Membrane technology for water purification

Environmental Chemistry Letters https://doi.org/10.1007/s10311-017-0699-y

REVIEW

Membrane technology for water purification Lavanya Madhura1 · Suvardhan Kanchi2 · Myalowenkosi I. Sabela2 · Shalini Singh1 · Krishna Bisetty2 · Inamuddin3,4 Received: 12 December 2017 / Accepted: 13 December 2017 © Springer International Publishing AG, part of Springer Nature 2017

Abstract Managing higher water demands is a grand challenge of the twenty-first century due to pollution and climate change that are decreasing the amount of drinkable water. There is therefore a need for improved techniques to purify contaminated waters. Nanotechnology provides materials of unprecedented properties, which can be used to clean water. This article reviews recent developments in nanotechnology for wastewater treatment using novel polymeric membrane materials. The use of polymeric membrane materials and polymer brushes are discussed. Keywords Nanomaterials · Membranes · Polymer brushes · Water purification

Introduction The development of global water is closely associated with global climate change and the growth of the world population, which is estimated to be approximately 2-fold from 3.4 billion in 2009 to 6.3 billion people in 2050 (Bruinsma 2009). The demand for freshwater is drastically growing because 70% of the world freshwater is allocated to produce food in the agriculture sector. The World Water Development Report (WWDR) suggests that 64 billion cubic meters of freshwater is gradually used each year (Report 2014). Approximately, 70% of the population in European countries and dry areas of North Africa do not have suitable access to the freshwater supply. Innovative technologies been designed in the USA for the purification and saving of water, demonstrated the difficulty of fatigued water reservoirs since more water is extracted than refilled. In China more than

* Inamuddin [email protected] Shalini Singh [email protected] 1

Department of Operations and Quality Management, Durban University of Technology, Durban, South Africa

2

Department of Chemistry, Durban University of Technology, Durban, South Africa

3

Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

4

Centre of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia

85% of major cities suffer from a shortage of water, since the biggest rivers are immeasurably polluted and even their use for agriculture purposes has to be restricted. Scheme and co-worker designed a multimodel assessment on the regional and global scarcity of freshwater due to the climate change. Due to global warming, rise in 2 °C above the normal temperature will lead to the additional decreases in the water resources by ~ 15% global population and thereby increase the people living under absolute water scarcity by at least 40% as illustrated in Figs. 1, 2 (Schewe et al. 2014). For the last 50 years the USA has recorded their worst droughts. In contrast, water is highly polluted in countries with heavy rainfall, due to soil erosion and soil runoff into the surface and ground waters. The drinking water hygiene is affected by the rise in air and raw water temperatures in storage systems, resulting in detrimental transmittable diseases. For example, legionella bacteria may be grown at 40 °C in warm water and lead to the Legionnaire’s disease (Schewe et al. 2014). In Singapore, NEWater plant is a pillar for water sustainability strategy due to its high-grade reclaimed water. The clean water is produced from treated used water that is further purified using advanced membrane technologies and ultraviolet disinfection, it is ultra-clean and safe to drink. NEWater has passed more than 150,000 scientific tests and is well within World Health Organization requirements (PUB 2017). The schematic illustration is shown in Fig. 3. Micropollutants such as pesticides, pharmaceuticals, inorganic molecules and other aromatic organic compounds flow into the water bodies, mainly in industrialized and

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Environmental Chemistry Letters

Fig. 1 Relative change in annual discharge at 2 °C compared with present day, under Representative Concentration Pathways (RCP8.5) a color description shows the multimodel mean change, and saturation shows the agreement on the sign of change across all global hydrological models (GHM)-global climate models (GCM) combinations b ratio of GCM variance to total variance; in red (blue) areas,

GHM (GCM) variance predominates. GHM variance was computed across all GHMs for each GCM individually, and then averaged over all GCMs; vice versa for GCM variance. Greenland has been masked. Reproduced with permission of Tsuru et al. (2013), Copy right 2014, Nature

developing countries. Therefore, conventional cleansing procedures like ozonation and chlorination were adopted; however, with the influx of pollutants above these methods can further produce high amounts of toxic by-products. The advancement of nanotechnology has led research and development in the design of novel and cutting-edge materials for the development of sensitive, effective and modern water and wastewater technology. This review was mainly focused on the performance of nanopolymers, polymer brushes and their characterization followed by applications to water purification. Additionally, the membrane technology was also discussed in alignment with water purification.

focused on nanotechnology to improve the selectivity and flux efficiency by mounting antifouling layers. This section is basically dedicated on the state-of-art in the field of engineered-nanomembrane filtration.

Membranes and separation processes Membrane separation processes are rapidly developing for water and wastewater treatment due to its significant role in water purification. Depending on the molecule and pore size, membranes act as a physical barrier for substances. Therefore, membrane technology is a promising and reliable area of research for water and wastewater treatment as shown in Fig. 4. Some of the commercially available membranes and their specifications are tabulated in Table 1. In recent decades, novel research activity in water purification is mainly

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Fundamentals of membrane process In membrane processes, certain components are separated/ rejected from a sample solution by using pressure, electrical and concentration differences as a driving force. The flow of a sample solution through the membrane results in the difference in applied pressure of the solvent and the solute in aqueous system, where the solutes are retained. Table 2 represents the overview of different pressure-driven membrane processes and their main applications. As these membranes have very tiny pores, it is possible to separate/reject analytes from the water using higher pressures.

Fabrication and modification of membranes In recent years, membrane fabrication and modification have attracted more attention globally, in generating superior filtration techniques for the separation of organic and inorganic substances from the solutions. The pronounced improvement in the nanofiltration membranes occurred due to the creativity in adopting the modified methods


Fig. 2 a, b Adverse impact of climate change on renewable water resources at different levels of global warming. Markers show the percentage of the world population living in 0.5° × 0.5° grid cells where the 31-y average of annual discharge falls short of the 1980– 2010 average by more than 1σ (standard deviation of annual discharge during 1980–2010), or by more than 20%, under the Representative Concentration Pathways 8.5 climate scenario and SSP2 population scenario. The five global climate models are displayed in separate

vertical columns (in the order in which they are listed in materials and methods; note that only four global climate models have sufficient coverage of the 3 °C warming level), and the 11 global hydrological models are displayed in unique colors. The black boxes give the interquartile range, and the horizontal black lines the median, across all global climate models and global hydrological models. Reproduced with permission of Tsuru et al. (2013), Copyright 2014, Nature

such as interfacial polymerization and incorporation of nanoparticles into polymeric membrane (Van der Bruggen et al. 2008). Though there is a significant development in the applications of nanofiltration processes in industry, a few limitations prevail. For instance, these need to be an enhancement of separation and rejection efficiency, a reduction in membrane fouling and improvement in lifetime of the membrane (Van der Bruggen et al. 2008). Organic solutes, biological solids and colloids are distinctive foulants in membrane technology. Different innovative fabrication and modification techniques have been reported in the literature in order to fabricate membranes that have significant improvement in terms of fouling susceptibility. These techniques are mainly grouped into interfacial polymerization and incorporation of nanoparticles. Interfacial polymerization involves phase inversion and produces thin-film composite membranes into polymeric membranes. Recently, there has been more focus on the

assimilation of nanoparticles into thin-film layers leading to thin-film nanocomposite (Van der Bruggen et al. 2008). Nanofiltration membranes Nanofiltration membranes are pressure-driven processes in which particles and molecules less than 0.5–1 nm are rejected by the membrane. These membranes are characterized by a distinctive charge-based repulsion mechanism, allowing the separation of various ions (Jagadevan et al. 2012; Qu et al. 2013; Sharma and Sharma 2012). Nanofiltration membranes are used to reduce the color, odor, hardness and separate heavy metal ions from water systems. Desalination (conversion of seawater into potable water) is another demandable area of application since comparable desalination technologies are very cost-intensive.

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Fig. 3 Schematic illustration of NEWater treatment plant established in 1970s at Singapore. Reproduced with permission of PUB (2017). Copyright 2017, NEWater

Fig. 4 Schematic illustration of membrane separation process

Nanocomposite membranes The design and synthesis of novel nanomaterials that is nanoparticles incorporated into the polymeric membrane have been an interesting area in recent years for material science researchers. Due to the remarkable properties such as selectivity, mechanical properties and enhancement in membrane permeable capacity, these membranes provided opportunities for researchers to develop more effective and better materials for water treatment and purification (Gilman 1999; Godovski 1995; Okada and Usuki 1995). Recent reports suggested that titanium dioxide, zinc oxide, silver and silica nanoparticles are generally

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incorporated into the nanofiber membranes. For example, a hybrid of silica nanoparticles/poly(vinylidene fluoride) membranes are highly stable at high temperature and more selective for water treatment (Yu et al. 2009b). However, the zinc oxide nanoparticles incorporated into the chitosan film exhibited excellent antibacterial and mechanical properties (Li et al. 2010). Other examples include decoration of silica nanoparticles onto the polysulfone membranes which rapidly enhanced the gas permeability nature of the membrane (Ahn et al. 2008) and aluminum oxide nanoparticles integrated with polyethersulfone membranes for the effective decrease of flux during the membrane process due to its high porous nature and pseudo steady-state

Environmental Chemistry Letters Table 1 Commercially available membranes and their specifications (Dekker 2007; Martin and Kohli 2003; Van der Bruggen et al. 2008) Membrane

Manufacturer

MWCO (Da)

Max. temp. (°C)

pH range

Composition top layer

UTC20 Desal5DL Desal51HL NTR7450 N30F NFPES10 ESNA1 SPIRAPRO SR3D TFC SR100 NFG NFW NFX DK CK TR60 XN45 TS40 NF90 TS80 NF200 NF270

Toray, Tokyo, Japan GE Osmonics, Le Mee sur Seine, Frankrijk GE Osmonics Le Mee sur Seine, Frankrijk Nitto-Denko, Somicon AG, Basel, Switzerland Nadir, Wiesbaden, Germany Nadir, Wiesbaden, Germany Nitto-Denko, Somicon AG, Basel, Switzerland Koch, Wilmington, Massachusetts, USA Koch, Wilmington, Massachusetts, USA Koch, Wilmington, Massachusetts, USA Synder, Vacaville, CA, USA Synder, Vacaville, CA, USA Synder, Vacaville, CA, USA GE Osmonics, Le Mee sur Seine, Frankrijk GE Osmonics, Le Mee sur Seine, Frankrijk Toray, Tokyo, Japan TriSep, Goleta, CA, USA TriSep, Goleta, CA, USA Dow Filmtec, Midland, Michigan, USA TriSep, Goleta, CA, USA Dow Filmtec, Midland, Michigan, USA Dow Filmtec, Midland, Michigan, USA

180 150–300 150–300 600–800 400 1000 100–300 200 200 200 600–800 300–500 150–300 200 2000 400 500 200 200–400 150 200–400 200–400

35 90 50 40 95 95 45 50 50 50 50 50 50 50 30 35 45 50 45 45 45 45

3.0–10.0 1.0–11.0 3.0–9.0 2.0–14.0 0–14.0 0–14.0 2.0–10.0 3.0–10.0 4.0–10.0 4.0–10.0 4.0–10.0 3.0–10.5 3.0–10.5 3.0–9.0 5.0–6.5 3.0–8.0 2.0–11.0 3.0–10.0 3.0–10.0 2.0–11.0 3.0–10.0 2.0–11.0

Polypiperazineamide Cross-linked aromatic polyamide Cross-linked aromatic polyamide Sulfonated polyethersulfone Permanently hydrophilic polyethersulfone Permanently hydrophilic polyethersulfone Composite polyamide Proprietary thin-film composite polyamide Proprietary thin-film composite polyamide Proprietary thin-film composite polyamide Proprietary polyamide thin-film composite Proprietary polyamide thin-film composite Proprietary polyamide thin-film composite Polyamide Cellulose acetate Cross-linked polyamide composite Polyamide Polypiperazineamide Polyamide thin-film composite Polyamide Polyamide thin-film composite Polyamide thin-film composite

MWCO Molecular weight cut off

Table 2 Summary of the different membrane operated using pressure as a main parameter (Simpson et al. 1987) Membranes

Pressure (bar)

Pore radius (nm)

Mechanism

Matrix separation

Applications

Microfiltration Ultrafiltration

0.1–2 1–5

50–10,000 1–50

Sieving Sieving

Food and pharmaceutical industries Food and pharmaceutical industries

ca.1

Sieving: Donnan-exclusion

Not porous

Diffusion of solution

Particles Organic molecules (> 10,000– 100,000 g mol−1 MW) Organic molecules (200– 1000 g mol−1 MW) Organics and salts

Nanofiltration

Reverse osmosis

5–20

10–100

permeability (Maximous et al. 2009). The polybenzimidazole membranes incorporated with silica nanoparticles membranes exhibited very high permeability for gas molecules. Therefore, this membrane can be used for selective separation of gases (Sadeghi et al. 2009). Zhang and co-workers reported the synthesis of nanofiltration membranes using titanium/polyethyleneimine hybrid layer by the Mineralization Method (Zhang et al. 2014). In this study, inorganic precursors, namely tetra-n-butyl titanate

Textile industry

Food and dairy industries

and tetraethoxysilane, were used to fabricate polyethyleneimine–titania/silica composite membranes (Zhang et al. 2014). The literature suggested that the particles added to the polymer matrix have increased the stability of the polymeric membrane against change in permeable-selectivity with temperature (Hu et al. 1997). Yin and co-workers reported a novel thin-film nanocomposite with the hybrid of Mobil Composition of Matter (MCM-41)-silica nanoparticles (Yin et al. 2012). This experiment was conducted

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with a high pressure cross-flow filtration system (pressure range: 50–500 psi) to evaluate water flux and solute rejection as represented in Fig. 5. The scanning electron microscopic and transmission electron microscopic images demonstrated that Mobil Composition of Matter-41 nanoparticles had near-spherical shape with a particle size of 100 nm. Interestingly, it was found that highly ordered hexagonal array and streak structure were also observed inside particles. The transmission electron microscopic image of spherical silica nanoparticles also showed a uniform spherical shape with an average particle size around 100 nm, but without internal pores as shown in Fig. 6. As observed in transmission electron microscopic images, the cross sections of thin-film composite (TFC) and thin-film nanocomposite (TFN-0.05) membranes indicated that the polyamide thin-film layer in both membranes had a thickness between 300 and 500 nm. The leaf-like structure of polyamide thin-film layer was consistent as shown in SEM (see Fig. 5). The dark spots appeared in the polyamide thinfilm layer of thin-film nanocomposite indicated the presence of Mobil Composition of Matter-41 nanoparticles and their aggregation could be identified on the surface of thin-film layer in the transmission electron microscopic cross-sectional image as shown in Fig. 7. In this study, Mobil Composition of Matter-41 was decorated with silica nanoparticles, enhanced the membrane performances like roughness, zeta potential and permeability due to its porous nature.

In some reports, two types of nanoparticles were incorporated onto the membrane, which led to its considerable enhancement in quality. For instance, a new mixed polymeric membrane, namely polyethersulfone and self-produced polyaniline/iron(III) oxide nanoparticles, was prepared using phase inversion method by varying the amount of nanoparticles. Interestingly, it was noted that the membrane with 0.1 wt% nanoparticles indicated the maximum removal of copper(II) from the lowest pure water flux. This is caused by nanoparticles located in the superficial pores of the membrane during preparation, i.e., surface pore blockage. The field emission scanning electron microscopy and atomic force microscopic studies revealed that the separation mechanism occurred through adsorption process. To better understand the adsorption process, authors have demonstrated the various isotherm models such as Langmuir, Freundlich and Redlich–Peterson. Based on the isothermal results, the Redlich–Peterson model offered superior fitness indicating relatively complex adsorption mechanism (Daraei et al. 2012). Stawikowska and co-workers reported an electron microscopic technique to pick up the high quality microscopic imaging of nanostructured nanofiltration membranes. The membrane was integrated with osmium dioxide nanoparticles and resulted in a highly porous structure (Stawikowska et al. 2013). The hybrid of polymeric membrane/nanoparticles has some drawbacks. For instance, the dispersion of

Fig. 5 Schematic diagram of a high pressure cross-flow filtration system equipped with temperature control, feed tank and mechanical pump controlled by speed control with an automatic sample collector

coupled to conductivity meter. Reproduced with permission of Dutta et al. (2008a), Yin et al. (2012), Copy right 2012, Elsevier

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Environmental Chemistry Letters Fig. 6 Microscopic images of nanofillers a scanning electron microscopic image and b transmission electron microscopic image of porous Mobil Composition of Matter No. 41-nanoparticles; and c transmission electron microscopic image of non-porous spherical silica nanoparticles. Reproduced with permission of Yin et al. (2012), Copyright 2012, Elsevier

Fig. 7 Cross-sectional transmission electron microscopic images of a thin-film composite membrane and b thin-film nanocomposite 0.05 membrane. Reproduced with permission of Yin et al. (2012), Copyright 2012, Elsevier

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the nanoparticles into the polymer is one of the limiting factors. The crucial and first step in the modification of membranes with nanoparticles is aggregation or dispersion control of nanoparticles over the membrane which is very difficult. The incorporation process can only be achieved with nanoparticles below 100 nm in diameter due to their easy surface interactions. Despite researchers understanding the theories behind surface interaction processes, the factors that are contributing to the enhancement of agglomeration of nanoparticles remain unclear. Nevertheless, agglomeration can be increased with the increase in the nanoparticles concentration (Yu et al. 2009a), besides the other parameters, namely pH effect and ionic strength of the solution (Gilbert et al. 2009). Additionally, carbon nanotubes (Vatanpour et al. 2014), halloysite nanotubes (Zhu et al. 2014) and electrospun nanofiber (Bui et al. 2011) are gaining interest in the fabrication of nanomembranes due to their easy functionalization process. By applying all these novel practices in the synthesis of membranes, better and superior quality nanofiltration membranes and other types of membranes can be achieved soon. In water treatment, interfacial polymerization has become very important and a routine method to produce thin layer active films for nanofiltration and reverse osmosis. It is based on the formation of a thin polymerization layer on surface of the membrane due to the reaction of two monomers. This is also known as a copolymerization process. In recent decades, much attention was gained on the thin-film membrane technology through interfacial polymerization due to significant improvement in fouling resistance, selectivity, easy to handle, self-inhibitive reaction and a 50-nm range thickness of the membrane (Seman et al. 2011). A review of the literature reveals that several monomers such as isophthaloyl chloride, tannic acid and polyvinylamine have reacted with trimesoyl chloride, bisphenol-A, m-phenylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine and piperazidine to form active thin-film layers for effective water treatment. Abu Seman and co-workers designed polyester based thin-film composite membranes by reacting two monomers, bisphenol-A with tetramethyl bisphenol-A to form a polymer. This reaction was carried out in different concentrations and interfacial polymerization times using 0.5 to 2.0% w/v for 10 s, 30 s, 60 s, respectively, in organic solution of trimesoyl chloride (TMC)-hexane(0.15% w/v). In this study, humic acid was used as a model solution to study unmodified polyethersulfone NFPES10 and modified polyester thin-film composite polyethersulfone membranes in terms of irreversible fouling process at pH 7.0 and pH 3.0. Interestingly, it was found that in a neutral environment, polyester thin-film composite membranes synthesized from bisphenol-A exhibited less irreversible fouling process by the humic acid molecules when compared to unmodified

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NFPES10 and tetramethyl bisphenol-A-polyester series. This irreversible fouling process was mainly due to the existence of an electrostatic repulsion force between highly negative charged humic acid and less negatively charged bisphenolA-polyester layer at pH 7.0. Nevertheless, some rough surface membranes were subjected to severe fouling by humic acid molecules at pH 3.0 in an acidic environment. Under the experimental conditions, carboxylic acid groups present on the humic acid start losing their charge and subsequently a smaller macromolecular configuration of humic acid was observed due to the increase in hydrophobicity and reduced inter-chain electrostatic repulsions. Therefore, the humic acid molecules were preferentially deposited on the surface of the membrane and led to the blockage of pits, resulting in severe fouling (Seman et al. 2011). Later a two-step interfacial polymerization method was reported by Tsuru and co-workers in which trimesoyl chloride (TMC)/m-phenylenediamine (MPD) was used to prepare polyamide (PA) membranes (Tsuru et al. 2013). In the first step, trimesoyl chloride/hexane solutions were sprayed onto the m-phenylenediamine-impregnated polysulfone (PSf) for 10–60 s. In the second step, the product obtained from the first method was exposed to trimesoyl chloride/ hexane solutions as shown in Fig. 8. Interestingly, the results suggested that the water permeable capacity was increased with the increase in spray time (Tsuru et al. 2013). The interfacial polymerization process was used to fabricate nanofiltration polyester thin-film composite membranes using

Fig. 8 Schematic procedure for one-step and spray-assisted, 2-step interfacial polymerization. Reproduced with permission of Tsuru et al. (2013), Copyright 2013, Elsevier


commercial polyethersulfone membrane. In this study, various bisphenol-A concentrations were prepared in aqueous solution with different interfacial polymerization times to explain the effective synthesis of polyester based nanocomposite nanofiltration membranes. These membranes were successfully confirmed by Fourier transform infrared spectroscopy—attenuated total reflectance. (Seman et al. 2010). Polyetherimide/amino-functionalized silica nanocomposite membrane was used as a support via interfacial polymerization for the synthesis of thin-film nanocomposite membrane. In this case, the stable nanocomposite membranes were achieved with different amounts of modified silica ranging from 0 to 20 wt% (Namvar-Mahboub and Pakizeh 2013). Li et al. (2014) developed four types of thin-film composite membranes using interfacial polymerization with water-soluble monomers, namely piperazidine, diethylenetriamine, tetraethylenepentamine and diethylenetriamine, triethylenetetramine in trimesoyl chloride as a solvent. It was noted that the solubility of water-soluble monomer in organic solvent has played a vital role in manipulating the membrane structure, charge properties and thus the separation performance of membranes (Li et al. 2014). Self‑assembling membranes Self-assembly is type of arrangement of nanomaterial on their own into patterns or structures (Whitesides and Grzybowski 2002). Researchers are focusing on the synthesis of nanostructures, especially for gas permeation, by means of self-assembly of block copolymers (block copolymer membranes). The structure of these membranes can be systematically controlled by the process parameters, so that the targeted membranes will have specific characteristics, for example: homogeneous nanopores. With self-assembling processes, high-density cylindrical nanopores can be synthesized and used for micro/nanofluidic devices and for water filtration (Qiu et al. 2013). Ultrafiltration membranes are generally prepared from self-assembling methods, which enhances the sensitivity, selectivity and permeate efficiency. It should be noted that self-assembling methods cannot be adopted in large-scale membrane production, as these membranes are made in small quantities in the research laboratory (Pendergast et al. 2013). Nanofiber membranes In presence of electric field, polymers or ceramics are used to produce fibers by electrospinning method (Cloete et al. 2010). They are produced as nanofibers in the range of nanometers in size and used in separation and filtration processes. Due to their smaller size, nanofibers ensure high specific surface porosity and thus have a high surface to mass ratio. Nanofibers are highly flexible to be tailored to

selective membranes and exhibit interconnected open pore structures (Wegmann et al. 2008). These membranes are highly useful in air treatment than for water and wastewater treatment. Therefore, it includes a very high research opportunity in membrane technology (Ramakrishna et al. 2006). NanoCeram® (Argonide Corporation, Sanford, FL, USA) is a first patented nanofiber with high surface area (300–600 m2 g−1), produced by a proprietary sol–gel reaction and applied in a filter cartridge in presence of electropositive filter medium. This is a white and free-flowing powder with ~ 2 nm in diameter and 10–100 nm in length. NanoCeram was integrated with glass and cellulose nonwoven sheets used to separate proteins, virus, bacteria and micropollutants using an electrostatic force attraction. Therefore, NanoCeram can be used in prefiltration of ultrapure water system in laboratory or in industrial wastewater treatment (Karim et al. 2009). Feng et al. (2013) confirmed that the 10, 20 and 40 wt% of tetramethyl orthosilicate can be used in polyvinylidene fluoride to enhance the membrane parameters using electrospinning and thermal treatment. Due to thermal treatment, it is expected to increase the mechanical properties and hydrophobicity by increasing the mass fraction of tetramethyl orthosilicate in polyvinylidene fluoride. Therefore, membranes made up of hydrophobic nanofibers might become useful for the separation of organic pollutants, leading to higher flux efficiency. The nanofibers incorporated with nano-titanium dioxide and nanosilver improve the antifouling ability of the membrane. Immobilization of biofilm onto the surface of nanofibers was used to degrade antibiotics from wastewaters. The immobilization of biofilm on nanofibers might create an advantage of comparability with the dimensions of microorganisms, the surface morphology and biocompatibility. Therefore, bionanofibers are a perfect platform for the construction of microbial biofilms, resulting in fast biodegradation due to a higher rate of carrier ingrowth. Polyethylene oxide, polyurethane and polylactic acid are commonly available in nanofiber membranes (Sharma and Sharma 2012). Aquaporin‑based membranes Aquaporins are omnipresent in living cells and poreforming proteins. Under specific conditions, they produce highly selective water channels that can reject most ionic molecules. These membranes have high water permeability and selective rejection, which make them perfect materials for producing innovative high flux biomimetic membranes. Aquaporin-based membranes are fused in vesicles to maintain stability. Since stand-alone aquaporin membranes are mechanically too weak for their proposed practical applications like osmosis. Aquaporins are embedded in a polymeric matrix or deposited onto polymeric substrates such as nanofiltration membranes (Tang et al. 2013). The first

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commercially available biomimetic membrane with aquaporins is Aquaporin Inside™ (Aquaporin A/S, Copenhagen, Denmark) and can withstand pressures up to 10 bar and allow a water flux (Tang et al. 2013). Xie et al.(2013) developed an in situ “surface imprinting” polymerization method based on aquaporins into self-assembled polymer vesicles to produce a dense hydrophobic polymer layer (Xie et al. 2013). This membrane possesses high selectivity, mechanical strength as necessary to withstand pressure-driven water filtration processes. Table 3 summarizes the most important properties, applications and innovative approaches for membranes and membrane processes in nanotechnology and water and wastewater treatment processes (Gehrke et al. 2015). Photocatalysis Titanium dioxide is effective and frequently used as a photocatalyst for the treatment of air and water due its easy availability and low-price. Titanium dioxide is insoluble in water, is a non-hazardous material and has a high resistance to acids, bases and solvents. The photocatalytic activity depends on the side of the particles in which the surface area to volume ratio is large. On the other hand, due to its semi-conducting nature, titanium dioxide is a very good photocatalyst. In titanium dioxide, electron mobility can be observed under certain conditions. For this electron mobility, the correct amount of energy is required. When titanium dioxide is irradiated with UV–Visible light, at 400 nm it absorbs UV–Visible light in the form of photons to mobilize electrons from ground to excited states.

Recently, several researchers have studied the combination of separation and catalytic processes (using a membrane photocatalytic reactor) in order to both purify the water and retain the catalytic particles (Menendez-Flores et al. 2008; Pichat et al. 2007; Tan et al. 2003). When employing highly efficient nanoparticles, a suitable filtration system such as nanofiltration has to be established in order to safeguard the complete rejection of possibly toxic nanoparticles. Therefore, it is necessary to develop an extensive and cost-effective technology including high pressure pumps for water treatment. A solution for rejection of photocatalytic nanoparticles is their immobilization on defined materials by use of suitable coating processes, such as physical or chemical vapor deposition, as well as wet chemical coating processes. When using a microfilter material as a substrate, a beneficial multibarrier effect comprising mechanical filtration and chemical decontamination was obtained. The dirt particles and larger microorganisms were rejected by microfiltration membranes at the same time that viruses, spores and contaminants are chemically eliminated and degraded as shown in Fig. 9. The rapid growth in world population leads to major water concerns such as worsening of water quality, scarcity of water, climate change and to name a few. Additionally, it is very difficult to maintain clean environments, due to waterborne diseases which appear among the public. In recent years, personal care products, endocrine disruptors, surfactants and pharmaceuticals are categorized as emerging intractable contaminants due to their poor degradation capacity. Since 2000, there is a significant increase in the treatment of pharmaceutical wastewaters (Fent et al. 2006; Heberer 2002; Miege et al. 2009; Tixier et al. 2003). Results

Table 3 Properties, applications and innovative approaches for nanomembranes (Gehrke et al. 2015) Nanomembranes

Nanofiltration membranes Nanocomposite membranes

Self-assembling membranes Nanofiber membranes

Aquaporin-based membranes

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Properties

Applications

Novel approaches

Positive

Negative

Charge-based repulsion, relative low pressure, high selectivity Increased hydrophilicity, water permeability, fouling resistance and thermal–mechanical robustness Homogenous nanopores, tailor-made membranes High porosity, tailor-made, higher permeate efficiency, bactericidal

Membrane blocking (concentration polarization)

Reduction of hardness, Seawater desalination color, odor, heavy metals

Resistant bulk material required when using oxidizing nanomaterial, possibly release of nanoparticles Small quantities available (laboratory scale) Pore blocking, possibly release of nanofibers

Bio nanocomposite memHighly dependent on branes type of composite, e.g., Reverse osmosis, removal of micro pollutants

High ionic selectivity and permeability

Mechanical weakness

Ultrafiltration

Process scale-up

Filter cartridge, ultrafiltration, prefiltration, water treatment, stand-alone filtration device Low-pressure desalination

Composite nanofiber membranes, bionanofiber membranes Stabilization processes (surface in printing, embedding in polymers)

Environmental Chemistry Letters Fig. 9 Multibarrier effect of photocatalytic titanium dioxide particles in combination with microfiltration for the treatment of water contaminated with microbes. Reproduced with permission of Gehrke et al. (2015), Copyright 2015, Dovepress

obtained from the above studies has raised the awareness of the impact of pharmaceutical compounds on the human and aquatic systems, in spite of observing them in trace quantities ranging from ng L−1 to µg L −1 (Kümmerer 2009; Mompelat et al. 2009). The survey of the literature suggests that the existing sewage techniques are not sufficiently specific, selective and sensitive to remove all micropollutants from the wastewaters, especially pharmaceutical compounds (Suárez et al. 2008). Therefore, it is important to develop a promising and cost-effective technology for the treatment of pharmaceutical compounds from wastewaters (Ternes 1998; Zhang et al. 2008). Based on the physical and chemical properties of organic compounds, some of these pollutants are non-degradable or poorly degradable. However, advanced oxidation processes (AOPs) are known to be suitable mechanisms for the degradation of organic pollutants especially for pesticides. Ozonation, photocatalysis, Fenton and photo-Fenton, electrochemical oxidation, sonolysis, wet air oxidation and ultrasound radiation are some of advanced oxidation processes used in the treatment of water and wastewaters. Dalrymple and co-workers discussed the presence of reactive oxygen species, which reacts with

non-degradable pollutants in water and wastewaters. This is one of the characterization methods for advanced oxidation processes (Dalrymple et al. 2007). Reactive oxygen species or free radicals are oxidative in nature and therefore these species mineralize the pollutants into simple and harmless molecules. Free radicals are based on atoms or molecules with one or more unpaired electrons such as hydroperoxyl radical (HO•2), hydroxyl radical ( HO•), alkoxyl radical ( RO•) or superoxide anion radical (O2•−). These species have gained more attention due to their strong oxidation capacity. Among all species mentioned above, HO• exhibits a high potential of 2.8 V when compared to HO•2, RO• and O2•−. It has fair interaction capability with an extensive variety of pollutants without adding any additives, and rate constant in the range of 1 06–109 mol L−1 s−1 (Andreozzi et al. 1999). Kanakaraju and co-workers developed a semiconductor based photocatalysis with TiO2 and explained its distinctive properties over other advanced oxidation processes for the removal of non-degradable pollutants from wastewaters (Kanakaraju et al. 2013). According to International Union of Pure and Applied Chemistry (IUPAC), photocatalysis is defined as the

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“initiation or change in rate of the reaction of in presence of substance, when it exposed to ultraviolet, visible or infrared radiation”. The photocatalyst absorbs light quanta and undergoes chemical transformation (Braslavsky 2007). Fujishima and co-worker invented a photochemical splitting of water in 1972. This process occurs in photochemical cell consisting of rutile titanium as a photoanode and platinum as a counter electrode (Fujishima et al. 2008). This invention led to the novel applications such as anti-cancer therapy, organic synthesis, air purification, disinfection and self-cleaning surfaces. The most interesting application of this invention is water treatment, by allowing organic molecules such as pesticides, dyes and pharmaceuticals to undergo degradation in the presence of a photocatalyst. The significant features of an ideal photocatalyst should be photostable, cost-effective, chemically and biologically inactive, non-toxic and should be excited with ultraviolet, visible or infrared radiations. Although several oxides and sulfates of chalcogenide semiconductor photocatalyst such as ferric oxide, cerium oxide, zinc sulfide, cadmium sulfide, zinc oxide, tungsten trioxide, titanium dioxide and tin dioxide are available, these materials do not meet the characteristics of an ideal photocatalyst. The most known and frequently used photocatalyst is titanium dioxide for the treatment of pharmaceuticals in wastewaters due its photostable, non-toxic, cost-effective and biologically and chemically inactive (Friedmann et al. 2010). In the literature, several researchers has proposed the mechanistic aspects of titanium dioxide induced photocatalytic process for the degradation of organic pollutants (Augugliaro et al. 2012; Chong et al. 2010; Fox and Dulay 1993; Hoffmann et al. 1995; Legrini et al. 1993). Interfacial photocatalytic reactions and photogenerated holes have been observed in the photocatalytic degradation process due to the exposure of titanium dioxide to ultraviolet, visible or infrared radiation with sufficient energy (Friedmann et al. 2010) as shown in Fig. 10. In photocatalytic process, titanium dioxide is generally exposed to near ultraviolet light in the range of wavelength from 300 to 400 nm light with artificial ultraviolet lamps or by a small section of the solar spectrum or by sunlight (Bahnemann 2004).

Polymer brushes as state‑of‑art tools for membrane separation Assemblies of polymer chains that are bound by one end to planar or spherical surfaces, or bound to linear polymer chains, are referred to as polymer brushes. When the polymer side chains are bound to a linear polymer the steric crowding of the side chains forces the polymer backbone to be more extended, resulting in a cylindrical worm-like structure that can reach lengths up to a few hundred nanometers. These polymers also show little evidence of any entanglement even

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at ultra-high molecular weights. The structural characteristics of brush polymers have led to increased interest in their utilization for applications such as drug delivery, templates for inorganic particles, molecular actuators and as precursors for various carbon nanostructures. Interestingly, endothelial glycocalyx layer is a synthetic polymer made up of glycoproteins and proteoglycans. It is a biological analog (Pries et al. 2000; Weinbaum et al. 2007) as shown in Fig. 11. The endothelial glycocalyx layer acts as a water penetrability exchange modulator in the capillary, as an interaction regulator for blood cells, and as a mechanotransducer of fluid shear stress (Deng et al. 2012; Weinbaum et al. 2007). Nucleoporin protein brush inside the pores of the typical protein selector is one of the best biological examples (Grünwald et al. 2011). Likewise, synthetic modification of polymer brushes is an outstanding method to tailoring surface properties (Azzaroni 2012; Banerjee et al. 2011). Physisorption and chemisorption are two major surface modification methods (Granville and Brittain 2005). Firstly, physisorption is a type of coating in which polymers are interacted with its surface by either electrostatic or van der Waals forces. In this case, the relative bind energy is low. The major constraint of this process involves low interaction energy between polymer and its surface, which can result in the desorption of the adsorbed film and ultimately results in delamination. Secondly, chemisorption involves the covalent bonding between the polymer and its surface. This method can be accomplished by grafting a polymer from the surface (Edmondson et al. 2004; Ohno et al. 2005, 2010; Pyun et al. 2003) or grafting to the surface (Ionov et al. 2004; Minko et al. 2002; Zdyrko and Luzinov 2011). In this process, the grafting-to adopt includes the interaction of preformed end-functionalized polymers with appropriate moieties exposed on a surface. The foremost disadvantage of this process involves the characteristic diffusion limitation affecting the grafting reaction (Zhao and Brittain 2000). In contrast, grafting-from method includes attaching or creating of starting species on the surface, followed by its growth as a monomer and its continuous incorporation into the growing chain. The starting species contains specific molecules or free radicals. The grafting-from method is less stalled by diffusion as compared with the grafting-to method, since comparatively small monomeric units, rather than an entire polymer, are needed to diffuse to the end of the growing chain. Therefore, the grafting-from method will be the foremost surface modification technique exploited to modify the membranes in surface chemistry. Polymer brushes are versatile, as these are hitched to a surface from one end, and have various properties from other polymers in solution. The properties include colloid stabilization (Pincus 1991), responsiveness to external stimuli (Kidoaki et al. 2001; Li et al. 2007; Motornov et al. 2003; Sanjuan and Tran 2008; Sidorenko et al. 1999; Stuart et al.

Environmental Chemistry Letters Fig. 10 Elementary reactions in titanium dioxide photocatalysis with corresponding timescales. Reproduced with permission of Gehrke et al. (2015), Copyright 2010, Elsevier

2010) and wetting behavior (Stuart et al. 2010). Precisely, articular cartilage proteoglycans are potential candidates for the reduction of roughness between joints in the body (Inn and Wang 1996; Klein et al. 1994). In addition to their chemical structure, morphology of polymer brushes are highly responsive materials to external stimuli such as temperature, ionic strength or pH. Thus, numerous characterization methods are used to analyze the morphology of the polymer brushes. Other physical properties such as surface grafting density, molecular weight, thickness and polydispersity of the polymer brushes can be characterized using appropriate methods (Barbey et al. 2009). Recently, the “graftingthrough” method has been developed to attain better control over the polydispersity of grafted brushes in which cellulose dialysis membranes are supplied as monomers through the surface (Sejoubsari et al. 2016) as shown in Fig. 12.

In this method, brushes are created using atom transfer radical polymerization (ATRP) initiators which are attached to the permeate-facing surface of the dialysis membrane and then reacting with the monomers. The local monomer concentration closer to the membranes increases in the graftingthrough method, allowing smaller brushes to grow faster than longer brushes. The resulting morphology of brushes shows a thicker layer with less roughness compared to the brushes obtained from grafting-from method.

Characterization techniques on polymer brush surfaces The characterization of polymer brush surfaces is highly essential to find out the enhancement of separation processes. The characterization of polymer brushes grafted

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Fig. 11 Representation of proteoglycans and glycoproteins on the surface of endothelial cells (ECs). Caveolin-1 associates with regions high in cholesterol and sphingolipids in the membrane (darker circles, left) and forms cave-like structures, caveolae (right). Glypicans along with their HS chains localize in these regions. Transmembrane syndecans are shown to cluster in the outer edge of caveolae. Besides heparan sulfate (HS), syndecans also contain chondroitin sulfate (CS), further down the core protein. A glycoprotein with its short oligosaccharide branched chains and their associated SA “caps” are displayed in the middle part of the figure. Hyaluronic acid (HA) is a very long glycosaminoglycan (GAG) that weaves into the glycoca-

lyx and binds with CD44. Transmembrane CD44 can have chondroitin sulfate, heparan sulfate and oligosaccharides attached to it and it localizes in caveolae. Plasma proteins, along with cations and cationic amino acids (red circles), are known to associate with glycosaminoglycans. a The cytoplasmic domains of syndecans can associate with linker molecules, which connect them to cytoskeletal elements. b Oligomerization of syndecans helps them make direct associations with intracellular signaling effectors. c A series of molecules involved in endothelial NOS signaling localize in caveolae. Reproduced with permission of Gehrke et al. (2015), Copyright 2007, Annual Reviews

to polymeric membranes is difficult; therefore, several researchers attempted to characterize polymer brush layers on proxy surfaces made up gold, silica and other materials, from which polymer chains are simply sliced or have various properties which make the characterization step easier (Advincula 2005).

should be noted that the information on chemical properties and surface morphology of brushes are highly significant to better understand the separation performance. The atomic force microscopy (AFM) is the most widely used technique for the characterization of polymer brushes. With this technique surface morphology, surface roughness and root mean square (RMS) height of the brush of poly(N-isopropylacrylamide) grafted from a silicon surface were studied (Tu et al. 2004). Besides, the above-mentioned properties, heights of polyelectrolyte brushes in different solvents (Farhan et al. 2005) change in thickness of a poly(N-Isopropylacrylamide) grafted layer due to thermoresponse (Kidoaki et al. 2001), and polydispersity of poly (N, N-dimethylacrylamide) (Goodman et al. 2004). This was studied using the atomic force microscopy technique. The conformational changes of poly(4-vinylpyridine) grown from gold surface at various pH’s have been studied with atomic force microscopy (Li

Atomic force microscopy The different properties of polymer brushes including topography and surface structure, brush density, stiffness, polydispersity, chemical composition and structure, conformation and swelling, and polymerization kinetics were characterized using single technique. However, to evaluate the surface grafting density of brushes (chains/nm2), a combination of techniques is required and it is difficult to characterize the desired properties (Barbey et al. 2009). It

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Environmental Chemistry Letters Fig. 12 Schematic representation of polymer brush growing approaches (monomers are shown by red circles, the red lines denote growing polymer chains, the green triangles represent surface bound initiator on the left, and active polymer chain ends on the right, and substrates are shown by blue rectangles). a Illustration of the “grafting-from” approach. b Illustration of “graftingthrough” approach. Reproduced with permission of Gehrke et al. (2015), Copyright 2016, The American Chemical Society

et al. 2007). In atomic force microscopy, the brushes can be mechanically compressed by the probe tip and may lead to brush thickness, which is one of the disadvantage of this technique (Barbey et al. 2009). Ellipsometry This is a spectroscopic technique used to measure the change in light polarization upon transmission or reflection. The data obtained can be fitted to a mathematical model. This is a unique technique to measure the thickness of a brush above a surface. Ellipsometry has been used to detect the height of a poly(N-Isopropylacrylamide), poly(methyl methacrylate), polyampholyte and polystyrene brushes in water at different temperatures, on a silicon surface and grafted to a silicon wafer through a poly(glycidyl methacrylate) anchoring layer, respectively (Iyer et al. 2003; Sanjuan and Tran 2008; Tu et al. 2004; Yamamoto et al. 2000). Infrared spectroscopy Infrared spectroscopy (IR) is primary and often used technique for the identification of functional groups present on a polymer surface (Kurosawa et al. 2004; Yu et al. 2007). The crystalline nature of the polymer can be determined by small shifts in the peak positions in IR spectra, for example crystallinity of bolaamphiphiles on polymeric substrates (Böhme et al. 1999). The presence of functionalities introduced onto a surface of polymer is specially characterized with attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR). It is also used to record the

chemical properties of polymer composition to a depth of a few micron from the surface away from the solution. Vinyl monomer-grafted brushes were analyzed with attenuated total reflectance–Fourier transform infrared spectroscopy to confirm the type of bonding that were present in the monomer. Therefore, it was confirmed that no carbon–carbon double bond peaks are noticed and it can be concluded that the interaction is physisorption instead of chemisorption (Kim et al. 2002; Liu et al. 2007). Beside Fourier transform infrared spectroscopy and attenuated total reflectance–Fourier transform infrared spectroscopy, other spectroscopic techniques such as 1H and 13C nuclear magnetic resonance (NMR) can be used to confirm the moles of monomer grafted per unit area of membrane (Liu et al. 2007). Surface plasmon resonance The conformational changes of brushes grafted onto a silicon (Lee et al. 2007) and poly(4-vinylpyridine) grown from gold surfaces on gold surfaces at various pH (Li et al. 2007) were determined with surface plasmon resonance (SPR) Technique. Based on the change in frequency of the crystal resonator mass per unit area deposited on a quartz crystal was measured with quartz crystal microbalance with dissipation monitoring (QCM-D) technique (Dutta and Belfort 2007; Dutta et al. 2008a, b). The viscoelastic properties such as storage and loss of moduli of the adsorbed material can be determined using quartz crystal microbalance (QCM) frequency. The quartz crystal microbalance was also used to record the frequency shift with the change pH in a system comprising of a poly(acrylic acid) brush grafted onto

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the crystal (Kurosawa et al. 2004). This technique had also provided information on the breakdown of polyelectrolyte brushes in an ionic solution (Moya et al. 2007). The quartz crystal microbalance is a suitable technique to measure the conformational changes of brushes, rather than providing the physical dimensions of brushes. X‑ray photoelectron spectroscopy The chemical composition of brushes grafted onto a surface is determined using X-ray photoelectron spectroscopy (XPS). This technique is more sensitive compared to attenuated total reflectance–Fourier transform infrared spectroscopy, as in-depth chemical information up to ~ 10 nm from the surface of brushes is measured. The chemical composition of silicon surface grafted with poly(N-Isopropylacrylamide) (Tu et al. 2004) and poly(allyl alcohol) grafter layer was brominated after a modification (Kurosawa et al. 2004) has been identified with X-ray Photoelectron Spectroscopy. Besides X-ray photoelectron spectroscopy technique, X-ray Reflectivity (XRR) was used to determine the dewetting of polystyrene (Henn et al. 1996) and the thickness of native oxide layers and polymer brush layers on a silicon surface (Tu et al. 2004). Size exclusion chromatography Molecular weight distribution and molecular weight of the polymer is analyzed with size exclusion chromatography (SEC). It should to be noted that size exclusion chromatography is suitable to determine the average molecular weight of the homopolymer chain (Yamamoto et al. 2000). The main drawback of this technique is that it can be difficult to separate polymers brushes from the surface of a membrane without destroying the polymer brush, since very strong acids are generally used (Barbey et al. 2009). Consequently, analogous studies may need to be accomplished on proxy substrates to separate polymers brushes from the surface without the use of strong acids. For example, thiolated terminal group brushes may be grown on the gold surface and can be easily separated by reducing with iodine (Dyer et al. 2005). Finally, the recovered polymer brushes can be passed through a size exclusion chromatography system to analyze the polydispersity index and molecular weight. Dynamic scanning calorimetry Dynamic scanning calorimetry (DSC) is well suited to determine the percentage crystallinity of grafted brushes, whereas a change in pore size after grafting is identified with mercury porosimetry (Liu et al. 2007). This technique is highly informative to study the mechanical properties such as tensile strength before and after grafted surfaces (Sun

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et al. 2010). The grafting kinetics and membrane morphology (Freger et al. 2002) was also accurately investigated using transmission electron microscopy. This was predominantly correct when surface plasma treatment was used on grafted polymer surface, as a number of variables such as plasma head distance from surface, plasma power and irradiation time were involved in the characterization process (Moses and Cohen 2014; Wang et al. 2008). Ellipsometry and quartz crystal microbalance with Dissipation Monitoring techniques can be used to study the polymerization kinetics (Barbey et al. 2009), these characterization methods have found frequent application in the analysis of polymer brushes.

Challenges associated with characterization of polymer brush surfaces As mentioned in previous sections, the direct characterization of polymer brush surfaces can be very difficult. This led researchers to distinguish one polymeric layer from another, when they have similar physical properties. For example, ellipsometry is used to measure height of the brush by differentiating refractive index between the grafted polymer layer and substrate. As was discussed, refractive index is one of the key property to measure the height of the brush. If the refractive indices are similar, as in the case of polymeric brushes over a polymeric membrane, an accurate measurement of brush height may not be possible. The challenge involved with size exclusion chromatography to measure the molecular weight of polymer chain needs to be separated from the underlying substrate before analysis. This can be overcome if the polymer chain is chemically bonded with the polymer brush surface, as selective cleavage takes place with reducing agent (Iodine) or with strong acid treatment. However, it is not feasible when grafting brushes from polymeric membrane materials. The scanning electron microscopy is easier to analysis the cross section of polymer brush layer if the layer is thick, but it is challenging when the layer is too thin. Thus, polymer brush layer characterization is essential to understand the surface morphology to enhance membrane’s separation performance; however, it is a difficult task to undertake. Characterization of polymer brush layers is necessary to understand the morphological impact of the brush structure on a membrane’s separation performance, but in general is a difficult.

Polymer brushes for water purification Polymer brushes were adopted to enhance the salt rejection of brackish water purification membranes while reducing the scaling tendency of these membranes. Poly(acrylamide) (PAAm) and poly(methacrylic acid) (PMAA) are hydrophilic


polymer brushes, grafted from polyamide membranes on a polysulfone support using atmospheric pressure polymerization (APP) (Lin et al. 2010). Interfacial polymerization is used to synthesize polyamide membranes on polysulfone supports. The synthesized pre-grafted polyamide membrane from interfacial polymerization process has approximately 30% rejection toward sodium chloride when filtering a 1000ppm sodium chloride solution. Thus, polyamide membranes act as a nanofiltration membrane. In this study, scaling with respect to the mineral gypsum, brackish water purification and antifouling capacity with regard to alginic acid and bovine serum albumin (BSA) were also investigated. Additionally, zeta potential measurements were carried out on the polyamide brush membranes and commercial reverse osmosis membranes. The zeta potential measurements were calculated by recording the streaming potential of 10 mM potassium chloride solution at pH 6.5. The pKa of the carboxylic acids of poly(methacrylic acid) is ~ 4.5–5. Then at pH 6.5, most of the carboxylic acid groups undergo deprotonation, resulting in the generation of greater negative surface zeta potential for the poly(methacrylic acid) brush membrane as compared with the commercial polyamide membrane. The poly(methacrylic acid) and poly(acrylamide) membranes are compared and evaluated with commercial polyamide membrane (LFC1-Hydranautics, Oceanside, CA) which is known as a reverse osmosis membrane in terms of their grafting solution concentrations. The poly(methacrylic acid) brush membranes have a large negative zeta potential due to its charged and polyelectrolytic nature, whereas poly(acrylamide) contains very small negative zeta potential which is expected since it is a neutral polymer. The captive air bubble method was used to measure the contact angle of the brush membranes (Zhang et al. 1989). This method was well suited to monitor the solvent diffusion into membrane structure though capillary action, thereby varying the measured contact angle with time (Taniguchi and Belfort 2002). It was noted that a higher contact angle for poly(acrylamide) brush membrane was observed when compared to poly(methacrylic acid) brush membrane. This is predictable due to the more hydrophilic nature of the charged poly(methacrylic acid) brush membrane and is in good alignment with the zeta potential measurements. When compared to commercial polyamide membrane, poly(methacrylic acid) and poly(acrylamide) brush membranes had very low contact angles due to their more hydrophilic surfaces after brush membrane modifications. The scaling experiment results obtained from poly(methacrylic acid) and poly(acrylamide) brush membrane have a much longer induction time before the beginning of flux decline as compared to commercial polyamide reverse osmosis membrane (LFC1). The induction time for commercial polyamide membrane has approximately 2.2 h before it undergo scaling process (normalized flux declines), whereas for the

poly(methacrylic acid) brush membrane, it was found to be 6.2 h at the concentration of 2.35 M. Remarkably, during the testing period at various concentrations (0.2, 0.3 M) the poly(acrylamide) brush membrane does not exhibit any flux decline over the period of 8 h. Interestingly, it was found that the presence of poly(methacrylic acid) brush significantly diminishes the adsorption of gypsum to the membrane surface. The poly(methacrylic acid) and poly(acrylamide) brush membranes had comparable flux declines as the commercial polyamide reverse osmosis membrane when filtering the BSA solution. In the case of alginic acid filtration, poly(methacrylic acid) brush membrane and commercial polyamide membrane showed the same flux declines; however, poly(methacrylic acid) showed much greater flux decline. The commercial polyamide reverse osmosis and poly(methacrylic acid) brush membranes produced the same performance regarding bovine serum albumin and alginic acid fouling, but poly(methacrylic acid) had a much better anti-scaling performance. The poly(methacrylic acid) and poly(acrylamide) brush membranes had higher permeabilities with a value of 2.0–3.4 × 10−10 m s−1 Pa−1 and 1.8–2.8 × 10−10 m s−1 Pa−1, respectively, which was found to be greater than the commercial polyamide reverse osmosis membrane (1.5 × 10−10 m s−1 Pa−1). Finally, these polymer brush membranes are not only capable of decreasing the scaling capacity and maintaining biocompatibility, but are also capable of achieving the comparable salt rejection values. This is noteworthy, since a polymer brush membrane can convert a nanofiltration membrane into reverse osmosis membrane with low salt rejection as like commercial reverse osmosis membranes. The gypsum scaling was reduced by grafting poly(methacrylic acid) brushes onto polyamide nanofiltration support membranes (Kim et al. 2010). The atmospheric pressure polymerization method was used to graft poly(methacrylic acid) brushes. Ellipsometry has been used to measure the thickness of the poly(methacrylic acid) layer as a function of grafting time for different monomer concentrations and two different reaction temperatures. Interestingly, it was found that the thickness of polymer brushes increased with increase in monomer concentration and reaction time. However, brush thickness decreased with increased temperature from 60 to 70 °C. This trend in the results was due to increased chain transfer and homopolymerization in solution as reaction time is increased. This allows surface chains to be terminated at higher rate and also with change in concentration, the membrane surface roughness increases. The rejection of ~ 95% toward sodium chloride was exhibited by poly(methacrylic acid) brush membranes as compared with polyamide support membrane (~ 30%) using a 1000 ppm sodium feed solution. Therefore, it is concluded that poly(methacrylic acid) rush membranes

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have a greater permeability than that of commercial polyamide reverse osmosis membrane while maintaining a comparable salt rejection. Wastewater purification and water softening The promising method for the removal of by-products from the wastewaters (Hong et al. 2006) and water softening (Hong et al. 2007) were used by layer-by-layer (LBL) adsorption of polyelectrolyte films on porous membranes. In this method, alternating polyelectrolyte layers were adsorbed onto a surface due to electrostatic attraction, which is similar to the grafting-to surface modification technique (Howarter and Youngblood 2009). By adopting layer-by-layer deposition, poly(styrene sulfonate) (PSS) and poly(diallyldimethyl ammonium chloride) (PDADMAC) were adsorbed onto porous alumina substrates with 0.2 μm diameter pore. The selectivity of these membranes enhanced by threefold higher flux than commercial membranes toward Cl−/F− and Br−/F− (Hong et al. 2007). The selectivity of A/B was well defined as the permeability of “A” over permeability of “B.” The fluoride rejection percentage was greater than 70% and was constant over the pressure of 3.6 to 6 bar. Based on the hydrodynamic radius, the selectivity studies of Cl− and Br− over F− and F− as compared with Cl− and Br− were carried out. Membranes are promising tools in the separation of monovalent ions from the drinking waters. It should be noted that separation of F− with ion exchange resins can be difficult because of the greater binding tendency of resins with Cl− compared to F−. The layer-by-layer deposition method was used to synthesize nanofiltration membranes namely, poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) onto porous alumina membranes for selective removal of amino acids, sucrose and dyes from sodium chloride solutions (Hong et al. 2006). Due to the hazardous nature of dyes, textile effluents cannot be released directly into the environment. Therefore, it is essential to separate dyes from the water systems using membranes. These membranes should have a low salt rejection, since increased salt rejection increases the osmotic pressure that needs to be overcome to drive the separation. The polyelectrolyte membranes system, namely poly(styrene sulfonate)/ poly(allylamine hydrochloride), showed selectivity of 130 and 2200 for sodium chloride/sucrose and sodium chloride/dye, respectively. The rejection rates for sucrose, dye, sodium chloride and glutamine were achieved and found to be 99.4, > 99,