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Molecular simulation of the ion exchange behavior of Cu2+, Cd2+ and Pb2+ ions on different zeolites exchanged with sodium
Journal of Environmental Chemical Engineering 7 (2019) 103040
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Molecular simulation of the ion exchange behavior of Cu2+, Cd2+ and Pb2+ ions on different zeolites exchanged with sodium
T
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Hossein Khanmohammadia, Behrouz Bayatib, , Javad Rahbar- Shahrouzia, Ali-Akbar Babaluoa, Asma Ghorbanib a b
Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran Chemical Engineering Department, Ilam University, Ilam, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Ion exchange Heavy metal Zeolite Molecular dynamics
In this research, the molecular dynamics simulation of separating metal ions from water at 298 K using LAMMPS software for LTA, FAU, LTN, THO, NAT and EDI zeolites and electrolyte solutions containing Cu2+, Cd2+ and Pb2+ individually and in pairs of ions have been studied. Among the single ions, the lowest ion exchange rate was Pb2+ and the highest was Cd2+, as well as the adsorption rate and ion exchange, which was directly related to the size of zeolite cavities. The FAU zeolite with the largest cavity diameter had the fastest adsorption rate. The LTA structure with the adsorption of 69% of cadmium and FAU with 46% adsorption had the highest and lowest ion exchange rate. Furthermore, the highest ion displacement in different types of zeolites was related to copper ion due to its low weight. In two cations electrolytes solutions, all the structures tended to adsorb cadmium ion EDI structures with 100% cadmium adsorption and 85% copper, FAU with 78% cadmium adsorption and 40% copper have been shown the highest and lowest selectivity and uptake of cadmium relative to copper, respectively. In addition, for electrolyte containing cadmium and lead, EDI structure with 100% cadmium adsorption against 60% lead and THO structure with 70% cadmium adsorption versus 38% lead had the highest and lowest selectivity for cadmium. In copper-leaded electrolytes, copper was superior to selected. EDI adsorbed 100% copper versus 60% lead, and FAU had a 40% copper uptake, compared to 65% lead.
1. Introduction Since provision of water for drinking, domestic, agricultural, industrial have always been a critical issue, water and wastewater pollutants today are among the greatest concerns of humankind [1–4]. These contaminations are of various types such as chemical and pharmaceutical, colored, nuclear, and so on. These contaminants also come from a variety of sources such as industrial and domestic wastewater [5–7]. Heavy metal ions are one of these types of pollutants, which are among the most dangerous ones [8–10]. These ions inflict enormous damage to the environment and the natural processes involved [11,12]. These hazardous materials also cause various diseases and disorders in humans [13–16]. Therefore, it is necessary to eliminate or reduce their content in water to safe limits. There are many methods for separating heavy metal ions from wastewater, including chemical separation, filtration, membrane separation, electrochemical purification, adsorption, and ion exchange processes [13,14,17–20]. Today, the ion exchange method is widely accepted due to reasonable cost, process simplicity, low risk, and metal ⁎
recovery. Porous materials such as zeolites are widely used in the ion exchange process. The zeolites are crystalline and hydrated aluminosilicates of alkali and alkaline earth metals, especially sodium, calcium, magnesium, strontium and barium, which contain three-dimensional networks consisting of [SiO4] and [AlO4]- four faces. These materials have a pore size of 0–50 nm [21]. Due to the presence of non-structural positive ions, including Ca2+, K+ and Na+ ions, zeolites are very common in ion exchange processes and show high ion exchange efficiency. Hence, the use of these materials is growing to eliminate the contaminations of water and wastewater, especially the removal of heavy metal ions. So far, several experimental and theoretical studies have been carried out on the removal of heavy metals by means of ion exchange processes using zeolites. Ren et al. [22] have studied the separation of Pb and Cr from aqueous solutions by modifying natural zeolite. They reported that zeolite modification by cetylpyridium bromide increases the ion absorption capacity because the size of the natural zeolite pore size is uniform, small and suitable for adsorption of mentioned ions. Wen et al. [23] investigated the separation of metal ions from Pb, Cu, Zn and Cd using modified zeolites by NaOH and HCl.
Corresponding author. E-mail address: [email protected] (B. Bayati).
https://doi.org/10.1016/j.jece.2019.103040 Received 26 November 2018; Received in revised form 16 March 2019; Accepted 19 March 2019 Available online 20 March 2019 2213-3437/ © 2019 Published by Elsevier Ltd.
Journal of Environmental Chemical Engineering 7 (2019) 103040
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Nomenclature
uij
gαβ N Nα Nβ P q r
α β σij εij ε0 εr
Radial distribution function analysis The total number of particles The number of atoms of the α component The number of atoms in the β component Density The charges on sites The distance between atoms
Interaction potentials the Lennard-Jones and Coulombic models One of the atoms of lead, cadmium or copper An atom in the structure of zeolite Lennard-Jones parameters Lennard-Jones parameters The permittivity of vacuum (8.8542 × 10−12 C2/N m2) Dielectric constant
Six different types of zeolites including LTA, FAU, LTN, EDI, THO, and NAT are chosen as the base zeolite and sodium ion was located as a nonstructural cation inside the zeolite structure to improve the ion exchange capability of the mentioned zeolites. Using the CVVF force field, which has the ability to minimize energy in mineral polymers, the sodium ions were located in places with the lowest possible energy levels. The electrolyte solution consists of 500 molecules of water and one heavy metal cation equivalent to the positive charge of sodium in the zeolite network. Given the presence of water molecules that slow down the movement of cations due to the filling of space and motor paths, we consider the presence of water in the electrolyte and replace it with the replacement of the steady-state dielectric (The electrical effect of water). Fig. 1 shows the structure of zeolites and the simulation box schematically. In the simulation box, zeolite is located in the center of the box, and on both sides, there are water molecules and metal ions. Structural specifications of the consıdered zeolites are summarized in Table 1 [30]. As can be seen in Table 1, FAU zeolite has the largest pore size for penetrating a molecule inside zeolite, and the smallest belongs to the LTN zeolite. In the following, the data on the input file of the LAMMPS software was completed. Molecular dynamics simulations of metal ions separation from electrolyte solution by zeolites have been done using LAMMPS software. An NVT ensemble was used in all these molecular dynamic simulations. For the short range site-site interaction potentials the Lennard-Jones (LJ) and Coulombic models are used as the following form:
Their results showed that the acid treatments decreased the amount of ion separation, but in the case of zeolite modified by NaOH, the results were quite the opposite, and the structure of natural zeolite and its cavities become even more uniform. Pepe et al. [24] studied the removal from between Ba2+ cations and aqueous solutions by natural zeolite with various initial concentrations by ion exchange process. They concluded that the removal of barium ion occurs by increasing its initial concentration in shorter times. It is due to the increased interaction between the structure of zeolite and the negative charge density of discontinuous electrons and barium positive ions. Mondale et al. [25] used irionite, cobacite, clinoptilolite and mordenite natural zeolites to remove heavy metal ions, including silver ions, lead, cadmium, zinc, copper, nickel, and mercury. Moreover, removal of various metal ions from water and waste using A-type zeolite were studied by Mang et al. [26]. They reported that this type of zeolite is capable to adsorb 227.77 mg/g of Pb2+ ions and 123.05 mg/g of Ag+ ions. In addition, Ismail et al. [27] studied the ion exchange properties of zeolite A. They found that pH and ion concentration were parameters that affected the ion exchange phenomenon. Feltcher et al. [28] synthesized Na-X and Na-Y zeolites to study the removing of palladium and platinum ions in aqueous solutions. They concluded that zeolite Na-Y has a higher selectivity for both ions than zeolite Na-X. In recent years, molecular simulation has been used in the study of various aspects of zeolites, including the structure of zeolite, the effect of the presence of water molecules within the structure of zeolite, the flexibility of structure-forming molecules and ion exchange. For instance, Louisferma et al. [29] used the Monte Carlo method to investigate the distribution of cations inside zeolite Y structure. They reported that there is a very good match between simulation results and experimental data. In the present work, to determine the effect of different zeolite structures on ion exchange, the molecular dynamics simulation is performed. Different zeolites with different properties for removal of heavy metal ions of various sizes were studied. Also, multi-component systems were considered to determine the effect of competitive conditions between heavy metal ions. In brief, the innovation of the work includes: (1) The thickness of the membrane and the concentration of the electrolyte solution, which is the main driving force of the ion exchange, are an important parameters. Therefore, the effects of different concentration of electrolyte and different thickness of membrane on ion exchange ratio and diffusivity of ions have been studied. (2) The ion exchange behavior in a mixture can be different from pure state, and the presence of another ion affects how ion exchange is different. Therefore, in this study, the behavior of ion exchange as pure and mixed is investigated. (3) Another point is that in less research, molecular dynamic simulations of heavy metal ion exchange for a wide variety of zeolites structure have been studied.
12
σij uij (r ) = 4εij ⎡ ⎛ ⎞ ⎢ ⎣⎝ r ⎠
qi qj σij 6 −⎛ ⎞⎤+ ⎥ r 4 πε ⎝ ⎠⎦ 0 rεr
(1)
where r is the distance between atoms, σij and εij are the LJ parameters, qi and qj represent the charges on sites i and j, respectively. ε0 = 8.8542 × 10−12 C2.N−1. m−2 is the permittivity of vacuum [31]. In this work, the implicit method was used to reduce the computational time. As mentioned before, in the present work, water molecules were not directly introduced into simulations and water was considered as a medium with a dielectric constant εr = 78 [31,32]. Site charge values were adapted from the paper of Salmas et al. [32]. The LAMMPS software calculates the forces imposed on each particle at any time t from Eq. (2), and then calculates the coordinates of d2r the particle position at any t from the motion equation F= m 2 . Again, dt Eq. (1) i for s used for new r in the range of the direct cutoff radius Rcut = 12 Å. This will be repeated until the energy changes of the system and the exchange rate of the ions reach the equilibrium. →
→
Fi = −∇V(r1,...,rN)
(2)
In the potential model, cations were considered as hard spheres, including partial charge, and the bond formation with the anions in the electrolyte is neglected. The parameters for all the species in the simulation boxes are shown in Table 2. Using the straight cut radius is very effective in reducing the simulation time. Considering that the shortest side of the simulation box is 24 Å, the cutoff radius was considered as half that length. Except sodium ions LAMMPS software calculates the forces imposed on each
2. Simulation methodology In this study, the molecular dynamic simulation has been performed to separate metal ions from water using the LAMMPS program. The simulation box is created using the Material Studio software, which has a graphical interface as well as a comprehensive database for zeolites. 2
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Fig. 1. Schematic of (a) zeolite with LTA, FAU, LTN, EDI, THO, and NAT structures and (b) simulation box for LTA zeolite.
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from molecular simulations were shown in Fig. 2(a). The results show that ion exchange is performed and the membrane is in equilibrium with the electrolyte solution. The amount of sodium ion exchange with electrolyte solution was approximately 50% of its initial value in zeolite. Also, the obtained result for Na ion exchange ratio were comparable with the experimental results of Salmas et al. [32] (Fig. 2(b)). In the study of Salmas et al., the results were related to the proportion of the amount of sodium ion exchange with electrolyte solution to the initial amount of sodium in zeolite, but in this study, the simulation results were considered as the proportion of the sodium in the structure of zeolite to the initial amount of sodium in zeolite. As can be seen, the results of the present study were in good agreement with the results provided by Salmas et al. In addition, the results show that LTA zeolite does not tend to have much Cl− ions and does not allow them to enter into their structure, and some which adsorbed are in contact with zeolite by electrolyte solution. In this study, the molecular simulation of ion exchange of Pb2+, Cu2+and Cd2+ ions have been studied and all simulations have been done for all three ions. Just because of the number of figures, they are not all in the text. Silver ion has been investigated for validation of simulations, Because there were no simulations results about Pb2+, Cu2+and Cd2+ ions, and no data was available for comparison of ion exchange in these zeolites. In order to investigate the effect of cavity size, length of zeolite membrane, as well as ion concentration in electrolyte solution, simulation boxes with zeolite membrane size of 1, 2 and 3 times the cells of unit zeolite LTA and electrolyte solutions with concentrations of 0.5, 1, 1.5 and 2 cadmium ions are constructd, respectively. During
Table 1 Structural characteristics of LTA, FAU, EDI, THO, NAT and LTN zeolites [30]. Zeolite type
Cell composition
Largest Cavity Diameter (Å)
Pore Limiting Diameter (Å)
FAU LTA LTN THO NAT EDI
Si96Al96O384 Si96Al96O384 Si384Al384O1536 Si20Al20O80 Si24Al16O80 Si6Al4O20
11.24 11.05 10.13 5.15 4.52 5.72
7.35 4.21 2.08 3.26, 3, 3.69 2.99, 2.99, 4.38 3.2, 3.2, 3.44
Table 2 Interaction potential parameters and ion radius [32]. Atom/Ion
z
ε (kJ/mol)
σ (Å)
Si4+ Al3+ O Na+ Pb2+ Cd2+ Cu2+ Cl−
+1.540 +1.110 −0.9125 +1.000 +2.000 +2.000 +2.000 −1.000
0.263 0.019 0.426 0.099 0.662 0.228 0.005 0.100
3.914 3.936 2.760 2.584 3.829 2.537 3.114 4.400
particle at any time t from E that should be able to move and participate in the ion exchange process, other atoms constituting the structure of zeolite were held constant during the simulation period. Intermittent boundary conditions were used for all three coordinate directions. All simulations were carried out at 298 K. The velocity of each particle in the simulation box is calculated using the Verlet algorithm [33]. All simulations were performed for 10 ns with time step of 1 fs, including 2 ns for equilibration and 8 ns for production. The structural information of the species was stored every 5 ps for analysis, including the number of atoms in the structure of zeolite and within the solution, the mean square displacement (MSD) of the ions in the structure of zeolite in various directions and the radial distribution function (RDF). To determine the distribution of distances between two types of α and β atoms, a radial distribution function analysis is used which is based on the statistical mechanics of its mathematical relation as follows [31]: N
gαβ (r ) =
Nβ
α N ∑ ∑ 〈δ (r − |rk − ri|)〉 ρNα Nβ i = 1 k
(3)
where N, ρ and r are the total number of particles, density and distance, respectively. δ is the three-dimensional Dirac delta function. Nα stands for the number of atoms of the component α (one of the atoms of lead, cadmium or copper) while Nβ indicates the number of atoms in the β component (an atom in the structure of zeolite). In fact, the radial distribution function specifies the tendency between the atoms to be located next to each other. If the molecules approach each other, the intensity of the RDF peak increases. To confirm the ion exchange ratios, the radial distribution function graphs (RDFs) for zeolite were prepared with different structures in single ion mode. 3. Results and discussion In this section, the results of molecular dynamics simulation for separation of metal ions (Pb2+, Cd2+ and Cu2+) from aqueous solution as single and multi-ion systems by LTA, FAU, LTN, THO, NAT, and LTN zeolite structures have been discussed. In order to investigate the ion exchange as a single ion, LTA zeolite with Si/Al molar ratio = 1 was considered. The simulated membrane consisted of three unit cells which containing 288 of sodium ions and aluminum atoms. The electrolyte solution, including Cl− and Ag+ ions, was 288 ions on both sides of the membrane. The ion exchange results
Fig. 2. Ion exchange for LTA zeolite and AgCl electrolyte solution at 370 K. (a) the simulated result in this work, and (b) the experimental result reported by salmas et al. 4
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reduces the availability of empty spaces and zeolite adsorption sites. Therefore more time was needed for ion exchange. Therefore, in the same time period and in the same concentration, the ion exchange rate also decreased, so that the results in Fig. 3 show that when the membrane thickness is 3 times of that of the unit cell, ion exchange was more than 90% for a solution of electrolyte at a concentration of 0.5. In order to perform molecular simulations of ion exchange between different zeolites and various electrolyte solutions including bivalent capacitive lead, copper and cadmium ions, single ion and pair of ions, the membrane thickness is 2 times the zeolite unit cell and the concentration of electrolyte solution containing bicarbonate cations is considered to be 2 times higher than sodium. Previous studies have shown that the type of zeolite structure can affect their behavior in ion exchange. Therefore, zeolites were used with different structures of LTA, FAU, LTN, THO, NAT and EDI to evaluate ion exchange in solutions containing Pb2+, Cd2+ and Cu2+ ions. The reason for choosing these zeolite structures is that the functional properties of some of them have not been studied yet and also have different pore sizes. In Fig. 4, ion exchange ratios for single-ion systems in LTA, FAU, LTN, THO, NAT and EDI structures are observed at 298 K for electrolyte solutions containing Pb2+, Cd2+ and Cu2+ ions. The results clearly indicate that the type of structure can play an important role in time to achieve equilibrium. The FAU zeolite has the largest pore size of 35.7 Å. Thus, the ions penetrate more quickly than other structures, and reach the equilibrium and LTN structure needed longest time to reach the ion exchange equilibrium because it has the smallest pore for ion diffusion. Therefore, it can be concluded that the larger the zeolite cavity is, the greater the ion exchange rate and ion exchange rate for adsorption of metal ions. As it can be seen, all zeolite structures tend to exchange with Cd2+ ion in electrolyte solution, which has the highest ionic exchange among metal ions, and the lowest amount of ion exchange associated with the Pb2+ ion in solution. The simulation results were summarized in Table 3. The results show that the LTA/Na zeolite has the highest (69%) and the FAU structure has the lowest (46%) cadmium adsorption. For single Pb2+ and Cu2+ ions, LTA and EDI zeolites have the best performance. Contrary, the results also show that THO and NAT zeolites have a very slight tendency to exchange lead ions from electrolyte solution, i.e. less than 10% ion exchange. Since the lead Van der Waals radius is larger than other ions and is close to the size of the THO and NAT zeolites cavities, therefore, the repulsion between the lead ion and the zeolite wall is high and allows it to be exchange and transmitted less. Consequently, the results of the simulation of metal ions exchange by zeolite with different structures indicate that the tendency to carry out the ion exchange process for zeolites is Cd2+ > Cu2+ > Pb2+. In fact, ionic exchange is carried out in the liquid phase and has no significant dependence on molecular mass. These adsorption behaviors depend on the structure of zeolite and the interaction between zeolite and metal ions. In other words, the porosity of the zeolite structure plays a decisive role in the attraction and repulsion of metal ions with a zeolite structure. Therefore, the mean square displacement and the radial distribution function of metal ions in the structure of zeolites are discussed below. Fig. 5 shows the MSD of single metal ions in LTA, FAU, LTN, THO, NAT and EDI zeolite structures. The results show that over time, metal ions move in all of the zeolite structures, but their displacement depends on the structure of the zeolite and the type of ions. The results also show that copper ion has the largest displacement in all zeolite structures, which is due to the low atomic mass of copper ions and can easily move inside the structure of zeolite. In addition, the smallest MSD belongs to lead metal ion, which is the same in all structures except LTA zeolite. The LTA zeolite has shown the best result for the exchange of lead ions among other zeolites because of the size of its cavities for conduction with relatively large van der Waals radius of lead ion. However, the amount of lead adsorbed in it is less than that of
Fig. 3. Ion exchange ratio versus time for LTA zeolite in different concentration of electrolyte for membrane thickness of (a) one unit cell, (b) two unit cell and (c) three unit cell.
simulation, ion exchange dynamics was analyzed by a parametric calculation, which called ion exchange ratio time. This parameter is defined as the ratio of the number of metal ions inserted into the zeolite structure to the initial number of these ions in the electrolyte solution. As the results in Fig. 3, the rate and amount of ion exchange are strongly influenced by the thickness of zeolite and the concentration of electrolyte solution. As shown in Fig. 3(a), ion exchange is rapidly performed in less than 0.5 ns for the thickness of one unit cell of LTA and when the concentration of the electrolyte solution was 0.5, the ion exchange was 100% because it filled a number of zeolite sites while there was still an empty site. However, with increasing concentrations, the ion exchange rate decreased and eventually concentration of 2 did not exceed 60% ion exchange. By increasing the thickness and thus increasing the sodium atoms in the zeolite structure, the availability of metal ions in the electrolyte is decreased due to the increased interaction with a greater number of sodium ions and an increase in the pathway inside zeolite. It 5
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Fig. 4. Ion exchange ratio for single-ion systems for LTA, FAU, LTN, THO, NAT and LTN zeolite structures with a thickness of 2 times the cell unit and concentration of 2 times the sodium.
cadmium. The reason is that, despite the proper interaction between these zeolites and lead ions, metal cation has a relatively comfortable movement in the structure of LTA, but suitable sites for lead adsorption are fewer than those can adsorb the cadmium ions. Diffusion of copper ion in EDI zeolite is higher than other zeolites. In other words, the pore size and porosity of this type of zeolite, as well as gravity and repulsion between copper ion and zeolite atoms, allow more free movement of copper than other structures. The Self-diffusion coefficients evaluated for single metal ions in LTA, FAU, LTN, THO, NAT and EDI zeolites using the slope of MSD vs time (Fig. 5) are shown in Table 4. As can be seen, the highest diffusion coefficients were related to copper ion in all different types of zeolite. The obtained results are comparable with results which presented for diffusion of ions in zeolites. In Fig. 6, the RDF diagrams for Cd2+, Cu2+, and Pb2+ ions in LTA,
Table 3 Simulation results of ion exchange for different types of zeolite. Zeolite
EDI FAU LTA NAT THO LTN
Adsorbed ions Pb2+
Cu2+
Cd2+
0.46 0.37 0.53 0.07 0.09 0.33
0.61 0.43 0.52 0.38 0.41 0.43
0.68 0.46 0.69 0.49 0.53 0.51
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Fig. 5. The mean square displacement curves of metal ions for single-ion systems for zeolite with LTA, FAU, LTN, THO, NAT and LTN structures with a thickness of 2 times the cell unit and concentration of 2 times sodium concentration.
zeolite, first sharp peak is located at a distance of 3.5 Å. A further review of the RDF diagrams shows that for each zeolite and in single-ion mode, the ion that has the largest ion exchange has the highest first peak. In other words, in zeolite structures whose mean ion exchange ratio was Cd2+ > Pb2+ > Cu2+. The same result was repeated for the height of the first peak in the RDF diagrams. This means that since the RDF peaks of the cadmium ion are longer, the probability of finding cadmium ions around aluminum atoms is also higher, which is consistent with the highest ion exchange rate for the ions. The RDF graphs of cadmium ions for these zeolites have higher peaks, confirms the results obtained by calculating the ion exchange ratio. Another notable point is that the RDF first peak for Cd2+ and Cu2+ ions occurs at very close locations, but the first peak of lead ion is always in different radius and, of course, larger than the other two ions. This is due to the closer proximity size of cadmium and copper ions. Since the van der Waals radius of lead ion is higher than other ions, so far away from the Al atoms located in the zeolite structure, and the peaks of this ion are farther away and it was located more than 4 Å. In order to investigate the effect of Cd2+, Cu2+ and Pb2+ ions on the adsorption and selectivity of two-ion systems with the same ratio of sodium ion in electrolyte solution were simulated by molecular
Table 4 Self-Diffusion Coefficients of Cd+2, Cu+2 and Pb+2 ions in different types of zeolite with a thickness of 2 times the cell unit and concentration of 2 times sodium concentration. zeolite
EDI LTN FAU THO LTA NAT
Self-Diffusion (10−9 m2/s) Cd+2
Cu+2
Pb+2
19.79 10.79 7.14 22.56 3.58 17.67
40.96 17.40 13.87 27.18 21.66 21.91
21.84 9.28 7.12 11.40 8.60 10.44
FAU, LTN, THO, NAT and EDI zeolite structures are presented, in which the tendency of each of the lead, cadmium and copper atoms for being along with Al in the structure of zeolite is investigated. The first sharp peak is related to the Cd2+ ion, which is located within the 3–4 Å of the aluminum atom inside the zeolite structure, and this behavior is visible in all of the structures of zeolite other than the LTN structure. For LTN 7
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Fig. 6. Radial distribution function for Cd2+, Cu2+, and Pb2+ ions with a concentration of 2 times the sodium in LTA, FAU, LTN, THO, NAT and LTN zeolite structures with a thickness of 2 times the cell unit.
will be less intrinsic to absorb ions than others. In addition, for electrolyte solutions containing Pb / Cd, the results show that zeolite structures tend to exchange cadmium ions relative to lead, and in most structures, approximately 80% of cadmium ions are exchanged, which is approximately equal to 100% for EDI structure. However, the exchange value for lead ions would not exceed 60%, so the results confirm that this structure of zeolite tends to attract iodine cadmium to lead ion. Furthermore, the results for electrolyte solution including lead ion and copper ions indicated that most zeolite structures tend to be more attractive to copper adsorption and exchange about 80%, which is about 100% for EDI zeolite. However, for FAU zeolite, the copper removal rate was approximately 40%.
dynamics method to study the effect of simultaneous presence of metal ions in electrolyte solution. The simulation results of ion exchange ratios for a two-ion electrolytic solutions Cu / Cd, Pb / Cd and Pb / Cu with LTA, FAU, LTN, THO, NAT and EDI zeolite structures are shown in Fig. 7. The results show that the selectivity of zeolites are strongly dependent on their molecular structure, so that in an EDI structure and electrolyte containing Cd / Cu, approximately 100% of cadmium ions and 85% copper ions were exchanged that had the highest selectivity to cadmium. For other zeolites except FAU, there was almost the same behavior. In the FAU structure, the cadmium ion selectivity is higher than copper, but this zeolite cannot exchange all cadmium ions, as well as about 40% copper ion exchange. Since in this structure the size of the cavity in which it penetrates is more and almost twice as high as the other zeolites (7.35 Å), the gravitational effect of the zeolite structure 8
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Fig. 7. Ion exchange ratio in binary system for zeolite with LTA, FAU, LTN, THO, NAT and LTN structures.
4. Conclusion
structure, ie, the size of the cavities and the number of adsorption sites. In addition, under competitive conditions, the ionic exchange for an electrolyte containing two metal cation, the repulsing effect of cations on each other has also been effective.
Simulation results of the ion exchange process in single-ion systems for all zeolite structures showed that the minimum and maximum ion exchange rate is related to Pb2 + ion and Cd2 + respectively. The simulation results of the metal ions adsorption by different structures zeolites show that the tendency to carry out the ion exchange process for all zeolites is Cd2+ > Cu2+ > Pb2+. In fact, these adsorption behaviors depend on the structure of zeolite and the interaction of zeolite with metal ions. The results also show copper ion has a higher displacement is in all zeolite structures. It can be due to the low weight of copper ion which can easily move inside the zeolite structure. Furthermore, the selectivity behavior of zeolites under the same conditions and the same Al / Si ratio strongly depends on the molecular
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Molecular simulation of the ion exchange behavior of Cu2+, Cd2+ and Pb2+ ions on different zeolites exchanged with sodium
T
⁎
Hossein Khanmohammadia, Behrouz Bayatib, , Javad Rahbar- Shahrouzia, Ali-Akbar Babaluoa, Asma Ghorbanib a b
Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran Chemical Engineering Department, Ilam University, Ilam, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Ion exchange Heavy metal Zeolite Molecular dynamics
In this research, the molecular dynamics simulation of separating metal ions from water at 298 K using LAMMPS software for LTA, FAU, LTN, THO, NAT and EDI zeolites and electrolyte solutions containing Cu2+, Cd2+ and Pb2+ individually and in pairs of ions have been studied. Among the single ions, the lowest ion exchange rate was Pb2+ and the highest was Cd2+, as well as the adsorption rate and ion exchange, which was directly related to the size of zeolite cavities. The FAU zeolite with the largest cavity diameter had the fastest adsorption rate. The LTA structure with the adsorption of 69% of cadmium and FAU with 46% adsorption had the highest and lowest ion exchange rate. Furthermore, the highest ion displacement in different types of zeolites was related to copper ion due to its low weight. In two cations electrolytes solutions, all the structures tended to adsorb cadmium ion EDI structures with 100% cadmium adsorption and 85% copper, FAU with 78% cadmium adsorption and 40% copper have been shown the highest and lowest selectivity and uptake of cadmium relative to copper, respectively. In addition, for electrolyte containing cadmium and lead, EDI structure with 100% cadmium adsorption against 60% lead and THO structure with 70% cadmium adsorption versus 38% lead had the highest and lowest selectivity for cadmium. In copper-leaded electrolytes, copper was superior to selected. EDI adsorbed 100% copper versus 60% lead, and FAU had a 40% copper uptake, compared to 65% lead.
1. Introduction Since provision of water for drinking, domestic, agricultural, industrial have always been a critical issue, water and wastewater pollutants today are among the greatest concerns of humankind [1–4]. These contaminations are of various types such as chemical and pharmaceutical, colored, nuclear, and so on. These contaminants also come from a variety of sources such as industrial and domestic wastewater [5–7]. Heavy metal ions are one of these types of pollutants, which are among the most dangerous ones [8–10]. These ions inflict enormous damage to the environment and the natural processes involved [11,12]. These hazardous materials also cause various diseases and disorders in humans [13–16]. Therefore, it is necessary to eliminate or reduce their content in water to safe limits. There are many methods for separating heavy metal ions from wastewater, including chemical separation, filtration, membrane separation, electrochemical purification, adsorption, and ion exchange processes [13,14,17–20]. Today, the ion exchange method is widely accepted due to reasonable cost, process simplicity, low risk, and metal ⁎
recovery. Porous materials such as zeolites are widely used in the ion exchange process. The zeolites are crystalline and hydrated aluminosilicates of alkali and alkaline earth metals, especially sodium, calcium, magnesium, strontium and barium, which contain three-dimensional networks consisting of [SiO4] and [AlO4]- four faces. These materials have a pore size of 0–50 nm [21]. Due to the presence of non-structural positive ions, including Ca2+, K+ and Na+ ions, zeolites are very common in ion exchange processes and show high ion exchange efficiency. Hence, the use of these materials is growing to eliminate the contaminations of water and wastewater, especially the removal of heavy metal ions. So far, several experimental and theoretical studies have been carried out on the removal of heavy metals by means of ion exchange processes using zeolites. Ren et al. [22] have studied the separation of Pb and Cr from aqueous solutions by modifying natural zeolite. They reported that zeolite modification by cetylpyridium bromide increases the ion absorption capacity because the size of the natural zeolite pore size is uniform, small and suitable for adsorption of mentioned ions. Wen et al. [23] investigated the separation of metal ions from Pb, Cu, Zn and Cd using modified zeolites by NaOH and HCl.
Corresponding author. E-mail address: [email protected] (B. Bayati).
https://doi.org/10.1016/j.jece.2019.103040 Received 26 November 2018; Received in revised form 16 March 2019; Accepted 19 March 2019 Available online 20 March 2019 2213-3437/ © 2019 Published by Elsevier Ltd.
Journal of Environmental Chemical Engineering 7 (2019) 103040
H. Khanmohammadi, et al.
Nomenclature
uij
gαβ N Nα Nβ P q r
α β σij εij ε0 εr
Radial distribution function analysis The total number of particles The number of atoms of the α component The number of atoms in the β component Density The charges on sites The distance between atoms
Interaction potentials the Lennard-Jones and Coulombic models One of the atoms of lead, cadmium or copper An atom in the structure of zeolite Lennard-Jones parameters Lennard-Jones parameters The permittivity of vacuum (8.8542 × 10−12 C2/N m2) Dielectric constant
Six different types of zeolites including LTA, FAU, LTN, EDI, THO, and NAT are chosen as the base zeolite and sodium ion was located as a nonstructural cation inside the zeolite structure to improve the ion exchange capability of the mentioned zeolites. Using the CVVF force field, which has the ability to minimize energy in mineral polymers, the sodium ions were located in places with the lowest possible energy levels. The electrolyte solution consists of 500 molecules of water and one heavy metal cation equivalent to the positive charge of sodium in the zeolite network. Given the presence of water molecules that slow down the movement of cations due to the filling of space and motor paths, we consider the presence of water in the electrolyte and replace it with the replacement of the steady-state dielectric (The electrical effect of water). Fig. 1 shows the structure of zeolites and the simulation box schematically. In the simulation box, zeolite is located in the center of the box, and on both sides, there are water molecules and metal ions. Structural specifications of the consıdered zeolites are summarized in Table 1 [30]. As can be seen in Table 1, FAU zeolite has the largest pore size for penetrating a molecule inside zeolite, and the smallest belongs to the LTN zeolite. In the following, the data on the input file of the LAMMPS software was completed. Molecular dynamics simulations of metal ions separation from electrolyte solution by zeolites have been done using LAMMPS software. An NVT ensemble was used in all these molecular dynamic simulations. For the short range site-site interaction potentials the Lennard-Jones (LJ) and Coulombic models are used as the following form:
Their results showed that the acid treatments decreased the amount of ion separation, but in the case of zeolite modified by NaOH, the results were quite the opposite, and the structure of natural zeolite and its cavities become even more uniform. Pepe et al. [24] studied the removal from between Ba2+ cations and aqueous solutions by natural zeolite with various initial concentrations by ion exchange process. They concluded that the removal of barium ion occurs by increasing its initial concentration in shorter times. It is due to the increased interaction between the structure of zeolite and the negative charge density of discontinuous electrons and barium positive ions. Mondale et al. [25] used irionite, cobacite, clinoptilolite and mordenite natural zeolites to remove heavy metal ions, including silver ions, lead, cadmium, zinc, copper, nickel, and mercury. Moreover, removal of various metal ions from water and waste using A-type zeolite were studied by Mang et al. [26]. They reported that this type of zeolite is capable to adsorb 227.77 mg/g of Pb2+ ions and 123.05 mg/g of Ag+ ions. In addition, Ismail et al. [27] studied the ion exchange properties of zeolite A. They found that pH and ion concentration were parameters that affected the ion exchange phenomenon. Feltcher et al. [28] synthesized Na-X and Na-Y zeolites to study the removing of palladium and platinum ions in aqueous solutions. They concluded that zeolite Na-Y has a higher selectivity for both ions than zeolite Na-X. In recent years, molecular simulation has been used in the study of various aspects of zeolites, including the structure of zeolite, the effect of the presence of water molecules within the structure of zeolite, the flexibility of structure-forming molecules and ion exchange. For instance, Louisferma et al. [29] used the Monte Carlo method to investigate the distribution of cations inside zeolite Y structure. They reported that there is a very good match between simulation results and experimental data. In the present work, to determine the effect of different zeolite structures on ion exchange, the molecular dynamics simulation is performed. Different zeolites with different properties for removal of heavy metal ions of various sizes were studied. Also, multi-component systems were considered to determine the effect of competitive conditions between heavy metal ions. In brief, the innovation of the work includes: (1) The thickness of the membrane and the concentration of the electrolyte solution, which is the main driving force of the ion exchange, are an important parameters. Therefore, the effects of different concentration of electrolyte and different thickness of membrane on ion exchange ratio and diffusivity of ions have been studied. (2) The ion exchange behavior in a mixture can be different from pure state, and the presence of another ion affects how ion exchange is different. Therefore, in this study, the behavior of ion exchange as pure and mixed is investigated. (3) Another point is that in less research, molecular dynamic simulations of heavy metal ion exchange for a wide variety of zeolites structure have been studied.
12
σij uij (r ) = 4εij ⎡ ⎛ ⎞ ⎢ ⎣⎝ r ⎠
qi qj σij 6 −⎛ ⎞⎤+ ⎥ r 4 πε ⎝ ⎠⎦ 0 rεr
(1)
where r is the distance between atoms, σij and εij are the LJ parameters, qi and qj represent the charges on sites i and j, respectively. ε0 = 8.8542 × 10−12 C2.N−1. m−2 is the permittivity of vacuum [31]. In this work, the implicit method was used to reduce the computational time. As mentioned before, in the present work, water molecules were not directly introduced into simulations and water was considered as a medium with a dielectric constant εr = 78 [31,32]. Site charge values were adapted from the paper of Salmas et al. [32]. The LAMMPS software calculates the forces imposed on each particle at any time t from Eq. (2), and then calculates the coordinates of d2r the particle position at any t from the motion equation F= m 2 . Again, dt Eq. (1) i for s used for new r in the range of the direct cutoff radius Rcut = 12 Å. This will be repeated until the energy changes of the system and the exchange rate of the ions reach the equilibrium. →
→
Fi = −∇V(r1,...,rN)
(2)
In the potential model, cations were considered as hard spheres, including partial charge, and the bond formation with the anions in the electrolyte is neglected. The parameters for all the species in the simulation boxes are shown in Table 2. Using the straight cut radius is very effective in reducing the simulation time. Considering that the shortest side of the simulation box is 24 Å, the cutoff radius was considered as half that length. Except sodium ions LAMMPS software calculates the forces imposed on each
2. Simulation methodology In this study, the molecular dynamic simulation has been performed to separate metal ions from water using the LAMMPS program. The simulation box is created using the Material Studio software, which has a graphical interface as well as a comprehensive database for zeolites. 2
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Fig. 1. Schematic of (a) zeolite with LTA, FAU, LTN, EDI, THO, and NAT structures and (b) simulation box for LTA zeolite.
3
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from molecular simulations were shown in Fig. 2(a). The results show that ion exchange is performed and the membrane is in equilibrium with the electrolyte solution. The amount of sodium ion exchange with electrolyte solution was approximately 50% of its initial value in zeolite. Also, the obtained result for Na ion exchange ratio were comparable with the experimental results of Salmas et al. [32] (Fig. 2(b)). In the study of Salmas et al., the results were related to the proportion of the amount of sodium ion exchange with electrolyte solution to the initial amount of sodium in zeolite, but in this study, the simulation results were considered as the proportion of the sodium in the structure of zeolite to the initial amount of sodium in zeolite. As can be seen, the results of the present study were in good agreement with the results provided by Salmas et al. In addition, the results show that LTA zeolite does not tend to have much Cl− ions and does not allow them to enter into their structure, and some which adsorbed are in contact with zeolite by electrolyte solution. In this study, the molecular simulation of ion exchange of Pb2+, Cu2+and Cd2+ ions have been studied and all simulations have been done for all three ions. Just because of the number of figures, they are not all in the text. Silver ion has been investigated for validation of simulations, Because there were no simulations results about Pb2+, Cu2+and Cd2+ ions, and no data was available for comparison of ion exchange in these zeolites. In order to investigate the effect of cavity size, length of zeolite membrane, as well as ion concentration in electrolyte solution, simulation boxes with zeolite membrane size of 1, 2 and 3 times the cells of unit zeolite LTA and electrolyte solutions with concentrations of 0.5, 1, 1.5 and 2 cadmium ions are constructd, respectively. During
Table 1 Structural characteristics of LTA, FAU, EDI, THO, NAT and LTN zeolites [30]. Zeolite type
Cell composition
Largest Cavity Diameter (Å)
Pore Limiting Diameter (Å)
FAU LTA LTN THO NAT EDI
Si96Al96O384 Si96Al96O384 Si384Al384O1536 Si20Al20O80 Si24Al16O80 Si6Al4O20
11.24 11.05 10.13 5.15 4.52 5.72
7.35 4.21 2.08 3.26, 3, 3.69 2.99, 2.99, 4.38 3.2, 3.2, 3.44
Table 2 Interaction potential parameters and ion radius [32]. Atom/Ion
z
ε (kJ/mol)
σ (Å)
Si4+ Al3+ O Na+ Pb2+ Cd2+ Cu2+ Cl−
+1.540 +1.110 −0.9125 +1.000 +2.000 +2.000 +2.000 −1.000
0.263 0.019 0.426 0.099 0.662 0.228 0.005 0.100
3.914 3.936 2.760 2.584 3.829 2.537 3.114 4.400
particle at any time t from E that should be able to move and participate in the ion exchange process, other atoms constituting the structure of zeolite were held constant during the simulation period. Intermittent boundary conditions were used for all three coordinate directions. All simulations were carried out at 298 K. The velocity of each particle in the simulation box is calculated using the Verlet algorithm [33]. All simulations were performed for 10 ns with time step of 1 fs, including 2 ns for equilibration and 8 ns for production. The structural information of the species was stored every 5 ps for analysis, including the number of atoms in the structure of zeolite and within the solution, the mean square displacement (MSD) of the ions in the structure of zeolite in various directions and the radial distribution function (RDF). To determine the distribution of distances between two types of α and β atoms, a radial distribution function analysis is used which is based on the statistical mechanics of its mathematical relation as follows [31]: N
gαβ (r ) =
Nβ
α N ∑ ∑ 〈δ (r − |rk − ri|)〉 ρNα Nβ i = 1 k
(3)
where N, ρ and r are the total number of particles, density and distance, respectively. δ is the three-dimensional Dirac delta function. Nα stands for the number of atoms of the component α (one of the atoms of lead, cadmium or copper) while Nβ indicates the number of atoms in the β component (an atom in the structure of zeolite). In fact, the radial distribution function specifies the tendency between the atoms to be located next to each other. If the molecules approach each other, the intensity of the RDF peak increases. To confirm the ion exchange ratios, the radial distribution function graphs (RDFs) for zeolite were prepared with different structures in single ion mode. 3. Results and discussion In this section, the results of molecular dynamics simulation for separation of metal ions (Pb2+, Cd2+ and Cu2+) from aqueous solution as single and multi-ion systems by LTA, FAU, LTN, THO, NAT, and LTN zeolite structures have been discussed. In order to investigate the ion exchange as a single ion, LTA zeolite with Si/Al molar ratio = 1 was considered. The simulated membrane consisted of three unit cells which containing 288 of sodium ions and aluminum atoms. The electrolyte solution, including Cl− and Ag+ ions, was 288 ions on both sides of the membrane. The ion exchange results
Fig. 2. Ion exchange for LTA zeolite and AgCl electrolyte solution at 370 K. (a) the simulated result in this work, and (b) the experimental result reported by salmas et al. 4
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reduces the availability of empty spaces and zeolite adsorption sites. Therefore more time was needed for ion exchange. Therefore, in the same time period and in the same concentration, the ion exchange rate also decreased, so that the results in Fig. 3 show that when the membrane thickness is 3 times of that of the unit cell, ion exchange was more than 90% for a solution of electrolyte at a concentration of 0.5. In order to perform molecular simulations of ion exchange between different zeolites and various electrolyte solutions including bivalent capacitive lead, copper and cadmium ions, single ion and pair of ions, the membrane thickness is 2 times the zeolite unit cell and the concentration of electrolyte solution containing bicarbonate cations is considered to be 2 times higher than sodium. Previous studies have shown that the type of zeolite structure can affect their behavior in ion exchange. Therefore, zeolites were used with different structures of LTA, FAU, LTN, THO, NAT and EDI to evaluate ion exchange in solutions containing Pb2+, Cd2+ and Cu2+ ions. The reason for choosing these zeolite structures is that the functional properties of some of them have not been studied yet and also have different pore sizes. In Fig. 4, ion exchange ratios for single-ion systems in LTA, FAU, LTN, THO, NAT and EDI structures are observed at 298 K for electrolyte solutions containing Pb2+, Cd2+ and Cu2+ ions. The results clearly indicate that the type of structure can play an important role in time to achieve equilibrium. The FAU zeolite has the largest pore size of 35.7 Å. Thus, the ions penetrate more quickly than other structures, and reach the equilibrium and LTN structure needed longest time to reach the ion exchange equilibrium because it has the smallest pore for ion diffusion. Therefore, it can be concluded that the larger the zeolite cavity is, the greater the ion exchange rate and ion exchange rate for adsorption of metal ions. As it can be seen, all zeolite structures tend to exchange with Cd2+ ion in electrolyte solution, which has the highest ionic exchange among metal ions, and the lowest amount of ion exchange associated with the Pb2+ ion in solution. The simulation results were summarized in Table 3. The results show that the LTA/Na zeolite has the highest (69%) and the FAU structure has the lowest (46%) cadmium adsorption. For single Pb2+ and Cu2+ ions, LTA and EDI zeolites have the best performance. Contrary, the results also show that THO and NAT zeolites have a very slight tendency to exchange lead ions from electrolyte solution, i.e. less than 10% ion exchange. Since the lead Van der Waals radius is larger than other ions and is close to the size of the THO and NAT zeolites cavities, therefore, the repulsion between the lead ion and the zeolite wall is high and allows it to be exchange and transmitted less. Consequently, the results of the simulation of metal ions exchange by zeolite with different structures indicate that the tendency to carry out the ion exchange process for zeolites is Cd2+ > Cu2+ > Pb2+. In fact, ionic exchange is carried out in the liquid phase and has no significant dependence on molecular mass. These adsorption behaviors depend on the structure of zeolite and the interaction between zeolite and metal ions. In other words, the porosity of the zeolite structure plays a decisive role in the attraction and repulsion of metal ions with a zeolite structure. Therefore, the mean square displacement and the radial distribution function of metal ions in the structure of zeolites are discussed below. Fig. 5 shows the MSD of single metal ions in LTA, FAU, LTN, THO, NAT and EDI zeolite structures. The results show that over time, metal ions move in all of the zeolite structures, but their displacement depends on the structure of the zeolite and the type of ions. The results also show that copper ion has the largest displacement in all zeolite structures, which is due to the low atomic mass of copper ions and can easily move inside the structure of zeolite. In addition, the smallest MSD belongs to lead metal ion, which is the same in all structures except LTA zeolite. The LTA zeolite has shown the best result for the exchange of lead ions among other zeolites because of the size of its cavities for conduction with relatively large van der Waals radius of lead ion. However, the amount of lead adsorbed in it is less than that of
Fig. 3. Ion exchange ratio versus time for LTA zeolite in different concentration of electrolyte for membrane thickness of (a) one unit cell, (b) two unit cell and (c) three unit cell.
simulation, ion exchange dynamics was analyzed by a parametric calculation, which called ion exchange ratio time. This parameter is defined as the ratio of the number of metal ions inserted into the zeolite structure to the initial number of these ions in the electrolyte solution. As the results in Fig. 3, the rate and amount of ion exchange are strongly influenced by the thickness of zeolite and the concentration of electrolyte solution. As shown in Fig. 3(a), ion exchange is rapidly performed in less than 0.5 ns for the thickness of one unit cell of LTA and when the concentration of the electrolyte solution was 0.5, the ion exchange was 100% because it filled a number of zeolite sites while there was still an empty site. However, with increasing concentrations, the ion exchange rate decreased and eventually concentration of 2 did not exceed 60% ion exchange. By increasing the thickness and thus increasing the sodium atoms in the zeolite structure, the availability of metal ions in the electrolyte is decreased due to the increased interaction with a greater number of sodium ions and an increase in the pathway inside zeolite. It 5
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Fig. 4. Ion exchange ratio for single-ion systems for LTA, FAU, LTN, THO, NAT and LTN zeolite structures with a thickness of 2 times the cell unit and concentration of 2 times the sodium.
cadmium. The reason is that, despite the proper interaction between these zeolites and lead ions, metal cation has a relatively comfortable movement in the structure of LTA, but suitable sites for lead adsorption are fewer than those can adsorb the cadmium ions. Diffusion of copper ion in EDI zeolite is higher than other zeolites. In other words, the pore size and porosity of this type of zeolite, as well as gravity and repulsion between copper ion and zeolite atoms, allow more free movement of copper than other structures. The Self-diffusion coefficients evaluated for single metal ions in LTA, FAU, LTN, THO, NAT and EDI zeolites using the slope of MSD vs time (Fig. 5) are shown in Table 4. As can be seen, the highest diffusion coefficients were related to copper ion in all different types of zeolite. The obtained results are comparable with results which presented for diffusion of ions in zeolites. In Fig. 6, the RDF diagrams for Cd2+, Cu2+, and Pb2+ ions in LTA,
Table 3 Simulation results of ion exchange for different types of zeolite. Zeolite
EDI FAU LTA NAT THO LTN
Adsorbed ions Pb2+
Cu2+
Cd2+
0.46 0.37 0.53 0.07 0.09 0.33
0.61 0.43 0.52 0.38 0.41 0.43
0.68 0.46 0.69 0.49 0.53 0.51
6
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Fig. 5. The mean square displacement curves of metal ions for single-ion systems for zeolite with LTA, FAU, LTN, THO, NAT and LTN structures with a thickness of 2 times the cell unit and concentration of 2 times sodium concentration.
zeolite, first sharp peak is located at a distance of 3.5 Å. A further review of the RDF diagrams shows that for each zeolite and in single-ion mode, the ion that has the largest ion exchange has the highest first peak. In other words, in zeolite structures whose mean ion exchange ratio was Cd2+ > Pb2+ > Cu2+. The same result was repeated for the height of the first peak in the RDF diagrams. This means that since the RDF peaks of the cadmium ion are longer, the probability of finding cadmium ions around aluminum atoms is also higher, which is consistent with the highest ion exchange rate for the ions. The RDF graphs of cadmium ions for these zeolites have higher peaks, confirms the results obtained by calculating the ion exchange ratio. Another notable point is that the RDF first peak for Cd2+ and Cu2+ ions occurs at very close locations, but the first peak of lead ion is always in different radius and, of course, larger than the other two ions. This is due to the closer proximity size of cadmium and copper ions. Since the van der Waals radius of lead ion is higher than other ions, so far away from the Al atoms located in the zeolite structure, and the peaks of this ion are farther away and it was located more than 4 Å. In order to investigate the effect of Cd2+, Cu2+ and Pb2+ ions on the adsorption and selectivity of two-ion systems with the same ratio of sodium ion in electrolyte solution were simulated by molecular
Table 4 Self-Diffusion Coefficients of Cd+2, Cu+2 and Pb+2 ions in different types of zeolite with a thickness of 2 times the cell unit and concentration of 2 times sodium concentration. zeolite
EDI LTN FAU THO LTA NAT
Self-Diffusion (10−9 m2/s) Cd+2
Cu+2
Pb+2
19.79 10.79 7.14 22.56 3.58 17.67
40.96 17.40 13.87 27.18 21.66 21.91
21.84 9.28 7.12 11.40 8.60 10.44
FAU, LTN, THO, NAT and EDI zeolite structures are presented, in which the tendency of each of the lead, cadmium and copper atoms for being along with Al in the structure of zeolite is investigated. The first sharp peak is related to the Cd2+ ion, which is located within the 3–4 Å of the aluminum atom inside the zeolite structure, and this behavior is visible in all of the structures of zeolite other than the LTN structure. For LTN 7
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Fig. 6. Radial distribution function for Cd2+, Cu2+, and Pb2+ ions with a concentration of 2 times the sodium in LTA, FAU, LTN, THO, NAT and LTN zeolite structures with a thickness of 2 times the cell unit.
will be less intrinsic to absorb ions than others. In addition, for electrolyte solutions containing Pb / Cd, the results show that zeolite structures tend to exchange cadmium ions relative to lead, and in most structures, approximately 80% of cadmium ions are exchanged, which is approximately equal to 100% for EDI structure. However, the exchange value for lead ions would not exceed 60%, so the results confirm that this structure of zeolite tends to attract iodine cadmium to lead ion. Furthermore, the results for electrolyte solution including lead ion and copper ions indicated that most zeolite structures tend to be more attractive to copper adsorption and exchange about 80%, which is about 100% for EDI zeolite. However, for FAU zeolite, the copper removal rate was approximately 40%.
dynamics method to study the effect of simultaneous presence of metal ions in electrolyte solution. The simulation results of ion exchange ratios for a two-ion electrolytic solutions Cu / Cd, Pb / Cd and Pb / Cu with LTA, FAU, LTN, THO, NAT and EDI zeolite structures are shown in Fig. 7. The results show that the selectivity of zeolites are strongly dependent on their molecular structure, so that in an EDI structure and electrolyte containing Cd / Cu, approximately 100% of cadmium ions and 85% copper ions were exchanged that had the highest selectivity to cadmium. For other zeolites except FAU, there was almost the same behavior. In the FAU structure, the cadmium ion selectivity is higher than copper, but this zeolite cannot exchange all cadmium ions, as well as about 40% copper ion exchange. Since in this structure the size of the cavity in which it penetrates is more and almost twice as high as the other zeolites (7.35 Å), the gravitational effect of the zeolite structure 8
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Fig. 7. Ion exchange ratio in binary system for zeolite with LTA, FAU, LTN, THO, NAT and LTN structures.
4. Conclusion
structure, ie, the size of the cavities and the number of adsorption sites. In addition, under competitive conditions, the ionic exchange for an electrolyte containing two metal cation, the repulsing effect of cations on each other has also been effective.
Simulation results of the ion exchange process in single-ion systems for all zeolite structures showed that the minimum and maximum ion exchange rate is related to Pb2 + ion and Cd2 + respectively. The simulation results of the metal ions adsorption by different structures zeolites show that the tendency to carry out the ion exchange process for all zeolites is Cd2+ > Cu2+ > Pb2+. In fact, these adsorption behaviors depend on the structure of zeolite and the interaction of zeolite with metal ions. The results also show copper ion has a higher displacement is in all zeolite structures. It can be due to the low weight of copper ion which can easily move inside the zeolite structure. Furthermore, the selectivity behavior of zeolites under the same conditions and the same Al / Si ratio strongly depends on the molecular
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