A photocatalytic TiO2/graphene bilayer membrane design for water desalination: a molecular dynamic simulation.

Molecular dynamics study on water desalination through functionalized nanoporous graphene

The performance of a desalination membrane depends on a specific pore size suitable for both water permeability and salt rejection. To increase membrane permeability, the applied pressure should be increased, which creates the need to improve membrane stability. In this research article, a molecular dynamics (MD) simulation was performed using ReaxFF module from Amsterdam Modeling suite (AMS) software to simulate water desalination efficiency using a single and multi-layer graphene membrane. The graphene membrane with different pore sizes and a multi-layer graphene membrane with descending pore size in each layer were designed and studied under different pressures. The stability of the membrane was checked using Material Studio 2019 by studying the dynamics summary. The single-layer graphene membrane was evaluated under pressures ranging from 100 to 500 MPa, with the salt rejection ranging from 95% to 82% with a water permeability of 0.347 × 10−9 to 2.94 × 10−9 (mm.g.cm−2s−1.bar−1), respectively. Almost 100% salt rejection was achieved for the multi-layer graphene membrane. This study successfully demonstrated the design and optimization of graphene membrane performance without functionalization.

In this study, BiOCl based nanocomposites were used as photocatalytic membranes for a simulated study on water desalination in reverse osmosis membrane systems. Through molecular dynamic simulation, the molecular structure of BiOCl, BiOCl/Ag2S and BiOCl/Bi2O3 heterojunctions were designed and their electronic properties, mechanical properties, and membrane performance for water desalination were evaluated for the first time. The molecular structure was created, and a geometry optimization task was used to optimize it. Material Studio 2019 CASTEP was used for simulation of the electronic and mechanical properties and water desalination was performed by ReaxFF software under pressures between 0 and 250 MPa. The novel BiOCl based nanocomposites showed improved electronic and mechanical properties and, most importantly, improvements in salt rejection and water permeability as compared to well-known materials such as graphene and MoS2. BiOCl and BiOCl/Ag2S had a bandgap around two, which is the ideal bandgap for semiconductor photocatalysts. A salt rejection of 98% was achieved under an applied pressure of 10 MPa. Salt rejection was higher for BiOCl/Bi2O3, while water permeability was higher for BiOCl/Ag2S. The monolayer BiOCl was unstable under pressures higher than 50 MPa, but the mechanical stability of BiOCl/Ag2S increased twofold and increased fourfold for BiOCl/Bi2O3, which is even higher than MoS2. However, between the three nanocomposites, BiOCl/Ag2S was found to be the most ideal photocatalytic nanocomposite membrane.

Microstructural design of crown nanopores in graphene membrane for efficient desalination process

Reverse osmosis (RO) technology is currently the most progressive, energy-saving and efficient membrane separation technology . Meanwhile, graphene becomes a promising candidate for fabricating the RO membranes in water desalination due to its high salt rejection and water flux. The concept of “temporal selectivity” is first proposed in our previous work in terms of the time difference between the penetration time of an ion passing through the pore and the tangential slipping time for the ion sliding across the pore. Nevertheless, the temporal selectivity mechanism of multilayered graphene membrane remains ambiguous. In this paper, the RO process of saltwater through porous graphene column RO membrane is studied by using molecular dynamics (MD) simulations method, and the effects of rotating angular velocity and the thickness of RO membrane on desalination performance of seawater are considered first. The MD results show that the salt rejection increases with the rotation speed of porous membrane increasing while the water flux initially increases and then decreases . Meanwhile, the interfacial slip velocity increases linearly with angular velocity increasing. On the other hand, the increasing thickness of porous graphene membrane can enhance the selectivity and reduce the permeability of water molecules. As expected, the tri-layered porous graphene RO membrane can achieve high salt rejection at low interfacial slip velocity. In order to ensure high selectivity and energy conservation and efficient, the pore structure of the porous graphene RO membrane is optimized. The results show that the optimized nanopores can increase the water flux significantly, whereas the salt rejection is not changed appreciably. It is found that the pore size of the innermost layer membrane near the feed region has the most significant effect on the water flux. The water flux increases sharply with the increase of pore diameter and the salt rejection remains totally higher than 80%. Moreover, the RO membrane with a special Type 3 structure exhibits excellent performance in seawater desalination, specifically, the ultrahigh water flux reaches 20029 L·cm<sup>–2</sup>·d<sup>–1</sup> and the super salt rejection arrives at 94%. The research results further clarify and verify the mechanism of the temporal selectivity in RO process, and improve the water flux under the condition of the same membrane thickness by designing gradient hole. The findings can conduce to the in-depth theoretical understanding of porous graphene-based membranes and designing and developing the large-scale seawater desalination devices and water filtration equipment.

Polyamide (PA) membranes possess properties that allow for selective water permeation and salt rejection, and these are widely used for reverse osmotic (RO) desalination of sea water to produce drinking water. In order to design high-performance RO membranes with high levels of water permeability and salt rejection, an understanding of microscopic PA membrane structures is indispensable, and this includes water transport and ion rejection mechanisms on a molecular scale. In this study, two types of virtual PA membranes with different structures and densities were constructed on a computer, and water molecular transport properties through PA membranes were examined on a molecular level via direct reverse/forward osmosis (RO/FO) filtration molecular dynamics (MD) simulations. A quasi-non-equilibrium MD simulation technique that uses applied (RO mode) or osmotic (FO mode) pressure differences of several MPa was conducted to estimate water permeability through PA membranes. A simple NVT (Number, Volume, and Temperature constant ensemble)-RO MD simulation method was presented and verified. The simulations of RO and FO water permeability for a dense PA membrane model without a support layer agreed with the experimental value in the RO mode. This PA membrane completely rejected Na+ and Cl− ions during a simulation time of several nano-seconds. The naturally dense PA structure showed excellent ion rejection. The effect that the void size of PA structure exerted on water permeability was also examined.

Most water in the world is as saline water in seas and oceans. Desalination technology is a promising method to solve the global water crisis. Recently, many attentions have been paid to the graphene-based membranes in water desalination due to their low production cost and high efficiency. In this paper, molecular dynamics simulations are employed to investigate the effect of functionalized graphene nanosheet (GNS) membranes on the performance of salt separation from seawater in terms of water permeability and salt rejection. For this purpose, the hydrogenated (–H) and fluorinated (–F) pores were created on the GNS membrane. Then, the functionalized graphene membrane was placed in the middle of the simulation box in an aqueous ionic solution containing Na+ and Cl− ions. The applied pressure (in the range of 10–100 MPa) was used as the driving force for transport of water molecules across the reverse osmosis (RO) graphene-based membrane in order to obtain the water permeability and salt rejection. Also, radial distribution functions (RDFs) of ion–water and water–water as well as the water density map around the membrane were obtained. The results indicated that the hydrophilic chemical functions such as fluorine (–F) can improve the water permeability at low pressures.

Water desalination using pressure-driven flow in hexagonal boron nitride (h-BN) and charged nanoporous graphene membranes are investigated using molecular dynamics (MD) simulations. Nanoporous h-BN membranes with pore diameters of 10.1 A, 12.2 A, and 14.7 A were selected to compare with the desalination performance of uncharged nanoporous graphene membranes with similar pore diameters. Salt rejection efficiency of uncharged graphene membranes is superior to that of h-BN, and the pressure drop for both materials exhibits the same inverse-cubic dependence on the pore diameter, regardless of the hydrophobic versus hydrophilic nature of graphene and h-BN, respectively. Charged graphene membranes with pore diameters of 15.9 A, 18.9 A, and 20.2 A were also considered, and 15.9 A pore diameter with total fixed charge of 12e was found to be the optimum setting for single-layer graphene membrane, resulting in 100% and 98% rejection efficiencies for Na+ and Cl− ions, respectively. The corresponding pressure drop is 51.8% lower than that obtained with 9.9 A pore diameter uncharged graphene with 100% salt rejection. To maintain perfect salt removal, 15.9 A pore diameter charged bilayer graphene membranes with 12e total charge on the first layer, and − 1e on the second one was simulated at different separation distances between the two membranes. The associated pressure drop is 35.7% lower than that obtained in 9.9 A pore diameter uncharged base-line case. These findings confirm the potential application of using charged bilayer nanoporous graphene membranes in improving the performance of reverse osmosis desalination systems.

The asymmetric membranes based on cellulose acetate are mainly applied for separations in aqueous systems and in reverse osmosis processes, although they can also be used in the so‐called salinity process of energy generation. These applications require membranes with considerable water permeability and high salt rejection. In this paper the improvements resulting from the addition of two different types of fibers on the permeability performance of the membranes (water permeability and salt rejection) as well as on mechanical properties are presented. Concerning the water permeability and salt rejection, the influence of four different contents of cellulosic fibers (CF) and anionic diethylaminoethyl cellulose (DEAE) fibers has been studied and the optimum value was chosen after measuring water permeability and salt rejection of the membranes. To study the mechanical performance, membranes with six different contents of these two types of fibers were produced. Both permeability and mechanical test results obtained for membranes with different contents of fibers were compared with the ones for the membranes produced from the same solution but without fibers. In terms of permeability tests, the membranes with 0.5 wt % CF fibers present the best results, with water permeability 22.8% higher than the membrane without fibers, while the salt rejection only decreases by 7.3%. Concerning the mechanical properties, the best membrane would be the one with 3 wt % CF fibers, however the membrane with 0.5 wt % CF fibers still present a toughness 18.9% higher than the membrane without fibers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Molecular insight into water desalination through functionalized graphenylene nanosheet membranes

Because of the high water flux and excellent ion rejection, the pores graphene is considered as a promising candidate material for fabricating the membranes in reverse osmosis (RO) process. Unfortunately, water molecules cannot pass through the perfect graphene, and how to effectively create a large number of nanopores with controllable size remains a challenge, which seriously prevents the practical application and development of graphene membrane for desalination. Recently, the emergence of pillared graphene (PGN) might open a new way for designing the graphene-based membranes, which can compensate for the deficiency of porous graphene membrane. The PGN has been extensively studied in gas storage and separation, and its RO characteristics and mechanism still remain unclear because the limitation of large area preparation in desalination. In this paper, the RO process of seawater through PGN membranes is investigated by molecular dynamics simulations, and the influences of the pressure within feed solution, temperature and the shearing of membrane on the desalination properties are considered. It is found that the water flux increases linearly with the pressure within feed solution increasing, and the PGN membrane with nanopore diameter of 0.8 nm can conduct water molecules but completely rejects high-concentration ions. As the diameter of nanopores increases to 1.2 nm, the rise of temperature can increase the permeability of water molecules, whereas the salt rejection is not appreciably sensitive to the temperature. Particularly, the shearing membrane can improve the salt rejection and hinder the water molecules from permeating at the same time. The designed PGN membrane exhibits excellent performance of water purification, and the ultrahigh water flux obtained in this work reaches 56.15 L·cm<sup>–2</sup>·day<sup>–1</sup>·MPa<sup>–1</sup> with a salt rejection of 88.9%. Subsequently, the hydrogen bond dynamics is calculated in order to explain the variation of water permeability under different conditions. The result shows that the rise of temperature reduces the stability of hydrogen bonds and leads the water flux to increase, while the increase of shearing speed will enhance the stability of hydrogen bonds and inhibit water seepage. Furthermore, the analysis results of hydrogen bond and ionic hydration structure show that the shear motion on RO membrane will improve the stability of ionic hydration shell, which makes it more difficult for the ions to pass through nanopores by weakening the hydration shell. On the contrary, rising temperature will impair the strength of ionic hydration shell, leading more ions to pass through the RO membrane. The simulation results can provide an in-depth understanding of the desalination performance of PGN membrane under different key conditions, and further demonstrate the promising applications of graphene-based membrane in water desalination.

At present, there is a general contradiction between permeability and selectivity of reverse osmosis (RO) membranes for desalination; a membrane with higher water permeability will give a lower salt rejection or selectivity, and vice versa. In this work, single-layer nanoporous graphene is used as RO membrane to investigate the effects of pore shape to reduce this contradiction by molecular dynamics simulations. Two kinds of pores (round and rectangular pores) with different sizes are simulated. For round pore, although the water permeability increases with the increase of the pore size, the salt rejection rate drops rapidly. For rectangular pore, reasonable designed pore structure can achieve improved water permeability and high salt rejection of graphene membrane by keeping one-dimensional length (i.e. the width) of the pore less than the size of the hydrated ions and increasing the other dimensional length. The restriction of one dimension can prevent the passage of hydrated ions through the pore effectively. This ‘one-dimensional restriction’ provides a simple strategy for designing RO membrane with variable pore structures to obtain a better desalination performance.

The performance of graphene oxide framework (GOF) membranes for water desalination is assessed using classical molecular dynamics (MD) simulations. The coupling between water permeability and salt rejection of GOF membranes is studied as a function of linker concentration n, thickness h and applied pressure ΔP. The simulations reveal that water permeability in GOF-(n,h) membranes can be tuned from ∼5 (n = 32 and h = 6.5 nm) to 400 L cm(-2) day(-1) MPa(-1) (n = 64 and h = 2.5 nm) and follows a Cnh(-αn) law. For a given pore size (n = 16 or 32), water permeability of GOF membranes increases when the pore spacing decreases, whereas for a given pore spacing (n = 32 or 64), water permeability increases by up to two orders of magnitude when the pore size increases. Furthermore, for linker concentrations n ≤ 32, the high water permeability corresponds to a 100% salt rejection, elevating this type of GOF membrane as an ideal candidate for water desalination. Compared to experimental performance of reverse osmosis membranes, our calculations suggest that under the same conditions of applied pressure and characteristics of membranes (ΔP ∼ 10 MPa and h ∼ 100 nm), one can expect a perfect salt rejection coupled to a water permeability two orders of magnitude higher than existing technologies, i.e., from a few cL cm(-2) day(-1) MPa(-1) to a few L cm(-2) day(-1) MPa(-1).

Porous graphene sheets can be considered as an ultrathin membrane in reverse osmosis water desalination processes. In this paper, employing the molecular dynamics simulation method, the performance of multilayer porous graphene membranes with different pore sizes, layer separation, and layer number were investigated. We found that salt rejection and water flux through the membrane significantly depend on the graphene pore size and number of graphene layers, and controlling these parameters could improve the filtration process. It was shown that our 2-layer designed graphene membranes with the pore radius of 3.3 Å and layer separation of 20 Å, can reject more than 86% of ions. Also, no filtration process had occurred for graphene layer separation less than 5 Å. The results of this study that are described by ion hydration radius and water velocity distributions can be used to improve the knowledge of water desalination at the molecular level, which leads to design more efficient multilayer graphene membranes for water purification.

Almost any societies facing with achieving and developing the most efficient method of water treatment. The nanotechnology innovative achievement, provided a variety of nonporous membrane for water treatment application. High permeability of water and high rejection of salt are essential properties of these membranes and have been investigated as an efficient way for water desalination. All of these systems have been designed in a way that ions do not pass and water molecules permeate through the nanopores, so clean water is obtained through this process. But, in this research, we employed permeation of heavy metallic ions through the passivated nanopores under electric field to purify water from such contaminations. We employed molecular dynamics calculations to investigate the removal of heavy metallic ions from water using 2D boron nitride and graphene membrane. A nanopore of 6 Å diameter is located in the middle of graphene and 2D boron nitride membranes, the pores passivated by fluorine atoms which is shown to favor cations permeation. Potential mean force calculation, water density profile, number of ions permeation and required time for an ion to pass through nanopore both in graphene and 2D boron nitride membranes have been obtained and the results indicate, that 2D boron nitride has a better performance in cation permeability than graphene.