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Review
10 (
2_suppl
); S1821-S1834
doi:
10.1016/j.arabjc.2013.07.009

Alteration of polyethersulphone membranes through UV-induced modification using various materials: A brief review

, ,
 Research Centre for Sustainable Process Technology (CESPRO), Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, University Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia

⁎Corresponding author. Tel.: +60 3 8921 6410; fax: +60 3 8921 6148. drawm67@gmail.com (Abdul Wahab Mohammad) wahabm@eng.ukm.my (Abdul Wahab Mohammad)

Received: , Accepted: ,
© The Author(s) 2013
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Polyethersulphone (PES) membranes have been widely applied in various separation applications such as microfiltration, ultrafiltration and nanofiltration. This has occurred as these membranes are easy to form, have good mechanical strength and good chemical stability (resistant to acidic or alkaline conditions) due to the presence of aromatic hydrocarbon groups in the structure. PES membranes are commonly fabricated through the phase inversion method due to the simplicity of the process. However, PES membranes are generally hydrophobic, which usually requires them to be modified before application. In most cases, these methods can reduce the hydrophobicity of the membrane surface and thus reduce membrane fouling during application. This review will further discuss the recently developed UV-induced modifications of PES membranes. The UV-induced grafting method is easy to apply to existing PES membranes, with or without the need for a photo-initiator. Additionally, nanoparticles entrapped in PES membranes subsequently exposed to UV-irradiation have been reported to possess photo-catalytic activity. However, UV-irradiation methods still require special care in order to produce membranes with the best performance.

Keywords

Nanoparticle
UV irradiation
Monomers
UV-grafting
Hydrophilicity
Surface modification
1

1 Introduction

In the last few decades, with increasing population and urbanisation, the huge amount of wastewater produced has become a great concern for many countries. These concerns need a more efficient and high quality technique for the treatment of wastewater. Polymeric membranes are the most recognised materials used in various separation processes (Robeson, 2012) due to their effective separation performance and lower production or maintenance costs in comparison with inorganic materials such as ceramic and metallic-based membranes. Types of well-known polymeric membrane materials used and investigated in depth include polysulphone (Abu-Thabit et al., 2010; Homayoonfal et al., 2010; Li et al., 2012; Namvar et al., 2013; Prakash et al., 2008; Shah and Murthy, 2013; Sueyoshi et al., 2012; Yoo et al., 2003), cellulose nitrate (Shieh and Chung, 2000; Soylak and Cay, 2007; Sun et al., 2007), polyvinyl chloride (Hu et al., 2011; Wenjuan et al., 2011; Xia et al., 2010, 2011; Zhang et al., 2009), polyvinylidene fluoride (De Gusseme et al., 2011; Feng et al., 2008; Puspitasari et al., 2010; Souzy et al., 2012; Venault et al., 2012b), polyvinyl alcohol (Han et al., 2008; Liu et al., 2013b; Singha et al., 2009; Wu et al., 2006; Zhang et al., 2006), cellulose acetate (Lv et al., 2007; Sossna et al., 2007; Zavastin et al., 2010), polyethersulphone (Cao et al., 2010; Fang et al., 2009; Qian et al., 2009; Rahimpour and Madaeni, 2010; Susanto et al., 2009; Zhao et al., 2011), and so forth. Most of the time, these polymeric materials are suitable to be applied in pressure-driven applications, for instance reverse osmosis, nanofiltration, ultrafiltration, microfiltration, dialysis and in some circumstances pervaporation.

To be used for long-term pressure-driven liquid-based separation processes, polymeric membrane materials should possess properties such as excellent mechanical or tensile strength, good anti-fouling resistance, high selectivity, high permeability and good control of the pore size distribution over the entire membrane surface area (Akar et al., 2013). This reduces the production and maintenance costs over the long term (Strathmann, 2001) and ensures sustainability. Thus, researchers have been working on modifying and improving various polymeric membranes for better tensile strength(Lin et al., 2009; Seol et al., 2007), better fouling resistance (Li et al., 2009; Liu et al., 2009; Mansourpanah et al., 2009; Peeva et al., 2010), higher rejection or selectivity (Ben-David et al., 2010; Cano-Odena et al., 2011; Li et al., 2007; Malaisamy et al., 2011) and higher flux or permeability (Shen et al., 2011a; Susanto et al., 2009; Yu et al., 2010; Zou et al., 2011). Polymeric membranes have some physical and chemical disadvantages in comparison to inorganic materials such as low resistance towards oxidising agents and pH-dependent performance (Van Wagner et al., 2009; Wang et al., 2011; Zhai et al., 2003), and they usually cannot withstand long-term exposure to a high temperature which causes deterioration of the polymeric structures. Polymeric membranes which are resistant to acids (Tanninen et al., 2004), bases, solvents (Vandezande et al., 2008), oxidising agents (such as hydrogen peroxide, chlorine (Glater et al., 1994), fluorine, and so on), pressure-induced compaction, and extreme temperature (Deligöz and Yilmazoglu, 2011; Jun et al., 2011) could be an added benefit and advantage for polymeric membranes to be well-commercialised and applied in various fields.

Polyethersulphone (PES) membranes have been focused on in this review due to their recently increased popularity in the polymeric membrane research field. PES membranes have been fabricated, applied and modified for microfiltration, ultrafiltration and nanofiltration purposes.

In microfiltration (MF) application, researchers employed gamma-ray irradiation during the grafting of PES powder with acrylic acid (Deng et al., 2008). They prepared the membranes using the modified PES powder with different degrees of grafting (DG) through phase inversion method. They confirmed that the water permeability and porosity of the produced MF membranes increased with an increase in DG. Besides, they also reported that the properties of produced MF membranes (using modified PES powder) were highly dependent on the pH value of the aqueous solution. The increased degrees of swelling and decreased flux were reasoned with enhanced ionisation of grafted polyacrylic acid side chains in elevated pH value of aqueous solution. PES MF membranes were also produced using N-methyl-2-pyrrolidone (NMP) and 2-methoxy ethanol (2-ME) non-solvent as an additive in the casting solution (Shin et al., 2005). In this specific work, vapour-induced phase inversion and non-solvent induced phase inversion were employed during the membrane preparation. The non-solvent additive and the exposure time at the relative humidity of 74% were found to have significant effects towards the membrane porosity. High performance PES MF membranes with a high flux and stable hydrophilic property were also produced using induced phase separation coupled with non-solvent induced phase separation method (Susanto et al., 2009). It was observed that the non-solvent content (tri-ethylene glycol) and the exposure time were important to obtain high-flux membranes, whereas the Pluronic was important in improving the membrane surface hydrophilicity.

To be used in the UF process, PES membranes were produced using phase inversion process and casting solution containing PES, N,N-dimethylformamide (DMF) as solvent and polyethylene glycol (PEG) of different molecular weights (Idris et al., 2007). It was observed that higher molecular weight of PEG can increase the membrane porosity and pure water permeability. Besides, PEG concentration in the casting solution was found to have significant effect towards the membrane solution flux and solute separation capability. PES UF membranes were also fabricated using various kinds of solvents such as dimethyl sulphoxide (DMSO), NMP, and DMF (Arthanareeswaran and Starov, 2011). Researchers observed that the membrane pure water permeability produced using solvent DMSO was the highest, followed by NMP and DMF. Researchers claimed that the formation of PES membrane active separation layer was due to the thermodynamic properties of the mixture and membrane formation kinetics. Besides, the diffusion rate of the solvent and non-solvent can determine the structures of macro-porous bottom sub-layer of the PES membranes (Lau et al., 2013). Researchers also produced PES-silver UF membranes through phase inversion method using polyvinylpyrollidone as dispersant in the casting solution (Basri et al., 2011). The distribution and entrapment of the silver particles in the membrane structure contributed to good anti-bacterial property and showed minimum silver loss during operation. PES UF membranes were also improved using acrylonitrile and acrylic acid, followed by membrane surface grafting using bovine serum albumin (Fang et al., 2009). The PES membrane produced using this modification technique has good blood compatibility while having increased water flux. Instead of flat-sheet membranes, PES hollow-fibre membranes were prepared using dry-wet spinning method which was followed by heat-treatment process (Gholami et al., 2003). The heat-treated PES hollow-fibre membranes were found to have increased solute separation without visible changes in the hollow-fibre dimension.

Besides MF and UF, PES membranes also have been widely produced purposely for NF applications. PES NF membranes were previously produced using various amounts of polyvinylpyrrolidone (Ismail and Hassan, 2007). Researchers successfully increased the membrane fluxes in the presence of additive but also recorded a decline in the salt rejection capability. The presence of the additive also contributed to a reduction in the effective membrane thickness. PES NF membranes were also produced by blending PES and polyimide in DMF solvent in the presence of self-produced modifiers (Mansourpanah et al., 2010). In this work, researchers claimed that the solution flux decline was reduced when the self-produced modifier was introduced into the polymeric membrane structure. Researchers also tried to use PES NF membranes to remove the ammonia–nitrogen from the aquaculture system (Ali et al., 2010). The PES NF membranes were produced using various shear rates during the membrane casting stage. The best shear rate in terms of highest flux and rejection ability was determined to be 200 s−1 with 68% of the ammonia–nitrogen removal. To understand the salt rejection property in PES NF membranes, researchers also conducted zeta-potential measurements using single salt solutions (Na2SO4 and KCl) at different pHs and concentrations (Ismail and Hassan, 2007). They observed that when the membrane was used at its iso-electric point, there is no preferential adsorption of chloride ions or potassium ions when KCl solution was used. Thus, they postulated that in this case only the dissociation of sulphonic acids from the PES membrane surface will affect the membrane electrostatic properties.

The PES molecular structure which consists of two benzene rings connected by alternate sulphonyl (SO2) groups and ether (–O–) linkages, contributed to the greater chemical stability of these compounds (Guan et al., 2005; Richard Bowen et al., 2001). Chemical stability means that common PES membranes can be continuously exposed to solutions of pH 1–13, which is an added advantage in cleaning processes. The existence of aromatic groups might also improve the physical properties of PES membranes (Guan et al., 2005; Li et al., 2002).

2

2 Common PES membrane fabrication methods

PES-based membranes, which are usually produced through the phase inversion technique, involve controlled conversion of a polymeric solution from the liquid phase into the solid phase. PES membranes are commonly fabricated according to several steps as shown in Fig. 1. In the first step of PES membrane preparation, a solvent is commonly used, such as N-methyl-2-pyrrolidone (NMP) (Maximous et al., 2009) or dimethylacetamide (DMAC) (Liu et al., 2013a; Rahimpour et al., 2010b, c; Razmjou et al., 2011), which is heated and stirred continuously until the solution reaches a homogeneous temperature. In the next step, PES beads are added in slowly until all the beads are completely dissolved in the solvent. The homogeneous solution is left for a few hours in order to completely release any bubbles, commonly designated as the degassing process. Next, the casting of the PES membranes is done using a casting machine or hand casting with a predetermined thickness. Casting of the membrane is followed by immersing the PES polymeric solution into gelation or coagulation media. The coagulation bath is usually composed of water as the major component, but sometimes a combination of few components is used. For instance, a coagulation bath may use a combination of isopropanol (30%) and water (70%) or a mixture of water/isopropanol/acrylic acid or 2-hydroxyethylmethacrylate (Rahimpour et al., 2010c). Normally, PES membranes should remain in the gelation media or water bath to facilitate the removal of residual solvent. The final stage in PES membrane fabrication is membrane storage. Membranes are usually pressed between sheets of filter paper at ambient temperature during the drying process (Rahimpour et al., 2010b, c) or immersed in water with the addition of formaldehyde (or sometimes methanol) to prevent bacterial growth.

Common PES fabrication steps.
Figure 1 Common PES fabrication steps.

Based on the PES membrane fabrication method as described above, alteration or adjustment of the PES membranes can be easily done through modification of the chemical composition or fabrication conditions during the membrane fabrication process. Additionally, alterations or adjustments of PES-based membranes can also be done after the formation of the membranes, which usually involves surface modification. Common surface modification methods included surface coating, surface grafting or surface chemical reactions (Venault et al., 2012a).

3

3 PES membrane modification methods applied in previous studies

PES has turned out to be an important polymeric material (Villaverde-de-Sáa et al., 2012) in membrane fabrication. However, the hydrophobicity of PES membranes contributes to lower membrane performance and leads to poor anti-fouling properties and lower membrane permeability, which seriously limit the lifespan and applications of PES membranes. Surface modification methods of PES membranes, whether chemical or physical, in order to reduce membrane surface hydrophobicity, have become the focus of research in recent years. One of the membrane surface modification methods introduced was through the sulphonation reaction (Klaysom et al., 2011). This method has successfully introduced the negatively charged sulphonic acid group onto the PES membrane surface through the sulphonation reaction. The molecular structures of the non-modified and sulphonated PES are displayed in Fig. 2. Sulphonation is a reaction where the cross-linking between polymeric membranes (usually polysulphone or PES) occurs with concentrated sulphuric or sulphonic-based acids. PES membranes generated by sulphonation are more hydrophilic, have better selectivity and are resistant to fouling (Baroña et al., 2007; Klaysom et al., 2011; Lau and Ismail, 2009; Rahimpour et al., 2010a).

Molecular structures of (a) non-modified PES structure and (b) sulphonated PES structure (Adapted from (Klaysom et al., 2011) with permission).
Figure 2 Molecular structures of (a) non-modified PES structure and (b) sulphonated PES structure (Adapted from (Klaysom et al., 2011) with permission).

Besides, another chemical modification method introduced to modify the PES membrane surface was through carboxylation (Wang et al., 2011). Carboxylation is a practice where a carboxyl group is introduced in the polymeric membrane matrix (Sajitha et al., 2002; Wang et al., 2011). In PES, the carboxyl group would become a substituent to replace the hydrogen atom at an aromatic hydrocarbon. In the carboxylation method, the PES molecules can be acetylated and oxidised at specific temperatures as described in Fig. 3. Another chemical modification method which is similar to carboxylation and sulphonation is the nitration process. Similarly, the nitration process is used to introduce amine functional groups to polymeric membranes to modify the membrane material (Botvay et al., 1999; Conceição et al., 2009).

PES carboxylation (Adapted from (Wang et al., 2011) with permission).
Figure 3 PES carboxylation (Adapted from (Wang et al., 2011) with permission).

Another method which has been applied in the PES membrane surface modification is plasma treatment. In membrane surface application, plasma treatment implies the alteration of membrane performance by plasma which is produced after ionisation of water vapour or gases (argon, carbon dioxide, oxygen, nitrogen, hydrogen gas and so on) through electrical discharge at an elevated frequency (Gröning et al., 1996; Kull et al., 2005; Pal et al., 2008ab; Steen et al., 2002; Tyszler et al., 2006; Wavhal and Fisher, 2002). Plasma would then be allowed to flow perpendicularly along the membrane surface.

Incorporation of the additives into the PES membrane matrices offers another efficient way to improve the membrane characteristics (Arthanareeswaran et al., 2010; Basri et al., 2010, 2011; Maximous et al., 2010; Ng et al., 2011, 2013; Rahimpour et al., 2008a; Shen et al., 2011b; Yuliwati and Ismail, 2011; Zhao et al., 2008). Incorporation of additives into polymeric matrices could alter the porosity and hydrophilicity of polymeric membranes. Additives added into the polymeric membrane matrix could be inorganic nanoparticles such as silica (Huang et al., 2012), titanium dioxide (Ngang et al., 2012), silver (Diagne et al., 2012; Zhang et al., 2012), zirconium dioxide, selenium and copper (Akar et al., 2013). Besides, polymeric materials also have been added as blended materials in polymer-based membranes such as Pluronic, polyethylene glycol, polyvinylpyrrolidone, and so on.

Research has been carried on in search of a better membrane modification method to improve the PES-based membranes in the past few years (Zhao et al., 2013). Amongst all the modification methods suggested, UV-induced modification of PES membranes has attracted much attention in the recent years due to several reasons which will be discussed in the following section.

4

4 UV-induced modification of PES membranes

Another familiar membrane alteration method uses UV radiation, which has acquired popularity due to the ease of the modification process (Peeva et al., 2012). UV radiation also has been used in UV photo-grafting (Rahimpour, 2011) modifications. UV-induced modification has been applied in many recent studies (Bilongo et al., 2010; Homayoonfal et al., 2010; Yu et al., 2010; Zhou et al., 2010). Details on UV-induced modification and the effects on membrane performance will be discussed in the following sections.

The reasons for performing membrane modification varied and depend on the application and suitability in certain fields. Generally, membrane modification often improves certain properties of the membrane, but these are compensated for with a reduction in certain aspects of membrane performance. Thus, the discussion here focuses more on how the UV-induced modification variables affect the performance of modified PES membranes. It is worthwhile to build on previous research to achieve further improvements in modification processes in the future.

In general, UV-induced modification can be divided into two categories: UV-grafting using monomers and UV-induced modification using nanoparticles. The following discussion focuses on how other researchers have modified PES membranes using UV irradiation, some of the improvements achieved through this method and significant observations which may be useful for future investigations. PES has been focused on in this review as previous research has revealed that PES is easier to modify through UV irradiation (Kaeselev et al., 2001). PES is much more sensitive towards the UV source in comparison to the other polymeric membranes such as polysulphone membranes in UV-induced polymerisation and PES can achieve the desired degree of grafting with less energy usage.

4.1

4.1 UV-grafting using monomers

In a recent paper (Rahimpour, 2011), a variety of monomers was used (which are considered hydrophilic) with various weight percentages: 2-hydroxyethylmethacrylate (HEMA) and acrylic acid (AA) as monomers of acrylic, and ethylene diamine (EDA) and 1,3-phenylenediamine (mPDA) as amino monomers for the alteration of PES membrane surfaces (Mw = 58,000 g/mol). During the modification process, the PES membranes were immersed briefly into the amino and acrylic monomer solutions. Next, the membranes immersed in the monomer solution were irradiated by a UV source. The molecular structures of the applied hydrophilic monomers are shown in Fig. 4. PES membranes were generated by the phase immersion procedure. The doping solution was made by adding PES beads to the solvent in the presence of a pore forming agent (in this specific research, this was polyvinylpyrrolidone). The coagulation bath mixture was a combination of 80% water and 20% 2-propanol by volume. The PES membranes had been customised by coating the membrane surface by a dipping procedure. The membranes were then irradiated by a UV source. In the membrane surface coating process, the membranes were immersed briefly (half an hour) in the amino and acrylic monomer solutions (5 wt.% EDA, 1 and 6 wt.% AA, HEMA and mPDA). The immersed membranes were positioned in a Plexiglas (PMMA) tube and irradiated by a 160 W UV source (wavelength 259 nm, intensity 24.3 mW/cm2) for 5 min. The membranes that had been exposed to UV radiation were rinsed with clean water for the removal of excess or unused hydrophilic monomers. Subsequent to rinsing, the drying process of the as-generated or modified PES membranes was done at ambient temperature. In order to determine the degree of modification (abbreviated as DM), Eq. (1) was used:

(1)
DM ( % ) = ( W 2 - W 1 ) / W 1 where the weights of the membrane prior and subsequent to the grafting process are W1 and W2, respectively.
Molecular structure of monomers applied in PES membrane modification (Adapted from (Rahimpour, 2011) with permission).
Figure 4 Molecular structure of monomers applied in PES membrane modification (Adapted from (Rahimpour, 2011) with permission).

It has been reported that the hydrophobicity on the modified PES membrane surface is reduced when exposed to UV-irradiation due to the existence of hydrophilic monomers (Rahimpour, 2011). However, the unchanged PES membrane surface displayed the highest hydrophobicity. In particular, the HEMA (6 wt.%) customised membrane showed the best or lowest hydrophobicity amongst the customised membranes. This study summarised that the reduction in the hydrophobicity of customised membranes irradiated by UV and grafted with amino and acrylic monomers can be used to demonstrate the polymerisation of the monomers with the PES membrane surface. The grafting of these monomers through UV irradiation increased the hydrophilicity of the membrane surface. This experiment used AFM analysis to distinguish those PES membrane surfaces with and without modification. The captured AFM images of the membrane surface are shown in Fig. 5. The AFM scan area used in this study was 10 × 10 μm. Rougher surfaces were observed for the PES membranes polymerised with the hydrophilic monomers (refer Table 1 for surface roughness values).

AFM images of the membrane surface for (a) the unmodified PES membrane, (b) PES membrane grafted with AA (6 wt.%), (c) PES membrane grafted with HEMA (6 wt.%), (d) PES membrane grafted with mPDA (6 wt.%) and (e) PES membrane grafted with EDA (5 wt.%) (Adapted from (Rahimpour, 2011) with permission).
Figure 5 AFM images of the membrane surface for (a) the unmodified PES membrane, (b) PES membrane grafted with AA (6 wt.%), (c) PES membrane grafted with HEMA (6 wt.%), (d) PES membrane grafted with mPDA (6 wt.%) and (e) PES membrane grafted with EDA (5 wt.%) (Adapted from (Rahimpour, 2011) with permission).
Table 1 Surface roughness recorded for unmodified membrane and membranes modified with hydrophilic monomers.
Membrane Roughness (nm)
Sa Sq Sz
Unmodified 2.64 3.34 22.6
Modified with:
1 wt.% AA – – –
6 wt.% AA 38 50.4 328
1 wt.% HEMA – – –
6 wt.% HEMA 87.9 113 747
1 wt.% mPDA – – –
6 wt.% mPDA 70.8 87.4 545
5 wt.% EDA 40.7 51.9 398

Surface roughness parameters of the membranes are expressed in terms of:

Mean roughness (Sa),

Root mean square of the Z data (Sq), and

Mean different between the five highest peaks and lowest valleys (Sz).

(Adapted from (Rahimpour, 2011) with permission).

To examine the consequences of the polymerisation of monomers onto the PES membrane through UV-irradiation, non-skim milk and pure water filtration were done using the original and customised PES membranes (Rahimpour, 2011). The PES membranes showed a reduction in pure water flux for any concentration of the monomers polymerised onto the membrane surface by UV-irradiation. An elevated concentration of monomers produced membranes with a greater decline in pure water flux. The explanation given was that the monomer chains polymerised onto the PES membranes formed narrower pores (a higher monomer concentration produced narrower surface mean pore sizes (Rahimpour, 2011). Based on Fig. 6, the photo-grafted PES membrane produced lower milk water fluxes. It was obvious that an increase in monomer concentration led to lower milk-water fluxes. It has been proposed that membranes grafted with monomers and with higher DM values produce membranes with reduced milk-water and pure water fluxes.

Milk water fluxes produced by a PES membrane following surface polymerisation with hydrophilic monomers (Adapted from (Rahimpour, 2011) with permission).
Figure 6 Milk water fluxes produced by a PES membrane following surface polymerisation with hydrophilic monomers (Adapted from (Rahimpour, 2011) with permission).

Monomer-grafted PES membranes had better milk protein rejection as shown in Fig. 7 (Rahimpour, 2011). Additionally, a higher monomer concentration led to higher rejection. This was explained by the observation that the reduction in pore size increased protein rejection.

Milk protein rejections by PES membranes following surface polymerisation with hydrophilic monomers (Adapted from (Rahimpour, 2011) with permission).
Figure 7 Milk protein rejections by PES membranes following surface polymerisation with hydrophilic monomers (Adapted from (Rahimpour, 2011) with permission).

In 2002, a similar research was used to investigate the consequences of UV-irradiation on PES with and without any modifying agents (Kaeselev et al., 2002). The modifying agents used were 2-acrylamidoglycolic acid monohydrate (AAG), N-vinyl-2-pyrrolidine (NVP) and 2-acrylamido-2-methyl-1-propanesulphonic acid (AAP). Characterisation using FTIR and XPS was performed for the determination of grafting in this study. AAG showed a better grafting tendency with the PES membrane compared to AAP or NVP. The PES ultrafiltration membranes were also tested for separation performance using a dextran solution. Without any monomers, the irradiated PES membrane showed reduced solution flux when the energy of irradiation was 200 mJ/cm2. However, the highest rejection of dextran was achieved when the UV irradiation energy in the absence of monomers was 200 mJ/cm2. However, when the intensity was increased from 200 to 300 mJ/cm2, dextran solution fluxes were found to be increased along with a drastic reduction in dextran rejection. This confirmed that a deterioration of membrane separation performance occurred when excess UV energy was used in the membrane modification. This could provide useful information for researchers when choosing their UV irradiation energy range during the membrane modification process. Modification of membranes with the addition of monomers indicated an increase in the separation performance. However, all membranes displayed a drastic reduction in solution fluxes and further improvements are required.

In 2010, there was a similar study on the modification of nanofiltration membranes (NFPES10, supplied by Hoechst Company) using acrylic acid and N-vinylpyrrolidone (Seman et al., 2012). The researchers claimed that a shorter exposure time to the UV source was not sufficient for the N-vinylpyrrolidone to be grafted with the PES membrane when the concentration of the monomer was too low. However, when the N-vinylpyrrolidone concentration was higher and exposed to the UV source for a longer period, higher water permeability was observed. The explanation given was that this might be due to trunk polymer scission, exhaustion of the monomers from the membrane surface or detachment of previously grafted monomers from the membrane surface. Moreover, the degree of grafting (DG) results revealed that the value of DG was in a linear relationship with the UV irradiation duration up to a maximum point, and then started to reduce with prolonged irradiation times. Over-irradiation could break the monomers that had already been grafted on the PES membrane, thus leading to lower DG values. Another observation from this research was that the concentration of monomers required might depend on the pH of the separation process. For instance, a high acrylic acid concentration was required for grafting when the modified membrane was designated to be used in an acidic environment in the presence of humic acids at pH 3. Additionally, a moderate N-vinylpyrrolidone concentration performed well for separation processes in a neutral environment at pH 7.

These studies have shown that the type of monomer used in membrane grafting not only alters the membrane structure and membrane characteristics, but also determines the membrane’s suitability to be used for different solutions or environments, whether acidic, neutral or alkaline. Irradiation intensity, duration and distance also might play a significant and important role on the monomer grafting process or degree of grafting. The selection of monomers to be used in membrane surface modification through UV-irradiation should be considered in view of all these parameters as they are dependent on various factors related to membrane performance.

For instance, UV-induced surface modification using monomers can improve the hydrophilicity or wettability of the PES membrane surface. One of the studies published in 2004 revealed that customised PES membrane filtration performance varied on the type of monomers used (Taniguchi and Belfort, 2004). This work categorised the monomers according to their charge property, such as neutrally charged, e.g. 2-hydroxyethyl methacrylate (HEMA) and N-2-vinyl-pyrrolidone (NVP); weakly charged, e.g. 2-acrylamidoglycolic acid (AAG) and acrylic acid (AA), and strongly negatively charged, e.g. 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPS) and 3-sulphopropyl methacrylate (SPMA). Thus, in this study, the researchers concluded that the type of monomer could affect the filtration performance and would exhibit a different degree of grafting. Based on this study, the monomers should be selected according to the application where the membranes would be used for better separation performance and anti-fouling properties (Mansourpanah and Momeni Habili, 2013).

4.2

4.2 Nanoparticle modification of PES membranes with UV irradiation (deposition and entrapment methods)

Nanoparticles have been incorporated into the membrane matrix as done in our previous work (Ng et al., 2011), where the silica nanoparticles and polysulphone membrane composition had been modified and optimised accordingly. Through the incorporation of silica nanoparticles into the polysulphone membrane matrix using blending method during the dope formulation, we observed an increase in membrane permeability and real salt rejection. This observation might be attributed to the surface charges of the silica nanoparticles which repulsed the permeation of ions.

Besides, silver chloride nanoparticles also were used to produce composite membrane through in situ micro-emulsion polymerisation (Wu et al., 2013). Researchers claimed that the AgCl nanoparticles can stay in a well-dispersed manner in the produced membranes. In this section, another newly developed method to incorporate the nanoparticles into the membrane matrices using UV-irradiation is discussed.

In another study (Rahimpour et al., 2008b), PES membranes with entrapped and non-entrapped TiO2 nanoparticles at various concentrations were fabricated by the phase immersion method. Membrane separation performance was investigated based on milk retention and water fluxes. The coating of the nanoparticles was conducted on a PES membrane prepared by immersing the membrane briefly into a nanoparticle colloidal suspension and UV-irradiated with various adjustments. The nanoparticle-covered membranes were modified by briefly immersing the pure membrane in various percentages of titanium dioxide nanoparticle suspensions for various times. Distilled water was used for the nanoparticle suspension by adding a suitable concentration of titanium dioxide nanoparticles. The well-mixed nanoparticle suspensions were prepared before use for this application. Finally, distilled water was used to rinse the membranes which were then exposed to the UV source for a variable period of time. The outcome regarding flux performance for pure and UV-irradiated nanoparticle-entrapped membranes is shown in Fig. 8. The researchers reported that the UV-irradiated nanoparticle-entrapped PES membrane showed better solution (milk) flux in comparison to the pure PES membrane and the nanoparticle-entrapped membrane without UV irradiation. The researchers also reported that UV irradiation did not cause degradation of the PES membrane morphology. Furthermore, the nanoparticle-entrapped membrane and the UV-irradiated membranes exhibited a similar protein retention ability, which signifies that the integrity of the membrane had been conserved.

Pure PES and UV-irradiated nanoparticle-entrapped PES membrane flux performance using non-skim milk (Adapted from (Rahimpour et al., 2008b) with permission).
Figure 8 Pure PES and UV-irradiated nanoparticle-entrapped PES membrane flux performance using non-skim milk (Adapted from (Rahimpour et al., 2008b) with permission).

When a substance whose electrical conductivity is intermediate between that of a metal and an insulator, also referred to as semi-conductor, is exposed to energy equivalent to or larger than the band gap energy, under normal circumstances, an electron can be moved to the conduction band from the capacity band (Rahimpour et al., 2008b). Consequently, pairs of electrons and holes can be produced on the surface of the semi-conductor. Due to the fact that titanium dioxide is a kind of semi-conductor, the electrons produced due to UV-irradiation causes O2 molecules from the surroundings to generate the superoxide radical anion ( O 2 - ). After being irradiated by a UV source, the holes produced react with the surrounding water vapour to produce OH radicals. Impurities such as organic compounds are then disintegrated and eliminated due to these strong oxidant reagents. The pathway of the photo-catalysis has been proposed as shown in Fig. 9.

Proposed pathway of photo-catalysis (Adapted from (Rahimpour et al., 2008b) with permission).
Figure 9 Proposed pathway of photo-catalysis (Adapted from (Rahimpour et al., 2008b) with permission).

The next observable fact was the super-hydrophilicity in this research (Rahimpour et al., 2008b). In this situation, formed holes and electrons would react in a dissimilar manner. The electrons produced due to UV-irradiation have a tendency to decrease the oxidation number of titanium from titanium (IV) to the titanium (III) state, and oxidised O 2 - anions are produced as a result of the holes generated. As proposed, oxygen vacancies will be generated on the membrane surface when oxygen atoms are released. The vacant locations will be taken up, and adsorbed OH groups can be produced from the surrounding water molecules on the membrane surface. This further reduces the hydrophobicity of the membrane. The super-hydrophilicity process is shown in Fig. 10.

Super-hydrophilicity process (Adapted from (Rahimpour et al., 2008b) with permission).
Figure 10 Super-hydrophilicity process (Adapted from (Rahimpour et al., 2008b) with permission).

Nanoparticle-entrapped PES membranes which were exposed to UV-irradiation exhibited elevated flux which can be contributed to by photo-catalysis and super-hydrophilicity (Rahimpour et al., 2008b). TiO2 nanoparticles are destructive to organic matter as they generate strong oxidant reagents. This limits the accumulation of proteins, fats and any other organic matter present in the non-skim milk on the surface of the membrane. Flux was therefore improved by limiting membrane fouling.

Another method used is coating the surface of a polymeric membrane with inorganic nanoparticles (Rahimpour et al., 2008b). The immersion of the membrane in a TiO2 nanoparticle suspension was done for one hour at ambient temperature. Next, water was used to flush the membrane irradiated by a UV source (160 W) for half an hour. A significant change in contact angles was used to reveal that the nanoparticles had been deposited on the surface of the membrane. Fig. 11 shows the SEM images of the control and TiO2-coated membranes. Agglomeration of nanoparticles was observed in this study. Deposition of nanoparticles on the membrane surface was explained by the existence of OH bonds in the membrane structure. Deposition of nanoparticles on the membrane surface occurred by forming a strong bond with the membrane and the inhibited removal of nanoparticles from the membrane surface.

SEM photos of (a) the control membrane surface and (b) the 0.03 wt.% TiO2-coated PES membrane surface (Adapted from (Rahimpour et al., 2008b) with permission).
Figure 11 SEM photos of (a) the control membrane surface and (b) the 0.03 wt.% TiO2-coated PES membrane surface (Adapted from (Rahimpour et al., 2008b) with permission).

Higher flux decline was observed with membranes not coated with nanoparticles when the membranes were tested with a non-skim milk solution. This observation was claimed by the authors as an improvement in the membrane antifouling characteristics after irradiation with a UV source and coating with TiO2 nanoparticles (Rahimpour et al., 2008b). However, differences in the amount of TiO2 did not significantly alter the membrane antifouling properties. This may have occurred due to pore blockage by the TiO2 nanoparticles with elevated loadings. Additionally, increased irradiation time also led to performance deterioration of the nanoparticle-deposited membrane. This likely occurred due to the formation of too many aggregated radicals on the membrane surface after exposure to the UV source at elevated energy and for an extended duration without any monomers. Peroxide groups can be produced on the membrane surface as a result of the response between OH radicals. Membrane performance after prolonged exposure to a UV source was diminished, probably due to alterations to the membrane surface. Moreover, it was found that prolonged membrane immersion in the nanoparticle colloidal suspension produces pore blockages which actually reduce membrane performance. In addition, increased UV source energy could also deteriorate the membrane performance, as previously reported.

In another similar research work, TiO2 nanoparticles were deposited onto the poly(vinylidene fluoride)/sulphonated polyethersulphone membrane which followed by UV-irradiation to activate the photo-catalytic property (Rahimpour et al., 2012). The researchers used scanning electron microscope to confirm the presence of the nanoparticles on the membranes. Besides, they found that the nanoparticles’ modified membrane showed better anti-fouling property when tested using bovine serum albumin solution. Lastly, they also reported that this modification method can produce membrane with better anti-bacterial property.

In brief, the deposition of nanoparticles onto the membrane surface might contribute to improved antifouling properties. However, the suitability of this method in other separation processes is still worth further investigation besides focusing on ultrafiltration applications. Furthermore, controlling the parameters of this process is still a questionable issue; optimisation might be the solution for this problem.

5

5 Summary

PES membranes have been successfully employed in diverse filtration processes and applications. PES is recognised as an excellent material in the manufacture of membranes as it is chemically resistant, possesses good mechanical properties and is thermally stable. PES membranes have been categorised as hydrophobic membranes in comparison with other types of membranes such as cellulose acetate, polyamide, polyimide, and so forth. The hydrophobicity of the membrane has been claimed by some researchers to reduce membrane fouling in certain separation processes. However, in some studies, modification of PES can significantly improve the membrane antifouling properties by reducing the hydrophobicity of the membrane (Möckel et al., 1999). The modification of PES membranes should be thoroughly investigated in the near future for better incorporation into various applications and purposes. PES membranes have been well-commercialised for the desalination process. However, in view of the progressive developments and research done on the forward osmosis application, PES membranes might be offered as a new kind of material for forward osmosis membranes. The requirements for a forward osmosis membrane, such as high salt rejection, high water permeability, low concentration polarisation, good chemical and thermal stability, are the characteristics possessed by PES membranes, except that PES membranes still need modification to improve their antifouling performance. Thus, the discussion above can be referred to for membrane modification selection, especially involving UV irradiation modifications.

Strongly charged monomers, for example, could be effective in the desalination process where ion separation is the main concern. The repulsion and attraction of ions in solution due to the charged membrane could prevent the ions from passing through the membrane (Donnan effect), thus improving the separation performance. Neutral or weakly charged membranes, however, could be appropriate for neutral solute separation where the charge of the membrane deteriorates the filtration performance.

In view of the easier operability of UV-induced surface modification, various modification conditions have been studied using polyethersulphone and polysulphone membranes. In general, when the monomer concentration increases during UV-induced grafting, membrane rejection will increase accordingly until a maximum concentration, which is highly dependent on the type of monomer, the UV-source intensity, the base membrane materials and the degree of grafting. Membrane separation performance will start to deteriorate when the monomer concentration exceeds the maximum concentration due to excess pore plugging. Consequently, the concentration of monomers used in surface modification is one of the main concerns; this parameter should be thoroughly investigated and controlled for all UV-induced surface modifications to obtain the optimised performance such as the greatest rejection of solutes, highest permeability, lowest fouling tendency due to solutes and thus lower operating pressure or energy consumption.

Besides monomers, nanoparticles also have been used and incorporated into PES membranes. So, UV-grafting of nanoparticles onto the PES membrane surface, as discussed above, could provide an alternative route to modification which might contribute to higher ion rejection during the salt filtration process. When nanoparticles are deposited onto the membrane surface, the charges possessed by the nanoparticles could inhibit ions from passing though the separation layer. Additionally, by using certain nanoparticles such as titanium dioxide or silver nanoparticles, the membrane anti-bio-fouling properties are increased, possibly since nanoparticles are destructive to organic matter by the generation of strong oxidant reagents. This could be useful for future nanofiltration membrane modification where it can be used to lessen the fouling tendency of the nanofiltration membrane and simultaneously increase the rejection of the membranes towards freely moving ions in aqueous solution by imposing repulsion in the presence of the surface charges of the nanoparticles on the membrane surface. However, the concentration of nanoparticles to be used in surface coating is also a critical issue that needs much investigation and attention as excess nanoparticles could cause pore blocking in polymeric membranes, thus reducing membrane permeability.

Acknowledgement

The authors of this work wish to gratefully acknowledge the financial support for this work by the UKM Research Grant (DIP) through the project No. DIP-2012-01.

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