Nanotechnology Approach in Nanofiltration Membrane Fabrications for Environmental Applications

Transcription

1 Nanotechnology Approach in Nanofiltration Membrane Fabrications for Environmental Applications ABDUL WAHAB MOHAMMAD a,b, NG LAW YONG b a Centre for Sustainable Process Technology (CESPRO) b Department of Chemical and Process Engineering Faculty of Engineering and Built Environment Universiti Kebangsaan Malaysia UKM Bangi, Selangor MALAYSIA wahabm@eng.ukm.my Abstract: - Membrane process is a technology that has been widely used for various separations within the various industries. There are variety of pressure-driven membrane processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Over the last decade, NF membranes have found wide usage for environmental applications focusing on water and wastewater treatment. However problems still persist especially regarding fouling-prone membranes, low flux and low selectivity. Over the last 5 years, various studies have been conducted to fabricate NF membranes with superior performance. Towards this end, nanotechnology approach in membrane fabrications and modifications has been proposed quite prominently. The focus of improvement has been on three areas namely (i) Membrane Selectivity or Solute Rejection Improvement, (ii) Membrane surface roughness and hydrophilicity improvement, and (iii) Membrane Fouling and Biofouling Control. These three areas will definitely impact the use of NF membranes especially for water and wastewater treatment and desalination. Key-Words: - separations, membranes, environmental applications, fouling, nanoparticles 1 Introduction Membrane technology is one of the upcoming separation technology that has been used in various fields. The use of membrane separation process is considered more efficient compared with the conventional separation processes. Hence, there has been increasing applications of membranes in various sectors of industry, medicine, food, chemical, textile and water quality improvement. [1]. Membrane separation technology has now expanded within the global market with reported usage reaching more than twelve billion US dollars [2]. The nanofiltration (NF) membrane is a sub-set of pressure-driven membrane with properties in between reverse osmosis (RO) and ultrafiltration (UF) membranes. NF offers several advantages such as low operation pressure, high flux, high retention of multivalent anion salts and an organic molecular above 300, relatively low investment and low operation and maintenance costs. Because of these advantages, the applications of NF worldwide have increased especially for environmental applications [3]. There are many interesting challenges ahead in order to make NF membrane processes more efficient technically and economically. A recent work [3] discussed several key unresolved problems that slow down large-scale applications of membranes. They have identified six challenges for NF membranes where solutions are still scarce: (1) avoiding membrane fouling, and possibilities to remediate, (2) improving the separation between solutes that can be achieved, (3) further treatment of concentrates, (4) chemical resistance and limited lifetime of membranes, (5) insufficient rejection of pollutants in water treatment, and (6) the need for modelling and simulation tools. The first four challenges essentially require improved NF membranes to be fabricated and produced. Over the last five years, various approach utilizing nanotechnology has been proposed in order to improve the performance of NF membranes. The main objective of this paper is to give an overview of the various fabrication and modification techniques that have been reported. 2 NF Membrane Fabrication and Modification ISBN:

2 Membrane fabrication and modification still become the limelight in membrane technology research amongst membrane researchers around the world in terms of materials or the methods used for several reasons: 1. Newly developed materials which are produced in large quantity made the material costs become competitive and feasible to be used in membrane fabrication and modification. Thus, more options are available during membrane material selection process. 2. Fouling problems still posed a serious threat to the development of the membrane application compared to other conventional processes. Thus, researchers are still trying to troubleshoot the issue by newly found materials or membrane modification methods in order to prevent or to reduce the membrane fouling. Reduction in the membrane fouling could contribute to less energy consumption, lower operating cost and higher throughput. 3. While there are getting more applications used membrane technology, suitability of the membrane materials with the application conditions (like ph, temperature, mechanical strength and hydrophilicity) attracted more efforts to be spent on the production of new membrane types. This could ensure a prolonged membrane lifetime when these membranes are used in wide variety of the production applications. This paper will expose the readers with various materials and methods used in the NF membrane fabrication and modification in order to increase the membrane selectivity or solute rejection capability. Secondly, we would further discuss this topic in terms of membrane surface roughness and hydrophilicity improvement. At last but not least, we would continue the discussion based on the membrane fabrication and modification done in order to reduce the membrane fouling and for membrane bio-fouling control. 3 Membrane Selectivity or Solute Rejection Improvement In year 2007, polyelectrolytes have been incorporated into the membrane modification as reported by a group of researchers from South Korea [4]. In this specific research work, they tried to remove the fluoride ions up to the drinking standard, by using layer-by-layer deposition of positively and negatively charged polyelectrolytes. They claimed that by using the poly(styrene sulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC), 4.5-bilayers already sufficient to improve the chloride/fluoride and bromide/fluoride ions selectivities at least three times along with the solution fluxes that are at least three times higher than those of commercial NF membranes. They also observed that the selectivity of the polyelectrolytes-modified NF membranes was highly related to the number of deposited layers. This study thus encouraged more research works to be conducted with the aim of solving the problems faced during NF separation. Application of polyelectrolytes for NF application was continued for water softening in year 2008 [5]. The researchers used five bi-layers of PSS/poly(allylamine hydrochloride) (PAH) on porous alumina supports which can produce 95% rejection of MgCl 2 along with a Na + /Mg 2+ selectivity of 22. Besides, they found that PSS/PDADMAC films can produce higher fluxes (due to the films swelling) but lower Mg2+ rejection (< 45 %). They proposed that the ph and supporting electrolyte concentration of polyelectrolyte solutions played a vital responsibility in determining the separation performances of the membranes produced. They suggested that when the PAH was used to cover the outermost membrane surface, its Mg 2+ rejection and Na + /Mg 2+ selectivity can be enhanced by increasing the ionic strength of the PAH solutions. In brief, they observed that this kind of membrane can be employed for water softening as they rejection divalent ions better than monovalent ions. In the same year, macrocyclic compound and a polyelectrolyte were used in NF study by applying layer-by-layer assembly method [6]. They revealed that when the aza-crownether compound was complexed with copper acetate, it removed almost all the divalent anions but only partially removed the monovalent ions. Yet, the solution fluxes throughout this experimental work still considered as low and not highly feasible at the moment to be used for other applications. In opposite, another similar work which employed PDADMAC and sulfonated poly(ether ether ketone) for layer-bylayer modification method [7], showed good retentions of up to 99% for charged solutes in the filtration of isopropanol solutions. Researchers found them excellent to be use in polar aprotic ISBN:

3 solvents, like dimethylformamide and tetrahydrofuran. In the year 2007, new kind of NF membranes were produced by using synthesized rigid star amphiphiles (RSAs) of nanosized [8]. They prepared the NF membrane by direct percolation of methanol solutions of the RSAs through asymmetric polyethersulfone support which had been firstly conditioned with methanol and cross-linked polyvinyl alcohol. This research group discovered that the membrane can retain its water contaminants rejection capability with better water permeability. They postulated that the RSAs can produce a uniformed plus extremely thin active layer atop the membrane support after lining its nanopores with sizes analogous to those of the RSAs. Once more in the year 2007, separation performance of polyamide (PA)/polyvinylidene fluoride (PVDF) hollow fiber composite NF membranes was performed [9]. The researchers included an additional stage to the conventional interfacial polymerization process in order to step up the separation performance. According them, conventional interfacial polymerization process includes submerging the support membrane in aqueous phased followed by oil phase. Next, in their reported work, they submerged the membrane into aqueous phase again. They stated that the separation performance was enhanced and the rejections for the hollow fiber composite NF membrane for Na2SO4, MgCl2, KCl, NaCl, PEG600 and PEG1000 were 92.3%, 7.0%, 9.5%, 14.2%, 88.4% and 89.3%, respectively. In year 2008, nanoparticles were used in order to fabricate membrane with high and stable rejection of salt solution [10]. TiO2 nanoparticles were dispersed in the aqueous phased of m-phenyl diamine (MPD) and the organic phase of trimesoyl chloride (TMC) in which the interfacial reaction occurred. Various effects such as curing temperature and curing time on the membrane performances were evaluated. It was observed that after 2 days of NF operation, certain amount of nanoparticles still can remain on the surface of the membrane. Besides, rejection of more than 95 % towards the MgSO4 was recorded. In year 2008 as well, NF composite membranes prepared via electro-polymerization were carried out by a group of researchers [11]. The researchers cut a circular piece of conductive support and installed at the base of the electrochemical cell. It was then sealed with an O-ring and functioned as anode, while the current was collected through the dry outer edge clamped with a concentric stainless steel ring attached to a potentiostat. A circular piece of platinum mesh of a diameter slightly smaller than the inner diameter of the cell was positioned horizontally within the cell above the base and functioned both as a counter electrode. They filled the cell with some monomer and the supporting electrolyte, which enclosed both electrodes and the relevant electrochemical procedure. The obtained composite membranes were washed thoroughly with deionized water. They revealed that the studied materials are not chemically steady and possessed averaged selectivity only. Yet, they suggested that this kind of membrane modification can be improved in the future through optimization process for better separation performances. Interfacial polymerization has been broadly used in year 2009, to increase the membrane rejection capability. In one of the recent work, interfacial polymerization of polyamide consisting of various ratios of piperazine and bipiperidine (as selective layer) was produced [12]. They compared the performances of the fabricated membranes with commercialized membranes. It was claimed that the self-generated membrane can produce 38 % of permeate flux more than the commercial NF270 membrane with identical rejection capability. In another similar work [13], researchers argued that they successfully produced thermally stable membrane through the interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) on poly(phthalazinone ether amide) (PPEA) ultrafiltration membrane. The flux of the membrane that they produced improved accordingly to the pressure and/or temperature, while retaining its dye rejection over 99.3 %. Researchers also tried to produce NF membrane through UV-initiated graft polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto the polysulfone ultrafiltration membranes as reported in year 2011 [14]. They argued that the NF membrane modified through photo-grafting in 1.5 M DMC solution for 5 minutes demonstrated almost the same rejection capability while increased its solution fluxes when the operating pressure was increased from 0.2 MPa to 0.8 MPa. 4 Membrane surface roughness and hydrophilicity improvement In the year 2007, another kind of membrane surface modification, low-temperature plasma grafting, also was conducted by another group of researchers [15]. They prepared the NF membrane from polyacrylonitrile UF membrane after went through ISBN:

4 the Ar low-temperature plasma treatment. It was followed by grafting the monomer styrene in vapor phase. They successfully produced higher hydrophobicity membrane. In opposite, higher hydrophilicity NF membranes were produced by grafting hydrophilic copolymers onto an UF membrane by another group of researchers [16]. They applied UV-irradiation and an in situ graft copolymerization of hydrophilic monomers (poly(acrylic acid and poly(acrylic acid-co-sodium allyl sulfonate)) on cardo polyetherketone membranes. Besides low-temperature plasma grafting, atom transfer radical polymerization (ATRP) was used to synthesize graft copolymers consist of poly(vinylidene fluoride-co-chlorotrifluoroethylene) backbone and poly(styrene sulfonic acid) side chains, P(VDF-co-CTFE)-g-PSSA [17]. P(VDF-co- CTFE)-g-PSSA was used to coat the surface of the P(VDF-co-CTFE) ultrafiltration support membrane. They found that the membrane surface hydrophilicity was increased which was supported by the contact angle measurement. Besides, the membrane rejection and solution flux were increased by increasing the PSSA content. Other research work tried ion beam irradiation to diminish the membrane surface roughness [18]. During the ion beam irradiation, ions penetrated through the membrane surface and eliminated tall peaks and deep valleys. As a result, they effectively reduced the membrane surface roughness. They conducted the atomic face microscopy (AFM) analysis to verify the reduction of the membrane surface roughness. However, they concluded that the overall membrane pore size distribution, hydrophilicity and selectivity of the membrane were not altered after the ion beam irradiation. In year 2009, however, TiO 2 nanoparticles were used to be assembled onto the NF membrane surface [19]. The membranes prepared were radiated by UV light after the nanoparticles deposition. The membranes exhibited better surface hydrophilicity and can resist fouling better. In the same year, researchers found that the redox initiated graft polymerization constantly contributed to the production of more hydrophilic membranes [20]. They found that longer reaction time (but not monomer concentration and initiator effect) can reduce the membrane surface hydrophobicity. Though the rejection of the raffinose was enhanced, the water flux was reduced after the grafting process. At the same time, they found out that the addition of sulfonated polyethersulfone to the polymer blend has positive effect towards the membrane surface hydrophilicity. Microwave irradiation was also employed in NF membrane preparation in year 2009 [21]. They used trimesoyl chloride (TMC) and piperazine (PIP) as the reagents to coat the polyethersulfone membrane with polyamide through interfacial polymerization. The membranes were then exposed to different duration and power of microwave irradiation. It was observed that higher irradiation power of the microwave could contribute to a decline in the membrane rejection due to the polymer degradation. However, this method exhibited positive effect to reduce the surface roughness due to the densification of the skin layer. 5 Membrane Fouling and Biofouling Control In year 2007, nanocomposite membranes were prepared by using Ag nanoparticles and polyamide membrane through in situ interfacial polymerization [22]. The application of the Ag nanoparticles was proved to have anti-biofouling capability when tested with Pseudomonas. However, addition of the Ag nanoparticles was not significant to improve the solution fluxes and salt rejection capabilities. Similar work was also done by another group of researchers where they applied TiO 2 layers which were chemically connected to the polyamide-based membrane [23] The TiO 2 layers were introduced in this study as TiO 2 compounds are distinctively known as photo-catalysts which can degrade organics in the presence of UV light. The membrane generated was found to be stable after numerous times of use and the membrane displayed better antifouling properties in the presence or the absence of UV light in the long term. In the year 2007 also, hollow fibre NF membranes were developed by applying UVgrafting using sodium p-stryrene sulfonate (NaSS) as a vinyl monomer [24]. As a common practice during UV-grafting, irradiation time, the presence of photo-initiator and the quantity of UV energy received were evaluated in this specific research work. The membrane was initially wetted by water, dipped in monomer aqueous solution, passed through two UV systems and then washed with RO water. When the modified membrane was tested with dye solution, researchers found that the similar charges between the membrane surface and the dye molecules could increase the repulsion between them thus lower the membrane fouling tendency. In year 2008, ion beam irradiation was applied in the modification of the sulfonated polysulfone membrane surfaces [25]. They found that this ISBN:

5 technique can be used to break some sulfonic and C- H bonds while forming new C-S bonds. Besides, the modified membranes possessed decreased surface roughness and smaller flux decline during the fouling test. However, this method did not affect the membrane hydrophobicity, pore size distribution and membrane rejection efficiencies. As discussed previously, newly found materials have gained more popularity in the membrane modification to reduce the membrane fouling. This can be verified when the polyelectrolyte coatings were used in NF membrane preparation as reported in 2010 [26] to reduce the membrane fouling. Researchers designed a positively NF membrane by chemical modification of P84 copolyimide using branched polyethylenimine (PEI). By applying dynamic coating method, a thin layer of hydrophilic polymers was adsorbed onto the membrane surface. They concluded that the coated layer maybe erasable or in opposite depending on the type of hydrophilic polymers used. They studied 3 types of coating materials, which were polyvinyl alcohol (PVA), polyacrylic acid (PAA) and polyvinyl sulfate (PVS). They observed that only negatively charged polyelectrolytes like PAA and PVS could be adsorbed onto the membrane surface through electrostatic forces thus forming more stable coating. In contrary, neutral polymer like PVA which adsorbed onto the membrane surface through hydrogen bonding, became easily detached from the membrane surfaces during acid cleaning process. They claimed that this can help to remove the membrane surface foulants thus lower membrane surface fouling and it can refresh the membrane surface for repeating use. This could be used to prolong the membrane lifetime and thus reduce the operation cost in the future. One of the latest works reported in 2012 employed multi-walled carbon nanotubes (MWCNTs) coated by anatase titanium dioxide (TiO 2 ) nanoparticles (which were synthesized via the precipitation of TiCl 4 precursor on the acid oxidized MWCNTs) to be used in the preparation of nanocomposite polyethersulfone (PES) membranes [27]. The photo-catalytic activity of TiO 2 coated MWCNTs was illustrated in the following Fig. 1. UV irradiation on the TiO 2 contributed to the formation of holes and electrons. The electrons generated would reduce Ti (IV) cations to the Ti O 2 - anions. Thus, oxygen atoms are removed and a group of oxygen vacancies are created on the surface. As a result, water molecules from the surroundings would take up the vacancy and the OH groups adsorbed would contribute to the formation of membrane surface with higher hydrophilicity. In addition, the radicals produced can avoid the proteins deposition therefore reduced membrane surface fouling. Throughout this experiment, researchers found that TiO 2 nanoparticles used possessed low agglomeration during the application as the nanoparticles exhibited good compatibility with the polymeric components. Incorporation of these additives into the PES membrane matrix leaded to an increased pure water flux, higher hydrophilicity, better anti-biofouling and decreased membrane surface roughness. Fig. 1. Postulated photo-catalytic activity of TiO 2 coated MWCNTs Adapted with permission from [27] 6 Conclusion Since year 2007 up to 2012, we noticed that the researchers from all over the world are heading towards the realization of NF membranes with the best attributes. They attempted to construct NF membranes with the best compatibility with membrane operational conditions so as to minimize the membrane surface fouling, reduce cost, amplify the throughput and so forth. As NF process has been categorized as one of the best membrane candidates to replace the conventional desalination or as pretreatment processes before the reverse osmosis processes, it encourages more interests from the researchers in recent years. In hope to increase the NF membrane selectivity and solutes rejections, researchers used polyelectrolytes coating to enhance the membrane surface charge density. Besides, researchers used nanoparticles to increase the NF membrane solution flux while trying to retain its rejection capability. ISBN:

6 For the first time, researchers also employed electropolymerization to improve the membrane rejection capability but found some limitations that need to be solved. During the last few years, researchers also worked on improving the membrane surface roughness and hydrophilicity. Low-temperature plasma grafting was utilized in year 2007 as discussed, to increase the membrane hydrophobicity in order to suit its application. Besides, UVirradiation and an in situ graft co-polymerization of hydrophilic monomers were performed in the same year to enhance the membrane surface hydrophilicity. Ion beam irradiation was also integrated to reduce the membrane surface roughness. In addition, nanoparticles also were used to increase the membrane surface hydrophilicity. Lastly, membrane bio-fouling control was mainly done by the incorporation of the nanoparticles into the membrane matrix. The nanoparticles normally used for bio-fouling control are silver and titanium dioxide nanoparticles. Besides, UV-grafting by using sodium p-stryrene sulfonate (NaSS) was also done to reduce the membrane surface fouling by improving the membrane surface charges. References: [1] Mulder, M., Basic Principles of Membrane Technology. Netherlands: Kluwer Academic Publisher, [2] Sutherland, K., Profile of The International Membran Industry, Edisi ke-3. Elsevier Ltd., [3] B. Van der Bruggen, M. Mänttäri, M. Nyström Drawbacks of applying nanofiltration and how to avoid them: A review, Separation and Purification Technology, Vol. 63, No. 2, 2008, pp [4] Hong, S.U., R. Malaisamy, and M.L. Bruening, Separation of fluoride from other monovalent anions using multilayer polyelectrolyte nanofiltration membranes. Langmuir, Vol. 23, No. 4, 2007, pp [5] Ouyang, L., R. Malaisamy, and M.L. Bruening, Multilayer polyelectrolyte films as nanofiltration membranes for separating monovalent and divalent cations. Journal of Membrane Science, 310(1 2), 2008, pp [6] El-Hashani, A., A. Toutianoush, and B. Tieke, Use of layer-by-layer assembled ultrathin membranes of dicopper-[18]azacrown-n6 complex and polyvinylsulfate for water desalination under nanofiltration conditions. Journal of Membrane Science, 318(1 2), 2008, pp [7] Li, X., et al., Solvent-Resistant Nanofiltration Membranes Based on Multilayered Polyelectrolyte Complexes. Chemistry of Materials, Vol. 20, No. 12, 2008, pp [8] Lu, Y., et al., Nanofiltration Membranes Based on Rigid Star Amphiphiles. Chemistry of Materials, Vol. 19, No. 13, 2007, pp [9] Liu, J.-Q., et al., An improved process to prepare high separation performance PA/PVDF hollow fiber composite nanofiltration membranes. Separation and Purification Technology, Vol. 58, No. 1, 2007, pp [10] Lee, H.S., et al., Polyamide thin-film nanofiltration membranes containing TiO2 nanoparticles. Desalination, 219(1 3), 2008, pp [11] Gloukhovski, R., et al., Thin-film composite nanofiltration membranes prepared by electropolymerization. Journal of Applied Electrochemistry, Vol. 38, No. 6, 2008, pp [12] Yoon, K., B.S. Hsiao, and B. Chu, High flux nanofiltration membranes based on interfacially polymerized polyamide barrier layer on polyacrylonitrile nanofibrous scaffolds. Journal of Membrane Science, 326(2), 2009, pp [13] Wu, C., et al., Preparation, characterization and application of a novel thermal stable composite nanofiltration membrane. Journal of Membrane Science, 326(2), 2009, pp [14] Deng, H., et al., High flux positively charged nanofiltration membranes prepared by UVinitiated graft polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto polysulfone membranes. Journal of Membrane Science, 366(1 2), 2011, pp [15] Chen, J., et al., Nanofiltration membrane prepared from polyacrylonitrile ultrafiltration membrane by low-temperature plasma: 5. Grafting of styrene in vapor phase and its application. Surface and Coatings Technology, 201(15), 2007, pp [16] Qiu, C., Q.T. Nguyen, and Z. Ping, Surface modification of cardo polyetherketone ultrafiltration membrane by photo-grafted copolymers to obtain nanofiltration membranes. Journal of Membrane Science, 295(1 2), 2007, pp ISBN:

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