Water desalination by a designed nanofilter of graphene-charged carbon nanotube: A molecular dynamics study

doi:10.1016/j.desal.2015.02.040

Desalination

Volume 365, 1 June 2015, Pages 176-181

https://doi.org/10.1016/j.desal.2015.02.040 Get rights and content

Highlights

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Using MD method a high efficiency nanofilter is designed to produce fresh water.
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The nanofilter is made of graphene sheets and a positive charged carbon nanotube.
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In the desalination the amount of CNT charge and the piston velocity are important.
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The water flow through the CNT depended on the CNT surface charge.

Abstract

Using molecular dynamics simulations, we show that a designed graphene-charged carbon nanotube (CNT) membrane can act as a nanofilter with the efficiency more than 90%, to separate Na⁺/Mg^+ 2/Fe^+ 3 and Cl⁻ ions from NaCl, MgCl₂ and FeCl₃ solutions. It is observed that there are two significant factors in ion separation process: the magnitude of charge density which covers the surface of the CNT and the velocity of the movable wall. The ion separation is improved by increasing the magnitude of surface charge density and decreasing the velocity of the movable wall. Also, it is found that, reducing the positive surface charge of the CNT will increase the percentage of passed water molecules through the CNT.

Graphical abstract

As the movable wall pushed the solution to pass through the charged CNT, positive and negative ions are separated. Positive ions remained in the first box and negative ions entered into the second box. Also by placing an extra porous graphene sheet in the second box, water molecules were separated from negative ions and entered into the third box.

Introduction

The rarity of fresh water resources is one of the most important problems in our world. Desalination of sea water is the process of removing salts and dissolved minerals from water to produce fresh water. The major desalination processes employ membrane and thermal technologies. Therefore, designing membranes to remove salts and other minerals from saline water have been investigated in many computational studies. For this purpose, carbon nanostructures, such as carbon nanotubes (CNTs) and graphene sheets are usually used. These structures have been used to design nanoscale devices, such as nanomotors and nanopumps [1], [2], [3], [4], [5], [6], [7], [8]. For example, Gong et al. [9] computationally designed the nanopump, composed of a CNT, two graphene sheets and three positive charges, by mimicking the biological channels in a cellular membrane. They showed that, the water flow through the designed nanopump depends on the spatial positions of the positive charges. In another study, Jae Hyun Park et al. [10] employed molecular dynamics (MD) simulation method and used a charged Y-junction carbon nanotube to separate K⁺ and Cl⁻ ions from a KCl solution. Corry [11] studied the dependence of a nanotube diameter on the transportation of water and ions through carbon nanotube membranes, via the potential mean force calculation. He showed that ions cannot pass through the narrow carbon nanotubes, such as (5,5), due to the large energy barrier, while they can pass through the wider carbon nanotubes, such as (7,7). Recently, Cohen-Tanugi et al. [12] demonstrated that hydrophilic and hydrophobic functionalized porous graphene membranes can be used to filter salt from water. They found that desalination performance depended critically on the pore size, chemical functionalization and applied pressure to graphene. Sint et al. [13] used the MD simulations method to investigate the transportation of different ions through functionalized pore graphene under application of external electric field. They obtained that the shape and size of the nanopores and the number and functional ligands attached to the nanopores are critical factors for ion selectivity and passage rates. In another computational study [14], the transportation of water through a porous graphene membrane and a thin carbon nanotube membrane was compared. It was found that the larger porous graphene membranes were more efficient in water transportation than the CNT membranes. Taghavi et al. [15], by using MD simulations, demonstrated that a charged CNT can be used to separate Na⁺ and Cl⁻ ions from a NaCl solution and encapsulate ions from a surrounding solution. In another study, Majumder et al. [16] put negatively charged functional groups at the CNT entrance and found that the flux of positive ions increased; they also claimed that this effect is reduced at higher ionic concentration. Furthermore, in an experimental study, Zhang et al. [17] showed that graphene/carbon nanotube (GR/CNT) electrodes provided higher efficiency in desalination performance compared to graphene and functionalized carbon nanotube under the same experimental conditions. Wang et al. [18] experimentally described the procedure for the preparation of GR/CNT composites by a modified exfoliation approach, and used the prepared GR/CNT composites as capacitive deionization (CDI) electrodes. Their results implied that the GR/CNT composite electrodes exhibited excellent desalination behavior in comparison with graphene and commercial activated carbon. In another experimental study [19], the graphene-coated hollow mesoporous carbon spheres were used as an efficient electrode material, which can be employed in CDI desalination technology.

In this paper, by using the MD simulations method, we designed a new high efficiency graphene-charged CNT membrane, as a nanofilter, and show that the nanofilter can separate up to 90% of salt ions (Na⁺/Mg^+ 2/Fe^+ 3 and Cl⁻) from water molecules and produce fresh water. The efficiency of the nanofilter for salt rejection is nearly 90%, while others reported this amount at about 60% [11], [20].

Section snippets

Simulation details

Our proposed nanofilter was composed of three main sections, as schematically shown in Fig. 1. The first section, referring to a movable wall or a rigid piston, was made of a square graphene sheet with an area of 4 × 4 nm². The second section of the nanofilter was made of two square graphene sheets of the same area at 4 × 4 nm² with central circular pores, which were connected to each other with a (10,10) CNT of length 3 nm. The surface of the CNT was covered by different surface charge densities. The

Results and discussion

Initially, the velocity of the rigid piston was kept at V = 0 and the system was equilibrated for 1 ns. Also, in the equilibration phase, the water molecules and the collection of the positive and negative ions were not allowed to enter the CNT. During equilibration, the total energy of the system was monitored whether the system had reached the equilibrium state. Following the equilibration phase, for each case, 0.5, 1 and 2 ns simulations were carried out for three different piston velocities; V₁ =

Conclusion

In this study, using the MD simulation method, the water desalination was investigated. For this purpose, the graphene-charged CNT membrane was used to separate Na⁺ and Cl⁻ ions from a NaCl solution. In our simulations, the surface of the CNT was covered by different surface charge densities of σ = 0.13, 0.26, 0.39, 0.52 C/m². Also, three different piston velocities V = 10, 5 and 2.5 nm/ns were considered. The conclusion is that the ion separation process was improved by increasing the magnitude of

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Water desalination by a designed nanofilter of graphene-charged carbon nanotube: A molecular dynamics study

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Simulation details

Results and discussion

Conclusion

Desalination

Biophys. J.

J. Mol. Graph.

Electrochim. Acta

Chem. Phys. Lett.

J. Comput. Phys.

Water transport inside a single-walled carbon nanotube driven by a temperature gradient

Nanotechnology

Subnanometer motion of cargoes driven by thermal gradients along carbon nanotubes

Science

Molecular dynamics modelling of an electrical-driven linear nanopump

Mol. Simul.

Directed motion of C60 on a graphene sheet subjected to a temperature gradient

Phys. Rev. E

Nanopumping using carbon nanotubes

Nano Lett.

Nanopumping molecules via a carbon nanotube

Nano Res.