An efficient graphyne membrane for water desalination
Graphical abstract
Introduction
Although the earth is mostly covered with water, 98% of the available water resources are in the form of saline water which contains various minerals or pollutants [1]. Access to clean water for a large number of people around the world is becoming more critical than access to health, education and even food. In different parts of the world, especially in developing countries, water-related issues such as water scarcity and water pollution is increasing. A promising solution to this challenge is to desalt the sea saline water [[2], [3], [4], [5], [6]]. Conventionally, three approaches are used: thermal distillation, electrodialysis and classic reverse osmosis (RO). However, these technologies currently provide only 1% of the world's fresh water [7,8]. All of these methods consume a larger amount of energy and also, their output is very limited due to the low water permeability [9]. For example, RO is a process that had the most efficient desalination to date [10]. Unfortunately, the water transfer rate is very low in classical RO membranes working based on a solution-diffusion process [2].
Novel nanoporous materials were proposed as a replacement for classic RO membranes to solve this problem. Due to the rapid development of technology, nanoporous materials can be a good choice for water purification. Nanoporous membranes can provide water permeation with a fast convective process.
Nowadays, molecular dynamics (MD) simulations have demonstrated that many of these nano materials like carbon nanotube-based membranes, high-aligned and high-density CNT arrays, zeolite and boron nitride nanotubes are inefficient in water desalination application [[11], [12], [13], [14], [15], [16], [17]]. Hydrodynamically, the flux through the membrane scales inversely with its thickness [2]. Therefore, the best membrane for desalination is a membrane with the minimum thickness, which is the thickness of one atom, mechanically strong and without failure even if under high applied pressure [18]. Cohen et al. [2] introduced the application of one-atom-thick nanoporous graphene in separation processes. Several computational studies have predicted that nanoporous graphene would provide water permeability as high as three folds compared with commercially available RO membranes [19]. According to the results reported by Cohen et al. [20], by three times increase the in water permeability, the applied pressure, and the required energy decrease by 44% and 15% respectively in RO plants for seawater desalination. Although studies have identified other applications for this porous material, such as DNA sequencing [21], or as an electrocatalyst [22], the potential of this material for desalination and purification of water has attracted great attention [23]. Also, some researchers [[24], [25], [26]] have used multilayer graphene and electric field for water desalination.
With the success of graphene in water desalination, researchers have focused on one-atom-thick single layers for water desalination. Among these single layers, Graphyne, another family of carbon allotropes, has demonstrated to be more efficient than graphene [27,28] because of its intrinsic pores.
Previous studies have shown that graphyne sheets exhibit novel electronic properties such as high electron mobility and direction-dependent dirac cone [29]. Wu et al. [30] investigate the mechanical properties of graphyne nanostructures by MD simulations with AIREBO potential function. Their results demonstrated that the numbers of acetylenic linkages between neighboring hexagonal rings in graphyne-n have significant effects on the mechanical properties. Cranford and Buehler [31] and Cranford et al. [32] have obtained mechanical properties of single-layer graphyne by MD simulations. They have shown that the Young's moduli and strength of graphyne are up to 350 GPa and 20 GPa, respectively.
Further studies have suggested that graphyne-2 (or graphdiyne) may be used for precise H purification [33]. Kou et al. [34] have examined the passage of water molecules through a single-layer graphyne-3 membrane by MD simulations. Their results demonstrated that graphyne-3, with the same diameter holes, has a higher water flow than the CNT (5, 5). Water molecules can pass through graphyne-3 but do not cross graphyne-1 and graphyne-2. Xue et al. [28] have shown that graphyne membranes can provide complete rejection of nearly all ions in seawater including Na+, Cl−, Mg2+, K+, and Ca2+. Graphyne-1 is impermeable to the water molecules and ions due to its small pore sizes [28]. The α-graphyne, β-graphyne and graphyne-3 can rapidly desalt seawater at 100% salt rejection, indicating that these intact graphynes can be as effective as high-performance RO desalination membranes [28]. Zhu et al. [27] have shown that water molecules can pass through graphyne-n membranes with n ≥ 3 and ion rejection is carried out for n ≤ 6. They have shown that water permeability is up to three orders of magnitude higher than classic RO membranes and up to ten times of nanoporous graphene membranes.
According to Table 1, the performance of graphyne-4 in ion rejection is much better than nanoporous graphene. It desalts seawater at 100% salt rejection and its water permeability is also greater than the nanoporous graphene.
Whether there is a way to passively improve the permeability of the system without reducing the performance of ion rejection, is the raised question. This is an important task as increasing the permeability even a few percents can be very valuable, especially in large scales. Secondly, in the passive methods, there is no need for using external devices and this means a substantial decrease in energy consumption and maintenance cost. Therefore, further investigations on the subject are necessary.
Sint et al. [37] have shown that nanopore graphene monolayer grafted with functional groups (FGs) can have an important role in the screening and selection of ions. As a polymer is a large molecule, or macromolecule, composed of many repeated units, functionalized graphene or graphyne membranes can be called polymers. Layek and Nandi [38] have concluded that the polymer functionalized graphene significantly improves engineering properties of graphene such as thermal, mechanical, optical, electronic, and magnetic properties. Park et al. [39], have shown that functionalized graphene oxide (FOG) improve the thermal, electrical and mechanical properties.
Chen et al. [40] have shown that functionalized nanoporous graphene (pyridinic-like nitrogen doped graphenes) have significant potential as desalination membranes. Raju et al. [41] have examined the water permeability and ion rejection through a single-layer bare and hydrogenated graphyne-n (n = 2, 3, 4) by MD simulations.
In the present study, we attempted to improve the water permeability through the monolayer graphyne family membranes by applying various FGs. We investigated the effects of applying Hydrogen (H), Fluorine (F), Carboxyl (COO−) and Amine (NH3+) FGs, to the graphyne pores.
Section snippets
Graphyne
Graphyne is a new family of carbon allotropes. It is a one-atom-thick planar sheet with different forms of extended conjunction between acetylene and benzene groups [[42], [43], [44]]. Baughman et al. [45] have suggested various forms of graphyne by inserting carbon triple bonds (acetylene bonds) (─C≡C─) between benzene rings in graphene, including three highly symmetric forms: α, β, and γ-graphyne as depicted in Fig. 1.
The γ-graphyne, which its structure is built upon firm connection of
Results and discussion
In the present study, ion rejection is defined as the percentage of ions remaining after passing half of the water molecules (Eq. (5)). In Eq. (5), is the number of ions remaining after passing half of the water molecules and is the initial number of ions in the saline water.
Also, to adjust the effect of the applied pressure and the area of the membrane, water permeability (L/cm2/day/MPa) was used [2,27].
To verify the present numerical approach, the present
Conclusion
The best water for drinking and industrial use is pure water without any ions. Today, various methods are used for water desalination. Novel nanoporous and one-atom-thick materials are proposed as efficient membranes for desalination. Among these membranes, graphyne has inherent-adjustable pores, so its performance in ion rejection is much better than other nanoporous membranes (like graphene) and its water permeability can be improved by applying FGs, especially H, F, NH3+, and COO−. We showed
Acknowledgements
This study was financially supported by Mashhad water and wastewater Co., Mashhad, Iran, under Contract no. 96101691 and Iran Nanotechnology Innovation Council (INIC), Tehran, Iran, under Project code 115755. INIC is supported by the scientific and technological department of the presidential office. Also, the authors gratefully acknowledge the High Performance Computing (HPC) Center of Sharif University of Technology, Tehran, Iran, for providing us with their computing facilities to perform
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