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Reverse osmosis (RO) is currently the most important desalination technology and it is experiencing signiﬁcant growth. The objective of this paper is to review the historical and current development of RO membrane materials which are the key determinants of separation performance and water productivity, and hence to deﬁne performance targets for those who are developing new RO membrane materials. The chemistry, synthesis mechanism(s) and desalination performance of various RO membranes are discussed from the point of view of membrane materials science. The review starts with the ﬁrst generation of asymmetric polymeric membranes and ﬁnishes with current proposals for nano-structured membrane materials. The paper provides an overview of RO performance in relation to membrane materials and methods of synthesis. To date polymeric membranes have dominated the RO desalination industry. From the late 1950s to the 1980s the research effort focussed on the search for optimum polymeric membrane materials. In subsequent decades the performance of RO membranes has been optimised via control of membrane formation reactions, and the use of poly-condensation catalysts and additives. The performance of stateof-the-art RO membranes has been highlighted. Nevertheless, the advances in membrane permselectivity in the past decade has been relatively slow, and membrane fouling remains a severe problem. The emergence of nano-technology in membrane materials science could offer an attractive alternative to polymeric materials. Hence nano-structured membranes are discussed in this review including zeolite membranes, thin ﬁlm nano-composite membranes, carbon nano-tube membranes, and biomimetic membranes. It is proposed that these novel materials represent the most likely opportunities for enhanced RO desalination performance in the future, but that a number of challenges remain with regard to their practical implementation. © 2010 Elsevier B.V. All rights reserved.
Ceramic/Inorganic membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mixed matrix membranes (MMM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Nano-particle/polymeric membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Carbon nano-tube/polymeric membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Biomimetic RO membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and future developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fig. 1. Cumulative desalination capacity from 1960 to 2016 [12,13].
RO membrane market is dominated by thin ﬁlm composite (TFC) polyamide membranes consisting of three layers: A polyester web acting as structural support (120–150 ␮m thick). This conﬁguration offers high speciﬁc membrane surface area. The selective barrier layer is most often made of aromatic polyamide. Hence. (a) Improvement in salt rejection. easy scale up operation. the former has higher salt rejection and net pressure driving force . Therefore. Although the latter has superior chlorine resistance. The spiral wound membrane module conﬁguration is the most extensively used design in RO desalination. enhanced durability and easy cleaning characteristics [1. There are four major membrane module suppliers which provide RO membranes for large scale desalination plants.24]. have optimised the inter-connection between module design and ﬂuidic transport characteristics. Data from .23. Hydranautics and DOW (FILMTECTM ) have 16-in. Data from .K. / Journal of Membrane Science 370 (2011) 1–22 3 Fig. namely DOW. Recently. between the barrier layer and the support layer. Hydranautics and Toyobo.26].2 ␮m) . Toray. state-of-the-art seawater desalination RO membrane modules from each supplier are tabulated in Table 1 in order to provide a benchmark of current SWRO performance. feed channels and vessels. improvements in the dimensions of spacers. A speciﬁc comparison of the various products is not attempted as the data corresponds to different test or operating conditions [29–36]. a microporous interlayer (about 40 ␮m). Data from . low replacement costs and.6 nm to achieve salt rejection consistently higher than 99%. Although the spiral wound conﬁguration was developed decades ago. Polyamide spiral wound membranes dominate RO/Nanoﬁltration (NF) market sales with a 91% share.P. it is the least expensive module conﬁguration to produce from ﬂat sheet TFC membrane [25. spiral wound modules with the MegaMagnum® trade name. thereby allowing prevention of the growth of microorganisms and algae via chlorine injection. Asymmetric cellulose acetate (CA) hollow ﬁbre membranes hold a distant second spot . for example via interfacial polymerization of 1. Membrane pore size is normally less than 0. thereby decreasing both fouling and pressure losses. modules which are being piloted in cooperation with the national water agency in . (b) Reduction in membrane cost. (c) Reduction in energy consumption of RO. Lee et al.3-phenylenediamine (also known as 1. a micro-porous interlayer of polysulfonic polymer is added to enable the ultra-thin barrier layer to withstand high pressure compression. With improved chemical resistance and structural robustness. it offers reasonable tolerance to impurities. inter-changeability. The polyester support web cannot provide direct support for the barrier layer because it is too irregular and porous.3-benzenediamine) and the tri-acid chloride of benzene (trimesoyl chloride) . The thickness of the barrier layer is reduced to minimize resistance to the permeate transport. and an ultra-thin barrier layer on the upper surface (0. Koch Membrane Systems have released 18-in. as well as the materials of construction. most importantly. 2. In addition. Research on the design of modular elements is currently focusing on optimization of hydrodynamics in order to minimise the concentration polarization effect. larger modular elements are desirable for increased desalination capacity.
Hence mega-sized desalination plants must be developed if we are to provide new clean water supplies to billions of people.40–99. Spain [30.35 at Tuas SWRO Plant. 25 ◦ C. It focused on composite RO membranes.40 (2. TM820C Toyobo 16-in.75 [a] 99. higher salt rejection can possibly reduce the number of RO passes necessary to achieve appropriate product water quality. Studies have shown that such module designs can further decrease the cost of desalination by approximately 20% [37.38]. The material molecular composition of these membranes is shown in Table 2. pH 8 and 8% recovery [29. and it has a production rate of about 110 million m3 year−1 .000 people. although initial work on the narrow gap membrane project was not fruitful. stringent water standard and brine management . 2.g. Developments in membrane material and module optimization can signiﬁcantly contribute to the reduction of all three aspects. 2. Electricity (energy). ease of handling and improved performance in selectivity and permeability. 25 ◦ C and 30% recovery . especially boron species. this plant can supply fresh water to less than 100. This new morphology produced a water ﬂux of at least an order of magnitude higher than the initial symmetric membrane . recovery. [c] Test condition: 35 g L−1 NaCl solution. 25 ◦ C. and it has been reported that 99% of boron rejection is required in the Middle East region for one-pass RO process to comply with the WHO water drinking standard . Reduction in fouling.7–24. Israel. a smaller plant foot print and a reduced use of cleaning chemicals.1. the announcement of the Loeb-Sourirajan CA membrane was of historical importance as it ﬁrst made RO possible in practice . Next.80 [b] 99.6 [a] 60.88) [e] at Llobregat SWRO Plant. Challenges and trends in RO desalination technology development Sheikholeslami recently concluded that the future challenges in the desalination industry include feed water characterization.50–99.0–67. Conventional desalination RO membranes – polymeric materials Polymeric RO membranes have dominated commercial applications since the very ﬁrst RO desalination plant. the reduction in energy consumption would be considerable. 55 bar. / Journal of Membrane Science 370 (2011) 1–22 Table 1 Some of the state-of-the-art SWRO membrane modules in application. HB10255 Material & module TFC cross linked fully aromatic polyamide spiral wound TFC cross linked fully aromatic polyamide spiral wound TFC cross linked fully aromatic polyamide spiral wound Asymmetric cellulose tri-acetate hollow ﬁbre Permeate ﬂux (m3 day−1 ) 28. Currently the largest SWRO plant in the world is in Ashkelon. focusing on the chemistry of the membrane materials.33] 4. materials development. Additionally. 3 highlights the major development of asymmetric RO membranes up to the 1980s. In the decade after the announcement of the Loeb-Sourirajan membrane. To be of commercial interest. In the late 1950s.17 (2. 55 bar. In 1993 Petersen  offered a comprehensive review of the same subject. etc. pH 7 and 10% recovery .70–99. [e] The number in brackets is the energy consumption for the RO membrane unit. [b] Test condition: 32 g L−1 NaCl solution. SW30HRLE Hydranautics 8-in. labour and chemicals make up about 87% of the total RO cost . Lee et al.0 [a] 24.P. PUB. 85% for agricultural irrigation. Japan  Note: [a] Test condition: 32 g L−1 NaCl solution. backwashing chemicals. Moreover higher permeability would lead to a reduction in membrane area. Considering the global average water consumption per capita of 1243 m3 year−1 (5% for domestic use. Early membrane chemistry development and asymmetric membranes In 1949 a report entitled The Sea as a Source of Fresh Water initiated research activities on salt-rejecting membranes . further research on CA materials was focused on the improvement of the membrane transport . renewable energy source. achieving 98% rejection. 54 bar. Membrane module brand name DOW FILMTECTM 8-in. of the order of <10 mL m−2 h−1 . and 10% for industrial use) . For a complete study of early RO membrane development.75 [a] 99. pre-treatment processes. as the energy cost represents half of the total water production cost. Due to their technological maturity they offer low-cost fabrication.60–99. covering activity from the inception of composite RO membranes up to approximately 1985. process development. but that the permeate ﬂux was very disappointing.35]. The highest boron rejection membrane offered in the market can only achieve 93% boron rejection at optimum conditions.0[c] Salt rejection (%) 99. Fig. and (ii) the evolution of more controlled conditions for membrane formulation to enhance membrane functionality and durability (late 1980s to date) . is important because it directly reduces the costs of membrane replacement.3. Singapore.00 at Fukuoka SWRO Plant.4 K. is necessary. Furthermore. feed water quality.32) [e] at Perth SWRO Plant. This chronological description provides the readers with a quick overview of RO membranes formed by different mechanisms and their impact on the desalination industry over the years. readers are however advised to refer to Petersen . Reid and Breton reported that a hand-cast thin symmetrical cellulose acetate (CA) membrane could retain salt effectively. particularly via the development of chlorine-tolerant membranes. and energy to overcome the additional osmotic pressure. SWC4+ Toray 8-in. In this context.60[c] [d] Speciﬁc energy consumption (kWh m−3 ) 3. Australia  4. This section will brieﬂy highlight the early development of membrane chemistry and graphical illustrations are used to visualize the performance improvement of RO membranes. the capital investment and operating costs of RO plants must be further reduced to achieve this. A CA asymmetric membrane was formed with a dense 200 nm thin layer over a thick micro-porous body. One of the earliest review studies on polymeric RO membrane materials was reported by Cadotte . signiﬁcant improvement in the rejection of low molecular weight compounds. Generally the development of membrane materials can be divided into two periods according to research activity: (i) the search for a suitable material (chemical composition) and membrane formation mechanism (1960s to late 1980s). Although the operating pressure in current systems is already close to the thermodynamic limit and a further reduction would have a modest impact on performance .) at different desalination plants. 1. any novel membrane must outperform the materials and modules listed in Table 1. Singapore  5. process design. the biggest challenge would be making RO desalination affordable for poorer countries. [d] These numbers should not be compared explicitly because of different operating parameters (e. Unarguably. and consequently a reduction in membrane replacement costs.6[b] 19.
67 m3 m−2 day−1 Salt rejection: 99.5% NaCl solution 3.K. as well as offering greater resistance to compaction . However.2% Test: > 80 bar.105% NaCl solution 5. as well as having higher resistance to chemical and biological attack compared to the initial cellulose diacetate (CDA) material. Although there was an intensive search for alternative membrane polymers. Polybenzimidazoline . e. the CA structure shown is CDA.Loeb-Sourirajan CA  Flux: 0.67 m3 m−2 day−1 Salt rejection: 97. CA remained the best membrane material for RO until 1969.13 m3 m−2 day−1 Salt Rejection: 95% Test: > 6 bar. Flux: 0. Cellulose Acetate . Aromatic Polyamide . limited the durability and range . Flux: 0. 0. 3. 0. CTA is prone to compaction resulting in severe loss of ﬂux even at moderate operating pressures of 30 bar or less . Flux: 0. the susceptibility of the acetate group to hydrolysis in both acidic and alkaline conditions.5% Test: 30 ◦ C. as well as sensitivity to microbial contamination. A blend of CDA and CTA ﬁnally offered higher permeability and selectivity than CA membranes.g. rather than CTA or mixed-CA. such as control degree of mixed ester substitution for the hydroxyl groups found in cellulose to monitor the performance of the CA membranes . 0. 4% NaCl solution 2. However. > 100 bar.P. properties and simpliﬁcation of manufacturing to bring the technology to industrial application .07 m3 m−2 day−1 Salt Rejection: 92% Test: > 45 bar.5% NaCl solution Note: The chemical structures listed represent segments of these membranes. The cellulose triacetate (CTA) membrane was developed for its stability in a wider range of temperatures and pH. Chemical type & description Chemical structure 5 1. More studies have been performed. Polypiperzine-amide .35 m3 m−2 day−1 Salt rejection: 99% Test: > 100 bar. / Journal of Membrane Science 370 (2011) 1–22 Table 2 Notable asymmetric RO membranes. It does not show all possible forms of the structure. Lee et al.Polyamide-hydrazide  Flux: 0. Polyoxadiazole .36% NaCl solution 4.
Though it has relatively low ﬂux and salt rejection. The reduced presence of amidic hydrogen also improves the resistance to chlorine attack .2. Despite the low ﬂux. In contrast. readers are referred to . of applications .73]. These anisotropic membrane morphologies are now referred to as composite membranes. 4. Thus.1. Francis cast the ﬁrst TFC membrane by ﬂoat-casting a CA ultra-thin ﬁlm on the water surface followed by annealing and lamination onto a pre-formed CA microporous support . polyoxadiazole is found to have superior mechanical and temperature stability but its salt rejection and permeability is not commercially attractive for RO applications [52. The ﬁrst non-cellulosic asymmetric membrane to gain attention was developed by Richter and Hoehn and consisted of an aromatic polyamide (PA) asymmetric hollow-ﬁbre membrane . Lee et al.67]. the durability. Furthermore. densiﬁcation in the middle transition layer of the CA asymmetric membrane occurs under pressure . Similarly. they are susceptible to pressure compaction and chlorine attack [51. mechanical. For detailed discussion about the variation in reactants for PA asymmetric membranes. and even less are commercially attractive in terms of the right combination of permeability and salt rejection. a stronger material with higher chemical stability was obviously needed and although many alternative polymers were tested in the 1960s the resulting improvements were insigniﬁcant.60]. However the susceptibility of polyamides to attack by disinfectants such as chlorine (halogens) and ozone was observed after the prolonged use of the B-9 Permasep® membrane. Whilst polybenzimidazoline (PBIL) membranes developed by Teijin show excellent permselectivity even in harsh operating conditions. a wide variety of polymers can be tested for the barrier layer and support layer separately. However. which outperformed the CA spiral wound elements in terms of ﬂux per unit module volume.69].2.68. this membrane was not commercialised due to its relatively low salt rejection (≤95%) . its commercial success can be attributed to the highly effective packing of the hollow ﬁbres. The presence of the sulphonic and phenyl groups in sulfonated polysulfone was expected to enhance permeability. / Journal of Membrane Science 370 (2011) 1–22 Fig. Membranes produced with this technique never gained commercial interest because their asymmetric counterparts offered a better ﬂux for lower manufacturing costs. reasonable ﬂux and most importantly.6 K. polysulfone was found to be the optimum material for the support layer due to its resistance to compaction. the former for mechanical support and the latter for optimal salt rejection and permeate ﬂux.61. The ﬁrst patented product based on this concept was named NS-200. Thin ﬁlm composite (TFC) membrane Only a few soluble polymers can form asymmetric structures in one-step casting. carboxylated polysulfone which gives a promising ﬂux also suffers from uncompetitive salt rejection [66. They have comparable permselectivity to the asymmetric CA membrane.P. This led to twostep casting methods that enabled individual optimization of the materials used for the micro-porous support ﬁlm and for the barrier layer. for application in brackish water desalination. Chlorineresistant asymmetric membranes based on polypiperazine-amides have subsequently been developed (Table 6) [50. 2. chemical and biological stability however the salt rejection was below the acceptable level necessary for commercialisation . a product of the reaction of furfuryl alcohol. its stability in an acidic environment which enables further development of the TFC membrane by acid polycondensation and interfacial polymerisation . Early development of TFC membrane As shown in Fig. After an extensive empirical study.62]. In addition. This was subsequently commercialised by Du Pont under the trade name of B-9 Permasep® . The development of asymmetric RO membrane. stability and versatility are greater than CA or aromatic polyhydrazides . 2. A dip-coating method involving acid polycondensation of lowmolecular-weight hydroxyl-containing compounds was proposed to overcome scaling up problems in ﬂoat-casting technology [60. . 3.
This enhancement has led to PA-300 spiral wound modular elements being installed in the TFC SWRO plant at Jeddah . namely the Solrox membrane. including both aliphatic and aromatic diamines. often a polysulfone support. it was susceptible to chlorine attack. Yasuda’s group was particularly active in plasma polymerization and a membrane formed by acetylene.. acetylene/water/carbon monoxide combinations [76–82]. . and good stability in high temperature. A summary of notable TFC RO membranes is shown in Table 3. It also demonstrated superior rejection of organic compounds. where fragmentation of monomer vapour is induced by the energy of the gas plasma.5tris(hydroxyethyl)isocyannuric acid instead of polyoxyethylene. polythylenimine reacted with toluene di-isocyanate (Table 3) by Cadotte was a major technological milestone in the history of RO processes . Initial attempts at interfacial polymerization of monomeric amines. / Journal of Membrane Science 370 (2011) 1–22 7 Fig. The development of thin ﬁlm composite RO membrane.3. However NS-100 membranes have virtually no resistance to chlorine. Another membrane prepared by acid polycondensation was the PEC-1000 TFC RO membrane produced by Toray Industries. The development of NS-100.K. and polypyrrolidine where the amino/carboxy-groups can be controlled to vary amphotericity and selectivity. 2. water and nitrogen performed particularly well in sea water desalination test. implying the shielding effect of divalent cations can signiﬁcantly decrease the monovalent ion rejection.2.94]. as it could withstand the alkaline conditions created by the use of caustic as an acid acceptor in the interfacial polymerisation process. allylamine. with terephthaloyl chloride. A range of polymers have been tested and good permselectivity have be obtained from vinylene carbonate/acrylonitirile.95. Despite research into plasma-polymerized ﬁlms and application in gas separation .5 m3 m−2 day−1 at 100 bar operation. Lee et al. RC-100 has a high resistance to bio-fouling which has resulted in the successful installation at Umm Lujj II and other desalination plants . strong Donnan effects were observed. Interfacial polymerisation synthesis of TFC membrane The use of polysulfone as a support layer opened the way to interfacial polymerisation to produce RO membranes.96]. It used 1. It was the ﬁrst successful non-cellulosic membrane with comparable ﬂux and monovalent salt rejection. 4.P. vinyl acetate/acrylonitrile. The PA-300 material showed improved ﬂux of about 1 m3 m−2 day−1 and an improved salt rejection 99. acetylene/water / nitrogen. Inc. Nevertheless.4% at 70 bar. and they have a pronounced surface brittleness as a result of a highly cross-linked structure. Despite its extremely high salt and organic compound rejection with adequate ﬂux. The barrier layer can be formed by plasma polymerization. only one commercial RO membrane has been commercialized using this technique. sulphuric acid and polyoxyethylene (Table 3) . acidic and alkaline environments [93. Plasma polymerized RO membranes mostly have low chlorine resistance due to their nitrogen-enriched chemical structure. Another commercialised product formed by interfacial polymerization of polymeric amines is polyepiamine with 2 versions designated as PA-300 and RC-100 (Table 3) [85. with 99% salt rejection for a ﬂux of 1. There are two other noteworthy interfacially polymerised TFC membranes. and atomic polymerization where the monomer is propagated onto a cool surface. Sulfonated polysulfone membranes have since been developed due to their stability in oxidizing environments . It showed excellent salt rejection but suffered from irreversible swelling and hydrolysis of the sulphate linkage. namely polyvinylamine which that offers high ﬂux. as compared to NS100. On the other hand.2. did not produce membranes with attractive .
 Flux: 0. Lee et al. Polyamide via polyethylenimine Name: NS-100 . 3.8% Test: > 100 bar. Polyether-Polyfurane .9% Test: > 69 bar. Flux: 0. Flux: 0.8 Table 3 Notable TFC RO membranes.Name: PEC-1000 . Polyfurane . 3.7 m3 m−2 day−1 Salt Rejection: 99% Test: > 100 bar. 3. / Journal of Membrane Science 370 (2011) 1–22 Chemical structure 1. Flux: 0.P.5% NaCl solution 2.5% NaCl solution .5% NaCl solution -Excellent chlorine resistance 4.5 m3 m−2 day−1 Salt Rejection: 99. Sulfonated Polysulfone .Name: Hi-Flux CP . 3.Name: NS-200 . Chemical type & description K.06 m3 m−2 day−1 Salt Rejection: 98% Test: > 69 bar.5% NaCl solution -Excellent organic rejection 3.8 m3 m−2 day−1 Salt Rejection: 99.
Lee et al. Flux: 0.P.7% Test: > 40 bar.Name: NS-300 . 0.0 m3 m−2 day−1 Salt Rejection: 98. 3. Flux: 3.7% Test: > 40 bar. 3.K.8 m3 m−2 day−1 Salt Rejection: 99.3 m3 m−2 day−1 Salt Rejection: 68% Test: > 100 bar. Flux: 1.5% NaCl solution .0 m3 m−2 day−1 Salt Rejection: 99. Conductivity = 5000 ␮S cm−1 7. Polyamide via polyepiamine . Polyvinylamine .Name: PA-300 or RC-100 . / Journal of Membrane Science 370 (2011) 1–22 Table 3 (Continued ) Chemical type & description Chemical structure 9 5.5% NaCl solution 6. Polypyrrolidine .4% Test: > 69 bar.Name: WFX-X006 .5% NaCl solution 8. Polypiperazine-amide . Flux: 2.
3.1 .3% Test: > 15 bar.Name: FT-30 - Flux: 1. e. Cross linked Fully Aromatic Polyamide – 3 .g. the NS-100 structure shown is the polyamide version. 0. 0. It does not show all possible forms of the structure.26 m3 m−2 day−1 Salt Rejection: > 98% Test: > 55 bar.P.10 Table 3 (Continued ) Chemical type & description K.2% NaCl solution 12.Name: UTC series - Flux: 0. rather than the polyurea equivalent. Cross linked Fully Aromatic Polyamide .Name: X-20 - Flux: 1 m3 m−2 day−1 Salt Rejection: 99.8 m3 m−2 day−1 Salt Rejection: 98.2 .2% NaCl solution 10. . Cross linked Aralkyl Polyamide Name: A-15 - Flux: 0.5% NaCl solution 11. 0. Cross linked Fully Aromatic Polyamide . Lee et al.0 m3 m−2 day−1 Salt Rejection: 99% Test: > 15 bar. / Journal of Membrane Science 370 (2011) 1–22 Chemical structure 9.5% Test: > 15 bar.2% NaCl solution Note: The chemical structures listed represent segments of these membranes.
research and development towards new polymeric materials for RO membranes has declined dramatically. e.3. These improvements are the results of surface modiﬁcation. It was the ﬁrst spiral wound membrane element capable of competing with the Du Pont asymmetric hollow ﬁbre polyamide B-9 Permasep® membranes. asymmetric membrane products are still based on the conventional CA materials. UTC-20 by Toray Industries .3-benzenediamine.103]. the CPA2 membrane produced by Hydranautics . has undoubtedly played an important role . DOW FILMTECTM .99]. it can achieve excellent rejection of divalent anions such as sulphate at high ﬂux.g.e. 1-isocyanato-3. NTR-7250 by Nitto Denko . i. heat curing was avoided. Sundet also patented the use of isocyanato aromatic acyl halides (e. researchers have been combining the use of various analytical techniques. The success of FT-30 led to the release of a number of similar products . and the UTC70 by Toray Industries . interfacial polymerisation of monomeric aromatic amines. In addition. Membrane FT-30 (Table 3) was prepared by the interfacial reaction between 1.113.62.g. the membrane structure. Fig. NF-40 series by DOW FILMTECTM . On the other hand. Lee et al. Although not completely resistant to chlorine attack. Unlike other interfacial polymerization methods. membranes supplied by Toray are based on UTC-70. showed superior resistance to fouling and chlorine due to its relatively neutral surface charge and stronger polyamide-urea bond linkage . For example. whilst at the same time maintaining high salt rejection due to the increase in effective membrane area . The development of RO membrane by reaction optimisation and postsynthesis surface modiﬁcations: (a) Dow Filmtec seawater series and (b) Toray brackish water series. Hydranautics membranes are based on NCM1. for example the Toyobo HollosepTM range of products is based on CTA and is the dominant asymmetric RO membrane. as well as more effective design of the module structure [41. thermal and chemical resistance.g. FT-30 shows a degree of tolerance to chlorine which is sufﬁcient to withstand accidental exposure to this chemical . In seawater desalination tests. Studies have shown that this rough ‘ridge and valley’ surface feature is closely related to the increased effective surface area for water transport and thus water ﬂux .e. cyclohexane-1. resulting in an aralkyl polyamide membrane giving better ﬂux [91. associated with advancements in membrane characterization techniques. i. Nevertheless. and insight into. This latter membrane. / Journal of Membrane Science 370 (2011) 1–22 11 salt rejection performance . TEM. It has been difﬁcult to track post-1990 development of commercially important RO membranes due to greatly reduced patenting activity by membrane manufacturers. and closer monitoring of interfacial polymerization reaction parameters. 2. with 99. producing a very unique surface characteristic.102. with trimesoyl chloride giving the best results [89.P.114]. Atomic Force Microscopy (AFM) has been a useful tool which has conﬁrmed that surface roughness of a membrane can greatly enhance permeability. rather than the smooth or slightly grainy surface obtained from aliphatic amines . The biggest manufacturers of desalination membranes.e. FT-30 yielded ﬂuxes of nearly 1 m3 m−2 day−1 . currently sells products based on FT-30.5-benzenedicarbonyl chloride) as cross-linking agents for 1.5-tricarbonyl chloride. this polypiperazine-amide membrane exhibited strong Donnan exclusion effects due to the anionic charged surface via the presence of carboxylic groups. ATR-FTIR. XPS. and a series of products based on this membrane have been commercialised by DOW FILMTECTM . and Trisep membranes are based on X-20. This makes it sufﬁciently attractive for practical usage in nanoﬁltration (NF) and the membrane has been designated as NS-300 (Table 3). To reveal the chemical composition and post-treatment that has been performed on commercial RO membranes. A range of NF membranes based on similar chemistry have been commercialised. Rutherford back scattering spectrometry is a powerful tool for elemental composition analysis at different layers and physicochemical characterisation [117–120].111]. e. The aromatic polyamide structure of FT-30 provides a high degree of resistance to compression. 5. better understanding of. originally released in 1972.2% salt rejection operating at 55 bar. i. in order to produce a membrane containing both amide and urea linkages that excels in both ﬂux and salt rejection (Table 8) . Despite the fact that no new polymeric membranes has been commercialized recently. and the recovery of fresh water can be over 60%. The Permasep A-15 TFC membrane (Table 3) was prepared by reacting 1. which has been described as a ‘ridge and valley’ structure. water permeability has been at least doubled. Cadotte discovered that membranes with excellent permselectivity can be produced using monomeric aromatic amines and aromatic acyl halides containing at least three carbonyl halide groups. Membrane post-synthesis modiﬁcations and control of interfacial polycondensation reactions After the revolutionary success of the introduction of crosslinked fully aromatic polyamide TFC RO membranes into the market. 5). and streaming potential measurement has also been used to gain a better understanding of both physical and chemical structure of . the performance of RO membranes has still improved dramatically (Fig. Current products from major manufacturers of RO desalination membranes are still based on the original chemistry discovered during the 1980s. and acid acceptor and surfactants were not required because polymerisation and crosslinking were both rapid even when acyl halide was supplied at lower concentrations. as well as a wide pH operating range. Crowdus has concluded that this membrane has signiﬁcant impact on the design and cost of RO desalination . Cadotte revisited this case and optimised the polymerisation conditions [61.3-benzenediamine with a saturated cross-linking agent.3-benzenediamine with trimesoyl chloride.3.K. which is identical to CPA2. A combination of various analytical techniques. designated X-20.
Mickols patented the addition of a ‘complexing agent’ into the acyl chloride (normally trimesoyl chloride) solution. A 70% ﬂux improvement is attained by soaking composite membranes in solutions containing various organic species. especially N. This minimises the concurrent hydrolysis and allows sufﬁcient reaction between the acyl halide and amines to take place for enhanced membrane formation. nucleation rate. Mickols patented post-treatment of a membrane surface with ammonia or alkyl compounds.N-dimethylformamide.and plasma-induced grafting. 6 shows micrographs of RO membranes produced using different additives which result in different permselectivities. protonation. For a more complete review of academic research activities on surface modiﬁcation of TFC membranes readers are directed to . or polyhydric alcohol to the amine solution can improve membrane permeability without signiﬁcant change in salt rejection [152–156]. The use of amine salts such as the triethylamine salt of camphorsulfonic acid. Membrane surface analysis showed that the ﬂuorine ratio was increased as a result of the treatment. / Journal of Membrane Science 370 (2011) 1–22 the membrane and how it relates to the membrane performance [121–123]. glycerol. a polymeric brush layer is formed by free radical graft polymerisation using methacrylic acid or acrylamide monomers. reaction time. sulphur-containing compounds. The reported stable performance over time has been  attributed to a poly(vinyl alcohol) (PVA) coating on the surface of conventional fully aromatic polyamide membranes . Diffusion of the monomer amine reactants has been enhanced resulting in the formation of a thinner barrier layer and improved water ﬂux .N-dimethylaminoethyl methacrylate) have also shown superior resistance against chlorine attack [123.1. reported that the use of atmospheric gas plasma surface activation and graft polymerisation on the surface of conventional polyamide TFC membranes can greatly enhance anti-fouling properties . redox. One important example was the launching of Hydranautics LFC series in 1996 .3. Mixtures of alcohol (ethanol and iso-propanol) and acid (hydroﬂuoric and hydrochloric acid) in water have also been used to improve ﬂux and rejection due to the partial hydrolysis and skin modiﬁcation initiated by the alcohol and acid . solvent solubility. and the triethylamine salt of camphorsulfonic acid [131. reactant diffusion coefﬁcients. Whilst increasing the ﬂux without altering the chemical structure. For example. the monomer reactants used are not readily available and the preparation method is complex [125–128]. water soluble polymers. Other surface modiﬁcation techniques including the use of free radical-. which produces more surface charge and eventually enhances the hydrophilicity and water ﬂux remarkably. The early success of Tomaschke  and Chau  in using additives in the casting solution (amine reactants) has led to intensive research in using different species of additives. 2. The inclusion of additives into the casting solution plays a major role in alteration of monomer solubility. these membranes are designed to minimise the adsorption of organic foulants. Optimisation of polymerisation reactions Another area of intense research study is the optimization of interfacial polymerisation reaction mechanisms. more speciﬁcally surface-modifying macromolecules. In this method the additive can move toward the active . Whilst there are proactive academic research activities in this ﬁeld. Fig.144].134]. Therefore posttreatment to chemically modify the membrane surface properties is preferred. Many patents disclose that the addition of alcohols. sodium lauryl sulphate. The membrane was soaked in a 15% solution of hydroﬂuoric acid for seven days and exhibited about a 4-fold improvement in ﬂux and slightly higher salt rejection. Post-treatment of membranes using an aqueous solution of poly (vinyl alcohol) and a buffer solution can effectively improve the abrasion resistance as well as the ﬂux stability of the membrane [133. this method however suffers from leaching of the hydrophilizing components over time causing the loss of any ﬂux enhancement . and the introduction of the LFC3-LD in 2005  targeting applications in wastewater treatment/reclamation.160]. Lee et al. etching of the surface resulted in a thinner barrier layer . and they can also act to scavenge inhibitory reaction byproducts . the miscibility of water and hexane has been improved by the addition of dimethyl sulfoxide into the casting solution. Most widely used are phosphate-containing compounds such as triphenyl phosphate.138]. whereas argon plasma treatment can greatly enhance chlorine resistance by increasing the extent of cross-linking at the nitrogen sites [143. are currently used to covalently attach some useful monomers onto the membrane surface which has been covered in . In addition.3. Chau added polar aprotic solvents. from Kwak et al. Lin et al. and characteristics of the micro-porous support [146–150]. radiation-. coatings have been the preferred method to tackle fouling issues. Gas plasma treatment is also used to induce surface modiﬁcation: water permeability is improved by oxygen plasma treatment due to the introduction of hydrophilic carboxylate groups. and various chemical and physical techniques have been developed.2. particularly in mineral fouling test. Various water soluble solvents such as acids and alcohols have been used to treat the membrane surface. into the casting solutions which eventually gave higher residues of carboxylate content and thus increased the water permeability. . Recently the introduction of active additives.P. The presence of hydrogen bonding is claimed to encourage interaction between the acid and water. As mentioned in reference . polymer molecular weight range. Although there has been some success in synthesizing membranes using monomer reactants with incorporated hydrophilic groups (such as carboxylate) and eliminated amidic hydrogen. Instead of mixing additives into the amine reactant solutions.132]. This brush layer can effectively reduce the ability of foulants to adhere to the surface. Hydrophilization has also been achieved by coating the membrane surface with more hydrophilic compounds. which can modify and eliminate the repulsive interaction of acyl chloride with other compounds by removing the halides formed during amide bond formation. photochemical-. After gas plasma surface activation. in particular permeate ﬂux is increased [159. including kinetics. ethers. Coatings of PVA and poly(N. Cahill et al. particularly ethylenediamene and ethanolamine. In addition. both atmospheric gas plasma treatment and graft polymerisation are readily adaptable to large scale membrane manufacture.g. As a result a more cross-linked membrane was formed with an improvement of the salt rejection without compromising the ﬂux. into the reactants has been reported. this review has focused on the most inﬂuential engineering developments that have been adapted into commercial products. as an additive in the aqueous amine reaction solution enabled post-reaction drying at temperatures higher than 100 ◦ C. e.140]. solution composition. Neutrally charged. Recently hydrophilic dendritic polymers have been reported to have successfully modiﬁed a membrane surface to reduce fouling effects [139. has reviewed the use of various analytical tools for membrane characterisation . curing time. Recently. achieved both enhanced ﬂux and salt rejection . Surface modiﬁcation A major area of membrane post-treatment research involves hydrophilization. hydrolysis. which can give an increase in permeability and chlorine resistance.12 K. diffusivity. 2. as proven in various fouling tests where this membrane has outperformed the commercial low-fouling membrane LFC1. Dramatic ﬂow enhancement was achieved via chemical treatment of a FT-30 membrane.
P. these membranes have shown comparable contaminants rejection performance with double the ﬂux. Due to the high manufacturing cost. high operating temperatures. (c) Flux: 1.7%.163]. (Reprinted with permission from Kwak et al. As illustrated in Fig. Though more membrane modules have been released. The composite multi-layer dendrimer structure allows the control of narrower pore size distribution. Salt rejection: > 99. and hence alter the surface chemistry to obtain desirable properties. Considering the similarity of polymeric NF and RO membrane morphology.1. 6. especially the membrane permeability. use is currently limited to applications where polymeric membranes cannot be used.2. 3. 7(a). 3. Recently. (b) Flux: 1. or any mixture of these materials. advances in nanotechnology have led to the development of nano-structured materials which may form the basis for new RO membranes. In this section. titania. In respect to commercial NF membranes. zirconia.15 m3 m−2 day−1 . 5. Salt rejection: > 96%. and highly reactive 3. > 15 bar for 0. Scale bar is 600 nm for all ﬁgures. Fig. silica.4%. Note: Tested at 20 ◦ C. Lee et al. Ceramic/Inorganic membranes Ceramic membranes are mostly made from alumina. radioactive/heavily contaminated feeds. the development of membranes that have been discussed in the previous two reviews will be brieﬂy highlighted with a focus on their possibility to be engineered into commercial RO membranes.161]. Li and Wang have included inorganic membranes and thin ﬁlm nano-composite membranes in a recent review whereas Mauter and Elimelech have discussed the perspective of carbon nanotube membranes as high ﬂux ﬁlters [2. with an average roughness in the range of 1–2 nm as compared with the higher values of roughness for commercial NF membranes (20–70 nm). with a thickness of about 20 nm. this membrane exhibited an extremely smooth surface.52 m3 m−2 day−1 . discussion about structured polymeric membranes synthesized via a new route. 7(b) shows one of the RSA molecules that were synthesized by various cyclization approaches as membrane building block materials. Polymeric membrane by rigid star amphiphiles A nanoﬁltration membrane based on rigid star amphiphiles (RSA) has been reported recently [162. FE-SEM micrographs of RO membranes surface of various permselectivity. further investigation is needed to verify the suitability for RO process. in particular its salt rejection is still unknown. through an asymmetric polyethersulfone support that had been previously conditioned with methanol and a cross-linked poly-vinyl alcohol.K. the participation of hydrophilic surface-modifying macromolecules. in the interfacial polycondensation reaction has improved the membrane ﬂux and stability of salt rejection over time . / Journal of Membrane Science 370 (2011) 1–22 13 Fig. However.e. As shown in Fig. such as poly(ethylene glycol) end-capped oligomers. surface during the polymerization. . only polymeric membranes have been employed for industrial use. The coverage of all proposed novel desalination RO membranes in this section is aimed to provide a general overview of these materials and to draw a fair comparison to their possibility to be developed into commercial RO membranes. carbon-derived nanoporous membranes and biomimetic membrane are included. most of them are improved by increasing membrane area per module. Salt rejection: > 98. At the same time. i. the membranes were prepared by direct percolation of methanol solutions of the RSAs. In SEM and AFM analysis.1%..2% NaCl solution. the advances in conventional polymeric RO membrane has been rather limited since the late 1990s. For example. this new route of polymeric membrane synthesis may offer a better alternative in tuning the membrane structure.16 m3 m−2 day−1 . Novel desalination RO membranes Since the operation of the ﬁrst RO desalination plant.85 m3 m−2 day−1 . The RSA membrane barrier layer is ultrathin. (a) Flux: 1. (d) Flux: 1. 1999) . Salt rejection: > 98.
in a test using a feed containing mixed ion species. i. / Journal of Membrane Science 370 (2011) 1–22 Fig.14 K. Membrane elements have been developed from simple tubular modules to monolithic honeycombtype structures which offer higher packing efﬁciency. environments . (a) RSA membrane synthesis process (Adapted from Lu et al. 2007) . which dominates the wettability and membrane surface charge. 8 shows the sub-nm inter-crystalline pores within the zeolite structure that allow the passage of water molecules and rejects the salt .56 nm.e. The Si / Al ratio. Although the ﬁrst RO test with a zeolite membrane was unsuccessful. Generally ceramic membranes are made up of a macro-porous support layer and a meso. and slip-casting of powder suspensions or sol–gel processing of colloidal suspensions for deposition of the active layer. The industrial use of ceramic membranes in domestic water production is rare but their process robustness has attracted the attention of researchers for both membrane distillation [166. It is also reported that rejection of bivalent cations was higher than for monovalent ions. Theoretical calculations have shown that ions can be completely excluded by zeolite membranes with pore sizes smaller than the size of the hydrated ion. and (b) MFI Zeolite.P. The ﬁrst experimental attempt at RO of a NaCl solution using a MFI silicalite-1 zeolite membrane showed 77% salt rejection and a water ﬂux as low as 0. (Reprinted with permission from Baerlocher et al. Fig.. These results show that the ﬁltration mechanism is not only dependent on size exclusion. Lee et al. both salt rejection and water ﬂux were too low to be of practical use. Micro-porous ceramic membrane structure: micro-porous channel in the crystalline structure (a) Type A Zeolite.or micro-porous active layer.and ultra-ﬁltration applications whereas ceramic membranes for nanoﬁltration are under development . A-type zeolite membranes Exhibit 0.003 m3 m−2 day−1 at 21 bar. 8. Early results on the use of ceramic membranes for RO desalination have recently been reported by a group of researchers from the New Mexico Institute of Mining and Technology . In other words the rejection of sodium ions in a mixed ion solution was lower than that for a pure solution of NaCl. 7. Fig. Currently commercial ceramic membranes are widely used in micro.4 nm pores and MFI-type membranes 0. but also on Donnan exclusion due to the charged double layer induced by adsorbed ions on the pore or the intercrystalline walls . 2007)  and (b) one of the RSA molecules tested in . Given the potential of desalting oil ﬁeld water. The state of the art ceramic membrane preparation techniques include paste extrusion for supports.167] and pervaporation . subsequent work has been conducted to improve both by modifying the zeolite structure. this group has experimentally investigated the RO separation mechanism and the feasibility of application of ceramic membranes. The Al content in the membrane can alter the surface hydrophilicity and therefore .. has been optimised to give improved ﬂux and salt rejection. and being motivated by molecular dynamic simulation results showing 100% of ion rejection by perfect all-Si ZK-4 zeolite membranes .
(Reprinted with permission from Gogotsi et al. These particles have been reported to be very hydrophilic (contact angle < 5◦ ). 10).5. widely used for disinfection and decomposition of organic compounds . This is followed by a series of complex processes involving template removal. CDC offers good control of pore size. further research is necessary to test the feasibility and practicality of CDC membranes for RO desalination.7 nm. 2007) . organic fouling has caused almost 25% loss in ﬂux after only 2 h of operation. The concept of MMM. Feeds of higher salinity are expected to cause shrinkage of the double layer due to the screening effect of counter ions on the surface charge. RO membranes with various zeolite loadings were prepared and consequent changes in membrane .1.e. The synthesis of CDC membranes by formation of a thin CDC ﬁlm on top of a porous ceramic support has been reported . Hence. 10.3. No signiﬁcant loss of TiO2 nano-particles from the membrane was observed after a continuous 7-day RO trial [184.P. with negatively charged 0. for example via manipulation of chlorination temperature as shown in Fig. the combination of organic and inorganic material. Schematic cross-section of zeolite nanocomposite membrane (Reprinted with permission from Jeong et al.. Zeolite nano-particles have also been used to prepare MMMs (Fig. and long operational experience of polymeric membranes. 3. especially with the aid of UV excitation.4 nm pores which are highly repulsive to anions. / Journal of Membrane Science 370 (2011) 1–22 15 Fig. Mixed matrix membranes (MMM) Fig. biological and thermal stability of inorganic membranes . whilst zeolite membranes are claimed to have high organic rejection. These were subsequently dip-coated onto an interfacially polymerized fully cross-linked polyamide TFC membrane with a surface layer functionalised with carboxylate groups . A homogeneous dispersion of zeolite particles is achieved using ultrasonication before the standard interfacial polymerization is carried out. Nano-particle/polymeric membranes Titanium oxide (TiO2 ) is a well known photocatalytic material. Though the improvement of zeolite membranes has been tremendous in the past 10 years. Despite the fact that MMMs were developed for water / ethanol separation via pervaporation in the 1990s. with a 2 ␮m thick zeolite membrane with 50:50 Si/Al ratio rejecting 92. This preliminary study introduces a route to producing asymmetric CDC membranes with average pore sizes of about 0. i. Testing with E.5% NaCl should be investigated to evaluate potential usage for desalting oil ﬁeld seawater. coupled with the superior chemical. afﬁnity with water . showing superior selectivity to conventional polymeric membranes . Moreover. though full recovery of ﬂux was achieved after chemical washing . Nevertheless. the incorporation of inorganic materials into organic RO TFC membranes only started in the early 2000s . The carboxylate groups are necessary for the self-assembly of TiO2 within the barrier layer via an adsorption mechanism.185]. and these properties make it interesting as an anti-fouling coating. Lee et al. Controlled pore size distribution of carbide derived carbon (CDC) materials has been reported . an undesirable increase in effective intercrystalline pore size would facilitate ion transport and therefore reduce rejection efﬁciency. providing excellent organic (>99%) and salt rejection (97. First zeolite nano-particles are synthesized via a templated hydrothermal reaction. 9. their performance and economics are still no match for polymeric membranes. is not new. Anatase TiO2 nano-particles (<10 nm) have been prepared by the controlled hydrolysis of titanium tetra-isopropoxide. The zeolite nano-particles are dissolved into a cross-linking agent solution (trimesoyl chloride dissolved in hexane) before the interfacial polycondensation reaction takes place. showing a potential for monovalent salt exclusion. the thickness of the membrane has been further reduced to 0. UOP developed a silicalite-cellulose acetate MMM for gas separation in 1980. Carbon is another candidate for formation of sub-nm porous material.K. This is different to dipping the previously formed membrane into a nano-particle-containing solution. These tests were carried out with a low NaCl concentration (0.7 ␮m.177]. the high packing density.3. 3. without compromising the ﬂux and salt rejection performance of the original membrane. This combined effort generated a remarkable improvement. This value can be even higher when the higher density and lower packing effectiveness are considered. The main objective of MMM is to combine the beneﬁts offered by each material. The zeolite membrane thickness is still at least 3 times higher than the current state of the art polymeric RO membranes..129 kg m−2 h−1 at 28 bar . causing higher resistance to water ﬂux.1%) and standard seawater desalination tests at 3.3%) as well as nearly 4 times improvement in water ﬂux [176. In a recent report from the same group. shape and uniformity. 2003) . Differential pore size distribution for CDC synthesized membrane at different chlorination temperature measured by methyl chloride adsorption. carbonization. Defects in the crystal structure are minimised by secondary growth of a zeolite layer on zeolite seeded onto a porous ␣-alumina substrate . sodium exchange and calcination .9% of sodium ions with a water ﬂux of 1. coli-containing feed water has shown superior anti-biofouling properties. The resultant NaA-type zeolite particles are in the size range of 50–150 nm with a Si/Al ratio of 1. Consequently ceramic membranes require at least 50 times higher membrane area than polymeric ones to achieve an equivalent production capacity. good permselectivity. as with the TiO2 nanocomposite membrane. 9.
This subsequently leads to the formation of a vapour layer between the surface and the bulk ﬂow that facilitates the water transport in a slug ﬂow manner [198. 11 illustrates the changed surface properties and membrane separation performance as a result of variation in zeolite nanoparticle loading.187]. Transport of ions of various valences has been studied in double walled CNTs of 1–2 nm diameter. prepared by cCVD. This conclusion is based on the observation that solution pH and electrostatic screening length signiﬁcantly inﬂuences ion rejection [191. Recently. 12. Effect of zeolite loading dosage on (a) surface properties. / Journal of Membrane Science 370 (2011) 1–22 Fig.193. (b) SEM micrograph showing cross-section of CNP membrane .3.199]. experimental analysis of water transport in a solid polystyrene ﬁlm membrane incorporating 7 nm diameter multi-wall CNTs was published. The factor of ﬂow enhancement over the predicted value is about 20. characteristics were observed. The authors suggest that this could be a result of enhanced Donnan exclusion by the zeolite particles and changes of membrane morphology [186. much smaller than in previous cases .2. water transport through template-grown carbon nanopipes (CNPs) of about 44 nm in diameter was investigated. which provides ‘shielding’ of the bulk water molecules such that they ﬂow faster . i. The origin of this extremely fast water transport in CNTs is not completely clear. Majumder’s group also demonstrated two methods Fig. This membrane contained double wall CNTs with <2 nm diameter. Shortly after this. pore misalignment. which governs macroscale hydrodynamics . and seemingly contradictory explanations have been reported by simulation studies on this topic [195–197]. Experimental results of ﬂuid ﬂow in a CNT membrane was ﬁrst reported in 2004 .. 2007) . this study suggests that the ion exclusion mechanism in CNTs is dominated by electrostatic interactions (Donnan exclusion) rather than steric effects.194]. 12(b)). yielding an amorphous (or turbostratic) graphitic structure (Fig. These CNPs were synthesized using non-catalytic CVD. The observed ﬂow velocity was four to ﬁve orders of magnitude higher than the value expected from the Haagen–Poiseuille equation. Well aligned multi-wall CNTs were grown by catalytic chemical vapour deposition (cCVD) on the surface of quartz substrates.16 K. The authors suggest that this might be due to the different surface chemistry and structure compared to the CNTs in the previously cited articles. 3. Fig. Although the ion rejection is not sufﬁciently high for desalination. Ion transport through the CNT channels has been investigated both experimentally and computationally. Carbon nano-tube/polymeric membranes Carbon nano-tubes (CNTs) have caught the attention of many researchers due to the similarity between their ﬂuid transport properties and those of water transport channels in biological membranes . On the other hand. and (b) separation performance (Reprinted with permission from Jeong et al. the membranes were smoother. and branching.201]. These were spin coated with polystyrene to seal the inter-tube gaps and plasma etching was used to open the tips of the CNTs. it also has been argued that the frictionless water ﬂow is due to the formation of a layer of liquid water molecules on the CNT walls. more hydrophilic and more negatively charged with increasing nano-particle loading.e.P. It has been suggested that the development of a strong hydrogen-bonding network between the water molecules and the atomically smooth hydrophobic inner nanotube wall causes spontaneous imhibition. The fast water transport observed has led to many ongoing scientiﬁc discussions [188. functionalized with negatively charged groups. and showed ﬂow velocities of three to four orders of magnitude higher than the theoretical calculation . Another ﬂuid ﬂow experiment with a CNT membrane synthesized using nanofabrication techniques has been reported (Fig. The presence of the template eliminates structural imperfections such as tortuosity. 11. 12(a)). Lee et al. The MMM membrane exhibited 90% of ﬂux and a slight improvement in salt rejection relative to the hand cast TFC membrane without zeolite nano-particles. (a) Schematic of CNT membrane reported in . .
2010) . namely alteration of pore size by CNT tip-functionalization. / Journal of Membrane Science 370 (2011) 1–22 17 Fig.4. which are often present at both ends of CNTs. studies on much larger surface area membranes are needed before large scale manufacturing methods can be developed. A company named Aquaporin was founded in Denmark in 2005 in order to develop these membranes for practical industrial use. the size distribution of the CNT diameters is still not small enough to complement the simulation studies performed. The patent also discloses the two different orientations of the membrane: (i) a lipid bilayer incorporating the aquaporins is sandwiched between two hydrophilic porous support layers such as mica. such as identiﬁcation of appropriate support materials. they represent an ideal opportunity for the production of ultra pure water [208–210]. Further study is needed to incorporate aquaporins into the phospholipid bilayer for practical use in water puriﬁcation. Furthermore. A molecular dynamic simulation of RO using CNTs has been performed depending solely on physical size exclusion mechanisms. Membranes incorporating bacterial Aquaporin Z proteins have been reported to show superior water transport efﬁciency relative to conventional RO membranes . 14) . This has shown that 0. In this way CNTs can be effectively embedded onto the barrier layer formed by conventional interfacial polymerization on a micro-porous polyethersulfone support . A continuous phospholipid bilayer was successfully formed on a. such as catalytic growth of CNTs onto expensive substrates. and in this patent the CNTs are functionalized with octadecylamine. Fig. The company has recently been awarded a patent on the method of fabricating membranes incorporating aquaporins (Fig. A test is disclosed in the patent which compares membranes fabricated with and without embedded CNTs.. or (ii) a lipid bilayer incorporating aquaporins . and fully covered a NTR-7450. were not considered in this simulation. and CNT tip opening via etching. Aquaporins were incorporated into the walls of self-assembled polymer vesicles constituted of tri-block co-polymer. Rather than using triblock polymers the aquaporins are reconstituted into lipid bilayers fabricated using the LangmuirBlodgett method.8 nm in diameter. with the CNTs used being 0. 13). a vesicle fusion method. 3. and voltage based gate control [202. to demonstrate the enhanced ﬂow generated by the CNT pathways. The use of an NF membrane as a biomimetic membrane support has been reported . which are proteins functioning as water-selective channels in biological cell membranes . extremely high salt rejection is expected from aquaporins since their functional biological performance is to only allow the passage of water molecules.69% as compared with 96. and even identiﬁcation of an appropriate range of operating conditions must be carried out to develop this membrane for practical use.K. as the formation of charged double layer can improve the salt rejection . more work is needed in the development of efﬁcient synthesis methods to align arrays of single-walled CNTs. these studies have demonstrated the potential of altering pore properties to enhance selectivity. An initial permeability test was carried out on the aquaporin-triblock polymer vesicles by stopped-ﬂow light-scattering experiments. Mauter and Elimelech summarize previous research efforts into the development of CNT membranes for desalination. polymer ﬁlling of the inter-tube spaces. 13. 2010) . understanding of the resistance to membrane fouling. for formulation of composite polymeric membranes (Fig. Schematic cross-section of CNTs embedded TFC membrane (Adapted from Ratto et al.205]. The CNTs need to be functionalized to obtain better solubility in organic solvents. The water permeates through the membrane via both the conventional polymeric barrier layer and the embedded CNT pathways.. using the vesicle fusion approach. Although monovalent salt rejection is not tested in either case. The results reported at least an order of magnitude improvement in permeability compared to commercially available TFC RO membranes . With CNTs present a slightly higher salt rejection was achieved (97. The study concludes that whilst CNT membranes are promising for ﬂux enhancement.P. or they are spin-coated. The resulting membrane can be easily adapted into current ﬁltration and RO systems. and in many other gas separation studies. 14.203]. and forecast the necessary work on the next generation of CNT membranes . and also the development of tip-functionalization for more efﬁcient salt rejection. Many practical issues. polysulfone or cellulose. preferably cross-linking agent solutions (trimesoyl of isophthalic chlorides). substrate removal. The effect of their presence would probably be able to expand the CNT size regime. To overcome this problem. These studies have so far been limited to investigating water permeability properties across a barrier layer composed of aquaporins and triblock polymers. involves multiple complex steps. depending on the assumed packing density of the CNTs [204. a patent has disclosed the blending of CNTs into solutions. Lee et al.19%) and a near doubling of water ﬂux (44 L m−2 day−1 bar−1 as compared with 26 L m−2 day−1 bar−1 ) was obtained. The fabrication of the CNT/polymeric membrane in the experimental studies described above [189–191]. Effects of charged functionalities. Hence. poly(2-methyl-2-oxazoline)-blockpoly(dimethylsiloxane)-block-poly(2-methyl-2-oxazoline). Biomimetic RO membranes The excellent water transport properties of biological membranes has led to the study of membranes incorporating aquaporins. to alter the selectivity of different ion species. Schematic cross-section of Aquaporin embedded membrane (Adapted from Jensen et al.8 nm CNTs can completely reject the salt whilst giving at least a 4 fold ﬂux improvement over current state of the art TFC RO membranes. However as the membrane disc that was synthesized was only 47 mm in diameter. Although a salt separation test has yet to be reported. with sub-nanometre diameters.
which has been dominating for more than 30 years. and controlling morphological changes by monitoring polymerisation reactions (1990s). the potential performance can be further improved by more dense and ordered packing of the CNTs. there are many fundamental scientiﬁc and technical aspects that have to be addressed before the potential beneﬁts may be realized. As with zeolite nano-composite membranes. Whilst nanotechnology is leading the way in the development of novel RO membranes for desalination. It is so far the most efﬁcient technology for wastewater reclamation (tertiary treatment). more resistance to biological and chemical attack is also very desirable. potentially achieving single pass RO desalination. Various nano-structured RO membranes have been proposed to offer attractive permeability characteristics. Unless combined savings from the enhanced permeability and superior membrane properties are proven. Coupling this with the ability to handle a wide range of water sources makes it a strong candidate to tackle current and future water shortage problems. shows a promising and scalable production technique that is almost identical to conventional RO membrane production . This research is expected to beneﬁt the desalination industry by lowering the energy cost and membrane area required. Theoretically. they appear to be more readily adaptable to commercial use due to their similarity to current commercial RO membranes. whereas severe fouling and concentration polarization are reported. particularly via the introduction of 16 and 18-in. In addition. and is one of the best performing technologies for desalting brackish water and sea water. Identiﬁcation of a suitable membrane support is another technical problem preventing practical implementation. However the manufacturing cost of the suggested 0. The evolutionary improvement of membranes solely prepared from polymeric materials seems to be approaching saturation. Acknowledgement The authors would like to acknowledge the University of Bath for supporting this work via the award of an Overseas Research Student Excellence Scholarship to Kah Peng Lee. The contents . Alongside the advancements in other aspects of RO technology. and increasing plant capacity. and operational robustness is still ongoing. An example target is the development of single-pass RO using multifunctional membranes. elimination of crystalline defects. Instead of incorporation of CNTs into polymer solutions. Despite the almost absolute salt rejection. their cost effectiveness needs to be investigated because signiﬁcant additional expense is expected to arise from the complex synthesis of nano-particles. nano-structured RO membranes appear to be in the same position as the initial development phase of polymer RO membranes. high ion and organic contaminant rejection. the search for multifunctional membrane materials that offer higher permeability. health and safety issues around the use of nano-materials have to be addressed in the domestic water industry. albeit with the latter now being a successful competitor in the market. Polymeric membrane fabricated via rigid star-shaped amphiphilic molecules is one of the ﬁrst real breakthroughs since the interfacially polymerised RO/NF membrane. The former is obviously a highly desirable solution in densely populated regions since it solves two problems simultaneously. namely wastewater treatment and enhancement of fresh water supply. which requires de-chlorination of the RO feed and rechlorination of the RO permeate. however strong competition is foreseen. Furthermore. and simpliﬁcation of nano-particle synthesis procedures are also needed. the packing density of aquaporins in a membrane can signiﬁcantly affect the overall permeability performance. Although the increased permeability of zeolite thin ﬁlm nano-composite membranes is the lowest when compared with membranes incorporating CNTs and aquaporins. The durability of the membrane is therefore expected to be relatively low. Further technical studies such as optimisation of nano-particle size and dosage. Nevertheless. excessive pressure is required to overcome the resistance arising when membranes become fouled. and are also possibly the most expensive. biomimetic membranes offer the highest permeability. Another key limitation of commercial RO membranes is the degradation by chlorine. the development of such membranes is only in the initial stages and many problems are yet to be overcome. The novel membrane materials are expected to outperform current RO membranes especially those listed in Table 1. there is no report of an RO membrane fabricated by this means so far. particularly with respect to the use of nano-particles. At this stage these novel technologies are still too expensive for practical application. the fabrication process for these membranes are amongst the most complex of all the systems analyzed in this review. (ii) selecting suitable polymerisation reactants based on a better understanding of polymer chemistry. In either case the patent does not disclose any numerical data regarding the ﬂux and salt rejection performance of the membranes. fully cross linked aromatic TFC membrane (1970s to 1980s). disclosed in the patent granted to NanOasis. RO membrane elements. / Journal of Membrane Science 370 (2011) 1–22 is assembled over a hydrophobic porous support membrane such as a porous PTFE ﬁlm. Most importantly. In addition. the incorporation of CNTs into membranes for water production appears economically unfavourable. The cost of CNT tip functionalization and the energy required to homogeneously disperse the CNTs also have to be added. The development of polymeric membrane materials has gone through three main stages: (i) empirical trial-and-error testing of polymers.800 per gram and upwards . Currently. However. In summary it promises to make signiﬁcant reductions in both capital investment and operating costs.18 K. and (iii) closely monitoring membrane morphology with the aid of advanced characterization tools. The two major practical challenges are the high cost of nano-structured materials. Despite major earlier breakthroughs such as the Loeb-Sourirajan asymmetric membrane (1960s). In addition. these bio-materials are relatively unstable. and hence the development of novel RO membranes with improved salt rejection and permeability at a reasonable cost is still the key focus of RO desalination technology. the incorporation of CNTs into membranes. and many scientists believe that nanotechnology could possibly bring revolutionary advancements to the desalination industry. Although it offers the possibility of engineering membrane structure in the nano-scale. Additionally we acknowledge the UK EPSRC (grant EP/G045798/1) and the EU (grant PIRG03-GA-2008-230876) for funding support. but this conclusion is solely derived from transport velocity measurements of water molecules through individual aquaporin channels rather than conventional membrane permeability tests. Nonetheless. Lee et al. 4. the development of membrane materials has undeniably made RO desalination more economic by increasing performance and efﬁciency. providing lower membrane maintenance costs. the evolutionary improvement of a commercial RO membrane has been rather slow during the ﬁrst decade of this century. Conclusions and future developments RO desalination has more than half a century of industrial operation. However.P. and the extra energy required to effectively disperse the nano-particles into the barrier layer. and the difﬁculty in scaling up nano-membrane manufacturing processes for commercial use. simplifying pre-treatment processes. eliminating the need for pre-treatment.8 nm single walled CNTs for RO membranes can range from US$1. with particularly severe fouling having been observed. in situ growth of CNTs via ceramic templating could offer a better method of engineering these novel membranes.
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