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Molecular dynamics simulation of electric field driven water and heavy metals transport through fluorinated carbon nanotubes
Accepted Manuscript Molecular dynamics simulation of electric field driven water and heavy metals transport through fluorinated carbon nanotubes
Abbas Panahi, Ali Shomali, Mohammad Hossein Sabour, Ebrahim Ghafar-Zadeh PII: DOI: Reference:
S0167-7322(18)35317-0 https://doi.org/10.1016/j.molliq.2019.01.084 MOLLIQ 10315
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
17 October 2018 11 January 2019 14 January 2019
Please cite this article as: Abbas Panahi, Ali Shomali, Mohammad Hossein Sabour, Ebrahim Ghafar-Zadeh , Molecular dynamics simulation of electric field driven water and heavy metals transport through fluorinated carbon nanotubes. Molliq (2018), https://doi.org/10.1016/j.molliq.2019.01.084
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Molecular Dynamics Simulation of Electric Field Driven Water and Heavy Metals Transport through Fluorinated Carbon Nanotubes
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Department of Electrical Engineering and Computer Science, York University, Toronto, Canada
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Department of Aerospace Engineering, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran
School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran, P. O. Box: 11155-4563
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Abbas Panahi 1, 2, Ali Shomali3, Mohammad Hossein Sabour *, 1 and Ebrahim Ghafar-Zadeh2
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Abstract
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In this study the fluorinated carbon nanotubes (FCNTs) were analyzed to investigate
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the proficiency of this type of functionalization on removal of heavy metals from
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water. Electrokinetics desalination properties of (FCNTs) embedded in silicon
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membrane has been investigated using molecular dynamics simulation. In terms of flow rate enhancement, it was shown that with increment of voltage difference from
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0 to 3 V, the water flux hit a peak of 378 H2O/ns (at 2 Volts) and 415 H2O/ns at (2.4 Volts) for FCNT and pristine carbon nanotubes (PCNT), respectively. Interestingly, FCNTs retarded the decline of flux condition which allows more water transport for higher voltages. The number of Zn2+ transport events were more than Hg2+, which shows the higher efficiency of this membranes on the removal of Zn2+ cations. Molecular dynamics simulation suggests FCNTs as a possible prospective volunteer 1
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gives the highest water flux compared to pristine carbon nanotubes.
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Keywords: Fluorinated carbon nanotubes, functionalized carbon nanotubes,
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electric field, heavy metals, molecular dynamics, water transport, fluorination Corresponding authors: Mohammad H. Sabour, PhD, Peng, Assistant Professor, Aerospace Department, Faculty of
New Sciences & Technologies, University of Tehran, North Kargar Ave., Tehran, IRAN, Zip Code: 1439955941, Tel.: (98)21-86093308, Fax: (98)21-88617081 Cell.: (98)912-727-2087 (1) 514-437-2942, Email: [email protected]
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ACCEPTED MANUSCRIPT Introduction The numerous industrial, domestic, agricultural, restorative and innovative applications of heavy metals, have increased their widespread delivery in nature that,
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altogether, rises concerns over their potential dire consequences for human health
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and the earth. There are many consumptions of these compounds in industrial activities, houses, and hospitals. All of these activities enhance the presence of heavy
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metals, for example, Cd2+, Cu2+, Ni2+, Hg2+, and Zn2+ in water which origin
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dangerous problems influencing different organs of the human body that is mainly
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due to their poisonous quality and in long-term causes cancer diseases [1-3]. Along this line, through basic research and practical endeavors, new trend in the eradication
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of these heavy metals from the water were under intense considerations.
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Extreme industrial activities in these days, will leave dangerous materials like heavy
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metals through various procedures which all of these materials before going through
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filtration steps will be introduced to nature and eventually come into human food chain [4-7]. The routines of Hg2+ ions to become dangerous in nature is smooth and may cause many harms to the environment. Its vapor and the existence of its natural substance in nature, significantly impact the nature of the soil and its quality in a long period of times [8-9]. The final products of such chemical reactions and evolutions such as oxidation, will results in divalent mercury [10]. On the other hand, 3
ACCEPTED MANUSCRIPT due to the existence of some sorts of microbes in the soil, through some chemical reactions, dangerous forms of mercury will be produced [11]. For this reason and to eliminate harmful heavy metals, intensive researches have focused on some of these
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contaminations, such as arsenic, cadmium, chromium, lead, and mercury [12-17].
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Regarding efficient desalination of water, the most important component is a
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membrane which should be designed to eliminate almost all ions from water. Some
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research works utilized organic and inorganic materials which predominantly they
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focused on CNTs [17-22], graphene [23-25], and metal-organic framework materials [25-28]. Among all mentioned materials, carbons nanotubes have grabbed a great
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attention in term of their viable applications in membranes for desalination of water,
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both through experiments and also computational studies [29-30]. There is also a
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tradeoff between a high water flux and the highest possible ions rejection. However,
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some types of above-mentioned membrane materials, do not meet all of these expectations [25, 24, 28]. Whereas CNTs have revealed a high water flow rate and
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acceptable ion rejection potential, the fabrication of a CNT based membrane have been remained as a challenging procedure which needs advanced Nano/micro machining processes [31]. From the height of other standpoints, some 3D materials like zeolite showed very good ion rejection characteristics but low water flux [3133]. CTFs (covalent triazine frameworks) demonstrated to be good at conducting water but very low in rejecting ions [34]. While graphene nanomaterials used as 4
ACCEPTED MANUSCRIPT water desalination membranes, and showed the same water flux features in parallel with demonstrating better ion rejection features compared to CTFs [34]. Thanks to recent nanofabrication processes that made it possible to create nanopore in
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graphene monolayer using highly energetic heavy ions hitting on its surface [35].
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This valuable achievements has leveled the way for further research on the usage of
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these nanomaterials in gas separation [36] and the most interesting one, DNA
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sequencing [37].
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Between all the membranes, it has been proved and accepted that CNTs are fast transporters of water and also in some range of diameter completely ban ions
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passages [38-40]. A lot of research works addressed the appropriateness of these
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nanomaterials for water treatment applications [39-40]; furthermore, Boron nitride
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nanotubes were also reported to be a volunteer for these purposes [41-43].
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Notwithstanding that some researchers have studied the functionalized CNTs under different conditions [44-46], some aspects of water physical characteristics inside
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extremely confined spaces like CNTs, however, are still unclear [47-48]. Water molecules, to become motivated to flux in larger volume inside CNTs, needs motivation. These stimulations could be even pressure difference [49-51], concentration difference, [52], and last but not least electric field [53-55]. Electric field has some interesting effects on the water molecules when they are confined or are in dynamic near nanomaterials such as carbon nanotubes, carbon Nano sheet and 5
ACCEPTED MANUSCRIPT etc. Some of these effects are electro freezing phenomena in the confined wall, [5558], and concentration-induced electric field inside cell membranes [59]. Pressure difference was used to derive water molecules in different works while
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examining different nanotubes, such as carbon nanotubes (CNTs) or boron nitride
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nanotubes (BNNTs), and it was concluded that higher pressure difference could
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increase water transport considerably [60-61]. Taking in mind the nature of polar
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features of water molecules, the impacts of the electric field, nevertheless, needs
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more study to be completely grasped.
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Some reports covered functionalized carbon nanotubes for desalination of water and removal of heavy metals from water [61-64]. Fluorinated carbon nanotubes were
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also investigated for the removal of desalination purposes using electric fields [44];
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furthermore, the fluorinated carbon nanomaterials nanopore were reported in some
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other research works [44].
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There have been considerable research works on fluorinated carbon nanotubes both experimentally and theoretically [65-68]. Fluorine atoms improve the solubilization of CNTs in water. Due to the properties of fluorine atom bounds with carbon atoms, it has become the target of research for exploring the possible application of fluorine functionalization of carbon nanotubes in desalination and other applications. The
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ACCEPTED MANUSCRIPT fluorinated carbon nanotubes could be utilized for altering physio-chemical properties of the surface features to become more hydrophilic [66, 68]. There are numerous experimental approaches for functionalization of carbon
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nanotubes by fluorine groups [68-73]. In recent studies, molecular dynamics
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simulation was used as an efficient method to discover the effectiveness of
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nanofiltration membranes in water desalination technologies regarding both
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electrokinetics and pressure difference methods. Different nanoporous membranes
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fabricated with carbon, boron nitride, and silicon carbon nanotubes embedded in silicon or silicon nitride membranes have been extensively researched in recent
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years. Furthermore, the effectiveness of carbon nanotubes with different functional
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groups was established in presence of hydrostatic pressure difference and electric
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field. Molecular dynamics simulation, due to its potential in giving an atomic insight
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into the transportation of ions through nanomembranes is of high importance in designing novel membranes and evaluates their efficiency without excess expenses
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on experimental costs. Electric field in various research works has been used to investigate the transport of different ions through carbon nanotubes, boron nitride nanotubes, boron nitride nanosheets and graphene nanoporous. [73-75]. Azamat et al investigated separation of Li+, Mg2+, and Cl- through nanostructure membranes using electric field in which the carbon nanotubes transported more water molecules as electric field increased. The results showed that Li+ and Mg2+ ions permeated 7
ACCEPTED MANUSCRIPT through (7, 7) nanotubes while Cl- ions didn’t. It was revealed that ion permeation with different ratios through (7, 7) nanotubes was fulfilled in the presence of an electric field; furthermore, it was demonstrated to be selective [76, 77].
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Herein, fluorinated carbon nanotubes were examined for removal of heavy metals
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from water. FCNTs were put inside a silicon membrane and the solution were under
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the action of the horizontal electric field to derive water and ions. Voltage applied
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on both side of system, varied from 0 to 3 V for PCNTs and FCNTs with same radii
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and length. This study with different simulation data paves the way for potential application of FCNTs as a new membrane for eliminating heavy metals from water,
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therefore its potential application will be in desalination and waste water treatment.
Figure 1. Snapshot of sub-nanometer fluorinated carbon nanotubes simulated for transport study of heavy metals and water solution. A) pristine carbon nanotubes, b) fluorinated carbon nanotubes, c) fluorinated CNTs embedded in silicon membrane, d) fluorine atoms at the gates
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ACCEPTED MANUSCRIPT 1. Computational Methods In this paper, we have scrutinized the transport properties of water molecules through both functionalized and pristine carbon nanotubes. The FCNTs were
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functionalized with fluorine atoms at both ends of carbon nanotubes. We used CNTs
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with (8, 8) chirality which corresponds to radii of 0.82 nm. The FCNTs and PCNTs
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were created with VMD, [78], and then their structure were optimized using
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Gaussian 03, with the B3LYP level of the theory and 6-31G basis sets; the main
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reason for minimizing the system with Gaussian was to design the most appropriate diameter for silicon nanopore before MD simulation. This is due to this fact that if
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the carbon atoms be very close to silicon atoms, an error happens in running the
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molecular dynamics, therefore optimizing the nanotubes help us to choose the most
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appropriate diamter of silicon nanopore [79]. The output “mol2” structure file from
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Gaussian were used as input file in online SwissParam website to find unknown parameters and force fields, that its computation method is reported in [80]. These
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parameters are listed in Table 1. After creating the nanotubes, they were embedded in a silicon membrane. The thickness of the silicon membrane was assumed to be 1.6 nm. The simulation system is demonstrated in Figure 2.
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Figure 2. The simulation box including two water boxes with a dimension of 5nm×5nm×10nm at both sides of a silicon slab with dimensions of 5nm×5nm×1.6nm. In each side, HgCl2 and ZnCl2 were solvated with certain concentrations and the electric field was applied along the z-axis of the simulation box. Each water boxes contains 10000 water molecules and 0.35 M ZnCl2 or HgCl2 ions.
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Figure 2 demonstrates the simulated system that consists of a silicon membrane in
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which the nanotubes (FCNTs and PCNTs) are located at the middle of the
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membrane. At both sides of the membrane, two water boxes are located which contain 0.35 M HgCl2 and 0.35 M ZnCl2. The system for each of these solutions
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(HgCl2+Water or ZnCl2+Water) was simulated separately, in better words there
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exists only Hg2+ or Zn2+ in solution, and in order to evaluate the system for getting
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their characteristics, the process of each simulation was repeated three times and the results were averaged to ensure the accuracy of results. The water boxes size was 5nm×5nm×10nm in x, y, and z-directions, consequently the overall dimension of system is 5nm×5nm×21.6nm including the membrane at the middle. Also, the electric field was applied horizontally and corresponding voltage difference changed from 0 to 3V.
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ACCEPTED MANUSCRIPT Table 1. Potential parameters for FCNT and PCNT used in the simulation CNT
FCNT
V(bond)=Kb(b-b0)2
C-C
F-C
b0 (A)
1.400
1.3420
Kb(kcal/mol. A2)
469.000
468.572
V(angle)=Kθ (θ-θ0)
C-C-C
C-C-F
θ0 (degree)
35.00
118.0650
Kθ(kcal/mol. rad2)
120.00 C-C-C-C
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[1+cos(nΦ- Φ0)]
78.730
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V(dihedral)=KΦ
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Potential Parameters
C-C-C-F
180.00
180.00
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Φ0
KΦ (kcal/mol) ε (Kcal/mol)
3.5
C
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0.0860
0.135000
1.910
1.630000
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Rmin/2 (A֯)
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V(LJ)
3.100
Charges
C (C-F) = 0.19
F (C-F)= -0.19
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In Table 1, the simulation parameters are listed. The simulation box dimensions were
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5×5×21.6 nm in x, y, and z-directions, respectively. The number of water molecules
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in each water boxes were the same (about 10000 molecules in each side), that a representation of the simulated system is shown in Figure 2. To explore the properties of CNTs for ion conduction, all of two water boxes with a length of about 10 nm were subjected to solvation of 0.35 M MgCl2 and ZnCl2 ions in each one for every simulation. In order to prevent the simulation box to move around and prevent the undesirable vibrations, its center of mass was set to a constant coordinate. The
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ACCEPTED MANUSCRIPT periodic boundary condition (PBC) was applied to all directions and the area of water boxes (along z-direction) was held constant. There were no interatomic bonds between silicon and carbon atoms; however, there were electrostatic interactions and
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also Lennard Jones (LJ) interactions. The charges of carbon atoms in CNTs were
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zero, and the carbon atom’s charges at two terminals of FCNTs were calculated to
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be C=0.19 and F= -0.19.
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To study the properties of fluorinated carbon nanotubes for removal of heavy metals
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from water, a molecular dynamics simulation was initiated to examine the ionrejection properties of both pristine and fluorinated carbon nanotubes. Here, NAMD
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2.9, [81], was applied for MD simulation in which CHARMM 27, [82], force field
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used to calculate the interaction forces. Although it is of paramount importance to
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choose an appropriate force field for simulation of carbon nanotubes, particularly,
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when it comes to determine their mechanical properties and characteristics, but in this simulation the mechanical properties of both PCNTs and FCNTs are not
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regraded, and their vibration do not affect the water transport considerably; therefore, we have treated the carbon nanotubes with CHARMM flexible force fields like other research works which considered this force field for investigating the water transport inside CNTs [74-77]. First, carbon nanotubes were constructed then embedded inside a silicon slab. Afterward, the system was placed between two water boxes which were neutralized with 0.35 M MgCl2 and ZnCl2. For modeling the water 12
ACCEPTED MANUSCRIPT molecules in this simulation, TIP3P, [83], model was used in which the HOH angle is 104.52° and partial charges were +0.417 for hydrogen and -0.834 for oxygen respectively. After the minimization process, the equilibration of the system was
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performed for 4 ns and at 300 K temperature, then the results of the simulation were
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considered for the rest 16 ns.
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In all dimensions PBC was applied and also the particle mesh Ewald (PME)
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summation, [84], was used with a grid size of 1 Å for long-range electrostatic
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interactions. The cutoff equal to 12 Å applied for van der Waals interactions. Also a 14.5 Å pairlistdist distance was assigned to search only within this distance for atoms
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which may interact by electrostatic or van der Waals interactions. In all simulations,
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the cutoff distance was set to 12 Å, with the aim of minimizing the overall simulation
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time. The Langevin dynamics is used to appoint a constant temperature in a NVT
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ensemble in minimization time in which the Langevin coupling coefficient, assigned to be 5/ps. After the minimization process, the molecular dynamics was conducted
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in NPT ensemble throughout of the simulation time. Furthermore, for in order to prevent the system undergoing a change from liquid to a solid phase, or from liquid to gaseous phase, all simulations were performed at 300 K. Each system was simulated for 20 ns at 300 K employing an electric field in the z-direction, with voltage variation ranging from 0 to 3 V. In this simulation 20 ns MD runs including
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𝑉 𝐿𝑧
(1)
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𝐸 = −23.0605492
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bonded potential energy as an extra term, [85], which is defined as:
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Where, E, represents electric field (in kcal/mol Å), V is for equivalent potential
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differences (in volts) and 𝐿𝑧 symbolizes the size of the system along the z-axis (in Angstrom), respectively. We should consider that 1 kcal. mole-1 A-1 e-1 = 4.35×108
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V/M [96].
To understand the energy barriers in front of cations and onions when passing
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through membranes, a PMF analysis has been conducted which clearly demonstrates
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why ions transport events in FCNTs are remarkably different from PCNTs. The PMF
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analysis is very important when the ion transport inside nanopore is under investigation. The PMF for ion could be demonstrated by Wi (z) which is calculated by integrating the force exerted on ions when are passing through the nanopore which is reversible work done by ions in the system when are passing through nanotubes based on thermodynamics integration method for calcualting PMF.
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(2)
𝑊𝑖 (𝑧) − 𝑊𝑖 (𝑧0 ) = − ∫ 〈𝐹𝑖 (𝑧)𝑑𝑧〉
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𝑧0
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In which z0 represents the reference position of the ions. Along the length of carbon
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nanotubes and at different positions, the mean force distribution was calculated by
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sampling the force experienced by ions. To achieve that, the ions moved through location from -10 Å to 20 Å in 0.2 Å increments (the length of each window in
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reaction coordinate) and the z component of the ions was held by a 16 kcal mol/Å2
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harmonic constraints, whereas the ion was free to move radially in each window.
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Before starting any simulation, the ions are fixed in a specific position for about 20
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fs, for be completely surrounded by water molecules.
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3. Results and Discussions
Molecular dynamics simulation has been conducted to investigate the transport of
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water and heavy metals through both fluorinated and pristine carbon nanotubes. The permeation of water molecules and ions through both of the nanotubes (PCNTs and FCNTs) were analyzed. Molecular dynamics results showed that heavy metals in a specific value of electric field start to break the energy barriers at the gate of CNT and pass through the nanotubes, which continuous of this trend is not possible
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ACCEPTED MANUSCRIPT without the external electric field. This type of functionalization (fluorination) is of high importance, if the goal be gather all heavy metals ions just in one side. Fluorination, from one prospective promotes water flow that is mainly because of
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creating hydrogen bonds with water molecules at the entrance of carbon nanotubes
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and also causes the fast departure of molecules from another side. To understand
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the flow of water through both types of nanotubes, diagrams of water flow versus
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time is demonstrated in Figure 3 which clarifies the spontaneous flow of water versus time. The simulated system contains 0.35 M of zinc chloride and mercury
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chloride solutions in two water boxes placed at the sides of a carbon nanotube
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embedded in a silicon hole with diameter of 1.2 nm. In another try, a membrane with
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functionalized pore was considered for analysis. An external electric field applied along the carbon nanotube’s longitudinal axis, which was perpendicular to the
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horizontal projected area of CNTs, and the size (diameter) of a nanopore in the
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membrane was around 1.2nm which is adequate for transporting Zn2+, Hg 2+ and Cl-
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ions. The important result from this simulation is that by applying the electric field, ions start to permeate from the functionalized pore after a specific threshold of electric field. In relevance to this study, Zhu Qiang et al, [86], using molecular dynamics simulation studied the effect of electric field on water molecules and concluded that the electric field considerably changes the orientation of water molecules dipole moment and eventually change the value of dielectric constant. 16
ACCEPTED MANUSCRIPT They also reported that by increasing the electric field, the dipole-field interaction energy increases which are the induced energy from the external field on water molecules. This energy intensifies the internal energy of molecules, which lead to
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more intermolecular interaction and could be a possible reason for the higher flux of
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water in confined spaces. The increment of electric field does not increase the water
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flux in all ranges and this could be studied by hydrogen bonding analysis on the
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system along CNT’s horizontal axis. For a better understanding of the effect of fluorination on water transport, we have examined the hydrogen bonding, ion
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transport, water flux, water density, water occupation inside CNTs, the potential of
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mean force (PMF) and radial distribution function (RDF).
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3.1 Water flow inside carbon nanotubes
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In this study electric field was used for exploring the water and heavy metals solution
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transport inside pristine and fluorinated carbon nanotubes. This phenomenon that electric field is directly applied to move aqueous solution through nanopore is called,
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electroosmotic flow (EOF) which is happening by making a voltage difference between two electrodes placed at both sides of the system. This voltage difference is produced by placing the anode and cathode at each side of the membrane and altering the voltage to apply the external driving force for a solution to migrate through nanpore. In EOF systems, it is needed that a clear description of the transportation phenomena be presented, because it deals with the infinitesimal 17
ACCEPTED MANUSCRIPT transport of water and ions through a small region that having a control over this area, will affect the overall transport considerably. Having a better control of such small system when the only way is to control the pressure, may become a great
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challenge. However, using electric field for such regulation could be done precisely
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as our results and other research works show [87, 88]. Pressure difference needs
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micro-instruments that their fabrication and installing in the system will impose a
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great difficulty on the construction of the whole system. Moreover, the problems associated with the uniformity of flow profile in pressure-induced nanoflow
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channels are the other disadvantages of osmotic systems [54, 55]. Here by means of
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molecular dynamics simulation, the water, and heavy metal ions were simulated for
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analysis of their transport characteristics inside carbon nanotubes which the system undergoes extremely confined spaces. In micro and nanofluidic systems, pristine and
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functionalized carbon nanotubes could be used for different purposes, wherever the
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fast flow of liquids are regarded.
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Molecular dynamics simulation shows this interesting result that fluorinated carbon nanotubes are a better transporter of water molecules compared to pristine carbon nanotubes. In this simulation, the electric field has been used as a motivation for the water molecules, to surge inside carbon nanopore. When water molecules are feeling an electric field, their dipole moment undergoe changing in its main direction and this is sensed by all molecules in solution that orders their direction toward this field. 18
ACCEPTED MANUSCRIPT While changing their course of dipoles at the gate of nanopore, where all molecules facing a potential barrier for moving inside which needs more enrergy. In order to model these systems for application in nanofluidic systems, it is vital that we have a
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time-dependent record of water flow, since this shows how the fluid transports with
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time. Molecular dynamics results of the water flow inside both PCNTs and FCNTs
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reveal an important flow pattern.
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In all electric fields that are applied horizontally along the centerline of NTs, FCNTs
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showed a continuous flow of water, whereas pristine carbon nanotubes in some occasions completely showed zero flow, as it is demonstrated in Figure 3. FCNTs
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due to their different charges with heavy metals ions, absorbs Zn2+ and Hg2+ near the
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mouth of carbon nanotubes and lower the potential barrier in front of ions to move
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inside. This, in turn, causes a uniform flow of water inside. Also, there are evidences
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that the effects of electric field on water flow in FCNTs are considerably more than PCNTs. This could be due to the effects of fluorine atoms induced electric field
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which is ascribed to the changes in their overall dipole moment course. Recently it was discussed by researchers, [44], that the residence of ions at the mouth of nanotubes is the main cause of flow interruption while in FCNTs, due to the pulling effects from fluorine atoms at the gate, ions surge inside nanotubes and the flow will become continues.
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Figure 3. Water flow versus time. Diagram of water molecules permeation through fluorinated and
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pristine carbon nanotubes in the presence of horizontal electric field. Electrical field are applied horizontally along the centerline of nanotubes and its value maintained constant in simulation time.
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FCNTs showed approximately non-zero flow in the presence of electrical field, but the same did
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not happened for PCNTs that it undergone abrupt zero-flow.
This could be seen in Figure 3 where the flow of water in the presence of electrical fields were gone through a non-interrupted pattern, while the same cannot be said for PCNTs. The other reason for such difference in flow pattern is the locallyinduced electrical field by fluorine atoms vibration at the gate which is discussed in following chapters.
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ACCEPTED MANUSCRIPT Having a precise look at the Figure 3 reveals that the water flow versus time for different electric field values on the system, specifically, when E-field is zero. It has experienced a nearly increasing trend in water flow but with ignorance of some
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fluctuations. Nonetheless, the water in PCNTs abruptly falls down to zero and then
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increases. This is the main advantage of FCNTs over PCNTs in the condition of
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existing no external stimuli. In better words, without any external force and by only
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the intermolecular interactions, fluorinated CNTs could transport water molecules more than carbon nanotubes with same radii and length. One possible prediction
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about the mechanism of water flow enhancement in FCNTs could be that fluorine
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atoms’ vibrations is responsible for such phenomena that gives energy to water
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molecules and also makes a small electric field near the mouth, as a result, motivates more water inside whilst making disturbances at the entrance. However, the same
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was not happened for higher values of electric fields, in which the electrical field
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plays the main rule in water transport. In many works, [41, 43, 51, 60], this was
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proved that electric field can increases the water flow through nanopore like CNTs and BNNTs when is applied on the system. Azamat et al concluded that by increasing electric field, the flow will be increased, they reported that in CNT (8,8), flow can be increased with the increment of electric field from 1 Kcal/mole A.e to 5 Kcal/mole A.e [60].
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ACCEPTED MANUSCRIPT In this study, for all values of voltage differences, the flow of water inside FCNTs were more than PCNTs, and this is an important advantage of functionalization with fluorine atoms which make us sure in terms of higher water flux. In Figure 4, for
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better judgment about the effect of fluorination, the mean flux has been calculated
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and is shown versus applied electric field. It is conspicuous that for all values of
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electric field the water flux through FCNTs are more than PCNTs, yet the fluorine
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atoms at the entrance of nanotubes decrease the entrance efficient diameter. The water flux trend in Figure 4, showed a peak, more interestingly, the peak for FCNTs
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were higher and happened at the higher electrical fields value. This results motivated
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us to find its main reason which directed us to put the light on the physics of water
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interaction with carbon nanotubes where embraces many contributory factors. Regarding a similar and relevant work, Konstantinos Ritos et al, [89], in a thought-
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provoking work, studied the effects of electric field on water transport inside carbon
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nanotubes. They reported that when water is going to pass through PCNTs under the
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action of electric field, the water flux will be increased as the electric field is intensified until a certain value of electric field, beyond which, the water flux profile will start to decline. They justified their result with considering the interconnected influences of hydrogen bonding dynamics and water molecules density. They argued that, this peak depends on the density and the hydrogen bonding before the entrance of water molecules inside CNTs. In their discussions it was investigated that the 22
ACCEPTED MANUSCRIPT sudden decrease of density and increase of hydrogen bonds will results in solidification of water at the gate of nanotubes and suddenly decrease the water flux [89]. Here in this work, when the carbon atoms at the entrance are fluorinated, the
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hydrogen atoms of water molecules start to hydrogen bonding with fluorine atoms
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and this will interrupt the hydrogen bonding network of water and always retain us
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sure that this number will not upsurge to form a complete structure of ice.
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Subsequently, the decrement of water flux will be retarded to a higher electric field which could help to, both, having a control on transport and directing more water
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through nanopore by applying greater electrical fields. According to simulation
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evidences, the surge in water flux and its declining trend, were mainly due to the fast
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and sudden jump of a cation into the nanotubes and also long residence of cations at
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the mouth of carbon nanochennel, respectively [89].
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Recent studies showed that the effects of electric field on changing the direction of water molecules orientation cause its anomalous behavior, and shift the equilibrium
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state toward filling carbon nanotubes [90]. The electric field causes a uniform water structure inside carbon nanotubes and results in an ordered structure which induces the liquid-to-solid phase transition [91-93]. These phenomena have been recognized as the underlying causes behind appearing a peak in the water flow pattern in nanotubes.
23
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Figure 4. Averaged water flux through fluorinated and pristine carbon nanotubes when electric field is applied to the system. The flow will reach a peak and then start to decline. FCNTs has
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caused a retardation in the lessening of water flux compared to PCNTs. FCNTs showed higher water transport capability compared to PCNTs even after the maximum peak which after that the
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go through a continual increment.
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flow will start to hit its minimum or plateau. The water flux versus electrical field will not always
The interaction between water molecules and CNTs plays an important role in
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physics of higher flux of water (compared to not application of electric field) when
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electric field is applied. Winarto et al. [94], studied a system of PCNT (8, 8) between graphene sheets, with and without the action of electric field. They concluded that the Lennard Jones (LJ) potential energy of water molecules inside CNT is lower than the water molecules in the reservoir which this energy difference facilitates the water transport inside PCNTs. According to a report of Wintaro [94], the LJ potential energy inside PCNT (8, 8) increased from 4.71 kj/mol to 17.98 kj/mol while it 24
ACCEPTED MANUSCRIPT decreased from 19.57 kj/mol to 19.51 kj/mol in the reservoir. When electric field (1 V/nm) was applied along the PCNT (8,8), the columbic potential energy difference (ΔU) changed from 14.86 kj/mole (no electric field) to -10.02 kj/mol which this
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implies that when the system undergoes an electric field the electrostatic interaction
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is the only driving force (the dominant one) for water molecules to surge inside
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PCNT (8,8). The other important contributory factor affecting the water flow inside
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carbon nanotubes is the dipole potential (DP) energy. This energy exists when an electric field is applied to the system, and has a linear dependence on the strength of
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electric field. When the electric field is applied, the dipole vibration in the reservoir
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is higher than the water molecules inside PCNTs and therefore the DP in the
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reservoir is greater than the DP inside PCNT, consequently, this difference motivates
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the water molecules to move inside carbon nanotubes [94].
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The other possible implication is that, having in mind the connection of fluorine atoms to carbon atoms at the entrance of nanotubes and their free vibrations, a
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locally-generated electrical field will be produced which is in reverse direction of main electric field, therefore this internal electric field reduce the external-induced electric field in the neighborhood, and enhance the DP potential energy difference. Thus, the water molecule finds it more favorable to move inside FCNT, and due to the opposite effect rising from fluorine atom at the counterpart exit FCNT gate, will go out faster. This is presented as an inhomogeneous distribution of electric potential 25
ACCEPTED MANUSCRIPT which is discussed in following chapters (see Figure 6). Contemplating more on the derived data by Winarto, The DP energy is lower than columbic potential energy, therefore, the difference in electrostatic energy in PCNT is the dominant motive for
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the fast occupation of carbon nanotubes by water when electric field is applied.
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Figure 5 demonstrates the LJ and electrostatic potential energy difference of water
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molecules inside the PCNT and FCNT with the water in the reservoir. As it shows,
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the LJ potential difference in FCNTs, when the system experiences no electrical field
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(E=0), is higher than the PCNT (8, 8) which is in tune with findings of Wintaro et al [94]. This could be the reason why the water flux in FCNTs is higher than PCNTs
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when there is no external electrical field. The ΔU (LJ) of PCNT (8, 8) is about 22.45
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kj/mol while it is approximately 23.97 kj/mol for FCNT (8, 8).
Figure 5. The Lennard johns and electrostatic potential energy difference between the water molecules inside carbon nanotubes and the water molecules in reservoir. (a): The Lennard johns (LJ) potential energy difference (∆ LJ), (b): electrostatic potential energy difference (∆U)
26
ACCEPTED MANUSCRIPT Figure 5-b depicts the electrostatic energy difference for a system under electrical field (V=1.2 Volts) in which FCNTs and PCNTs are under consideration. As it is evident, the potential energy difference between inner water inside PCNT (8,8) is
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more than FCNT (8,8) which leads to this results that the electrostatic effects of
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fluorine atoms on the terminals of FCNTs and their effects on the water inside and
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water in the reservoir has resulted in a decrease in electrostatic potential energy. For
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PCNT (8, 8) the averaged electrostatic potential energy difference is about 20.84 kj/mol while the corresponding value for FCNT (8, 8) is -11.15 kj/mol. The energy
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analysis of system showed that the differences in electrostatic energy has influenced
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the transport events of water considerably and one possible explanation could be this
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energy variations.
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To have a better understanding of the electrical field effects on electrostatic
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distribution in CNTs, an electrostatic study has been done to measure the values of the electrical field, in the vicinity of CNTs and more importantly, at the middle plane
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of its symmetric axis. The results of electrostatic distribution are shown in Figure 6. As it can be seen, some disturbances has been occurred in the middle of carbon nanotubes that is due to effects of fluorine atoms at the gates.
27
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Figure 6. The electrostatic profiles of the electrically induced system which shows the fluctuations of the electrical fields near the entrance of FCNTs and a regular pattern of electrostatic energy distribution in the same value of an the electrical field in PCNT.
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The electrical field distribution is somehow uniform in PCNT but it is not unvarying
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along the FCNTs. This might be due to effects of the locally-generated electrical
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field from fluorine atoms at both ends of FCNT that disturbed the homogeneity of
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its distribution that influence the flow of water and hydrogen bonding.
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In order to demonstrate the effect of fluorinations on water transport, the density counters of water when passing through PCNT and FCNT for two different electric
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fields is calculated and demonstrated in Figure 7. It shows that the water density for FCNTs in higher electric field value is more than PCNTs that proves the higher flux of water through FCNTs reported in Figure 4. In FCNTs, denser water arrangement could be found near the entrance, at the middle and in the vicinity of nanotubes walls of FCNTs compared to PCNTs. The density profiles show that in a specific electric field the water storage inside FCNTs is considerably higher than PCNTs which open 28
ACCEPTED MANUSCRIPT the discussion for investigating the properties of these NTs for water storage applications. It should be noted that all densities are calculated based on the NAMD standard which is NH2O/A3. In other words, these graphs show the averaged occupied
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space (AOS) of water inside and outside the NTs. As it is clear, there are two water
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layer inside PCNTs and FCNTs which one is a line of water molecules along the
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centerline of nanotubes and the outer is a circular ring of water molecules. This
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presented at the final section of this article.
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results is supported with radial distribution function (RDF) outcomes which is
Figure 7. The averaged spatial occupation of water inside PCNT (8, 8) and FCNT (8, 8) for different electric field values. Here the higher water occupation density near the nanotubes entrance is evident. (a) PCNT (8, 8), V=1.6V- (b) FCNT (8, 8), V=1.8V
In order to evaluate the effect of functionalization on the removal of heavy metals, a study has been conducted on the overall permeation of ions through NTs. 29
ACCEPTED MANUSCRIPT Considering the PBC on the system, only the number of ions transports which permeated for one time is calculated, and the rest were omitted from calculations. The ions were solvated in the water and then the system was initialized under
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temperature of 300 K. It was concluded that the permeation of Zn2+ cations through
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FCNTs were more than the permeation of Hg2+ ions through PCNTs and this shows
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that this membrane could be used if we want to gather all ions in one side of water
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boxes (see Figure 8). However, by increment of electric field, the overall permeation of ions increased which could be due to the higher force of electric field on ions. Hg
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cations do not transport through NTs below 1.4 V voltage difference. The number
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of Zn2+ ions which permeated successfully was more than the Hg2+ numbers in the
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same conditions of thermodynamics state and the external forces. Azamt et al, also witnessed the increment of ions transport through NTs with enhancing the electric
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field strength [41,43]. The origins of higher flux capability of FCNTs, when are
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subjected to the external electric field, has been also reported by Panahi and Sabour
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[44]. The effectiveness of FCNTs comes to attention when we want to force to accumulate all ions in one side and in one reservoir. The molecular dynamics helps us to clarify the physics and also evaluate the efficient electrical field for complete transportation. According the results in Figure 8, the effect of the external electrical field showed that increment of the applied voltage from 0 to 3 V, could intensify the Zn2+ transport through FCNTs which it increased from 4 in 1.2 voltage difference to 30
ACCEPTED MANUSCRIPT about 15 permeation events in 3 V. The diagram apparently demonstrates that in all range of electrical field the permeation events of Zn cations were more than Hg cations both for PCNTs and FCNTs. Approximately there is a linear dependency
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between voltage and ions permeation which is of high importance regarding the ionic
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current analysis and conductivity analysis of nanopore, but they are not presented
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here because of having nearly same trend as ions migration records.
Figure 8. The effect of the the electrical field on heavy metals transport through FCNTs and PCNTs in the same precondition of the external electrical field and also thermodynamics. The transport records for Zn2+ were more than Hg2+ in all electrical fields. The FCNTs showed lower restriction toward allowing cations to migrate to other box at the other side. There is somehow a linear dependency between voltage and the number of ions permeated along nanopore.
This is extensively discussed in the research papers, that water and ions during their endeavor to move inside the carbon nanotubes, will face a potential barrier, and in 31
ACCEPTED MANUSCRIPT order to move inside, accordingly, they need to overcome this potential fence which has been generated due to the effects of small size effects [41,42,44]. To calculate the amount of energy barrier that all molecules should break to move inside, the
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potential of mean force has to be calculated; afterwards, this should be mapped along
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the central axis of carbon nanotubes to show the potential along z distances. In
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other words, the ions selectivity of nanopores could be analysed with the amount
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of energy barrier that the molecules or ions should break to surge inside. This barrier plays an important rule in designing nanofluidic devices. The potential of mean force
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(PMF) analysis, tells that how much is the discrimination of these nano-sized
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channels to different ions. For such purpose, a PMF analysis has been performed to
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investigate these properties of FCNT-Silicon and PCNT-Silicon membranes. This analysis is shown in Figure 9. In Figure 9-a the maximum of PMF for all ions in the
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system when are transporting through PCNT (8,8) and FCNT (8,8) is shown. The
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maximum energy barrier happened at the middle of carbon nanotubes because the
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system was symmetric.
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Figure 9. Potential of mean force analysis results for anions and cations when they are trying to move inside carbon nanotubes. (a) the maximum PMF for all ions at the middle of nanotubes, (b)
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the local PMF in which the Cl(-) shows the higher PMF value, that indicates, it face a great potential barrier when arrives near NTs. Hg(2+) and Zn(2+) showed different PMF value for
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entrance to NTs. Cations face lower energy barrier when face FCNTs compared to PCNTs.
The PMF analysis showed that there is a high potential barrier in front of chlorine
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ions for entering inside carbon nanotubes (PCNTs and FCNTs) and this is why there
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were no permeation events for chlorine ions. The positive sign of PMF demonstrates
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the unpreferable state of permeation and the negative PMF shows the preferable permeation for ions. PMF analysis proves the results of ions transportation phenomena of Hg and Zn permeation. The Zn cations transportation numbers are more than Hg2+ and this is because of the higher potential barrier in front of Hg compared to Zn. The maximum PMF value has been occured in the middle of CNTs and also the potential energy of permeation raise at the entrance of nanotubes. Due
33
ACCEPTED MANUSCRIPT to the existence of fluorine atoms at another side of FCNTs, the maximum PMF value at the middle of CNTs are lower than the same value for PCNTs. Also, it has some fluctuations which are mainly from both interaction with fluorine rings at both
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terminals of FCNTs. The maximum value for Zn and Hg at the middle of FCNTs
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and PCNTs are 67.254 Kcal/mol for Zg2+ permeation through PCNT, and 48.802
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Kcal/mol for Hg2+ transport through FCNT, both at the middle of nanotubes. The
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results in Figure 9, are in line with results of PMF, which this convey very important information about the efficiency of FCNT membranes. The results imply that the
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possibility of Cl-1 transport through FCNTs is considerably lower compared to
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PCNTs in the same thermodynamic conditions. Besides these interesting results,
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some basic questions rise which may spark dispute on the phenomena of water transport and the effects of the electrical field on the water and ions transport. To
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understand the effects of some aspects of the hydrogen bonding on water transport
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inside carbon nanotubes, analysis of a local distribution of hydrogen bonds inside
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carbon nanotubes, along the its center line, and under different values of the electrical field for both PCNTs&FCNTs is performed (see Figure 10). The hydrogen bonds distribution show that there are some fluctuations in hydrogen bonding before the entrance of water molecules inside the FCNTs and PCNTs.
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Figure 10. Hydrogen bonds axial distribution along the center line of CNTs for two values of voltage differences applied on PCNT and FCNT with chirality of (8,8) and length of 2 nm. The increment in voltage has led to growth in number of local hydrogen bonds
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Figure 10 demonstrates how the averaged hydrogen bonds are distributed along
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carbon nanotubes’ centerline under the action of the horizontal electrical field. It
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shows this fact that when the electrical field is increased, the number of water molecules inside carbon nanotubes (with regards to Figure 11) will start to rise, at
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that point, with increment in the water density, the number of hydrogen bonds will
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also increase. Nevertheless, according to the density profiles in Figure 11, the normalized number of hydrogen bonds has decreased dramatically. On the other hand, inside carbon nanotubes, by enhancing the electrical field the hydrogen bonds will be manipulated which causes a decrease in overall hydrogen bonds. This is due to the effects of the electrical field on the orientation of water molecules, which force them to be directed along the main courses of the electrical field near the entrance 35
ACCEPTED MANUSCRIPT of carbon nanotubes. Hydrogen bonding in FCNTs with V=0 V state was lower than FCNTs with V=1.2 Voltage difference strength which is due to the influence of the electrical field on the water arrangements. The main reason behind a higher number
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of hydrogen bonds for FCNTs compared to PCNTs (near the entrance of CNTs) is
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due to fluorine atoms hydrogen bonding with water hydrogens. The hydrogen bonds
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with fluorine atoms and hydrogen of water molecules has resulted in a local
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increment in hydrogen bonds near the mouth of FCNTs compared to PCNTs. The structure of averaged hydrogen bonds in both sides of FCNTs are not the same, and
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this is due to the electrostatic forces from the the electric field. Besides, fluorine
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atoms due to hydrogen bonding with hydrogens of water molecules disturbed their
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partnering in making hydrogen bonds with other water molecules in the reservoir, and this will result in a disturbance in the structure of water near the entrance to
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carbon nanotubes and consequently, cause changes in physical properties of water
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near the mouth of nanotubes.
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Electrostatic simulation showed that, electrical field strength is not homogeneous all through the FCNTs length, while its pattern is nearly uniform in PCNT. This could be due to internal electrical field that is generated inside carbon nanotubes due to fluctuation of fluorine atoms at the terminals of carbon nanotubes which generates a local electrical field in the opposite of the main direction of the external electrical field (left side), and a local electrical field along the course of the main electrical 36
ACCEPTED MANUSCRIPT field (right side). These internally generated electric fields would bring disturbances in the electrostatic field along the system and results in an inhomogeneous electrical field distribution that eventually increases the difference between the DP inside and
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outside the NTs. Consequently, this makes it more favorable for water molecules to
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fill the FCNT (8, 8).
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To show how the electrical field affects the number of hydrogen bonds and also the
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density of water inside carbon nanotubes, after running all system in the same
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precondition of thermodynamics state, the number of hydrogen bonds with respect to time was calculated and also the number of molecules were derived from
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simulation using a TCL code in TkConsol of VMD software. Hydrogen bonds and
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density analysis showed (see Figure 11) that when the electric field goes up, the
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number of hydrogen bonding increases, and also the density of water inside PCNTs and FCNTs upsurge as well. The density of water inside FCNTs increased from 854
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Kg/m3 to about 886 Kg/m3 at the voltage difference of 3 Volts. When the electrical
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field increases, it will exert more internal force on atoms in the system and also change the dipole orientations which this, in turn, causes the molecules to force each other to open more spaces. Due to these extra forces, the system will be expanded and this will push water molecules to surge inside nanotubes more than when sensing no potential difference. As it can be seen, fluorination of CNTs considerably affects the number of hydrogen bonds inside carbon nanotubes. 37
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Figure 11. Water molecules density and hydrogen bonds inside FCNT and PCNT with chirality of (8, 8) and length of 2nm. Density inside FCNT is more than PCNT and also hydrogen bonds inside FCNTs were more than PCNT.
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The instantaneous number of water molecules inside FCNT and the number of water
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molecules were determined for this nanotube and demonstrated in Figure 12. For
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different values of electric field, this parameter has been calculated using a TCL code in VMD software. It clearly demonstrates the effect of electric field on the
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number of water molecule inside FCNTs and also gives a general picture of the
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averaged presence of water inside these nanotubes. Figure 12, demonstrates how this type of functionalized nanotubes could be used for water storage applications.
38
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Figure 12. (a): Axial density of water along the centerline of carbon nanotubes, (b): The instantaneous flow of water inside FCNT. The effect of electric field on the increasing pattern of total number of water molecules inside these nanotubes is demonstrated. When water molecules enter into the FCNT and PCNT their density will be reduced from 1000 kg/m3 to about 850 kg/m3. (Vertical dashed line: the entrance of carbon nanotubes)
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According to Figure 12-b, beyond a specific electric field, the number of water molecules inside FCNT do not change considerably, which is a very important result
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when the water storage inside the nanotubes are concerned. Also, it is demonstrated
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(Figure 12-a), that the density of water inside nanotubes will be suddenly decreased
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from 1000 kg/m3 in the reservoir to about 850 kg/m3 in carbon nanotubes. The water density in FCNT (8, 8) was more than that PCNT (8, 8) in the condition of V=1.4 Volt and V=0 Volt. Recently it was demonstrated that the central reason for occupation of inner space of carbon nanotubes by water molecules was the electrostatic potential energy difference between the water molecules inside CNTs and reservoir [94]. Figure 6 shows that the fluorination of CNTs has affected the 39
ACCEPTED MANUSCRIPT electrostatic potential energy distribution along the CNT and this has introduced a non-uniformity in the system which caused a potential difference inside the carbon nanotubes. This difference will make the more potential difference between water
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inside and the water in the reservoir, for this feature causes more water occupation
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inside the FCNT.
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In another attempt, the effect of an electrical field on the orientation of water
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molecules dipoles is investigated. The main reason behind this study, is to
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understand the sensitivity of water molecules orientation toward external electrical field. To do so, dipole orientation of water was calculated and averaged in a cubic
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of 16×16×4 Å 3 (x,y and z direction) along the carbon nanotubes length. Before
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applying electric field along the FCNT central line, there was some fluctuations in
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the main course of water molecules dipoles. After applying 1.2 V voltage difference
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along the horizontal axis of FCNT, with ignorance of some small fluctuations, the vector of water dipole has directed along the CNTs centerline. Figure 13 shows how
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the electrical field changes the orientations of water molecules. The effect of electric field on the changing of water molecules dipole was stronger in the middle of FCNT and near the terminals, however, some fluctuations happened, which are related to effects of fluorine atoms on the water molecules. This could be also due to the strength of fluorine atoms on hydrogen bonding with water molecules. Li Zhang et al, [95], studied the effects of placing charges on two terminals of boron-nitride 40
ACCEPTED MANUSCRIPT nanotubes (BNNTs), on water flux. They concluded that the electrostatics interaction forces at the mouth of BNNTs cause a retardation in the process of transportation of water molecules. Without applying the external electric field, there were not the
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external force and the water molecules spent more time at the mouth of BNNTs to
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change their orientation for entering and exiting to/from nanotubes at both terminals.
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This influences the dynamics of water molecules which slow down their movability
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to enter the nanopore.
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Figure 13. Effect of electric field on the dipole direction and water molecule orientation. The analysis was applied to FCNT for a voltage difference of 0 and 1.2 V. (a): PCNT (8,8) , (b): FCNT (8,8)
The considerable changes in the course of dipole moment in water molecules happened which is shown in Figure 13 which could be analyzed by considering the relation between the induced-dipole and the external electric field. The dipole and electric field interaction are related as follow: 41
ACCEPTED MANUSCRIPT ⃗⃗⃗ = −𝑀 ⃗⃗ ∙ ⃗⃗⃗⃗⃗⃗⃗⃗ 𝑊 𝐸𝑒𝑥𝑡
(3)
Equation (2) indicates that with an increment in the external electric field strength (Eext), the dipole-field interaction energy (W) increases. This parameter addresses
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the organization of the system to some extent. Electric field causes a decrement in
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the dielectric constant of the water medium and also it increases the amount of total
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energy of the system. One reason for the higher flux of water under the action of the
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electrical field could be due to the effect of the electrical field on the total energy of the system. By intensifying the strength of the electrical field the dipole-field
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interaction energy of the system increases and therefore it causes an increment in the
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total energy of the system. Then this added energy origin more vibration and motion
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of water molecules which could flow them inside carbon nanotubes. Also by
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enhancing the external electric field, this effects on the local electric field, and increase it. Therefore, by exerting an electric field on the system, the electrostatic
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force in every part of the system increases and this will exert a local force on the
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molecules. Figure 13 demonstrates the influence fluorine atoms on the dipole orientation at the gates of FCNTs, furthermore it shows that the electrical field has more effect on organization of water inside PCNTs. In better words there isn’t the disturbances originated from fluorine atoms; therefore, water molecules responded to electrical field without any disruption.
42
ACCEPTED MANUSCRIPT Electric field affects the orientation and internal energy of the system considerably. In order to have a deeper understanding of the structural geometry of water when is confined in carbon nanotubes, the radial distribution function (RDF) analysis is
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performed which is beneficial in understanding the structur and arrangement of
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water inside carbon nanotubes with and without the external electric field. The RDF
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analysis could clarify the structure of water inside CNTs in terms of layering and the
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magnitudes of radial density. As Figure 14 demonstrates the RDF analysis of watercarbon atoms, two water shell layer happened inside carbon nanotubes for all voltage
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values.
Figure 14. Radial distribution function (RDF) analysis of water inside a) PCNT (8, 8) with corresponding peaks at the right and b) FCNT (8, 8) with corresponding peaks at right. The radial density of water molecules inside FCNT is dominantly more than PCNTs in the same value of electric field.
43
ACCEPTED MANUSCRIPT Figure 14 demonstrates the layered structure of water inside FCNT and PCNT for different values of the electrical field. The first layer of water happened around 3.33.5 Å from the carbon layer inside nanotubes. The second layer of water molecules
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was created almost at 7.9-8.2 Å from the carbon layer. At higher electric field the
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RDF value was more and this was mainly due to denser water arrangement inside
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these carbon nanotubes. However, there were some fluctuations in RDF values for
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FCNTs when the voltage of 1.8 V was applied. Same as other publications, [41,43,51,60,74,75,76,77 ], The RDF value increased when the electrical field
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enhanced for both PCNTs and FCNTs regardless of some fluctuations in some parts
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of nanotubes. The principal reason for such increment in the RDF of water-carbon
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inside carbon nanotubes were attributed to the accumulative existence of water inside FCNTs and PCNTs [41, 43]. This proves that density will increases as
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electrical field enhances.
The results and discussions outlined above put us one step closer to understand the
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water permeation through fluorinated carbon nanotubes under action of electrical field. It was proved that there is not an increasing trend in water flux pattern inside carbon nanostructure even when it is functionalized. The fast-paced acceleration in manufacturing of nano and micro devices in recent years have made us hopeful about application of these membranes in near future. In terms of this research, there is a demand for a more precise, accurate and specifically focused study on unraveling 44
ACCEPTED MANUSCRIPT the physics behind the hydrogen dynamics of FCNTs in interaction with electrical field stimuli since it play an important role in water transport inside nanopore under electrical field.
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Conclusion
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Fluorinated carbon nanotubes were studied for the purpose of removal of heavy metals from water. The system consists of Hg2+ and Zn2+ which studied for both
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PCNT and FCNT in different electric field values. It was concluded that fluorinated
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carbon nanotubes are a good transporter of water molecules and also heavy metals
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cations. This was supported by doing water flux analysis and ions transportation
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study. FCNTs had a greater influence on increment of water flux in all electrical field values. Stimulatingly, with increment of voltage difference from 0 to 3 V, the
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water flux hit a peak of 378 H2O/ns (at 2 Volts) and 415 H2O/ns at (at 2.4 Volts) for
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PCNTs and FCNTs, respectively. This might be the prominent benefit of FCNTs
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over PCNTs in terms of higher water flux in higher electrical field values. It is shown that the electrostatic energy difference was the main reason of higher water flux in FCNTs compared to the water transport in PCNTs. FCNTs facilitated the transports of cations more appropriately compared to PCNTs and its records for Hg 2+ was more than Zn2+ in all electrical fields which the PMF study justified this trend in ion migration. According to the results, FCNTs are the best choice when we want to
45
ACCEPTED MANUSCRIPT gather all cations in one side of the system and also the best reservoir for water storage purposes compared to PCNTs. All in all, FCNTs are introduced for the purposes of heavy metal removal when at the same time a high water transport rate
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is regarded.
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Data availability
The raw/processed data required to reproduce these findings cannot be shared at this
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time as the data also forms part of an ongoing study. However, for running any
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similar works and getting the NAMD input files, the reader could ask the
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corresponding author by Email.
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Acknowledgment
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The Faculty of New Sciences and Technologies at University of Tehran is highly appreciated, for providing the authors with high speed computational facility for
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conducting this research.
References
[1] Alby D., Charnay C., Heran M., Prelot B., Zajac J.Recent developments in nanostructured inorganic materials for sorption of cesium and strontium: Synthesis and shaping, sorption capacity, mechanisms, and selectivity—A review, Journal of Hazardous Materials, Volume 344, 2018 46
ACCEPTED MANUSCRIPT [2] Fenglian Fu, Qi Wang, Removal of heavy metal ions from wastewaters: A review, Journal of Environmental Management, Volume 92, Issue 3, 2011, pp. 407-418
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[3] Ayhan Demirbas, Heavy metal adsorption onto agro-based waste materials: A review, Journal of Hazardous Materials, Volume 157, Issues 2–3, 2008, pp. 220-229
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[4] Anita Singh, Rajesh Kumar Sharma, Madhoolika Agrawal, Fiona M. Marshall, Health risk assessment of heavy metals via dietary intake of foodstuffs from the wastewater irrigated site of a dry tropical area of India, Food and Chemical Toxicology, Volume 48, Issue 2, 2010, pp. 611-619
AN
US
[5] Ping Zhuang, Murray B. McBride, Hanping Xia, Ningyu Li, Zhian Li, Health risk from heavy metals via consumption of food crops in the vicinity of Dabaoshan mine, South China, Science of The Total Environment, Volume 407, Issue 5, 2009, pp. 1551-1561
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[6] Monu Arora, Bala Kiran, Shweta Rani, Anchal Rani, Barinder Kaur, Neeraj Mittal, Heavy metal accumulation in vegetables irrigated with water from different sources, Food Chemistry, Volume 111, Issue 4, 2008, pp. 811-815
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[7] Fabrication of carboxylated cellulose nanocrystal/sodium alginate hydrogel beads for adsorption of Pb(II) from aqueous solution, International Journal of Biological Macromolecules, Volume 108, 2018
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[8] Sang-Jo Kim, Hyun-Kyung Lee, Abimbola C. Badejo, Won-Chan Lee, Hyo-Bang Moon Species-specific accumulation of methyl and total mercury in sharks from offshore and coastal waters of Korea, Marine Pollution Bulletin, Volume 102, Issue 1, 2016, pp. 210-215 [9] Tetsuya Endo, Yohsuke Hisamichi, Koichi Haraguchi, Yoshihisa Kato, Chiho Ohta, Nobuyuki Koga, Hg, Zn and Cu levels in the muscle and liver of tiger sharks (Galeocerdo cuvier) from the coast of Ishigaki Island, Japan: Relationship between metal concentrations and body length Marine Pollution Bulletin, Volume 56, Issue 10, 2008, pp. 1774-1780
47
ACCEPTED MANUSCRIPT [10] The removal of fluoride from aqueous solution by a lateritic soil adsorption: Kinetic and equilibrium studies, Ecotoxicology and Environmental Safety, Volume 149, 2018
IP
T
[11] Patricia Giovanella, Lucélia Cabral, Fátima Menezes Bento, Clesio Gianello, Flávio Anastácio Oliveira Camargo, Mercury (II) removal by resistant bacterial isolates and mercuric (II) reductase activity in a new strain of Pseudomonas sp. B50A, New Biotechnology, Volume 33, Issue 1, 2016, pp. 216-223
US
CR
[12] Karen A. Merritt, Aria Amirbahman, Mercury methylation dynamics in estuarine and coastal marine environments — A critical review, Earth-Science Reviews, Volume 96, Issues 1–2, 2009, pp. 54-66
AN
[13] Lee S., Kim D.-H., Kim K.-W., The enhancement and inhibition of mercury reduction by natural organic matter in the presence of Shewanella oneidensis MR-1, Chemosphere, Volume 194, 2018
ED
M
[14] Mahbub K., Krishnan K., Naidu R., Megharaj M., Development of a whole-cell biosensor for the detection of inorganic mercury, Environmental Technology and Innovation, Volume 8, 2017
CE
PT
[15] Efficient, stable and selective adsorption of heavy metals by thiofunctionalized layered double hydroxide in diverse types of water, Chemical Engineering Journal, Volume 332, 2018
AC
[16] Kyzas G.Z., Deliyanni E.A., Bikiaris D.N., Mitropoulos A.C., Graphene composites as dye adsorbents: Review, Chemical Engineering Research, and Design, Volume 129, 2018 [17] Sheng L., Jin Y., He Y., Huang Y., Yan L., Zhao R., Well-defined magnetic surface imprinted nanoparticles for selective enrichment of 2,4dichlorophenoxyacetic acid in real samples, Talanta, Volume 174, 2017 [18] Lu F., Astruc D., Nanomaterials for removal of toxic elements from water, Coordination Chemistry Reviews, Volume 356, 2018
48
ACCEPTED MANUSCRIPT [19] Removal of vanadium from wastewater by multi-walled carbon nanotubes, Fullerenes Nanotubes, and Carbon Nanostructures, Volume 25, 2017
IP
T
[20] Thines R.K., Mubarak N.M., Nizamuddin S., Sahu J.N., Abdullah E.C., Ganesan P., Application potential of carbon nanomaterials in water and wastewater treatment: A review, Journal of the Taiwan Institute of Chemical Engineers, Volume 72, 2017
US
CR
[21] K. Pillay, E.M. Cukrowska, N.J. Coville, Multi-walled carbon nanotubes as adsorbents for the removal of parts per billion levels of hexavalent chromium from aqueous solution, Journal of Hazardous Materials, Volume 166, Issues 2–3, 2009, pp. 1067-1075
ED
M
AN
[22] Shitong Yang, Jiaxing Li, Dadong Shao, Jun Hu, Xiangke Wang, Adsorption of Ni(II) on oxidized multi-walled carbon nanotubes: Effect of contact time, pH, foreign ions and PAA, Journal of Hazardous Materials, Volume 166, Issue 1, 2009, pp. 109-116
AC
CE
PT
[23] Park C.M., Heo J., Wang D., Su C., Yoon Y., Heterogeneous activation of persulfate by reduced graphene oxide–elemental silver/magnetite nanohybrids for the oxidative degradation of pharmaceuticals and endocrine disrupting compounds in water, Applied Catalysis B: Environmental, Volume 225, 2018 [24] Wang C., Yang F., Tang Y., Yang W., Zhong H., Yu C., Li R., Zhou H., Li Y., Mao L.Graphene quantum dots nanosensor derived from 3D nanomesh graphene frameworks and its application for fluorescent sensing of Cu2+in rat brain, Sensors and Actuators, B: Chemical, Volume 258, 2018 [25] Zhao Q., Zhu X., Chen B., Stable graphene oxide/poly(ethyleneimine) 3D aerogel with a tunable surface charge for high-performance selective
49
ACCEPTED MANUSCRIPT removal of ionic dyes from water, Chemical Engineering Journal, Volume 334, 2018
CR
IP
T
[26] Stolyarchuk N.V., Kolev H., Kanuchova M., Keller R., Vaclavikova M., Melnyk I.V., Synthesis and sorption properties of bridged polysilsesquioxane microparticles containing 3-mercaptopropyl groups in the surface layer, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 538, 2018
AN
US
[27] Hashemi B., Zohrabi P., Raza N., Kim K.-H., Metal-organic frameworks as advanced sorbents for the extraction and determination of pollutants from environmental, biological, and food media, TrAC - Trends in Analytical Chemistry, Volume 97, 2017
CE
PT
ED
M
[28] Akbar Bagheri, Mohsen Taghizadeh, Mohammad Behbahani, Ali Akbar Asgharinezhad, Mani Salarian, Ali Dehghani, Homeira Ebrahimzadeh, Mostafa M. Amini, Synthesis and characterization of magnetic metal-organic framework (MOF) as a novel sorbent, and its optimization by experimental design methodology for determination of palladium in environmental samples, Talanta, Volume 99, 2012, pp. 132-139
AC
[29] De Velasco-Maldonado P.S., Hernández-Montoya V., Montes-Morán M.A., Vázquez N.A.-R., Pérez-Cruz M.A., Surface modification of a natural zeolite by treatment with cold oxygen plasma: Characterization and application in water treatment, Applied Surface Science, Volume 434, 2018
[30] Bartczak P., Klapiszewski Ł., Wysokowski M., Majchrzak I., Czernicka W., Piasecki A., Ehrlich H., Jesionowski T., Treatment of model solutions and wastewater containing selected hazardous metal ions using a chitin/lignin hybrid material as an effective sorbent, Journal of Environmental Management, Volume 204, 2017 50
ACCEPTED MANUSCRIPT
T
[31] Jason K. Holt,*Aleksandr Noy, Thomas Huser, David Eaglesham, andOlgica Bakajin, Fabrication of a Carbon Nanotube-Embedded Silicon Nitride Membrane for Studies of Nanometer-Scale Mass Transport, Nano Letters, 2004, 4 (11), pp 2245–2250
US
CR
IP
[32] Burakov A.E., Galunin E.V., Burakova I.V., Kucherova A.E., Agarwal S., Tkachev A.G., Gupta V.K., Adsorption of heavy metals on conventional and nanostructured materials for wastewater treatment purposes: A review, Ecotoxicology and Environmental Safety, Volume 148, 2018
ED
M
AN
[33] K.S.HuiC.Y.H.ChaoS.C.Kot, Removal of mixed heavy metal ions in wastewater by zeolite 4A and residual products from recycled coal fly ash, Journal of Hazardous Materials, Volume 127, Issues 1–3, 9 December 2005, Pages 89-101
AC
CE
PT
[34] Sun N., Zhang Y., Ma L., Yu S., Li J., Preparation and characterization of chitosan/purified attapulgite composite for sharp adsorption of humic acid from aqueous solution at low temperature, Journal of the Taiwan Institute of Chemical Engineers, Volume 78, 2017
[35] HaiboPeng, MengliSun, FengfeiLiu, DiYang, DuofeiZhang, WeiYuan, XinDu, HaoChen, LanxiWang, TieshanWang, Potential effect on the interaction of highly charged ion with graphene, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Volume 407, 15 September 2017, Pages 291-296
[36] Queiroga J.A., Souza D.F., Nunes E.H.M., Silva A.F.R., Amaral M.C.S., Ciminelli V.S.T., Vasconcelos W.L., Preparation of alumina tubular 51
ACCEPTED MANUSCRIPT membranes for treating sugarcane vinasse obtained in ethanol production, Separation and Purification Technology, Volume 190, 2018
CR
IP
T
[37] Gustavo A. Rivas, María D. Rubianes, Marcela C. Rodríguez, Nancy F. Ferreyra, Guillermina L. Luque, María L. Pedano, Silvia A. Miscoria, Concepción Parrado, Carbon nanotubes for electrochemical biosensing, Talanta, Volume 74, Issue 3, 15 December 2007, pp. 291-307
AN
US
[38] Rizzuto C., Pugliese G., Bahattab M.A., Aljlil S.A., Drioli E., Tocci E.,Multiwalled carbon nanotube membranes for water purification, Separation and Purification Technology, Volume 193, 2018
PT
ED
M
[39] Zhang X., Wang Z., Tang C.Y., Ma J., Liu M., Ping M., Chen M., Wu Z., Modification of microfiltration membranes by alkoxysilane polycondensation induced quaternary ammonium compounds grafting for biofouling mitigation, Journal of Membrane Science, Volume 549, 2018
AC
CE
[40] YoungbinBaek, CholinKim, Dong KyunSeo,, Tae-woo Kim, Jeong SeokLee, YongHyupKim, Kyung, Hyun Ahn, Sang, SeekBae, Sang, CheolLee, JaelimLim, KyunghyukLee, Jeong Yoon, High performance, and antifouling vertically aligned carbon nanotube membrane for water purification, Journal of Membrane Science, Volume 460, 15 June 2014, Pages 171-177
[41] Azamat J., Khataee A., Removal of nitrate ion from water using boron nitride nanotubes: Insights from molecular dynamics simulations, Computational and Theoretical Chemistry, Volume 1098, 2016
52
ACCEPTED MANUSCRIPT [42] Tripathy M.K., Jena N.K., Samanta A.K., Ghosh S.K., Chandrakumar K.R.S., Theoretical investigations on Zundel cation present inside boronnitride nanotubes: Effect of confinement and hydrogen bonding, Chemical Physics, Volume 446, 2015
US
CR
IP
T
[43] Azamat J., Khataee A., Joo S.W., Molecular dynamics simulation of trihalomethanes separation from water by functionalized nanoporous graphene under induced pressure, Chemical Engineering Science, Volume 127, 2015
M
AN
[44] A. Panahi, M. H. Sabour, Electrokinetics desalination of water using fluorinated carbon nanotubes embedded in silicon membrane: Insights from molecular dynamics simulation, Chemical Engineering Science, Volume 173, 14 December 2017, Pages 60-73
CE
PT
ED
[45] Sheikhi M., Shahab S., Khaleghian M., Kumar R., Interaction Between New Anti-cancer Drug Syndros and CNT(6,6-6) Nanotube for Medical Applications: Geometry Optimization, Molecular Structure, Spectroscopic (NMR, UV/Vis, Excited state), FMO, MEP and HOMO-LUMO Investigation, Applied Surface Science, Volume 434, 2018
AC
[46] Tao Feng, Mingming Li, Jingjie Zhou, Haining Zhuang, Feng Chen, Ran Ye, Osvaldo Campanella, Zhongxiang Fang, Application of molecular dynamics simulation in food carbohydrate research—a review, Innovative Food Science & Emerging Technologies, Volume 31, 2015, pp. 1-13
[47] Kowalczyk P., Miyawaki J., Azuma Y., Yoon S.-H., Nakabayashi K., Gauden P.A., Furmaniak S., Terzyk A.P., Wisniewski M., Włoch J., Kaneko K., Neimark A.V., Molecular simulation aided nanoporous carbon design for 53
ACCEPTED MANUSCRIPT highly efficient low-concentrated formaldehyde capture, Carbon, Volume 124, 2017
CR
IP
T
[48] Matsumoto H., Tsuruoka S., Hayashi Y., Abe K., Hata K., Zhang S., Saito Y., Aiba M., Tokunaga T., Iijima T., Hayashi T., Inoue H., Amaratunga G.A.J., Water transport phenomena through membranes consisting of vertically-aligned double-walled carbon nanotube array, Carbon, Volume 120, 2017
M
AN
US
[49] Fatemeh Khalili-Araghi, James Gumbart, Po-Chao Wen, Marcos Sotomayor, Emad Tajkhorshid, Klaus Schulten, Molecular dynamics simulations of membrane channels and transporters, Current Opinion in Structural Biology, Volume 19, Issue 2, 2009, pp. 128-137
PT
ED
[50] Hannah Ebro, Young Mi Kim, Joon Ha Kim, Molecular dynamics simulations in membrane-based water treatment processes: A systematic overview, Journal of Membrane Science, Volume 438, 2013, pp. 112-125
AC
CE
[51] Hosseini M., Azamat J., Erfan-Niya H., Improving the performance of water desalination through ultra-permeable functionalized nanoporous graphene oxide membrane, Applied Surface Science, Volume 427, 2018
[52] Thierry O. Wambo, Roberto A. Rodriguez, Liao Y. Chen, Computing osmotic permeabilities of aquaporins AQP4, AQP5, and GlpF from nearequilibrium simulations, Biochimica et Biophysica Acta (BBA) Biomembranes, Volume 1859, Issue 8, 2017, pp. 1310-1316
54
ACCEPTED MANUSCRIPT [53] Zhang X.-X., Li X.-H., Chen M., Role of the electric double layer in the ice nucleation of water droplets under an electric field, Atmospheric Research, Volumes 178-179, 2016
CR
IP
T
[54] Dong Hyun Jung, Jung HwanYang, Mu Shik Jhon, The effect of an the external electric field on the structure of liquid water using molecular dynamics simulations, Chemical Physics, Volume 244, Issues 2–3, 15 June 1999, Pages 331-337
M
AN
US
[55] C.E. López-Plascencia, M. Martínez-Negrete-Vera, R. GaribayAlonso, Reactive force field study of the molecular structure of water under thermal and electric effects: Water splitting phenomenon, International Journal of Hydrogen Energy, Volume 42, Issue 8, 2017, pp. 4774-4781
PT
ED
[56] In-Chul Yeh and Max L. Berkowitz, the Dielectric constant of water at high electric fields: Molecular dynamics study, The Journal of Chemical Physics 110, 7935 (1999)
AC
CE
[57] Breton M., Mir L.M., Investigation of the chemical mechanisms involved in the electropulsation of membranes at the molecular level, Bioelectrochemistry, Volume 119, 2018
[58] Xinfu Xia and Max L. Berkowitz, Electric-Field Induced Restructuring of Water at a Platinum-Water Interface: A Molecular Dynamics Computer Simulation, Phys. Rev. Lett. 74, 3193 – Published 17 April 1995
55
ACCEPTED MANUSCRIPT [59] Wang Y., He Z., Gupta K.M., Shi Q., Lu R., Molecular dynamics study on water desalination through functionalized nanoporous graphene, Carbon, Volume 116, 2017
CR
IP
T
[60] Jafar Azamat, Alireza Khataee, Sang Woo Joo, Functionalized graphene as a nanostructured membrane for removal of copper and mercury from aqueous solution: A molecular dynamics simulation study, Journal of Molecular Graphics and Modelling, Volume 53, 2014, pp. 112-117
AN
US
[61] Sadollah Ebrahimi, Influence of curvature on water desalination through the graphene membrane with a Si-passivated nanopore, Computational Materials Science, Volume 124, 2016, pp. 160-165
PT
ED
M
[62] Soumitra Kar, R.C. Bindal, P.K. Tewari, Carbon nanotube membranes for desalination and water purification: Challenges and opportunities, Nano Today, Volume 7, Issue 5, 2012, pp. 385-389
AC
CE
[63] Chang Hoon Ahn, Youngbin Baek, Changha Lee, Sang Ouk Kim, Suhan Kim, Sangho Lee, Seung-Hyun Kim, Sang Seek Bae, Jaebeom Park, Jeyong Yoon, Carbon nanotube-based membranes: Fabrication and application to desalination, Journal of Industrial and Engineering Chemistry, Volume 18, Issue 5, 2012, pp. 1551-1559
[64] P.S. Goh, A.F. Ismail, B.C. Ng, Carbon nanotubes for desalination: Performance evaluation and current hurdles, Desalination, Volume 308, 2013, pp. 2-14
56
ACCEPTED MANUSCRIPT [65] F. Chamssedine, K. Guérin, M. Dubois, E. Disa, E. Petit, Z. El Fawal, A. Hamwi, Fluorination of single-walled carbon nanotubes at low temperature: Towards the reversible fluorine storage into carbon nanotubes, Journal of Fluorine Chemistry, Volume 132, Issue 12, 2011, pp. 1072-1078
CR
IP
T
[66] Jatindranath Maiti, Nitul Kakati, Seok Hee Lee, Young Soo Yoon, Fluorination of multiwall carbon nanotubes by a mild fluorinating reagent HPF6, Journal of Fluorine Chemistry, Volume 135, 2012, pp. 362-366
M
AN
US
[67] Mi-Seon Park, Kyung Hoon Kim, Young-Seak Lee, Fluorination of single-walled carbon nanotube: The effects of fluorine on structural and electrical properties, Journal of Industrial and Engineering Chemistry, Volume 37, 2016, pp. 22-26
PT
ED
[68] E.T.Mickelsona, Huffman Rinzlerab, Smalleyab Haugea, J.L.Margrave, Fluorination of single-wall carbon nanotubes, Chemical Physics Letters, Volume 296, Issues 1–2, 30 October 1998, Pages 188-194
AC
CE
[69] Z. Gu, H. Peng, R. H. Hauge, R. E. Smalley, andJ. L. Margrave*, Cutting Single-Wall Carbon Nanotubes through Fluorination, Nano Letters, 2002, 2 (9), pp 1009–1013, DOI: 10.1021/nl025675
[70] Kay Hyeok An, Jeong Goo Heo, Kwan Goo Jeon, Dong Jae Bae, Chulsu Jo, Cheol Woong Yang, Chong-Yun Park, and Young Hee Lee, X-ray photoemission spectroscopy study of fluorinated single-walled carbon nanotubes, Appl. Phys. Lett. 80, 4235 (2002)
57
ACCEPTED MANUSCRIPT [71] Masafumi Kobayashi, Tetsuya Inoguchi, Takashi Iida, Takashi Tanioka, Hiroshi Kumase, Yasushi Fukai, Development of direct fluorination technology for application to materials for lithium battery, Journal of Fluorine Chemistry, Volume 120, Issue 2, 1 April 2003, pp. 105-110
CR
IP
T
[72] Jae-Ho Kim, Susumu Yonezawa, Masayuki Takashima, Reaction between cerium trifluoride and elemental fluorine, Journal of Fluorine Chemistry, Volume 120, Issue 2, 1 April 2003, pp. 111-116
M
AN
US
[73] Min-Jung Jung, Euigyung Jeong, Young-Seak Lee, The surface chemical properties of multi-walled carbon nanotubes modified by thermal fluorination for the electric double-layer capacitor, Applied Surface Science, Volume 347, 2015, pp. 250-257
CE
PT
ED
[74] JafarAzamat, Alireza Khataee, Sang WooJoo, Separation of copper and mercury as heavy metals from aqueous solution using functionalized boron nitride nanosheets: A theoretical study, Journal of Molecular Structure, Volume 1108, 15 March 2016, Pages 144-149
AC
[75] Jaber Jahanbin Sardroodi, Jafar Azamat, Alireza Rastkar, Negar Rad Yousefnia, The preferential permeation of ions across carbon and boron nitride nanotubes, Chemical Physics, Volume 403, (2012), pp. 105-112 [76] Jafar Azamat, Ali Balaei, Mehdi Gerami, A theoretical study of nanostructure membranes for separating Li+ and Mg2+ from Cl−, Computational Materials Science, Volume 113, (2016), Pages 66-74 [77] Alireza Khataee, Golchehreh Bayat, Jafar Azamat, Molecular dynamics simulation of salt rejection through silicon carbide nanotubes as a nanostructure membrane, Journal of Molecular Graphics and Modelling, Volume 71, 2017, pp. 176-183 58
ACCEPTED MANUSCRIPT [78] W. Humphrey, A. Dalke, K. Schulten, VMD: visual molecular dynamics, J. Mol.Graphics 14 (1996) 33–38
ED
M
AN
US
CR
IP
T
[79] Gaussian 03, Revision C.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.48 Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Wallingford CT, 2004
CE
PT
[80] V. Zoete, M. A. Cuendet, A. Grosdidier, O. Michielin, SwissParam, a Fast Force Field Generation Tool For Small Organic Molecules, J. Comput. Chem, 2011, 32(11), 2359-68. PMID: 21541964, DOI: 10.1002/jcc.21816
AC
[81] L. Kalé, R. Skeel, M. Bhandarkar, R. Brunner, A. Gursoy, N. Krawetz, J. Phillips, A. Shinozaki, K. Varadarajan, K. Schulten, NAMD2: greater scalability for parallel molecular dynamics, J. Comput. Phys. 151 (1999) 283– 312. [82] A.D. MacKerell, D. Bashford Bellott, R.L. Dunbrack, J.D. Evanseck, M.J. Field, S.Fischer, J. Gao, H. Guo, S. Ha, D. Joseph-McCarthy, L. Kuchnir, K. Kuczera, F.T.K.Lau, C. Mattos, S. Michnick, T. Ngo, D.T. Nguyen, B. Prodhom, W.E. Reiher, B.Roux, M. Schlenkrich, J.C. Smith, R. Stote, J. Straub, M. Watanabe, J.Wiórkiewicz-Kuczera, D. Yin, M. Karplus, All-atom
59
ACCEPTED MANUSCRIPT empirical potential for molecular modeling and dynamics studies of proteins, J. Phys. Chem. B 102(1998) 3586–3616. [83] W.L. Jorgensen, J. Chandrasekhar, J.D. Madura, R.W. Impey, M.L. Klein, Comparison of simple potential functions for simulating liquid water, J. Chem.Phys. 79 (1983) 926–935.
CR
IP
T
[84] T. Darden, D. York, L. Pedersen, Particle mesh Ewald: a N·log (N) method forEwald sums in large systems, J. Chem. Phys. 98 (1993) 10089– 10092
US
[85] J. Azamat a, B. S. Sattary, A. Khataee, S. W.Joo, Journal of Molecular Graphics and Modelling. 61. (2015) 13–20
AN
[86] Zhu Qiang, Kan Zi-qui, Ma Jing, Electrostatic interaction of water in external electric field: molecular dynamics simulation, J. Electrochem. (2017), 23(4):391-399
ED
M
[87] V. Vijayaraghavan, C.H. Wong, Transport characteristics of water molecules in carbon nanotubes investigated by using molecular dynamics simulation, Computational Materials Science, Volume 89, 2014, pp. 36-44
CE
PT
[88] Amal Kanta Giri, Filipe Teixeira, M. Natália D.S. Cordeiro, Structure and kinetics of water in highly confined conditions: A molecular dynamics simulation study, Journal of Molecular Liquids, Volume 268, 2018, pp. 625636
AC
[89] Ritos K, Borg MK, Mottram NJ, Reese JM. Electric fields can control the transport of water in carbon nanotubes. Phil. Trans. R. Soc. A (2016) 374: 20150025 [90] Vaitheeswaran, S.; Rasaiah, J.C.; Hummer, G. Electric field and temperature effects on water in the narrow nonpolar pores of carbon nanotubes. J. Chem. Phys. 2004, 121, 7955. [91] He, Y.; Sun, G.; Koga, K.; Xu, L. Electrostatic field-exposed water in nanotube at constant axial pressure. Sci. Rep. 2014, 4, 6596
60
ACCEPTED MANUSCRIPT [92] Winarto; Takaiwa, D.; Yamamoto, E.; Yasuoka, K. Structures of water molecules in carbon nanotubes under electric fields. J. Chem. Phys. 2015, 142, 124701. [93] Fu, Z.; Luo, Y.; Ma, J.; Wei, G. Phase transition of nanotube-confined water driven by electric field. J. Chem. Phys. 2011, 134, 154507.
CR
IP
T
[94] Winarto , Eiji Yamamoto, Kenji Yasuoka, Water Molecules in a Carbon Nanotube under an Applied Electric Field at Various Temperatures and Pressures, Water, (2017), 9, 437
US
[95] Li Zhang, Jiachen Li, Guoteng Peng, Lijun Liang, Zhe Kong, JiaWei Shen, Linjie Jia, Xinping Wang, Wei Zhang, Charge-tunable water transport through boron nitride nanotubes, J. Molecular Liquids, https://doi.org/10.1016/j.molliq.2017.12.065
AC
CE
PT
ED
M
AN
[96] Alek Aksimentiev, Jeffrey Comer, Bionanotechnology Tutorial, University of Illinois at Urbana-Champaign Beckman Institute for Advanced Science and Technology, Theoretical and Computational Biophysics Group, April 2011
61
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Graphical Abstract
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FCNTs showed continuous water flow transport while the PCNTs didn’t Water flux in FCNTs and PCNTs hit a peak in a specific electric field Permeation of Zn2+ through FCNT and PCNT were more than Hg2+
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Abbas Panahi, Ali Shomali, Mohammad Hossein Sabour, Ebrahim Ghafar-Zadeh PII: DOI: Reference:
S0167-7322(18)35317-0 https://doi.org/10.1016/j.molliq.2019.01.084 MOLLIQ 10315
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
17 October 2018 11 January 2019 14 January 2019
Please cite this article as: Abbas Panahi, Ali Shomali, Mohammad Hossein Sabour, Ebrahim Ghafar-Zadeh , Molecular dynamics simulation of electric field driven water and heavy metals transport through fluorinated carbon nanotubes. Molliq (2018), https://doi.org/10.1016/j.molliq.2019.01.084
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Molecular Dynamics Simulation of Electric Field Driven Water and Heavy Metals Transport through Fluorinated Carbon Nanotubes
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Department of Electrical Engineering and Computer Science, York University, Toronto, Canada
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Department of Aerospace Engineering, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran
School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran, P. O. Box: 11155-4563
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Abbas Panahi 1, 2, Ali Shomali3, Mohammad Hossein Sabour *, 1 and Ebrahim Ghafar-Zadeh2
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Abstract
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In this study the fluorinated carbon nanotubes (FCNTs) were analyzed to investigate
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the proficiency of this type of functionalization on removal of heavy metals from
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water. Electrokinetics desalination properties of (FCNTs) embedded in silicon
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membrane has been investigated using molecular dynamics simulation. In terms of flow rate enhancement, it was shown that with increment of voltage difference from
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0 to 3 V, the water flux hit a peak of 378 H2O/ns (at 2 Volts) and 415 H2O/ns at (2.4 Volts) for FCNT and pristine carbon nanotubes (PCNT), respectively. Interestingly, FCNTs retarded the decline of flux condition which allows more water transport for higher voltages. The number of Zn2+ transport events were more than Hg2+, which shows the higher efficiency of this membranes on the removal of Zn2+ cations. Molecular dynamics simulation suggests FCNTs as a possible prospective volunteer 1
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gives the highest water flux compared to pristine carbon nanotubes.
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Keywords: Fluorinated carbon nanotubes, functionalized carbon nanotubes,
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electric field, heavy metals, molecular dynamics, water transport, fluorination Corresponding authors: Mohammad H. Sabour, PhD, Peng, Assistant Professor, Aerospace Department, Faculty of
New Sciences & Technologies, University of Tehran, North Kargar Ave., Tehran, IRAN, Zip Code: 1439955941, Tel.: (98)21-86093308, Fax: (98)21-88617081 Cell.: (98)912-727-2087 (1) 514-437-2942, Email: [email protected]
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ACCEPTED MANUSCRIPT Introduction The numerous industrial, domestic, agricultural, restorative and innovative applications of heavy metals, have increased their widespread delivery in nature that,
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altogether, rises concerns over their potential dire consequences for human health
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and the earth. There are many consumptions of these compounds in industrial activities, houses, and hospitals. All of these activities enhance the presence of heavy
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metals, for example, Cd2+, Cu2+, Ni2+, Hg2+, and Zn2+ in water which origin
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dangerous problems influencing different organs of the human body that is mainly
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due to their poisonous quality and in long-term causes cancer diseases [1-3]. Along this line, through basic research and practical endeavors, new trend in the eradication
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of these heavy metals from the water were under intense considerations.
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Extreme industrial activities in these days, will leave dangerous materials like heavy
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metals through various procedures which all of these materials before going through
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filtration steps will be introduced to nature and eventually come into human food chain [4-7]. The routines of Hg2+ ions to become dangerous in nature is smooth and may cause many harms to the environment. Its vapor and the existence of its natural substance in nature, significantly impact the nature of the soil and its quality in a long period of times [8-9]. The final products of such chemical reactions and evolutions such as oxidation, will results in divalent mercury [10]. On the other hand, 3
ACCEPTED MANUSCRIPT due to the existence of some sorts of microbes in the soil, through some chemical reactions, dangerous forms of mercury will be produced [11]. For this reason and to eliminate harmful heavy metals, intensive researches have focused on some of these
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contaminations, such as arsenic, cadmium, chromium, lead, and mercury [12-17].
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Regarding efficient desalination of water, the most important component is a
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membrane which should be designed to eliminate almost all ions from water. Some
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research works utilized organic and inorganic materials which predominantly they
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focused on CNTs [17-22], graphene [23-25], and metal-organic framework materials [25-28]. Among all mentioned materials, carbons nanotubes have grabbed a great
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attention in term of their viable applications in membranes for desalination of water,
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both through experiments and also computational studies [29-30]. There is also a
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tradeoff between a high water flux and the highest possible ions rejection. However,
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some types of above-mentioned membrane materials, do not meet all of these expectations [25, 24, 28]. Whereas CNTs have revealed a high water flow rate and
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acceptable ion rejection potential, the fabrication of a CNT based membrane have been remained as a challenging procedure which needs advanced Nano/micro machining processes [31]. From the height of other standpoints, some 3D materials like zeolite showed very good ion rejection characteristics but low water flux [3133]. CTFs (covalent triazine frameworks) demonstrated to be good at conducting water but very low in rejecting ions [34]. While graphene nanomaterials used as 4
ACCEPTED MANUSCRIPT water desalination membranes, and showed the same water flux features in parallel with demonstrating better ion rejection features compared to CTFs [34]. Thanks to recent nanofabrication processes that made it possible to create nanopore in
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graphene monolayer using highly energetic heavy ions hitting on its surface [35].
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This valuable achievements has leveled the way for further research on the usage of
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these nanomaterials in gas separation [36] and the most interesting one, DNA
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sequencing [37].
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Between all the membranes, it has been proved and accepted that CNTs are fast transporters of water and also in some range of diameter completely ban ions
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passages [38-40]. A lot of research works addressed the appropriateness of these
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nanomaterials for water treatment applications [39-40]; furthermore, Boron nitride
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nanotubes were also reported to be a volunteer for these purposes [41-43].
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Notwithstanding that some researchers have studied the functionalized CNTs under different conditions [44-46], some aspects of water physical characteristics inside
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extremely confined spaces like CNTs, however, are still unclear [47-48]. Water molecules, to become motivated to flux in larger volume inside CNTs, needs motivation. These stimulations could be even pressure difference [49-51], concentration difference, [52], and last but not least electric field [53-55]. Electric field has some interesting effects on the water molecules when they are confined or are in dynamic near nanomaterials such as carbon nanotubes, carbon Nano sheet and 5
ACCEPTED MANUSCRIPT etc. Some of these effects are electro freezing phenomena in the confined wall, [5558], and concentration-induced electric field inside cell membranes [59]. Pressure difference was used to derive water molecules in different works while
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examining different nanotubes, such as carbon nanotubes (CNTs) or boron nitride
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nanotubes (BNNTs), and it was concluded that higher pressure difference could
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increase water transport considerably [60-61]. Taking in mind the nature of polar
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features of water molecules, the impacts of the electric field, nevertheless, needs
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more study to be completely grasped.
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Some reports covered functionalized carbon nanotubes for desalination of water and removal of heavy metals from water [61-64]. Fluorinated carbon nanotubes were
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also investigated for the removal of desalination purposes using electric fields [44];
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furthermore, the fluorinated carbon nanomaterials nanopore were reported in some
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other research works [44].
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There have been considerable research works on fluorinated carbon nanotubes both experimentally and theoretically [65-68]. Fluorine atoms improve the solubilization of CNTs in water. Due to the properties of fluorine atom bounds with carbon atoms, it has become the target of research for exploring the possible application of fluorine functionalization of carbon nanotubes in desalination and other applications. The
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ACCEPTED MANUSCRIPT fluorinated carbon nanotubes could be utilized for altering physio-chemical properties of the surface features to become more hydrophilic [66, 68]. There are numerous experimental approaches for functionalization of carbon
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nanotubes by fluorine groups [68-73]. In recent studies, molecular dynamics
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simulation was used as an efficient method to discover the effectiveness of
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nanofiltration membranes in water desalination technologies regarding both
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electrokinetics and pressure difference methods. Different nanoporous membranes
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fabricated with carbon, boron nitride, and silicon carbon nanotubes embedded in silicon or silicon nitride membranes have been extensively researched in recent
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years. Furthermore, the effectiveness of carbon nanotubes with different functional
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groups was established in presence of hydrostatic pressure difference and electric
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field. Molecular dynamics simulation, due to its potential in giving an atomic insight
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into the transportation of ions through nanomembranes is of high importance in designing novel membranes and evaluates their efficiency without excess expenses
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on experimental costs. Electric field in various research works has been used to investigate the transport of different ions through carbon nanotubes, boron nitride nanotubes, boron nitride nanosheets and graphene nanoporous. [73-75]. Azamat et al investigated separation of Li+, Mg2+, and Cl- through nanostructure membranes using electric field in which the carbon nanotubes transported more water molecules as electric field increased. The results showed that Li+ and Mg2+ ions permeated 7
ACCEPTED MANUSCRIPT through (7, 7) nanotubes while Cl- ions didn’t. It was revealed that ion permeation with different ratios through (7, 7) nanotubes was fulfilled in the presence of an electric field; furthermore, it was demonstrated to be selective [76, 77].
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Herein, fluorinated carbon nanotubes were examined for removal of heavy metals
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from water. FCNTs were put inside a silicon membrane and the solution were under
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the action of the horizontal electric field to derive water and ions. Voltage applied
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on both side of system, varied from 0 to 3 V for PCNTs and FCNTs with same radii
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and length. This study with different simulation data paves the way for potential application of FCNTs as a new membrane for eliminating heavy metals from water,
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therefore its potential application will be in desalination and waste water treatment.
Figure 1. Snapshot of sub-nanometer fluorinated carbon nanotubes simulated for transport study of heavy metals and water solution. A) pristine carbon nanotubes, b) fluorinated carbon nanotubes, c) fluorinated CNTs embedded in silicon membrane, d) fluorine atoms at the gates
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ACCEPTED MANUSCRIPT 1. Computational Methods In this paper, we have scrutinized the transport properties of water molecules through both functionalized and pristine carbon nanotubes. The FCNTs were
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functionalized with fluorine atoms at both ends of carbon nanotubes. We used CNTs
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with (8, 8) chirality which corresponds to radii of 0.82 nm. The FCNTs and PCNTs
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were created with VMD, [78], and then their structure were optimized using
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Gaussian 03, with the B3LYP level of the theory and 6-31G basis sets; the main
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reason for minimizing the system with Gaussian was to design the most appropriate diameter for silicon nanopore before MD simulation. This is due to this fact that if
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the carbon atoms be very close to silicon atoms, an error happens in running the
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molecular dynamics, therefore optimizing the nanotubes help us to choose the most
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appropriate diamter of silicon nanopore [79]. The output “mol2” structure file from
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Gaussian were used as input file in online SwissParam website to find unknown parameters and force fields, that its computation method is reported in [80]. These
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parameters are listed in Table 1. After creating the nanotubes, they were embedded in a silicon membrane. The thickness of the silicon membrane was assumed to be 1.6 nm. The simulation system is demonstrated in Figure 2.
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Figure 2. The simulation box including two water boxes with a dimension of 5nm×5nm×10nm at both sides of a silicon slab with dimensions of 5nm×5nm×1.6nm. In each side, HgCl2 and ZnCl2 were solvated with certain concentrations and the electric field was applied along the z-axis of the simulation box. Each water boxes contains 10000 water molecules and 0.35 M ZnCl2 or HgCl2 ions.
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Figure 2 demonstrates the simulated system that consists of a silicon membrane in
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which the nanotubes (FCNTs and PCNTs) are located at the middle of the
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membrane. At both sides of the membrane, two water boxes are located which contain 0.35 M HgCl2 and 0.35 M ZnCl2. The system for each of these solutions
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(HgCl2+Water or ZnCl2+Water) was simulated separately, in better words there
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exists only Hg2+ or Zn2+ in solution, and in order to evaluate the system for getting
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their characteristics, the process of each simulation was repeated three times and the results were averaged to ensure the accuracy of results. The water boxes size was 5nm×5nm×10nm in x, y, and z-directions, consequently the overall dimension of system is 5nm×5nm×21.6nm including the membrane at the middle. Also, the electric field was applied horizontally and corresponding voltage difference changed from 0 to 3V.
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ACCEPTED MANUSCRIPT Table 1. Potential parameters for FCNT and PCNT used in the simulation CNT
FCNT
V(bond)=Kb(b-b0)2
C-C
F-C
b0 (A)
1.400
1.3420
Kb(kcal/mol. A2)
469.000
468.572
V(angle)=Kθ (θ-θ0)
C-C-C
C-C-F
θ0 (degree)
35.00
118.0650
Kθ(kcal/mol. rad2)
120.00 C-C-C-C
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[1+cos(nΦ- Φ0)]
78.730
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V(dihedral)=KΦ
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Potential Parameters
C-C-C-F
180.00
180.00
n
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Φ0
KΦ (kcal/mol) ε (Kcal/mol)
3.5
C
F
0.0860
0.135000
1.910
1.630000
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Rmin/2 (A֯)
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V(LJ)
3.100
Charges
C (C-F) = 0.19
F (C-F)= -0.19
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In Table 1, the simulation parameters are listed. The simulation box dimensions were
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5×5×21.6 nm in x, y, and z-directions, respectively. The number of water molecules
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in each water boxes were the same (about 10000 molecules in each side), that a representation of the simulated system is shown in Figure 2. To explore the properties of CNTs for ion conduction, all of two water boxes with a length of about 10 nm were subjected to solvation of 0.35 M MgCl2 and ZnCl2 ions in each one for every simulation. In order to prevent the simulation box to move around and prevent the undesirable vibrations, its center of mass was set to a constant coordinate. The
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ACCEPTED MANUSCRIPT periodic boundary condition (PBC) was applied to all directions and the area of water boxes (along z-direction) was held constant. There were no interatomic bonds between silicon and carbon atoms; however, there were electrostatic interactions and
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also Lennard Jones (LJ) interactions. The charges of carbon atoms in CNTs were
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zero, and the carbon atom’s charges at two terminals of FCNTs were calculated to
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be C=0.19 and F= -0.19.
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To study the properties of fluorinated carbon nanotubes for removal of heavy metals
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from water, a molecular dynamics simulation was initiated to examine the ionrejection properties of both pristine and fluorinated carbon nanotubes. Here, NAMD
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2.9, [81], was applied for MD simulation in which CHARMM 27, [82], force field
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used to calculate the interaction forces. Although it is of paramount importance to
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choose an appropriate force field for simulation of carbon nanotubes, particularly,
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when it comes to determine their mechanical properties and characteristics, but in this simulation the mechanical properties of both PCNTs and FCNTs are not
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regraded, and their vibration do not affect the water transport considerably; therefore, we have treated the carbon nanotubes with CHARMM flexible force fields like other research works which considered this force field for investigating the water transport inside CNTs [74-77]. First, carbon nanotubes were constructed then embedded inside a silicon slab. Afterward, the system was placed between two water boxes which were neutralized with 0.35 M MgCl2 and ZnCl2. For modeling the water 12
ACCEPTED MANUSCRIPT molecules in this simulation, TIP3P, [83], model was used in which the HOH angle is 104.52° and partial charges were +0.417 for hydrogen and -0.834 for oxygen respectively. After the minimization process, the equilibration of the system was
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performed for 4 ns and at 300 K temperature, then the results of the simulation were
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considered for the rest 16 ns.
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In all dimensions PBC was applied and also the particle mesh Ewald (PME)
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summation, [84], was used with a grid size of 1 Å for long-range electrostatic
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interactions. The cutoff equal to 12 Å applied for van der Waals interactions. Also a 14.5 Å pairlistdist distance was assigned to search only within this distance for atoms
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which may interact by electrostatic or van der Waals interactions. In all simulations,
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the cutoff distance was set to 12 Å, with the aim of minimizing the overall simulation
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time. The Langevin dynamics is used to appoint a constant temperature in a NVT
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ensemble in minimization time in which the Langevin coupling coefficient, assigned to be 5/ps. After the minimization process, the molecular dynamics was conducted
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in NPT ensemble throughout of the simulation time. Furthermore, for in order to prevent the system undergoing a change from liquid to a solid phase, or from liquid to gaseous phase, all simulations were performed at 300 K. Each system was simulated for 20 ns at 300 K employing an electric field in the z-direction, with voltage variation ranging from 0 to 3 V. In this simulation 20 ns MD runs including
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ACCEPTED MANUSCRIPT the biasing electric field was done which the equilibration phase lasted for 4 ns that after this period of time, 16 ns production run started. In NAMD electric field is modeled as an external force which is added to non-
𝑉 𝐿𝑧
(1)
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𝐸 = −23.0605492
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bonded potential energy as an extra term, [85], which is defined as:
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Where, E, represents electric field (in kcal/mol Å), V is for equivalent potential
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differences (in volts) and 𝐿𝑧 symbolizes the size of the system along the z-axis (in Angstrom), respectively. We should consider that 1 kcal. mole-1 A-1 e-1 = 4.35×108
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V/M [96].
To understand the energy barriers in front of cations and onions when passing
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through membranes, a PMF analysis has been conducted which clearly demonstrates
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why ions transport events in FCNTs are remarkably different from PCNTs. The PMF
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analysis is very important when the ion transport inside nanopore is under investigation. The PMF for ion could be demonstrated by Wi (z) which is calculated by integrating the force exerted on ions when are passing through the nanopore which is reversible work done by ions in the system when are passing through nanotubes based on thermodynamics integration method for calcualting PMF.
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(2)
𝑊𝑖 (𝑧) − 𝑊𝑖 (𝑧0 ) = − ∫ 〈𝐹𝑖 (𝑧)𝑑𝑧〉
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𝑧0
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In which z0 represents the reference position of the ions. Along the length of carbon
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nanotubes and at different positions, the mean force distribution was calculated by
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sampling the force experienced by ions. To achieve that, the ions moved through location from -10 Å to 20 Å in 0.2 Å increments (the length of each window in
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reaction coordinate) and the z component of the ions was held by a 16 kcal mol/Å2
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harmonic constraints, whereas the ion was free to move radially in each window.
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Before starting any simulation, the ions are fixed in a specific position for about 20
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fs, for be completely surrounded by water molecules.
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3. Results and Discussions
Molecular dynamics simulation has been conducted to investigate the transport of
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water and heavy metals through both fluorinated and pristine carbon nanotubes. The permeation of water molecules and ions through both of the nanotubes (PCNTs and FCNTs) were analyzed. Molecular dynamics results showed that heavy metals in a specific value of electric field start to break the energy barriers at the gate of CNT and pass through the nanotubes, which continuous of this trend is not possible
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ACCEPTED MANUSCRIPT without the external electric field. This type of functionalization (fluorination) is of high importance, if the goal be gather all heavy metals ions just in one side. Fluorination, from one prospective promotes water flow that is mainly because of
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creating hydrogen bonds with water molecules at the entrance of carbon nanotubes
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and also causes the fast departure of molecules from another side. To understand
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the flow of water through both types of nanotubes, diagrams of water flow versus
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time is demonstrated in Figure 3 which clarifies the spontaneous flow of water versus time. The simulated system contains 0.35 M of zinc chloride and mercury
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chloride solutions in two water boxes placed at the sides of a carbon nanotube
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embedded in a silicon hole with diameter of 1.2 nm. In another try, a membrane with
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functionalized pore was considered for analysis. An external electric field applied along the carbon nanotube’s longitudinal axis, which was perpendicular to the
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horizontal projected area of CNTs, and the size (diameter) of a nanopore in the
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membrane was around 1.2nm which is adequate for transporting Zn2+, Hg 2+ and Cl-
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ions. The important result from this simulation is that by applying the electric field, ions start to permeate from the functionalized pore after a specific threshold of electric field. In relevance to this study, Zhu Qiang et al, [86], using molecular dynamics simulation studied the effect of electric field on water molecules and concluded that the electric field considerably changes the orientation of water molecules dipole moment and eventually change the value of dielectric constant. 16
ACCEPTED MANUSCRIPT They also reported that by increasing the electric field, the dipole-field interaction energy increases which are the induced energy from the external field on water molecules. This energy intensifies the internal energy of molecules, which lead to
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more intermolecular interaction and could be a possible reason for the higher flux of
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water in confined spaces. The increment of electric field does not increase the water
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flux in all ranges and this could be studied by hydrogen bonding analysis on the
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system along CNT’s horizontal axis. For a better understanding of the effect of fluorination on water transport, we have examined the hydrogen bonding, ion
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transport, water flux, water density, water occupation inside CNTs, the potential of
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mean force (PMF) and radial distribution function (RDF).
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3.1 Water flow inside carbon nanotubes
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In this study electric field was used for exploring the water and heavy metals solution
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transport inside pristine and fluorinated carbon nanotubes. This phenomenon that electric field is directly applied to move aqueous solution through nanopore is called,
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electroosmotic flow (EOF) which is happening by making a voltage difference between two electrodes placed at both sides of the system. This voltage difference is produced by placing the anode and cathode at each side of the membrane and altering the voltage to apply the external driving force for a solution to migrate through nanpore. In EOF systems, it is needed that a clear description of the transportation phenomena be presented, because it deals with the infinitesimal 17
ACCEPTED MANUSCRIPT transport of water and ions through a small region that having a control over this area, will affect the overall transport considerably. Having a better control of such small system when the only way is to control the pressure, may become a great
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challenge. However, using electric field for such regulation could be done precisely
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as our results and other research works show [87, 88]. Pressure difference needs
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micro-instruments that their fabrication and installing in the system will impose a
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great difficulty on the construction of the whole system. Moreover, the problems associated with the uniformity of flow profile in pressure-induced nanoflow
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channels are the other disadvantages of osmotic systems [54, 55]. Here by means of
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molecular dynamics simulation, the water, and heavy metal ions were simulated for
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analysis of their transport characteristics inside carbon nanotubes which the system undergoes extremely confined spaces. In micro and nanofluidic systems, pristine and
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functionalized carbon nanotubes could be used for different purposes, wherever the
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fast flow of liquids are regarded.
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Molecular dynamics simulation shows this interesting result that fluorinated carbon nanotubes are a better transporter of water molecules compared to pristine carbon nanotubes. In this simulation, the electric field has been used as a motivation for the water molecules, to surge inside carbon nanopore. When water molecules are feeling an electric field, their dipole moment undergoe changing in its main direction and this is sensed by all molecules in solution that orders their direction toward this field. 18
ACCEPTED MANUSCRIPT While changing their course of dipoles at the gate of nanopore, where all molecules facing a potential barrier for moving inside which needs more enrergy. In order to model these systems for application in nanofluidic systems, it is vital that we have a
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time-dependent record of water flow, since this shows how the fluid transports with
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time. Molecular dynamics results of the water flow inside both PCNTs and FCNTs
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reveal an important flow pattern.
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In all electric fields that are applied horizontally along the centerline of NTs, FCNTs
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showed a continuous flow of water, whereas pristine carbon nanotubes in some occasions completely showed zero flow, as it is demonstrated in Figure 3. FCNTs
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due to their different charges with heavy metals ions, absorbs Zn2+ and Hg2+ near the
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mouth of carbon nanotubes and lower the potential barrier in front of ions to move
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inside. This, in turn, causes a uniform flow of water inside. Also, there are evidences
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that the effects of electric field on water flow in FCNTs are considerably more than PCNTs. This could be due to the effects of fluorine atoms induced electric field
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which is ascribed to the changes in their overall dipole moment course. Recently it was discussed by researchers, [44], that the residence of ions at the mouth of nanotubes is the main cause of flow interruption while in FCNTs, due to the pulling effects from fluorine atoms at the gate, ions surge inside nanotubes and the flow will become continues.
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Figure 3. Water flow versus time. Diagram of water molecules permeation through fluorinated and
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pristine carbon nanotubes in the presence of horizontal electric field. Electrical field are applied horizontally along the centerline of nanotubes and its value maintained constant in simulation time.
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FCNTs showed approximately non-zero flow in the presence of electrical field, but the same did
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not happened for PCNTs that it undergone abrupt zero-flow.
This could be seen in Figure 3 where the flow of water in the presence of electrical fields were gone through a non-interrupted pattern, while the same cannot be said for PCNTs. The other reason for such difference in flow pattern is the locallyinduced electrical field by fluorine atoms vibration at the gate which is discussed in following chapters.
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ACCEPTED MANUSCRIPT Having a precise look at the Figure 3 reveals that the water flow versus time for different electric field values on the system, specifically, when E-field is zero. It has experienced a nearly increasing trend in water flow but with ignorance of some
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fluctuations. Nonetheless, the water in PCNTs abruptly falls down to zero and then
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increases. This is the main advantage of FCNTs over PCNTs in the condition of
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existing no external stimuli. In better words, without any external force and by only
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the intermolecular interactions, fluorinated CNTs could transport water molecules more than carbon nanotubes with same radii and length. One possible prediction
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about the mechanism of water flow enhancement in FCNTs could be that fluorine
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atoms’ vibrations is responsible for such phenomena that gives energy to water
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molecules and also makes a small electric field near the mouth, as a result, motivates more water inside whilst making disturbances at the entrance. However, the same
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was not happened for higher values of electric fields, in which the electrical field
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plays the main rule in water transport. In many works, [41, 43, 51, 60], this was
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proved that electric field can increases the water flow through nanopore like CNTs and BNNTs when is applied on the system. Azamat et al concluded that by increasing electric field, the flow will be increased, they reported that in CNT (8,8), flow can be increased with the increment of electric field from 1 Kcal/mole A.e to 5 Kcal/mole A.e [60].
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ACCEPTED MANUSCRIPT In this study, for all values of voltage differences, the flow of water inside FCNTs were more than PCNTs, and this is an important advantage of functionalization with fluorine atoms which make us sure in terms of higher water flux. In Figure 4, for
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better judgment about the effect of fluorination, the mean flux has been calculated
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and is shown versus applied electric field. It is conspicuous that for all values of
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electric field the water flux through FCNTs are more than PCNTs, yet the fluorine
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atoms at the entrance of nanotubes decrease the entrance efficient diameter. The water flux trend in Figure 4, showed a peak, more interestingly, the peak for FCNTs
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were higher and happened at the higher electrical fields value. This results motivated
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us to find its main reason which directed us to put the light on the physics of water
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interaction with carbon nanotubes where embraces many contributory factors. Regarding a similar and relevant work, Konstantinos Ritos et al, [89], in a thought-
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provoking work, studied the effects of electric field on water transport inside carbon
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nanotubes. They reported that when water is going to pass through PCNTs under the
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action of electric field, the water flux will be increased as the electric field is intensified until a certain value of electric field, beyond which, the water flux profile will start to decline. They justified their result with considering the interconnected influences of hydrogen bonding dynamics and water molecules density. They argued that, this peak depends on the density and the hydrogen bonding before the entrance of water molecules inside CNTs. In their discussions it was investigated that the 22
ACCEPTED MANUSCRIPT sudden decrease of density and increase of hydrogen bonds will results in solidification of water at the gate of nanotubes and suddenly decrease the water flux [89]. Here in this work, when the carbon atoms at the entrance are fluorinated, the
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hydrogen atoms of water molecules start to hydrogen bonding with fluorine atoms
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and this will interrupt the hydrogen bonding network of water and always retain us
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sure that this number will not upsurge to form a complete structure of ice.
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Subsequently, the decrement of water flux will be retarded to a higher electric field which could help to, both, having a control on transport and directing more water
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through nanopore by applying greater electrical fields. According to simulation
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evidences, the surge in water flux and its declining trend, were mainly due to the fast
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and sudden jump of a cation into the nanotubes and also long residence of cations at
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the mouth of carbon nanochennel, respectively [89].
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Recent studies showed that the effects of electric field on changing the direction of water molecules orientation cause its anomalous behavior, and shift the equilibrium
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state toward filling carbon nanotubes [90]. The electric field causes a uniform water structure inside carbon nanotubes and results in an ordered structure which induces the liquid-to-solid phase transition [91-93]. These phenomena have been recognized as the underlying causes behind appearing a peak in the water flow pattern in nanotubes.
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Figure 4. Averaged water flux through fluorinated and pristine carbon nanotubes when electric field is applied to the system. The flow will reach a peak and then start to decline. FCNTs has
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caused a retardation in the lessening of water flux compared to PCNTs. FCNTs showed higher water transport capability compared to PCNTs even after the maximum peak which after that the
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go through a continual increment.
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flow will start to hit its minimum or plateau. The water flux versus electrical field will not always
The interaction between water molecules and CNTs plays an important role in
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physics of higher flux of water (compared to not application of electric field) when
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electric field is applied. Winarto et al. [94], studied a system of PCNT (8, 8) between graphene sheets, with and without the action of electric field. They concluded that the Lennard Jones (LJ) potential energy of water molecules inside CNT is lower than the water molecules in the reservoir which this energy difference facilitates the water transport inside PCNTs. According to a report of Wintaro [94], the LJ potential energy inside PCNT (8, 8) increased from 4.71 kj/mol to 17.98 kj/mol while it 24
ACCEPTED MANUSCRIPT decreased from 19.57 kj/mol to 19.51 kj/mol in the reservoir. When electric field (1 V/nm) was applied along the PCNT (8,8), the columbic potential energy difference (ΔU) changed from 14.86 kj/mole (no electric field) to -10.02 kj/mol which this
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implies that when the system undergoes an electric field the electrostatic interaction
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is the only driving force (the dominant one) for water molecules to surge inside
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PCNT (8,8). The other important contributory factor affecting the water flow inside
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carbon nanotubes is the dipole potential (DP) energy. This energy exists when an electric field is applied to the system, and has a linear dependence on the strength of
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electric field. When the electric field is applied, the dipole vibration in the reservoir
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is higher than the water molecules inside PCNTs and therefore the DP in the
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reservoir is greater than the DP inside PCNT, consequently, this difference motivates
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the water molecules to move inside carbon nanotubes [94].
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The other possible implication is that, having in mind the connection of fluorine atoms to carbon atoms at the entrance of nanotubes and their free vibrations, a
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locally-generated electrical field will be produced which is in reverse direction of main electric field, therefore this internal electric field reduce the external-induced electric field in the neighborhood, and enhance the DP potential energy difference. Thus, the water molecule finds it more favorable to move inside FCNT, and due to the opposite effect rising from fluorine atom at the counterpart exit FCNT gate, will go out faster. This is presented as an inhomogeneous distribution of electric potential 25
ACCEPTED MANUSCRIPT which is discussed in following chapters (see Figure 6). Contemplating more on the derived data by Winarto, The DP energy is lower than columbic potential energy, therefore, the difference in electrostatic energy in PCNT is the dominant motive for
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the fast occupation of carbon nanotubes by water when electric field is applied.
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Figure 5 demonstrates the LJ and electrostatic potential energy difference of water
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molecules inside the PCNT and FCNT with the water in the reservoir. As it shows,
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the LJ potential difference in FCNTs, when the system experiences no electrical field
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(E=0), is higher than the PCNT (8, 8) which is in tune with findings of Wintaro et al [94]. This could be the reason why the water flux in FCNTs is higher than PCNTs
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when there is no external electrical field. The ΔU (LJ) of PCNT (8, 8) is about 22.45
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kj/mol while it is approximately 23.97 kj/mol for FCNT (8, 8).
Figure 5. The Lennard johns and electrostatic potential energy difference between the water molecules inside carbon nanotubes and the water molecules in reservoir. (a): The Lennard johns (LJ) potential energy difference (∆ LJ), (b): electrostatic potential energy difference (∆U)
26
ACCEPTED MANUSCRIPT Figure 5-b depicts the electrostatic energy difference for a system under electrical field (V=1.2 Volts) in which FCNTs and PCNTs are under consideration. As it is evident, the potential energy difference between inner water inside PCNT (8,8) is
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more than FCNT (8,8) which leads to this results that the electrostatic effects of
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fluorine atoms on the terminals of FCNTs and their effects on the water inside and
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water in the reservoir has resulted in a decrease in electrostatic potential energy. For
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PCNT (8, 8) the averaged electrostatic potential energy difference is about 20.84 kj/mol while the corresponding value for FCNT (8, 8) is -11.15 kj/mol. The energy
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analysis of system showed that the differences in electrostatic energy has influenced
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the transport events of water considerably and one possible explanation could be this
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energy variations.
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To have a better understanding of the electrical field effects on electrostatic
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distribution in CNTs, an electrostatic study has been done to measure the values of the electrical field, in the vicinity of CNTs and more importantly, at the middle plane
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of its symmetric axis. The results of electrostatic distribution are shown in Figure 6. As it can be seen, some disturbances has been occurred in the middle of carbon nanotubes that is due to effects of fluorine atoms at the gates.
27
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Figure 6. The electrostatic profiles of the electrically induced system which shows the fluctuations of the electrical fields near the entrance of FCNTs and a regular pattern of electrostatic energy distribution in the same value of an the electrical field in PCNT.
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The electrical field distribution is somehow uniform in PCNT but it is not unvarying
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along the FCNTs. This might be due to effects of the locally-generated electrical
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field from fluorine atoms at both ends of FCNT that disturbed the homogeneity of
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its distribution that influence the flow of water and hydrogen bonding.
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In order to demonstrate the effect of fluorinations on water transport, the density counters of water when passing through PCNT and FCNT for two different electric
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fields is calculated and demonstrated in Figure 7. It shows that the water density for FCNTs in higher electric field value is more than PCNTs that proves the higher flux of water through FCNTs reported in Figure 4. In FCNTs, denser water arrangement could be found near the entrance, at the middle and in the vicinity of nanotubes walls of FCNTs compared to PCNTs. The density profiles show that in a specific electric field the water storage inside FCNTs is considerably higher than PCNTs which open 28
ACCEPTED MANUSCRIPT the discussion for investigating the properties of these NTs for water storage applications. It should be noted that all densities are calculated based on the NAMD standard which is NH2O/A3. In other words, these graphs show the averaged occupied
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space (AOS) of water inside and outside the NTs. As it is clear, there are two water
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layer inside PCNTs and FCNTs which one is a line of water molecules along the
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centerline of nanotubes and the outer is a circular ring of water molecules. This
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presented at the final section of this article.
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results is supported with radial distribution function (RDF) outcomes which is
Figure 7. The averaged spatial occupation of water inside PCNT (8, 8) and FCNT (8, 8) for different electric field values. Here the higher water occupation density near the nanotubes entrance is evident. (a) PCNT (8, 8), V=1.6V- (b) FCNT (8, 8), V=1.8V
In order to evaluate the effect of functionalization on the removal of heavy metals, a study has been conducted on the overall permeation of ions through NTs. 29
ACCEPTED MANUSCRIPT Considering the PBC on the system, only the number of ions transports which permeated for one time is calculated, and the rest were omitted from calculations. The ions were solvated in the water and then the system was initialized under
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temperature of 300 K. It was concluded that the permeation of Zn2+ cations through
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FCNTs were more than the permeation of Hg2+ ions through PCNTs and this shows
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that this membrane could be used if we want to gather all ions in one side of water
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boxes (see Figure 8). However, by increment of electric field, the overall permeation of ions increased which could be due to the higher force of electric field on ions. Hg
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cations do not transport through NTs below 1.4 V voltage difference. The number
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of Zn2+ ions which permeated successfully was more than the Hg2+ numbers in the
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same conditions of thermodynamics state and the external forces. Azamt et al, also witnessed the increment of ions transport through NTs with enhancing the electric
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field strength [41,43]. The origins of higher flux capability of FCNTs, when are
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subjected to the external electric field, has been also reported by Panahi and Sabour
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[44]. The effectiveness of FCNTs comes to attention when we want to force to accumulate all ions in one side and in one reservoir. The molecular dynamics helps us to clarify the physics and also evaluate the efficient electrical field for complete transportation. According the results in Figure 8, the effect of the external electrical field showed that increment of the applied voltage from 0 to 3 V, could intensify the Zn2+ transport through FCNTs which it increased from 4 in 1.2 voltage difference to 30
ACCEPTED MANUSCRIPT about 15 permeation events in 3 V. The diagram apparently demonstrates that in all range of electrical field the permeation events of Zn cations were more than Hg cations both for PCNTs and FCNTs. Approximately there is a linear dependency
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between voltage and ions permeation which is of high importance regarding the ionic
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current analysis and conductivity analysis of nanopore, but they are not presented
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here because of having nearly same trend as ions migration records.
Figure 8. The effect of the the electrical field on heavy metals transport through FCNTs and PCNTs in the same precondition of the external electrical field and also thermodynamics. The transport records for Zn2+ were more than Hg2+ in all electrical fields. The FCNTs showed lower restriction toward allowing cations to migrate to other box at the other side. There is somehow a linear dependency between voltage and the number of ions permeated along nanopore.
This is extensively discussed in the research papers, that water and ions during their endeavor to move inside the carbon nanotubes, will face a potential barrier, and in 31
ACCEPTED MANUSCRIPT order to move inside, accordingly, they need to overcome this potential fence which has been generated due to the effects of small size effects [41,42,44]. To calculate the amount of energy barrier that all molecules should break to move inside, the
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potential of mean force has to be calculated; afterwards, this should be mapped along
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the central axis of carbon nanotubes to show the potential along z distances. In
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other words, the ions selectivity of nanopores could be analysed with the amount
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of energy barrier that the molecules or ions should break to surge inside. This barrier plays an important rule in designing nanofluidic devices. The potential of mean force
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(PMF) analysis, tells that how much is the discrimination of these nano-sized
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channels to different ions. For such purpose, a PMF analysis has been performed to
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investigate these properties of FCNT-Silicon and PCNT-Silicon membranes. This analysis is shown in Figure 9. In Figure 9-a the maximum of PMF for all ions in the
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system when are transporting through PCNT (8,8) and FCNT (8,8) is shown. The
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maximum energy barrier happened at the middle of carbon nanotubes because the
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system was symmetric.
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Figure 9. Potential of mean force analysis results for anions and cations when they are trying to move inside carbon nanotubes. (a) the maximum PMF for all ions at the middle of nanotubes, (b)
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the local PMF in which the Cl(-) shows the higher PMF value, that indicates, it face a great potential barrier when arrives near NTs. Hg(2+) and Zn(2+) showed different PMF value for
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entrance to NTs. Cations face lower energy barrier when face FCNTs compared to PCNTs.
The PMF analysis showed that there is a high potential barrier in front of chlorine
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ions for entering inside carbon nanotubes (PCNTs and FCNTs) and this is why there
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were no permeation events for chlorine ions. The positive sign of PMF demonstrates
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the unpreferable state of permeation and the negative PMF shows the preferable permeation for ions. PMF analysis proves the results of ions transportation phenomena of Hg and Zn permeation. The Zn cations transportation numbers are more than Hg2+ and this is because of the higher potential barrier in front of Hg compared to Zn. The maximum PMF value has been occured in the middle of CNTs and also the potential energy of permeation raise at the entrance of nanotubes. Due
33
ACCEPTED MANUSCRIPT to the existence of fluorine atoms at another side of FCNTs, the maximum PMF value at the middle of CNTs are lower than the same value for PCNTs. Also, it has some fluctuations which are mainly from both interaction with fluorine rings at both
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terminals of FCNTs. The maximum value for Zn and Hg at the middle of FCNTs
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and PCNTs are 67.254 Kcal/mol for Zg2+ permeation through PCNT, and 48.802
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Kcal/mol for Hg2+ transport through FCNT, both at the middle of nanotubes. The
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results in Figure 9, are in line with results of PMF, which this convey very important information about the efficiency of FCNT membranes. The results imply that the
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possibility of Cl-1 transport through FCNTs is considerably lower compared to
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PCNTs in the same thermodynamic conditions. Besides these interesting results,
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some basic questions rise which may spark dispute on the phenomena of water transport and the effects of the electrical field on the water and ions transport. To
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understand the effects of some aspects of the hydrogen bonding on water transport
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inside carbon nanotubes, analysis of a local distribution of hydrogen bonds inside
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carbon nanotubes, along the its center line, and under different values of the electrical field for both PCNTs&FCNTs is performed (see Figure 10). The hydrogen bonds distribution show that there are some fluctuations in hydrogen bonding before the entrance of water molecules inside the FCNTs and PCNTs.
34
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Figure 10. Hydrogen bonds axial distribution along the center line of CNTs for two values of voltage differences applied on PCNT and FCNT with chirality of (8,8) and length of 2 nm. The increment in voltage has led to growth in number of local hydrogen bonds
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Figure 10 demonstrates how the averaged hydrogen bonds are distributed along
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carbon nanotubes’ centerline under the action of the horizontal electrical field. It
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shows this fact that when the electrical field is increased, the number of water molecules inside carbon nanotubes (with regards to Figure 11) will start to rise, at
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that point, with increment in the water density, the number of hydrogen bonds will
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also increase. Nevertheless, according to the density profiles in Figure 11, the normalized number of hydrogen bonds has decreased dramatically. On the other hand, inside carbon nanotubes, by enhancing the electrical field the hydrogen bonds will be manipulated which causes a decrease in overall hydrogen bonds. This is due to the effects of the electrical field on the orientation of water molecules, which force them to be directed along the main courses of the electrical field near the entrance 35
ACCEPTED MANUSCRIPT of carbon nanotubes. Hydrogen bonding in FCNTs with V=0 V state was lower than FCNTs with V=1.2 Voltage difference strength which is due to the influence of the electrical field on the water arrangements. The main reason behind a higher number
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of hydrogen bonds for FCNTs compared to PCNTs (near the entrance of CNTs) is
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due to fluorine atoms hydrogen bonding with water hydrogens. The hydrogen bonds
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with fluorine atoms and hydrogen of water molecules has resulted in a local
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increment in hydrogen bonds near the mouth of FCNTs compared to PCNTs. The structure of averaged hydrogen bonds in both sides of FCNTs are not the same, and
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this is due to the electrostatic forces from the the electric field. Besides, fluorine
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atoms due to hydrogen bonding with hydrogens of water molecules disturbed their
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partnering in making hydrogen bonds with other water molecules in the reservoir, and this will result in a disturbance in the structure of water near the entrance to
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carbon nanotubes and consequently, cause changes in physical properties of water
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near the mouth of nanotubes.
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Electrostatic simulation showed that, electrical field strength is not homogeneous all through the FCNTs length, while its pattern is nearly uniform in PCNT. This could be due to internal electrical field that is generated inside carbon nanotubes due to fluctuation of fluorine atoms at the terminals of carbon nanotubes which generates a local electrical field in the opposite of the main direction of the external electrical field (left side), and a local electrical field along the course of the main electrical 36
ACCEPTED MANUSCRIPT field (right side). These internally generated electric fields would bring disturbances in the electrostatic field along the system and results in an inhomogeneous electrical field distribution that eventually increases the difference between the DP inside and
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outside the NTs. Consequently, this makes it more favorable for water molecules to
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fill the FCNT (8, 8).
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To show how the electrical field affects the number of hydrogen bonds and also the
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density of water inside carbon nanotubes, after running all system in the same
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precondition of thermodynamics state, the number of hydrogen bonds with respect to time was calculated and also the number of molecules were derived from
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simulation using a TCL code in TkConsol of VMD software. Hydrogen bonds and
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density analysis showed (see Figure 11) that when the electric field goes up, the
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number of hydrogen bonding increases, and also the density of water inside PCNTs and FCNTs upsurge as well. The density of water inside FCNTs increased from 854
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Kg/m3 to about 886 Kg/m3 at the voltage difference of 3 Volts. When the electrical
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field increases, it will exert more internal force on atoms in the system and also change the dipole orientations which this, in turn, causes the molecules to force each other to open more spaces. Due to these extra forces, the system will be expanded and this will push water molecules to surge inside nanotubes more than when sensing no potential difference. As it can be seen, fluorination of CNTs considerably affects the number of hydrogen bonds inside carbon nanotubes. 37
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Figure 11. Water molecules density and hydrogen bonds inside FCNT and PCNT with chirality of (8, 8) and length of 2nm. Density inside FCNT is more than PCNT and also hydrogen bonds inside FCNTs were more than PCNT.
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The instantaneous number of water molecules inside FCNT and the number of water
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molecules were determined for this nanotube and demonstrated in Figure 12. For
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different values of electric field, this parameter has been calculated using a TCL code in VMD software. It clearly demonstrates the effect of electric field on the
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number of water molecule inside FCNTs and also gives a general picture of the
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averaged presence of water inside these nanotubes. Figure 12, demonstrates how this type of functionalized nanotubes could be used for water storage applications.
38
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Figure 12. (a): Axial density of water along the centerline of carbon nanotubes, (b): The instantaneous flow of water inside FCNT. The effect of electric field on the increasing pattern of total number of water molecules inside these nanotubes is demonstrated. When water molecules enter into the FCNT and PCNT their density will be reduced from 1000 kg/m3 to about 850 kg/m3. (Vertical dashed line: the entrance of carbon nanotubes)
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According to Figure 12-b, beyond a specific electric field, the number of water molecules inside FCNT do not change considerably, which is a very important result
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when the water storage inside the nanotubes are concerned. Also, it is demonstrated
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(Figure 12-a), that the density of water inside nanotubes will be suddenly decreased
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from 1000 kg/m3 in the reservoir to about 850 kg/m3 in carbon nanotubes. The water density in FCNT (8, 8) was more than that PCNT (8, 8) in the condition of V=1.4 Volt and V=0 Volt. Recently it was demonstrated that the central reason for occupation of inner space of carbon nanotubes by water molecules was the electrostatic potential energy difference between the water molecules inside CNTs and reservoir [94]. Figure 6 shows that the fluorination of CNTs has affected the 39
ACCEPTED MANUSCRIPT electrostatic potential energy distribution along the CNT and this has introduced a non-uniformity in the system which caused a potential difference inside the carbon nanotubes. This difference will make the more potential difference between water
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inside and the water in the reservoir, for this feature causes more water occupation
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inside the FCNT.
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In another attempt, the effect of an electrical field on the orientation of water
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molecules dipoles is investigated. The main reason behind this study, is to
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understand the sensitivity of water molecules orientation toward external electrical field. To do so, dipole orientation of water was calculated and averaged in a cubic
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of 16×16×4 Å 3 (x,y and z direction) along the carbon nanotubes length. Before
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applying electric field along the FCNT central line, there was some fluctuations in
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the main course of water molecules dipoles. After applying 1.2 V voltage difference
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along the horizontal axis of FCNT, with ignorance of some small fluctuations, the vector of water dipole has directed along the CNTs centerline. Figure 13 shows how
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the electrical field changes the orientations of water molecules. The effect of electric field on the changing of water molecules dipole was stronger in the middle of FCNT and near the terminals, however, some fluctuations happened, which are related to effects of fluorine atoms on the water molecules. This could be also due to the strength of fluorine atoms on hydrogen bonding with water molecules. Li Zhang et al, [95], studied the effects of placing charges on two terminals of boron-nitride 40
ACCEPTED MANUSCRIPT nanotubes (BNNTs), on water flux. They concluded that the electrostatics interaction forces at the mouth of BNNTs cause a retardation in the process of transportation of water molecules. Without applying the external electric field, there were not the
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external force and the water molecules spent more time at the mouth of BNNTs to
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change their orientation for entering and exiting to/from nanotubes at both terminals.
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This influences the dynamics of water molecules which slow down their movability
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to enter the nanopore.
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Figure 13. Effect of electric field on the dipole direction and water molecule orientation. The analysis was applied to FCNT for a voltage difference of 0 and 1.2 V. (a): PCNT (8,8) , (b): FCNT (8,8)
The considerable changes in the course of dipole moment in water molecules happened which is shown in Figure 13 which could be analyzed by considering the relation between the induced-dipole and the external electric field. The dipole and electric field interaction are related as follow: 41
ACCEPTED MANUSCRIPT ⃗⃗⃗ = −𝑀 ⃗⃗ ∙ ⃗⃗⃗⃗⃗⃗⃗⃗ 𝑊 𝐸𝑒𝑥𝑡
(3)
Equation (2) indicates that with an increment in the external electric field strength (Eext), the dipole-field interaction energy (W) increases. This parameter addresses
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the organization of the system to some extent. Electric field causes a decrement in
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the dielectric constant of the water medium and also it increases the amount of total
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energy of the system. One reason for the higher flux of water under the action of the
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electrical field could be due to the effect of the electrical field on the total energy of the system. By intensifying the strength of the electrical field the dipole-field
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interaction energy of the system increases and therefore it causes an increment in the
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total energy of the system. Then this added energy origin more vibration and motion
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of water molecules which could flow them inside carbon nanotubes. Also by
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enhancing the external electric field, this effects on the local electric field, and increase it. Therefore, by exerting an electric field on the system, the electrostatic
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force in every part of the system increases and this will exert a local force on the
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molecules. Figure 13 demonstrates the influence fluorine atoms on the dipole orientation at the gates of FCNTs, furthermore it shows that the electrical field has more effect on organization of water inside PCNTs. In better words there isn’t the disturbances originated from fluorine atoms; therefore, water molecules responded to electrical field without any disruption.
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ACCEPTED MANUSCRIPT Electric field affects the orientation and internal energy of the system considerably. In order to have a deeper understanding of the structural geometry of water when is confined in carbon nanotubes, the radial distribution function (RDF) analysis is
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performed which is beneficial in understanding the structur and arrangement of
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water inside carbon nanotubes with and without the external electric field. The RDF
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analysis could clarify the structure of water inside CNTs in terms of layering and the
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magnitudes of radial density. As Figure 14 demonstrates the RDF analysis of watercarbon atoms, two water shell layer happened inside carbon nanotubes for all voltage
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values.
Figure 14. Radial distribution function (RDF) analysis of water inside a) PCNT (8, 8) with corresponding peaks at the right and b) FCNT (8, 8) with corresponding peaks at right. The radial density of water molecules inside FCNT is dominantly more than PCNTs in the same value of electric field.
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ACCEPTED MANUSCRIPT Figure 14 demonstrates the layered structure of water inside FCNT and PCNT for different values of the electrical field. The first layer of water happened around 3.33.5 Å from the carbon layer inside nanotubes. The second layer of water molecules
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was created almost at 7.9-8.2 Å from the carbon layer. At higher electric field the
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RDF value was more and this was mainly due to denser water arrangement inside
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these carbon nanotubes. However, there were some fluctuations in RDF values for
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FCNTs when the voltage of 1.8 V was applied. Same as other publications, [41,43,51,60,74,75,76,77 ], The RDF value increased when the electrical field
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enhanced for both PCNTs and FCNTs regardless of some fluctuations in some parts
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of nanotubes. The principal reason for such increment in the RDF of water-carbon
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inside carbon nanotubes were attributed to the accumulative existence of water inside FCNTs and PCNTs [41, 43]. This proves that density will increases as
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electrical field enhances.
The results and discussions outlined above put us one step closer to understand the
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water permeation through fluorinated carbon nanotubes under action of electrical field. It was proved that there is not an increasing trend in water flux pattern inside carbon nanostructure even when it is functionalized. The fast-paced acceleration in manufacturing of nano and micro devices in recent years have made us hopeful about application of these membranes in near future. In terms of this research, there is a demand for a more precise, accurate and specifically focused study on unraveling 44
ACCEPTED MANUSCRIPT the physics behind the hydrogen dynamics of FCNTs in interaction with electrical field stimuli since it play an important role in water transport inside nanopore under electrical field.
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Conclusion
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Fluorinated carbon nanotubes were studied for the purpose of removal of heavy metals from water. The system consists of Hg2+ and Zn2+ which studied for both
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PCNT and FCNT in different electric field values. It was concluded that fluorinated
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carbon nanotubes are a good transporter of water molecules and also heavy metals
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cations. This was supported by doing water flux analysis and ions transportation
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study. FCNTs had a greater influence on increment of water flux in all electrical field values. Stimulatingly, with increment of voltage difference from 0 to 3 V, the
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water flux hit a peak of 378 H2O/ns (at 2 Volts) and 415 H2O/ns at (at 2.4 Volts) for
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PCNTs and FCNTs, respectively. This might be the prominent benefit of FCNTs
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over PCNTs in terms of higher water flux in higher electrical field values. It is shown that the electrostatic energy difference was the main reason of higher water flux in FCNTs compared to the water transport in PCNTs. FCNTs facilitated the transports of cations more appropriately compared to PCNTs and its records for Hg 2+ was more than Zn2+ in all electrical fields which the PMF study justified this trend in ion migration. According to the results, FCNTs are the best choice when we want to
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ACCEPTED MANUSCRIPT gather all cations in one side of the system and also the best reservoir for water storage purposes compared to PCNTs. All in all, FCNTs are introduced for the purposes of heavy metal removal when at the same time a high water transport rate
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is regarded.
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Data availability
The raw/processed data required to reproduce these findings cannot be shared at this
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time as the data also forms part of an ongoing study. However, for running any
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similar works and getting the NAMD input files, the reader could ask the
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corresponding author by Email.
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Acknowledgment
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The Faculty of New Sciences and Technologies at University of Tehran is highly appreciated, for providing the authors with high speed computational facility for
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conducting this research.
References
[1] Alby D., Charnay C., Heran M., Prelot B., Zajac J.Recent developments in nanostructured inorganic materials for sorption of cesium and strontium: Synthesis and shaping, sorption capacity, mechanisms, and selectivity—A review, Journal of Hazardous Materials, Volume 344, 2018 46
ACCEPTED MANUSCRIPT [2] Fenglian Fu, Qi Wang, Removal of heavy metal ions from wastewaters: A review, Journal of Environmental Management, Volume 92, Issue 3, 2011, pp. 407-418
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[3] Ayhan Demirbas, Heavy metal adsorption onto agro-based waste materials: A review, Journal of Hazardous Materials, Volume 157, Issues 2–3, 2008, pp. 220-229
CR
IP
[4] Anita Singh, Rajesh Kumar Sharma, Madhoolika Agrawal, Fiona M. Marshall, Health risk assessment of heavy metals via dietary intake of foodstuffs from the wastewater irrigated site of a dry tropical area of India, Food and Chemical Toxicology, Volume 48, Issue 2, 2010, pp. 611-619
AN
US
[5] Ping Zhuang, Murray B. McBride, Hanping Xia, Ningyu Li, Zhian Li, Health risk from heavy metals via consumption of food crops in the vicinity of Dabaoshan mine, South China, Science of The Total Environment, Volume 407, Issue 5, 2009, pp. 1551-1561
ED
M
[6] Monu Arora, Bala Kiran, Shweta Rani, Anchal Rani, Barinder Kaur, Neeraj Mittal, Heavy metal accumulation in vegetables irrigated with water from different sources, Food Chemistry, Volume 111, Issue 4, 2008, pp. 811-815
CE
PT
[7] Fabrication of carboxylated cellulose nanocrystal/sodium alginate hydrogel beads for adsorption of Pb(II) from aqueous solution, International Journal of Biological Macromolecules, Volume 108, 2018
AC
[8] Sang-Jo Kim, Hyun-Kyung Lee, Abimbola C. Badejo, Won-Chan Lee, Hyo-Bang Moon Species-specific accumulation of methyl and total mercury in sharks from offshore and coastal waters of Korea, Marine Pollution Bulletin, Volume 102, Issue 1, 2016, pp. 210-215 [9] Tetsuya Endo, Yohsuke Hisamichi, Koichi Haraguchi, Yoshihisa Kato, Chiho Ohta, Nobuyuki Koga, Hg, Zn and Cu levels in the muscle and liver of tiger sharks (Galeocerdo cuvier) from the coast of Ishigaki Island, Japan: Relationship between metal concentrations and body length Marine Pollution Bulletin, Volume 56, Issue 10, 2008, pp. 1774-1780
47
ACCEPTED MANUSCRIPT [10] The removal of fluoride from aqueous solution by a lateritic soil adsorption: Kinetic and equilibrium studies, Ecotoxicology and Environmental Safety, Volume 149, 2018
IP
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[11] Patricia Giovanella, Lucélia Cabral, Fátima Menezes Bento, Clesio Gianello, Flávio Anastácio Oliveira Camargo, Mercury (II) removal by resistant bacterial isolates and mercuric (II) reductase activity in a new strain of Pseudomonas sp. B50A, New Biotechnology, Volume 33, Issue 1, 2016, pp. 216-223
US
CR
[12] Karen A. Merritt, Aria Amirbahman, Mercury methylation dynamics in estuarine and coastal marine environments — A critical review, Earth-Science Reviews, Volume 96, Issues 1–2, 2009, pp. 54-66
AN
[13] Lee S., Kim D.-H., Kim K.-W., The enhancement and inhibition of mercury reduction by natural organic matter in the presence of Shewanella oneidensis MR-1, Chemosphere, Volume 194, 2018
ED
M
[14] Mahbub K., Krishnan K., Naidu R., Megharaj M., Development of a whole-cell biosensor for the detection of inorganic mercury, Environmental Technology and Innovation, Volume 8, 2017
CE
PT
[15] Efficient, stable and selective adsorption of heavy metals by thiofunctionalized layered double hydroxide in diverse types of water, Chemical Engineering Journal, Volume 332, 2018
AC
[16] Kyzas G.Z., Deliyanni E.A., Bikiaris D.N., Mitropoulos A.C., Graphene composites as dye adsorbents: Review, Chemical Engineering Research, and Design, Volume 129, 2018 [17] Sheng L., Jin Y., He Y., Huang Y., Yan L., Zhao R., Well-defined magnetic surface imprinted nanoparticles for selective enrichment of 2,4dichlorophenoxyacetic acid in real samples, Talanta, Volume 174, 2017 [18] Lu F., Astruc D., Nanomaterials for removal of toxic elements from water, Coordination Chemistry Reviews, Volume 356, 2018
48
ACCEPTED MANUSCRIPT [19] Removal of vanadium from wastewater by multi-walled carbon nanotubes, Fullerenes Nanotubes, and Carbon Nanostructures, Volume 25, 2017
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[20] Thines R.K., Mubarak N.M., Nizamuddin S., Sahu J.N., Abdullah E.C., Ganesan P., Application potential of carbon nanomaterials in water and wastewater treatment: A review, Journal of the Taiwan Institute of Chemical Engineers, Volume 72, 2017
US
CR
[21] K. Pillay, E.M. Cukrowska, N.J. Coville, Multi-walled carbon nanotubes as adsorbents for the removal of parts per billion levels of hexavalent chromium from aqueous solution, Journal of Hazardous Materials, Volume 166, Issues 2–3, 2009, pp. 1067-1075
ED
M
AN
[22] Shitong Yang, Jiaxing Li, Dadong Shao, Jun Hu, Xiangke Wang, Adsorption of Ni(II) on oxidized multi-walled carbon nanotubes: Effect of contact time, pH, foreign ions and PAA, Journal of Hazardous Materials, Volume 166, Issue 1, 2009, pp. 109-116
AC
CE
PT
[23] Park C.M., Heo J., Wang D., Su C., Yoon Y., Heterogeneous activation of persulfate by reduced graphene oxide–elemental silver/magnetite nanohybrids for the oxidative degradation of pharmaceuticals and endocrine disrupting compounds in water, Applied Catalysis B: Environmental, Volume 225, 2018 [24] Wang C., Yang F., Tang Y., Yang W., Zhong H., Yu C., Li R., Zhou H., Li Y., Mao L.Graphene quantum dots nanosensor derived from 3D nanomesh graphene frameworks and its application for fluorescent sensing of Cu2+in rat brain, Sensors and Actuators, B: Chemical, Volume 258, 2018 [25] Zhao Q., Zhu X., Chen B., Stable graphene oxide/poly(ethyleneimine) 3D aerogel with a tunable surface charge for high-performance selective
49
ACCEPTED MANUSCRIPT removal of ionic dyes from water, Chemical Engineering Journal, Volume 334, 2018
CR
IP
T
[26] Stolyarchuk N.V., Kolev H., Kanuchova M., Keller R., Vaclavikova M., Melnyk I.V., Synthesis and sorption properties of bridged polysilsesquioxane microparticles containing 3-mercaptopropyl groups in the surface layer, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 538, 2018
AN
US
[27] Hashemi B., Zohrabi P., Raza N., Kim K.-H., Metal-organic frameworks as advanced sorbents for the extraction and determination of pollutants from environmental, biological, and food media, TrAC - Trends in Analytical Chemistry, Volume 97, 2017
CE
PT
ED
M
[28] Akbar Bagheri, Mohsen Taghizadeh, Mohammad Behbahani, Ali Akbar Asgharinezhad, Mani Salarian, Ali Dehghani, Homeira Ebrahimzadeh, Mostafa M. Amini, Synthesis and characterization of magnetic metal-organic framework (MOF) as a novel sorbent, and its optimization by experimental design methodology for determination of palladium in environmental samples, Talanta, Volume 99, 2012, pp. 132-139
AC
[29] De Velasco-Maldonado P.S., Hernández-Montoya V., Montes-Morán M.A., Vázquez N.A.-R., Pérez-Cruz M.A., Surface modification of a natural zeolite by treatment with cold oxygen plasma: Characterization and application in water treatment, Applied Surface Science, Volume 434, 2018
[30] Bartczak P., Klapiszewski Ł., Wysokowski M., Majchrzak I., Czernicka W., Piasecki A., Ehrlich H., Jesionowski T., Treatment of model solutions and wastewater containing selected hazardous metal ions using a chitin/lignin hybrid material as an effective sorbent, Journal of Environmental Management, Volume 204, 2017 50
ACCEPTED MANUSCRIPT
T
[31] Jason K. Holt,*Aleksandr Noy, Thomas Huser, David Eaglesham, andOlgica Bakajin, Fabrication of a Carbon Nanotube-Embedded Silicon Nitride Membrane for Studies of Nanometer-Scale Mass Transport, Nano Letters, 2004, 4 (11), pp 2245–2250
US
CR
IP
[32] Burakov A.E., Galunin E.V., Burakova I.V., Kucherova A.E., Agarwal S., Tkachev A.G., Gupta V.K., Adsorption of heavy metals on conventional and nanostructured materials for wastewater treatment purposes: A review, Ecotoxicology and Environmental Safety, Volume 148, 2018
ED
M
AN
[33] K.S.HuiC.Y.H.ChaoS.C.Kot, Removal of mixed heavy metal ions in wastewater by zeolite 4A and residual products from recycled coal fly ash, Journal of Hazardous Materials, Volume 127, Issues 1–3, 9 December 2005, Pages 89-101
AC
CE
PT
[34] Sun N., Zhang Y., Ma L., Yu S., Li J., Preparation and characterization of chitosan/purified attapulgite composite for sharp adsorption of humic acid from aqueous solution at low temperature, Journal of the Taiwan Institute of Chemical Engineers, Volume 78, 2017
[35] HaiboPeng, MengliSun, FengfeiLiu, DiYang, DuofeiZhang, WeiYuan, XinDu, HaoChen, LanxiWang, TieshanWang, Potential effect on the interaction of highly charged ion with graphene, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Volume 407, 15 September 2017, Pages 291-296
[36] Queiroga J.A., Souza D.F., Nunes E.H.M., Silva A.F.R., Amaral M.C.S., Ciminelli V.S.T., Vasconcelos W.L., Preparation of alumina tubular 51
ACCEPTED MANUSCRIPT membranes for treating sugarcane vinasse obtained in ethanol production, Separation and Purification Technology, Volume 190, 2018
CR
IP
T
[37] Gustavo A. Rivas, María D. Rubianes, Marcela C. Rodríguez, Nancy F. Ferreyra, Guillermina L. Luque, María L. Pedano, Silvia A. Miscoria, Concepción Parrado, Carbon nanotubes for electrochemical biosensing, Talanta, Volume 74, Issue 3, 15 December 2007, pp. 291-307
AN
US
[38] Rizzuto C., Pugliese G., Bahattab M.A., Aljlil S.A., Drioli E., Tocci E.,Multiwalled carbon nanotube membranes for water purification, Separation and Purification Technology, Volume 193, 2018
PT
ED
M
[39] Zhang X., Wang Z., Tang C.Y., Ma J., Liu M., Ping M., Chen M., Wu Z., Modification of microfiltration membranes by alkoxysilane polycondensation induced quaternary ammonium compounds grafting for biofouling mitigation, Journal of Membrane Science, Volume 549, 2018
AC
CE
[40] YoungbinBaek, CholinKim, Dong KyunSeo,, Tae-woo Kim, Jeong SeokLee, YongHyupKim, Kyung, Hyun Ahn, Sang, SeekBae, Sang, CheolLee, JaelimLim, KyunghyukLee, Jeong Yoon, High performance, and antifouling vertically aligned carbon nanotube membrane for water purification, Journal of Membrane Science, Volume 460, 15 June 2014, Pages 171-177
[41] Azamat J., Khataee A., Removal of nitrate ion from water using boron nitride nanotubes: Insights from molecular dynamics simulations, Computational and Theoretical Chemistry, Volume 1098, 2016
52
ACCEPTED MANUSCRIPT [42] Tripathy M.K., Jena N.K., Samanta A.K., Ghosh S.K., Chandrakumar K.R.S., Theoretical investigations on Zundel cation present inside boronnitride nanotubes: Effect of confinement and hydrogen bonding, Chemical Physics, Volume 446, 2015
US
CR
IP
T
[43] Azamat J., Khataee A., Joo S.W., Molecular dynamics simulation of trihalomethanes separation from water by functionalized nanoporous graphene under induced pressure, Chemical Engineering Science, Volume 127, 2015
M
AN
[44] A. Panahi, M. H. Sabour, Electrokinetics desalination of water using fluorinated carbon nanotubes embedded in silicon membrane: Insights from molecular dynamics simulation, Chemical Engineering Science, Volume 173, 14 December 2017, Pages 60-73
CE
PT
ED
[45] Sheikhi M., Shahab S., Khaleghian M., Kumar R., Interaction Between New Anti-cancer Drug Syndros and CNT(6,6-6) Nanotube for Medical Applications: Geometry Optimization, Molecular Structure, Spectroscopic (NMR, UV/Vis, Excited state), FMO, MEP and HOMO-LUMO Investigation, Applied Surface Science, Volume 434, 2018
AC
[46] Tao Feng, Mingming Li, Jingjie Zhou, Haining Zhuang, Feng Chen, Ran Ye, Osvaldo Campanella, Zhongxiang Fang, Application of molecular dynamics simulation in food carbohydrate research—a review, Innovative Food Science & Emerging Technologies, Volume 31, 2015, pp. 1-13
[47] Kowalczyk P., Miyawaki J., Azuma Y., Yoon S.-H., Nakabayashi K., Gauden P.A., Furmaniak S., Terzyk A.P., Wisniewski M., Włoch J., Kaneko K., Neimark A.V., Molecular simulation aided nanoporous carbon design for 53
ACCEPTED MANUSCRIPT highly efficient low-concentrated formaldehyde capture, Carbon, Volume 124, 2017
CR
IP
T
[48] Matsumoto H., Tsuruoka S., Hayashi Y., Abe K., Hata K., Zhang S., Saito Y., Aiba M., Tokunaga T., Iijima T., Hayashi T., Inoue H., Amaratunga G.A.J., Water transport phenomena through membranes consisting of vertically-aligned double-walled carbon nanotube array, Carbon, Volume 120, 2017
M
AN
US
[49] Fatemeh Khalili-Araghi, James Gumbart, Po-Chao Wen, Marcos Sotomayor, Emad Tajkhorshid, Klaus Schulten, Molecular dynamics simulations of membrane channels and transporters, Current Opinion in Structural Biology, Volume 19, Issue 2, 2009, pp. 128-137
PT
ED
[50] Hannah Ebro, Young Mi Kim, Joon Ha Kim, Molecular dynamics simulations in membrane-based water treatment processes: A systematic overview, Journal of Membrane Science, Volume 438, 2013, pp. 112-125
AC
CE
[51] Hosseini M., Azamat J., Erfan-Niya H., Improving the performance of water desalination through ultra-permeable functionalized nanoporous graphene oxide membrane, Applied Surface Science, Volume 427, 2018
[52] Thierry O. Wambo, Roberto A. Rodriguez, Liao Y. Chen, Computing osmotic permeabilities of aquaporins AQP4, AQP5, and GlpF from nearequilibrium simulations, Biochimica et Biophysica Acta (BBA) Biomembranes, Volume 1859, Issue 8, 2017, pp. 1310-1316
54
ACCEPTED MANUSCRIPT [53] Zhang X.-X., Li X.-H., Chen M., Role of the electric double layer in the ice nucleation of water droplets under an electric field, Atmospheric Research, Volumes 178-179, 2016
CR
IP
T
[54] Dong Hyun Jung, Jung HwanYang, Mu Shik Jhon, The effect of an the external electric field on the structure of liquid water using molecular dynamics simulations, Chemical Physics, Volume 244, Issues 2–3, 15 June 1999, Pages 331-337
M
AN
US
[55] C.E. López-Plascencia, M. Martínez-Negrete-Vera, R. GaribayAlonso, Reactive force field study of the molecular structure of water under thermal and electric effects: Water splitting phenomenon, International Journal of Hydrogen Energy, Volume 42, Issue 8, 2017, pp. 4774-4781
PT
ED
[56] In-Chul Yeh and Max L. Berkowitz, the Dielectric constant of water at high electric fields: Molecular dynamics study, The Journal of Chemical Physics 110, 7935 (1999)
AC
CE
[57] Breton M., Mir L.M., Investigation of the chemical mechanisms involved in the electropulsation of membranes at the molecular level, Bioelectrochemistry, Volume 119, 2018
[58] Xinfu Xia and Max L. Berkowitz, Electric-Field Induced Restructuring of Water at a Platinum-Water Interface: A Molecular Dynamics Computer Simulation, Phys. Rev. Lett. 74, 3193 – Published 17 April 1995
55
ACCEPTED MANUSCRIPT [59] Wang Y., He Z., Gupta K.M., Shi Q., Lu R., Molecular dynamics study on water desalination through functionalized nanoporous graphene, Carbon, Volume 116, 2017
CR
IP
T
[60] Jafar Azamat, Alireza Khataee, Sang Woo Joo, Functionalized graphene as a nanostructured membrane for removal of copper and mercury from aqueous solution: A molecular dynamics simulation study, Journal of Molecular Graphics and Modelling, Volume 53, 2014, pp. 112-117
AN
US
[61] Sadollah Ebrahimi, Influence of curvature on water desalination through the graphene membrane with a Si-passivated nanopore, Computational Materials Science, Volume 124, 2016, pp. 160-165
PT
ED
M
[62] Soumitra Kar, R.C. Bindal, P.K. Tewari, Carbon nanotube membranes for desalination and water purification: Challenges and opportunities, Nano Today, Volume 7, Issue 5, 2012, pp. 385-389
AC
CE
[63] Chang Hoon Ahn, Youngbin Baek, Changha Lee, Sang Ouk Kim, Suhan Kim, Sangho Lee, Seung-Hyun Kim, Sang Seek Bae, Jaebeom Park, Jeyong Yoon, Carbon nanotube-based membranes: Fabrication and application to desalination, Journal of Industrial and Engineering Chemistry, Volume 18, Issue 5, 2012, pp. 1551-1559
[64] P.S. Goh, A.F. Ismail, B.C. Ng, Carbon nanotubes for desalination: Performance evaluation and current hurdles, Desalination, Volume 308, 2013, pp. 2-14
56
ACCEPTED MANUSCRIPT [65] F. Chamssedine, K. Guérin, M. Dubois, E. Disa, E. Petit, Z. El Fawal, A. Hamwi, Fluorination of single-walled carbon nanotubes at low temperature: Towards the reversible fluorine storage into carbon nanotubes, Journal of Fluorine Chemistry, Volume 132, Issue 12, 2011, pp. 1072-1078
CR
IP
T
[66] Jatindranath Maiti, Nitul Kakati, Seok Hee Lee, Young Soo Yoon, Fluorination of multiwall carbon nanotubes by a mild fluorinating reagent HPF6, Journal of Fluorine Chemistry, Volume 135, 2012, pp. 362-366
M
AN
US
[67] Mi-Seon Park, Kyung Hoon Kim, Young-Seak Lee, Fluorination of single-walled carbon nanotube: The effects of fluorine on structural and electrical properties, Journal of Industrial and Engineering Chemistry, Volume 37, 2016, pp. 22-26
PT
ED
[68] E.T.Mickelsona, Huffman Rinzlerab, Smalleyab Haugea, J.L.Margrave, Fluorination of single-wall carbon nanotubes, Chemical Physics Letters, Volume 296, Issues 1–2, 30 October 1998, Pages 188-194
AC
CE
[69] Z. Gu, H. Peng, R. H. Hauge, R. E. Smalley, andJ. L. Margrave*, Cutting Single-Wall Carbon Nanotubes through Fluorination, Nano Letters, 2002, 2 (9), pp 1009–1013, DOI: 10.1021/nl025675
[70] Kay Hyeok An, Jeong Goo Heo, Kwan Goo Jeon, Dong Jae Bae, Chulsu Jo, Cheol Woong Yang, Chong-Yun Park, and Young Hee Lee, X-ray photoemission spectroscopy study of fluorinated single-walled carbon nanotubes, Appl. Phys. Lett. 80, 4235 (2002)
57
ACCEPTED MANUSCRIPT [71] Masafumi Kobayashi, Tetsuya Inoguchi, Takashi Iida, Takashi Tanioka, Hiroshi Kumase, Yasushi Fukai, Development of direct fluorination technology for application to materials for lithium battery, Journal of Fluorine Chemistry, Volume 120, Issue 2, 1 April 2003, pp. 105-110
CR
IP
T
[72] Jae-Ho Kim, Susumu Yonezawa, Masayuki Takashima, Reaction between cerium trifluoride and elemental fluorine, Journal of Fluorine Chemistry, Volume 120, Issue 2, 1 April 2003, pp. 111-116
M
AN
US
[73] Min-Jung Jung, Euigyung Jeong, Young-Seak Lee, The surface chemical properties of multi-walled carbon nanotubes modified by thermal fluorination for the electric double-layer capacitor, Applied Surface Science, Volume 347, 2015, pp. 250-257
CE
PT
ED
[74] JafarAzamat, Alireza Khataee, Sang WooJoo, Separation of copper and mercury as heavy metals from aqueous solution using functionalized boron nitride nanosheets: A theoretical study, Journal of Molecular Structure, Volume 1108, 15 March 2016, Pages 144-149
AC
[75] Jaber Jahanbin Sardroodi, Jafar Azamat, Alireza Rastkar, Negar Rad Yousefnia, The preferential permeation of ions across carbon and boron nitride nanotubes, Chemical Physics, Volume 403, (2012), pp. 105-112 [76] Jafar Azamat, Ali Balaei, Mehdi Gerami, A theoretical study of nanostructure membranes for separating Li+ and Mg2+ from Cl−, Computational Materials Science, Volume 113, (2016), Pages 66-74 [77] Alireza Khataee, Golchehreh Bayat, Jafar Azamat, Molecular dynamics simulation of salt rejection through silicon carbide nanotubes as a nanostructure membrane, Journal of Molecular Graphics and Modelling, Volume 71, 2017, pp. 176-183 58
ACCEPTED MANUSCRIPT [78] W. Humphrey, A. Dalke, K. Schulten, VMD: visual molecular dynamics, J. Mol.Graphics 14 (1996) 33–38
ED
M
AN
US
CR
IP
T
[79] Gaussian 03, Revision C.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.48 Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Wallingford CT, 2004
CE
PT
[80] V. Zoete, M. A. Cuendet, A. Grosdidier, O. Michielin, SwissParam, a Fast Force Field Generation Tool For Small Organic Molecules, J. Comput. Chem, 2011, 32(11), 2359-68. PMID: 21541964, DOI: 10.1002/jcc.21816
AC
[81] L. Kalé, R. Skeel, M. Bhandarkar, R. Brunner, A. Gursoy, N. Krawetz, J. Phillips, A. Shinozaki, K. Varadarajan, K. Schulten, NAMD2: greater scalability for parallel molecular dynamics, J. Comput. Phys. 151 (1999) 283– 312. [82] A.D. MacKerell, D. Bashford Bellott, R.L. Dunbrack, J.D. Evanseck, M.J. Field, S.Fischer, J. Gao, H. Guo, S. Ha, D. Joseph-McCarthy, L. Kuchnir, K. Kuczera, F.T.K.Lau, C. Mattos, S. Michnick, T. Ngo, D.T. Nguyen, B. Prodhom, W.E. Reiher, B.Roux, M. Schlenkrich, J.C. Smith, R. Stote, J. Straub, M. Watanabe, J.Wiórkiewicz-Kuczera, D. Yin, M. Karplus, All-atom
59
ACCEPTED MANUSCRIPT empirical potential for molecular modeling and dynamics studies of proteins, J. Phys. Chem. B 102(1998) 3586–3616. [83] W.L. Jorgensen, J. Chandrasekhar, J.D. Madura, R.W. Impey, M.L. Klein, Comparison of simple potential functions for simulating liquid water, J. Chem.Phys. 79 (1983) 926–935.
CR
IP
T
[84] T. Darden, D. York, L. Pedersen, Particle mesh Ewald: a N·log (N) method forEwald sums in large systems, J. Chem. Phys. 98 (1993) 10089– 10092
US
[85] J. Azamat a, B. S. Sattary, A. Khataee, S. W.Joo, Journal of Molecular Graphics and Modelling. 61. (2015) 13–20
AN
[86] Zhu Qiang, Kan Zi-qui, Ma Jing, Electrostatic interaction of water in external electric field: molecular dynamics simulation, J. Electrochem. (2017), 23(4):391-399
ED
M
[87] V. Vijayaraghavan, C.H. Wong, Transport characteristics of water molecules in carbon nanotubes investigated by using molecular dynamics simulation, Computational Materials Science, Volume 89, 2014, pp. 36-44
CE
PT
[88] Amal Kanta Giri, Filipe Teixeira, M. Natália D.S. Cordeiro, Structure and kinetics of water in highly confined conditions: A molecular dynamics simulation study, Journal of Molecular Liquids, Volume 268, 2018, pp. 625636
AC
[89] Ritos K, Borg MK, Mottram NJ, Reese JM. Electric fields can control the transport of water in carbon nanotubes. Phil. Trans. R. Soc. A (2016) 374: 20150025 [90] Vaitheeswaran, S.; Rasaiah, J.C.; Hummer, G. Electric field and temperature effects on water in the narrow nonpolar pores of carbon nanotubes. J. Chem. Phys. 2004, 121, 7955. [91] He, Y.; Sun, G.; Koga, K.; Xu, L. Electrostatic field-exposed water in nanotube at constant axial pressure. Sci. Rep. 2014, 4, 6596
60
ACCEPTED MANUSCRIPT [92] Winarto; Takaiwa, D.; Yamamoto, E.; Yasuoka, K. Structures of water molecules in carbon nanotubes under electric fields. J. Chem. Phys. 2015, 142, 124701. [93] Fu, Z.; Luo, Y.; Ma, J.; Wei, G. Phase transition of nanotube-confined water driven by electric field. J. Chem. Phys. 2011, 134, 154507.
CR
IP
T
[94] Winarto , Eiji Yamamoto, Kenji Yasuoka, Water Molecules in a Carbon Nanotube under an Applied Electric Field at Various Temperatures and Pressures, Water, (2017), 9, 437
US
[95] Li Zhang, Jiachen Li, Guoteng Peng, Lijun Liang, Zhe Kong, JiaWei Shen, Linjie Jia, Xinping Wang, Wei Zhang, Charge-tunable water transport through boron nitride nanotubes, J. Molecular Liquids, https://doi.org/10.1016/j.molliq.2017.12.065
AC
CE
PT
ED
M
AN
[96] Alek Aksimentiev, Jeffrey Comer, Bionanotechnology Tutorial, University of Illinois at Urbana-Champaign Beckman Institute for Advanced Science and Technology, Theoretical and Computational Biophysics Group, April 2011
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US
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FCNTs showed continuous water flow transport while the PCNTs didn’t Water flux in FCNTs and PCNTs hit a peak in a specific electric field Permeation of Zn2+ through FCNT and PCNT were more than Hg2+
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