Next-Generation Nanoporous Materials: Progress and Prospects for

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Next-Generation Nanoporous Materials: Progress and Prospects for Reverse Osmosis and Nanofiltration Derek M. Stevens, Jessica Y. Shu, Matthew Reichert, and Abhishek Roy* Dow Water and Process Solutions, 7600 Metro Boulevard, Edina, Minnesota 55439, United States

ABSTRACT: Reverse osmosis and nanofiltration are highly adopted, growing technologies that are used to remove salts and other small molecules in water treatment. Both of these technologies primarily use cross-linked polyamide membranes to achieve the desired separation. Many novel nanoporous materials are being developed as alternatives or complements to polyamides, including graphene, graphene oxide, block copolymers, liquid crystals, aquaporins, and other biologically inspired molecular channels. This article evaluates each of these technologies by (i) reviewing the current progress in each area, (ii) identifying key needs for immediate research, and (iii) evaluating considerations for commercial development. The economic benefits of these technologies in reverse osmosis applications are further reviewed to help frame the expected commercial value proposition.

1. INTRODUCTION Water is one of our planet’s most precious resources, and one of life’s most basic and essential components. If current predictions about population growth prove true, the world’s population will reach 8.5 billion by 2030.1 That will increase the global demand for water by 30%. In fact, some analysts predict that available water supplies will satisfy only 60% of the demand at that time, maybe less. According to the World Economic Forum, that would make water the biggest risk factor for global stability, behind only financial crises and unemployment.2 How is that possible, with so much of the world’s surface covered with water? One of the main reasons is the fact that only 2.5% of the world’s water is freshwater. Creating the fresh water necessary to quench our thirst is an unprecedented challenge. Science and technology will play a significant role in addressing this pressing issue besides water infrastructure development and involvement of society and policies. The intent of the current paper is to review future and present state-of-the-art water purification technologies and provide the readers an insight into the challenges and opportunities with upcoming technologies. Before we begin, let us look at the broader water purification technology spectrum. As seen from Figure 1, water purification technologies exist to filter contaminants ranging from suspended solids down to dissolved ions. Coarse filtration and ultrafiltration are strictly mechanical treatment, operating by size exclusion. As the size of the solute decreases, additional mechanisms begin to play a role. In the ideal model of reverse osmosis, for example, separation is achieved through differences in the transmembrane mass transport rates of species due to differences in membrane/solute/solvent interactions and in the partitioning or solubility across the membrane/solvent interface. Thermodynamic interactions © XXXX American Chemical Society

between solutes and the separation media govern selectivity when filtering at the molecular scale. The main topic of this paper will be centered on, but not limited to, reverse osmosis (RO) and nanofiltration (NF) technologies. The current state-of-the-art membrane technology for RO/NF is based on a cross-linked polyamide membrane chemistry, which provides separation of water and salt with the application of pressure greater than the osmotic pressure of the feed. It was in the early 1980s when John Cadotte developed interfacial polyamide (PA) chemistry on thin film composites for reverse osmosis applications.3 Even today, nearly four decades later, polyamides remain the leading technology in the industry. The chemistry involves an interfacial polymerization between a difunctional amine in the water phase and a trifunctional acid chloride in the organic phase. The most commonly employed amines are m-phenylenediamine (MPD) in RO membranes and piperazine in NF membranes, while trimesoyl chloride (TMC) is the most common acid chloride. The kinetics are very fast (few seconds), leading to a cross-linked three-dimensional structure with residual end-groups as amine and COOH. This barrier layer, having a thickness of ca. 200 nm, is part of a three-layered composite: the bottom layer is a polyethylene terephthalate (PET) reinforcing web to provide mechanical support, followed by an asymmetric polysulfone (PS) layer with surface porosity in Received: June 13, 2017 Revised: August 15, 2017 Accepted: September 9, 2017

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DOI: 10.1021/acs.iecr.7b02411 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Technology spectrum for water purification.

Figure 2. Manufacturing of cross-linked polyamide membranes: (a) schematic process diagram, (b) chemistry, and (c) spiral-wound module. Module figure reproduced with permission from ref 22. Copyright 2010, Taylor and Francis, London.

the tens of nanometers, and finally the polyamide. The polyamide layer is responsible for separation. At a macroscopic level, one can easily appreciate the dynamic nature of a polyamide’s structure through its separation performance, which can change notably in response to external stimuli such as feed temperature,4,5 pH,6 composition,7 strength7 and ionic state.8 At a microscopic level, polyamide membranes have a complex, heterogeneous,9 cross-linked morphological structure, the properties of which can be difficult to characterize and correlate to performance. The characterization of a definable pore size and structure in a polyamide is challenging to measure directly,10 but so-called “effective pore sizes” of the membranes can be inferred through size exclusion measurements.11 Simpler measurements such as water uptake have been performed,12 but are complicated by the submicrometer thickness of the active layer and the fact that it is bonded to an underlying polysulfone support. RO and NF polyamides are negatively charged at

practical operating conditions, a product of the balance between dissociated free acid and amine end groups. Characterization of membrane charge has been well-studied in the literature,13,14 as it can be critical for both solute rejection6 as well as interaction with foulants.15,16 The mechanism of transport in reverse osmosis is classically described through the solution-diffusion model,17 which treats the membrane as homogeneous and does not directly take into account effects of charge or any pores with convective transport. More-complete descriptions incorporating these effects exist,18−20 which become increasingly important as the membrane becomes more open (i.e., more permeable), particularly in the NF space.21 Figure 2 provides a high-level overview of each layer in a polyamide thin film composite and how this chemistry is processed and implemented commercially. It is a continuous process where the PS support is phase inverted on the PET web and subsequently coated with aqueous amine. Following amine B


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Industrial & Engineering Chemistry Research application, the next step is the TMC application from an organic phase. The polymerization starts at the interface between the organic and aqueous phases, and within 2−3 s, the trilayer composite membrane structure is formed. To complete the process, the membrane is first taken through an extraction step to remove residual monomers and solvents, followed by an oven application to drive the polymerization further, and finally is rewound into large rolls. In the next step, the trilayer membrane structure is typically manufactured into spiral wound modules to provide the best tradeoff between productivity and efficiency. Feed passes through the module and over the membrane surface in a cross-flow orientation. Ions are rejected by the PA to the concentrate stream, and purified water passing through the composite is guided through spiral permeate channels, ultimately exiting through the center of the module. 1.1. Current Technology Levers in Reverse Osmosis. RO is an energy-intensive process, and over the last 30 years, most membrane chemistry development has focused on making energy-efficient membranes. Besides energy savings, the quality of permeate is also of prime importance. Permeate quality can dictate outright feasibility of the application, system size (through the number of required passes) as well as operating conditions such as flux and recovery, which are necessary to meet quality limits. In the industrial space, ion exchange resins are often used as polishers after RO to achieve a particular permeate quality for boilers and other such high-purity applications. The better the quality of water from RO permeate, the lower the chemical requirement for ion exchange regenerations and, hence, the higher the chemical savings. Water permeability (A, or A-value) and salt permeability (B, or B-value) are the two primary properties used to evaluate the performance of an RO membrane and are derived through the solution-diffusion model.17 Volumetric water flux (Jw) is driven by net driving pressure, or the difference between net hydraulic pressure (ΔP) and net osmotic pressure (Δπ), while the mass flux of dissolved solute (js) is driven by concentration differences (ΔC):

Jw = A(ΔP − Δπ )

(1)

js = B(ΔC)

(2)

Figure 3. Tradeoff between salt permeability and water permeability in SW and BW RO membranes sold by Dow, as extracted from commercially published module specifications. The typical rejections given on the upper x-axis reflect common specifications of today’s commercialized offering groups (SW, BW by energy grade, and NF) under their respective test conditions, which are conducted at similar flows (8000−12 000 gallons per day in an 8-in. module) but at varying feed pressure and salinity. As an added comparison, the secondary y-axis maps water permeability to the corresponding feed pressure needed to achieve identical flow in a brackish water environment: 11 500 gallons per day for a 440 ft2 spiral module operated at 15% recovery with a feed of 2000 ppm of NaCl.

different membranes is not a linear function of the A-value, because of inherent thermodynamic (i.e., Δπ), module, and system limitations, upon which we will expand later in this paper. As an illustration of these diminishing returns, the solid black curve in Figure 3 plots, as a function of A-value, the pressure required for reasonably constructed modules to produce an equivalent amount of water when presented with the same brackish water feed. 1.2. Next-Generation Needs and Research. As the membranes become more energy-efficient, we approach a space where, because of several fundamental limitations, further improvements in water permeability may not result in similar energy or system savings. The shape of the required feed pressure curve in Figure 3 is just one example of these diminishing returns, upon which we expound further in Section 5.2. Customer focus is shifting to other significant value propositions related to having a membrane that is less prone to fouling, resistant to scaling, stable to chlorine, stable to extreme pH, and many more. Areas where these properties could be beneficial are industrial wastewater reuse, high recovery applications, acid and base recovery and specialty applications. For example, a membrane with enhanced chlorine stability could enable the end user to add chlorine to their feed and eliminate biofouling. This is a technological gap, as current polyamide-based RO membranes are not stable under prolonged exposure to chlorine. In order to meet those application needs, improvements in material properties are desired. Several novel nanoporous materials are being investigated which, in addition to high permeability and rejection, offer potential solutions to many of these other challenges. 1.3. Next-Generation Nanoporous Materials. Research into nanoporous materials as next-generation, ultrahigh permeability filtration membranes has blossomed. A literature search over the last 5−10 years will produce no shortage of reviews and perspectives covering the scope of these technologies. Current publications cover material including today’s commercially available polyamide and cellulose acetate

The higher the A-value, the higher the productivity of water; similarly, the lower the B-value, the higher the salt rejection. The SI units for A and B are m/(s Pa) and m/s, respectively, but across the industry are almost universally expressed in more workable units of flux and pressure. In this paper, we refer to Avalue in units of Lmh/bar (or liters per square meter per hour per bar, L/m2/h/bar) and the B-value in units of Lmh. Figure 3 represents a plot of A vs B for a sampling of commercially available polyamide membranes sold into either seawater (SW) or brackish water (BW) applications, with their respective A- and B-values extracted from the industry standard test conditions appropriate to the membrane in question (e.g., 32 000 ppm of NaCl for all SW; 2000 ppm of NaCl for most BW). Membranes with higher permeability require less energy to operate, but exhibit a tradeoff in salt permeability. Consequently, low energy membranes do not provide the same quality of water as high energy membranes. Typical SW membranes provide 99.8% rejection of a high TDS feedwater, while brackish water membranes operated at lower TDS provide rejections ranging from 99% to 99.7%, depending on the A-value, as well as the unique distinctions of the membrane chemistry employed. Furthermore, the energy savings that one can achieve with C


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Figure 4. Selection of next-generation materials holding promise as ultrahigh-water-permeability reverse osmosis (RO) or nanofiltration (NF) membranes: (a) single or few layer nanoporous graphene (top) and stacked graphene oxide (bottom) membranes [reproduced with permission from ref 47, Copyright 2015, The Royal Society of Chemistry, London]; (b) aquaporins and biologically inspired nanochannels [illustration by Typoform, reproduced from ref 78, Copyright 2014, The Royal Swedish Academy of Sciences]; and (c) block copolymer membrane fabricated by combining selfassembly with non-solvent-induced phase separation [reprinted with permission from ref 79, Copyright 2014, Macmillan Publishers, Ltd., Basingstoke, U.K.].

Table 1. High-Level Comparison of Biologically Inspired, Self-Assembling Organic and Graphene/Carbon Membranes

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Table 2. Snapshot of the Separation Capabilities of Commercial RO, NF, and UF Membranes Today, along with a Selection of Promising Results Reported Using Next-Generation Materials

Figure 5. Representative, not comprehensive, summary of biologically inspired membrane research. (a) Illustration by Typoform (taken from ref 78, Copyright 2014, The Royal Swedish Academy of Sciences). (b) Image reproduced from ref 105 (Copyright 2007, National Academy of Sciences, Washington, DC). (c) Image reprinted, with permission, from ref 106 (Copyright 2007, American Chemical Society, Washington, DC). (d) Image reproduced from ref 107 (Copyright 2011, Wiley, New York). (e) Reprinted with permission from ref 108 (Copyright 2012, Elsevier, Amsterdam); stability work performed by Qi et al.99 (f) Reproduced from ref 109 (Copyright 2015, MDPI AG, Basel, Switzerland).

membranes,23−29 alternative semipermeable polymers,23,25,26,28 block copolymers,24,27,30−32 zeolites,23−25,33−36 inorganic nanoparticles,23−26,29,33,35−37 carbon nanotubes,23−26,29,33−39 nanoporous graphene,27,33−35,38−51 graphene oxide,26,27,34−39,41,42,44−62 other two-dimensional (2D) inorganics,50 aquaporins or biomimetic channels,23−27,35,63−69 and liquid crystals.27,70 Many of these reports cover multiple materials.23−27,29,33−39,61,62,71 Several authors have also examined the expected impact that such low-energy membranes might have on industrial-scale filtration processes.72,73 The reader is referred to these many manuscripts for in-depth examination of each technology. In this paper, we take a look at several of these promising materials from an industrial perspective. A high level view is provided of the basic material structure, expected potential, results demonstrated to date, and comparison to current commercial offerings, followed by an examination of multiple challenges related to scale, thermodynamics, and particularly fouling, which are not always detailed in the context of these new materials. While the attention of this review is devoted to desalination by RO and NF, various alternative desalination technologies should be mentioned, several of which have been investigated with and could benefit from novel membrane materials. Forward osmosis (FO), which is the the

opposite of RO, uses a concentrated draw solution to pull clean water from a saline feed and may be a beneficial approach to desalination in certain situations.74,75 Pressure-retarded osmosis (PRO) operates similarly with a feed and concentrated draw, but relies on the pressure created by osmosis to generate power.76 Membrane distillation recovers water from a saline feed by passing its vapor through a hydrophobic membrane.77 Our review encompasses 2D materials such as nanoporous graphene (NP G) and graphene oxide (GO), aquaporins (AQP), block copolymers (BCP), and liquid crystals (LC), as well as their potential for application in RO and NF (see Figure 4). Table 1 provides a first look, summarizing key characteristics for each technology and our interpretation of experimentally demonstrated advantages and disadvantages against today’s commercial polyamides. Details of each technology, elaborating on the information outlined in the table, are provided in subsequent sections. Table 2 highlights some of the best experimental membranes reported to date using these materials, along with the operational range of some commercialized ultrafiltration (UF) and polyamide (PA) RO and NF membranes. Contrary to Figure 3, solute selectivity in the rest of this paper is gauged only by measured rejection under the given test condition reported, E


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at the entrance of the pore; this filter is composes of a cluster of amino acids that helps to bind water molecules, while excluding other molecules. Thus, the primary sequence of the protein, which dictates its structure and function, codes for both high water permeability (achieved through hydrophobicity of the interior of the channel), and high selectivity (achieved through mechanisms of charge, hydrogen bonding, and size exclusion). Lipid bilayers are somewhat permeable to water due to simple diffusion, with a characteristic diffusion coefficient for water permeability for many biological membranes of 10 kcal/ mol). Unlike diffusional water permeability, the channelmediated water transport through AQPs exhibits low activation energy (Ea ≈ 3 kcal/mol) and osmotic water permeability (∼0.2 cm/s). This corresponds to a rate of ∼2 × 109 water molecules per second per pore unit, making AQPs a particularly impressive water channel evolved by nature.110 Since the discovery of AQPs in 1993,111 there has been considerable interest in utilizing this protein for membrane technology. Traditional synthetic polymeric RO/NF membranes typically demonstrate a tradeoff between A- and B-values, whereas nature has evolved a high selectivity/high permeability water filter that has the potential to surpass status-quo performance when harnessed appropriately. Over the past decade, many researchers have focused on this objective, with the first report of AQP’s potential applicability in water desalination membranes made in a landmark paper in 2007, which demonstrated the reconstituion of Aquaporin Z (AqpZ, a bacterial AQP from Escherichia coli) in amphiphilic triblock copolymer ABA vesicles.105 The permeability of AQP-incorporated ABA polymersomes was up to ∼800 times greater than that of unmodified polymersomes, while complete rejection of the solutes glucose, glycerol, salt, and urea was also demonstrated. Given that the water permeability for each AqpZ water channel was estimated to be ∼3 × 10−14 cm3/s and assuming the same AQP packing density in supported membranes that was achieved in polymersomes, an increase in membrane permeability of 2 orders of magnitude was anticipated over traditional polyamide membranes. Motivated by the idea that AQPs could enable the realization of semipermeable membranes with 100% rejection of all species except water, this study spawned interest by many different groups to develop methods by which AQPs could be embedded in solute-impermeable, defect-free, stable matrices capable of withstanding pressure-driven long-term operation over a range of feedwater conditions for water filtration. The current challenge in developing biomimetic AQP membrane technology is providing a means for efficient and practical scaleup of such a membrane, while also maintaining and effectively harnessing the unique functionality of biological proteins within synthetic environments or matrices. The enormous potential of AQP-incorporated desalination membranes has attracted extensive research efforts worldwide in recent years. Ideal AQP-based membranes are (1) highly water permeable, (2) highly selective and defect-free, (3) mechanically stable (able to withstand pressure-driven operations characteristic of RO/NF applications), (4) chemically and biologically stable (able to withstand long-term operation under real water conditions), and (5) practical to scaleup at reasonable cost. Many different approaches to achieve AQP-incorporated membranes that meet these objectives have been reported in recent years and have been summarized in several comprehensive reviews.63−66 For the preparation of these membranes, AQPs must be

because of the various solutes used and difficulty in extracting true solute permeability values under varying experimental conditions. The spectrum of polyamides used today ranges from high energy seawater membranes, which give 99.8+% NaCl rejection at permeabilities down to 1 Lmh/bar, up to the lowest-energy brackish RO membranes, which have permeabilities up to 10 Lmh/bar and ∼98%−99% NaCl rejection. As they filter salts, the effective pore size of a semipermeable RO membrane is 4. Total rejection of PEO molecules with a molecular weight of 4 kDa (∼2 nm radius of gyration) at pH 5.5 was also reported. Additional performance studies of membranes produced via SNIPS have been demonstrated by Clodt et al.170 The authors investigated the performance attributes of multiple PS-b-P4VP nanostructured films produced via SNIPS, demonstrating the exclusion of PEG between molecular weights of 100 and 1000 kDa over a range of pore sizes and rejection layer thicknesses. The results are compared to those of commercial poly(ether sulfone) membrane, and potential advantages are highlighted. Yu et al. report a remarkably high A-value of up to 430 Lmh/ bar for a blended PS-b-PAA/PS-b-P4VP block copolymer membrane (also produced via SNIPS) that rejects 100% of protoporphyrin IX, but some passage of lysine, indicating a pore size near 1.5 nm.32,87 In addition, they provide dissipative particle

dynamics simulations that corroborate the experimental pore size results. In this particular case, 50 cm2 samples were produced, demonstrating the potential for this method to be scaled to even larger active areas. 3.3. Outlook. Currently, block copolymer and liquid crystal membranes have the ability to filter out larger molecules, as would be encountered in ultrafiltration applications. However, liquid crystal membranes have only shown fluxes that are orders of magnitude below those of BCP and polyamide membranes. BCP membranes are shown to be comparable to conventional PES in flux and superior in rejection. Yet, for the few cases where desalination data are given for self-assembled membranes, the performance does not approach replicating that of RO. This is to be expected, given the relatively large minimum pore size for these materials (1−10 nm), compared to the size of dissolved ions (on the order of a few Angstroms). However, the biggest challenge facing block copolymer and liquid crystal self-assembled membranes is the cost of materials, compared to conventional materials for UF and NF applications. Moreover, while the processability of these systems appears to be readily tailorable to large-scale manufacturing, to date, sample sizes are limited to ∼100 cm2 or smaller. With some investment in scale-up technologies, it is possible that this gap could be overcome. However, it remains to be seen if the material price and any increase in processing cost is worth the benefit to the flux and rejection to UF and NF for the removal of large molecules, as well as the added oxidation resistance during aggressive cleaning cycles. Applications for block copolymer membranes in the desalination space are limited, because of the relative pore size that is currently achievable. In addition, while these materials have potential use in NF and UF spaces, they may be limited by high production costs. Also, while scale-up methods have been identified for block copolymer and liquid crystal membranes, to date, there has not been a demonstration of a commercially relevant product that utilizes enough membrane area as to be feasible in any application where UF, NF, and RO currently serve. However, block copolymer membranes have shown some promise as mediators of selective transport for solutes. In one case, Qiu et al. demonstrated that, by functionalizing a PS-bP4VP block copolymer membrane (formed by SNIPS) with a cationic molecule, they could tune selectivity of BSA versus Bovine hemoglobin (Bhb).173 At pH 4.7, only Bhb would pass through while at pH 7, only BSA would move through the membrane. Even without functionalization, the membrane showed selectivity for BSA transport versus the protein globulin-γ as a function of solution pH. If further developed, such a technology has clear implications for drug delivery and medicine. Switchable pH-responsive block copolymer membranes have also been developed utilizing metal-block copolymer complexes with SNIPS.174 In this case, membranes demonstrated a large change in flux as a function of pH, with 50 Lmh/bar at pH 2 and >900 Lmh/bar at pH 7. The authors were able to show, using SEM, that this corresponded to a change in pore diameter from 2 nm to 18 nm. Such techniques open the door to further development and modifications that have the potential to lead to selective solute transport. In addition, there are potential applications for block copolymer membranes in the pharmaceutical industry,175 batteries,176,177 CO2 recovery operations,178 biofuel separations,179 and many others. It is possible that the economics of the J


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Figure 8. Representative, not comprehensive, summary of graphene and graphene oxide research activities. (a) AFM image of single-layer graphene: periphery, SiO2 surface; central area, 0.8 nm height (reprinted, with permission, from ref 181, Copyright 2004, AAAS, Washington, DC). (b) Schematic model of desalination through nanoporous graphene (reprinted with permission from ref 93, Copyright 2012, American Chemical Society, Washington, DC). (c) Schematic illustration of covalently cross-linked GO nanosheets (reprinted with permission from ref 102, Copyright 2013, American Chemical Society, Washington, DC). (d) Lockheed Martin press release, March 18, 2013 (data taken from ref 182). (e) Scanning transmission electron microscopy images of pores created in a graphene membrane by ion bombardment, followed by chemical etching (reprinted with permission from ref 95, Copyright 2015, American Chemical Society, Washington, DC). (f) Schematic (top) and photograph (bottom) of GO nanosheets confined by epoxy and assembled into a stacked membrane (reprinted by permission from ref 89, Copyright 2017, Macmillan Publishers, Ltd., Basingstoke, U.K.).

fullerenes, the lattice in graphene is, by definition, only a single atomic layer thick, making it a truly 2D material with a multitude of potential applications.41,180,183,184 The aromatic rings of the honeycomb lattice are too small for the passage of molecules such as salts, water, and even gases.185 For a perfect sheet of graphene to be used as a filtration membrane, larger nanometerscale pores must be created or other defects must intentionally be present to allow passage of water while rejecting targeted solutes. Sheets of single or few layer graphene are commonly made through chemical vapor deposition (CVD) onto an inorganic crystalline substrate, a process which can be scalable to large areas.186−188 Other methods, such as exfoliation of graphite,189 are possible, but present greater challenges in large area scalability. If grown on a nonporous substrate, graphene sheets must be subsequently transferred onto a porous support that will allow for the flow of water, with the original substrate subsequently etched away. While impressive transfer areas have been demonstrated,188 the process typically imparts defects such as wrinkles, cracks, or regions of incomplete transfer.44,190 As grown, CVD graphene films can have grain boundaries spaced as little as 50 μm apart94 and intrinsic nanopores that may or may not be ideally sized for filtration.190 The creation of intentional nanopores for water transport has been approached from multiple directions,44,88,189 all of which present challenges in forming an ideal monodisperse distribution of optimally sized pores that can block salt while still passing water.47 Transfer defects, grain boundaries, and nonideal pores are all sources of leakage that compromise the selectivity of the membrane. Therefore, management of unwanted holes and gaps is the

separations process may favor a more expensive membrane, which also has added solvent resistance, is readily cleanable, and can be functionally modified to promote selective separations. In these cases, where application spaces may prove to be lucrative, the specific utility of block copolymer membranes may justify their development costs.

4. CARBON AND SIMILAR NANOSTRUCTURES 2D materials, such as graphene, are highly attractive for water purification, because they hold the promise of providing the absolute minimum resistance to water transport. Following the rapid appearance of graphene in materials research,180 the topic of 2D materials for water filtration has skyrocketed in the last 10 years, with Figure 8 summarizing some key research milestones. These materials can be fashioned into membranes as (i) composite blends with polymers or other matrix structures, (ii) stacks of sheets, or (iii) truly 2D single layer or few layer continuous membranes. Here, we provide an overview of nanoporous graphene used as a truly 2D membrane, graphene oxide as a thick stack of sheets, and composite blends using each of these. Other 2D materials not formed from carbon are also mentioned. Although not touched upon here, we should note that 2D sheets are not the only way to incorporate carbon and other types of inorganic nanostructures into a membrane. Nanoparticle composites and carbon nanotubes are both active areas of interest that are mentioned here but, for the sake of brevity, are not covered in great detail. 4.1. Nanoporous Graphene. Graphene consists of an atomically thin sp2 hybridized hexagonal carbon lattice. Unique to other forms of carbon such as graphite, carbon nanotubes, and K


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Industrial & Engineering Chemistry Research foremost consideration in all nanoporous graphene films evaluated for water filtration. Computational studies have demonstrated high predicted rates of water transport and salt rejection through nanoporous graphene membranes.43 The chemical structure of the walls becomes more important as the diameter of the pore is reduced to dimensions relevant to salt rejection.191,192 At these scales, ion hydration, and interactions with pore walls all become important, and ion passage is not as simple as comparing the diameter of the ion to the diameter of the pore. Functionalization of the pore can be used to tune transport,43,193 with functional groups such as negatively charged nitrogen and fluorine,194 amine,193 oxygen,195 hydroxyl,93,193 carboxyl,193,196 carbonyl,196 hydrogen,93,194,195 and silicon,94 all having been either modeled41,93,193−197 or experimentally evaluated41,88,94,191 in applications including both water filtration and gas separation. Cohen-Tanugi and Grossman have predicted optimum pore diameters for salt separation of 5.4 and 4.6 Å for hydrogenated pores and hydroxylated pores, respectively.43 The predicted water permeabilities of such membranes constructed with 10% porosity ranged from ∼1600 Lmh/bar to ∼2800 Lmh/bar (66−112 gfd/psi). On the experimental side, graphene membranes have made many strides over the last five years. Early efforts by O’Hern et al. at growing reasonable active areas and introducing monodisperse pores were plagued by transfer defects, which obviated use of the films for filtration.88,190 Several authors have been able to successfully validate high water permeation rates by reducing the active area of measurement to within the grain size of the graphene sheets.94,198 Surwade et al., for example, masked nanoporous graphene over a 5 μm hole, with forward osmosis measurements showing a water permeability of ∼250 Lmh/bar and ∼100% KCl rejection.94 Several recent works have attempted to seal up unintended defects and measure performance over larger areas. O’Hern et al. created a 24 mm2 nanofiltration graphene membrane with ∼70% MgSO4 rejection in FO measurements, using single-layer graphene transferred to a polycarbonate track etch membrane.95 Defects were sealed by a combination of atomic layer deposition and interfacial polymerization, and pores were subsequently created by ion bombardment.95 The graphene/track etch composite had a water permeability of 0.14 Lmh/bar, with the graphene layer itself being ∼10 times greater (1.41 Lmh/bar). In a type of hybrid example between graphene and the graphene oxide discussed in the next section, Park et al. have tested 25-mm-diameter membranes composed of a few layers of weakly oxidized graphene flakes (10−30 μm each) on a microporous support, with the gaps between flakes sealed shut by sintered silica.199 Nanopores, believed to be in the nanometer to subnanometer range, were bored into the film by copper-assisted nitric acid etching, with the copper deposited on the membrane via thermal evaporation. The resulting membranes showed high pure water permeability (∼1400 Lmh/bar). Multivalent cation rejections of 20%−25% in a single pass were observed, with portions of the removal attributed both to size exclusion as well as absorption. This approach of using multiple stacked layers is attractive to limit film defects in nanoporous graphene and aid in solute rejection, as described by recent modeling.200 Finally, the Lockheed Martin Corporation has been developing a patented nanoporous graphene membrane under the tradename Perforene.182,201 The technology is currently under development for desalination, and initial applications requiring coarser filtration in the oil and gas industry are reportedly being investigated.202

4.2. Graphene Oxide. Pore uniformity and defect management are the two main factors limiting accelerated development of nanoporous graphene membranes. The constraints imposed by the large size and hydrophobicity of graphene sheets also make processing a challenge. In an oxidized state, graphene becomes hydrophilic, allowing dispersions of it to be managed in solution. Film formation from such dispersions becomes much more straightforward, and, consequently, an abundance of research using graphene oxide (GO) membranes for separations has been published. GO membranes used in water filtration are composed of layered GO flakes coated onto a microporous support surface. The flow profile in a GO laminate film is one of percolation, with lateral flow along hydrophobic channels combined with periodic perpendicular transport through spacing between adjacent nanosheets, as well as holes of imperfection within sheets. These holes are often created inadvertently during the oxidation process or subsequent reduction reactions.203,204 The net combination of these varied transport routes, along with the ever-present concern of larger-scale film defects, determines the water permeability and solute separation capability of a GO membrane. Water in such nanoconfined spaces behaves uniquely different from its typical bulk properties. Transport in the undisturbed hydrophobic sections between sheets takes place with very low friction,205 akin to that within carbon nanotubes.206,207 Low friction and capillary effects are cited to explain fast water transport in GO membranes.205,208 However, hydrogen bonding interactions of water with oxygen functionalities on the sheets may act against this benefit by hindering water transport.205,209 Wei et al. have suggested that these interactions may retard much of the low friction benefits, and instead have proposed other avenues of porosity (wrinkles, holes, interedge spacing between sheets) may be responsible for the high rate of water transport.210 GO flakes can be prepared by chemical oxidation of graphite followed by exfoliation, for example, by sonication, to release individual or few layer nanosheets into aqueous suspension.102,211,212 The sheets can range in size from a few hundred nanometers up to a few micrometers.212−214 Characterization is challenged by the complex and perhaps variable nature of the final product. Oxidation is believed to take the form of epoxide and hydroxyl groups within the basal plane and carbonyl and carboxyl groups along the edges of the now-hydrophilic sheets.211,214,215 This hydrophilicity, in turn, can pose problems to film stability in aqueous solutions, and more robust membranes can be prepared by reduction through thermal, chemical, or electrochemical methods. Graphene in this state is often referred to as reduced graphene oxide (or rGO).214,216 Li et al. discovered partial reduction of dispersed graphene oxide, termed chemically converted graphene (CCG), using hydrazine removes all oxidation sites except carboxylic acid functional groups, affording electrostatically stabilized aqueous suspensions.217 New routes to produce more ideal reduction products continue to be investigated. 218 Several functionalization reactions can also be performed with GO.219,220 GO suspensions, either fully oxidized or reduced, can be processed into films using various laboratory techniques,52 with many showing good promise for translation to large industrialscale processes. Film thicknesses ranging from just several atomic layers up to several micrometers have been reported. The thicker micrometer-scale laminates can be readily prepared by vacuum filtration, with the suction of the vacuum acting to orient the sheets parallel to the underlying support.212 Other, morecomplex routes involve a layer-by-layer construction, in which L


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ultimately limited by the d-spacing or gap height between adjacent parallel nanosheets. Even in the absence of corrugations, several layers of water are believed to exist between adjacent GO layers, with regions of oxidation serving as periodic spacers.205,208 A fully wetted GO film has a d-spacing in the neighborhood of 13.5 Å, which is too large to effectively block salts.89 That being said, the height of this gap can be tuned by several factors, including humidity89 or degree of hydration,235 pH,60 osmotic strength,102,221,223,236 and the presence or absence of contaminants in the water,237 as well as the degree of oxidation of the sheets.205,218,222,238 A negative charge on the sheets can also serve as an additional barrier to salt through Donnan exclusion. In the LBL approaches mentioned earlier, a variety of covalent or electrostatic linkers can be inserted between sheets to aid in stability and help control channel height. As an example, idealized 10-nm-thick graphene oxide framework membranes consisting of boronic acid linkers have been modeled with 100% NaCl rejection and A-values of 200−1000 Lmh/bar.100 Experimentally, however, LBL membranes have, thus far, not produced spacing capable of sieving monovalent salts.102,223,224,239 Chemical reduction to shrink the gap height may block salts but has shown to also significantly reduce water permeability.60,218,240 This tradeoff creates a strong possibility that, unless gap height can be efficiently controlled, GO membranes will only find utility in NF.27 Interesting methods of managing gap height were recently proposed by Abraham et al.89,241−248 In one approach, they were able to control the gap height between 6.4 Å and 9.8 Å by first equilibrating the membrane to a desired humidity and then locking the structure in place with epoxy. Laboratory filtration experiments, conducted in parallel through cross sections of ∼3 mm long GO stacks, demonstrated that salt permeabilities could be tuned in this manner, by several orders of magnitude. The solute transport was thermally activated, suggesting that the shedding of hydration water from the shell of the ions dictates their entrance into the nanochannels. This sort of membrane, requiring lateral permeation through a cross-section, would be difficult to scale up in practice. In their second approach, the authors demonstrated a simpler method in which d-spacing could also be controlled by blending normal graphene flakes with GO. Films 5 μm thick were tested under forward osmosis, giving a flux of 0.5 Lmh under a driving pressure of ∼75 bar. The membranes showed 97% NaCl rejection, which is quite impressive, given the low operating flux. Permeability could be increased by reducing the thickness of the stacks, with a trade-off in NaCl rejection. 4.3. Other 2D Materials. A wide variety of other materials can be fashioned in 2D geometries, including transition-metal dichalcogenides,249 transition-metal carbides/carbonitrides, and boron nitride. Dervin et al. have published a recent review for further reading on the topic.50 Molybdenum disulfide250,251 and tungsten disulfide252 have both garnered particular interest as water filtration membranes. As with nanoporous graphene, these materials must also rely on appropriately sized defects or intentionally placed pores to allow water flux. Again, similar to graphene, stacks of many sheets together allow for ease of processing and defect management. In one recent study of note, stacks of MoS2 sheets 1.7 μm thick have been prepared with permeabilities of 245 Lmh/bar and 89% rejection of Evan’s Blue dye.253 4.4. Hybrids and Composites. An alternative approach to making membranes with 2D materials is to incorporate them as a component or additive within the matrix of a more traditional

GO sheets alternate with a covalent or electrostatic bridging molecule.62 The general ease of processing is reflected by a long list of publications involving experimental membranes, only a few of which are cited here.101,102,208,221−224 CCG films, for example, have been shown to orient in well-aligned stacks in a form of hydrogel with retained water.225 The sheets are believed to be intrinsically corrugated. Corrugation, although it may not be present in all examples of GO,222 is tunable by temperature and appears to persist in assembled films, providing channels for transport.221,226 Graphene and its analogues are highly absorbent materials,227,228 and experiments must be done carefully to differentiate between separation by rejection from separation by absorption.101,102,229 While our attention here deals with the separation of dissolved solutes such as salts, the unique properties of graphene and GO have also attracted significant interest for the separation and recovery of oil and organic solvents from water, both as sorbents as well as antifouling membranes.230−233 By and large, experimental publications involving dissolved solutes have produced membranes with high water permeabilities that function within or slightly outside the nanofiltration regime. Huang et al. prepared GO membranes which, under certain conditions, showed 71 Lmh/bar water permeability and 85% rejection of Evan’s Blue dye.221 However, permeability decreased precipitously in the presence of sodium chloride (which passed with no rejection) and at basic or acidic pH values, presumably due to feed-induced alterations in the nanosheet gap spacing. Sizeable losses in permeability were also observed with increasing feed pressure, with the change attributed to flattening of the nanochannels formed by corrugations of the GO flakes. Akbari et al. have used shear alignment of highly concentrated dispersions of GO nanosheets to make well-ordered films with tunable thickness.101 They used a doctor-blade coating method that is well scalable to industrial processes and also demonstrated large area production with a gravure printer. The GO dispersions were concentrated using hydrogel beads to remove water from more dilute starting concentrations, and the films were partially reduced with hydrazine treatment to provide aqueous stability. Membranes 150 nm thick showed water permeabilities of 71 Lmh/bar with 90% rejection of Methyl Red and, surprisingly, a fairly consistent 30%−40% rejection of both monovalent and divalent salts. Permeability was sustained above 40 Lmh/bar over extended operation with BSA foulant, with recovery to initial levels obtained readily with cleaning. Wang et al. have made layer-by-layer (LBL) GO structures that behaved quite similar to commercial NF, with a water permeability of ∼16 Lmh/bar and salt rejections of 20% for monovalent NaCl and >80% for divalent salts.224 Another LBL example has demonstrated >95% rejection of some divalent cations with a somewhat lower water permeability (4.7 Lmh/bar).234 Considering the abundance of preparation techniques being investigated, it is not surprising to see the permeability and selectivity of GO membranes (whether they be pure, reduced, assembled with layers of other materials, or otherwise) vary over several orders of magnitude. For closer inspection, Amadei and Vecitis have tabulated the range of reported A-values and constructions for GO membranes in detail.59 While many GO membranes have provided ultrahigh water permeability, their selectivities for ionic solutes, thus far, have not been competitive against current RO and NF membranes. Joshi et al. have proposed that many of these problems have been a result of defects introduced during the casting of the thin films. Defects aside, the degree to which a GO film can sieve solutes is M


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Figure 9. Filtration and water permeability range of membrane processes, with shapes providing a rough approximation of the water permeability and size selectivity of: commercialized RO, NF, and UF used in industrial and municipal applications (rectangles); typical, approximate ranges of published experimental results achieved thus far with novel nanoporous membranes (ovals, citations provided throughout this text); publications of some of the best reported rejections with nanoporous membranes (stars, citations provided in Table 2); and modeling results of idealized nanoporous membranes, for materials in which theoretical predictions were available (triangles, citations provided in Table 2).

of the polymer matrix more rapidly. GO may also affect the interfacial polymerization itself. Several investigators have found reduced PA thicknesses when graphene is incorporated into the matrix,242,245,247 which may serve to reduce the polyamide contribution to resistance. Molecular scale changes to the PA structure also cannot be ruled out, perhaps leading to changes in void fraction, network cross-linking, or the distribution of functional groups in the PA. Improvements reported in fouling and chlorine resistance are particularly promising, as these phenomena are two of the major pain points facing commercial RO membranes today. Reduced fouling can be ascribed to the smoothness of the graphene sheets and their regions of high hydrophilicity. Graphene also appears to have antimicrobial properties, which retard the rate of biological growth.47 Rather than blending, properties such as fouling, which are dependent entirely on surface interactions, may be better served by applying GO (or other such materials) as a secondary coating. These films maximize the coverage or protection of the underlying membrane while using the minimum required nanomaterial. When used as a barrier coating above polyamides, GO has provided advantages in fouling,257−263 biological growth,263 and chlorine resistance.257,264 Somewhat nonintuitively, other benefits have also been seen by embedding GO below the polyamide in the underlying support structure. These membranes may form a stronger support that is more resilient against compaction,265 alter the subsequent polymerization reaction,266 or provide a better structure parameter for use in FO.267 Finally, in pressure-retarded osmosis composites, GO layers have been applied to the backside of the membrane to block foulant from entering and plugging the porous side of the support.268

membrane polymer, such as a cross-linked polyamide. In addition to graphene/graphene oxide, such hybrid or composite membranes have also been made using other additives, such as carbon nanotubes,254,255 inorganic nanoparticles, and zeolites. Further information on several of these approaches is provided in the reviews cited at the beginning of this paper. Although resistance from the matrix material will undoubtedly place some constraints on the water permeability of the composite, such architectures are scalable and may allow one to circumnavigate issues of film formation, defect management, and stability, while still realizing a portion of the benefits of the 2D material. These benefits may include more than just an increase in water permeability. As will be elaborated below, in comparison to polyamides, graphene has high mechanical strength, is more stable to chemical and oxidative attack, forms smooth hydrophilic surfaces resistant to fouling, and has documented antimicrobial properties. Liu et al. have written a comprehensive and timely review on the integration of graphene oxide into polyamide RO membranes.61 GO can be added as a blend to either layer of the oil/water interfacial polymerization or as a surface coating to an already-prepared composite. When graphene oxide is blended into the matrix of the polyamide, a long list of publications have observed benefits in water permeability, solute rejection,244,245 organic fouling,241,244−247 biological fouling,242,247 mechanical strength (as evidenced, for example, by membrane compaction),243,246 and chlorine resistance.242−244,246,256 Many of these same benefits have been observed with the blending of carbon nanotubes. The mechanisms underlying these improvements are not always so straightforward. Higher water permeability may certainly arise from the low friction water channels that exist between adjacent sheets of GO,248 but these channels will not necessarily exist if layers are separated in the polymer matrix. Whether present as individual sheets or small stacks, the GO may only present itself as a barrier to perpendicular flow through the substrate if the sheets are aligned parallel to the substrate, which is a barrier albeit with perhaps ultrafast water flow in the lateral direction. A nonparallel alignment might reduce total resistance, or perhaps any alignment allows water to find lower resistance pathways out

5. PROSPECTS AND CHALLENGES 5.1. Tailored Applications. To put all of this discussion into one perspective, Figure 9 plots the water permeability as a function of pore size and associated application space for various materials, as reported in the literature. It is expected that there will be new data coming in the future that might provide a more comprehensive picture down the road. The figure provides a N


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Industrial & Engineering Chemistry Research picture of current state-of-the-art materials for RO, NF and UF technology to compare with the novel materials discussed here. For the novel materials, the plot shows model predictions (where available), typical ranges of experimental results, and the list of best reported experimental data cited in Table 2. As seen, there is a difference between model predictions versus actual experimental results. Much of this gap is due to the defects and processing challenges associated with handling these new materials in early stage research versus already established processes for current industry standard materials. One would expect significant research directed toward this area to close the gap. These novel materials hold promise both in terms of high water permeability and breadth of application. The immediate application space could be classical RO/NF or UF, but many new specialty application spaces could emerge because of their characteristics. The following two sections will explore additional challenges and opportunities solely within the RO space. Besides these, consideration could be given in exploring application spaces such as bioprocessing, special solute removal, UF or NF applications, solvent or oil extraction, as well as acid recovery and various segments requiring improved fouling resistance. 5.2. Limitations of High Permeability. Filtration processes such as RO, which operate via gradients in chemical potential, face inherent limitations in thermodynamics and energy efficiency. Most of these issues have been described at length in several manuscripts, including separate evaluations in recent years by Elimelech and Phillip,72 Cohen-Tanugi et al.,269 and Shrivastava et al.73 In RO, the net driving pressure from feed/ brine to permeate (ΔP−Δπ) must always be greater than zero to achieve water flux. As such, the feed osmotic pressure and desired recovery set limits on the lowest possible operating pressure. The combination of these thermodynamic constraints, the need to operate at some reasonable flux, and losses associated with equipment and module operation leads to diminishing marginal savings in energy as the permeability of the membrane is increased. Examples of these effects in seawater desalination are provided in Figure 10. Diminishing returns have clearly manifested by the time the permeability of a membrane enters the brackish water regime (A ≈ 0.15 gfd/psi (3.7 Lmh/bar) or higher). An important note to keep in mind is that Figure 10 represents only membrane energy, which, as we see later in the next section, is just one component in the total cost of water desalination. As the promise of high-permeability membranes is evaluated, operational losses in particular are worth a deeper understanding. Consider a spiral-wound RO module standard to today’s industry (Figure 2c). The module consists of multiple leaves of membrane sandwiched between spacer materials, which provide channels for brine and permeate flow. Filtered water passing through the membrane travels down the permeate channel to the central collection tube. Shrivastava et al. note such modules suffer from frictional losses on the brine and permeate sides of the membrane. These losses directly reduce net driving pressure and further erode the incremental benefit of higher membrane Avalue. For spiral-wound modules, Johnson and Busch22 have published a technical summary that thoroughly describes permeate channel pressure drop, while a practical exercise on brine side pressure drop has been provided by Eriksson.271 Permeate pressure drop, feed pressure drop, and concentration polarization are often not considered in the open literature but play an ever more dominant role in module performance as the A-value increases. Shrivastava et al. consider these aspects in their

Figure 10. Energy limits in seawater reverse osmosis. (a) Evolution in specific energy consumption for SWRO since the 1970s; the horizontal dashed line represents the thermodynamic minimum energy at 50% recovery (reprinted with permission from ref 72, Copyright 2011, AAAS, Washington, DC). (b) Specific energy requirement for SWRO, as a function of increasing membrane water permeability (reprinted from ref 270, Copyright 2013, International Desalination Association (IDI), Topsfield, MA).

analysis of thermodynamic limitations (Figure 10b).73,270 A more recent assessment by Shi et al. calls out these losses individually and demonstrates the diminishing returns of higher A-values when each limitation is considered.272 Equally important, but less often discussed in these terms, are any relevant layer resistances. Many of the novel materials described in this review would realistically be fashioned as composite membranes consisting of a thin, high-permeability discriminating layer over a porous support. The resistance, or inverse permeability, of this composite is at least equal to the sum of the unique resistances in each layer. Additional constrictions may arise at the interface between these layers, as, for example, the flow of water out of the discriminating layer could be restricted by the finitely distributed surface pores of the support.273,274 While the permeability and pore structure of supports can certainly be engineered for improvement, their use will always take up a larger fraction of the total resistance as the permeability of the discriminating layer is increased. The support thus sets a ceiling on the permeability of the composite. For example, polysulfone UF membranes typically used as RO supports can have a permeability in the neighborhood of 750 Lmh/bar (∼2 × 10−9 m/s/Pa).103 These materials can further constrain energy savings in operation, because they are susceptible to compaction under elevated temperature and pressure. Figure 11 highlights a simple exercise that illustrates several of these limitations. Figure 11a shows how dominant the support becomes as the water permeability of the discriminating layer is increased. Additional interfacial losses are neglected here for O


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Figure 11. Examples highlighting resistances common to RO membranes, which become increasingly important for high water permeability membranes: (a) impact on composite permeability if the discriminating layer is constructed on a typical polymeric support that is susceptible to compaction; (b) localized membrane flux profiles, as a function of distance from the permeate tube, of a spiral-wound module with 30 in. leaves, calculated assuming an average flux of 25 Lmh in all cases; (c) impact of membrane permeability on module efficiency and performance response to fouling. Efficiencies were calculated assuming a spiral-wound module with 30 in. leaves and typical permeate channel friction. Response to fouling calculated by applying the same fouling resistive layer of 0.07 bar/Lmh to all membranes. Such a layer would reduce the flux of a standard brackish water membrane (3.7 Lmh/bar) by 20%, which is a relatively unremarkable occurrence in real waters.

Figure 12. Cost balance for novel membrane materials, with costs outlined in the red box on the left and potential gains outlined in the black box on the right.

simplicity but could be particularly important in nanoporous (rather than semipermeable) composites, where defined channels in the discriminating layer need to line up with pores in the support. Figures 11b and 11c examine the impact of composite permeability on permeate frictional losses, which, in this case, are represented numerically by module efficiency (η). Calculations were performed using equations and a typical permeate channel friction parameter provided by Johnson and Busch.22 The element efficiency (Figure 11b) decreases exponentially with the A-value, showing essentially the same behavior recently modeled by Shi.272 A composite membrane of 250 Lmh/bar or more rolled into a spiral-wound element with these specifications has an efficiency of 15%, giving it an effective permeability in the module of only 12 Lmh/bar (A × η). Similarly, the feed-side pressure drop and polarization are both more important with increasing A-value, as they make up an increasingly larger fraction of the required net driving pressure. This frictional penalty paid to extract permeate from a leaf manifests itself as a flux imbalance over the radial direction of a spiral-wound element. As the permeability of the composite increases, a growing fraction of the total product flow occurs closest to the permeate tube (Figure 11b). In addition to the energy cost, high flux near the product water tube will make the membrane more prone to fouling. Flux imbalance is a common design concern in the axial direction down an element, vessel, or series of stages. High fluxes in the front of a system promote faster rates of fouling while lower fluxes in the back reduce recovery. Implementation of an ultrahigh permeability membrane in a spiral-wound module will make flux imbalance in the radial direction an important design concern.

Fouling is one of the most widely studied and industrially relevant topics in membrane water separation processes today,275−277 and its growing importance with high-permeability membranes cannot be overstated. Consider a typical BWRO membrane (composite permeability of ∼3.7 Lmh/bar) operating in a moderately challenging feedwater. A 20% loss in flow from fouling would be a rather unremarkable event in such a water. As seen in Figure 11c, with the same series resistance argument just made for the porous support, one can easily show that the same amount of fouling in an ultrahigh permeability membrane (125 Lmh/bar) would reduce flux by 90%. The losses just described are not fixed, but rather are dependent on the structure of the support, the design of the filtration module, and the fouling propensity of the membrane. The latter highlights the need for a high-permeability membrane to also have significantly improved fouling resistance. The former represent module engineering challenges that must be addressed to make a commercialized ultrahigh-permeability membrane as efficient as possible. Efficiency must strike a balance with capital and materials cost. For example, spiral module losses can be improved by shortening leaves, widening feed or permeate channels, or other such efforts, but may result in less area per module, greater concentration polarization, or bigger modules, which add to cost and footprint. Instead of spiral designs, RO and NF modules can be made in a hollow fiber geometry, as is done today, for example, with cellulose triacetate membranes. Hollow fiber designs can have up to 10 times the surface-area-to-volume ratio of a spiral module, but accomplish this by having thin fibers with smaller-diameter lumens.278,279 Hollow fibers are certainly an attractive approach for ultralow-energy membranes, but frictional losses do not disappear280,281 and always will become P


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Figure 13. Roll-to-roll manufacturing process, including potential substeps.

self-assembled systems, the cost barrier may be higher, since completely new processes may need to be developed. The next step in Figure 13 details any postreaction modification steps, such as etching holes, chemical vapor deposition (CVD), or coatings that must be applied to impart or add functionality to the membrane. Current polyamide membranes typically include a coating step, which is achieved by roll-coating waterborne materials onto the face side of the web and subsequent drying. More-complicated processes, such as etching and CVD, have the potential to add significant cost to the process. After the membrane is manufactured, it must be rolled into an element and tested for quality control. The primary concern for novel materials for these steps is arguably the ability of the membrane to withstand rolling and unrolling, folding, gluing, and contact with feed screen without acquiring too many defects. This presents an additional and important challenge to researchers that often goes entirely overlooked. All of these costs must be balanced and exceeded by the benefits that novel materials provide. These are listed as energy savings, improved separation capability, chemical resistance, and fouling resistance. To understand the significance of performance improvements in these spaces, Figure 14 shows a

more paramount with higher A-value. An analysis that considers the costs and benefits of manufacturability, area, pressure drop, fouling propensity and cleanability must be balanced against other design alternatives (such as spiral) to determine the best construction. 5.3. Manufacturing Challenges and Cost. In order for the successful adoption of novel membrane technologies and materials, the economic advantage to both the manufacturers and users must outweigh the costs of development and production. Figure 12 outlines the main elements of this balance. In Figure 12, the product development costs are 3-fold: research and development (R&D) to establish baseline performance advantages, R&D to establish long-term performance, and, lastly, development of a new or modified production process that is amenable to large-scale manufacturing of the new membrane. It is clear that the current state of research into novel materials is invested heavily in establishing baseline performance advantages, with tangential efforts into process engineering by virtue of the fact that researchers must be able to make the materials they wish to test. However, there are no significant efforts that are published that detail investigations into larger-scale production, and much of the current literature relies on benchtop membrane casting. This is a tricky problem to overcome; in order to attract significant investment, baseline and long-term performance advantages must first be established for the novel materials, but without larger scale production, the performance variability and defects that are introduced by handmade materials make this a real challenge. The second source of expense is the bulk materials costs associated with the new membranes. In the case of graphene and GO, these may be less significant, compared to the other costs, unless additional materials are needed for processing steps. For block copolymer, biomimetic, and others, this may be a crucial factor in determining profitability, compared to current polyamide technology. Lastly, capital expenses can take the form of either key changes to the polyamide process outlined in Figure 2a, or if the process is significantly different from current manufacturing methods, it could mean incurring the full cost of building a factory to produce the membrane. For the latter case, one then encounters an additional opportunity cost of deciding to invest in novel membranes rather than established technologies. Capital expenses can be broken out into separate categories depending on the fabrication step, which is outlined in Figure 13. Raw materials must initially be converted to the base membrane to be used in element fabrication. This requires a support web or support layer, which provides mechanical strength to the separation layer, as well as a substrate for the next step of solvent deposition and/or chemical reaction. Once this is completed, solvent must be removed through a drying or extraction process. For materials that are a composite of polyamide and some other novel ingredient (e.g., GO, AQPs), this process may be extremely similar to established methods but with an extra extraction step to deal with the new materials. For

Figure 14. Cost breakdown for a SWRO desalination plant (adapted, with permission, from ref 282, Copyright 2013, Elsevier, Amsterdam).

breakdown, reproduced from Ghaffour et al., of the incurred costs for desalinating seawater with RO.282 The sectors of this cost breakdown that are generally targeted for improvement by R&D efforts in the literature are the energy costs and chemical costs (through reduced cleanings), which together only represent 25% of the total desalination cost. The chemical cost represents all chemicals, including antiscalants and pH adjustment, as well as cleaning chemicals. Reducing the cleaning frequency will only affect a portion of the 6% shown. Arguably, membrane module costs are also a subject of research, because of the variety of novel materials being used; however, to date, no work has pointed toward a significant cost reduction in this space. Q


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As the chart shows, annualized capital expenses present the most significant challenge for water producers. The average total cost of desalinated water, aggregated by Ghaffour from several sources, is in the range of $0.5−1.2/m3,282 while the energy cost of the RO operation, which, as we noted earlier, is subject to engineering challenges and inherent thermodynamic limits, is ∼$0.19/m3 (2.12 kWh/m3, as reported by Shrivastava,73 at 0.09 $/kWh). This breakdown points to a need for more-efficient system designs, and not necessarily improved membrane materials. However, it is possible that improved separation efficiency could lead to savings in capital expenses. Highpermeability membranes could reduce construction costs by lowering design operating pressures, but are constrained, as seen earlier, by the thermodynamic requirement to overcome osmotic pressure (unconcentrated SW has a typical osmotic pressure of >400 psi). Operating high-permeability membranes at higher flux than today’s systems could reduce footprint, but increases susceptibility to fouling, as noted in the previous section. Given these constraints, there are still promising opportunities for reductions in capital in certain areas, such as the residential and food and beverage markets. Given the limited importance of the targeted areas for improvement by research in novel materials, with regard to the total cost of producing water, coupled with the associated costs of developing new technology, it remains a significant challenge for researchers to demonstrate a technology that will be disruptive in this space. However, it is by no means impossible. The best technology should be able to couple improvements in energy, chemical use (through antifouling performance), material costs, and new designs, as well as a simple and reliable manufacturing process to push the limits on reducing the overall cost of clean water.

Abhishek Roy: 0000-0002-2801-5971 Notes

The authors declare the following competing financial interest(s): All four co-authors are employed by Dow Water and Process Solutions, which manufactures and sells reverse osmosis, nanofiltration and ultrafiltration membranes, as well as ion exchange resins, to the water treatment industry. Biographies Dr. Abhishek Roy is the Senior R&D Manager and leads the reverse osmosis (RO) membrane chemistry activities in Dow Energy and Water Solutions, a business division of The Dow Chemical Company. Over the last several years, Abhishek and his team were instrumental in developing several generations of membrane chemistries that have contributed to addressing the problem of scarcity of water, water reuse, and water-energy nexus. These technologies have been commercialized to more than 10 new products in the past few years. A notable contribution is the DOW FILMTEC ECO RO product family, which reduced energy consumption by 30% and at the same time, improved the quality of recovered water by 40%sustainable offerings that delivered both ecological and economical value to the market. Abhishek Roy is the recipient of prestigious Gordon Moore Medal from Society of Chemical Industries (2016) and a winner of Dow’s prestigious Sustainability Innovator Award, and he has been selected as the outstanding recent graduate alumnus from Virginia Tech’s College of Science. Abhishek earned his Ph.D. from Virginia Tech, under the guidance of Prof. James E. McGrath in the macromolecular science and engineering program. He has coauthored more than 100 scientific articles, including 30 peer-reviewed journal publications (with more than 1000 citations) and book chapters. He is a coinventor of 40 patent applications with more than 10 U.S. patents of them granted to date.

6. FUTURE RESEARCH DIRECTIONS The water filtration industry is always looking for membranes with higher selectivity and more efficient water transport. While significant hurdles still remain, an exciting number of nanoporous, next-generation materials are being investigated that have the potential to supplant the energy and separation capabilities of industry standard polyamides for reverse osmosis and nanofiltration. Revisiting the discussion and last several figures of this manuscript, such ultrahigh permeability materials are not expected to “change the game” of either energy consumption or total water cost in mainstream RO desalination. Although certainly still welcome and needed, the benefits of success in energy will be measured on a percentage basis, and, for the most part, really require only percentage-scale gains in permeability to achieve. As noted by others,283,284 greater reward can perhaps be realized through enhanced designs and operating strategies, or the alleviation of operational pains through fouling resistance, pH tolerance, or chlorine tolerance. Several of the materials discussed here appear to show advantage in these areas. Although certainly studied by many, such secondary properties of nanoporous materials, to date, have garnered less fanfare, and understandably so, since efforts in many of these materials are still at the stage of achieving workable membranes. As fabrication techniques continue to improve, however, we look with excitement toward understanding the full potential of each of these materials.

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Dr. Derek M. Stevens is a Research Scientist at Dow Water & Process Solutions in Edina, MN. Derek is currently a member of the Technical Service and Development organization, with a focus on application development in industrial waters and providing solutions to customers. Derek joined Dow Water and Process Solutions in 2010, in the Reverse Osmosis/Nanofiltration R&D group. His work at Dow has encompassed both fundamental and applied process and product development efforts for both brackish and seawater reverse osmosis membranes. Derek earned his Ph.D. in Chemical Engineering from the University of Minnesota in 2010.

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*E-mail: [email protected]. R


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(5) Jin, X.; Jawor, A.; Kim, S.; Hoek, E. M. V. Effects of feed water temperature on separation performance and organic fouling of brackish water RO membranes. Desalination 2009, 239, 346. (6) Hoang, T.; Stevens, G.; Kentish, S. The effect of feed pH on the performance of a reverse osmosis membrane. Desalination 2010, 261, 99. (7) Bartels, C.; Franks, R.; Rybar, S.; Schierach, M.; Wilf, M. The effect of feed ionic strength on salt passage through reverse osmosis membranes. Desalination 2005, 184, 185. (8) Hyung, H.; Kim, J.-H. A mechanistic study on boron rejection by sea water reverse osmosis membranes. J. Membr. Sci. 2006, 286, 269. (9) Freger, V. Nanoscale Heterogeneity of Polyamide Membranes Formed by Interfacial Polymerization. Langmuir 2003, 19, 4791. (10) Fujioka, T.; Oshima, N.; Suzuki, R.; Price, W. E.; Nghiem, L. D. Probing the internal structure of reverse osmosis membranes by positron annihilation spectroscopy: Gaining more insight into the transport of water and small solutes. J. Membr. Sci. 2015, 486, 106. (11) Košutić, K.; Kaštelan-Kunst, L.; Kunst, B. Porosity of some commercial reverse osmosis and nanofiltration polyamide thin-film composite membranes. J. Membr. Sci. 2000, 168, 101. (12) Zhang, X.; Cahill, D. G.; Coronell, O.; Mariñas, B. J. Absorption of water in the active layer of reverse osmosis membranes. J. Membr. Sci. 2009, 331, 143. (13) Tang, C. Y.; Kwon, Y.-N.; Leckie, J. O. Probing the nano- and micro-scales of reverse osmosis membranesA comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements. J. Membr. Sci. 2007, 287, 146. (14) Coronell, O.; Mariñ as, B. J.; Zhang, X.; Cahill, D. G. Quantification of Functional Groups and Modeling of Their Ionization Behavior in the Active Layer of FT30 Reverse Osmosis Membrane. Environ. Sci. Technol. 2008, 42, 5260. (15) Childress, A. E.; Deshmukh, S. S. Effect of humic substances and anionic surfactants on the surface charge and performance of reverse osmosis membranes. Desalination 1998, 118, 167. (16) Azari, S.; Zou, L. Using zwitterionic amino acid l-DOPA to modify the surface of thin film composite polyamide reverse osmosis membranes to increase their fouling resistance. J. Membr. Sci. 2012, 401-402, 68. (17) Wijmans, J. G.; Baker, R. W. The solution-diffusion model: A review. J. Membr. Sci. 1995, 107, 1. (18) Paul, D. R. Reformulation of the solution-diffusion theory of reverse osmosis. J. Membr. Sci. 2004, 241, 371. (19) Coronell, O.; Mi, B.; Mariñas, B. J.; Cahill, D. G. Modeling the Effect of Charge Density in the Active Layers of Reverse Osmosis and Nanofiltration Membranes on the Rejection of Arsenic(III) and Potassium Iodide. Environ. Sci. Technol. 2013, 47, 420. (20) Soltanieh, M.; Gill, W. N. Review of Reverse Osmosis Membranes and Transport Models. Chem. Eng. Commun. 1981, 12, 279. (21) Bowen, W. R.; Mohammad, A. W.; Hilal, N. Characterisation of nanofiltration membranes for predictive purposesUse of salts, uncharged solutes and atomic force microscopy. J. Membr. Sci. 1997, 126, 91. (22) Johnson, J.; Busch, M. Engineering Aspects of Reverse Osmosis Module Design. Desalin. Water Treat. 2010, 15, 236. (23) Lee, K. P.; Arnot, T. C.; Mattia, D. A review of reverse osmosis membrane materials for desalinationDevelopment to date and future potential. J. Membr. Sci. 2011, 370, 1. (24) Pendergast, M. M.; Hoek, E. M. V. A review of water treatment membrane nanotechnologies. Energy Environ. Sci. 2011, 4, 1946. (25) Shenvi, S. S.; Isloor, A. M.; Ismail, A. F. A review on RO membrane technology: Developments and challenges. Desalination 2015, 368, 10. (26) Goh, P. S.; Matsuura, T.; Ismail, A. F.; Hilal, N. Recent trends in membranes and membrane processes for desalination. Desalination 2016, 391, 43. (27) Werber, J. R.; Osuji, C. O.; Elimelech, M. Materials for nextgeneration desalination and water purification membranes. Nat. Rev. Mater. 2016, 1, 16018.

Dr. Jessica Shu is an Associate Research Scientist in the Product R&D Group within the RO Value Center of Dow Water and Process Solutions, based in Edina, MN. Her primary area of focus is on new product development for the Residential Market. In this role, she is responsible for identifying new opportunities and value for DW&PS, and developing new products for commercial launch. Prior to joining Dow in 2012, she completed her Ph.D. in the Materials Science and Engineering Department at the University of California, Berkeley, where she studied the solution self-assembly behavior of peptide−polymer conjugates for drug delivery applications.

Dr. Matthew D. Reichert was formerly a Senior Engineer in the R&D department of Dow Water and Process Solutions. Matthew developed novel nanofiltration and reverse osmosis membrane production technologies, as well as investigated paths toward mitigating organic and biological fouling of reverse osmosis membranes. He is currently employed by Capital One as a Senior Manager in Commercial Lending.

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ACKNOWLEDGMENTS This invited contribution is part of the I&EC Research special issue recognizing the 2017 Class of Influential Researchers.

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