nanomaterials Article
Ionic Adsorption and Desorption of CNT Nanoropes Jun-Jun Shang 1 , Qing-Sheng Yang 1, *, Xiao-Hui Yan 1 , Xiao-Qiao He 2, * and Kim-Meow Liew 2 1 2
*
Department of Engineering Mechanics, Beijing University of Technology, Beijing 100124, China; [email protected] (J.-J.S.); [email protected] (X.-H.Y.) Department of Civil and Architectural Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, China; [email protected] Correspondence: [email protected] (Q.-S.Y.); [email protected] (X.-Q.H.); Tel.: +86-10-6739-6333 (Q.-S.Y.); +852-3442-4670 (X.-Q.H.)
Academic Editor: Thomas Nann Received: 20 July 2016; Accepted: 22 September 2016; Published: 28 September 2016
Abstract: A nanorope is comprised of several carbon nanotubes (CNTs) with different chiralities. A molecular dynamic model is built to investigate the ionic adsorption and desorption of the CNT nanoropes. The charge distribution on the nanorope is obtained by using a modified gradient method based on classical electrostatic theory. The electrostatic interactions among charged carbon atoms are calculated by using the Coulomb law. It was found here that the charged nanorope can adsorb heavy metal ions, and the adsorption and desorption can be realized by controlling the strength of applied electric field. The distance between the ions and the nanorope as well as the amount of ions have an effect on the adsorption capacity of the nanorope. The desorption process takes less time than that of adsorption. The study indicates that the CNT nanorope can be used as a core element of devices for sewage treatment. Keywords: nanorope; adsorption and desorption of ions; carbon nanotubes
1. Introduction With the rapid development of industry, heavy metal pollution and its control have become the major environmental problems [1,2]. Metal plating equipment, petroleum and chemical industry, battery industry, pesticide residues, and so on are producing more and more heavy metal which directly or indirectly emit to the environment [3], especially in the developing world. Heavy metal is different from organic pollutants [4]. When in water, it is not biodegradable and easy to accumulate in organisms. As we all know, Cu2+ , Hg2+ , Cd2+ , and Pb2+ are common toxic pollutants. Water, soil [5], and biology are difficult to detoxify once polluted by the heavy metal ions. It has been proved that many heavy metal ions are poisonous and carcinogenic substances and cause serious harm to ecological environments and humans, endangering life and health. Considering these serious public health implications, how to remove heavy metal ions from water has become a major challenge confronting humanity. Moreover, the most detrimental impurities in the feed water are high molar mass substances, except metal ions and organic pollutants. When the content of ions like AsIII , AsV , F− , NO3 − , etc. exceed an acceptable level, these ions are released into the environment, which causes serious environmental and health problems. One way to remove ions in industrial wastewater is adsorption [6,7], which is simple and inexpensive. Repeated utilization is an important consideration for evaluating the stand or fall of adsorption material. It is required that the materials should not only have good adsorption ability, but also have good desorption ability [8,9]. The structure and properties of carbon nanotubes (CNTs) may make it a good adsorption material [10,11]. CNTs, as a new type of nanomaterials, not only have excellent mechanical, thermal, and electrical properties, but also possess good chemical
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and thermal stability and a large specific surface area. They have important application value in adsorbing heavy metal ions effectively and in the extraction of minerals, especially rare earth elements [12]. Ajayan et al. [13] found that CNTs can be filled with lead without inducing any damage. When applying voltage to one end of the tubes, extra charges can be attached to tubes and then charge redistribution takes place [14]. The charge properties of CNTs can vary with the amount of charge [15]. The corresponding carbon quantum wires, CNT field emission devices, nanotubes tweezers, and carbon nanotube actuators [14,15] have been developed successively. A series of results show that charged CNTs have enormous development potential [16–18]. It has been shown that, due to the additional charges in CNTs, the ionic velocity in NaCl solutions markedly changes [18]. Alijani et al. [19] came up with a new application for magnetic CNTs, namely the removal of lead ions, and the results suggest that this sorbent is suitable for the removal and solidification of metal ions from polluted environmental samples. Doubt studied the changing structure and dynamic behavior of metal salt, Ru, Bi, Ag, Pt, Pd, and biomolecules when they are encapsulated in CNTs [20]. Guo et al. [21] investigated a CNT system decorated with transition metals such as Ni, Cu, Au, and Pt. Valentini et al. [22] concluded that water molecules in CNTs can be influenced by an electric field. Water molecules are very stable and have a high efficient adsorption in electric fields. When the electric field is removed, CNTs can recover their initial state. Although the adsorption of single CNTs has been widely investigated, their size makes them difficult for industrial applications. CNT assemblies including different CNT-based nanostructures are expected to deliver the superb properties of CNTs from nano-scale to macro-scale. It is important to investigate the adsorption ability of CNT-based nanostructures. Akin to the common geometry of synthetic fiber ropes [23], a model of a CNT nanorope was built by twisting a CNT bundle. In this paper, the ionic adsorption and desorption of CNT nanoropes were realized by changing the strength of the applied electric field. Firstly, molecular dynamic models of CNT nanoropes were built. Then, the charge distribution on the nanorope was obtained by using a gradient method based on classical electrostatic theory. Finally, the adsorption and desorption of the metal ions and negative ions of the charged nanoropes were investigated. 2. Molecular Dynamics Model of Nanoropes A CNT bundle including seven CNTs was generated in the quantum mechanics software Materials Studio. The initial length in the free state was 20.168 nm. Then, the bundle was twisted around a core axis to form a nanorope by using LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code [24]. In this paper, the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential function [25–27] is used to describe the interaction between carbon atoms. The potential consists of three terms. " # 1 LJ REBO TOR E = ∑ ∑ Eij + Eij + ∑ ∑ Ekijl , 2 i j 6 =i k 6=i,j l 6=i,j,k
(1)
where EijREBO represents the reactive empirical bond-order potential developed by Brenner et al. [26], EijLJ represents the longer-ranged interactions in a form similar to the standard Lennard–Jones potential, and EijTOR stands for the torsional interaction. The simulation process made use of the NVT ensemble (the system is isolated from changes in moles, volume, and temperature). In order to avoid the effects of thermal activation, the Nose/Hoover thermostat kept the temperature of the system at 1 × 10−3 K, and the velocity obeyed the Boltzmann distribution. The time step was set to be 1 fs, and the trajectory information was output every 500 steps. Three-dimensional non-periodic boundary conditions and rotation load were applied. Under the rotation, the CNTs were self-assembled into a nanorope under the action of Van der Waals force. After the full energy relaxation (1 ns) of the system, the model of nanorope was obtained. Figure 1 shows three nanoropes including CNTs with chiralities (3, 3), (6, 6), and (9, 9).
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(3, 3)
(6, 6)
(9, 9)
Figure1.1. Nanoropes Nanoropes including including carbon carbon nanotubes nanotubes (CNTs) (CNTs) with with chiralities chiralities (3, (3, 3) 3) (left), (left), (6, (6, 6) 6) (middle), (middle), Figure and(9, (9,9) 9)(right). (right). and
The charge distribution was calculated by using a modified gradient method [28] based on The charge distribution was calculated by using a modified gradient method [28] based on classical electrostatic theory [29,30]. It is assumed that the nanorope is subjected to an ideal electric classical electrostatic theory [29,30]. It is assumed that the nanorope is subjected to an ideal electric field, and the charges induced by the electric field distribute on the surface of CNTs. For a nanorope field, and the charges induced by the electric field distribute on the surface of CNTs. For a nanorope with n atoms, the electric potential at an arbitrary atomic position can be expressed by with n atoms, the electric potential at an arbitrary atomic position can be expressed by n
∑ qn / ( 4πε
V ( ri ) =
V ( ri )
j
∑ qj /
j =1 =
0
ri − r j
)
(2) (2)
4πε 0 ri − r j , ,
j =1 where q j (j = 1, 2, 3 …, n) denotes the charge on each carbon atom,
ri and r j represent the location
of carbon respectively, ε0 ison theeach permittivity of vacuum. equationsthe canlocation be written where q j (jatoms = 1, 2, i3and . . . ,j,n) denotes theand charge carbon atom, ri and rnj represent of in a matrix form as j,following: carbon atoms i and respectively, and ε0 is the permittivity of vacuum. n equations can be written in a matrix form as following: [ A]{q} = [Vx ] , (3) (3) [ A] {q} = [Vx ], where {q} and [Vx] stand for charge vector and electric potential vector. [A] is a diagonal matrix of where {q} and [Vxand ] stand for charge vector and electric potential vector. [A] is a diagonal matrix of n × n dimension, its elements can be written as n × n dimension, and its elements can be written as = aij 1/ 4πε 0 ri − r j aij = 1/ 4πε 0 ri − r j 2 (4) 2 2 q 2 y − y 2 z − z 2 , (4) = 1/=1/ − + + x x 4πε 4πε i j y j z 0 xi j− x j +i yi − +i zi − 0 j j , where x, y, and z represent the Cartesian coordinates of the carbon atoms. Apparently, Equation (4) where x, y, and z represent the Cartesian coordinates of the carbon atoms. Apparently, Equation (4) is is not applicable when i = j. In order to obtain the diagonal elements, the charges are supposed to be not applicable when i = j. In order to obtain the diagonal elements, the charges are supposed to be distributed uniformly on a triangular surface area surrounding the carbon atom. The area of a triangle distributed on a triangular surface area surrounding the carbon atom. The area of a triangle √ 2uniformly 2, where b = 0.142 nm is the length of the C–C bond. The charge density of the triangle 3 is s = 3 b is s = 3 3b , where b = 0.142 nm is the length of the C–C bond. The charge density of the triangle area areabecan be expressed d =The qi/s.electric The electric potential the atom can be expressed can expressed as d =as qi /s. potential of theofatom can be expressed as as
)
(
= V ( ri )i
V ( ri ) i
) (
) (
(
η ∫∫ xq q/ (/4πε4πεs sξqξ+ η+ η)dξddξdη, = , 2
s
i
0
2
0
i
2
2
)
(5) (5)
The diagonal elements can be obtained:s
The diagonal elements can be 1 / ( 4πε 0 s ) = aiiobtained:
(
∫∫ 1 / s
ξ 2 + η 2 dξ dη
)
s
=aii b =31/ ln(4πε 2+ (4πεξ 20 s+) η2 dξdη s /1/ 0 s)3 √ √ , ,
= b 3ln 2 + The total charges Q on a nanorope are
p
3 / (4πε 0 s)
(6) (6)
n
Q = ∑ qi i =1
,
(7)
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The total charges Q on a nanorope are n
Q=
∑ qi
(7)
i =1
The electric potential in the middle of the nanorope is taken as 0. The charge distribution of the nanorope can be obtained by solving Equations (3) and (7) using a MATLAB program. 3. Calculation of Adsorption of Metal and Negative Ions Based on the contact mechanics, the CNTs of the nanorope pile onto each other during the twisting process, and CNTs with larger chiralities produce more marked distortion. CNT nanoropes with small extrusion deformation possess more porosity, which is beneficial for ions entering. Here, the nanorope including CNTs with less chirality (3, 3) was used to simulate the adsorption of metal ions and negative ions. The molecular dynamics software LAMMPS was adopted to simulate the adsorption and desorption of ions. The NVT ensemble was used for the whole system, and the temperature was 300 K. The L-J pair style potential and Coulomb force were applied to describe the interaction between atoms [28]. The time step was set to be 1 fs and the simulation time was 200 ps. According to our previous study, the large strength of the electric field can cause the nanorope to rotate back to the CNT bundle over a long period of time [28]. In order to reduce the deformation of the nanorope caused by the applied electric field, the electric strength was set to be 1 × 10−5 V/Å. After adsorption, we increased the strength of the electric field to realize the desorption. In the present work, we denoted A+ with one positive charge to represent Na+ or other metal ions as well as B− with one negative charge to describe Cl− or other ions. The nanorope included 6888 carbon atoms, and its combination with 28 A+ or 28 B− was simulated. The conditions of the adsorption of the metal and negative ions are listed in Table 1. Table 1. The conditions of adsorption of metal and negative ions. Conditions
A+
B−
C-atom
Ensemble
Time
1 2
28 0
0 28
6888 6888
NVT NVT
200 ps 200 ps
The amount of total charge on the nanorope had an effect on ionic adsorption. The adsorption was trivial when there were few charges on the nanorope. As charges increased, a mutation would appear in a narrow scope. During the scope, the adsorption increased from 0% to 100%. In the simulation, a total charge of 4 × 10−4 e was chosen. The reason is that it is small to reduce the effect of an initial charge, and the nanorope possessed good adsorption ability with the charge. Trajectory tracking indicated that ions around the charged nanorope were influenced by the applied electrostatic field. The ionic configuration diagrams are shown in Figure 2 when the simulation time varies from 0 fs, to 400 fs, to 3.0 ps, and to 4.0 ps. The ions in a gray color are adsorbed into nanorope from the top and side directions. The top view shows that, when the ions expanded into nanorope, some may have existed in the gap between the CNTs, and others may have entered into the inside of the CNTs. The side view displays clearly how the ions filled the nanorope along the axial direction. In the initial stage of adsorption, ions were distributed on the surface of the nanorope. After a while, the ions accepted the geometric modulation of the nanorope, and most ions began to enter the rope. Some ions were pushed along the ropes in one direction, close to a CNT. Few ions drifted away due to the long distance from the nanorope.
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fs tt==00fs
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400fs fs tt==400
3.0ps ps tt==3.0
4.0ps ps tt==4.0
Figure2.2.Adsorption Adsorption of metalions. ions. Figure Figure 2. Adsorptionofofmetal metal ions.
Similarly,the thecharged chargednanorope nanoropeadsorbed adsorbednegative negativeions ionsas aswell. well.We Wereplaced replacedthe themetal metalions ionswith with Similarly, Similarly,ones, the charged nanorope adsorbedremained negative unchanged ions as well.to Weperform replacedthe thesimulation. metal ions with the negative and all other conditions The the negative ones, and all other conditions remained unchanged to perform the simulation. The the negative ones, and all other conditions remained unchanged towhich perform the simulation. The process process of the adsorption of negative ions is shown in Figure 3, is similar to that of the metal process of the adsorption of negative ions is shown in Figure 3, which is similar to that of the metal of theThe adsorption of negative ions is shown in Figure 3, whichmodulation is similar toand thatentered of the the metal ions. ions. ionsdistributing distributing onthe the surface accepted thegeometric geometric rope in ions. The ions on surface accepted the modulation and entered the rope in The ions distributing on theall surface accepted the geometric modulation and entered the rope in time. time. The difference is that negative ions could be adsorbed by the rope. time. The difference is that all negative ions could be adsorbed by the rope. The difference is that all negative ions could be adsorbed by the rope.
fs tt==00fs
400fs fs tt==400
3.0ps ps tt==3.0
4.0ps ps tt==4.0
Figure3.3.Adsorption Adsorption of negativeions. ions. Figure Figure 3. Adsorptionofofnegative negative ions.
Theadsorption adsorptionof of the thenegative negativeions ionswas wasbetter betterthan thanthat thatof ofmetal metalions, ions, which whichmay may be becaused causedby by The The adsorption of of thethe negative ions wasItbetter than that of metal ions, which may be caused by the charge distribution whole system. can be seen from Figures 2 and 3 that some adsorbed the charge distribution of the whole system. ItItcan be seen from Figures 22and 33that some adsorbed the charge distribution of the whole system. can be seen from Figures and that some adsorbed ions may may have have amassed amassed in in the the rope rope under under the the action action of of Van Van der der Waals Waals force force and and static electricity, electricity, ions ions may have amassed inincrease the rope under the actionWe of Van der Waals force and staticstatic electricity, which which can lead to a local in ionic density. also found that the arrangement of adsorbed which canto lead to a increase local increase in ionic density. We found also found thatarrangement the arrangement of adsorbed can lead a local in ionic density. We also that the of adsorbed ions ionswas wasinfluenced influencedby bythe thegeometrical geometricalstructure structureof ofthe thenanorope. nanorope.The Thecontrol controlof ofthe theionic ionicadsorption adsorption ions was storage influenced by the geometrical of the The control ofofthe adsorption and of the the nanorope couldstructure be realized realized bynanorope. changing the the strength strength theionic electric field or orand by and storage of nanorope could be by changing of the electric field by storage of the nanorope could be realized by changing the strength of the electric field or by applying applying different charges. applying different different charges. charges. 4.Adsorption Adsorption Capacityof of Nanorope 4. 4. AdsorptionCapacity Capacity ofNanorope Nanorope The relative distance between ions and and nanorope has has an important important influence on on the adsorption adsorption The Therelative relativedistance distancebetween betweenions ions andnanorope nanorope hasan an importantinfluence influence onthe the adsorption capacityof of thenanoropes. nanoropes. Theions ions distantfrom from thenanorope nanorope couldnot not beadsorbed. adsorbed. In thissection, section, capacity capacity ofthe the nanoropes.The The ionsdistant distant fromthe the nanoropecould could notbe be adsorbed.InInthis this section, we describe how 24 metal ions were used to investigate the influence of the distance on the adsorption we wedescribe describehow how24 24metal metalions ionswere wereused usedtotoinvestigate investigatethe theinfluence influenceofofthe thedistance distanceon onthe theadsorption adsorption capacity of of the the nanorope. nanorope. The The ions ions were were in in random random distribution distribution around around the the rope rope along along the the length capacity capacity of the nanorope. The ions were in random distribution around the rope along thelength length directionand and formedaacircle circlein inaacross crosssection. section.The Thedistance distancebetween betweenthe theions ionsand and thenanorope nanoropewere were direction direction andformed formed a circle in a cross section. The distance between the ionsthe and the nanorope 2 Å, 4 Å, 6 Å, 7 Å, and 8 Å, as is depicted in Figure 4. Figure 4 indicates that each ion with a distance 2were Å, 4 Å, 6 Å, 7 Å, and7 8Å,Å,and as is8 depicted in Figurein 4. Figure Figure 44.indicates each ion with a distance 2 Å, 4 Å, 6 Å, Å, as is depicted Figure 4that indicates that each ion with less than 6 Å could be adsorbed by the nanorope. However, with the increase in distance, the less than 6 Å could be adsorbed by the nanorope. However, with the increase in distance, the adsorption of the nanorope was significantly reduced, especially as the distance exceeded 6 Å. The adsorption of the nanorope was significantly reduced, especially as the distance exceeded 6 Å. The
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a distance less than Å could be adsorbed by the nanorope. However, with the increase6in distance, Nanomaterials 2016, 6, 6177 Nanomaterials 2016, 6, 177 of 9 6 of 9 Nanomaterials 2016, 6, 177 6 of 9 6 Å. the adsorption of the nanorope was significantly reduced, especially as the distance exceeded numerical results show that the maximal adsorption rate of 7 Å was 91.7%, and the adsorption rate numerical results showshow that the maximal adsorption rate of 7 rate Å was and91.7%, the adsorption The numerical results that the maximal adsorption of 91.7%, 7 Å was and the rate adsorption numerical results show that the was maximal adsorption rate of 7 that, Å was 91.7%, and the adsorption rate could plunge when the distance 8 Å. It can be concluded when the distance exceeds a certain could plunge whenwhen the distance was 8 Å.was It can when the distance exceeds a certainexceeds rate could plunge the distance 8 be Å.concluded It can be that, concluded that, when the distance could plunge when the distance was 8 Å. It can be concluded that, when the distance exceeds a certain value, the charged nanorope cancan no longer adsorb ions.ions. Figure 5ions. visually demonstrates thedemonstrates variation value, the charged nanorope no longer adsorb Figure 5 visually demonstrates the variation a certain value, the charged nanorope can no longer adsorb Figure 5 visually value, the charged nanorope can no longer adsorb ions. Figure 5 visually demonstrates the variation the ofof the adsorption rates of the nanorope withwith the distance between the nanorope and the ions. the adsorption rates of the nanorope the distance between the nanorope and the ions. variation ofadsorption the adsorption rates of the nanorope the distance between theand nanorope and the ions. of the rates of the nanorope with thewith distance between the nanorope the ions.
D=2Å DD= =22ÅÅ
D=4Å D == 44 Å Å D
D=6Å DD==66ÅÅ
D=7Å DD = 7= Å 7Å
D=8Å D =D8 =Å8 Å
Figure 4. Relation between distance and adsorption. Figure 4. 4. Relation andadsorption. adsorption. Figure Relationbetween between distance distance and
Figure 4. Relation between distance and adsorption.
100 100
100
80
80
Adsorption rate(% ) ) Adsorption rate(%
Adsorptionrate(%)
80 60 60
60
40 40
20
40 20
0
20 0 0 1 0
0
0
2
3
1
2
1
2
4 3
5 4
6 5
Distance(Å) 3 Distance(Å 4 5)
7 6
8 7
6
9 8
7
9
8
9
Figure 5. Variation of adsorption rate with Distance(Å ) distance. Figure 5. Variation of adsorption rate with distance.
Figure 5. Variation of adsorption rate with distance. Figure 5.by Variation of adsorption with distance. Although ions can be adsorbed a nanorope within arate certain distance, the amount of ions Although ions can be adsorbed by a nanorope within a certain distance, the amount of ions should be limited. To determine the ionic adsorption capacity of a nanorope, 120 ions were located Although canTo bedetermine adsorbed by aionic nanorope within a certain the of ions should should be ions limited. the capacity a distance, nanorope, 120amount ions located ions can be adsorbed by 6aadsorption nanorope aofcertain distance, theofwere amount aroundAlthough a nanorope with a distance less than Å, as shownwithin in Figure 6. Along the length the CNTof ions around a nanorope with a distance less than 6 Å, as shown in Figure 6. Along the length of the CNT benanoropes, limited. To determine the ionic adsorption capacity of a nanorope, 120 ions were located around should bethe limited. To determine ionic adsorption a nanorope, ions were ions were distributed the at random. Comparedcapacity with the of example with 24120 metal ions, thelocated nanoropes, the ions were distributed atas random. Compared with the example with metal ions, the a nanorope awas distance less than and 6 Å, shown 6. of24the CNT adsorption rate nowith longer 100%, thethan process needed more time. around awith nanorope a distance less 6 Å,in asFigure shown inAlong Figurethe 6. length Along the length ofnanoropes, the CNT adsorption rate was no longer 100%, and the process needed more time. the ions were distributed at random. Compared with the example with 24 metal ions, the adsorption
nanoropes, the ions were distributed at random. Compared with the example with 24 metal ions, the rateadsorption was no longer 100%, the100%, process moreneeded time. more time. rate was noand longer andneeded the process
t = 0 fs t = 0 fs
t = 0 fs
t = 2 ps t = 2 ps
t = 7 ps t = 7 ps
Figure 6. Ionic adsorption capacity of a nanorope. Figure 6. Ionic adsorption capacity of a nanorope.
t = 2 ps
t = 7 ps
Figure 6. Ionic adsorption capacity of a nanorope. Figure 6. Ionic adsorption capacity of a nanorope.
t = 14 ps t = 14 ps
t = 14 ps
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5. Desorption Desorptionof ofNegative NegativeIons Ions 5. Desorption of of ions ions is is aa reversible reversible process process of of adsorption, adsorption, and and the the ions ions need need to to be be released released from from Desorption the nanorope. nanorope. Here, Here, the the rope rope with with 28 28 adsorbed adsorbed negative negative ions ions was was used used to to simulate simulate the the desorption the desorption process. It It was was found found that, that, with with the the increase increase of of the the applied applied electric electric field, field, the the electric electric field field force force was was process. larger than than the the Van Van der der Waals Waals force and the the electrostatic electrostatic force, force, which which led led to to the the desorption desorption of of ions. ions. larger force and The strength strength of of the and other other simulation simulation conditions conditions were were the the same same as as The the electric electric field field was was set set to to 22 V/Å, V/Å, and the adsorption thethe releasing process of the It canItbecan seenbethat thethat twisted the adsorptionones. ones.Figure Figure7 illustrates 7 illustrates releasing process ofions. the ions. seen the nanorope rotated back to some extent, the and ionsthe promptly deviateddeviated from thefrom rope.the Werope. can see twisted nanorope rotated back to someand extent, ions promptly We that can full that desorption was shorter the when strength the electric field was greater. However, the see full desorption waswhen shorter theofstrength of the electric field was greater.because However, highest strength of the electricoffield subjected to the nanorope wasnanorope never more 3 V/Å because the highest strength the electric field subjected to the wasthan never more[28], thanwe 3 had [28], to control thetostrength of the electric field in a certain to satisfy desorption demand. V/Å we had control the strength of the electric field inrange a certain range the to satisfy the desorption After the desorption process, theprocess, nanorope rebuilt by a twist load, which demand. After the desorption thewas nanorope wasapplying rebuilt by applying a twistdemonstrated load, which the recycle of the demonstrated thenanorope. recycle of the nanorope.
t = 0 fs
t = 5 fs
t = 10 fs
t = 20 fs
t = 25 fs
Figure Figure 7. 7. Desorption Desorption of of negative negative ions. ions.
6. Conclusions 6. Conclusions In this paper, we built a molecular dynamics model of a nanorope by twisting a CNT bundle. In this paper, we built a molecular dynamics model of a nanorope by twisting a CNT bundle. The charge distribution of the nanorope was calculated using a modified gradient method based on The charge distribution of the nanorope was calculated using a modified gradient method based on classical electrostatic theory. For the charged nanorope, the adsorption and desorption of metal ions classical electrostatic theory. For the charged nanorope, the adsorption and desorption of metal ions and negative ions were achieved by adjusting the strength of the applied electric field. Two factors and negative ions were achieved by adjusting the strength of the applied electric field. Two factors that that influenced the adsorption capacity of the nanorope were found: One is the relative distance influenced the adsorption capacity of the nanorope were found: One is the relative distance between between the ions and the rope, and the other is the amount of ions. The results indicate that the the ions and the rope, and the other is the amount of ions. The results indicate that the nanorope nanorope cannot absorb ions distant from it. As the amount of ions increase, adsorption reaches its cannot absorb ions distant from it. As the amount of ions increase, adsorption reaches its limit, and limit, and process time is prolonged. The strength of the electric field can be taken as an adjusting process time is prolonged. The strength of the electric field can be taken as an adjusting switch, making switch, making it possible to provide a standard system for the adsorption and desorption of ions at it possible to provide a standard system for the adsorption and desorption of ions at the nano scale. the nano scale. This is very significant for applications in sewage treatment. This is very significant for applications in sewage treatment. Acknowledgements: This work was supported by the National Natural Science Foundation of China under Nos. Acknowledgments: Thiswhich workiswas supported by the National Natural Science Foundation of China under Nos. 11502007 and 11472020, gratefully acknowledged. 11502007 and 11472020, which is gratefully acknowledged. Author Contributions: Jun-Jun Shang and Xiao-Hui Yan performed the simulation and participated in Author Contributions: Jun-Jun Shang and Xiao-Hui Yan performed the simulation and participated in manuscript manuscript Qing-Sheng Yang analyzed the data and wrote the Xiao-Qiao paper. Xiao-Qiao HeKim-Meow and Kim-Meow preparation.preparation. Qing-Sheng Yang analyzed the data and wrote the paper. He and Liew Liew provided suggestions the concept and research methodology, and participated data analysis. provided suggestions on theon concept and research methodology, and participated in datainanalysis. Conflicts of The authors authors declare declare no no conflict conflict of of interest. interest. Conflicts of Interest: Interest: The
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Ionic Adsorption and Desorption of CNT Nanoropes Jun-Jun Shang 1 , Qing-Sheng Yang 1, *, Xiao-Hui Yan 1 , Xiao-Qiao He 2, * and Kim-Meow Liew 2 1 2
*
Department of Engineering Mechanics, Beijing University of Technology, Beijing 100124, China; [email protected] (J.-J.S.); [email protected] (X.-H.Y.) Department of Civil and Architectural Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, China; [email protected] Correspondence: [email protected] (Q.-S.Y.); [email protected] (X.-Q.H.); Tel.: +86-10-6739-6333 (Q.-S.Y.); +852-3442-4670 (X.-Q.H.)
Academic Editor: Thomas Nann Received: 20 July 2016; Accepted: 22 September 2016; Published: 28 September 2016
Abstract: A nanorope is comprised of several carbon nanotubes (CNTs) with different chiralities. A molecular dynamic model is built to investigate the ionic adsorption and desorption of the CNT nanoropes. The charge distribution on the nanorope is obtained by using a modified gradient method based on classical electrostatic theory. The electrostatic interactions among charged carbon atoms are calculated by using the Coulomb law. It was found here that the charged nanorope can adsorb heavy metal ions, and the adsorption and desorption can be realized by controlling the strength of applied electric field. The distance between the ions and the nanorope as well as the amount of ions have an effect on the adsorption capacity of the nanorope. The desorption process takes less time than that of adsorption. The study indicates that the CNT nanorope can be used as a core element of devices for sewage treatment. Keywords: nanorope; adsorption and desorption of ions; carbon nanotubes
1. Introduction With the rapid development of industry, heavy metal pollution and its control have become the major environmental problems [1,2]. Metal plating equipment, petroleum and chemical industry, battery industry, pesticide residues, and so on are producing more and more heavy metal which directly or indirectly emit to the environment [3], especially in the developing world. Heavy metal is different from organic pollutants [4]. When in water, it is not biodegradable and easy to accumulate in organisms. As we all know, Cu2+ , Hg2+ , Cd2+ , and Pb2+ are common toxic pollutants. Water, soil [5], and biology are difficult to detoxify once polluted by the heavy metal ions. It has been proved that many heavy metal ions are poisonous and carcinogenic substances and cause serious harm to ecological environments and humans, endangering life and health. Considering these serious public health implications, how to remove heavy metal ions from water has become a major challenge confronting humanity. Moreover, the most detrimental impurities in the feed water are high molar mass substances, except metal ions and organic pollutants. When the content of ions like AsIII , AsV , F− , NO3 − , etc. exceed an acceptable level, these ions are released into the environment, which causes serious environmental and health problems. One way to remove ions in industrial wastewater is adsorption [6,7], which is simple and inexpensive. Repeated utilization is an important consideration for evaluating the stand or fall of adsorption material. It is required that the materials should not only have good adsorption ability, but also have good desorption ability [8,9]. The structure and properties of carbon nanotubes (CNTs) may make it a good adsorption material [10,11]. CNTs, as a new type of nanomaterials, not only have excellent mechanical, thermal, and electrical properties, but also possess good chemical
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and thermal stability and a large specific surface area. They have important application value in adsorbing heavy metal ions effectively and in the extraction of minerals, especially rare earth elements [12]. Ajayan et al. [13] found that CNTs can be filled with lead without inducing any damage. When applying voltage to one end of the tubes, extra charges can be attached to tubes and then charge redistribution takes place [14]. The charge properties of CNTs can vary with the amount of charge [15]. The corresponding carbon quantum wires, CNT field emission devices, nanotubes tweezers, and carbon nanotube actuators [14,15] have been developed successively. A series of results show that charged CNTs have enormous development potential [16–18]. It has been shown that, due to the additional charges in CNTs, the ionic velocity in NaCl solutions markedly changes [18]. Alijani et al. [19] came up with a new application for magnetic CNTs, namely the removal of lead ions, and the results suggest that this sorbent is suitable for the removal and solidification of metal ions from polluted environmental samples. Doubt studied the changing structure and dynamic behavior of metal salt, Ru, Bi, Ag, Pt, Pd, and biomolecules when they are encapsulated in CNTs [20]. Guo et al. [21] investigated a CNT system decorated with transition metals such as Ni, Cu, Au, and Pt. Valentini et al. [22] concluded that water molecules in CNTs can be influenced by an electric field. Water molecules are very stable and have a high efficient adsorption in electric fields. When the electric field is removed, CNTs can recover their initial state. Although the adsorption of single CNTs has been widely investigated, their size makes them difficult for industrial applications. CNT assemblies including different CNT-based nanostructures are expected to deliver the superb properties of CNTs from nano-scale to macro-scale. It is important to investigate the adsorption ability of CNT-based nanostructures. Akin to the common geometry of synthetic fiber ropes [23], a model of a CNT nanorope was built by twisting a CNT bundle. In this paper, the ionic adsorption and desorption of CNT nanoropes were realized by changing the strength of the applied electric field. Firstly, molecular dynamic models of CNT nanoropes were built. Then, the charge distribution on the nanorope was obtained by using a gradient method based on classical electrostatic theory. Finally, the adsorption and desorption of the metal ions and negative ions of the charged nanoropes were investigated. 2. Molecular Dynamics Model of Nanoropes A CNT bundle including seven CNTs was generated in the quantum mechanics software Materials Studio. The initial length in the free state was 20.168 nm. Then, the bundle was twisted around a core axis to form a nanorope by using LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code [24]. In this paper, the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential function [25–27] is used to describe the interaction between carbon atoms. The potential consists of three terms. " # 1 LJ REBO TOR E = ∑ ∑ Eij + Eij + ∑ ∑ Ekijl , 2 i j 6 =i k 6=i,j l 6=i,j,k
(1)
where EijREBO represents the reactive empirical bond-order potential developed by Brenner et al. [26], EijLJ represents the longer-ranged interactions in a form similar to the standard Lennard–Jones potential, and EijTOR stands for the torsional interaction. The simulation process made use of the NVT ensemble (the system is isolated from changes in moles, volume, and temperature). In order to avoid the effects of thermal activation, the Nose/Hoover thermostat kept the temperature of the system at 1 × 10−3 K, and the velocity obeyed the Boltzmann distribution. The time step was set to be 1 fs, and the trajectory information was output every 500 steps. Three-dimensional non-periodic boundary conditions and rotation load were applied. Under the rotation, the CNTs were self-assembled into a nanorope under the action of Van der Waals force. After the full energy relaxation (1 ns) of the system, the model of nanorope was obtained. Figure 1 shows three nanoropes including CNTs with chiralities (3, 3), (6, 6), and (9, 9).
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(3, 3)
(6, 6)
(9, 9)
Figure1.1. Nanoropes Nanoropes including including carbon carbon nanotubes nanotubes (CNTs) (CNTs) with with chiralities chiralities (3, (3, 3) 3) (left), (left), (6, (6, 6) 6) (middle), (middle), Figure and(9, (9,9) 9)(right). (right). and
The charge distribution was calculated by using a modified gradient method [28] based on The charge distribution was calculated by using a modified gradient method [28] based on classical electrostatic theory [29,30]. It is assumed that the nanorope is subjected to an ideal electric classical electrostatic theory [29,30]. It is assumed that the nanorope is subjected to an ideal electric field, and the charges induced by the electric field distribute on the surface of CNTs. For a nanorope field, and the charges induced by the electric field distribute on the surface of CNTs. For a nanorope with n atoms, the electric potential at an arbitrary atomic position can be expressed by with n atoms, the electric potential at an arbitrary atomic position can be expressed by n
∑ qn / ( 4πε
V ( ri ) =
V ( ri )
j
∑ qj /
j =1 =
0
ri − r j
)
(2) (2)
4πε 0 ri − r j , ,
j =1 where q j (j = 1, 2, 3 …, n) denotes the charge on each carbon atom,
ri and r j represent the location
of carbon respectively, ε0 ison theeach permittivity of vacuum. equationsthe canlocation be written where q j (jatoms = 1, 2, i3and . . . ,j,n) denotes theand charge carbon atom, ri and rnj represent of in a matrix form as j,following: carbon atoms i and respectively, and ε0 is the permittivity of vacuum. n equations can be written in a matrix form as following: [ A]{q} = [Vx ] , (3) (3) [ A] {q} = [Vx ], where {q} and [Vx] stand for charge vector and electric potential vector. [A] is a diagonal matrix of where {q} and [Vxand ] stand for charge vector and electric potential vector. [A] is a diagonal matrix of n × n dimension, its elements can be written as n × n dimension, and its elements can be written as = aij 1/ 4πε 0 ri − r j aij = 1/ 4πε 0 ri − r j 2 (4) 2 2 q 2 y − y 2 z − z 2 , (4) = 1/=1/ − + + x x 4πε 4πε i j y j z 0 xi j− x j +i yi − +i zi − 0 j j , where x, y, and z represent the Cartesian coordinates of the carbon atoms. Apparently, Equation (4) where x, y, and z represent the Cartesian coordinates of the carbon atoms. Apparently, Equation (4) is is not applicable when i = j. In order to obtain the diagonal elements, the charges are supposed to be not applicable when i = j. In order to obtain the diagonal elements, the charges are supposed to be distributed uniformly on a triangular surface area surrounding the carbon atom. The area of a triangle distributed on a triangular surface area surrounding the carbon atom. The area of a triangle √ 2uniformly 2, where b = 0.142 nm is the length of the C–C bond. The charge density of the triangle 3 is s = 3 b is s = 3 3b , where b = 0.142 nm is the length of the C–C bond. The charge density of the triangle area areabecan be expressed d =The qi/s.electric The electric potential the atom can be expressed can expressed as d =as qi /s. potential of theofatom can be expressed as as
)
(
= V ( ri )i
V ( ri ) i
) (
) (
(
η ∫∫ xq q/ (/4πε4πεs sξqξ+ η+ η)dξddξdη, = , 2
s
i
0
2
0
i
2
2
)
(5) (5)
The diagonal elements can be obtained:s
The diagonal elements can be 1 / ( 4πε 0 s ) = aiiobtained:
(
∫∫ 1 / s
ξ 2 + η 2 dξ dη
)
s
=aii b =31/ ln(4πε 2+ (4πεξ 20 s+) η2 dξdη s /1/ 0 s)3 √ √ , ,
= b 3ln 2 + The total charges Q on a nanorope are
p
3 / (4πε 0 s)
(6) (6)
n
Q = ∑ qi i =1
,
(7)
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The total charges Q on a nanorope are n
Q=
∑ qi
(7)
i =1
The electric potential in the middle of the nanorope is taken as 0. The charge distribution of the nanorope can be obtained by solving Equations (3) and (7) using a MATLAB program. 3. Calculation of Adsorption of Metal and Negative Ions Based on the contact mechanics, the CNTs of the nanorope pile onto each other during the twisting process, and CNTs with larger chiralities produce more marked distortion. CNT nanoropes with small extrusion deformation possess more porosity, which is beneficial for ions entering. Here, the nanorope including CNTs with less chirality (3, 3) was used to simulate the adsorption of metal ions and negative ions. The molecular dynamics software LAMMPS was adopted to simulate the adsorption and desorption of ions. The NVT ensemble was used for the whole system, and the temperature was 300 K. The L-J pair style potential and Coulomb force were applied to describe the interaction between atoms [28]. The time step was set to be 1 fs and the simulation time was 200 ps. According to our previous study, the large strength of the electric field can cause the nanorope to rotate back to the CNT bundle over a long period of time [28]. In order to reduce the deformation of the nanorope caused by the applied electric field, the electric strength was set to be 1 × 10−5 V/Å. After adsorption, we increased the strength of the electric field to realize the desorption. In the present work, we denoted A+ with one positive charge to represent Na+ or other metal ions as well as B− with one negative charge to describe Cl− or other ions. The nanorope included 6888 carbon atoms, and its combination with 28 A+ or 28 B− was simulated. The conditions of the adsorption of the metal and negative ions are listed in Table 1. Table 1. The conditions of adsorption of metal and negative ions. Conditions
A+
B−
C-atom
Ensemble
Time
1 2
28 0
0 28
6888 6888
NVT NVT
200 ps 200 ps
The amount of total charge on the nanorope had an effect on ionic adsorption. The adsorption was trivial when there were few charges on the nanorope. As charges increased, a mutation would appear in a narrow scope. During the scope, the adsorption increased from 0% to 100%. In the simulation, a total charge of 4 × 10−4 e was chosen. The reason is that it is small to reduce the effect of an initial charge, and the nanorope possessed good adsorption ability with the charge. Trajectory tracking indicated that ions around the charged nanorope were influenced by the applied electrostatic field. The ionic configuration diagrams are shown in Figure 2 when the simulation time varies from 0 fs, to 400 fs, to 3.0 ps, and to 4.0 ps. The ions in a gray color are adsorbed into nanorope from the top and side directions. The top view shows that, when the ions expanded into nanorope, some may have existed in the gap between the CNTs, and others may have entered into the inside of the CNTs. The side view displays clearly how the ions filled the nanorope along the axial direction. In the initial stage of adsorption, ions were distributed on the surface of the nanorope. After a while, the ions accepted the geometric modulation of the nanorope, and most ions began to enter the rope. Some ions were pushed along the ropes in one direction, close to a CNT. Few ions drifted away due to the long distance from the nanorope.
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fs tt==00fs
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400fs fs tt==400
3.0ps ps tt==3.0
4.0ps ps tt==4.0
Figure2.2.Adsorption Adsorption of metalions. ions. Figure Figure 2. Adsorptionofofmetal metal ions.
Similarly,the thecharged chargednanorope nanoropeadsorbed adsorbednegative negativeions ionsas aswell. well.We Wereplaced replacedthe themetal metalions ionswith with Similarly, Similarly,ones, the charged nanorope adsorbedremained negative unchanged ions as well.to Weperform replacedthe thesimulation. metal ions with the negative and all other conditions The the negative ones, and all other conditions remained unchanged to perform the simulation. The the negative ones, and all other conditions remained unchanged towhich perform the simulation. The process process of the adsorption of negative ions is shown in Figure 3, is similar to that of the metal process of the adsorption of negative ions is shown in Figure 3, which is similar to that of the metal of theThe adsorption of negative ions is shown in Figure 3, whichmodulation is similar toand thatentered of the the metal ions. ions. ionsdistributing distributing onthe the surface accepted thegeometric geometric rope in ions. The ions on surface accepted the modulation and entered the rope in The ions distributing on theall surface accepted the geometric modulation and entered the rope in time. time. The difference is that negative ions could be adsorbed by the rope. time. The difference is that all negative ions could be adsorbed by the rope. The difference is that all negative ions could be adsorbed by the rope.
fs tt==00fs
400fs fs tt==400
3.0ps ps tt==3.0
4.0ps ps tt==4.0
Figure3.3.Adsorption Adsorption of negativeions. ions. Figure Figure 3. Adsorptionofofnegative negative ions.
Theadsorption adsorptionof of the thenegative negativeions ionswas wasbetter betterthan thanthat thatof ofmetal metalions, ions, which whichmay may be becaused causedby by The The adsorption of of thethe negative ions wasItbetter than that of metal ions, which may be caused by the charge distribution whole system. can be seen from Figures 2 and 3 that some adsorbed the charge distribution of the whole system. ItItcan be seen from Figures 22and 33that some adsorbed the charge distribution of the whole system. can be seen from Figures and that some adsorbed ions may may have have amassed amassed in in the the rope rope under under the the action action of of Van Van der der Waals Waals force force and and static electricity, electricity, ions ions may have amassed inincrease the rope under the actionWe of Van der Waals force and staticstatic electricity, which which can lead to a local in ionic density. also found that the arrangement of adsorbed which canto lead to a increase local increase in ionic density. We found also found thatarrangement the arrangement of adsorbed can lead a local in ionic density. We also that the of adsorbed ions ionswas wasinfluenced influencedby bythe thegeometrical geometricalstructure structureof ofthe thenanorope. nanorope.The Thecontrol controlof ofthe theionic ionicadsorption adsorption ions was storage influenced by the geometrical of the The control ofofthe adsorption and of the the nanorope couldstructure be realized realized bynanorope. changing the the strength strength theionic electric field or orand by and storage of nanorope could be by changing of the electric field by storage of the nanorope could be realized by changing the strength of the electric field or by applying applying different charges. applying different different charges. charges. 4.Adsorption Adsorption Capacityof of Nanorope 4. 4. AdsorptionCapacity Capacity ofNanorope Nanorope The relative distance between ions and and nanorope has has an important important influence on on the adsorption adsorption The Therelative relativedistance distancebetween betweenions ions andnanorope nanorope hasan an importantinfluence influence onthe the adsorption capacityof of thenanoropes. nanoropes. Theions ions distantfrom from thenanorope nanorope couldnot not beadsorbed. adsorbed. In thissection, section, capacity capacity ofthe the nanoropes.The The ionsdistant distant fromthe the nanoropecould could notbe be adsorbed.InInthis this section, we describe how 24 metal ions were used to investigate the influence of the distance on the adsorption we wedescribe describehow how24 24metal metalions ionswere wereused usedtotoinvestigate investigatethe theinfluence influenceofofthe thedistance distanceon onthe theadsorption adsorption capacity of of the the nanorope. nanorope. The The ions ions were were in in random random distribution distribution around around the the rope rope along along the the length capacity capacity of the nanorope. The ions were in random distribution around the rope along thelength length directionand and formedaacircle circlein inaacross crosssection. section.The Thedistance distancebetween betweenthe theions ionsand and thenanorope nanoropewere were direction direction andformed formed a circle in a cross section. The distance between the ionsthe and the nanorope 2 Å, 4 Å, 6 Å, 7 Å, and 8 Å, as is depicted in Figure 4. Figure 4 indicates that each ion with a distance 2were Å, 4 Å, 6 Å, 7 Å, and7 8Å,Å,and as is8 depicted in Figurein 4. Figure Figure 44.indicates each ion with a distance 2 Å, 4 Å, 6 Å, Å, as is depicted Figure 4that indicates that each ion with less than 6 Å could be adsorbed by the nanorope. However, with the increase in distance, the less than 6 Å could be adsorbed by the nanorope. However, with the increase in distance, the adsorption of the nanorope was significantly reduced, especially as the distance exceeded 6 Å. The adsorption of the nanorope was significantly reduced, especially as the distance exceeded 6 Å. The
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a distance less than Å could be adsorbed by the nanorope. However, with the increase6in distance, Nanomaterials 2016, 6, 6177 Nanomaterials 2016, 6, 177 of 9 6 of 9 Nanomaterials 2016, 6, 177 6 of 9 6 Å. the adsorption of the nanorope was significantly reduced, especially as the distance exceeded numerical results show that the maximal adsorption rate of 7 Å was 91.7%, and the adsorption rate numerical results showshow that the maximal adsorption rate of 7 rate Å was and91.7%, the adsorption The numerical results that the maximal adsorption of 91.7%, 7 Å was and the rate adsorption numerical results show that the was maximal adsorption rate of 7 that, Å was 91.7%, and the adsorption rate could plunge when the distance 8 Å. It can be concluded when the distance exceeds a certain could plunge whenwhen the distance was 8 Å.was It can when the distance exceeds a certainexceeds rate could plunge the distance 8 be Å.concluded It can be that, concluded that, when the distance could plunge when the distance was 8 Å. It can be concluded that, when the distance exceeds a certain value, the charged nanorope cancan no longer adsorb ions.ions. Figure 5ions. visually demonstrates thedemonstrates variation value, the charged nanorope no longer adsorb Figure 5 visually demonstrates the variation a certain value, the charged nanorope can no longer adsorb Figure 5 visually value, the charged nanorope can no longer adsorb ions. Figure 5 visually demonstrates the variation the ofof the adsorption rates of the nanorope withwith the distance between the nanorope and the ions. the adsorption rates of the nanorope the distance between the nanorope and the ions. variation ofadsorption the adsorption rates of the nanorope the distance between theand nanorope and the ions. of the rates of the nanorope with thewith distance between the nanorope the ions.
D=2Å DD= =22ÅÅ
D=4Å D == 44 Å Å D
D=6Å DD==66ÅÅ
D=7Å DD = 7= Å 7Å
D=8Å D =D8 =Å8 Å
Figure 4. Relation between distance and adsorption. Figure 4. 4. Relation andadsorption. adsorption. Figure Relationbetween between distance distance and
Figure 4. Relation between distance and adsorption.
100 100
100
80
80
Adsorption rate(% ) ) Adsorption rate(%
Adsorptionrate(%)
80 60 60
60
40 40
20
40 20
0
20 0 0 1 0
0
0
2
3
1
2
1
2
4 3
5 4
6 5
Distance(Å) 3 Distance(Å 4 5)
7 6
8 7
6
9 8
7
9
8
9
Figure 5. Variation of adsorption rate with Distance(Å ) distance. Figure 5. Variation of adsorption rate with distance.
Figure 5. Variation of adsorption rate with distance. Figure 5.by Variation of adsorption with distance. Although ions can be adsorbed a nanorope within arate certain distance, the amount of ions Although ions can be adsorbed by a nanorope within a certain distance, the amount of ions should be limited. To determine the ionic adsorption capacity of a nanorope, 120 ions were located Although canTo bedetermine adsorbed by aionic nanorope within a certain the of ions should should be ions limited. the capacity a distance, nanorope, 120amount ions located ions can be adsorbed by 6aadsorption nanorope aofcertain distance, theofwere amount aroundAlthough a nanorope with a distance less than Å, as shownwithin in Figure 6. Along the length the CNTof ions around a nanorope with a distance less than 6 Å, as shown in Figure 6. Along the length of the CNT benanoropes, limited. To determine the ionic adsorption capacity of a nanorope, 120 ions were located around should bethe limited. To determine ionic adsorption a nanorope, ions were ions were distributed the at random. Comparedcapacity with the of example with 24120 metal ions, thelocated nanoropes, the ions were distributed atas random. Compared with the example with metal ions, the a nanorope awas distance less than and 6 Å, shown 6. of24the CNT adsorption rate nowith longer 100%, thethan process needed more time. around awith nanorope a distance less 6 Å,in asFigure shown inAlong Figurethe 6. length Along the length ofnanoropes, the CNT adsorption rate was no longer 100%, and the process needed more time. the ions were distributed at random. Compared with the example with 24 metal ions, the adsorption
nanoropes, the ions were distributed at random. Compared with the example with 24 metal ions, the rateadsorption was no longer 100%, the100%, process moreneeded time. more time. rate was noand longer andneeded the process
t = 0 fs t = 0 fs
t = 0 fs
t = 2 ps t = 2 ps
t = 7 ps t = 7 ps
Figure 6. Ionic adsorption capacity of a nanorope. Figure 6. Ionic adsorption capacity of a nanorope.
t = 2 ps
t = 7 ps
Figure 6. Ionic adsorption capacity of a nanorope. Figure 6. Ionic adsorption capacity of a nanorope.
t = 14 ps t = 14 ps
t = 14 ps
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5. Desorption Desorptionof ofNegative NegativeIons Ions 5. Desorption of of ions ions is is aa reversible reversible process process of of adsorption, adsorption, and and the the ions ions need need to to be be released released from from Desorption the nanorope. nanorope. Here, Here, the the rope rope with with 28 28 adsorbed adsorbed negative negative ions ions was was used used to to simulate simulate the the desorption the desorption process. It It was was found found that, that, with with the the increase increase of of the the applied applied electric electric field, field, the the electric electric field field force force was was process. larger than than the the Van Van der der Waals Waals force and the the electrostatic electrostatic force, force, which which led led to to the the desorption desorption of of ions. ions. larger force and The strength strength of of the and other other simulation simulation conditions conditions were were the the same same as as The the electric electric field field was was set set to to 22 V/Å, V/Å, and the adsorption thethe releasing process of the It canItbecan seenbethat thethat twisted the adsorptionones. ones.Figure Figure7 illustrates 7 illustrates releasing process ofions. the ions. seen the nanorope rotated back to some extent, the and ionsthe promptly deviateddeviated from thefrom rope.the Werope. can see twisted nanorope rotated back to someand extent, ions promptly We that can full that desorption was shorter the when strength the electric field was greater. However, the see full desorption waswhen shorter theofstrength of the electric field was greater.because However, highest strength of the electricoffield subjected to the nanorope wasnanorope never more 3 V/Å because the highest strength the electric field subjected to the wasthan never more[28], thanwe 3 had [28], to control thetostrength of the electric field in a certain to satisfy desorption demand. V/Å we had control the strength of the electric field inrange a certain range the to satisfy the desorption After the desorption process, theprocess, nanorope rebuilt by a twist load, which demand. After the desorption thewas nanorope wasapplying rebuilt by applying a twistdemonstrated load, which the recycle of the demonstrated thenanorope. recycle of the nanorope.
t = 0 fs
t = 5 fs
t = 10 fs
t = 20 fs
t = 25 fs
Figure Figure 7. 7. Desorption Desorption of of negative negative ions. ions.
6. Conclusions 6. Conclusions In this paper, we built a molecular dynamics model of a nanorope by twisting a CNT bundle. In this paper, we built a molecular dynamics model of a nanorope by twisting a CNT bundle. The charge distribution of the nanorope was calculated using a modified gradient method based on The charge distribution of the nanorope was calculated using a modified gradient method based on classical electrostatic theory. For the charged nanorope, the adsorption and desorption of metal ions classical electrostatic theory. For the charged nanorope, the adsorption and desorption of metal ions and negative ions were achieved by adjusting the strength of the applied electric field. Two factors and negative ions were achieved by adjusting the strength of the applied electric field. Two factors that that influenced the adsorption capacity of the nanorope were found: One is the relative distance influenced the adsorption capacity of the nanorope were found: One is the relative distance between between the ions and the rope, and the other is the amount of ions. The results indicate that the the ions and the rope, and the other is the amount of ions. The results indicate that the nanorope nanorope cannot absorb ions distant from it. As the amount of ions increase, adsorption reaches its cannot absorb ions distant from it. As the amount of ions increase, adsorption reaches its limit, and limit, and process time is prolonged. The strength of the electric field can be taken as an adjusting process time is prolonged. The strength of the electric field can be taken as an adjusting switch, making switch, making it possible to provide a standard system for the adsorption and desorption of ions at it possible to provide a standard system for the adsorption and desorption of ions at the nano scale. the nano scale. This is very significant for applications in sewage treatment. This is very significant for applications in sewage treatment. Acknowledgements: This work was supported by the National Natural Science Foundation of China under Nos. Acknowledgments: Thiswhich workiswas supported by the National Natural Science Foundation of China under Nos. 11502007 and 11472020, gratefully acknowledged. 11502007 and 11472020, which is gratefully acknowledged. Author Contributions: Jun-Jun Shang and Xiao-Hui Yan performed the simulation and participated in Author Contributions: Jun-Jun Shang and Xiao-Hui Yan performed the simulation and participated in manuscript manuscript Qing-Sheng Yang analyzed the data and wrote the Xiao-Qiao paper. Xiao-Qiao HeKim-Meow and Kim-Meow preparation.preparation. Qing-Sheng Yang analyzed the data and wrote the paper. He and Liew Liew provided suggestions the concept and research methodology, and participated data analysis. provided suggestions on theon concept and research methodology, and participated in datainanalysis. Conflicts of The authors authors declare declare no no conflict conflict of of interest. interest. Conflicts of Interest: Interest: The
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