content, flux decline (13%) for 20 nm coatings with 40% PFA content for short term permeation tests with DI (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

20

PAMAM and PAMAM–PEG

XLE

BaSO4-deposition

Tianchuang RO

PEG acrylate multilayers (containing PAA-Alk, PEG-Az or PEG-Alk)

ALD-Al2O3

LE-400

SW30, BW30LE

Coating agenta

Membrane

Table 3 (continued)

–

–

–

25

70, 100

Temp. (°C) of coating

PAA-Alk, PEG-Az, PEGAlk: 0.5 g/L

BaCl2: 0.05 M, Na2SO4: 0.05 M

AlMe3, Me = CH and H2O as precursors

Concentration of coating agentb

30 s

20 min

Alternate soaking process (ASP), each step 60 s

0.1 s AlMe3 pulse, 10 s N2 purge, 0.1 s H2O, 10 s N2 and 10, 50 and 100 coating cycle

Time of coating

1000 ppm

–

–

Seawater: NaCl: 30.83 g/L, CaCl2: 1.11 g/L, SA: 100 mg/L, Brackish water: NaCl: 2 g/ L, SA: 100 mg/L

BSA: 200 mg/L

Bacteria adhesion of P. aeruginosa

10 mM

500 mg/L

Other foulants testsc

NaCl aqueous solution test

100 psi

800 psi

5 bar

27.6 bar

Pressure

–

18.6

25

23

Temp. (°C)

–

Stirred cell

Cross flow

Cross flow

Permeation test unit

Ref.

water. Coating caused membranes [243] surface tightening, decreasing the roughness, the most hydrophilic surface was obtained with 10 and 50 ALD cycles at temperature 70 °C (16 and 27) and 10 ALD cycles at 100 °C (27), improved antifouling performance of the membrane, the lowest number of bacteria cells was adhered to surface. [244] Mineralized membranes became smoother, more hydrophilic (from 65 of raw membrane to 38.5), more negatively charged, enhanced water flux and salt rejection (from 24.7 L/ m2 h and 96.8% for raw membranes to 29.5 L/m2 h and 98.2% of mineralized membranes), improved antifouling property. [245] fouling resistance and CA reduction for coated membranes, for seawater membranes, the flux decline 9–17% and salt rejection increase small and for brackish water, no reduction in the flux and slight increase in salt rejection relative of the uncoated value Coated membranes had [249] reduced flux (from 2 0.11 mL/cm min for uncoated membranes to 0.08 mL/cm2 min for coated membranes) without salt rejection (about 99%), reduction in CA (from 60° to 15°). (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

Desalination xxx (xxxx) xxx–xxx MMA-HPOEM: methyl methacrylate-hydroxyl poly(oxyethylene) methacrylate, Triglyme: trimethylene glycol dimethyl ether, PEGA: poly(ethylene glycol) acrylate, GA: glutaraldehyde, PEG: poly(ethylene glycol), P(MDBAC-r-Am-r-HEMA): poly (methylacryloxyethyldimethyl benzyl ammonium chloride-r-acryl-amide-r-2-hydroxyethyl methacrylate), P(NIPAM-co-Am): poly(N-isopropylacrylamide-co-acrylamide), PVA: polyvinyl alcohol, PHMG: polyhexamethylene guanidine hydrochloride, P(NIPAm-co-AAc): N-isopropylacrylamide-co-acrylic acid, MPC-co-AEMA: 2-(methacryloyloxy) ethyl phosphorylcholine (MPC) copolymer with 2-aminoethylmethacrylate (AEMA), DMAP: N,N-dimethylaminopropylamine, SPGE: sorbitol polyglycidyl ether, PDA: polydopamine, L-DOPA: 3-(3,4-dihydroxyphenyl)-L-alanine, P(4-VP-co-EGDA): poly(4-vinylpyridine-co-ethylene glycol diacrylate), 3-BPA: 3-bromopropionic acid, pCBAA: poly(carboxybetaine acrylic acetate), HPOEM: hydroxyl poly(oxyethylene) methacrylate, P(4-VP-co-DVB): poly(4-vinylpyridine-co-divinylbenzene), PS: 1,3-propanesultone, (HEMA-co-PFDA): hydroxyethyl methacrylate (HEMA)-co-perfluorodecyl acrylate, PAA-Alk: poly(acrylic acid) with alkyne functionality, PEG-Az: PEG acrylate modified with azide, PEG-Alk: PEG acrylate modified with alkyne, PAMAM: polyamidoamine, HBP-PEG: hyperbranched polymers-poly(ethylene glycol). a PEO: polyethylene-oxide, PEI: polyethyleneimine, P(AMPS-co-Am-co-VI): 2-acrylamido-2-methyl propanesulfonic acid-co-acrylamide-co1-vinylimidazole (VI), PEBAX® 1657: polyether–polyamide block copolymer. b PEGDA: poly(ethylene glycol) diacrylate, HEA: 2-hydroxyethyl acrylate, AA: acrylic acid, HPK: 1-hydroxycyclohexyl phenyl ketone, AlMe3: trimethylaluminium, HB-PA: hyperbranched polyamide, PEG-DGE 526: Poly(ethylene glycol)diglycidyl ether with MW 526. c DTAB: dodecyltrimethylammonium bromide, NaClO: sodium hypochlorite, BSA: bovine serum albumin, AAS: aglinic acid sodium salt, HA: humic acid, SDS: sodium dodecyl sulfate, SA: sodium alginate. d Except [193,194] which are physical modification by “Surface adsorption”, all other references in this table are physical modification by “Surface coating”. e The configuration of all studied RO membranes in this table is flat sheet except [223], which is spiral wound.

[250] Coating increased surface hydrophilicity without any detrimental effect on salt rejection and with acceptable permeate flux reduction. SEPA CF II (GE Osmonics – 100 psi – 1000 ppm 30 s – HBP-PEG, PAMAM-PEG LE, XLE

Varying weight percent of HB-PA/PAMAM and stoichiometric PEG-DGE 526 in water

Time of coating Coating agenta Membrane

Table 3 (continued)

Concentration of coating agentb

Temp. (°C) of coating

NaCl aqueous solution test

Other foulants testsc

Pressure

Temp. (°C)

Permeation test unit

Performance

Ref.

M. Asadollahi et al.

loss due to solvent evaporation depends on solvent properties including surface tension, polarity, water miscibility and hydrogen-bonding capacity [197]. In surface coating 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. Also, in this method, the design of coating material is crucial to determine the physicochemical properties of coating [195]. The materials used for surface coating are usually hydrophilic polymers containing hydroxyl, carboxyl, amino groups, or enzyme. The presence of coating layer could significantly enhance hydrophilicity and reduce surface charge and roughness of membrane, rendering a better antifouling property. However, coating layer tended to offer an additional resistance to water permeation and render flux decline [23,158]. Coating the surface of polyamide TFC RO membranes by hydrophilic polymers was investigated by many researchers. Poly(ethylene glycol) (PEG) and PEG-based polymers are widely used for membrane surface treatment due to their extraordinary antifouling ability especially to resist protein adsorption (by both physical and chemical modification methods) [198–204]. PEG is an uncharged water-soluble polymer having good hydrophilicity, flexible long chains and large exclusion volume. In aqueous solutions, they have inherent affinity and a tendency to form a huge complex with the surrounding water molecules (via a hydrogen bond network), forming hydrated layers on the membrane surface. Foulants such as hydrophobic or large molecules cannot detect surfaces that are covered with the hydrated layers. Also, steric repulsion mechanism of PEG provides resistance to foulants. The steric repulsion effect is due to the loss of configurational entropy resulting from volume restriction and osmotic repulsion of PEG chains [198–200]. Although hydration and steric repulsion of PEG show excellent protein resistance and biofouling ability, some problems still remain unsolved PEG materials are susceptible to damage caused by oxidation and they have been reported to be only resistant to short-term fouling; however, the long-term fouling formation still occurs [205]. Choi et al. [198] used methyl methacrylate-hydroxy poly(oxyethylene) methacrylate (MMA-HPOEM), a comb-like amphiphilic copolymer. An amphiphilic copolymer with PEG side chains was synthesized and introduced on the membrane surface using the dip coating method. Although the MMA block and backbone of the polymer corresponding to the hydrophobic region adhered to the membrane surface, the PEG chains of the polymer extended to a water phase, swayed like a brush, and repelled various foulants in feed water. Experiments using various foulants (such as bovine serum albumin (BSA), Escherichia coli (E. coli), and seawater) showed that the modified membrane had improved antifouling property not seen on the virgin membrane. Also, the modified membrane had a less negative surface charge compared with the virgin membrane and coating layer was durable to chemical cleaning [198]. In another work, PEG-like hydrophilic polymer (trimethylene glycol dimethyl ether (triglyme)) was deposited onto aromatic polyamide RO membrane by plasma polymerization to reduce organic fouling tendency [201]. Plasma polymerization can be assumed to be one of the less damaging methods of membrane modification and that is a great advantage for the treatment of TFC membranes. Plasma polymerization is the electrical ionization of a monomer, which results in the generation of reactive monomer fragments. These fragments recombine on surfaces to form cross-linked structures. The advantages of plasma polymers over conventional polymers are that they show a much higher degree of cross-linking and adhere strongly to the substrate, the coatings are uniform and do not require the use of harsh solvents that may damage the substrate [201]. Triglyme contains a PEG/polyethyleneoxide (PEO)-like type of polymer functionality, which increased the hydrophilicity of the membrane surface and reduced organic fouling caused by protein absorption. Moreover, the modified membranes were easily cleaned. However, the plasma polymerization in their study caused an increased roughness, which is disadvantageous to the 21

Desalination xxx (xxxx) xxx–xxx

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pure water permeability, enhanced salt rejection, and improved fouling resistance to BSA [207]. Hydrophilic random copolymer poly(methylacryloxyethyldimethyl benzyl ammonium chloride-r-acrylamide-r-2-hydroxyethyl methacrylate) (P(MDBAC-r-Am-r-HEMA)) was used as a coating material to improve membrane antifouling performance and chlorine resistance [208]. The terpolymer was coated on a commercial RO membrane followed by GA chemical cross-linking. The method was efficient as the resulted membranes showed much better wettability, antimicrobial properties, chlorine and fouling resistances than the virgin membranes. In addition, the membranes prepared from higher coating concentration showed more prominent effects [208]. Liu et al. [209] evaluated the coating of hydrophilic copolymers poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAM-co-Am)) which was deposited on the RO membranes surface. The coating layer enhanced the intermolecular hydrogen bonding in the polyamide membrane and the enhanced intermolecular hydrogen bonding would impede the hydrolysis and chlorination of amides of the aromatic polyamide barrier layer. Also, the additional polymeric coating layer acted as a protective and sacrificial layer, which would hinder the attack of acid and free chlorine on the underlying polyamide film. The results showed that the acid stability and chlorine resistance of the modified polyamide membrane increased with increasing the coating solution concentration. Although the surface coating layer would offer additional resistance to water permeation, the hydrophilic coating layer increased the membrane surface hydrophilicity and promote water permeation and membrane performance [209]. Hydrophilic random copolymer (terpolymer) of 2-acrylamido-2methyl propanesulfonic acid, acrylamide and 1-vinylimidazole (P (AMPS-co-Am-co-VI)) was designed as a coating material to improve the chlorine tolerance of polyamide RO membrane [195]. Coated membranes became more hydrophilic, neutral, and smooth, which were advantageous to the improvement of membrane antifouling ability. With increasing the coating concentration, the water flux increased and then decreased because the coating layer increased both the surface hydrophilicity and filtration resistance and the salt rejection increased monotonically due to the defective pores on the membrane surface are blocked by the terpolymer. The chlorine resistance property of modified membrane was improved significantly and higher terpolymer concentration exhibited stronger chlorine tolerance, especially in acid environment [195]. Nikkola et al. [210] developed polyvinyl alcohol (PVA) and cationic polyhexamethylene guanidine hydrochloride (PHMG) coatings on the RO membranes surface. Coatings increased the hydrophilicity and decreased the surface roughness and the permeability properties compared to uncoated membrane. In addition, coating decreased the attachment of bacteria on the membranes surface. Among the coated membranes, the membranes with higher PHMG percentage tended to had higher bacteria repellent and antimicrobial performance [210]. In other works, the surface of polyamide RO membranes were modified by depositing a thermo-responsive polymer (TRP) N-isopropylacrylamide-co-acrylic acid copolymers (P(NIPAm-co-AAc)) [211] or P(NIPAM-co-Am) [212,213] with low critical solution temperature (LCST). These TRP copolymers are hydrophilic and highly water permeable. PNIPAM, one of the most extensively studied synthetic TRP smart polymers, exhibits a LCST at about 32–33 °C in aqueous solution. Below LCST, the free polymer chains are soluble in water and exist in an extended random coil conformation that is fully hydrated. On the contrary, above LCST, the chains hydrophobically fold as a result of dehydration and assemble to form a phase separating state. Furthermore, its phase transition behavior can be modulated by incorporating more hydrophilic or hydrophobic monomer in the polymer composition. The incorporation of hydrophilic moieties such as acrylamide (Am) will enhance its LCST, whereas the introduction of hydrophobic moieties such as butyl methacrylate (BMA) will lower the LCST [211,212]. It is

colloidal fouling resistance [201]. In addition, the homopolymer poly(ethylene glycol) acrylate (PEGA) was used as surface coating to enhance antifouling property of RO membranes [202]. PEGA coated RO membranes were cross-linked by the glutaraldehyde (GA) solution to enhance the durability of the coating layer. After the surface modification, the RO membranes showed a lower surface roughness, more hydrophilicity, and better performance compared to the unmodified RO membranes. The modified RO membranes exhibited better antifouling properties and recovered almost 100% of their initial water flux after physical cleaning. Sagle et al. [203] investigated the effect of coating of a series of cross-linked PEG-based hydrogels on the RO membranes. In their method, the liquid prepolymer mixture (monomer, crosslinker, and photoinitiator) was firstly coated on the surface of RO membrane and then photopolymerized by UV to form a water-insoluble coating. The PEG-based hydrogels were synthesized using PEGA, 2-hydroxyethyl acrylate (HEA), or acrylic acid (AA) as monomers, poly(ethylene glycol) diacrylate (PEGDA) as the crosslinker, and photoinitiator 1-hydroxycyclohexyl phenyl ketone (HPK). Water flux of coated membranes was less than that of uncoated membranes whiteout changing the salt rejection. Negatively charged membranes fouled extensively in the presence of positively charged surfactants and experienced minimal fouling in the presence of negatively charged surfactants. In addition, coated membranes were more resistant to fouling in surfactant feed solutions and in an oil/water emulsion. Louie et al. [204] performed another physical coating study of commercial polyamide RO membranes with PEBAX-1657, which was a very hydrophilic block copolymer of nylon-6 and PEG. 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 that showed the coated membranes had enhanced fouling resistance. However, the coating resulted in large water flux reduction, especially for high-flux RO membranes [204]. Also, Ethanol re-soaking was tested on PEBAX-1657 coated membranes as a means to recover flux loss caused by the coating process [197]. Ethanol was able to increase the water flux of PEBAX coated membranes by almost 70%; however, there was a decrease in salt rejection (from 96.8% to 95.4%). FTIR analysis confirmed that PEBAX remained on the membrane surface after ethanol soaking, so the observed increase in flux was not caused by removal of the coating layer. Instead, the flux increase was likely due to the recovery of hydrogen-bonding sites on the polyamide for water permeation by breaking interchain hydrogen bonds [197]. Xu et al. [206] investigated surface treatment of the polyamide RO membrane by chlorine, followed by supramolecular assembly of chitosan (CS) on the membrane surface. Controlled chlorination with dilute NaClO solution not only made the membrane surface more hydrophilic but also led to a more negative zeta potential. This characteristic was favorable for electrostatic deposition of a polycation on the membrane surface. Chitosan was selected as a polycation for deposition onto the polyamide membrane because of its moderate charge density, high hydrophilicity, mechanical, and chemical stabilities as well as good film forming properties. Chitosan coated membrane performed better than the original polyamide membrane with NaCl feed. Also, the modified membranes had good performance for rejection of divalent salts such as MgCl2 and Na2SO4 [206]. Polyamide RO membrane was also modified by surface coating of natural polymer sericin to improve antifouling property [207]. Sericin is a natural hydrophilic polymer and it is a water-soluble globular protein having polar side groups of hydroxyl, carboxyl, and amino groups. The deposition of sericin on the membrane surface was carried out through dip-coating and sericin was adsorbed on the membrane surface mainly by hydrogen bonding followed by in situ cross-linking with GA to insolubilize sericin. Sericin coating on membranes resulted in improved surface hydrophilicity, smoothed surface morphology, and enhanced negative charge. In addition the coated membranes had lower 22

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coating. PDA has been reported for several kinds of membranes, including electrodialysis (ED) [218], UF [219], and RO membranes [216,220–223]. Dopamine solutions for coating of RO membranes were prepared by dissolving dopamine hydrochloride at different concentrations in TrisHCl buffer (aqueous solution). Dopamine spontaneously polymerizes and forms PDA by contacting with oxygen in an alkaline aqueous solution [216]. Fig. 10 shows a mechanism for dopamine oxidative selfpolymerization. It was reported by Karkhanechi et al. [216] that surface modification with PDA did not affect the hydrophilicity, surface charge, and salt rejection appreciably. Also, the thickness of the PDA layer on the membrane surface was gradually increased with increasing modification time which cause increase in the diffusional resistance for water permeation and decreased the water flux. In addition, PDA enhanced the biofouling and bacterial adhesion resistance of RO membranes [216]. The effect of PDA deposition on RO membranes was investigated for pure water flux, flux during filtration of an oil/water emulsion, and NaCl rejection [220]. The filtration results indicated that PDA was not successfully deposited onto the membranes surface under acidic conditions and pure water flux decreased with increasing dopamine solution concentration and deposition time. Membranes modified with PDA at all dopamine concentrations, deposition times, and alkaline pH values were significantly more resistant to fouling in oil/water emulsion fouling tests than uncoated membranes. Also, NaCl rejection values in all membranes were within the manufacturer's specification [220]. McCloskey et al. used PDA deposition and the deposition of PDA was followed by PEG-NH2 grafting [221,222] on the RO membranes surface to improve fouling resistance of the membranes. The primary amine groups at the termini of these PEG chains reacted and formed covalent linkages with PDA; that is, PEG-NH2 covalently grafts to the PDA layer [221]. PDA modified RO membranes achieved 30–50% higher flux than an unmodified membrane after 1 h of oil emulsion filtration, and the PDAg-PEG RO membranes exhibited no flux decline and therefore no fouling during filtration. Furthermore, modified membranes exhibited an increase in irreversible fouling resistance, which enhanced cleaning cycle efficiency and lowered overall operating costs [221]. Also, the pure water flux of PDA-g-PEG RO membranes was considerably reduced relative to the unmodified membranes and the PDA modified membranes [221,222]. PDA deposition reduced BSA adhesion and additional BSA adhesion reduction was observed when PEG grafted [222]. Moreover, the deposition of PDA was carried out on RO membrane

thus expected that TRP surface coating layer will improve antifouling property and facilitate the removal of foulant that is located on a membrane surface [213]. The results indicated that coating of copolymers caused increase in membrane surface hydrophilicity, enhancement the membrane cleaning efficiency, improvement the fouling resistance of membrane to BSA, and good durability and high performance stability of coating layer [211–213]. Saeki et al. [214] developed a simple and easy modification method for coating phosphorylcholine polymer onto a RO membrane surface via electrostatic interaction to prevent bacterial adhesion. They used poly[2-methacryloyloxyethyl phosphorylcholine (MPC)-co-2-aminoethylmethacrylate (AEMA)] (P(MPC-co-AEMA)) as a cationic phosphorylcholine polymer. It was considered that hydrophilic P(MPC-coAEMA) was immobilized onto the membranes surface because of electrostatic interaction between the cationic amide groups of P(MPC-coAEMA) and the anionic carboxyl groups on the surface of RO membrane. The coating reduced the contact angle of membranes and changed the surface charge from a negative to a neutral value. The coated membranes showed decline in water permeability about 20% and increase in salt rejection and resistance to bacterial adhesion [214]. Kwon et al. [215] coated the surface of polyamide RO membrane by in-situ polymerization of hydrophilic sorbitol polyglycidyl ether (SPGE) immediately after interfacial polymerization of TFC membranes. N,Ndimethylaminopropylamine (DMAP), a tertiary amine, was used as both an anchor to hold the SPGE to the membrane surface and a ringopening agent since tertiary amine can initiate ring opening of the epoxy moiety of the SPGE polymer (Fig. 9). The primary amine group in DMAP can form chemical bonds with an unreacted acyl chloride group of nascent polyamide skin layer. Also, they used glycerol as a humectant prevented the membrane from drying out during the ring-opening reaction in an oven and increased membrane performance. With increasing SPGE concentration in the coating solution, the water flux declined but salt rejection increased. SPGE coating layer resulted in a membrane surface that was more neutral, hydrophilic, and smooth. Moreover, the modified surface was much less susceptible to chlorine attack and had enhanced chlorine stability [215]. Dopamine (3,4-dihydroxyphenethylamine) coating is a relatively new surface modification method. A polydopamine (PDA) layer is formed on any substrate that is immersed in dopamine solution. The polar groups in a PDA coating, such as hydroxyl and amine groups, improve the hydrophilicity, antifouling, and antibiofouling properties of a substrate [216,217]. The PDA layer tightly adheres to the surface by strong covalent and non-covalent interactions with the substrate, giving a highly stable PDA

Fig. 9. Schematic surface modification process: (a) polyamide TFC membrane, (b) DMAP-treated TFC membrane and (c) SPGE-treated TFC membrane [215].

23

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Fig. 10. Dopamine polymerization [216].

which may cause damage to delicate substrates such as polyamide TFC RO membrane. For instance, SAM requires specific surface functionality, which limits this method to gold substrates. Furthermore, the use of solvents in solution polymerization and ATRP not only damage delicate substrates of RO membranes but may also lead to increased surface roughness, which deteriorates anti-biofouling properties. Therefore, solvent-free deposition of antifouling coatings on RO membranes is the best strategy [230,231]. Some coating layers of zwitterionic materials had been used to modify the surface of RO membranes such as 2-(methacryloyloxy) ethyl phosphorylcholine (MPC) copolymer with 2-aminoethyl methacrylate (AEMA) (MPC-co-AEMA) [217], carboxylated polyethyleneimine (carboxylated PEI) [232], poly(carboxybetaine acrylic acetate) (PCBAA) [230,233], and pyridine-based sulfobetaine zwitterionic functional groups [234]. Karkhanechi et al. [217] modified a commercial RO membrane with PDA coating as a precursor layer and then polyzwitterionic (MPC-coAEMA) was covalently immobilized on the PDA surface that is shown in Fig. 11. The PDA precursor layer was confirmed to increase the stability of a polyzwitterionic (PZ) layer immobilized on the membrane surface because PDA layer containing reactive groups endows a versatile platform for further surface functionalization and immobilization. Surface modification with PZ layer clearly enhanced the biofouling resistance and lower bacterial attachment of RO membranes. In addition, water permeability and salt rejection were not affected by modification. In addition, the zwitterionic carboxylated PEI-coated membrane showed higher affinity to sodium chloride solution than deionized water, and presented an inhibitory effect to foulant adsorption under seawater conditions. This suggested that an effective antifouling layer specialized for seawater filtration could be prepared with zwitterionic materials [232]. Some researchers deposited zwitterionic structure on the RO membrane surface by initiated chemical vapor deposition (iCVD) [230,233,234]. iCVD is a process that allows thin-film deposition of a wide variety of polymers without the use of any solvent. It is a vacuumbased, vapor-phase, and free-radical polymerization technique which has shown great promise as a surface modification technique. Proceeding from volatile monomer units, polymer synthesis and film deposition occur simultaneously at modest vacuum and low temperature [235]. The unique features of this technique boast of several advantages which are shown in Table 4. One of the most important attributes of iCVD is that growth occurs on surfaces held at near room temperature in the absent of solvents. Thus, iCVD avoids damaging to either the chemical structure or porosity of polymeric membrane substrates [236]. For deposition of zwitterionic materials by iCVD [230,233,234], prior to deposition, the membranes were cleaned by high purity argon gas and were then treated with O2 plasma for 5 min. The purpose of oxygen plasma treatment was to create dangling bond in order to had strong interface between the membrane surface and the depositing

modules in situ and PDA was deposed on the membrane surface, feed spacers, and all wetted parts. Modules were used to purify saline hydraulic fracturing flowback water from wells in the Barnett shale play in north Texas [223]. The PDA coated RO membranes did not show enhanced flux or depressed transmembrane pressure difference relative to the unmodified module, likely due to the cleanliness of the RO feed after UF pretreatment. However, the salt rejection was higher and more stable in the modified modules than in the unmodified modules. The results demonstrated that PDA could be employed in the modification of industrial membrane modules and those modifications improved the fouling behavior of modules challenged with complex and highly fouling feed water [223]. Over recent years, a new type of antifouling and antibiofouling material, zwitterionic polymers, have attracted increasing attention because of their excellent anti adhesion properties against proteins and bacteria [224]. The antifouling characteristic of zwitterionic materials is strongly associated with a hydration layer near the surface [225,226]. In fact water is attached to hydrophilic materials through hydrogen bonding, whereas it binds to zwitterionic materials through ionic solvation. Therefore, compared to hydrophilic materials, higher energy is required to remove bound water from zwitterionic materials. Since adsorption of a protein or bacteria to the surfaces involves expulsion of water from both protein and surface, zwitterionic materials are more stable against protein adsorption than hydrophilic materials [227]. Zwitterionic polymers (polyampholytes) are polymers possessing both positively and negatively charged functional groups within the same segment side chains but maintain overall charge neutrality, such as polyphosphobetaine, polysulfonbetaine, and polycarboxybetaine [228]. The typical anionic group for zwitterion is a quaternary ammonium group, while the cationic groups include sulfonic, carboxylic, and phosphoric groups [229]. Several innovative techniques have been developed to synthesize zwitterionic coatings. The most common of these techniques include self-assembled monolayer (SAM) formation, solution polymerization with solvent evaporation, atom transfer radical polymerization (ATRP) and initiated chemical vapor deposition (iCVD) (Table 4). These methods (except iCVD) generally involve harsh process conditions

Table 4 Synthesis methods of the zwitterionic coatings [231]. Method

SAMs

Solution polymerization and solvent evaporation

ATRP

iCVD

Conformality nm-to-um scale thickness No specific surface functionality required Solvent free Small post treatment roughness

High × ×

Low × ✓

High × ×

High ✓ ✓

× ✓

× ×

× ✓

✓ ✓

24

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Fig. 11. The scheme for PZ immobilization on a PDA-modified RO membrane [217].

copolymer film. That strong bonding eliminated delamination of deposited copolymer film when placed in water. To obtain zwitterionic PCBAA coating [230,233], firstly the copolymer film poly(4-vinylpyridine-co-ethylene glycol diacrylate) (p(4-VP-co-EGDA)) deposited on RO membranes surface. EGDA and 4-VP monomers were heated to 85 °C and 55 °C respectively and tert-butyl peroxide (TBPO) as initiator was maintained at room temperature and three components fed into iCVD reactor. During thin film synthesis, the filament temperature was maintained at 200 °C to promote radicalization of the initiator, while the substrate was fixed at 20 °C. The conversion of as-deposited copolymer films (films synthesized by iCVD) to zwitterions was accomplished by exposing the film to vapors of the quaternizing agent 3-bromopropionic acid (3-BPA) and zwitterionic copolymer containing PCBAA was obtained [230,233]. The coating of membranes resulted in higher hydrophilicity and significant enhanced resistance against bacterial adhesion. Also, the salt rejection performance of the modified membranes indicated improved salt rejection. However, permeate flux was slightly compromised compared to virgin membranes [230]. Moreover, the coated membranes showed remarkable improvement in resisting biopolymers adsorption and reduced attachment of seawater bacterial species [233]. In Yang et al. report [234], the 4-VP and divinylbenzene (DVB) monomers were heated up to 50 °C and 65 °C respectively and delivered into the reactor and the films of poly(4-vinylpyridine-co-divinylbenzene) (PVD) was obtained. Films were deposited at a filament temperature of 250 °C and a stage temperature of 20 °C. Enhanced durability resulted from cross-linking by DVB and in situ grafting. The in situ reaction with 1,3-propanesultone (PS) vapors produced pyridinebased sulfobetaine zwitterionic functional groups. The coating on membranes showed good fouling resistance against dissolved chemicals, much greater resistance to bacterial attachment, and excellent chlorine resistance. Also, the salt rejection of the modified RO membranes was unaltered, confirming the benign nature of the solvent-free process [234]. Azari and Zou [237] coated amino acid 3-(3,4-dihydroxyphenyl)-Lalanine (L-DOPA) on the RO membranes. Comprising the negatively charged carboxyl group and positively charged amine group, qualifies LDOPA as a zwitterionic molecule, whereas dopamine does not. The results indicated that deposition of a compound structurally related to dopamine, L-DOPA, onto RO membranes caused the salt rejection to remain intact and the water flux increased with increasing the coating duration. Also, coated membranes improved fouling resistance of membranes to BSA adhesion, organic, and surfactant [237]. In other works, the copolymer films of hydrophilic hydroxyethyl methacrylate (HEMA) and the hydrophobic perfluorodecyl acrylate (PFDA) monomers was synthesized and (HEMA-co-PFDA) copolymer films deposited on RO membranes using iCVD technique that involves

free-radical polymerization initiated by gas-phase radicals [238–242]. HEMA and PFDA monomers were heated up to 70 °C and 80 °C, respectively, while the initiator TBPO was kept at room temperature. The filaments in the reactor were heated to approximately 220 °C. Monomer molecules were adsorbed on the substrate surface which was kept at room temperature, while the initiator molecules were thermally decomposed into initiator radicals by the heated filaments. These radicals then initiated the free-radical polymerization on the substrate surface by reacting with the vinyl bonds of the adsorbed monomer molecules [238,239]. In (HEMA-co-PFDA) copolymer coated membranes, iCVD resulted the coatings to be very smooth and conformal and copolymer coatings had negligible effect on permeate water flux and salt rejection. In addition, coating increased the hydrophilicity of membranes. Increase in the PFDA content in the films led to increase the contact angle and surface roughness. In addition, coated membranes had resistance to bacterial and cell adhesion [238–242]. Nikkola et al. [243] modified the surface of polyamide reverse osmosis membrane by atomic layer deposition (ALD) process using trimethylaluminium (AlMe3, Me = CH). Atomic layer deposition (ALD) technology has been used almost four decades for manufacturing inorganic coating layers, such as oxides, nitrides and sulfides, with thickness down to the nanometer range. Recent advancement in ALD technology has increased its potential to produce functional thin films on flexible and temperature-sensitive polymeric materials. The studies suggested that ALD processing parameters, such as temperature and number of ALD cycles, have clear impact on membrane performance. The ALD-Al2O3 depositions were carried out in a reactor and AlMe3 and H2O were used as precursors [243]. ALD coating caused tightening the surface, decreasing the surface roughness and affected on the hydrophilicity. In addition, the lowest number of cells was adhered on the surface and the antifouling performance of the membranes was improved. Zhou et al. adopted surface mineralization method to modify the surface of commercial polyamide reverse osmosis membrane (Fig. 12) [244]. BaSO4-based mineral coating was deposited on the surface of the polyamide RO membrane by an alternate soaking process (ASP) using aqueous solutions containing barium chloride (BaCl2) and sodium sulfate (Na2SO4). The degree of surface mineralization was changed by varying the numbers of ASP cycles. Mineralized membranes became smoother, more hydrophilic and more negatively charged, which were desirable for the purpose of improving fouling resistance to foulants of similar charge. Also, coated membranes had improved fouling resistance to BSA aqueous solution. The coating resulted in increased both pure water permeability and salt rejection, corresponding to decreased hydraulic resistance and salt permeability coefficient, respectively. The increased salt rejection was attributed to the enhanced repulsion force 25

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dendrons emanating from a central core which can be either a single atom or an atomic group. Amine-functional polyamidoamine (PAMAM) dendrimers and PAMAM–PEG multi-arm stars with difunctional PEG crosslinkers were coated on the surface of RO membranes. The coated membranes had acceptable reduction of permeate flux without deterioration of the salt rejection. Moreover, contact angle of modified membranes reduced significantly, which indicated the potential for increased resistance to fouling by hydrophobic foulants, such as biofoulants and organic pollutants [249]. Sarkar et al. [250] used PAMAM-PEG dendrimers and hyperbranched polymers-PEG (HBP-PEG) on the RO membrane surface. Hyperbranched polymers are highly branched, tree-like molecules. Among others, their architecturally related, characteristic properties include: (a) nanoscopic molecular sizes, (b) very high density of molecular functionality, (c) ability to encapsulate smaller molecular weight species within their highly branched nanoscopic molecular interiors; and (d) significantly lower viscosities than those of linear polymers of comparable molecular weights. The results indicated that coating increased surface hydrophilicity without any detrimental effect on salt rejection and with acceptable permeate flux reduction [250]. Fig. 12. Basic principle of BaSO4 mineralization on the surface of polyamide TFC RO membrane [244].

7.3.2. Chemical surface modification Surface modification has been considered as a useful method to improve the antifouling properties of TFC RO membranes without destroying bulk properties. Chemical surface modification includes different methods such as chemical coupling, free radical graft polymerization, atom transfer radical polymerization and etc. Among various methods, graft polymerization is regarded as one of the most promising methods to modify a membrane surface through covalent bonding interactions between the polymer chains with a new functionality and the membrane surface. This method is attractive due to its simplicity, robustness, high graft density, and wide range of monomers that can be polymerized, which can result in membranes with diverse physicochemical surface properties [251,252]. The grafted chains can be covalently attached to the membrane surface with either surface initiated (grafting from) or convergent (grafting to) approaches. The “grafting from” method utilizes the polymerization initiated from the membrane surface by attached (usually by covalent bonds) initiating groups. Molecules of a monomer from the solution penetrate through the already grafted polymer chains easily and grow form the membrane surface. In “grafting to” method the polymer chains are polymerized separately and later react with functional groups on the membrane surface and attach it. The “grafting to” method has the advantages of a well-defined polymer chain length and properties. However, the approach is uncommon in membrane modification, due to slow diffusion-limited kinetics and low grafting density. In contrast, the regular “grafting from” results in a wide distribution of chain lengths but can offer a higher density and degree of modification [253–255]. In Table 5, different materials which were used in chemical surface modification of polyamide TFC RO membranes, operating conditions, and performance of membranes are summarized.

between Cl− ion and the surface of the mineralized membrane. Wang et al. [245] were assembled PEG acrylate multilayers via the layer-by-layer (LbL) technique on a polyamide RO membrane and stabilized using click chemistry. LbL assembly is a thin film fabrication technique. The films are formed by depositing alternating layers of oppositely charged materials with wash steps in between. This can be accomplished by using various techniques such as immersion and spin [246]. This technique not only creates a charged skin layer but also allows for a better control of the thickness, charge density, and hydrophilicity of the active skin layer [247]. Click chemistry refers to a set of covalent reactions with high reaction yields that can be performed under extremely mild conditions. Click chemistry provides stable cross-links within the films, which means that they are highly stable and are not susceptible to disassembly under varying solution conditions (salt and pH), as it is typically observed for a range of electrostatically coupled films [248]. The results of experiments indicated that as the number of PEG bilayers increased, the hydrophobicity of the membrane surface decreased and the resistance to fouling increased. Also, coated brackish water membranes showed no reduction in the water flux and slight increase in salt rejection [245]. Sarkar et al. [249] used dendrimer-based coatings for modification of RO membranes (Fig. 13). Dendrimers are highly branched, globular, and nanoscopic macromolecules composed of two or more tree-like

7.3.2.1. Chemical coupling. The surface of polyamide TFC RO membranes contained carboxylic acid groups. It had been confirmed that polymerization reaction occurred in organic phase during interfacial polymerization process. In other words, amine should continually cross the water-organic interface, diffuse through the polyamide layer already formed, and then contact with acyl halide in the organic solvent side of the polyamide layer. With the increasing thickness of polyamide layer, there would be fewer and fewer amine groups on the organic phase side. The acyl chloride groups (eCOCl) without reacting with amine to form amide bonds would be eventually hydrolyzed to carboxyl acid groups (eCOOH). Meanwhile, since the hydrolysis reaction of acyl chloride was a relatively slow process, the

Fig. 13. Schematic of antifouling dendrimer-based coating on the polyamide RO membrane [249].

26

27

Reaction with polyamide

MA

EDC·HCl, NHS

RE4021TL

EDC·HCl: activating

−

Redox initiator

K2S2O8, K2S2O5

Commercial Dow FilmTech.

−

Redox initiator

K2S2O8, Na2S2O5

Hangzhou RO

IP of 2 wt % MPD and 0.1 wt % TMC TFC-HR

Type of effect of additive

Additiveb

Membranea

Chemical coupling

Grafting by iCVD

−

EDC·HCl: excess, NHS:

Chemical coupling

PEI with different

PDMMSA

MPEGNH2

AA, MAA, PEGMA, SPM

Grafting of NIPAm followed by AA

FRGP

FRGP

Grafting agentd

Method of graftingc

−

K2S2O8: 0.025 M, K2S2O5: 0.025 M

K2S2O8: 0.5 mol% of monomer, Na2S2O5: 0.5 mol% of monomer

Concentration of additive

PEI: 5 wt %

DMAEMA & EGDMA with DMAEMA content in film 0, 15, 35, 70, 100% to yield PDE followed by reaction with PS to obtain PDMMSA

MPEGNH2: 5.0% w/w

Room temp.

Filament temp.: 250, stage temp.: 20

EDC·HCl: 10 min,

MA: 20 min, reaction: 6h

5 min

−

−

Room temp.

NIPAm: 30, 60, 90,120 min, AA: 30, 60, 90, 120 min

NIPAm: 30, AA: 30

NIPAm: 2.0 wt%, AA: 0.5 wt%

Monomer: 0.1–1 M, EGDMA: 0.01 M

Time of grafting

Temp. (°C) of grafting

Concentration of grafting agente

−

2000 ppm

2000 ppm

−

−

−

−

500 mg/L

−

−

NaCl aqueous solution test

Heat curing

Table 5 Chemical surface modification of polyamide TFC RO membranes: materials, operating conditions, and performance of membranesg.

Lysozyme:

Bacterial adhesion test by E. coli

Tannic acid: 100 ppm, DTAB: 100 ppm

−

BSA: 200 mg/ L & 500 ppm NaCl

Other foulants testsf

1.55 MPa

80, 300 psi

1.6 MPa

−

0.5 MPa

Pressure

Cross flow

Cross flow

−

25

Cross flow

−

Improved both of the water [190] flux and salt rejection of the RO membrane by graft polymerization of NIPAm followed by AA (flux: 27–32.8 L/m2 h, rejection: 96.8–98.2%), improved the fouling resistance to BSA as the results of the enhanced membrane surface hydrophilicity and the increased membrane surface negative charge at neutral pH, increased chlorine resistance of membranes. [191] Reduction of roughness obtained with modified RO membranes, TEM microscopy of modified membranes revealed a reasonable correlation with the extent of grafting as determined by ATR–FT-IR. Modified membrane showed [200] more irregularities and unevenly distributed features arose, PEG-modified membrane had antifouling property and lower relative flux decline after fouling. [231] The iCVD coating on membrane was highly smooth, increasing the thickness of the coatings increased the surface roughness and decreased the permeation rates, for a 30 nm coating with the composition of 35% sulfobetaine, the permeation flux was reduced by ~ 15% compared to bare RO without changing the salt rejection, coating prevented the attachment of bacteria and significant antifouling performance and possibility prolonged usage of RO membranes. By grafting PEI, the surface [256] charge of membrane (continued on next page)

Cross flow

Ref.

Performance

Permeation test cell

25

−

25

Temp. (°C)

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

28

EDC·HCl, NHS

ACA, EDC, NHS

IP of 2 wt % MPD, 0.15 wt% SDS and

EDC

Beidouxing RO

RE4021TE, BW30FR, LFC3

Additiveb

Membranea

Table 5 (continued)

IP method for ACA and TMC, amine coupling reaction of EDC and NHS

EDC·HCl: activating agent, NHS: stabilizer

ACA: 0.1–10 wt %, EDC: 0.01–0.15 mol/ L, NHS: 0.01–0.15 mol/ L

EDC·HCl: excess, NHS: certain amount

EDC: 0.1 wt%

certain amount

agent, NHS: stabilizer

Coupling reagent

Concentration of additive

Type of effect of additive

lysozyme

PVAm (Grafting), PVA (Dip coating)

Chemical coupling

Chemical coupling

PEG derivate with end amine groups (Jeffamine ED600, ED2001)

MW of 600, 1800, 10,000, 70,000

Grafting agentd

Chemical coupling

Method of graftingc

Lysozyme: 1 mg/ mL

PVAm; 0.05, 0.10, 0.20, 0.25 w/v %, PVA: 1.0 w/v%

Excessive amount of ED600 or ED2001

Concentration of grafting agente

4

NHS: room temp.

4

Temp. (°C) of grafting

24 h

EDC·HCl: 10 min, NHS: 16 h, PVAm: 4h

24 h

NHS: 15 min, PEI: 12 h

Time of grafting

2000 ppm

0.05 wt%

−

1500 ppm

NaCl aqueous solution test

PVA: heat treatment at 90 °C for 10 min

−

Heat curing

Bacteria test by B. subtilis & 0.05 wt% NaCl

BSA, lysozyme, SA, DTAB, Fe (OH)3: 50 mL, (each of them contains 2000 mg/ L NaCl)

Milk solution: 100 ppm, DTAB: 100 ppm

1000 mg/ L, DTAC: 60 mg/L, CTAC: 60 mg/L, (each of them contains 2000 mg/ L NaCl)

Other foulants testsf

0.75 MPa

1.55 MPa

1.05 MPa

Pressure

Cross flow

Cross flow

−

Cross flow

Permeation test cell

25

25

Temp. (°C)

Ref.

changed from negative to positive, with increasing of the Mw of the PEI, the CA of the membrane decreased about 15° without no obvious change in the surface morphology, water flux and salt rejection increased (but the flux was lower than virgin membrane 41.7 L/m2 h), modification resulted in high antifouling property to positively charged pollutants. Modified membranes were [257] more hydrophilic and more resistant to fouling in protein and cationic surfactant, acceptable decrease in pure water flux (from 50 L/m2 h to 32–36 L/ m2 h) whiteout significant change in NaCl rejection (about 96%). [258] Grafted membrane surfaces became smoother by increasing PVAm concentration, modification decreased CA, reduced negative charge density (for 0.20 w/v% PVAm, CA of 37.1°), reduced water flux and increased salt rejection as PVAm concentration increase, higher flux recovery of the PVAmgrafted membrane relative the two commercial membranes for all foulants, neutral surface charge and steric repulsion of the PVAm-grafting caused better antifouling property for membrane than the PVAcoating. Lysozyme increased [259] membranes hydrophilicity, water flux decreased by about half of the unmodified membranes (0.27 m3/ m2 day for modified membrane) of 1 wt% ACA, (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

29

GA

AGE, K2S2O8

SW30XLE

AGE: activating agent, K2S2O8: initiator

Linkage agent

−

−

LE, XLE

Tianchuang RO

EDC·HCl & NHS activator agents, EDA: bonding agent

Type of effect of additive

EDC·HCl, NHS, EDA

Additiveb

RE4021TL

1 wt% TMC

Membranea

Table 5 (continued)

AGE: required amounts, K2S2O8: 60 mg in 150 mL

GA: 0.04 wt%

−

EDC·HCl: excess, NHS: certain amount, EDA: 5 wt%

Concentration of additive

Chemical coupling

Chemical coupling

Chemical coupling

Chemical coupling

Method of graftingc

Amino acid Lcysteine

PVA bonded to GA

PEGDE

IU

Grafting agentd

LCysteine: 733 in 150 mL solution.

PVA: 50, 100, 150, 200 mg/L

PEGDE: 1, 15% (w/ w)

IU: 5 wt%

Concentration of grafting agente

AGE: ambient temp., Lcysteine: 60

25

40

NHS: room temp.

Temp. (°C) of grafting

AGE: 15 min, Lcysteine: 40 min

GA: 5 min, PVA: 2 min

10 min

EDC·HCl: 10 min, NHS: 16 h, IU: 12 h

Time of grafting

BSA: 100 mg/ L, SDS: 200 mg/ L, DTAB: 10 mg/L

2000 ppm

500 mg/L

2.5 g/ 200 mL

−

At 50 °C for 5 min

−

BSA: 100 mg/ L, DTAB: 20 mg/L, (each of

NaClO: 1000 ppm free chlorine for 1 h exposure time, sterilization and the antibiofouling test by E. coli Oil-inwater emulsion (9:1 ndecane and surfactant): 150 mg/ L, DTAB: 150 mg/ L, SDS: 150 mg/L

Other foulants testsf

2000 ppm

NaCl aqueous solution test

−

Heat curing

50 bar

5 bar

10.3 bar

1.55 MPa

Pressure

22

25

25

25

Temp. (°C)

Stirred dead end, Cross flow

Cross flow

Cross flow

Cross flow

Permeation test cell

Ref.

the salt rejection was maintained the same (90%), improved antibacterial activity. [260] CA of IU-modified membranes (23°) were half of the CA of the virgin membranes, no obvious difference between the surface morphology after modification, grafting showed water flux reduction by 24.9% but no change in salt rejection, high antibiofouling and chlorine resistance properties, enhanced life time of membranes. [261] Modification decreased water flux and increased NaCl rejection, PEGDE grafting exhibited improved fouling resistance to charged surfactant and emulsion, improved ability to be cleaned after fouling compared to raw membranes, PEGDE molecular weight had a stronger effect on fouling resistance than PEGDE concentration. By increasing content of [262] PVA, surface roughness increased, CA decreased (from 60.5° (raw) to 40°), surface negative charge decreased, pure water flux increased when PVA content is < 150 mg/L, salt rejection increased from 98.05% to 98.42%, decreased water flux slightly, good durability of modified membranes, improved antifouling property and chlorine stability. [263] L-Cysteine grafting made the membrane surface smoother, CA dropped (to 34°), increased the salt rejection and declined the (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

30

Esterification of polyamide

−

−

IP of 20 g/ L MPD and 1 g/L TMC

DMAE

−

−

IP of 2.0 wt % MPD and 0.5 wt % TMC

IP of 2 wt % MPD and 0.1 wt % TMC

Type of effect of additive

Additiveb

Membranea

Table 5 (continued)

DMAE: 0.1–10 wt%

−

−

Concentration of additive

Chemical coupling

Chemical coupling

Chemical coupling

Method of graftingc

Coupling followed by reaction with CSA to obtain QAC and SA

PAMAM

MDMH

Grafting agentd

CSA: –

PAMAM: 10% w/v in methanol (PAMAM1) or water (PAMAM2) as solvent

MDMH: 2.0, 5.0, 10.0, 20.0 wt%

Concentration of grafting agente

DMAE: 4–20 h, reaction: 2h

3 min

−

DMAE: 25, reaction: room temp.

5 min,

Time of grafting

40

Temp. (°C) of grafting

−

At 80 °C for 2 min

At 103 °C for 10 min

Heat curing

2000 ppm

3.5 g/L

2000 ppm

NaCl aqueous solution test

NaClO: 1000–7000 ppm for 4 h exposure time, bacteria adhesion

BSA: 1 g/ L

NaClO: 100–2000 ppm free chlorine for 1 h exposure time, sterilization test by E. coli

them contains10 mM NaCl)

Other foulants testsf

1.6 MPa

50 bar

1.5 MPa

Pressure

−

Cross flow

−

25

−

Permeation test cell

25

Temp. (°C)

Ref.

permeability (from 95.3% and 0.85 L/m2 h bar for virgin membrane to 98.4% and 0.79 L/m2 h bar for grafted membrane), lower fouling propensity. After modification, the CA [264] decreased from 57.7° to 31.5–50.4°, increased water flux after chlorination whiteout change in salt rejection relative to unmodified membranes (flux: 95–125 L/m2 h relative to 82–112.5 L/m2 h with salt rejection 89–95%), higher chlorine resistance and antibiofouling properties. [265] Methanol led to the deposition of PAMAM clusters and water resulted in the formation of high coverage PAMAM film, PAMAM slightly smoother surface (from 49.25 to about 45) and positively charged over a wide range of pH, the significant increase in the water flux (24.8% TFC PAMAM1, 19.6% TFC PAMAM2), the salt rejection of the PAMAM2 membrane remained on a high level (98.0%) and comparable to the raw membrane (98.2%), the PAMAM1 membrane exhibited a decrease in salt rejection to 95.4%, the static protein adsorption on PAMAM2 membrane was much lower than for PAMAM1 and the raw membrane. [266] The modified membrane showed decrease in CA in comparison with the nascent membrane and the tertiaryamine membranes (from 60° to 28°), stronger chlorine resistance and the more powerful anti-biofouling (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

31

Redox initiator

Redox initiator

K2S2O8, K2S2O5

K2S2O8, Na2S2O5

K2S2O8, K2S2O5

K2S2O8, K2S2O3

SW30, BW30, SWC1, SWC2, CPA2, ESPA, TFCULP

SW-30, BW30, CPA-2

SWC2

Commercial

K2S2O8: 0.01 g, K2S2O3: 0.01 g in 100 mL

Redox initiator

Redox initiator

−

−

IP of 2 wt % MPD and 0.1 wt % TMC

K2S2O8, Na2S2O5

−

−

SWC4 +

BW-30, SW30, CPA-2

Type of effect of additive

Additiveb

Membranea

Table 5 (continued)

Redox initiator

Initiator: 1–2% of monomer concentration

FRGP

FRGP

FRGP

FRGP

−

K2S2O8: 0.01 M, Na2S2O5: 0.01 M

FRGP

Chemical coupling

Chemical coupling

Method of graftingc

K2S2O8: 2% of monomers, Na2S2O5: 1/3 of K2S2O8

−

−

Concentration of additive

SPM

MAA, Na salt VSA, K salt SPM, AMPS SPM, PEGMA

PEGMA, SPM, AMPS

MAA, PEGMA

Aldehydes

PAAsilane

Grafting agentd

30 min

−

−

Monomer: 0.2 M

−

−

−

Monomer: 1 M of

−

10–120 min

Aldehyde: 1–6 h, GA: 1–4 h

0.5 h

Time of grafting

20 min

Room temp.

Aldehyde: 20, 40, 60 80, GA: 25

Room temp.

Temp. (°C) of grafting

Room temp.

Monomer: 1M

Monomers: 10–20%

PAA: 0.2, 0.5, 1, 2.5, 10 wt % in methanol Formaldehyde: 37–40 wt % & phosphoric acid: 85 wt% (volume ratio of 1:50), followed by GA crosslinking

Concentration of grafting agente

−

−

−

Seawater of Gulf of Aqaba

−

32,000 ppm

−

−

1500 ppm

2000 ppm

−

−

3000 ppm

NaCl aqueous solution test

Drying at 60 °C

Heat curing

−

− Feed water of Barmer

−

−

−

−

21

−

25

−

Temp. (°C)

−

55 bar

225 psi

1.6 MPa

500 psi

Pressure

−

Fouling test by Nahal Taninim stream

NaClO; 500 ppm active chlorine for 1–20 h exposure time, BSA: 60 mg/ L & 2000 ppm NaCl, HCl: 1 M (pH = 0.0) for 1 h −

test by E. coli −

Other foulants testsf

−

−

−

−

[271]

[270]

[269]

[268]

[267]

Ref.

[272] Grafting improved the fouling resistance of membranes, PEGMA had stronger antifouling effect than SPM. [273] Surface modified membranes had better fouling resistance. (continued on next page)

Optimum modification exhibited a water flux of 37.5 L/m2 h and a salt rejection of 98.6%, (close to the raw one: 41.6 L/m2 h, 98.7%) with 2 h and 60 °C for formaldehyde reducing step and 3 h for GA time, no effect of modification on the surface roughness, reduced CA from (from 62.2° to 42.8°), stable acidresistance, better antifouling property to BSA, improved chlorine resistance both in the acidic and alkaline conditions. Modified membranes had higher negative zeta potential, for short time grafting the flux slightly reduced and rejection unchanged or improved. The receding CA was reduced in most membranes, after modification the permeability stayed the same or dropped by no > 0–25%, and salt rejection increased by an average 1% and as much as almost 3%. The grafting of monomers to surface of membranes elucidated by ATR-FTIR.

Cross flow

Stirred batch

Reduction in CA after grafting from 58° to 25°, no reduction in membrane performance.

properties.

Performance

Stirred flow

Permeation test cell

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

Thermal dissociation initiator

Redox initiator

Redox initiator

Redox initiator

K2S2O8

Na2S2O8, K2S2O5

K2S2O8, NaHSO3

K2S2O8, Na2S2O5

XLE-2540, BW302540

XLE

RE4021TE

water

Tianchuang RO

Type of effect of additive

Additiveb

Membranea

Table 5 (continued)

32

K2S2O8: 0.5 mol% of DMAEMA, Na2S2O5: 0.5 mol% of DMAEMA

K2S2O8: 1 mmol/L, NaHSO3:1 mmol/L

Na2S2O8: 2 mM, K2S2O5: 2 mM

K2S2O8: 1.0 wt %

Concentration of additive

FRGP

FRGP

FRGP

FRGP

Method of graftingc

Grafting of DMAEMA followed by reacted with 3BPA to

PSVBP

GMA

PVA

Grafting agentd

DMAEMA: 0.05, 0.1, 0.2, 0.3 mol/ L, 3-BPA: excess

SVBP: 2 mmol/L

GMA: 2.3, 4 mM

PVA: 100, 300, 500, 1000 mg/ L

Concentration of grafting agente

2 g/L

2000 ppm

− DMAEMA: 60 min, reaction: 48 h DMAEMA: 30, reaction:30

2000 ppm

−

−

500 mg/L

NaCl aqueous solution test

At 60 °C for 120 s

Heat curing

2–18 h

35 min

2 min

Time of grafting

Room temp., 60

20–30

25

Temp. (°C) of grafting

Lysozyme/BSA: 1000 mg/ L, antimicrobial test by E. coli

desalination plant BSA: 100 mg/ L, SDS: 200 mg/ L, DTAB: 10 mg/L, (each of them contains 500 mg/L NaCl), NaClO: 6250 ppm for 1–5 h exposure time, NaOH: at pH = 11 for 300 h Natural brackish water with 6000 ppm conductivity, Bisphenol-A, 15 ppm BSA: 100 mg/ L & 2000 ppm NaCl

Other foulants testsf

1.5 MPa

1.5 MPa

16 bar

5 bar

Pressure

25

25

22–25

25

Temp. (°C)

Cross flow

Cross flow

The modified membranes [275] showed lower flux and higher salt rejection (for 2.3 GMA: flux: 3.7 L/m2 bar h and rejection 99.5% and for 4 GMA: flux: 1.9 L/m2 bar h and rejection 99.8%) in comparison with unmodified membranes (flux: 4.9 L/ m2 bar h and rejection 98%) PSVBP grafting increased the [276] rejection from 98.0% to 99.7% with the trade-off 20% of the flux, higher polymerization temperature and longer time show more prominent effects, more negative charge onto the surface, decrease in CA, improved antifouling property in the short term and enhanced cleaning efficiency. The CBMA grafted [277] membrane surface was near neutrality, increase 22.55% in flux without change in salt rejection (unmodified membrane flux: 70.59 L/ m2 h and salt rejection 97.89%), better resistance to (continued on next page)

−

[274]

Ref.

By increasing PVA concentration, CA and surface roughness of grafted membranes decreased, grafted membranes surface charge became less, flux decreased slightly and salt rejection increased, chlorine stability and alkaline resistance improved, enhanced fouling resistance to BSA, SDS and DTAB.

Performance

Cross flow

Permeation test cell

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

33

Redox initiator

Cerium(IV)/ PVA

IP of 2% MPD and 0.1% w/v TMC

Initiator

Initiator

AIBA

RE2521TL

Redox initiator

AIBN

K2S2O5, K2S2O8

ESPA-1

Type of effect of additive

RE2521TL

Additiveb

Membranea

Table 5 (continued)

Ce (IV): 1.5 × 10− 3 mol/L, PVA: 1, 2 × 10− 2 mol/ L

AIBN: 0.4 wt% in ethyl alcohol

AIBA: 0.02 wt %

K2S2O5: 2 mM, K2S2O8: 2 mM

Concentration of additive

FRGP

FRGP

FRGP

FRGP

Method of graftingc

SPM, MBA

ADMH, MBA

ADMH

SPE, METMAC, sulfonated GMS

convert to CBMA polymer

Grafting agentd

0− 3

mol/

4 & 8 × 1-

2,

SPM: 2 & 2.8 × 10− -

ADMH: 0, 2 wt % & MBA: 0, 2 wt% in ethyl alcohol solution

ADMH: 5.0 wt%

SPE: 65 mM, METMAC: 30 mM, GMA: 2.5 mM

Concentration of grafting agente

20 min

15 min

−

20, 40, 60, 100 min

SPE: 60 min, METMAC: 35 min, GMA: 30 min

Time of grafting

−

70

−

Temp. (°C) of grafting

At 50 °C for 10 min

2000 ppm

2000 ppm

2000 ppm

−

At 60 °C for 20

–

NaCl aqueous solution test

−

Heat curing

NaClO: 1000 ppm free chlorine for 1 h exposure time, sterilization and the antibiofouling test by E. coli NaClO: 1000 ppm for 1–6 h exposure time, FeSO4:

NaClO: 100 ppm free chlorine for 1–25 h exposure time, sterilization test by E. coli

Salt solution: 1.5 g/L, bacteria deposition test by P. fluorescens

Other foulants testsf

1.72 MPa

1.5 MPa

1.5 MPa

−

Pressure

24

25

25

22–25

Temp. (°C)

Cross flow

Cross flow

−

Parallel plate flow, cross-flow

Permeation test cell

[281]

[280]

[278]

Ref.

[282] Modified membranes had smoother surface, reduced roughness, smaller CA (from 66° to 45°), variation in water flux (22–63 L/m− 2 h) and salt rejection (0–3%) for (continued on next page)

protein adsorption (antiadhesive), improved antimicrobial and easycleaning properties. modified membranes were more hydrophilic (the CA of raw membrane 45° reduce to 10°), surface roughness did not change considerably, reduced flux (by 20–40%) whiteout change salt rejection, bacterial deposition was faster for the METMAC and slower for modified membrane with SPE and still lower for the GMS. Grafting of ADMH increased hydrophilicity whiteout changing the surface charges, increased water fluxes and decreased salt rejections after modification (flux: 167–184.5 L/m2 h and rejection: 91.8–95.8% for modified and flux: 151.5 L/ m2 h and rejection: 96.6% for raw membranes), modification improved chlorine resistance and antimicrobial efficiency of membranes. ADMH and MBA grafted membranes had higher hydrophilicity, lower roughness, lower flux and slightly higher salt rejection (86.3 L/m2 h and 96.9% in comparison with 104.8 L/ m2 h and 96.2% for raw membrane), enhanced chlorine resistance and better antimicrobial efficiencies.

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

34

BIBB

IP of 2 wt % MPD, 0.15 wt% TMC and

K2S2O8, K2S2O5

SWC1

K2S2O8, K2S2O5

K2S2O8, K2S2O5

SWC1

SUL-H

Additiveb

Membranea

Table 5 (continued)

ATRP initiator

Redox initiator

Redox initiator

Redox initiator

Type of effect of additive

BIBB: 4.2 mmol in 20 mL of hexane

K2S2O8: 0.01 M, K2S2O5: 0.01 M

K2S2O8: 0.01 M, K2S2O5: 0.01 M

K2S2O8: 0.01 M, K2S2O5: 0.01 M

Concentration of additive

SI-ATRP

Sol-gel coating method with silane coupling agent

Sol-gel coating method with silane coupling agent

Sol-gel coating method with silane coupling agent

Method of graftingc

MPC

GPPTMS

MeTES, OcTES, OdTMS, PhTES, VTES

MeTES, OcTES, OdTMS, PhTES

Grafting agentd

Additive: 30 min, silane: –

Additive: 30 min, silane: –

BIBB: 1 h, MPC: 30, 60, 90, 120 min

−

−

−

Silane: 1.5 wt%

MPC: 10 mmol in 18 mL water/ methanol (2/8, v/v) solution

GPPTMS: 0.5, 1, 1.5, 2 wt % in ethanol

10 min

Time of grafting

70

Temp. (°C) of grafting

Silane: 1, 1.5, 2 w/v %

2 mol/L

1.3 × 10− -

3,

L, MBA: 1.9 & 9.5 × 10− -

Concentration of grafting agente

−

At 70 °C for 10 min

At 70 °C for 10 min

−

Heat curing

0.05 wt%

2 g/L

35 g/L

35 g/L

NaCl aqueous solution test

Bacteria adhesion test by S. paucimobilis

Casein solution: 100 ppm

NaClO: 2 g/L for 1–12 h exposure time

200 ppm& CaSO4: 70 ppm, BSA: 200 ppm, (each of them contains 2000 mg/ L NaCl) −

Other foulants testsf

0.75 MPa

15.5 kgf/ cm2

55 kgf/cm2

55 kgf/cm2

Pressure

−

25

25

25

Temp. (°C)

Cross flow

−

Flat-type

−

Permeation test cell

Ref.

[283] As the silane concentration increased, the water flux decreased, salt rejection increased (from 99.2 of virgin membrane to 99.6), CA increased, the surface roughness of MeTES and OdTMS membranes decreased and OcTES and PhTES increased. The silane-coated membrane [284] maintained a salt rejection of above 99.0% after an exposure of 25,000 ppm h (versus 15,000 ppm h for uncoated membrane) modification resulted reduce in water flux and increase in chlorine resistance. In modified membranes, the [285] fouling resistance improved and the flux decline index (m) and the modified fouling index (MFI) decreased for GPPTMS concentrations < 1.0 wt%, GPPTMS-modified membrane fouled less when the concentration of GPPTMS was over 1.5 wt%, as the concentration of the GPPTMS increased, the CA decreased (from 25° to 13°) and the permeate flux decreases significantly. With increasing [289] polymerization time, the grafted amount increased and the surface structure became smoother, the MPC grafted membranes had lower CA, flux and salt (continued on next page)

NaCl solution, improved antifouling properties and chlorine stability compared to virgin membranes.

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

IP of MPD and TMC

SW30HR, Toray RO

2 wt% DEA SW30HR, AD, NanoH2O

Membranea

ATRP initiator

35

EDA–BIBB: 3, 5, 10 mg/mL

BiBBr-initiatordopamine: 450 mg, 4.0 × 10− 3 mol in 100 mL water

Coating on membrane

BiBBrinitiatorPDA

EDA–BIBB

BiBBr: 9.8 × 10− 3 mol, TEA: 2 × 10− 2 mol,

Concentration of additive

ATRP initiator

Type of effect of additive

BIBB, TEA

Additiveb

Table 5 (continued)

SI-ATRP

ARGETATRP

SI-ATRP

Method of graftingc

PSBMA

MTAC

Polysulfobetaine

Grafting agentd

SBMA: 1.000 g in 10 mL methanol/ water solution

MTAC: 8.9 g in a 1:1 isopropanol: water mixture (150 mL, v/v)

3-SBMA: 10.0 g & TPMA: 0.02 g in methanol/ water (1:1 v/v; 155 mL)

Concentration of grafting agente

24 h

BiBBrinitiatorPDA: 10 min, MTAC: 1, 3, 6, 24 h

EDA–BIBB: 1 min, SBMA: 20, 60, 90 min

−

−

Time of grafting

Room temp.

Temp. (°C) of grafting

0.22 wt%

2000 ppm

−

EDA–BIBB: at 80 °C for 15 min,

2000 ppm

NaCl aqueous solution test

−

Heat curing

Seawater: 36,000 ppm, nutrient solution with a C:N:P ratio of 100:20:10, SA: 60 ppm & CaCl2: 10 mM & NaCl: 36,000 ppm NaClO: 10–1000 ppm for 24 h exposure time, nutrient solution: NaCl: 1.0 g, sodium acetate: 100.0 mg, MSP: 10.0 mg, NH4Cl: 20.0 mg, MgSO4: 20.0 mg dissolved in 500 mL water MgSO4: 0.22 wt%, OPD: 100 mg/ L, HA: 30 mg/L, BSA: 0.1 wt%, protein adsorption test by HRP-

Other foulants testsf

1.6–4.6 MPa

2400 kPa

2760, 4140, 5520 kPa

Pressure

−

−

−

Temp. (°C)

−

Stirred cell

Cross flow

Permeation test cell

[291]

[290]

Ref.

[292] PSBMA-grafted membrane had smoother surface and improved the membrane resistance to nonspecific protein adsorption, water flux increased by 65% and salt rejection decreased by 4–10% depending on loading pressures, the salt rejection increased as molecular size of foulants increased. (continued on next page)

By increasing time of grafting, CA increased slightly and pure water flux improved, for all PDA-gMTAC membranes water flux was increased up to 6 h followed by a decrease after deposition for 24 h and salt rejection were higher than the unmodified membranes (> 88%), grafted membranes showed antibiofouling capability and more resistance to chlorine attack.

rejection and exhibited high antibiofouling activity. Sulfobetaine modified membranes exhibited higher or equivalent fluxes and the same or small decrease in salt rejection compared to the unmodified membranes, increased hydrophilicity and smoothness of the membrane, 80% reduction in microbial abundance at the surface.

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

36

–

–

−

IP of 2.5% w/w MPD and 0.13% w/w TMC

−

–

−

IP of 2.5% w/w MPD and 0.13% w/w TMC

IP of 2.5% w/w MPD and 0.13% w/w TMC

Type of effect of additive

Additiveb

Membranea

Table 5 (continued)

−

−

−

Concentration of additive

APPIFRGP

APPIFRGP

APPIFRGP

Method of graftingc

PMAA, PAAm

PMAA

PMAA, PAAm

Grafting agentd

MAA: 0.57–2.35 M, acrylamide: 0.1–0.3 M

MAA: 5–20 vol %

MAA: 1.17 M, acrylamide: 3 M

Concentration of grafting agente

PMAA: 60, PAAm: 70

60–70

PMAA: 60, PAAm: 70

Temp. (°C) of grafting

−

0.5 h

30 min

Time of grafting

1000 ppm

−

1000 ppm

1000 ppm

−

−

NaCl aqueous solution test

Heat curing

CaCl2: 1000 ppm, supersaturated solution of Na2SO4, BSA: 50 ppm containing NaCl (0.409 g/ L) and CaCl2 (0.11 g/

CaCl2: 1000 ppm, supersaturated solution of Na2SO4

conjugated goat antihuman lgG Biofilm growth by secondary wastewater effluent

Other foulants testsf

345–2070 kPa

345–2070 kPa

138–1380 kPa

Pressure

25

25

−

Temp. (°C)

PFRO

PFRO

PFROg

Permeation test cell

Ref.

[293] In comparison with EPSA2 RO (commercial membrane) salt rejection for the PMAA and PAAm grafted membranes were greater (by about 1.2–1.8%), the permeability of the membranes were higher (21–58%), lower CA and surface roughness, enhanced biofouling resistance and cleaning effectiveness properties. [294] The PMAA membranes had higher permeability (by up to a factor of ∼ 2) relative to the low fouling commercial membrane and about the same salt rejection (95%), reduction in membrane mineral scaling propensity while retaining or even improving water permeability relative to RO membrane performance at the same salt rejection level, by increasing monomer concentration, the membrane roughness increased. [295] The PMAA and PAAm grafted membranes had a negatively charged and near neutral surfaces and lower mineral scaling, CA was lower for the PMAA membranes and higher for the PAAm membranes relative to the LFC1 membrane, grafted membranes had lower surface roughness, higher permeability (2–3.4 × 10− 10 m/s Pa for PMAA and 1.8–2.8 × 10− 10 m/s Pa for PAAm with similar NaCl (continued on next page)

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

−

−

HR95PP, HR98PP

37

Photoinitiator

−

−

BPh

−

SW30HR

ESPA-1

XLE

−

Type of effect of additive

Additiveb

Membranea

Table 5 (continued)

−

BPh: 10 mM in ethanol

−

−

Concentration of additive

UV irradiation

UV irradiation

Hydrophilization

Hydrophilization

Method of graftingc

PFPAPEG

Poly SPE

HF, FSA

Binary solutions of water & HF, HCl, H3PO4, HNO3, H2SO4, ethyl alcohol or IPA, ternary solutions of acids, alcohol & water

Grafting agentd

3 M, (n: MW

PFPAPEGn: 2 × 10− -

HCl: 18, 36%, H2SO4: 20, 24%, HF: 1, 15, 49%, ethanol: 50, 100%, HNO3: 70%, IPA: 100%, ternary solution: 5% HCl, HNO3, H2SO4, H3PO4 & 50% ethanol & 45% water HF: 5, 15 wt%, FSA: 1 wt %, FSA: 0.05 wt % & HF: 5, 15 wt% SPE: 0.3 M

Concentration of grafting agente

UV irradiation: 3 min

BPh: UVirradiated for 5 min, SPE: UV irradiation for 15 min

−

Ambient conditions

2–30 days

0–14 days

Time of grafting

−

−

Temp. (°C) of grafting

2 g/L

−

−

−

0.5 wt%

0.5 wt%

NaCl aqueous solution test

−

−

Heat curing

Bacteria deposition tests by P. fluorescens, tertiary effluent from a membrane bioreactor (MBR) Bacteria adhesion test by E. coli

−

−

L), alginic acid: 100 ppm

Other foulants testsf

−

−

−

225, 300, 400 psi

24

24

Temp. (°C)

200–400 psi

250, 350 psi

Pressure

Stirred cell

Cross flow

−

Single flat sheet cell

Permeation test cell

[300] PEPA-PEG reduced the CA (from 63° to 35° for PEG 5000 MW), declined pure water permeability and (continued on next page)

[299]

[298]

[297]

rejection of 94.1–95%) (LFC1: 1.5 × 10− 10 m/ s Pa), PAAm grafting imparted greater alginic acid fouling and similar resistance to BSA fouling relative to LFC1 membrane. Exposure the modified membranes to protic acid mild solvent caused increase in flux up to an order of magnitude without any loss of salt rejection, increase in hydrophilicity of the membrane surface.

Flux enhancements up to an order of magnitude without any reduction in the salt rejection (about 97%) and not to affect the life of the membrane in chemically treated membranes. SPE modified membrane became more hydrophilic (CA: 16° vs. 36° of pristine membrane), slightly less negative surface charge, enhanced resistance to initial bacteria adhesion, improvement in short-term resistance to organic fouling and biofouling.

Ref.

Performance

M. Asadollahi et al.

Desalination xxx (xxxx) xxx–xxx

−

Concentration of additive

UV irradiation

Method of graftingc

PAA

Grafting agentd

AA: 3–50 g/L

1000, 5000)

of PEG: 550,

Concentration of grafting agente

−

Temp. (°C) of grafting

UV irradiation: 1–10 min

Time of grafting

−

Heat curing

−

NaCl aqueous solution test

HA: 50 ppm, dye (RR261): 50 ppm, BSA: 50 ppm

Other foulants testsf

15 bar

Pressure

Room temp.

Temp. (°C)

Dead-end cell

Permeation test cell

increased NaCl rejection, increasing the MW of the polymer had a greater effect on the permeability and rejection and decreased the adhered bacteria, grafting prevented initial bacterial adhesion. Significant decrease of CA, from 51° for an unmodified membrane to 23–25° for the modified ones, the membrane surface becomes more compact and smoother (from 121 nm to 33.3 nm), improved flux (about 25–30%) and retention of HA, improved fouling resistance and lower irreversible fouling factors of the membrane after grafting of PAA.

Performance

[301]

Ref.

38

b

SDS: sodium dodecyl sulfate, DEA: diethanolamine. K2S2O8: potassium persulfate, Na2S2O5: sodium metabisulfite, K2S2O5: potassium metabisulfite, MA: maleic anhydride, EDC·HCl: N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride, NHS: N-hydroxysuccinimide, EDC: 1-ethyl-3-(3dimethyl amidopropyl) carbodiimide, ACA: 6-amino caproic acid, EDA: ethylenediamine, GA: glutaraldehyde, AGE: allyl glycidyl ether, DMAE: dimethylaminoethanol, Na2S2O8: sodium persulfate, AIBA: 2,2′-azobis(isobutyramidine) dihydrochloride, AIBN: 2,2′-azobisisobutyronitrile, PVA: polyvinyl alcohol, BIBB: 2-bromoisobutyryl bromide, TEA: triethylamine, BiBBr-initiator-PDA: 2-bromoisobutyryl bromide initiator-polydopamine, EDA–BIBB: Ethanediamine- Bromoisobutyryl bromide, BPh: Benzophenone. c FRGP: free radical graft polymerization, iCVD: initiated chemical vapor deposition, SI-ATRP: surface initiated-atom transfer radical polymerization, ARGET-ATRP: activators regeneration by electron transfer-atom transfer radical polymerization, APPI-FRGP: atmospheric pressure plasma-induced free-radical graft polymerization. d NIPAm: N-isopropylacrylamide, AA: acrylic acid, MAA: methacrylic acid, PEGMA: poly(ethylene glycol) methacrylate, SPM: sulfopropyl methacrylate, MPEG-NH2: amino poly(ethylene glycol) monomethylether, PDMMSA: poly[N,N-dimethylN-methacryloxyethyl-N-(3-sulfopropyl)], PEI: polyethyleneimine, PEG: poly(ethylene glycol), PVAm: polyvinylamine, PVA: polyvinyl alcohol, IU: imidazolidinyl urea, PEGDE: poly(ethylene glycol) diglycidyl ether, MDMH: 3-monomethylol-5,5dimethylhydantoin, PAMAM: polyamidoamine, CSA: 5-chloromethylsalicylaldehyde, QAC: quaternary ammonium cation, SA: salicylaldehyde, PAA-silane: poly(acrylic acid) chains with an end functionality of trimethoxysilane, AMPS: 2acrylamido-2-methyl propane sulfonate, VSA: vinylsulfonic acid, GMA: Glycidyl methacrylate, PSVBP: Poly (4-(2-sulfoethyl)-1-(4-vinylbenzyl) pyridinium betaine), DMAEMA: N,N′-dimethylaminoethyl methacrylater, 3- BPA: 3-bromopropionic acid, CBMA: Carboxybetaine methacrylate, SPE: 2-[(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide, METMAC: [2(methacryloylxy)ethyl]trimethylammonium chloride, GMS: glycidyl methacrylate, ADMH: 3-allyl-5,5dimethylhydantoin, MBA: Methylene-bis-acrylamide (or N,N′-Methylenebis (acrylamide)), MeTES: methyltriethoxysilane, OcTES: octyltriethoxysilane, OdTMS: octadecyltrimethoxysilane, PhTES: phenyltriethoxysilane, VTES: vinyltriethoxysilane, GPPTMS: 3-glycidoxypropyltrimethoxysilane, MPC: 2-methacryloyloxyethyl phosphorylcholine, MTAC: [2-(methacryloyloxy) thyl]trimethylammonium chloride, PSBMA: Poly(sulfobetaine methacrylate), PMAA: Poly(methacrylic acid), PAAm: polyacrylamide, HF: hydrofluoric acid, HCl: hydrochloric acid, H3PO4: phosphoric acid, HNO3: nitric acid, H2SO4: sulfuric acid, IPA: isopropyl alcohol, FSA: fluosilicic acid, SPE: [2(Methacryloyloxy)ethyl]dimethyl-(3 sulfopropyl)ammonium, PFPA-PEG: perfluorophenyl azide-terminated poly(ethylene glycol), PAA: poly(acrylic acid). e EGDMA: ethylene glycol dimethacrylate, PDE: poly[2-(dimethylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate], PS: 1,3-propane sultone, 3-SBMA: [3-(methacryloylamino)propyl]dimethyl(3-sulfopropyl)ammonium hydroxide, TPMA: Tris(2-pyridylmethyl)amine. f BSA: bovine serum albumin, DTAB: dodecyltrimethylammonium bromide, DTAC: dodecyl trimethyl ammonium chloride, CTAC: cetyl trimethyl ammonium chloride, NaClO: sodium hypochlorite, SDS: sodium dodecyl sulfate, SA: sodium alginate, MSP: sodium dihydrogen phosphate (monosodium phosphate), NH4Cl: ammonium chloride, MgSO4: magnesium sulfate, OPD: O-phenylene diamine, HA: humic acid. g PFRO: plate-and-frame RO. The configuration of all studied RO membranes in this table is flat sheet except [272] which is flat sheet and spiral wound and [284] which is spiral wound.

−

−

BW30

a

Type of effect of additive

Additiveb

Membranea

Table 5 (continued)

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Xu et al. [256] modified the surface of RO membranes by grafting polyethyleneimine (PEI) using carbodiimide-induced method as it is shown in Fig. 15. PEI is a cationic polyelectrolyte with branched chain, containing primary amine groups, secondary amine groups, and tertiary amine groups in the ratio of 1:2:1. As a polymer with one of the highest charge density currently available and the highly reactive groups (primary amine groups), PEI can easily react with epoxy, acid, acyl chloride, and isocyanate and has been widely used in surface modification. Therefore, PEI was grafted to the surface of RO membrane through amide bond in the presence of carbodiimide. EDC·HCl and NHS reacted with the amine group of PEI. Due to the introduction of the amine groups and the consumption of carboxyl groups on the membrane surface after the grafting of PEI, the surface charge of membrane changed from negative to positive, therefore, the grafted membranes showed high antifouling property to positively charged pollutants. Also, the hydrophilicity of the membrane was obviously improved by grafting of PEI [256]. PEG derivatives with end amine groups were also grafted on the surface of polyamide RO membrane based on the existing carboxylic acid groups on surface with the help of EDC [257]. Compared to the original membrane, the modified RO membranes were more hydrophilic and more resistant to fouling in protein and cationic surfactant feeding solutions. In addition, the modified membranes had acceptable decreases in pure water flux but no significant change in NaCl rejection [257]. Wu et al. [258] modified the surface of RO membranes with polyvinylamine (PVAm) grafting and PVA coating. PVAm is a positively charged and hydrophilic polymer and contains a large quantity of primary amine groups and amide groups. The primary amine groups of PVAm form covalent bonds with the carboxyl groups on the polyamide TFC RO membrane surface by using EDC·HCL and NHS. The results proved that the PVAm grafting membranes became more hydrophilic and smoother and the surface charge climbed from negative to positive values. The large quantity of polar amine groups in PVAm enhanced the surface hydrophilicity. The PVAm grafting had smaller influence on water flux and salt rejection than the PVA coating due to the lower hydraulic resistance of

Fig. 14. Modification of polyamide RO membranes by MPEG-NH2 grafting [200].

nascent membrane would contain numerous acyl halide groups on the surface. Therefore, polyamide TFC RO membrane surface has acyl chloride groups or free carboxylic acid groups and also primary amine groups on chain ends. These relatively active groups provide the possibility of surface modification via chemical coupling or reaction [200,215]. Kang et al. [200] used amino poly(ethylene glycol) monomethylether (MPEG-NH2) as grafting monomer by using acyl chloride groups according to Fig. 14. The fouling experiments showed that PEGmodified membranes had antifouling property due to enhanced hydrophilicity by hydrophilic PEG and less negative charge (by eliminating a portion of acyl halide by reaction with PEG). Also, the modified membranes had good steric repulsion effect of PEG. Some researchers grafted polymers and other materials to the surface of polyamide TFC RO membranes by using carbodiimide-induced method [256–260]. The carbodiimide is a coupling reagent for the activation of carboxylic acid groups, promoting the modification reaction. They used N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC·HCl) as the activating agent and N-hydroxysuccinimide (NHS) as the stabilizer. EDC·HCl firstly reacts with the carboxyl group on the polyamide TFC RO membrane surface and forms an unstable reactive o-acylisourea ester intermediate. Then NHS reacts with the O-acylisourea ester to form the semi-stable amine-reactive NHS-ester intermediate. Both of the unstable reactive o-acylisourea ester and the semi-stable amine-reactive NHS-ester intermediate could react with functional groups of polymers [256,257].

Fig. 15. Schematic of PEI grafting modification [256].

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Fig. 16. Schematic diagram of steric repulsion and hydraulic resistance for (a) the cross-linked PVA layer and (b) PVAm polymer brush layer [258].

usage of enzymes. ACA was used to prevent steric hindrance between membrane and enzymes. The immobilization of lysozyme onto the membranes resulted in a decrease in the water flux whiteout changing the salt rejection. Also, the modified membranes showed sufficient antibacterial activity and the antibacterial activity was retained for 5 months after storage at 5 °C [259]. Xu et al. modified the RO membranes with imidazolidinyl urea (IU) [260]. IU can endow the membrane with the regenerable antibiofouling and chlorine resistant properties for the total six NeH groups and two imide groups with high reaction activity with free chlorine to produce N-halamine in one molecule of IU. IU was grafted on the membrane surface by using EDC·HCL, NHS, and the ethylenediamine (EDA) acted as a bridge to bond IU to the aromatic polyamide membrane through amide bond. IU-modified membranes had significant decline in contact

the PVAm polymer brush layer than that of the cross-linked PVA coating layer (Fig. 16). The increase in salt rejection of the PVAmgrafted membrane was attributed to the electrostatic repulsion of the positively charged PVAm to the cation ions. Moreover, The PVAmgrafted membrane exhibited better antifouling property than the two commercially antifouling RO membranes with PVA coating layers [258]. Saeki et al. [259] carried out modification of polyamide RO membranes by covalently immobilized enzymes, lysozyme (Fig. 17). Firstly, the surface of polyamide layer was modified with 6-amino caproic acid (ACA) by interfacial polymerization between TMC and ACA. Next, lysozyme was immobilized onto the ACA-modified polyamide layer by an amine coupling reaction using EDC and NHS through a spacer molecule (ACA) for the long-term operation of membranes and reduction in

Fig. 17. Scheme of the preparation of the lysozyme-immobilized RO membrane [259].

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Fig. 18. Schematic diagram for the covalent attachment of PVA molecules onto the surface of polyamide membrane with GA as a linkage agent [262].

angle. Also, IU grafting caused water flux reduction and no apparent change in salt rejection. Experiments showed that the high antibiofouling and chlorine resistance properties of modified membranes were achieved [260]. In another work poly(ethylene glycol) diglycidyl ether (PEGDE) was grafted to the surface of the polyamide TFC RO membrane and the effect of PEG molecular weight and concentration on membrane performance were investigated [261]. After modification of membranes, the water flux decreased and NaCl rejection increased. Membranes modified with PEGDE generally demonstrated improved fouling resistance to charged surfactants, but experienced minimal changes in surface properties (hydrophilicity, surface charge, and roughness) and improved ability to be cleaned after fouling. Their investigations showed that PEGDE molecular weight had a stronger influence on fouling resistance than PEGDE concentration, suggesting modification with lower concentrations of higher molecular weight PEGDE for surface modification [261]. The surface of RO membrane was also modified through sequential surface treatment with GA aqueous solution followed by PVA aqueous solution to covalently attach the neutral hydrophilic PVA polymer molecules onto the surface of the polyamide TFC RO membrane [262]. As schematically depicted in Fig. 18, GA molecules were firstly bonded on the membrane surface through the reaction between the aldehyde group of GA and the amide linkages or unreacted end amino groups of polyamide backbones, then PVA macromolecules were bonded onto the membrane surface through the reaction between the unreacted aldehyde group of the bonded GA molecule and the hydroxyl groups of the PVA molecules. The surface modification caused increase in surface hydrophilicity and roughness but decrease in the surface negative charge. Modified membranes with both improved salt rejection and water flux could be obtained by controlling the content of PVA in aqueous solution. The covalent attachment of PVA mitigated membrane fouling mainly through decreasing the adsorptive interactions between foulant molecules and membrane surface. It further improved membrane chlorine stability [262].

Azari and Zou [263] grafted zwitterionic amino acid L-cysteine on the surface of polyamide TFC RO membrane. The modification was carried out through covalently bonding the thiol group of the L-cysteine to the allyl functionalized membrane by thiol-ene reaction. Thiol-ene reaction displays high rates with near quantitative, regioselective yields, and high tolerance towards various functional groups as well as water and oxygen. In addition, the formed thiol ether bonds provide high chemical stability. Allyl glycidyl ether (AGE) was applied as the activating agent on the membrane. Its epoxy end group is capable of reaction with the free carboxylic acid and primary amine groups on the polyamide membranes, while the alkene end group can react with thiol group of L-cysteine. Amine group is strongly hydrophilic therefore it can desirably react with the epoxide. Modification resulted in more hydrophilic and smoother surface. Although modification enhanced salt rejection of the membranes, a slight deterioration in the permeability of the modified membrane was observed. In addition, zwitterionic grafted membranes displayed lower fouling propensity [263]. Wei et al. [264] grafted hydantoin derivative, 3-monomethylol-5,5dimethylhydantoin (MDMH) onto the nascent aromatic polyamide membrane surfaces by the reactions with active groups in the surfaces. Hydantoin derivatives such as MDMH, 5,5-dimethylhydantoin (DMH), and 3-allyl-5,5-dimethylhydantoin (ADMH) are a group of chemicals bearing a heterocyclic ring structure, namely a hydantoin ring. These hydantoin derivatives are widely used as precursors to prepare N-halamine biocides. With two electron-donating methyls on α-carbons adjacent to the nitrogen atoms, the NeH groups in hydantoin derivatives have high reaction activity with free chlorine to produce N-halamines. The N-halamines have shown strong antimicrobial actions against a broad spectrum of microorganisms by inhibiting their growth. By sterilizing microorganisms, the N-halamines regenerate to the hydantoin derivatives as described in Fig. 19. N-halamines can also be transformed into hydantoin structures by hydrolysis [264]. MDMH molecules with active methylol groups were incorporated into the nascent membrane surfaces through the reactions with acyl chloride groups and/or carboxylic acid groups. After surface

Fig. 19. Reversible chlorination reaction [264].

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Fig. 20. Schematic of possible grafting sites in aromatic polyamide RO membrane [190].

sensitive NeH bonds of amide groups in polyamide layer into NCH2OH. Phosphoric acid was acted as a proton catalyst in the formaldehyde-reducing reaction. Also, GA, a common cross-linking agent, was introduced to crosslink the newborn-CH2OH groups via stable ether bonds. The results indicated that the performance of modified membranes was close to pristine membranes and the chlorine resistance of membranes both in the acidic and alkaline aging conditions was improved. Also, modification of membranes reduced the contact angle of membranes and resulted in better antifouling property towards protein [268].

modification with MDMH, the contact angles of membranes decreased obviously. Also, the results confirmed that the MDMH membranes had chlorine resistance because the grafted MDMH moieties with high reaction activity to free chlorine played as sacrificial pendant groups when membranes suffer from chlorine attacks. In addition, the modified membranes had antibiofouling properties [264]. It was reported that a hydrophilic hyperbranched PAMAM was bonded onto the surface of RO membrane by chemical coupling with carbonyl chloride groups of polyamide [265]. The use of the hyperbranched PAMAM is advantageous because only a low ratio of covalent bonding is needed to effectively cover the surface with PAMAM. The modifications led to a significant increase in the water flux. Concerning salt rejection and protein adsorption, the use of water as a solvent had been found to be advantageous over the use of methanol solvent. RO membrane surface was also modified with the quaternary ammonium cation (QAC) and salicylaldehyde (SA), in which QAC and SA were used as the hydrophilic material [266]. The residual acyl chloride groups on the surface of RO membrane can be converted into ester with tertiary amino alcohol. Therefore, dimethylaminoethanol (DMAE) used as the tertiary amino groups and was introduced onto the polyamide membrane surface based on the above esterification of the residual acyl chloride groups. Secondly, the tertiary amino groups on the membrane surface converted into the quaternary ammonium salt through the quaternarization with 5-chloromethylsalicylaldehyde (CSA). The results showed that QAC and SA were simultaneously linked on the surface of the membrane. The modified membrane indicated decrease in contact angle in comparison with the nascent membrane. In addition, modification caused stronger chlorine resistance and the more powerful antibiofouling properties [266]. Poly(acrylic acid) (PAA) chains with an end functionality of trimethoxysilane (PAA-silane) was grafted on the surface of RO membranes [267]. The experiments showed that modified membranes were more hydrophilic, which is expected to result in favorable fouling behavior. Also, the permselectivity of the modified RO membranes was not impacted by the addition of PAA chains. Lin et al. [268] used a two-step surface modification by formaldehydes and GA. Formaldehyde activation is kind of an easy, cheap, and relatively low nonspecific reduction reaction. Formaldehyde/ phosphoric acid reductant was employed to reduce the chlorine

7.3.2.2. Free radical graft polymerization (FRGP). Radical grafting is an effective way for polymer modification. In this process, the free radicals are produced from the initiators and transferred to the polymer to react with monomer, realizing the modification of membrane material. In general, the proposed grating site for aromatic polyamide chain is the hydrogen in amide bond. The most common redox-initiator for radical grafting of monomers onto polyamide TFC RO membranes surface is a redox system, composed of potassium persulfate and potassium metabisulfite, used to generate radicals [190,191,269–278]. They attack the polyamide backbone abstracting the hydrogen atom in amide bonding, thus initiating the graft polymerization by attachment of monomer to the membrane surface. Polymerization then occurs via propagation. In the redox system composed of potassium persulfate (K2S2O8) and potassium metabisulfite (K2S2O5), radicals such as sulfate radical (%SO4−), metabisulfite radical (%S2O5−), and hydroxyl radical (%OH) are generated [279]. The coverage of the surface for grafting increases with modification time, monomer concentration and initiator concentration [191]. The grafting sites in aromatic polyamide RO membrane are not only at the end groups of amine and carboxylic acid but also at the amide groups (schematically shown in Fig. 20). The redox initiated surface graft polymerization would reduce the chlorine-susceptible sites in aromatic polyamide chains on the membrane surface and the NeH groups in the grafted polymer chains would play as sacrificial pendant groups, as a result, the chlorine resistance of the modified membrane could be improved [190]. Cheng et al. [190] modified the surface of polyamide RO

Fig. 21. Schematic diagram of the surface modification of polyamide TFC RO membrane through graft polymerization of NIPAm followed by AA [190].

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[269]. The results showed that the membrane surface roughness reduced after modification of RO membranes irrespective of the type of monomer used, which improved the resistance to fouling [191,273]. The membranes modified with PEGMA and SPM indicated lower contact angle, implying more hydrophilic than the unmodified membranes [270]. Liu et al. [274] reported the chemical linkage of the neutral hydrophilic polymer PVA on the surface of a commercial polyamide RO membrane through a single step of thermally initiated free radical grafting and its effects on membrane properties. As schematically depicted in Fig. 22, free radicals of sulfate radical (%SO4−) generated through heating of potassium persulfate which caused abstraction of a hydrogen from the amide nitrogen and end amino group of the polyamide layer and abstraction of a hydrogen from the hydroxyl carbon of PVA. As a result, PVA chains grafted onto the polyamide backbones at the sites of amide linkages and/or end amino groups through combination of free radicals. It was found that membrane surface became smoother, more hydrophilic, and less charged after modification with PVA. Modified membranes with improved salt rejection and slightly declined water flux could be obtained through controlled surface grafting. Also, PVA grafted membranes had improved fouling resistances to protein, negatively and positively charged foulants in addition to enhanced chlorine stability and good alkaline resistance [274]. Bernstein et al. [275] applied in situ modification of RO elements using a sparingly soluble monomer glycidyl methacrylate (GMA) by

membranes through redox initiated graft polymerization of N-isopropylacrylamide (NIPAm) followed by AA (Fig. 21). The NIPAmpolymer chains covalently bonded with the membrane backbone. While for the following graft polymerization of monomer AA, the grafting sites could also be at the amide groups of the grafted NIPAm-polymer chains. Therefore, the AA-polymer chains covalently bonded with both of the membrane backbone and the grafted NIPAm-polymer chains. Both of the water flux and salt rejection of the polyamide RO membrane could be improved through controlled graft polymerization of NIPAm followed by AA. The modification enhanced the fouling resistance of the RO membrane to protein as the results of the enhanced membrane surface hydrophilicity and increased membrane surface negative charge at neutral pH. Also, the chlorine resistance of membranes was increased [190]. Various hydrophilic vinyl monomers were also grafted to the surface of RO membranes by redox-initiator such as AA, methacrylic acid (MAA), Poly(ethylene glycol) methacrylate (PEGMA), 3-sulfopropyl methacrylate (SPM), vinylsulfonic acid (VSA), and 2-acrylamido-2methylpropane-sulfonic acid (AMPS) [191,269–273]. A redox system, composed of K2S2O8 and K2S2O5, was used to generate radicals. Also, ethylene glycol dimethacrylate (EGDMA) was used as crosslinker [191]. In general, the membranes after grafting with those hydrophilic monomers showed less adsorption of foulants and were more easily cleaned than the unmodified membranes. Moreover, the MAA grafted membrane had a higher negative zeta potential over the whole pH range because of the higher degree of dissociation of carboxylic groups

Fig. 22. Schematic diagram for the graft of PVA molecule onto the surface of polyamide membrane with K2S2O8 as the thermal disassociation initiator [274].

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Fig. 23. Schematic of the salt responsive polyamide TFC RO membrane [276].

anionic carboxylate (COO−) group on its backbone. The water flux of grafted polymer CBMA membranes increased while their salt rejections were nearly the same. Modified membranes had better resistance to protein adsorption (antiadhesive), improved antimicrobial, and easy cleaning properties [277]. In other work, surface of RO membranes were modified by radical graft polymerization of zwitterionic and positively and negatively charged monomers including [2(Methacryloyloxy)ethyl]dimethyl-(3 sulfopropyl)ammonium (SPE), [2(methacryloylxy)ethyl]trimethylammonium chloride (METMAC), and sulfonated glycidyl methacrylate (GMS), respectively [278]. Modified membranes were more hydrophilic and roughness of membranes did not change. In addition, bacterial deposition was much faster for the positively charged membrane (METMAC), significantly slower for the membrane modified with the zwitterionic monomer (SPE), and still lower for the negatively charged (GMS) [278]. In other studies ADMH with a hydantoin ring in chemical structure was grafted to the surface of RO membranes by free radical graft polymerization using 2,2′-azobis(isobutyramidine) dihydrochloride (AIBA) [280] and 2,2′-azobisisobutyronitrile (AIBN) [281] as an initiator. Zhang et al. [281] also used N,N′-Methylenebis (acrylamide) (MBA) as a crosslinker, which linked two ADMH groups and reduced the self-inhibition of allyl unit on ADMH. As a result, the crosslinker MBA formed a net structure on grafted membrane surface. Moreover, the ethylene unit on MBA had higher reactivity than the allyl unit on ADMH in free radical graft polymerization. In both modification

radical graft polymerization. The modified membranes showed lower flux and higher salt rejection in comparison with unmodified membranes. Some zwitterionic materials also grafted to the surface of RO membranes by redox initiator [276–278]. A salt responsive zwitterionic polymer poly(4-(2-sulfoethyl)-1-(4-vinylbenzyl) pyridinium betaine) (PSVBP) were grafted by free radical polymerization of SVBP initiated by redox system. SVBP as a salt responsive polymer is insoluble in pure water but readily soluble in the presence of halide (Fig. 23). PSVBP itself can form a hydration layer on the membrane surface, alleviating foulants adsorption and accumulation. In addition, PSVBP chains folded, forming a collapsed hydration layer at pure water or low salinity conditions, and extended, forming a swelling hydration layer at high salinity conditions. In that case, the foulants can be released by tuning the salt concentration in the feed solution. PSVBP grafting increased the salt rejection but reduced the permeation flux. The modified membranes became more hydrophilic and more negative charge onto the surface. Furthermore, the antifouling property of membranes improved in addition to enhanced cleaning efficiency [276]. Also, the N,N-dimethylaminoethyl methacrylater (DMAEMA) covalently bonded polyamide RO membrane by redox initiated graft polymerization, and then the DMAEMA was transformed into zwitterionic polymer carboxybetaine methacrylate (CBMA) via surface quaternization reaction with 3-bromopropionic acid (3-BPA) [277]. The grafted CBMA have a cationic quaternary ammonium (N+) group and an

Fig. 24. Surface modification of polyamide TFC RO membrane by Ce(IV)-PVA redox system mediated polymerization of SPM and MBA [282].

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produces a stable chemical structure with SieOeN or SieOeC bonds, which form the amide linkages or unreacted COOH groups [284]. Potassium metabisulfite and potassium persulfate were used to etch the polyamide RO membrane surface as a redox initiator [283–285]. Four different silanes methyltriethoxysilane (MeTES), octyltriethoxysilane (OcTES), octadecyltrimethoxysilane (OdTMS), and phenyltriethoxysilane (PhTES) were used [283]. OdTMS has a longchain aliphatic functional (alkyl) group and flexible carbon chain structure. PhTES has a phenyl functional group and rigid structure. Both silanes are much more hydrophobic than MeTES or OcTES. The silanes selected in that study contain one alkyl or aryl and three alkoxy functional groups. Four different silane coupling agents contained methyl, octyl, octadecyl or phenyl functional groups, respectively. The experiments indicated that as the silane concentration increased, the water flux decreased and the salt rejection and the contact angle of membranes increased [283]. Moreover, in another study MeTES, OcTES, OdTMS, PhTES, and vinyltriethoxysilane (VTES) used as a coating reagent [284]. The results indicated that silane coated membranes had an increased resistance to chlorine because the alkyl and aryl groups protected the polyamide skeleton from chlorine attack. Silane coupling agent 3-glycidoxypropyltrimethoxysilane (GPPTMS) was also selected for modification of RO membranes [285]. The chemistry of GPPTMS has a hydrophilic epoxy group and flexible carbon chain. The fouling resistances of the GPPTMS modified RO membranes were improved by using a hydrophilic epoxy compound with hydrolyzed functional groups.

methods, the chlorine resistances of membranes were significantly improved and better antimicrobial efficiencies were achieved [280,281]. Rana et al. [282] reported the Ce(IV)/PVA redox system mediated rapid graft copolymerization of SPM, MBA, and attachment of crosslinked hydrophilic copolymer to the polyamide RO membrane surface (Fig. 24). Direct grafting of poly (SPM-MBA) chains to the polyamide layer of the membrane happened by Ce(IV), which formed an intermediate complex involving the amide groups of the active layer and thus resulted in the generation of free radicals through the abstraction of a hydrogen from the amide nitrogen of the polyamide layer. This caused the initiation of free radical polymerization of SPM monomer while MBA monomer provided cross-linking to the poly(SPM) chains. This resulted in the direct attachment of poly(SPM) as well as poly (SPM-co-MBA) to the polyamide barrier layer. Also, grafting of poly (SPM-co-MBA) chains to PVA backbone and the deposition of the grafted polymer PVA-g-poly(SPM-co-MBA) on membrane surface happened. In this process, Ce(IV) formed a complex with PVA in presence of nitric acid and generated free radicals on PVA backbone, which in turn initiated the polymerization of SPM and MBA monomers. Very soon, PVA-g-poly(SPM-co-MBA) became insoluble due to cross-linking and thus deposited on top of the active layer. The experiments indicated that modified membranes had smoother surface and higher hydrophilicity. Also, the modification improved antifouling properties and chlorine stability of membranes [282].

7.3.2.3. Sol-gel condensation of silane. Lee et al. investigated the effect of silane coupling agents on the performance of RO membranes on the basis of sol-gel coating method [283–285]. The chemical bond is formed by a two-step reaction on the active layer of the membrane surface. Fig. 25 shows the mechanism of sol-gel condensation of silane coupling agents. If electron-withdrawing groups are present on the polyamide chain, a chemical reaction would occur on the alkoxy group of silane compound by redox. Assuming that this occurs, the alkoxy group may act as a bridge to bond the silane compound to the polyamide through a chain of primary bonds that would lead to the strongest interfacial bond. This reaction has two steps: (1) the alkoxy is hydrolyzed to form a reactive silanol group and (2) a condensation reaction occurs between the silanol and the polyamide. Ultimately, it

7.3.2.4. Atom transfer radical polymerization (ATRP). Conventional radical polymerization is employed to produce many polymers, with different compositions. However, the control of structure in these polymers is very limited. Atom transfer radical polymerization (ATRP) is often used to accurately control the chain length of synthesized polymers [286–288]. In modifying the surface of polyamide TFC RO membranes by using ATRP, initiators must be immobilized on the membrane surface, but their direct immobilization on commercial RO membranes remains difficult. Some studies investigated using ATRP for surface modification of RO [289–292]. The surface of RO membrane was modified by covalently grafting

Fig. 25. Coating mechanism using sol-gel method with silane compounds for modification of RO membrane [284].

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Fig. 26. Preparation of the MPC polymer-grafted polyamide RO membrane [289].

industry) which is constantly reformed in the presence of excess reducing agent, allowing the reaction to take place in the presence of oxygen, and thus be more industrially friendly. The results showed that the PDA-g-MTAC membranes had antibiofouling capability and more resistance to chlorine attack [291]. In addition, a zwitterionic poly(sulfobetaine methacrylate) (PSBMA) was grafted on the surface of polyamide membrane using surface initiated-ATRP (SI-ATRP) [292]. The PSBMA-grafted membranes were obtained by immobilizing ethanediamine-BIBB (EDA-BIBB) initiator on the carbonyl chloride groups and then a following SBMA polymerization through SI-ATRP. PSBMA grafted membranes had improved resistance to protein adsorption. Also, modification greatly increased water flux whiteout significant change in salt rejection [292].

zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer on the surface using surface-initiated ATRP according to Fig. 26 [289]. Firstly, polyamide RO membranes modified with hydroxyl groups of diethanolamine (DEA). Then, the ATRP initiator, 2-bromoisobutyryl bromide (BIBB), was immobilized on the fabricated membrane by condensation between DEA hydroxyl groups and BIBB carboxylic bromide. Finally, surface-initiated ATRP was carried out using MPC and immobilized BIBB [289]. With increasing polymerization time, the grafted amount increased and the surface structure became smoother. The MPC grafted membranes had lower contact angle and decreased water flux and salt rejection. Moreover, modified membranes exhibited high antibiofouling activity. Markovic et al. [290] investigated RO membranes modification by reacting surface amine groups with an ATRP initiator, BIBB, followed by surface initiated polymerization of sulfobetaine from the surface using activators regenerated by electron transfer (ARGET). Sulfobetaine grafting created smooth and hydrophilic surface of membrane and microbial biofouling was reduced by at least 80% in aquaria experiments in addition to comparable flux and salt rejection with unmodified membranes [290]. Blok et al. [291] firstly modified the surface of membranes by BIBBinitiator-PDA coating and then [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MTAC) were grafted from the BIBB-initiator-PDA surface using ARGET-ATRP. ARGET-ATRP offers several advantages over traditional ATRP, particularly when used in an industrial setting. ARGET-ATRP uses smaller amounts of copper catalyst (reducing cost to

7.3.2.5. Plasma-induced graft polymerization. Some researchers investigated surface nano-structuring (SNS) of polyamide TFC RO membranes via atmospheric pressure plasma-induced graft polymerization to enhance the performance of membranes [293–295]. Plasma-induced polymerization is different to plasma polymerization. The earlier utilizes plasma to activate the surface to generate oxide or hydroxide groups, which can then be used in conventional polymerization methods [296] but the latter is used to deposit a polymer onto the surface. Also, plasma-induced polymerization is a two-step process while the plasma polymerization is a one-step process. SNS polyamide TFC RO membranes were prepared via three step

Fig. 27. Schematic of preparation of the SNS polyamide TFC RO membranes via plasma-induced surface activation followed by surface graft polymerization [295].

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Fig. 28. Surface modification of polyamide TFC RO membrane with (a) treatment of membranes with MA (b) iCVD deposition of copolymer PDE (c) reaction with PS [231].

(PAAm) because of their excellent biocompatibility and high water solubility [293–295]. It had been shown that this method enables the creation of a surface polymer layer of terminally anchored chains that were in the dense ‘brush regime’. The chains of the hydrophilic grafted polymer extended from the surface when exposed to a good solvent (water), thereby screening the surface while the Brownian motion of the free portion of the anchored chains reduced the ability of foulants to adhere to the surface [295]. The experiments demonstrated that SNS of polyamide RO membranes with PMAA and PAAm brush layers resulted in increased membrane permeability, greater mineral scaling resistance, and comparable fouling resistance for model organic (BSA and alginic acid)

process (Fig. 27). Firstly, the polyamide layer of RO membrane was synthesized by conventional polyamide interfacial polymerization. In the second step, the polyamide membrane surface was activated via exposure to an impinging atmospheric pressure plasma source produced using a mixture of hydrogen (1 vol%) and helium (99 vol%). After a suitable plasma treatment period, the polyamide membrane surface was briefly exposed to an impinging oxygen stream. The atmospheric plasma mixture, followed by surface oxygenation resulted in the formation of peroxides of surface density and surface graft polymerization was initiated from these surface active peroxide sites. The third step consisted of graft polymerization which was carried out in an aqueous solutions of MAA or acrylamide (AAm) to obtain a grafted surface of poly(methacrylic acid) (PMAA) and poly(acrylamide) 47

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Fig. 29. Synthesis of PFPA-terminated PEG brush polymers and attachment to RO membrane surface [300].

significant antifouling performance by coating was exhibited. Furthermore, the cross-linker in the copolymer and the MA grafting improved the coating stability compared with non-grafting coatings [231].

compared to low fouling high performance commercial RO membrane of similar salt rejection level. In addition, biofouling resistance and cleaning effectiveness of RO membranes enhanced via hydrophilic brush layers [293–295]. 7.3.2.6. Hydrophilization treatment. Another method which was studied to modify the performance of polyamide TFC RO membranes included chemical modification using hydrophilizing agents such as hydrofluoric, hydrochloric, sulfuric, phosphoric, and nitric acids [297] and liquid fluorinating agents like hydrofluoric acid (HF) and fluosilicic acid (FSA) [298]. At solvable sites along the polyamide chain, the reactions caused the partial hydrolysis to more hydrophilic eNH2 and –COOH, which was consistent with the contact angle measurement. The results indicated that the hydrophilicity of the membranes was increased after modification and increase in flux up to an order of magnitude without any loss of salt rejection [297,298]. However, the concentration of acid and the time of exposure must be well controlled to avoid the breakdown of polymeric structures, resulting in the decrease of salt rejection. Moreover, the antifouling property of treated RO membranes was not investigated in their study.

7.3.2.8. UV and gamma irradiation. In some researches, the modification of RO surface membranes carried out by UV irradiation [299–301]. Tirado et al. [299] used zwitterionic polymer for UV radical grafting. At first stage, the photoinitiator benzophenone (BPh) and UV irradiation caused to graft some BPh on the membrane surface and at second stage poly 2-[(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (PSPE) was grafted to the surface using UV activation from aqueous solution. Zwitterionic PSPE modified membrane became more hydrophilic and slightly less negative surface charge. Also, modification enhanced resistance to initial bacteria adhesion and improvement in short-term resistance to organic fouling and biofouling [299]. Also, the surface of RO membranes was modified by dipping into an aqueous solution containing perfluorophenyl azide terminated PEG (PFPA-PEG) species and then exposed to UV light under ambient conditions (Fig. 29) [300]. PFPA are known for their highly reactive azide group that allows PFPA derivatives to make chemical bonds with rather unreactive targets, such as graphene, carbon nanotubes, fullerenes, and organic polymers. The azide functionality is activated by photo excitation that expels nitrogen gas and affords a reactive singlet nitrene (a reactive intermediate) that inserts into eNHe and C]C bonds. The surface layer of RO membranes is contained these groups, thus providing a target for modification. The results determined that PFPA undergo UV irradiation generated nitrene and bonded covalently to the membrane through an aziridine linkage. PEPA-PEG modification increased the hydrophilicity of membranes, declined pure water permeability, and increased salt rejection. Moreover, PFPA photochemical reactions prevented initial bacterial adhesion and exhibited high fouling resistance [300]. Ngo et al. [301] carried out UV-photo-induced graft polymerization of AA onto the surfaces of polyamide RO membranes using an immersion method performed under ambient conditions as depicted in Fig. 30. The attractive features of UV-induced grafting are the easy and controllable introduction of graft chains with a high density and an

7.3.2.7. Grafting by iCVD. A random copolymer poly[2(dimethylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] (PDE) thin films were synthesized and deposited via iCVD and reacted with 1,3-propane sultone (PS) to obtain the zwitterionic structure for modification of polyamide TFC RO membrane surface (Fig. 28) [231]. The zwitterionic coatings were covalently grafted on to RO membranes. Firstly, maleic anhydride (MA) was heated to 65 °C and the vapor was delivered into the reactor. MA reacted with the secondary amide group in the RO barrier layer. Then 2-(Dimethylamino)ethyl methacrylate (DMAEMA) and EGDMA monomers were heated up to 55 and 80 °C, respectively, and delivered into the reactor and formed PDE layer. EGDMA was utilized to make the copolymer insoluble in water. After that, PDE was reacted with the PS to convert the DMAEMA group into a zwitterionic poly[N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopro pyl)] (PDMMSA or poly-(sulfobetaine) or pSB). The results illustrated that the iCVD zwitterionic coating on RO membrane was highly smooth and conformal. Also, increasing the thickness of the coatings increased the surface roughness and decreased the permeation rates. Moreover, the attachment of bacteria was prevented by modification and 48

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Fig. 30. Mechanism of the surface graft polymerization of AA onto the polyamide TFC RO membrane by UV irradiation [301].

dominating membranes in commercial RO membranes applications. Despite their advantages such as high salt rejection and water permeability, resistance to pressure compaction, wide operation temperature and pH ranges, and high stability to biological attack, the polyamide TFC RO membrane has two drawbacks: the vulnerability to fouling as a major problem and the susceptibility to a chlorine disinfectant, which is widely used in the control of biofouling. It is known from literatures that the separation performance and other properties such as fouling resistance of the polyamide TFC RO membrane are strongly affected by the membrane surface properties such as surface hydrophilicity, charge, and roughness. Membranes with a hydrophilic and smooth surface with similar charge to the foulant seem to possess good antifouling property. Therefore, a great deal of research efforts has been devoted to modify the membrane surface properties so as to improve the membrane properties through surface modifications including physical and chemical treatments. Despite the achievements, there are still some challenges encountered antifouling RO membranes:

exact localization on the polymeric membrane surface. Furthermore, the covalent attachment of the graft chains onto a polymer surface is stable. According to Fig. 30, firstly, the absorption of UV light caused abstraction of hydrogen atoms from amide groups on the base of the polyamide TFC RO membrane. This produced the radical sites required for grafting with AA monomer free radicals, which then formed the polymeric AA-grafted chains on the membrane surface. AA grafted membranes had significant increase in hydrophilicity and the membrane surface became more compact and smoother. Also, the experiments indicated improved fouling resistance and lower irreversible fouling factors of the membrane after grafting of AA [301]. Furthermore, effect of gamma irradiation at intermediate doses on the performance of RO membranes was investigated by using different doses of 0.2 and 0.5 MGy with a constant dose rate of 0.5 kGy h− 1 using gamma 60Co source [302]. Characterizations of RO membrane after irradiation showed that membrane resistance towards irradiation remained effective until a dose of 0.2 MGy. After this dose, membranes degradation occurred by an increase in the permeability (by factor 2) and a decrease in salt rejection from (99% to 95%) after 0.2 MGy. At 0.5 MGy, scissions of ester and amide bonds were identified as well as leaching of nitrogen and carbon compounds [302].

• Surface modification, either by physical or chemical treatments,

8. Conclusion

•

Reverse osmosis membrane is a very attractive technology which has been widely used in desalination to tackle water shortage because it is more energy efficient compared with other desalination methods. Aromatic polyamide TFC RO membranes are still now the most 49

usually leads to additional resistance as a result of layer formed after modification, which impedes the permeation of water through the membrane and thereby decreases the water flux. Therefore, the layer should be ultrathin and the trade-off of flux reduction and antifouling property should be optimized and balanced. Long-term operation of surface modified RO membranes have not been studied in depth in most cases. Good chemical and mechanical stability must be sustained for long-term operation when using those

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• •

[25] A.G. Fane, R. Wang, M.X. Hu, Synthetic membranes for water purification: status and future, Angew. Chem. Int. Ed. 54 (2015) 2–21. [26] H.T. Madsen, Membrane filtration in water treatment-removal of micropollutants, in: E. Sogaard (Ed.), Chemistry of Advanced Environmental Purification Processes of Water, Fundamentals and Applications, Elsevier, 2014, pp. 199–248. [27] Y.N. Wang, C.Y. Tang, Fouling of nanofiltration, reverse osmosis, and ultrafiltration membranes by protein mixtures: the role of inter-foulant-species interaction, Environ. Sci. Technol. 45 (2011) 6373–6379. [28] S.S. Shenvi, A.M. Isloor, A.F. Ismail, A review on RO membrane technology: developments and challenges, Desalination 368 (2015) 10–26. [29] K.P. Lee, T.C. Arnot, D. Mattia, A review of reverse osmosis membrane materials for desalination-development to date and future potential, J. Membr. Sci. 370 (2011) 1–22. [30] M. Schiffler, Perspectives and challenges for desalination in the 21st century, Desalination 165 (2004) 1–9. [31] A. Kumano, M. Sekino, Y. Matsui, N.A. Fujiwara, H. Matsuyama, Study of mass transfer characteristics for a hollow fiber reverse osmosis module, J. Membr. Sci. 324 (2008) 136–141. [32] G.M. Geise, D.R. Paul, B.D. Freeman, Fundamental water and salt transport properties of polymeric materials, Prog. Polym. Sci. 39 (2014) 1–42. [33] J. Wang, D.S. Dlamini, A.K. Mishra, M.T.M. Pendergast, M.C.Y. Wong, B.B. Mamba, V. Freger, A.R.D. Verliefde, E.M.V. Hoek, A critical review of transport through osmotic membranes, J. Membr. Sci. 454 (2014) 516–537. [34] A. Sagle, B. Freeman, Fundamentals of membranes for water treatment, in: J.A. Arroyo (Ed.), The Future of Desalination in Texas, II Texas Water Development Board, Austin, TX, 2004, pp. 137–153. [35] J. Wijmans, R. Baker, The solution-diffusion model: a review, J. Membr. Sci. 107 (1995) 1–21. [36] D. Li, H. Wang, Recent developments in reverse osmosis desalination membranes, J. Mater. Chem. 20 (2010) 4551–4566. [37] G.M. Geise, H.-S. Lee, D.J. Miller, B.D. Freeman, J.E. McGrath, D.R. Paul, Water purification by membranes: the role of polymer science, J. Polym. Sci. B Polym. Phys. 48 (2010) 1685–1718. [38] S. Zhao, L. Zou, C.Y. Tang, D. Mulcahy, Recent developments in forward osmosis: opportunities and challenges, J. Membr. Sci. 396 (2012) 1–21. [39] C.H. Worthley, Improving the Performance of Cellulose Acetate Reverse Osmosis Membranes (Ph.D thesis), Flinders University, Adelaide, South Australia, 2012. [40] J. E. Cadotte, Interfacially synthesized reverse osmosis membrane, US 4,277,344, 1981. [41] S.H. Maruf, D.U. Ahn, A.R. Greenberg, Y. Ding, Glass transition behaviors of interfacially polymerized polyamide barrier layers on thin film composite membranes via nano-thermal analysis, Polymer 52 (2011) 2643–2649. [42] V. Freger, Nanoscale heterogeneity of polyamide membranes formed by interfacial polymerization, Langmuir 19 (2003) 4791–4797. [43] G.–.R. Xu, J.–.N. Wang, C.–.J. Li, Strategies for improving the performance of the polyamide thin film composite (PA-TFC) reverse osmosis (RO) membranes: surface modifications and nanoparticles incorporations, Desalination 328 (2013) 83–100. [44] S.S. Sablani, M.F.A. Goosen, R. Al-Belushi, M. Wilf, Concentration polarization in ultrafiltration and reverse osmosis: a critical review, Desalination 141 (2001) 269–289. [45] A.G. Fane, P. Beatson, H. Li, Membrane fouling and its control in environmental applications, Water Sci. Technol. 41 (2000) 303–308. [46] Q. She, R. Wang, A.G. Fane, C.Y. Tang, Membrane fouling in osmotically driven membrane processes: a review, J. Membr. Sci. 499 (2016) 201–233. [47] N. Peña, S. Gallego, F.D. Vigo, S.P. Chesters, Evaluating impact of fouling on reverse osmosis membranes performance, Desalin. Water Treat. (2012) 1–11. [48] L.D. Tijing, Y.C. Woo, J.–.S. Choi, S. Lee, S.–.H. Kim, H.K. Shon, Fouling and its control in membrane distillation-a review, J. Membr. Sci. 475 (2015) 215–244. [49] B.V. Derjaguin, L. Landau, Theory of the stability of strongly charged lyophobic sols and of the adhesion of strongly charged particles in solutions of electrolytes, Prog. Surf. Sci. 43 (1993) 30–59. [50] E.J.W. Verwey, J.T.G. Overbeek, Theory of the stability of lyphobic colloids, J. Phys. Chem. 51 (1947) 631–636. [51] J.R. Lead, K.J. Wilkinson, Environmental colloids and particles: current knowledge and future developments, in: K.J. Wilkinson, J.R. Lead (Eds.), Environmental Colloids and Particles: Behavior, Separation and Characterization, 10 John Wiley & Sons, England, 2007, pp. 1–15. [52] W. Stumm, Aquatic colloids as chemical reactants. Surface structure and reactivity, Colloids Surf. A Physicochem. Eng. Asp. 73 (1993) 1–18. [53] P. Bacchin, P. Aimar, R.W. Field, Critical and sustainable fluxes: theory, experiments and applications, J. Membr. Sci. 281 (2006) 42–69. [54] J. Buffle, G.G. Leppard, Characterization of aquatic colloids and macromolecules. 2. Key role of physical structures on analytical results, Environ. Sci. Technol. 29 (1995) 2176–2184. [55] J. Buffle, K.J. Wilkinson, S. Stoll, M. Filella, J.W. Zhang, Generalized description of aquatic colloidal interactions: the three-colloidal component approach, Environ. Sci. Technol. 32 (1998) 2887–2899. [56] S.P. Nunes, K. Peinemann, Membrane Technology: In the Chemical Industry, fourth ed., John Wiley & Sons, 2010. [57] W. Guo, H.–.H. Ngo, J. Li, A mini-review on membrane fouling, Bioresour. Technol. 122 (2012) 27–34. [58] W. Zhang, J. Luo, L. Ding, M.Y. Jaffrin, A review on flux decline control strategies in pressure-driven membrane processes, Ind. Eng. Chem. Res. 54 (2015) 2843–2861. [59] C.A.C. van de Lisdonk, J.A.M. van Paassen, J.C. Schippers, Monitoring scaling in nanofiltration and reverse osmosis membrane systems, Desalination 132 (2000)

modified membranes. As compared with physical surface coating, chemical grafting would be a more favorable approach to achieve due to covalent linkage between membrane and modifiers. Many development methods for surface modification 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. 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.

Although the fouling cannot be thoroughly prevented even for antifouling RO membranes, much research and development are still needed to develop surface modified RO membranes whiteout trade-off effect and applicable for long-term and commercial uses. References [1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [2] N.H. Afgan, Sustainability concept for energy, water and environment systems, in: K. Hanjalić, R. Van de Krol, A. Lekić (Eds.), Sustainable Energy Technologies, Springer, 2008, pp. 25–49. [3] L.F. Greenlee, D.F. Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: water sources, technology, and today's challenges, Water Res. 43 (2009) 2317–2348. [4] T. Uemura, M. Henmi, Thin-film Composite Membranes for Reverse Osmosis, John Wiley & Sons, 2008. [5] C. Fritzmann, J. Löwenberg, T. Wintgens, T. Melin, State-of-the-art of reverse osmosis desalination, Desalination 216 (2007) 1–76. [6] M.A. Alghoul, P. Poovanaesvaran, K. Sopian, M.Y. Sulaiman, Review of brackish water reverse osmosis (BWRO) system designs, Renew. Sust. Energ. Rev. 13 (2009) 2661–2667. [7] B. Peñate, L. García-Rodríguez, Current trends and future prospects in the design of seawater reverse osmosis desalination technology, Desalination 284 (2012) 1–8. [8] S. Liyanaarachchi, L. Shu, S. Muthukumaran, V. Jegatheesan, K. Baskaran, Problems in seawater industrial desalination processes and potential sustainable solutions: a review, Rev. Environ. Sci. Biotechnol. 13 (2014) 203–214. [9] S.A. Kalogirou, Seawater desalination using renewable energy sources, Prog. Energy Combust. 31 (2005) 242–281. [10] P. Allison, C. Gasson, Desalination Markets 2005–2015: A Global Assessment and Forecast, Media Analytics Limited, Oxford, United Kingdom, 2004. [11] A.D. Khawaji, I.K. Kutubkhanah, J. Wie, Advances in seawater desalination technologies, Desalination 221 (2008) 47–69. [12] C. Charcosset, A review of membrane processes and renewable energies for desalination, Desalination 245 (2009) 214–231. [13] L.G. -Rodríguez, Renewable energy applications in desalination: state of the art, Sol. Energy 75 (2003) 381–393. [14] A. Subramani, M. Badruzzaman, J. Oppenheimer, J.G. Jacangelo, Energy minimization strategies and renewable energy utilization for desalination: a review, Water Res. 45 (2011) 1907–1920. [15] A. Subramani, J.G. Jacangelo, Emerging desalination technologies for water treatment: a critical review, Water Res. 75 (2015) 164–187. [16] N. Misdan, W.J. Lau, A.F. Ismail, Seawater reverse osmosis (SWRO) desalination by thin-film composite membrane-current development, challenges and future prospects, Desalination 287 (2012) 228–237. [17] T. Mezher, H. Fath, Z. Abbas, A. Khaled, Techno-economic assessment and environmental impacts of desalination technologies, Desalination 266 (2011) 263–273. [18] R.W. Baker, Membrane Technology and Applications, third ed., John Wiley & Sons, New York, 2012. [19] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301–310. [20] D. Bastani, N. Esmaeili, M. Asadollahi, Polymeric mixed matrix membranes containing zeolites as a filler for gas separation applications: A review, J. Ind. Eng. Chem. 19 (2013) 375–393. [21] S. Alzahrani, A.W. Mohammad, Challenges and trends in membrane technology implementation for produced water treatment: a review, J. Water Process Eng. 4 (2014) 107–133. [22] M.T. Ravanchia, T. Kaghazchi, A. Kargari, Application of membrane separation processes in petrochemical industry: a review, Desalination 235 (2009) 199–244. [23] D. Li, Y. Yan, H. Wang, Recent advances in polymer and polymer composite membranes for reverse and forward osmosis processes, Prog. Polym. Sci. 61 (2016) 104–155. [24] B.V.D. Bruggen, C. Vandecasteele, T.V. Gestel, W. Doyen, R. Leysen, A review of pressure-driven membrane processes in wastewater treatment and drinking water production, Environ. Prog. 22 (2003) 46–56.

50

Desalination xxx (xxxx) xxx–xxx

M. Asadollahi et al.

[93] M.E. Williams, A Review of Reverse Osmosis Theory, EET Corporation and William Engineering Services Company, 2003. [94] P. Bacchin, D.S. -Hassen, V. Starov, M.J. Clifton, P. Aimar, A unifying model for concentration polarization, gel-layer formation and particle deposition in cross flow membrane filtration of colloidal suspensions, Chem. Eng. Sci. 57 (2002) 77–91. [95] C.Y. Tang, T.H. Chong, A.G. Fane, Colloidal interactions and fouling of NF and RO membranes: a review, Adv. Colloid Interf. Sci. 164 (2011) 126–143. [96] A.S. Al-Amoudi, Factors affecting natural organic matter (NOM) and scaling fouling in NF membranes: a review, Desalination 259 (2010) 1–10. [97] Y. He, D.M. Bagley, K.T. Leung, S.N. Liss, B.-Q. Liao, Recent advances in membrane technologies for biorefining and bioenergy production, Biotechnol. Adv. 30 (2012) 817–858. [98] V. Chen, A. Fane, S. Madaeni, I. Wenten, Particle deposition during membrane filtration of colloids: transition between concentration polarization and cake formation, J. Membr. Sci. 125 (1997) 109–112. [99] M.F.A. Goosen, S.S. Sablani, H. Al-Hinai, S. Al-Obeidani, R. Al-Belushi, D. Jackson, Fouling of reverse osmosis and ultrafiltration membranes: a critical review, Sep. Sci. Technol. 39 (2005) 2261–2297. [100] R. Bian, K. Yamamoto, Y. Watanabe, The effect of shear rate on controlling the concentration polarization and membrane fouling, Desalination 131 (2000) 225–236. [101] F. Zamani, J.W. Chew, E. Akhondi, W.B. Krantz, A.G. Fane, Unsteady-state shear strategies to enhance mass-transfer for the implementation of ultrapermeable membranes in reverse osmosis: a review, Desalination 356 (2015) 328–348. [102] P. Bacchin, P. Aimar, V. Sanchez, Model for colloidal fouling of membranes, AIChE J 41 (1995) 368–376. [103] S.J. You, X.H. Wang, M. Zhong, Y.J. Zhong, C. Yu, N.Q. Ren, Temperature as a factor affecting transmembrane water flux in forward osmosis: steady-state modeling and experimental validation, Chem. Eng. J. 198–199 (2012) 52–60. [104] M.J. Campbell, R.P. Walter, R. McLoughlin, C.J. Knowles, Effect of temperature on protein conformation and activity during ultrafiltration, J. Membr. Sci. 78 (1993) 35–43. [105] B.J. Wang, T.C. Wei, Z.R. Yu, Effect of operating temperature on component distribution of West Indian cherry juice in a microfiltration system, LWT - Food Sci. Technol. 38 (2005) 683–689. [106] P. Le-Clech, V. Chen, T.A. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci. 284 (2006) 17–53. [107] M. Mänttäri, A. Pihlajamäki, M. Nyström, Effect of pH on hydrophilicity and charge and their effect on the filtration efficiency of NF membranes at different pH, J. Membr. Sci. 280 (2006) 311–320. [108] D. Nanda, K.-L. Tung, Y.-L. Li, N.-J. Lin, C.-J. Chuang, Effect of pH on membrane morphology, fouling potential, and filtration performance of nanofiltration membrane for water softening, J. Membr. Sci. 349 (2010) 411–420. [109] A. Escoda, P. Fievet, S. Lakard, A. Szymczyk, S. Déon, Influence of salts on the rejection of polyethyleneglycol by an NF organic membrane: pore swelling and salting-out effects, J. Membr. Sci. 347 (2010) 174–182. [110] X. Jin, X.F. Huang, E.M.V. Hoek, Role of specific ion interactions in seawater RO membrane fouling by alginic acid, Environ. Sci. Technol. 43 (2009) 3580–3587. [111] Y.H. Mo, A. Tiraferri, N.Y. Yip, A. Adout, X. Huang, M. Elimelech, Improved antifouling properties of polyamide nanofiltration membranes by reducing the density of surface carboxyl groups, Environ. Sci. Technol. 46 (2012) 13253–13261. [112] K. Listiarini, W. Chun, D.D. Sun, J.O. Leckie, Fouling mechanism and resistance analyses of systems containing sodium alginate, calcium, alum and their combination in dead-end fouling of nanofiltration membranes, J. Membr. Sci. 344 (2009) 244–251. [113] Y. Yu, S. Lee, S. Hong, Effect of solution chemistry on organic fouling of reverse osmosis membranes in seawater desalination, J. Membr. Sci. 351 (2010) 205–213. [114] H. Ma, C.N. Bowman, R.H. Davis, Membrane fouling reduction by backpulsing and surface modification, J. Membr. Sci. 173 (2000) 191–200. [115] B.V. Bruggen, M. Mänttäri, M. Nyström, Drawbacks of applying nanofiltration and how to avoid them: a review, Sep. Purif. Technol. 63 (2008) 251–263. [116] E.M. Vrijenhoek, S. Hong, M. Elimelech, Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes, J. Membr. Sci. 188 (2001) 115–128. [117] A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, sixth ed., John Wiley & Sons, Inc., New York, 1997. [118] C.H. Koo, A.W. Mohammad, F. Suja', M.Z.M. Talib, Review of the effect of selected physicochemical factors on membrane fouling propensity based on fouling indices, Desalination 287 (2012) 167–177. [119] N.N. Li, A.G. Fane, W.S.W. Ho, T. Matsuura, Advanced Membrane Technology and Applications, John Wiley & Sons, 2008. [120] L.J. Zeman, A.L. Zydney, Microfiltration and ultrafiltration: principles and applications, Marcel Dekker, 1996. [121] B. Kwon, S. Lee, J. Cho, H. Ahn, D. Lee, H.S. Shin, Biodegradability, DBP formation, and membrane fouling potential of natural organic matter: characterization and controllability, Environ. Sci. Technol. 39 (2005) 732–739. [122] B.S. Lalia, V. Kochkodan, R. Hashaikeh, N. Hilal, A review on membrane fabrication: structure, properties and performance relationship, Desalination 326 (2013) 77–95. [123] D. Rana, T. Matsuura, Surface modifications for antifouling membranes, Chem. Rev. 110 (2010) 2448–2471. [124] G. Kang, Y. Cao, Development of antifouling reverse osmosis membranes for water treatment: a review, Water Res. 46 (2012) 584–600. [125] K.C. Khulbe, C. Feng, T. Matsuura, The art of surface modification of synthetic

101–108. [60] A. Antony, J.H. Low, S. Gray, A.E. Childress, P. Le-Clech, G. Leslie, Scale formation and control in high pressure membrane water treatment systems: a review, J. Membr. Sci. 383 (2011) 1–16. [61] S. Shirazi, C.–.J. Lin, D. Chen, Inorganic fouling of pressure-driven membrane processes-a critical review, Desalination 250 (2010) 236–248. [62] J. Ayyavoo, T.P.N. Nguyen, B.-M. Jun, I.-C. Kim, Y.-N. Kwon, Protection of polymeric membranes with antifouling surfacing via surface modifications, Colloids Surf. A Physicochem. Eng. Asp. 506 (2016) 190–201. [63] R.J. Xie, M.J. Gomez, Y.J. Xing, P.S. Klose, Fouling assessment in a municipal water reclamation reverse osmosis system as related to concentration factor, J. Environ. Eng. Sci. 3 (2004) 61–72. [64] H.F. Ridgway, Biological Fouling of Separation Membranes Used in Water Treatment Applications, AWWA Research Foundation, 2003. [65] O. Habimana, A.J.C. Semião, E. Casey, The role of cell-surface interactions in bacterial initial adhesion and consequent biofilm formation of nanofiltration/reverse osmosis membranes, J. Membr. Sci. 454 (2014) 82–96. [66] J.S. Vrouwenvelder, D. van der Kooij, Diagnosis, prediction and prevention of biofouling of NF and RO membranes, Desalination 139 (2001) 65–71. [67] V. Kochkodan, N. Hilal, A comprehensive review on surface modified polymer membranes for biofouling mitigation, Desalination 356 (2015) 187–207. [68] J. Lee, I.S. Kim, Microbial community in seawater reverse osmosis and rapid diagnosis of membrane biofouling, Desalination 273 (2011) 118–126. [69] R.A. Al-Juboori, T. Yusaf, Biofouling in RO system: mechanisms, monitoring and controlling, Desalination 302 (2012) 1–23. [70] Microbial biofouling: unsolved problems, insufficient approaches, and possible solutions, in: H.-C. Flemming, H.-C. Flemming, J. Wingender, U. Szewzyk (Eds.), Biofilm Highlights, Springer, Berlin Heidelberg, 2011, pp. 81–109. [71] H.-C. Flemming, Biofouling in water systems-cases, causes and countermeasures, Appl. Microbiol. Biotechnol. 59 (2002) 629–640. [72] L.H. -Stoodley, J.W. Costerton, P. Stoodley, Bacterial biofilms: from the natural environment to infectious diseases, Nat. Rev. Microbiol. 2 (2004) 95–108. [73] A. Matin, Z. Khan, S.M.J. Zaidi, M.C. Boyce, Biofouling in reverse osmosis membranes for seawater desalination: phenomena and prevention, Desalination 281 (2011) 1–16. [74] J. Gutman, S. Fox, J. Gilron, Interactions between biofilms and NF/RO flux and their implications for control-a review of recent developments, J. Membr. Sci. 421–422 (2012) 1–7. [75] D. Kim, S. Jung, J. Sohn, H. Kim, S. Lee, Biocide application for controlling biofouling of SWRO membranes-an overview, Desalination 238 (2009) 43–52. [76] J. Powell, J. Luh, O. Coronell, Amide link scission in the polyamide active layers of thin-film composite membranes upon exposure to free chlorine: kinetics and mechanisms, Environ. Sci. Technol. 49 (2015) 12136–12144. [77] J. Glater, S.-K. Hong, M. Elimelech, The search for a chlorine-resistant reverse osmosis membrane, Desalination 95 (1994) 325–345. [78] Y.N. Kwon, J.O. Leckie, Hypochlorite degradation of crosslinked polyamide membranes I. Changes in chemical/morphological properties, J. Membr. Sci. 283 (2006) 21–26. [79] Y.N. Kwon, J.O. Leckie, Hypochlorite degradation of crosslinked polyamide membranes II. Changes in hydrogen bonding behavior and performance, J. Membr. Sci. 282 (2006) 456–464. [80] S. Surawanvijit, A. Rahardianto, Y. Cohen, An integrated approach for characterization of polyamide reverse osmosis membrane degradation due to exposure to free chlorine, J. Membr. Sci. 15 (2016) 164–173. [81] V.T. Do, C.Y. Tang, M. Reinhard, J.O. Leckie, Degradation of polyamide nanofiltration and reverse osmosis membranes by hypochlorite, Environ. Sci. Technol. 46 (2012) 852–859. [82] S. Avlonitis, W.T. Hanbury, T. Hodgkiess, Chlorine degradation of aromatic polyamide, Desalination 85 (1992) 321–334. [83] T. Kawaguchi, H. Tamura, Chlorine-resistant membrane for reverse osmosis. Preparation of chlorine-resistant polyamide composite membranes, J. Appl. Polym. Sci. 29 (1984) 3369–3379. [84] T. Uemura, M. Kurihara, Chlorine resistance of reverse osmosis membranes and changes in membrane structure and performance caused by chlorination degradation, Bull. Soc. Sea Water Sci. Japan 57 (2003) 498–507. [85] J. Glater, M.R. Zachariah, Mechanistic study of halogen interaction with polyamide reverse-osmosis membranes, ACS Symp. Ser. (1985) 345–358. [86] S.H. Son, J. Jegal, Preparation and characterization of polyamide reverse osmosis membranes with good chlorine tolerance, J. Appl. Polym. Sci. 120 (2011) 1245–1252. [87] S. Kim, E.M.V. Hoek, Modeling concentration polarization in reverse osmosis processes, Desalination 186 (2005) 111–128. [88] W.R. Bowen, F. Jenner, Theoretical descriptions of membrane filtration of colloids and fine particles: an assessment and review, Adv. Colloid Interf. Sci. 56 (1995) 141–200. [89] L. Song, M. Elimelech, Theory of concentration polarization in cross-flow filtration, J. Chem. Soc. 91 (1995) 3389–3398. [90] S. De, S. Mondal, S. Banerjee, Brief introduction to pressure-driven membranebased processes, in: S. De, S. Mondal, S. Banerjee (Eds.), Stevioside: Technology, Applications and Health, John Wiley & Sons, 2013, pp. 65–89. [91] K. Hu, J.M. Dickson, M. Zargar, B. Jin, S. Dai, Development and application of reverse osmosis for separation, in: K. Hu, J.M. Dickson (Eds.), Membrane Processing for Dairy Ingredient Separation, John Wiley & Sons, 2015, pp. 139–175. [92] T. Cath, A. Childress, M. Elimelech, Forward osmosis: principles, applications, and recent developments, J. Membr. Sci. 281 (2006) 70–87.

51

Desalination xxx (xxxx) xxx–xxx

M. Asadollahi et al.

3389–3398. [160] A. Dąbrowski, P. Podkoscielny, Z. Hubicki, M. Barczak, Adsorption of phenolic compounds by activated carbon-a critical review, Chemosphere 58 (2005) 1049–1070. [161] L.D. Temmerman, T. Maere, H. Temmink, A. Zwijnenburg, I. Nopens, Salt stress in a membrane bioreactor: dynamics of sludge properties, membrane fouling and remediation through powdered activated carbon dosing, Water Res. 63 (2014) 112–124. [162] J.G. Jacangelo, J. Demarco, D.M. Owen, S.J. Randtke, Selected processes for removing NOM-an overview, J. Am. Water Works Assoc. 87 (1995) 64–77. [163] A. Brehant, V. Bonnelye, M. Perez, Assessment of ultrafiltration as a pretreatment of reverse osmosis membranes for surface seawater desalination, Water Sci. Technol. Water Supply 3 (2003) 437–445. [164] A. Brehant, V. Bonnelye, M. Perez, Comparison of MF/UF pretreatment with conventional filtration prior to RO membranes for surface seawater desalination, Desalination 144 (2002) 353–360. [165] M. Herzberg, D. Berry, L. Raskin, Impact of microfiltration treatment of secondary wastewater effluent on biofouling of reverse osmosis membranes, Water Res. 44 (2010) 167–176. [166] G.K. Pearce, UF/MF pre-treatment to RO in seawater and wastewater reuse applications: a comparison of energy costs, Desalination 222 (2008) 66–73. [167] T. Al-Sarkal, H.A. Arafat, Ultrafiltration versus sedimentation-based pretreatment in Fujairah-1 RO plant: environmental impact study, Desalination 317 (2013) 55–66. [168] A.A. Al-Hajouri, A.S. Al-Amoudi, A.M. Farooque, Long term experience in the operation of nanofiltration pretreatment unit for seawater desalination at SWCC SWRO plant, Desalin. Water Treat. 51 (2013) 1861–1873. [169] W. Jian, A. Kitanaka, W. Nishijima, A.U. Baes, M. Okada, Removal of oil pollutants in seawater as pretreatment of reverse osmosis desalination process, Water Res. 33 (1998) 1857–1863. [170] I.S. Chang, P. Le Clech, B. Jefferson, S. Judd, Membrane fouling in membrane bioreactors for wastewater treatment, J. Environ. Eng. 128 (2002) 1018–1029. [171] H. Huang, K. Schwab, J.G. Jacangelo, Pretreatment for low pressure membranes in water treatment: a review, Environ. Sci. Technol. 43 (2009) 3011–3019. [172] Y.A.L. Gouellec, M. Elimelech, Calcium sulfate (gypsum) scaling in nanofiltration of agricultural drainage water, J. Membr. Sci. 205 (2002) 279–291. [173] A.S. Al-Amoudi, A.M. Farooque, Performance, restoration and autopsy of NF membranes used in seawater pretreatment, Desalination 178 (2005) 261–271. [174] J. Saqib, I.H. Aljundi, Membrane fouling and modification using surface treatment and layer-by-layer assembly of polyelectrolytes: state-of-the-art review, J. Water Process Eng. 11 (2016) 68–87. [175] R. Ning, Expanding issues in desalination, in: J.M. Arnal, B. García-Fayos, M. Sancho (Eds.), Membrane Cleaning, 3 2011, pp. 63–84. [176] J.P. Chen, S.L. Kim, Y.P. Ting, Optimization of membrane physical and chemical cleaning by a statistically designed approach, J. Membr. Sci. 219 (2003) 27–45. [177] A. Sagiv, R. Semiat, Backwash of spiral wound membranes, Desalination 179 (2005) 1–9. [178] J.C.-T. Lin, D.J. Lee, C. Huang, Membrane fouling mitigation: membrane cleaning, Sep. Sci. Technol. 45 (2010) 858–872. [179] S.S. Madaeni, T. Mohammadi, M.K. Moghadam, Chemical cleaning of reverse osmosis membranes, Desalination 134 (2001) 77–82. [180] N.M. D'Souza, A.J. Mawson, Membrane cleaning in the dairy industry: a review, Crit. Rev. Food Sci. Nutr. 45 (2005) 125–134. [181] S.S. Madaeni, S. Samieirad, Chemical cleaning of reverse osmosis membrane fouled by wastewater, Desalination 257 (2010) 80–86. [182] T.R. Tuladhur, Development of a Novel Sensor for Cleaning Studies, University of Cambridge, 2001 (Ph.D. Thesis). [183] A. Weis, M.R. Bird, M. Nystrom, The chemical cleaning of polymeric UF membranes fouled with spent sulphite liquor over multiple operational cycles, J. Membr. Sci. 216 (2003) 67–79. [184] M.G. Buonomenna, Nano-enhanced reverse osmosis membranes, Desalination 314 (2013) 73–88. [185] W.J. Lau, S. Gray, T. Matsuura, D. Emadzadeh, J.P. Chen, A.F. Ismail, A review on polyamide thin film nanocomposite (TFN) membranes: history, applications, challenges and approaches, Water Res. 80 (2015) 306–324. [186] E. Bet-moushoul, Y. Mansourpanah, K. Farhadi, M. Tabatabaei, TiO2 nanocomposite based polymeric membranes: a review on performance improvement for various applications in chemical engineering processes, Chem. Eng. J. 283 (2016) 29–46. [187] J. Yin, B. Deng, Polymer-matrix nanocomposite membranes for water treatment, J. Membr. Sci. 479 (2015) 256–275. [188] M.T.M. Pendergast, E.M.V. Hoek, A review of water treatment membrane nanotechnologies, Energy Environ. Sci. 4 (2011) 1946–1971. [189] D. Li, H. Wang, Thin film nanocomposite membranes for water desalination, in: M. Duke, D. Zhao, R. Semiat, M. Lu (Eds.), Functional Nanostructured Materials and Membranes for Water Treatment, John Wiley & Sons, 2013, pp. 163–194. [190] Q. Cheng, Y. Zheng, S. Yu, H. Zhu, X. Peng, Jia Liu, J. Liu, M. Liu, C. Gao, Surface modification of a commercial thin-film composite polyamide reverse osmosis membrane through graft polymerization of N-isopropylacrylamide followed by acrylicacid, J. Membr. Sci. 447 (2013) 236–245. [191] V. Freger, J. Gilron, S. Belfer, TFC polyamide membranes modified by grafting of hydrophilic polymers: an FT-IR/AFM/TEM study, J. Membr. Sci. 209 (2002) 283–292. [192] C.Y. Tang, Y.N. Kwon, J.O. Leckie, Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes. II. Membrane physiochemical properties and their dependence on

polymeric membranes, J. Appl. Polym. Sci. 115 (2010) 855–895. [126] S.A. Jeshi, A. Neville, An investigation into the relationship between flux and roughness on RO membranes using scanning probe microscopy, Desalination 189 (2006) 221–228. [127] M. Elimelech, X. Zhu, A.E. Childress, S. Hong, Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes, J. Membr. Sci. 127 (1997) 101–109. [128] N.L. Le, S.P. Nunes, Materials and membrane technologies for water and energy sustainability, Sustain. Mater. Technol. 7 (2016) 1–28. [129] N. Hilal, O.O. Ogunbiyi, N.J. Miles, R. Nigmatullin, Methods employed for control of fouling in MF and UF membranes: a comprehensive review, Sep. Sci. Technol. 40 (2005) 1957–2005. [130] X.M. Wang, T.D. Waite, Role of gelling soluble and colloidal microbial products in membrane fouling, Environ. Sci. Technol. 43 (2009) 9341–9347. [131] S. Jamaly, N.N. Darwish, I. Ahmed, S.W. Hasan, A short review on reverse osmosis pretreatment technologies, Desalination 354 (2014) 30–38. [132] W. Gao, H. Liang, J. Ma, M. Han, Z.-L. Chen, Z.-S. Han, Membrane fouling control in ultrafiltration technology for drinking water production: a review, Desalination 272 (2011) 1–8. [133] N. Prihasto, Q.F. Liu, S.H. Kim, Pre-treatment strategies for seawater desalination by reverse osmosis system, Desalination 249 (2009) 308–316. [134] N.P. Cheremisinoff, Handbook of Water and Wastewater Treatment Technologies, Elsevier, 2002. [135] E. Friedler, I. Katz, C.G. Dosoretz, Chlorination and coagulation as pretreatments for grey water desalination, Desalination 222 (2008) 38–49. [136] Hydranautics, Chemical Pretreatment for RO and NF. Technical Application Bulletin No. 111, Rev. 2003, B, Available from: http://www.membranes.com/ docs/tab/TAB111.pdf (accessed 17.05.08). [137] J. Park, Y. Kim, P.K. Kim, H.V. Daniels, Effects of two different ozone doses on seawater recirculating systems for black sea bream Acanthopagrus schlegeli (Bleeker): removal of solids and bacteria by foam fractionation, Aquac. Eng. 44 (2011) 19–24. [138] J.S. Baker, L.Y. Dudley, Biofouling in membrane systems-a review, Desalination 118 (1998) 81–89. [139] U. Vongunten, J. Holgne, Bromate formation during ozonation of bromide-containing waters: interaction of ozone and hydroxyl radical reaction, Environ. Sci. Technol. 28 (1994) 1234–1242. [140] M.K. da Silva, I.C. Tessaro, K. Wada, Investigation of oxidative degradation of polyamide reverse osmosis membranes by monochloramine solutions, J. Membr. Sci. 282 (2006) 375–382. [141] G. Petrucci, M. Rosellini, Chlorine dioxide in sea water for fouling control and post-disinfection in potable water works, Desalination 182 (2005) 283–291. [142] N. Voutchkov, Considerations for selection of seawater filtration pretreatment system, Desalination 261 (2010) 354–364. [143] L. Henthorne, B. Boysen, State-of-the-art of reverse osmosis desalination pretreatment, Desalination 356 (2015) 129–139. [144] Potable Organic Polymers-Types and Applications, Sweetwater Technologies, Government Engineering, 2006 Available from: http://www.govengr.com/ ArticlesNov06/potable.pdf (accessed 13.02.08). [145] C.R. O'Melia, Coagulation and sedimentation in lakes, reservoirs and water treatment plants, Water Sci. Technol. 37 (1998) 129–135. [146] J. Pizarro, N. Belzile, M. Filella, G.G. Leppard, J.C. Negre, D. Perret, J. Buffle, Coagulation sedimentation of submicron iron particles in a eutrophic lake, Water Res. 29 (1995) 617–632. [147] J.K. Edzwald, J. Haarhoff, Seawater pretreatment for reverse osmosis: chemistry, contaminants, and coagulation, Water Res. 45 (2011) 5428–5440. [148] V. Bonnelye, M.A. Sanz, J.–.P. Durand, L. Plasse, F. Gueguen, P. Mazounie, Reverse osmosis on open intake seawater: pre-treatment strategy, Desalination 167 (2004) 191–200. [149] F. Morenski, Current pretreatment requirements for reverse osmosis membrane applications, Official Proceedings of the 53rd International Water Conference, 1992, pp. 325–330. [150] K.T. Chua, M.N.A. Hawlader, A. Malek, Pretreatment of seawater: results of pilot trials in Singapore, Desalination 159 (2003) 225–243. [151] A.M. Elguera, S.O.P. Báez, Development of the most adequate pre-treatment for high capacity seawater desalination plants with open intake, Desalination 184 (2005) 173–183. [152] T. Peters, D. Pintó, E. Pintó, Improved seawater intake and pre-treatment system based on Neodren technology, Desalination 203 (2007) 134–140. [153] M. Petry, M.A. Sanz, C. Langlais, V. Bonnelye, J.-P. Durand, D. Guevara, W.M. Nardes, C.H. Saemi, The El Coloso (Chile) reverse osmosis plant, Desalination 203 (2007) 141–152. [154] C.H. Lai, Y.H. Chen, H.H. Yeh, Effects of feed water quality and pretreatment on NF membrane fouling control, Sep. Sci. Technol. 48 (2013) 1609–1615. [155] A. Drak, K. Glucina, M. Busch, D. Hasson, J.-M. Laîne, R. Semiat, Laboratory technique for predicting the scaling propensity of RO feed waters, Desalination 132 (2000) 233–242. [156] E.E.A. Ghafour, Enhancing RO system performance utilizing antiscalants, Desalination 153 (2003) 149–153. [157] E.G. Darton, Membrane chemical research: centuries apart, Desalination 132 (2000) 121–131. [158] D. Zhao, S. Yu, Review of recent advance in fouling mitigation of NF/RO membranes in water treatment: pretreatment membrane modification, and chemical cleaning, Desalin. Water Treat. 55 (2015) 870–891. [159] A. Sweity, Y. Oren, Z. Ronen, M. Herzberg, The influence of antiscalants on biofouling of RO membranes in seawater desalination, Water Res. 47 (2013)

52

Desalination xxx (xxxx) xxx–xxx

M. Asadollahi et al.

[220] S. Kasemset, A. Lee, D.J. Miller, B.D. Freeman, M.M. Sharma, Effect of polydopamine deposition conditions on fouling resistance, physical properties, and permeation properties of reverse osmosis membranes in oil/water separation, J. Membr. Sci. 425–426 (2013) 208–216. [221] B.D. McCloskey, H.B. Park, H. Ju, B.W. Rowe, D.J. Miller, B.D. Freeman, A bioinspired fouling-resistant surface modification for water purification membranes, J. Membr. Sci. 413–414 (2012) 82–90. [222] B.D. McCloskey, H.B. Park, H. Ju, B.W. Rowe, D.J. Miller, B.J. Chun, K. Kin, B.D. Freeman, Influence of polydopamine deposition conditions on pure water flux and foulant adhesion resistance of reverse osmosis, ultrafiltration, and microfiltration membranes, Polymer 51 (2010) 3472–3485. [223] D.J. Miller, X. Huang, H. Li, S. Kasemset, A. Lee, D. Agnihotri, T. Hayes, D.R. Paul, B.D. Freeman, Fouling-resistant membranes for the treatment of flowback water from hydraulic shale fracturing: a pilot study, J. Membr. Sci. 437 (2013) 265–275. [224] J. Liu, Y. Zhan, T. Xu, G. Shao, Preparation and characterizations of novel zwitterionic membranes, J. Membr. Sci. 325 (2008) 495–502. [225] S. Chen, J. Zheng, L. Li, S. Jiang, Strong resistance of phosphorylcholine selfassembled monolayers to protein adsorption: insights into nonfouling properties of zwitterionic materials, J. Am. Chem. Soc. 127 (2005) 14473–14478. [226] H. Kitano, T. Mori, Y. Takeuchi, S. Tada, M. Gemmei-Ide, Y. Yokoyama, M. Tanaka, Structure of water incorporated in sulfobetaine polymer films as studied by ATR-FTIR, Macromol. Biosci. 5 (2005) 314–321. [227] S. Chen, L. Li, C. Zhao, J. Zheng, Surface hydration: principles and applications toward low-fouling/nonfouling biomaterials, Polymer 51 (2010) 5283–5293. [228] F. Xuan, J. Liu, Preparation, characterization and application of zwitterionic polymers and membranes: current developments and perspective, Polym. Int. 58 (2009) 1350–1361. [229] Q. Li, J. Imbrogno, G. Belfort, X.–.L. Wang, Making polymeric membranes antifouling via “grafting from” polymerization of zwitterions, J. Appl. Polym. Sci. 132 (2015) (41781-1-41781-12). [230] H.Z. Shafi, Z. Khan, R. Yang, K.K. Gleason, Surface modification of reverse osmosis membranes with zwitterionic coating for improved resistance to fouling, Desalination 362 (2015) 93–103. [231] R. Yang, J. Xu, G.O. -Ince, S.Y. Wong, K.K. Gleason, Surface-tethered zwitterionic ultrathin antifouling coatings on reverse osmosis membranes by initiated chemical vapor deposition, Chem. Mater. 23 (2011) 1263–1272. [232] H. Choi, Y. Jung, S. Han, T. Tak, Y.–.N. Kwon, Surface modification of SWRO membranes using hydroxyl poly (oxyethylene) methacrylate and zwitterionic carboxylated polyethyleneimine, J. Membr. Sci. 486 (2015) 97–105. [233] H.Z. Shafi, A. Matin, Z. Khan, A. Khalil, K.K. Gleason, Surface modification of reverse osmosis membranes with zwitterionic coatings: a potential strategy for control of biofouling, Surf. Coat. Technol. 279 (2015) 171–179. [234] R. Yang, H. Jang, R. Stocker, K.K. Gleason, Synergistic prevention of biofouling in seawater desalination by zwitterionic surfaces and low-level chlorination, Adv. Mater. 26 (2014) 1711–1718. [235] R. Yang, A. Asatekin, K.K. Gleason, Design of conformal, substrate-independent surface modification for controlled protein adsorption by chemical vapor deposition (CVD), Soft Matter 8 (2012) 31–43. [236] A.M. Coclite, R.M. Howden, D.C. Borrelli, C.D. Petruczok, R. Yang, J.L. Yague, A. Ugur, N. Chen, S. Lee, W.J. Jo, A. Liu, X. Wang, K.K. Gleason, 25th anniversary article: CVD polymers: a new paradigm for surface modification and device fabrication, Adv. Mater. 25 (2013) 5392–5423. [237] S. Azari, L. Zou, Using zwitterionic amino acid l-DOPA to modify the surface of thin film composite polyamide reverse osmosis membranes to increase their fouling resistance, J. Membr. Sci. 401–402 (2012) 68–75. [238] A. Matin, Z. Khan, K.K. Gleason, M. Khaled, S.M.J. Zaidi, A. Khalil, P. Moni, R. Yang, Surface-modified reverse osmosis membranes applying a copolymer film to reduce adhesion of bacteria as a strategy for biofouling control, Sep. Purif. Technol. 124 (2014) 117–123. [239] G.O. -Ince, A. Matin, Z. Khan, S.M.J. Zaidi, K.K. Gleason, Surface modification of reverse osmosis desalination membranes by thin-film coatings deposited by initiated chemical vapor deposition, Thin Solid Films 539 (2013) 181–187. [240] A. Matin, H.Z. Shafi, Z. Khan, M. Khaled, R. Yang, K. Gleason, F. Rehman, Surface modification of seawater desalination reverse osmosis membranes: characterization studies & performance evaluation, Desalination 343 (2014) 128–139. [241] A. Matin, H. Shafi, M. Wang, Z. Khan, K. Gleason, F. Rahman, Reverse osmosis membranes surface-modified using an initiated chemical vapor deposition technique show resistance to alginate fouling under cross-flow conditions: filtration & subsequent characterization, Desalination 379 (2016) 108–117. [242] A. Matin, G.O. -Ince, Z. Khan, S.M.J. Zaidi, K. Gleason, D. Eggenspiler, Random copolymer films as potential antifouling coatings for reverse osmosis membranes, Desalin. Water Treat. 34 (2011) 100–105. [243] J. Nikkola, J. Sievänen, M. Raulio, J. Wei, J. Vuorinen, C.Y. Tang, Surface modification of thin film composite polyamide membrane using atomic layer deposition method, J. Membr. Sci. 450 (2014) 174–180. [244] C. Zhou, D. Ye, H. Jia, S. Yu, M. Liu, C. Gao, Surface mineralization of commercial thin-film composite polyamide membrane by depositing barium sulfate for improved reverse osmosis performance and antifouling property, Desalination 351 (2014) 228–235. [245] C. Wang, G.K. Such, A. Widjaya, H. Lomas, G. Stevens, F. Caruso, S.E. Kentish, Click poly (ethylene glycol) multilayers on RO membranes: fouling reduction and membrane characterization, J. Membr. Sci. 409–410 (2012) 9–15. [246] J.J. Richardson, M. Björnmalm, F. Caruso, Technology-driven layer-by-layer assembly of nanofilms, Science 348 (2015) (2491-1-2491-11). [247] T. Kouketsu, S. Duan, T. Kai, S. Kazama, K. Yamada, PAMAM dendrimer composite membrane for CO2 separation: formation of a chitosan gutter layer, J. Membr. Sci.

polyamide and coating layers, Desalination 242 (2009) 168–182. [193] M.C. Wilbert, J. Pellegrino, A. Zydney, Bench-scale testing of surfactant-modified reverse osmosis/nanofiltration membranes, Desalination 115 (1998) 15–32. [194] Y. Zhou, S. Yu, C. Gao, X. Feng, Surface modification of thin film composite polyamide membranes by electrostatic self deposition of polycations for improved fouling resistance, Sep. Purif. Technol. 66 (2009) 287–294. [195] J. Sun, L.–.P. Zhu, Z.–.H. Wang, F. Hu, P.–.B. Zhang, B.–.K. Zhu, Improved chlorine resistance of polyamide thin-film composite membranes with a terpolymer coating, Sep. Purif. Technol. 157 (2016) 112–119. [196] D.J. Miller, D.R. Dreyer, C.W. Bielawski, D.R. Paul, B.D. Freeman, Surface modification of water purification membranes: a review, Angew. Chem 56 (2017) 4662–4711. [197] J.S. Louie, I. Pinnau, M. Reinhard, Effects of surface coating process conditions on the water permeation and salt rejection properties of composite polyamide reverse osmosis membranes, J. Membr. Sci. 367 (2011) 249–255. [198] H. Choi, J. Park, T. Tak, Y.–.N. Kwon, Surface modification of seawater reverse osmosis (SWRO) membrane using methyl methacrylate-hydroxy poly (oxyethylene) methacrylate (MMA-HPOEM) comb-polymer and its performance, Desalination 291 (2012) 1–7. [199] J.M. Harris, Poly(Ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications, Plenum Press, New York, 1992. [200] G. Kang, M. Liu, B. Lin, Y. Cao, Q. Yuan, A novel method of surface modification on thin-film composite reverse osmosis membrane by grafting poly (ethylene glycol), Polymer 48 (2007) 1165–1170. [201] L. Zou, I. Vidalisa, D. Steelec, A. Michelmore, S.P. Low, J.Q.J.C. Verberk, Surface hydrophilic modification of RO membranes by plasma polymerization for low organic fouling, J. Membr. Sci. 369 (2011) 420–428. [202] Y. Kwon, S. Hong, H. Choi, J. Jung, J. Moon, T. Tak, Development of foulingresistant RO membranes using PEGA macromere, Desalin. Water Treat. 15 (2010) 54–61. [203] A.C. Sagle, E.M.V. Wagner, H. Ju, B.D. McCloskey, B.D. Freemana, M.M. Sharma, PEG-coated reverse osmosis membranes: desalination properties and fouling resistance, J. Membr. Sci. 340 (2009) 92–108. [204] J.S. Louie, I. Pinnau, I. Ciobanu, K.P. Ishida, A. Ng, M. Reinhard, Effects of polyether-polyamide block copolymer coating on performance and fouling of reverse osmosis membranes, J. Membr. Sci. 280 (2006) 762–770. [205] A. Roosjen, H.C. van der Mei, H.J. Busscher, W. Norde, Microbial adhesion to poly (ethylene oxide) brushes: influence of polymer chain length and temperature, Langmuir 20 (2004) 10949–10955. [206] J. Xu, X. Feng, C. Gao, Surface modification of thin-film-composite polyamide membranes for improved reverse osmosis performance, J. Membr. Sci. 370 (2011) 116–123. [207] S. Yu, G. Yao, B. Dong, H. Zhu, X. Peng, J. Liu, M. Liu, C. Gao, Improving fouling resistance of thin-film composite polyamide reverse osmosis membrane by coating natural hydrophilic polymer sericin, Sep. Purif. Technol. 118 (2013) 285–293. [208] L. Ni, J. Meng, X. Li, Y. Zhang, Surface coating on the polyamide TFC RO membrane for chlorine resistance and antifouling performance improvement, J. Membr. Sci. 451 (2014) 205–215. [209] M. Liu, Z. Chen, S. Yu, D. Wu, C. Gao, Thin-film composite polyamide reverse osmosis membranes with improved acid stability and chlorine resistance by coating N-isopropylacrylamide-co-acrylamide copolymers, Desalination 270 (2011) 248–257. [210] J. Nikkola, X. Liu, Y. Li, M. Raulio, H.–.L. Alakomi, J. Wei, C.Y. Tang, Surface modification of thin film composite RO membrane for enhanced anti-biofouling performance, J. Membr. Sci. 444 (2013) 192–200. [211] S. Yu, Z. Lü, Z. Chen, X. Liu, M. Liu, C. Gao, Surface modification of thin-film composite polyamide reverse osmosis membranes by coating N-isopropylacrylamide-co-acrylic acid copolymers for improved membrane properties, J. Membr. Sci. 371 (2011) 293–306. [212] D. Wu, X. Liu, S. Yu, M. Liu, C. Gao, Modification of aromatic polyamide thin-film composite reverse osmosis membranes by surface coating of thermo-responsive copolymers P (NIPAM-co-Am). I: preparation and characterization, J. Membr. Sci. 352 (2010) 76–85. [213] S. Yu, X. Liu, J. Liu, D. Wu, M. Liu, C. Gao, Surface modification of thin-film composite polyamide reverse osmosis membranes with thermo-responsive polymer (TRP) for improved fouling resistance and cleaning efficiency, Sep. Purif. Technol. 76 (2011) 283–291. [214] D. Saeki, T. Tanimoto, H. Matsuyama, Prevention of bacterial adhesion on polyamide reverse osmosis membranes via electrostatic interactions using a cationic phosphorylcholine polymer coating, Colloids Surf. A Physicochem. Eng. Asp. 443 (2014) 171–176. [215] Y.–.N. Kwon, S. Hong, H. Choi, T. Tak, Surface modification of a polyamide reverse osmosis membrane for chlorine resistance improvement, J. Membr. Sci. 415–416 (2012) 192–198. [216] H. Karkhanechi, R. Takagi, H. Matsuyama, Biofouling resistance of reverse osmosis membrane modified with polydopamine, Desalination 336 (2014) 87–96. [217] H. Karkhanechi, R. Takagi, H. Matsuyama, Enhanced antibiofouling of RO membranes via polydopamine coating and polyzwitterion immobilization, Desalination 337 (2014) 23–30. [218] M. Vaselbehagh, H. Karkhanechi, S. Mulyati, R. Takagi, H. Matsuyama, Improved antifouling of anion-exchange membrane by polydopamine coating in electrodialysis process, Desalination 332 (2014) 126–133. [219] S. Kasemset, Z. He, D.J. Miller, B.D. Freeman, M.M. Sharma, Effect of polydopamine deposition conditions on polysulfone ultrafiltration membrane properties and threshold flux during oil/water emulsion filtration, Polymer 97 (2016) 247–257.

53

Desalination xxx (xxxx) xxx–xxx

M. Asadollahi et al. 287 (2007) 51–59. [248] G.K. Such, J.F. Quinn, A. Quinn, E. Tjipto, F. Caruso, Assembly of ultrathin polymer films by click chemistry, J. Am. Chem. Soc. 128 (2006) 9318–9319. [249] A. Sarkar, P.I. Carver, T. Zhang, A. Merrington, K.J. Bruza, J.L. Rousseau, S.E. Keinath, P.R. Dvornic, Dendrimer-based coatings for surface modification of polyamide reverse osmosis membranes, J. Membr. Sci. 349 (2010) 421–428. [250] A. Sarkar, P.I. Carver, T. Zhang, A. Merrington, K.J. Bruza, J.L. Rousseau, S.E. Keinath, P.R. Dvornic, Surface modification of polyamide reverse osmosis membranes with hydrophilic dendritic polymers, in: A. Mueller, B. Guieysse, A. Sarkar (Eds.), New Membranes and Advanced Materials for Wastewater Treatment, ACS, 2009, pp. 187–207. [251] A. Bhattacharya, B.N. Misra, Grafting: a versatile means to modify polymers techniques, factors and applications, Prog. Polym. Sci. 29 (2004) 767–814. [252] M. Zhou, H. Liu, J.E. Kilduff, R. Langer, D.G. Anderson, G. Belfort, High throughput membrane surface modification to control NOM fouling, Environ. Sci. Technol. 43 (2009) 3865–3871. [253] S. Minko, Grafting on solid surfaces: “grafting to” and “grafting from” methods, in: M. Stamm (Ed.), Polymer Surfaces and Interfaces: Characterization, Modification and Applications, Springer, Berlin Heidelberg, 2008, pp. 215–234. [254] M. Ulbricht, Advanced functional polymer membranes, Polymer 47 (2006) 2217–2262. [255] M. Wang, J. Yuan, X. Huang, X. Cai, L. Li, J. Shen, Grafting of carboxybetaine brush onto cellulose membranes via surface-initiated ARGET-ATRP for improving blood compatibility, Colloids Surf. B: Biointerfaces 103 (2013) 52–58. [256] J. Xu, Z. Wang, J. Wang, S. Wang, Positively charged aromatic polyamide reverse osmosis membrane with high anti-fouling property prepared by polyethylenimine grafting, Desalination 365 (2015) 398–406. [257] G. Kang, H. Yu, Z. Liu, Y. Cao, Surface modification of a commercial thin film composite polyamide reverse osmosis membrane by carbodiimide-induced grafting with poly (ethylene glycol) derivatives, Desalination 275 (2011) 252–259. [258] J. Wu, Z. Wang, Y. Wang, W. Yan, J. Wang, S. Wang, Polyvinylamine-grafted polyamide reverse osmosis membrane with improved antifouling property, J. Membr. Sci. 495 (2015) 1–13. [259] D. Saeki, S. Nagao, I. Sawada, Y. Ohmukai, T. Maruyama, H. Matsuyama, Development of antibacterial polyamide reverse osmosis membrane modified with a covalently immobilized enzyme, J. Membr. Sci. 428 (2013) 403–409. [260] J. Xu, Z. Wang, L. Yu, J. Wang, S. Wang, A novel reverse osmosis membrane with regenerable anti-biofouling and chlorine resistant properties, J. Membr. Sci. 435 (2013) 80–91. [261] E.M.V. Wagner, A.C. Sagle, M.M. Sharma, Y.–.H. La, B.D. Freeman, Surface modification of commercial polyamide desalination membranes using poly (ethylene glycol) diglycidyl ether to enhance membrane fouling resistance, J. Membr. Sci. 367 (2011) 273–287. [262] Y. Hu, K. Lu, F. Yan, Y. Shi, P. Yu, S. Yu, S. Li, C. Gao, Enhancing the performance of aromatic polyamide reverse osmosis membrane by surface modification via covalent attachment of polyvinyl alcohol (PVA), J. Membr. Sci. 501 (2016) 209–219. [263] S. Azari, L. Zou, Fouling resistant zwitterionic surface modification of reverse osmosis membranes using amino acid L-cysteine, Desalination 324 (2013) 79–86. [264] X. Wei, Z. Wang, J. Chen, J. Wang, S. Wang, A novel method of surface modification on thin-film-composite reverse osmosis membrane by grafting hydantoin derivative, J. Membr. Sci. 346 (2010) 152–162. [265] D. Nikolaeva, C. Langner, A. Ghanem, M.A. Rehimd, B. Voit, J.M. –Haack, Hydrogel surface modification of reverse osmosis membranes, J. Membr. Sci. 476 (2015) 264–276. [266] T. Zhang, C. Zhu, H. Ma, R. Li, B. Dong, Y. Liu, S. Li, Surface modification of APATFC membrane with quaternary ammonium cation and salicylaldehyde to improve performance, J. Membr. Sci. 457 (2014) 88–94. [267] K. Gao, L.T. Kearney, R. Wang, J.A. Howarter, Enhanced wettability and transport control of ultrafiltration and reverse osmosis membranes with grafted polyelectrolytes, Appl. Mater. Interfaces 7 (2015) 24839–24847. [268] S. Lin, H. Huang, Y. Zeng, L. Zhang, L. Hou, Facile surface modification by aldehydes to enhance chlorine resistance of polyamide thin film composite membranes, J. Membr. Sci. 518 (2016) 40–49. [269] S. Belfer, Y. Purinson, R. Fainshtein, Y. Radchenko, O. Kedem, Surface modification of commercial composite polyamide reverse osmosis membranes, J. Membr. Sci. 139 (1998) 175–181. [270] J. Gilron, S. Belfer, P. Väisänen, M. Nyström, Effects of surface modification on antifouling and performance properties of reverse osmosis membranes, Desalination 140 (2001) 167–179. [271] S. Belfer, Y. Purinson, O. Kedem, Surface modification of commercial polyamide reverse osmosis membranes by radical grafting: an ATR-FTIR study, Acta Polym. 49 (1998) 574–582. [272] S. Belfer, J. Gilron, Y. Purinson, R. Fainshtain, N. Daltrophe, M. Priel, B. Tenzer, A. Toma, Effect of surface modification in preventing fouling of commercial SWRO membranes at the Eilat seawater desalination pilot plant, Desalination 139 (2001) 169–176. [273] A.V.R. Reddy, J.J. Trivedi, C.V. Devmurari, D.J. Mohan, P. Singh, A.P. Rao, S.V. Joshi, P.K. Ghosh, Fouling resistant membranes in desalination and water recovery, Desalination 183 (2005) 301–306. [274] M. Liu, Q. Chen, L. Wang, S. Yu, C. Gao, Improving fouling resistance and chlorine

[275]

[276]

[277]

[278]

[279] [280]

[281]

[282]

[283] [284] [285] [286] [287] [288]

[289]

[290]

[291]

[292]

[293] [294]

[295]

[296]

[297] [298] [299]

[300]

[301]

[302]

54

stability of aromatic polyamide thin-film composite RO membrane by surface grafting of polyvinyl alcohol (PVA), Desalination 367 (2015) 11–20. R. Bernsteina, S. Belfera, V. Fregerc, Improving performance of spiral wound RO elements by in situ concentration polarization-enhanced radical graft polymerization, J. Membr. Sci. 405–406 (2012) 79–84. J. Meng, Z. Cao, L. Ni, Y. Zhang, X. Wang, X. Zhang, E. Liu, A novel salt-responsive TFC RO membrane having superior antifouling and easy-cleaning properties, J. Membr. Sci. 461 (2014) 123–129. J. Wang, Z. Wang, J. Wang, S. Wang, Improving the water flux and bio-fouling resistance of reverse osmosis (RO) membrane through surface modification by zwitterionic polymer, J. Membr. Sci. 493 (2015) 188–199. R. Bernstein, S. Belfer, V. Freger, Bacterial attachment to RO membranes surfacemodified by concentration-polarization-enhanced graft polymerization, Environ. Sci. Technol. 45 (2011) 5973–5980. A.S. Sarac, Redox polymerization, Prog. Polym. Sci. 24 (1999) 1149–1204. X. Wei, Z. Wang, Z. Zhang, J. Wang, S. Wang, Surface modification of commercial aromatic polyamide reverse osmosis membranes by graft polymerization of 3allyl-5, 5-dimethylhydantoin, J. Membr. Sci. 351 (2010) 222–233. Z. Zhang, Z. Wang, J. Wang, S. Wang, Enhancing chlorine resistances and antibiofouling properties of commercial aromatic polyamide reverse osmosis membranes by grafting 3-allyl-5,5-dimethylhydantoin and N, N′-Methylenebis(acrylamide), Desalination 309 (2013) 187–196. H.H. Rana, N.K. Saha, S.K. Jewrajka, A.V.R. Reddy, Low fouling and improved chlorine resistant thin film composite reverse osmosis membranes by cerium (IV)/ polyvinyl alcohol mediated surface modification, Desalination 357 (2015) 93–103. N. Kim, D.H. Shin, Y.T. Lee, Effect of silane coupling agents on the performance of RO membranes, J. Membr. Sci. 300 (2007) 224–231. D.H. Shin, N. Kim, Y.T. Lee, Modification to the polyamide TFC RO membranes for improvement of chlorine-resistance, J. Membr. Sci. 376 (2011) 302–311. K.Y. Jee, D.H. Shin, Y.T. Lee, Surface modification of polyamide RO membrane for improved fouling resistance, Desalination 394 (2016) 131–137. K. Matyjaszewski, Atom transfer radical polymerization (ATRP): current status and future perspectives, Macromolecules 45 (2012) 4015–4039. K. Matyjaszewski, J. Xia, Atom transfer radical polymerization, Chem. Rev. 101 (2001) 2921–2990. J. Ran, L. Wu, Z. Zhang, T. Xu, Atom transfer radical polymerization (ATRP): a versatile and forceful tool for functional membranes, Prog. Polym. Sci. 39 (2014) 124–144. D. Saeki, T. Tanimoto, H. Matsuyama, Anti-biofouling of polyamide reverse osmosis membranes using phosphorylcholine polymer grafted by surface-initiated atom transfer radical polymerization, Desalination 350 (2014) 21–27. M.G. -Markovic, T.G. Barclay, K.T. Constantopoulos, E. Markovic, S.R. Clarke, J.G. Matisons, Biofouling resistance of polysulfobetaine coated reverse osmosis membranes, Desalination 369 (2015) 37–45. A.J. Blok, R. Chhasatia, J. Dilag, A.V. Ellis, Surface initiated polydopamine grafted poly ([2-(methacryoyloxy) ethyl] trimethylammonium chloride) coatings to produce reverse osmosis desalination membranes with anti-biofouling properties, J. Membr. Sci. 468 (2014) 216–223. Y. Zhang, Z. Wang, W. Lin, H. Sun, L. Wu, S. Chen, A facile method for polyamide membrane modification by poly (sulfobetaine methacrylate) to improve fouling resistance, J. Membr. Sci. 446 (2013) 164–170. K.J. Varin, N.H. Lin, Y. Cohen, Biofouling and cleaning effectiveness of surface nanostructured reverse osmosis membranes, J. Membr. Sci. 446 (2013) 472–481. M. Kim, N.H. Lin, G.T. Lewis, Y. Cohen, Surface nano-structuring of reverse osmosis membranes via atmospheric pressure plasma-induced graft polymerization for reduction of mineral scaling propensity, J. Membr. Sci. 354 (2010) 142–149. N.H. Lin, M. Kim, G.T. Lewis, Y. Cohen, Polymer surface nano-structuring of reverse osmosis membranes for fouling resistance and improved flux performance, J. Mater. Chem. 20 (2010) 4642–4652. J. Mansouri, S. Harrison, V. Chen, Strategies for controlling biofouling in membrane filtration systems: challenges and opportunities, J. Mater. Chem. 20 (2010) 4567–4586. A. Kulkarni, D. Mukherjee, W.N. Gill, Flux enhancement by hydrophilization of thin film composite reverse osmosis membranes, J. Membr. Sci. 114 (1996) 39–50. D. Mukherjee, A. Kulkarni, W.N. Gill, Flux enhancement of reverse osmosis membranes by chemical surface modification, J. Membr. Sci. 97 (1994) 231–249. M.L.M. Tirado, M. Bass, M. Piatkovsky, M. Ulbricht, M. Herzberg, V. Freger, Assessing biofouling resistance of a polyamide reverse osmosis membrane surfacemodified with a zwitterionic polymer, J. Membr. Sci. 520 (2016) 490–498. B.T. McVerry, M.C.Y. Wong, K.L. Marsh, J.A.T. Temple, C.M. -Jones, E.M.V. Hoek, R.B. Kaner, Scalable antifouling reverse osmosis membranes utilizing perfluorophenyl azide photochemistry, Macromol. Rapid Commun. 35 (2014) 1528–1533. T.H.A. Ngo, D.T. Tran, C.H. Dinh, Surface photochemical graft polymerization of acrylic acid onto polyamide thin film composite membranes, J. Appl. Polym. Sci. 133 (2016) (44418-1-44418-9). N. Combernoux, V. Labed, L. Schrive, Y. Wyart, E. Carretier, P. Moulin, Effect of gamma irradiation at intermediate doses on the performance of reverse osmosis membranes, Radiat. Phys. Chem. 124 (2016) 241–245.