Schematic diagram of the MFI zeolite morphology. Adapted with permission from [58] (Copyright 2004, John Wiley and Sons). 

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... may enable operation closer to the ideal osmotic pressure, thereby decreasing the energy cost for a given membrane area and capacity; alternatively, such membranes may reduce capital cost or plant size by requiring less membrane area for a given desalination capacity. However, the coupling between flux, salt rejection, and other properties such as mechanical stability and fouling resistance requires careful optimization of the membrane material, which is particularly challenging as improving one factor tends to adversely affect the others [17]. Researchers have extensively explored various combinations of polymeric materials, as well as different synthesis procedures and modification techniques to optimize membrane performance [18, 19]. Some of the issues have been addressed by providing macroscopic solutions, e.g., feedwater pre-treatment to minimize membrane fouling. These aspects are covered extensively in recent reviews on RO [17, 18]. While significant advances have been made in polymeric RO membrane technology, new approaches beyond the optimization of polymeric materials could be exploited to develop next-generation membranes. Advances in nanotechnology have enabled unprecedented control over the fabrication of nanostructured materials, and in particular, the ability to create well-defined, size-selective, nanostructured filtration membranes. In contrast to polymeric membranes with flexible chains that do not form well-defined pores, a rigid, size-selective membrane with pore sizes in the sub-nanometer regime is expected to allow water molecules to pass through, while impeding the passage of ions that have a larger effective diameter due to their hydration shells [47, 48]. For example, the diameter of a hydrated sodium ion is ∼ 7 . 6 A; ̊ theoretically, if the pore diameter is smaller than that of a solvated ion but larger than a water molecule, the pore could act as a molecular sieve. Since a significant energy barrier must be overcome to strip the ion of its solvation shell ( ∼ 1709 kJ mol − 1 for Na + ) [49, 50], applying a pressure greater than the osmotic pressure on the feedwater will force the water molecules through such pores while impeding the passage of ions. In addition to selectivity through steric exclusion, electrostatic and van der Waals interactions may also play an important role or may be harnessed to achieve the desired selectivity. Nanostructured materials that are promising for desalination include zeolites [51, 52], carbon nanotubes [45, 46], and graphene [53–55], which can be synthesized to have non-tortuous pores on the order of 1 nm or less and can be fabricated into macroscopic arrays. Figure 3 summarizes the current performance of nanostructured materials that are in research or are commercially available, compared to polymeric RO membranes. In the following sections, we discuss in detail the progress in the fabrication and understanding of the transport characteristics of these materials to achieve improved RO membranes for the development of efficient desalination. Zeolites are aluminosilicate minerals with a microstructure composed of 3–8 A ̊ pores (figure 4). Zeolite crystals occur naturally or can be synthesized in a laboratory environment using a high temperature furnace and an autoclave [56]. Crystal sizes can be controlled from a few nanometers to centimeters by varying synthesis temperature and time [56]. Properties such as adsorption characteristics, geometry, ion exchange capabilities, and catalytic behavior differ amongst the zeolite crystal families and can be tailored to a specific application by using the correct composition [56]. Porosity varies among zeolites, typically ranging between 30–40%. The combination of high porosity and high active surface area has led to significant zeolite research for catalytic applications. In commercial applications, the most common use for zeolites is as adsorbents during various chemical processes [57]. Since most zeolites have a tight pore distribution less than the diameter of a hydrated salt ion, a membrane created from these crystals has the potential to completely reject salt ions while permitting water molecules to permeate through. Molecular dynamics (MD) simulations have provided mechanistic insights into these processes. Murad and Lin investigated water–ion separation using a single ZK-4 zeolite with 4.4 A ̊ pore diameters in a NaCl/water solution [59, 60]. The solvated ions were too large to pass through the pores and only water molecules could flow through the zeolite. These MD simulations have since motivated researchers to fabricate zeolite-based membranes for RO and experimentally investigate the possibility of achieving high flux with excellent ion rejection. Li et al [51] used hydrothermal synthesis to develop 0.5–3 μ m thick membranes consisting of hydrophobic MFI (mordenite framework inverted) type zeolites with an average pore diameter of 5.6 A ̊ on a porous α -alumina support (figure 5(a)). Under an applied pressure of 2.07 MPa (20.7 bar) and with 0.1 M NaCl feedwater, the membranes rejected 76% of Na + ions while permitting a water flux of 0 . 112 kg ( m − 2 h − 1 )( ∼ 0 . 11 l ( m − 2 h − 1 )) . This lower rejection was attributed to ions transporting across nanometer-sized interstitial defects created during the membrane synthesis process. In later work, Li et al decreased the silicon / aluminum ratio of the zeolite, which decreased the hydrophobicity and increased the flux to 10 . 21 ( m − 2 h − 1 ) with an applied pressure of 3.5 MPa (35 bar) and 0.1 M NaCl feedwater [44]. Ion rejection also improved dramatically from ∼ 76% to 98.6% (figure 5). However, as the salt concentration of the ...