Synthesis of nanomaterial-incorporated pressure retarded osmosis membrane for energy generation

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Synthesis of nanomaterialincorporated pressure retarded osmosis membrane for energy generation

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Ozgur Arar*, Idil Ipek†, Sarper Sarp‡ Faculty of Science, Department of Chemistry, Ege University, Izmir, Turkey* Faculty of Engineering, Chemical Engineering Department, Ege University, Izmir, Turkey† Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea, United Kingdom‡

11.1 PRESSURE RETARDED OSMOSIS PROCESS FOR ENERGY GENERATION 11.1.1 INTRODUCTION Even though seawater reverse osmosis (RO) has been widely used to produce potable water in many countries, it requires a high-energy input. Furthermore, the disposal of brine from desalination plants can cause harmful effects on the environment. To overcome this situation, the integration of pressure retarded osmosis (PRO) and seawater RO may provide a green approach by producing energy and clean water at a lower cost with reduced CO2 emissions. A great quantity of renewable energy can be potentially generated when waters of different salinities are mixed together. It is estimated that the global energy production potential of PRO is on the order of 2000 TWh/year, while the estimated global energy production from all renewable sources is approaching 10,000 TWh/year (Achilli et al., 2009; Achilli and Childress, 2010; Cheng et al., 2017; Han et al., 2014; Helfer et al., 2014; Spiegler and El-Sayed, 2001). There are other notable methods to harvest energy from salinity difference, such as reverse electrodialysis, the electric double-layer capacitor, and the Faradaic pseudocapacitor. Additionally, in nature some organisms can convert salinity energy into bioelectricity. For example, the electric eel has the ability to generate considerable electric shocks with highly selective ion channels and pumps on its cell membrane. Similarly, reverse electrodialysis is a nonpolluting, sustainable technology that can Advanced Nanomaterials for Membrane Synthesis and Its Applications. https://doi.org/10.1016/B978-0-12-814503-6.00011-2 Copyright # 2019 Elsevier Inc. All rights reserved.

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convert the free energy generated by mixing two aqueous solutions into electrical power directly without any other equipment. Capacitive energy extraction is a new method based on the electrochemical double layer capacitor technology. A pseudocapacitor is based on charge storage brought about by the Faradaic charge transfer process of the capacitor electrodes. It has a higher energy density and a simpler structure compared to the electric double-layer capacitors. Among the aforementioned technologies, the membrane-based systems are the most suitable technologies for large-scale applications ( Jia et al., 2014; Yip and Elimelech, 2014). The osmotic pressure gradient energy released from the mixing of water streams with different salinities is an alternative resource of renewable energy. Employing a semipermeable membrane that controls the mixing process, the osmotic pressure gradient energy can be converted in terms of electrical power via PRO without causing an undesired environmental impact. This technique uses a semipermeable membrane to separate a less concentrated solution or solvent (for example, fresh water) from a more concentrated and pressurized solution (for example, seawater), allowing the solvent to pass to the concentrated solution side; the power is then obtained by depressurizing a portion of the diluted seawater through a hydroturbine. The lack of effective membranes with a desirable structure and performance, however, retards further development of PRO technology. Mainly, novel flat-sheet and hollow fiber polymeric membranes with the desired structure, mechanical robustness, and permeation characteristics have been developed for PRO applications. In order to improve the efficiency of these energy-harvesting systems, the performance of the membranes should be optimized. The semipermeable membrane should have a good water transport flux, good mechanical properties, and desirable ion selectivity. Since natural seawater and river water have been employed to harvest energy, the membrane should be resistant to fouling as well (Cheng et al., 2017; Han et al., 2013, 2014; Han and Chung, 2014; Jia et al., 2014; Klaysom et al., 2013; Straub et al., 2014; Thorsen and Holt, 2009). With the improvement of the membranes, this technique will result in entropic energy being produced in a much more efficient manner. Water salinity power is a completely environmentally friendly power source. With increasing efficiency, water salinity power may become a promising energy source (Cheng et al., 2017; Han et al., 2014; Jia et al., 2014). With the recent progress in material science, new perspectives have been considered for the new generation of membrane synthesis to be employed in PRO processes and applications. The main focus of this chapter is on the material science and membrane engineering of the synthesis and designing of nanomaterial-incorporated PRO membranes for energy generation.

11.1.2 PRESSURE RETARDED OSMOSIS PRO is a promising technology that can harvest renewable osmotic energy from salinity gradients. The osmosis pressure difference between river water and seawater is about 23 atm under ordinary conditions. Utilizing specific devices, the large-scale

11.1 Pressure retarded osmosis process for energy generation

salinity energy can be converted to mechanical energy or electricity directly. PRO is a process that can extract salinity-gradient energy by using semipermeable membranes that allow for the transportation of water from a low concentration solution, such as river, brackish, and wastewater, into a high-concentration draw solution, such as seawater and brine water. The migration of water from the low-concentration solution to the high-concentration solution could increase the static energy of the high-concentration side, which can be utilized to power the turbine. Theoretically, the maximum extractable energy during the irreversible mixing of a dilute stream with saline draw solutions is substantial, ranging from 0.75 to 14.1 kWh/m3 depending on the low-concentration stream (Achilli and Childress, 2010; Cheng et al., 2017; Elimelech and Phillip, 2011; Helfer et al., 2014; Jia et al., 2014; Lee et al., 1981; Sarp et al., 2016; Valladares Linares et al., 2014; Zhang et al., 2016). Fig. 11.1 shows a typical PRO process. Filtered fresh water and seawater are pumped into semipermeable membrane modules. In the modules, fresh water passes through the semipermeable membrane into the pressurized seawater. Then the mixing solution is split into two streams; one is depressurized by a hydropower turbine to generate power and the other passes through a pressure exchanger in order to pressurize the incoming seawater. The energy efficiency of the pressure exchanger and the membrane is very important for the energy cost of this renewable energy. A PRO power plant generates about 1 MW from each cubic meter per second of fresh water that passes through the membranes when high-energy efficient membranes and

Brackish water out Pressure exchanger Water filter

Sea water in

Membrane modules

Turbine for power

Brackish water out Water filter

Fresh water in

Fresh water out

FIG. 11.1 Simplified process layout for a typical osmotic power plant. Adapted from Jia, Z., Wang, B., Song, S., Fan, Y., 2014. Blue energy: current technologies for sustainable power generation from water salinity gradient. Renew. Sust. Energ. Rev. 31, 91–100.

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pressure exchangers are employed in the system ( Jia et al., 2014; Valladares Linares et al., 2014). The energy extracted by PRO is always limited by the properties of the membrane. The primary barrier preventing the widespread use of PRO is the lack of effective membranes, which are at the heart of osmotic power systems. Desirable PRO membranes should possess a substantially high-water flux and enough mechanical strength to withstand the operation pressure and provide a sustainable power output. They must also have a low reverse salt flux to maintain an effective osmotic driving force across the membrane, and low fouling propensity in order to sustain PRO operations using different feed streams. The efficiency of the PRO is affected by the permeable property of the membrane. It is important to note that the membrane is not completely impermeable to the solutes. Energy will be lost by the irreversible migration of salts to the freshwater, a loss which can be expressed as a reduction in the effective osmotic pressure. Additionally, the membrane is not capable of allowing an adequate flow. The bulky support layers of membranes cause severe concentration polarization, which significantly reduces flux and power density (Bui and McCutcheon, 2014; Huang et al., 2016; Jia et al., 2014; Yip et al., 2010; Yip and Elimelech, 2012).

11.2 THEORY 11.2.1 OSMOTIC PROCESSES The water flux in an osmosis process using a semipermeable thin film that only allows the passage of water, yet fully rejects other solutes or ions, can be described by: JV ¼ A  ðΔπ  ΔPÞ 2

(11.1) 2

where JV (L/m /h) is the volumetric water flux through the membrane, A (L/m /h/bar) is the water permeability of the membrane, Δπ (bar) and ΔP (bar) are the osmotic and hydrostatic pressure differences across the membrane, respectively (Chou et al., 2012; Sarp et al., 2016; Song et al., 2013; Van’t Hoff, 1888). When water naturally permeates from fresh water (lower salinity) to salty water (higher salinity), the process can be defined as forward osmosis (FO) or direct osmosis. However, if the salty water is pressurized beyond a certain hydrostatic pressure, which is equivalent to the value of the osmotic pressure difference, Δπ, the water flow through the membrane will be RO. PRO is an intermediate osmosis process between the FO and RO, where the hydrostatic pressure of the draw solution is lower than the osmotic pressure difference across the membrane, so that the water permeates from the fresh water side to the salty water side. The overview of water flux direction in the osmosis processes is shown in Fig. 11.2A (Chou et al., 2012; Sandler, 1999; Sarp et al., 2016; Valladares Linares et al., 2014). Based on Fig. 11.2B, the power density (W) (W/m2) is calculated as W ¼ JV  ΔP ¼ A  ðΔπ  ΔPÞ  ΔP

(11.2)

It can be seen from Fig. 11.2B that pressure energy is produced in the PRO process by transferring the water from a low-pressure side to a high-pressure side, while it is

11.2 Theory

FIG. 11.2 Water flux direction and energy consumption/production in FO, PRO, and RO. (A) Water flux vs pressure difference and (B) power density vs pressure difference. Adapted from Chou, S., Wang, R., Shi, L., She, Q., Tang, C., Fane, A.G., 2012. Thin-film composite hollow fiber membranes for pressure retarded osmosis (PRO) process with high power density. J. Membr. Sci. 389, 25–33.

consumed in the RO process to produce fresh water by overcoming the osmotic pressure of the salty water. The energy density is a major performance indicator in PRO processes, as it determines the amount of membrane area and thus the size of the PRO plant for any given energy production capacity (Chou et al., 2012; Sandler, 1999; Valladares Linares et al., 2014).

11.2.2 THEORETICAL POTENTIAL OF OSMOTIC PRESSURE GRADIENT ENERGY Osmotic pressure gradient energy is the free energy released during the mixing of waters with different salt concentrations. The amount of energy available from the mixing of two solutions can be theoretically calculated by applying Gibbs free energy. For example, the free energy available from mixing 1 m3 of salty water and 1 m3 of fresh water can be calculated as ΔGmix ¼ GB  ðGS + GF Þ

(11.3)

where Gmix (J/mol) is the change in Gibbs energy and GB, GS, and GF are the Gibbs energies of the resultant brackish water, salty water, and fresh water (J/mol), respectively. Assuming the solutions are ideal, the chemical potential (μi) of component i in the solution can be presented as

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μi ¼ μ0i + V i ΔP + RT ln xi + |zi |FΔφ

(11.4)

where μ is the molar free energy under standard conditions (J/mol), V i is the specific volume of component I (m3/mol), P is the pressure change compared to the atmospheric conditions (Pa), R is the gas constant (8.31441 J/K/mol), T is the absolute temperature (K), xi is the molar fraction of the component i, z is the valence of an ion (equiv./mol), F is the Faraday constant (96,485 C/equiv.), and Δφ is the electrical potential difference (V). Since there is no pressure change and charge transport, for the free energy difference that can be theoretically estimated from the chemical potential change of the system after and before mixing, the following formula should be applied: 0

ΔGmix ¼

X i

ðGi, S + Gi, F  Gi, B Þ ¼ RT

X

ci, S VS ln ð xi, S Þ + ci, F VF ln ð xi, F Þ  ci, B VB ln ð xi, B Þ

i

(11.5)

where c is the molar concentration (mol/L) and V is the volume (L). The change in Gibbs energy during mixing is negative since energy is released. Eq. (11.3) provides a good approximation for the theoretical amount of free energy obtainable from the mixing of two solutions (Han et al., 2014, 2017; Han and Chung, 2014).

11.2.3 CONCENTRATION POLARIZATION Concentration polarization is an important parameter affecting the efficiency of PRO. It is the accumulation or depletion of solutes near a membrane surface. As a result of water crossing the membrane, the solute is concentrated on the feed side of the membrane surface and diluted on the permeate side of the membrane surface. In PRO applications, the dense layer of the membrane faces the draw solution, and the porous support layer faces the feed solution (Fig. 11.3) (Achilli et al., 2009; Helfer et al., 2014; Hoover et al., 2013; Kim et al., 2015b). It is very important to select a membrane for PRO that is thin and deprived of a fabric support layer to allow for higher water flux. In this context, FO membranes, because of their thinner support layer, are significantly less susceptible to concentration polarization and thus are used more often in PRO studies (Helfer et al., 2014; McCutcheon and Elimelech, 2004, 2006).

11.2.4 THE MODEL DESCRIPTION OF PRO PROCESS In order to facilitate a working PRO process, the membrane must be configured in modules, as seen in RO processes, either in a spiral wound, hollow fiber or other forms, such as plate and frame configurations. An essential PRO module consists of the flow of the two water phases along both opposite surfaces of the membrane, creating a crossflow. This is different from RO, where there is crossflow all through the module on the salt water side of the membrane only. A section of a membrane in PRO operation is depicted in Fig. 11.4 (Elimelech and Phillip, 2011; McCutcheon and Elimelech, 2007; Thorsen and Holt, 2009).

11.2 Theory

FIG. 11.3 Representation of solvent flow in FO, PRO, and RO. Membrane orientation is indicated in each system by the thick black line representing the membrane dense layer. Adapted from Achilli, A., Cath, T.Y., Childress, A.E., 2009. Power generation with pressure retarded osmosis: an experimental and theoretical investigation. J. Membr. Sci. 343, 42–52.

The mass transport through the membrane is dominated by hydraulic flow. This is evident from the dimensions of the pores, which are large enough for the continuum theory to apply. The ideal osmotic process can be described by the thermodynamic equations for the water and salt fluxes. The simplest forms of these equations are: Jw ¼ A  ðΔπ  ΔPÞ

(11.6)

Js ¼ B  Δcs

(11.7)

FIG. 11.4 Structural model for the membrane and the concentration polarization films. Adapted from Thorsen, T., Holt, T., 2009. The potential for power production from salinity gradients by pressure retarded osmosis. J. Membr. Sci. 335, 103–110.

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Accordingly, the water flux, Jw (L/m2/h), is proportional to the water permeability, A (L/m2/h/bar), and the difference between the osmotic pressure difference, Δπ (bar), and the hydraulic pressure difference, Δp (bar), across the membrane skin. The salt flux, Js (G/m2/h), is proportional to the salt permeability, B (L/m2/h), and the salt concentration difference across the skin, Δcs (mol/L). The equations are valid for the skin only and describe the diffusive transport of water and salt through the skin material (Kim and Elimelech, 2013; Ramon et al., 2011; Thorsen and Holt, 2009). The presence of porous structures reduces the efficiency of the membrane in PRO. No skin is perfectly semipermeable, and some salt will diffuse through the skin. With saltwater on the skin side of the membrane, this salt will diffuse into the porous substructure toward the freshwater, and the salt concentration will thus increase on the freshwater side downstream in the module. For salt to escape out of the structure, a concentration gradient will develop, and consequently concentration polarization will arise within the porous structure. The porous structure can be described by its average thickness, x (m), the porosity, ϕ, the tortuosity, τ, and the pore length, lp (m). With a model of diffusion and hydraulic flow, the tortuosity is τ ¼ (lp/x)2 and the water velocity in the pores is vp (m/s) ¼ Jw  lp/ϕ/x. From Eq. (11.7) and a mass balance through the membrane, a relation can be found for the steadystate transport of salt through the skin and the porous structures, which can be understood as: B  Δcs ¼

    xØ dc xØ dc lp Ø  D dc c ¼ D  vp  c ¼ D  JW   Jw  c lp lp τ dx dlp dlp Øx

(11.8)

where cs (mol/L) is the salt concentration difference across the skin and D (m2/s) is the salt diffusion coefficient (Chou et al., 2013; Thorsen and Holt, 2009).

11.3 MEMBRANES FOR PRESSURE RETARDED OSMOSIS Membranes are at the heart of controlled mixing processes, so membrane properties and the design of the modules and stacks are expected to have a strong bearing on performance (Yip et al., 2011; Yip and Elimelech, 2014). The basic parameters that characterize the membrane are water permeability (A), salt permeability (B) and structural parameter of a membrane (S) (She et al., 2016; Thorsen and Holt, 2009). Most modern salt-rejecting (RO-like) membranes are composed of a very thin ( 100 nm) dense film over a porous structure, which offers mechanical support. This asymmetric structure, particularly the porous support, results in an additional resistance to mass transfer, which has been termed internal concentration polarization (ICP). The ICP is caused by two mechanisms: the first is the rejection and subsequent accumulation of salt present in the dilute stream; the second mechanism is the diffusive salt diffusion through the membrane driven by the concentration gradient from the concentrated to the dilute side. The occurrence of ICP results in a reduction of the available driving force for osmosis and may be regarded as an artificial source of

11.3 Membranes for pressure retarded osmosis

inefficiency in PRO energy conversion due to the construction of modern RO membranes. The development of a specific PRO membrane is now a necessity to overcome the limitations of the process. The best characteristics of membranes for PRO should be as follows (Chou et al., 2012, 2013; Fu et al., 2014; Ramon et al., 2011; Sun and Chung, 2013; Wan et al., 2017): (a)

(b) (c)

High density of the active layer for high-solute rejection; a thin membrane with minimum porosity of the support layer for low ICP and, therefore, higher water flux. Hydrophobicity for enhanced flux and reduced membrane fouling. High-mechanical strength to sustain hydraulic pressure.

11.3.1 ROLES OF NANOMATERIALS ON TFC/TFN MEMBRANES The development of PRO has been hindered for many years by the lack of a membrane capable of allowing an adequate flow. The performances of existing commercial membranes (asymmetric cellulose triacetate (CTA) membranes) are generally limited by their relatively low water permeability and salt rejection rate (Ma et al., 2012). It is essential to set the pH of the feed and draw solution in the range of 4–6 and the operation temperature should not be above 35°C for preserving the membranes (Niksefat et al., 2014). A number of fabrication strategies have been developed to increase the average membrane permeability while maintaining highsalt rejection. To date, although some asymmetric membranes were shown to exhibit a high performance, thin film composite (TFC) membranes have dominated the commercial RO markets due to their lower fabrication costs and higher stability, regardless of the feed quality and composition (Dumee et al., 2013). The Observatory NANO Briefing, which took place in June 2011, focused on the thematic issue of nanoenhanced membranes (NEM) for improved water treatment. The distinction between NEM and nanostructured membranes (NSM) was noted as follows: “There is a clear distinction between NEM where the membranes are functionalized with discrete nanoparticles or nanotubes and NSM where the term ‘nano’ refers only to the internal structure (pores) of the membrane.” Fig. 11.5 illustrates the structure of NEM (also known as TFN membranes) and typical TFC membranes. Over the past decade, nanotechnology has rapidly changed from an academic pursuit to a commercial reality; already nanotechnology concepts have led to new water treatment membranes that have exceeded state-of-the-art performance levels and have enabled new functionalities, such as high permeability, catalytic reactivity, and fouling resistance (Buonomenna, 2013). The various nanomaterials, such as zeolite, silver, titanium dioxide (TiO2), silicon dioxide (SiO2), and multiwalled carbon nanotubes (MWCNT) have been investigated due to their unique functionalities (e.g., hydrophilicity, antimicrobial functionality, and mechanical properties). More information on the development and performance of the TFC and TFN membranes is provided in the next section.

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FIG. 11.5 Conceptual illustration of (A) TFC and (B) TFN membrane structures. Adapted from Jeong, B.H., Hoek, E.M.V., Yan, Y., Subramani, A., Huang, X., Hurwitz, G., et al., 2007. Interfacial polymerization of thin film nanocomposites: a new concept for reverse osmosis membranes. J. Membr. Sci. 294, 1–7.

11.3.2 TFC/TFN MEMBRANES FOR ENERGY GENERATION 11.3.2.1 Typical TFC membranes Before the specialized PRO membranes, RO membranes were used for the PRO evaluations. Membranes could withstand a high-hydraulic pressure of 91.2 bar but showed relatively low peak power densities of <1.74 W/m2 (Han et al., 2014; Loeb, 1976). The main problem with using such membranes was ICP. RO membranes have a very thin salt-rejecting “skin” on one side of the membrane. The remainder of the membrane is a finely porous matrix that serves as a support for the skin. Salt would accumulate within the porous substrate of the membrane, leading to ICP and reduced water permeability (Gerstandt et al., 2008; Lee et al., 1981).

11.3 Membranes for pressure retarded osmosis

From the beginning of 2010, flat-sheet TFC membranes, mostly composed of two layers (a PA active layer and a highly porous support layer), have been actively developed due to their advantage on the PRO performance. Compared to the CTA membrane, TFC membraneshave a relatively higher salt retention rate due to their thin-film PA selective layer, while also having a lower ICP phenomenon, which leads to a higher water flux because of the higher porosity in the support layer (Kim et al., 2015a). TFC membranes improved the PRO performance by achieving higher power densities compared to CTA membranes. Yip et al. (2011) prepared TFC membranes with a polyamide active layer formed over a polysulfone (PSf ) microporous substrate for sustainable power generation from salinity gradients. The highly porous PSf support layer minimized ICP and allowed the transport properties of the active layer to be customized in order to enhance PRO performance. The synthetic seawater was used as a draw solution and river water was used as a feed solution. The results showed that when the TFC membrane was pretreated by a NaOCl solution followed by a NaOH solution, the maximum peak power density of 10 W/m2 was able to be achieved when river water and seawater were used as feed and draw solutions, respectively. Exposure of the PA active layer to chlorine (by NaOCl) alters its structure and morphology, resulting in increased water permeability and decreased selectivity of the membrane. By careful control of the reaction parameters (chlorine concentration and pH of reaction medium), water and salt permeabilities of the membrane active layer can be tailored. Chou et al. (2012) on the other hand examined the potential of hollow fiber membranes for power generation. The researchers fabricated the PRO membrane by interfacial polymerizing phenylenediamine (MPD) and trimesoyl chloride (TMC) on the surface of a polyethersulfone (PES) hollow fiber support. A power density of 10.6 W/m2 was obtained using this membrane when it was tested using wastewater brine (40 mM NaCl) as feed solution and seawater brine (1.0 M NaCl) as draw solution. Ingole et al. (2014) modified the surface of a TFC membrane using polydopamine (PDA). The PDA-coated TFC membrane was reported to produce a power density of 3.9 W/m2 when it was tested at 8 bar. This experiment was carried out using 0.6 M NaCl solution as the draw solution and deionized water as the feed solution.

11.3.2.2 TFN membranes incorporated with nanomaterials The TFN membrane is a new type of composite membrane prepared via an interfacial polymerization process. For the TFN membrane, nanomaterials are incorporated within thepolyamide dense layer or within the support layer of the membrane with the aim of improving the membrane separation characteristics. In recent years, the fabrication of TFN membranes specialized for the PRO process has actively been conducted worldwide. A great advantage of such a membrane is that its polyamide selective layer and porous support can be independently optimized to achieve a desired separation performance ( Jeong et al., 2007). A zeolite-incorporated polyamide layer was successfully prepared by Ma et al. (2012) on the surface of a PSf substrate and used in a PRO process.

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With the incorporation of only 0.1 w/v% zeolite, the resultant TFN membrane showed water flux of 30.7 L/m2/h when tested using 10 mM NaCl solution as feed solution and 1 M NaCl as draw solution. As a comparison, the TFC membrane without zeolite incorporation only exhibited a water flux of 21.5 L/m2/h under the same testing conditions. The significant enhancement in the water flux of the TFN membrane also indicates the higher power density that could be produced. In another work of Ma et al. (2013), the effect of draw solution concentration (0.5, 1, and 2 M NaCl) on the performance of a zeolite-incorporated TFN membrane for the PRO process was investigated. Results showed that greater draw solution concentration led to increased water flux due to greater osmotic pressure difference. When tested using 2 M NaCl solution as draw solution and DI water as feed solution, the resultant TFN membrane showed about 2.5 times higher water flux than that of a control TFC membrane, recording 86 L/m2/h. A zeolite-incorporated TFN membrane was also prepared and studied in the work of Salehi et al. (2018) for PRO applications. The effect of nanostructured zeolite (clinoptilolite) on the characteristics of a PES substrate and then the TFN membrane was studied. The findings showed that the TFN membrane made of optimized nanocomposite substrate (0.4 wt%) could improve approximately 50% of the flux of the control membrane (without zeolite) when tested under a PRO mode (10 mM NaCl as feed solution and 2 M NaCl as draw solution), resulting in water flux of 33.1 L/m2/h. Amini et al. (2013) fabricated a TFN membrane incorporated with amine functionalized multiwalled carbon nanotubes (F-MWCNTs) for a PRO process that used a feed solution containing 10 mM NaCl and draw solution containing 2 M NaCl. With the presence of F-MWCNTs, the resultant TFN membrane showed significantly higher water flux (95 L/m2/h) than that of the control TFC membrane (37 L/m2/h). This revealed the positive roles of nanomaterials in improving the characteristics of TFN membrane for power generation. Niksefat et al. (2014) developed a new type of TFN membrane by incorporating SiO2 into a polyamide active layer. The performance of the resultant membrane was measured using a 10 mM NaCl solution as feed solution and a 2 M NaCl solution as draw solution in PRO mode. The water flux of the TFN membranes was increased from 15.5 to 36.5 L/m2/h as the SiO2 concentration was increased from 0 to 0.1% (w/v). The control TFC membrane meanwhile only showed about 15 L/m2/h under the same testing conditions. Emadzadeh et al. (2014) modified the PSf substrate of TFN membranes by incorporating different amounts of TiO2 nanoparticles into the substrate matrix. The amount of TiO2 was varied from 0 to 1 wt% and its effect on the TFN membrane was studied in PRO mode using 10 mM NaCl and 2 M NaCl solution as feed and draw solutions, respectively. The best-performing membrane, incorporated with 0.5% TiO2, was reported to have water flux of 56.27 L/m2/h, about 87% higher compared to the control TFC membrane. Ghanbari et al. (2015a,b) modified the TFN membranes using TiO2/halloysite nanotube (HNT) for FO and PRO applications. In the PRO mode, which used 10 mM NaCl as feed solution and 2 M NaCl as draw solution, the water flux of

11.3 Membranes for pressure retarded osmosis

the TFN membrane incorporated with 0.05 w/v% TiO2/HNTs was recorded at 40.8 L/m2/h, which was remarkably higher than the control TFC membrane (24 L/m2/h). Ghanbari et al. (2015a) also prepared a TFN membrane containing only HNTs and investigated its performance in PRO mode. However, the water flux of the TFN membrane (33.6 L/m2/h) was lower compared to the TFN membrane incorporated with TiO2/HNTs. In another work, Emadzadeh et al. (2016) prepared a spindle-shaped anatase nanoporous TiO2 before utilizing it as nanofiller to fabricate TFN membranes. Similar to other relevant works, higher water flux was achieved by the TFN membrane (34.4 L/m2/h) compared to the control TFC membrane (22.1 L/m2/h). The PRO performance was determined under the same conditions, which used 10 mM NaCl as the feed solution and 0.5 M NaCl as the draw solution. Using nanomaterials as filler, Son et al. (2016) prepared a new type of TFN membrane in which the substrate was incorporated with CNT. The CNT was first functionalized by the oxidation method before being added into the polymer solution to form the CNT-blended support layer. The interfacial polymerization method was applied to establish the polyamide active layer to develop TFN membranes. Fig. 11.6 illustrates the structure of substrate incorporated with and without CNT. As can be seen, the presence of CNT helps to increase the water transport rate. With respect to the peak power density, the prepared TFN membrane could achieve 1.0 W/m2 at 6 bar using 0.5 M NaCl solution and deionized water as draw and feed solution, respectively. Zhao et al. (2017) immobilized carbon quantum dots onto the PDA layer of the PRO membrane to improve not only its power capability but also the antifouling resistance. The PRO operation at 15 bar confirmed that the carbon quantum dotsmodified membranes exhibited a much higher power density (11.0 vs 8.8 W/m2) and water recovery after backwashing (94% vs 89%) than the unmodified membrane. Shokrgozar Eslah et al. (2018) prepared graphene oxide (GO)-incorporated TFN membranes for water desalination. In this work, GO with different concentrations was embedded into the polyamide layer and its impacts on the membrane performance were studied. The water flux of the control TFC membrane was significantly improved from 18.3 to 34.7 L/m2/h upon addition of 0.1 wt% GO into the polyamide layer.

11.3.2.3 TFN membranes made of nanofiber Real-world applications of membranes, especially those involving natural water and wastewater, will require membranes to withstand significant stresses. Therefore, structural changes to the support layer, which are necessary in minimizing ICP, must not compromise their critical abilities to resist mechanical stress and further provide a suitable surface for the interfacial polymerization of a robust and selective active layer (Hoover et al., 2013). Tian et al. (2015) fabricated a novel TFN PRO membrane consisting of a tiered structure of polyetherimide nanofiber support reinforced by functionalized CNTs and an ultrathin polyamide selective top skin layer. The results revealed that the optimized membrane could endure a transmembrane pressure up to 24 bar and generate a

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FIG. 11.6 (A) Illustration of the membrane structure incorporated with and without CNTs; and (B) the SEM image of the membrane surface that contained CNTs. Adapted from Son, M., Park, H., Liu, L., Choi, H., Kim, J.H., Choi, H., 2016. Thin-film nanocomposite membrane with CNT positioning in support layer for energy harvesting from saline water. Chem. Eng. J. 284, 68–77.

peak power density as high as 17.3 W/m2 at 16.9 bar using 1.0 M NaCl as the draw solution and DI water as feed solution. SiO2 containing thin-film nanofiber composite (TNC) PRO membranes were prepared by Song et al. (2013) with aims of improving the power density of membranes. The polyacrylonitrile nanofiber incorporated with SiO2 was prepared followed by polyamide selective layer formation via an interfacial polymerization technique. The resultant TNC membrane was further posttreated before it was used for the PRO process. Results showed that the TNC PRO membrane was able to achieve a power density of 15.2 W/m2 using synthetic brackish water (80 mM NaCl) as feed solution and seawater brine (1.06 M NaCl) as draw solution. These two solutions possessed osmotic pressure of 3.92 and 51.8 bar, respectively. Currently, only a limited number of studies have examined the potential of nanofiber as support layer for the TFN membranes for PRO applications. However, the unique characteristics of the nanofiber membrane, such as large porosity and greater water flux in comparison to the typical asymmetric membrane made from the phase inversion method, would create more opportunities to enhance the performance of existing TFN membranes for power generation.

References

11.4 CONCLUSION PRO is one of the most promising renewable ocean energy sources, and the membrane is the heart of this process. To improve the performance of the PRO process, various composite membranes incorporated with nanomaterials have been developed by membrane scientists. The chapter showed that the performance of the PRO membrane could be improved by incorporating nanomaterials either into the membrane support layer or its polyamide top selective layer. Many studies have successfully demonstrated the enhanced performance of nanomaterial-incorporated TFN membranes for PRO operation, in comparison to the control TFC membrane, in terms of water flux and power density. Depending on the testing conditions (e.g., operating pressure and properties of draw and feed solution), the power density that could be produced by the existing nanomaterial-incorporated TFN membranes is in the range of 1.0–24.3 W/m2. In order to further improve the performance of the TFN membranes for PRO applications, more research is required to optimize the intrinsic characteristics of the membranes by developing a new method to introduce nanomaterials into the membrane morphology without affecting the structural integrity.

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